Pesticides Research, 57
A laboratory model system for determining the volatility of pesticides
Table of contents
Executive summary
In the present study the volatilisation potential of pesticides is discussed and illustrated based on both physical/chemical properties of the pesticides and on experimental volatilisation experiments. A laboratory model system for the direct determination of the volatilisation of pesticides from different surfaces has been developed. In this system it is possible to evaluate volatilisation of pesticides relative to each other. Experiments have been carried out with mecoprop-P (MCPP-P), mecoprop methylester (MCPP methyl), and lindane as test substances. The volatilisation of these pesticides has been tested in different volatilisation chambers, at different temperatures, airflows and from different surfaces. Furthermore, recommendations of important aspects that should be included in the development of a new guideline on assessing pesticide volatilisation are described. The main conclusions regarding the experimental set-up are drawn taking practical aspects of testing and the demand for simple cost-effective tests into account.
At the beginning of the experiments, MCPP-P and lindane were applied on a filter-paper corresponding to an application rate of 1.5; 15; 75, and 150 kg ai ha-1 for MCPP-P and 1; 10; 50; and 100 kg ai ha-1 for lindane; corresponding to the recommended application rate; 10´ ; 50´ ; and 100´ the recommended application rate. During the experimental duration of 24 hours only insignificant volatilisation of MCPP-P was measured (less than 1%) while the volatilisation of lindane varied from 3.6 to 75.6 % of the applied dose depending of the concentrations. However, the actual amount of volatilised lindane was relatively constant at concentrations above 10´ the recommended doses. The results indicate that MCPP-P does not appreciably volatilise from an artificial surface whereas lindane volatilises. In contrast to MCPP-P, a great deal of the applied MCPP methyl volatilised under the present test conditions. Thus, when MCPP methyl was applied at a rate equivalent to the recommended application rate, about 90% of the applied dose were volatilised during a 24 hours period. Furthermore, the results revealed comparable volatilisation of the test substances when the experiments were carried out in different volatilisation chambers with varying design.
Interpreting the volatilisation as a function of time from various volatilisation experiments with pesticides, it seems that the air sampling 1, 3, 6, and 24 hours after application asked for in the German guideline are reasonable. Normally, high volatilisation rates are seen within the first few hours after application, and the 24 hours values after application are measured at a time when the volatilisation process has decreased considerably.
To test the influence of the temperature on the volatilisation of the two pesticides lindane and MCPP methyl were applied to filter paper at the recommended dose and the volatilisation was tested at 15oC and 23oC under otherwise identical conditions. The volatilisation of lindane increased significantly with temperature (from about 46% to 80% of the applied dose) whereas only a slight increase was noted for MCPP methyl (from 90% at 15oC to 96% of the applied dose at 23oC).
At the recommended application rate, about 80% of the applied lindane volatilised, while at 10´ the recommended dose only about 20% of the applied amount volatilised corresponding to the actual amount of lindane volatilising increased with a factor of 2.4. When the amount of MCPP methyl was increased with a factor of 10, the amount of MCPP methyl volatilising increased with a factor of 8.9 illustrating the higher volatilisation potential of this compound.
Volatilisation of lindane and MCPP methyl was also tested at two different airflows. The two airflows were 0.17 and 0.67 m per sec and the application rate corresponded to the recommended application rate and 10 ´ this dose. The results revealed that an increase of the airflow by a factor of 4 does not significantly increase the amount of volatilisation of the two pesticides.
MCPP methyl and lindane were applied as active ingredients to the surface of two artificial surfaces, filter paper representing an absorbent surface and a bowl of stainless steel illustrating a non-absorbent surface. Volatilisation of the two pesticides after 24 hours were comparable when applied to the two selected surfaces. However, it seems that the volatilisation of MCPP methyl was faster at the non-absorbent surface, while the rate of volatilisation of lindane was comparable when applied at the two different surfaces. In contrast to the artificial surfaces only a small fraction (about 15% of the applied dose) of the pesticides volatilised when applied to a typical Danish soil.
Dansk sammendrag
I denne undersøgelse diskuteres fordampningspotentialet af pesticider ud fra pesticidernes iboende fysisk/kemiske egenskaber samt ud fra eksperimentelle fordampningsforsøg. Der er udviklet en eksperimentel forsøgsopstilling, hvor det er muligt at måle fordampningen af pesticider fra forskellige overflader. I denne forsøgsopstilling er det muligt at sammenligne fordampningspotentialet af forskellige pesticider. De eksperimentelle undersøgelser er blevet udført med mecoprop-P (MCPP-P), mecoprop methylester (MCPP methyl) og lindan, og fordampningen af disse pesticider er blevet undersøgt i forskellige fordampningskamre, ved forskellige temperatur, luftflow og fra forskellige overflader. Baseret på erfaringerne fra disse og andres undersøgelser er der udarbejdet forslag til, hvilke elementer der skal inddraget i forbindelse med udarbejdelsen af en ny guideline til vurdering af pesticiders fordampning.
Ved starten af forsøgsrækken blev MCPP-P tilført filtrerpapir i følgende koncentrationer 1,5; 15; 75 og 150 kg ai per ha. og lindan i 1, 10, 50, og 100 kg ai per ha, svarende til den anbefalede dosering samt 10´ , 50´ , og 100´ den anbefalede dosering. Gennem forsøgsperioden på 24 timer blev der kun registreret en ubetydeligt fordampning af MCPP-P (mindre end 1% af den tilførte mængde), mens fordampningen af lindan varierede fra 3,6 til 75,6% af den tilførte mængde (den totale mængde af lindan, der fordampede, var dog relativt konstant ved koncentrationerne over 10´ den anbefalede dosis). Disse resultater viser, at MCPP-P ikke forventes at fordampe fra sprøjtede overflader, mens lindan forventes at fordampe i store mængder. I modsætning til MCPP-P blev der iagttaget en meget stor fordampning af MCPP metyl. Således fordamper 90% af den tilførte mængde indenfor 24 timer, når MCPP metyl tilføres i en koncentration svarende til den anbefalede mængde. Der blev iagttaget samstemmende resultater i forsøg udført i forsøgskamre med varierende design.
Ved at gennemgå kinetikken hvormed forskellige pesticider fordamper gennem forsøgsperioden, virker prøveudtagningsintervallerne på 1, 3, 6 og 24 timer, som angives i den tyske guideline, rimelige. Normalt ses der en høj fordampning de første timer efter stoftilførslen, og efter 24 timer er fordampningen aftaget markant.
For at undersøge temperaturens indflydelse på fordampningen blev der udført forsøg med MCPP methyl og lindan ved både 15oC og 23oC. Ved disse forsøg blev der iagttaget en markant forøget fordampning af lindan (fra ca. 46% ved 15oC til ca. 80% af den tilførte mængde ved 23oC), mens der kun blev iagttaget en mindre stigning i fordampningen for MCPP methyl (fra 90% ved 15oC til ca. 96% ved 23oC).
Ved den anbefalede dosering fordampede ca. 80% af den tilførte mængde efter 24 timer, mens der ved en stoftilførsel på 10´ den anbefalede dosering kun blev iagttaget en fordampning på ca. 20% af den tilførte mængde svarende til, at den absolutte mængde af lindan, der fordampede, blev forøget med en faktor på 2,4. Når den tilførte mængde af MCPP methyl blev forøget med en faktor 10, blev den absolutte mængde MCPP methyl, der fordampede, forøget med en faktor 8,9 hvilket illustrerer, at dette stof fordamper lettere.
Fordampningen af lindan og MCPP methyl blev undersøgt ved to forskellige luftgennemstrømninger (ved en lufthastighed på henholdsvis 0,17 og 0,67 meter pr. sekund). Resultaterne viste, at en forøgelse af hastigheden på luftgennemstrømningen med en faktor 4 ikke forøgede den mængde af pesticid, der fordampede.
MCPP methyl og lindan blev tilført til to forskellige kunstige overflader, filtrerpapir, der illustrerer en absorberende overflade, og en metalbakke, som illustrerer en ikke absorberende overflade. Fordampningen af de to pesticider fra de to forskellige overflader var sammenlignelige efter 24 timer, men der blev iagttaget en hurtigere fordampning af MCPP methyl fra den ikke absorberende overflade. Ved tilførsel af lindan og MCPP methyl til en typisk dansk jordtype fordamper der til forskel fra de kunstige overflader kun en lille andel (ca. 15%) af den tilførte pesticidmængde.
1. Introduction
Since the late 1960s, losses of pesticides by volatilisation and subsequent atmospheric transport have been increasingly recognised as a process, sometimes of major importance, in the loss of pesticides from the areas where they are applied (a process limiting their effectiveness), and as a pathway for general environmental contamination (extensive lists of literature are provided in Nolting. et al. 1988; Gottschild, et al. 1995; Jansma and Linders 1995; Spencer et al. 1973; Hartley, 1969; Plimmer 1976; Willis et al. 1983). Therefore, volatilisation is assumed to be the reason why some pesticides are widely distributed, contributing to pollution of air, rain, soil, surface, and seawater. Recently, rather high concentrations of pesticides in rainwater have been determined (Buser, 1990; Gath et al. 1992; Gath et al. 1993; Siebers et al. 1994; Glotfelty et al. 1990; Schomburg et al. 1991; Nations and Hallberg 1992; Scharf and Bächmann 1993, Felding et al. 1999).
Volatilisation rates from plant or moist soil surfaces can be very large for more volatile compounds, with losses approaching 90% within a few days (Taylor, 1978; Jansma and Linders, 1995). Although pesticides range in volatility from fumigants, such as gaseous methyl bromide, to herbicides, with vapour pressures below 10–6 Pa, the same physical/chemical principles are assumed to govern their rates of volatilisation. However, even though factors, influencing volatilisation in principle, have been clarified and understood for a long time (Spencer et al. 1973; Hartley, 1969; Plimmer, 1976), many problems concerning volatilisation still remain unsolved.
Since volatilisation is an important factor in the fate of pesticides in the environment, it is necessary to have an experimental design to measure this process. In 1990 a German Guideline on assessing pesticide volatilisation was developed. This BBA guideline (Nolting et al. 1990) was prompted not only by the legal demands for protecting the air, but also by increasing public concern for pesticide residues found in precipitation and ground water. It was decided to start with a "liberal" guideline with only a few specific demands, which then later on could be refined, based on the results obtained. Since then a number of methods have been developed, ranging from very simple to high-tech designs.
The volatilisation rate is not only determined by the properties of the compound, the application rate, and the crop, but also by other factors like meteorological and soil conditions. As these other factors are highly variable field experiments for the same compound, application rate and crop can show highly variable results. This makes it difficult to determine whether the volatilisation rate of a particular compound is comparable with other compounds. Moreover, field experiments are rather expensive. For that reason it would be preferable if laboratory experiments could be undertaken that could complement field experiments and could indicate the volatilisation potential of different compounds relative to each other.
For this reason a simple laboratory model system has been designed for the present study, in which a wide range of outdoor conditions can be simulated. The finalised method has then been used to measure the volatility of selected pesticides from different surfaces and under different "climatic" conditions such as temperature, wind speed etc.
2. Materials and methods
2.1 Test system
A simple laboratory model system for the direct determination of the volatilisation of pesticides has been developed for this study. The system permits a direct determination of the volatilisation of a compound from a surface. Moreover, it gives the possibility to check the mass balance at the end of the study. Furthermore, the system permits determination of the volatilisation potential of different compound added on varying surfaces, at different airflows, temperatures etc.
2.1.1 Volatilisation chamber
Two chambers of different sizes have been used in the present study. Both chambers were made of stainless steel, and each chamber was made up of 3 parts:
The wind tunnel (1) in which a rectangular bowl of stainless steel (2) containing the test sample can be placed. By the open side (3), opposite to the air outlet, the bowl can be introduced and removed for changing of the test substrate. The open side also permits the cleaning of the chamber inner walls.
The test sample is placed in the rectangular metal bowl that is introduced into the volatilisation chamber immediately after application of the pesticides to the surface. After the bowl has been placed in the "wind tunnel", the open side of the chamber is closed with a plate of stainless steel in which the air inlet is placed. The air inlet is connected to an ORBO-42 (large) adsorption tube to remove potential contamination of pesticides from the inlet air.
The volatilised pesticides are collected on three ORBO-42 (large) adsorption tubes connected in series to the outlet of the chamber. A given air flow in the "wind tunnel" is ensured by a constant flow pump, SKC model 224-17SD connected at the end of each series of adsorption tubes. Each series of adsorption tubes can be replaced at different sampling intervals during an experiment.
2.1.2 System set-up
Test compounds
The volatilisation of mecoprop –P (MCPP-P), mecoprop methylester (MCPP methyl) and lindane have been tested in this study. A range finding test has been carried out with concentrations corresponding to the recommended dose, 10´ , 50´ , and 100´ the dose to determine which dose would be useful to apply in the final test. In the final test the pesticides were applied at an application rate equivalent to 1 and 10 kg ai ha-1 for lindane and 1.5 and 15 kg ai ha-1 for MCPP methyl corresponding to the recommended and 10´ the recommended dose.
Size of the chambers
Initially the experiments were performed in two volatilisation chambers of different sizes in order to test the influence of the size of the test chamber on the volatilisation of the pesticides.
One set of experiments was carried out in a "short" volatilisation chamber in which the "wind tunnel" was 12 cm long, 3.5 cm wide and 1.7 cm high. Another set of experiments was carried out in a "longer and lower" chamber in which the "wind tunnel" was 23 cm long, 3.6 cm wide and 1.0 cm high. The metal bowl in the "short" chamber had the following inner dimensions: 11.1 cm long, 2.25 cm wide and a depth of 0.3 cm. The dimension of the bowl in the "longer chamber" was 22.9 cm long, 2.4 cm wide and 0.3 cm deep.
Temperatures and air humidity
The volatilisation of the pesticides were tested at two different temperatures. Normally the experiments were carried out in a climatic room at a temperature of 23Co +/- 0.5o C and an air humidity of 50%. The volatilisation was also tested at 15oC +/- 0.5o C and in these cases the volatilisation chamber was placed in an incubator and the air was passed through an air glass bubbler containing a calcium chloride solution. The air arrived in the volatilisation chamber at about 50% relative humidity.
Air temperature and moisture were monitored during the study period using a Testo 600, Testoterm.
Airflow
Different airflows in the "wind tunnel" can be created by opening different numbers of air outlet from the volatilisation chamber which each was connected to a set of adsorption tubes and an air pump. This was done to ensure the recommended flow rate through the ORBO tubes. In the present study, an airflow rate of 0.08 m/sec and 0.31 m/sec was tested in the "short chamber". In the "longer and lower chamber", an airflow rate of 0.17 m/sec and 0.67 m/sec was tested.
The airflow was measured using a Termo-Anemometer, Alnor GGA-65P.
Application on different surfaces
The pesticides were applied to three different surfaces:
 | Filter paper (Whatman 1) illustrating an artificial absorbent surface. |
| Directly to the bowl of stainless steel illustrating a non-absorbent surface. |
| A coarse sandy soil from the Danish National Agricultural Research station at Jyndevad, typical Danish soil. The relevant soil characteristics are described in table 2.1. The test was carried out using 8.5 g sieved soil less than or equal to 2 mm which has been stored air-dried and re-equilibrated with deionised water just before the experiment to give an overall moisture content of about 35 per cent of the dry weight. |
Table 2.1
Soil characteristics for the Jyndevad soil.
Jordkarakteristika for Jyndevadjorden.
Clay |
3.9 % |
Silt |
4.1% |
Sand |
89.0% |
Humus |
3.0% |
Soil water capacity (a) |
34.0% |
PH |
6.5 |
2.2 Performance of the test
2.2.1 Chemicals and standards
All solvents used were of analytical grade. MCPP-P, MCPP methyl and lindane were obtained from Dr. Ehrenstorfer GmbH, Germany. Hexachlorbenzene-13C6 and dichlorprop-13C6 were obtained from Cambridge Isotope laboratories, Woburn, MA, USA. Standards were prepared using standard volumetric techniques.
2.2.2 Preparation and spiking of the samples
The test surface was placed in the rectangular metal bowl and spiked with the analytes dissolved in acetone. After evaporation of acetone at room temperature the metal bowl was placed in the volatilisation chamber. The application time was less than 3 minutes. For tests with filter paper the analytes were dissolved in 250 and 500 m l acetone for the "short" and "longer" chamber, respectively, and added by several applications by a Hammilton syringe. For tests in which the pesticides were applied directly to the metal bowl or to soil the analytes were dissolved in 3 ml acetone and applied to the metal bowl or to the soil by several applications.
Pre-test experiments
To investigate whether some part of the pesticides applied to the test surface would evaporate together with the acetone during application before the test sample was placed in the volatilisation chamber, spiked samples were placed at room temperature for 30 minutes after which they were analysed. To test the recovery from the adsorption tubes, the analytes were dissolved in 200 m l acetone and added to the ORBO-42 (large) adsorption tubes after which the tubes were connected to an air pump. Airflow of 2 l/h was maintained during 24 hours. After this period, the absorption tubes were analysed.
2.2.3 Sample preparation
Sampling and samples treatment
When several samplings were taking during an experiment the adsorption tubes were immediately replaced with a new trap set-up. At the end of the experiments each set of adsorption tubes was removed from the chamber and each of the three tubes in a set was analysed, separately.
At the end of an experiment the volatilisation chamber and the bowl including the test surface applied with the pesticides were extracted with 200.0 ml methylene chloride in a Pyrex bottle by shaking for 30 minutes. 2.0 ml of the extract was diluted and spiked with internal standard(s).
The adsorption tubes were extracted with 5.0 ml methylene chloride by shaking for 5 minutes. 2.0 ml of the extract was diluted and spiked with internal standard(s).
The sample extracts to be analysed for MCPP-P and lindane were spiked with 20 m l HCB-13C6 (500 ng/m l) and 20 m l dichlorprop-13C6 (500 ng/m l) as internal standards. MCPP-P and dichlorprop-13C6 were methylated with diazomethan before analysing.
The sample extracts to be analysed for MCPP methyl and lindane were spiked with 20 m l HCB-13C6 (500 ng/m l) as internal standard.
The extracts were analysed by GC-MS-SIM.
2.2.4 GC-MS-SIM analysis
The extracts were analysed by GC-MS-SIM in the electron impact mode (EI, 70 eV).
Ions for qualification and quantification were selected on the basis of the mass spectra of the components. The ions 219 and 221 amu were selected for lindane, 169, 228 and 230 amu for MCCP methyl, 168 and 254 amu for dichlorprop-13C6 and 290 and 292 for HCB-13C6. The quantification was done on the basis of the area ratio analyte/internal standard for the selected ions.
The mass scale was calibrated using PFTBA as a calibration gas.
The calibration curves were established on solutions in methylene chloride of the analytes. This was done on the basis of experiments showing that the extraction efficiency of the analytes from the filter paper was 92-96 % and from the adsorption tubes 87-93 %.
Injection technique: on-column injection (35° C). Precolumn: 1 meter 0.25 mm id, 0.25 m m RTX-5. Column: WCOT fused silica 50m ´ 0.25 mm id, 0.4 m m CP Sil 13CB. Temperature programming: 35° C (0.5 min)-280° C, 30° C/min. Carrier gas: Helium (25 psi). GC-MS: HP5971.
3. Results
The test design includes two independent tests (on different days). Thus, all data
represent the mean of two experiments. All experiments were carried out at 23° C unless something else has been stated. The detection limit for
all analytes was 0.05 m g.
3.1 Pre-test of the experimental set-up
To investigate whether some of the pesticides applied to the test surface would evaporate together with the acetone during application before the test sample was placed in the volatilisation chamber, spiked samples were placed at room temperature for 30 minutes after which they were analysed. The analyses showed that the pesticides did not evaporate together with the acetone and a recovery of more than 90% was found in these experiments.
The extraction efficiency from the adsorption tubes was tested and the results revealed that the recovery was more than 95%.
3.2 Results of the experiments in a short volatilisationchamber after 24 hours
3.2.1 Volatilisation of MCPP-P applied to filter paper at different concentrations
The volatilisation of MCPP-P applied to filter paper was tested at different concentrations: 0.015, 0.15, 0.75, and 1.5 mg MCPP-P/cm2, corresponding to the recommended doses, 10´ , 50´ , and 100´ the recommended dose. The results revealed that less than 0.06% of the applied pesticide volatised after 24 hours in the volatilisation chamber with airflow of 0.08 m/sec. At the end of the experiment between 89 and 110% of the applied pesticide could be extracted from the filter paper and the volatilisation chamber.
3.2.2 Volatilisation of lindane applied to filter paper at different concentrations
When lindane was applied to a filter paper at concentrations of 0.01, 0.1, 0.5, and 1.0 mg/cm2 corresponding to the recommended application rate, 10´ , 50´ , and 100´ the recommended dose a significant amount of the added pesticides volatised. The airflow was 0.08 m/sec. The results are depicted in table 3.1.
Table 3.1
Volatilisation of lindane after a 24 hours period. S.D. less than 10%.
Fordampningen af lindan efter en periode på 24 timer. S.D. mindre end 10%
Applied concentration of lindane |
Volatilisation of lindane (% of applied dose) |
Extraction from paper and chamber (in %) |
Total recovery |
Recommended dose 10´ recomm. dose 50´ recomm. dose 100´ recomm. dose |
75.6 31.2 8.1 3.6 |
24.0 60.0 80.8 86.8 |
99.6 91.2 88.9 90.4 |
Figure 3.1
The volatilisation of lindane relative to the applied amount (R2 = 0.98).
Fordampning af lindan relativt i forhold til tilført mængde (R2 = 0,98).
The volatilisation of lindane under the present experimental conditions corresponds to 7.6, 31.2, 40.5 and 36 m g lindane/cm2 for the recommended doses, 10´ , 50´ and 100´ the recommended doses, respectively. Thus, the results revealed that a comparable amount of lindane is volatised in concentrations corresponding to 10´ 50´ and 100´ the recommended doses. It was therefore decided to investigate the volatilisation of the pesticides at the recommended dose and 10´ the concentration in the further experiments.
3.2.3 Volatilisation of MCPP methyl and lindane applied to different artificial surfaces and at different airflows
Volatilisation of MCPP methyl from a non-absorbent surface
Since no volatilisation could be demonstrated in the MCPP-P experiments it was decided to start experiments with MCPP methyl. The results from these experiments revealed that a great deal of the applied MCPP methyl volatilised under the present test conditions. Application of MCPP methyl at a concentration corresponding to 10´ the recommended dose to a non-absorbent metal surface resulted in a volatilisation of the pesticide corresponding to 85.5% of the applied amount after 24 hours (air flow 0.08 m/sec). Extraction of pesticides from the metal surface and the chamber revealed 4.8 % of the applied dose resulting in a recovery of 90.3%.
Volatilisation of lindane from a non-absorbent surface
The volatilisation of lindane from a non-absorbent metal surface was tested with airflow of 0.08 m/sec and at a pesticide concentration corresponding to 10´ the recommended application. The results revealed that after 24 hours 30% of the applied dose was volatilised while 61% could be extracted from the metal surface and the chamber. This gave a recovery of 91%.
Volatilisation of lindane and MCPP methyl with an increased airflow
The volatilisation of lindane and MCPP methyl applied to filter paper at a concentration corresponding to 10´ the recommended dose was tested in the volatilisation chamber with an increased airflow (0.31 m/sec). The results revealed that after 24 hours 34.1% of the applied lindane were volatilised and 64% could be extracted from the paper and chamber giving a recovery of 98%. For MCPP methyl 95,8% of the applied dose was volatilised and 0.3% was found on the filter paper and in the chamber a recovery was 96%.
3.3 Results of volatilisation of lindane and MCPP methyl in a "longer and lower" volatilisation chamber
3.3.1 Volatilisation of lindane and MCPP methyl applied at recommended dose during an experimental test of 30 hours
Figure 3.2 illustrates the accumulated volatilisation of lindane and MCPP methyl from filter paper during a 30 hours period at a temperature of 23° C and an air humidity of 50%. The application rate was 0.01 mg/cm2 for lindane and 0.015 mg/cm2 for MCPP methyl, corresponding to the recommended application rate. The airflow in the chamber was 0.17 m/s. About 10% and 30% of the applied dose had volatilised during the first hour after application of lindane and MCPP methyl, respectively. After 8 hours about 50% of the applied dose of lindane was volatilised corresponding to 2/3 of the total amount which volatised during a 30 hours period. For MCPP methyl about 80% of the applied pesticide was volatised after 8 hours, corresponding to about 9/10 of the total amount during a 30 hours test. At the end of the experiment 14.5% and 4.1% of the applied pesticides could be extracted from the filter paper and the chamber for lindane and for MCPP methyl, respectively. Thus, the recovery in these experiments was 90% for both pesticides.
Figure 3.2
The figure illustrates the accumulated volatilisation of lindane and MCPP methyl from
filter paper during a 30 hours period. The application rate was 0.01 mg/cm2 for
lindane and 0.015 mg/cm2 for MCPP methyl. The airflow in the chamber was 0.17
m/s. All data represent the mean of at least two experiments (S.D. less than 5%).
Figuren illustrerer den akkumulerede mængde af fordampet lindan og MCPP methyl fra filterpapir over en periode på 30 timer. Stofferne blev tilført i en koncentration svarende til 0,01 mg/cm2 for lindan og 0,015 mg/cm2 for MCPP methyl. Luftgennemstrømningen i kammeret var 0,17 m/s. Alle data er middeltallet af mindst to forsøg (S.D. er mindre end 5%).
In table 3.2 the volatilisation is expressed as the amount of pesticide volatili-sed per hour during the experiment. The results revealed that the rate of volatilisation was significantly higher for MCPP methyl compared with the rate found for lindane during the first 6 hours after application. For MCPP methyl a very high but rapid decreasing volatilisation was seen during the first 8 hours. For lindane the rate of volatilisation was much lower, even though the same pattern was seen, in which a higher rate of volatilisation occurred during the first 8 hours after application. However in this case, the rate of decrease in the volatilisation was slower than the rate found for MCPP methyl and after 8 hours the rate of volatilisation of lindane was higher than the one found for MCPP methyl.
Table 3.2
Volatilisation of lindane and MCPP methyl per hour during a 30 hours period (S.D. less
than 10%).
Fordampning af lindan og MCPP methyl udtrykt pr. time i en periode på 30 timer (S.D. er mindre end 10%).
Hours after |
Volatilisation of lindane per hour (% of applied dose) |
Volatilisation of MCPP methyl per hour (% of applied dose) |
0-1 1-3 3-6 6-8 8-24 24-30 |
7.8 7.0 5.7 3.8 1.6 0.6 |
28.6 15.4 5.6 2.0 0.4 0.1 |
In order to test the influence of the temperature on the volatilisation of the two pesticides lindane and MCPP methyl were applied to filter paper at the recommended dose and the volatilisation was tested at 15° C and 23° C under otherwise identical test conditions.
As shown in figure 3.3, the volatilisation of lindane decreased significantly when the temperature was decreased from 23° C to 15° C. Each column illustrates the total amount of pesticide analysed after the test period. The lowest dark part of each column illustrates the amount of pesticide extracted from the filter paper and the volatilisation chamber, and the upper light part illustrates the amount of pesticides volatilised. The application rate was 0.01 mg/cm2 for lindane and 0.015 mg/cm2 for MCPP methyl. The airflow in the chamber was 0.17 m per sec. Thus, at 23° C about 80% of the applied lindane was volatised after 24 hours whereas only 46% was volatised at 15° C. In contrast the amount of MCCP, that volatised during a 24 hours period, only decreased from 96% at 23° C to 90% of the applied dose at 15° C.
Figure 3.3
The figure illustrates the volatilisation and total recovery (whole column) of lindane
and MCPP methyl at 15° C and 23°
C from filter paper after a 24 hours period. Each column illustrates the total amount of
pesticide analysed after the test period (total recovery). The upper light part
illustrates the amount of pesticides volatilised. All data represent the mean of at least
two experiments (S.D. less than 10%).
Figuren illustrerer fordampningen og den totale "recovery" af lindan og MCPP methyl ved 15° C og 23° C fra filterpapir efter en periode på 24 timer. Hver søjle illustrerer den totale mængde pesticid, der blev analyseret efter testperiodens ophør (total "recovery"). Den lyse del illustrerer den mængde pesticid, der er fordampet. Data repræsenterer et gennemsnit af mindst to forsøg (S.D. er mindre end 10%).
3.3.3 Volatilisation of lindane and MCPP methyl applied at two different doses
Figure 3.4 illustrates the volatilisation and the total recovery of lindane and MCPP methyl from filter paper after a 24 hours period. Each column illustrates the total amount of pesticide analysed after the test period (total recovery). The lowest dark part of each column illustrates the amount of pesticide extracted from the filter paper and the volatilisation chamber, and the upper light part illustrates the amount of pesticides volatilised. The airflow in the chamber was 0.17 m per sec. The application rate was 0.01 mg/cm2 and 0.1 mg/cm2 for lindane and 0.015 and 0.15 mg/cm2 for MCPP methyl, corresponding to the recommended application rate and 10´ the recommended application rate. At the recommended application rate 80.5% of the applied lindane volatilised corresponding to 0.439 mg, while at 10´ the recommended doses only about 19.6% of the applied amount volatised corresponding to 1.068 mg lindane. At the recommended application rate 95.9% of the applied MCPP methyl volatilised while the corresponding values were 85.5% at 10´ the recommended dose.
Figure 3.4
The figure illustrates the volatilisation and the total recovery of lindane and MCPP methyl at two different doses from filter paper after a 24 hours period. Each column illustrates the total amount of pesticide analysed after the test period (total recovery). The upper light part illustrates the amount of pesticides volatilised. The application rate was 0.01 mg/cm2 and 0.1 mg/cm2 for lindane and 0.015 and 0.15 mg/cm2 for MCPP methyl. All data represent the mean of at least two experiments (S.D. less than 8%).
Figuren illustrerer fordampningen og den totale recovery af lindan og MCPP methyl ved to forskellige doseringer 24 timer efter tilførsel til filterpapir. Hver søjle illustrerer den totale mængde pesticid, der blev analyseret efter testperiodens ophør (total "recovery"). Den lyse del illustrerer den mængde pesticid, der er fordampet. Doseringen var 0,01 mg/cm2 og 0,1 mg/cm2 for lindan og 0,015 mg/cm2 og 0,15 mg/cm2 for MCPP methyl. Data repræsenterer et gennemsnit af mindst to forsøg (S.D. var mindre end 8%).
3.3.4 Volatilisation of lindane and MCPP methyl at different air flow
Figure 3.5 illustrates the volatilisation and the total recovery of lindane and MCPP methyl from filter paper after a 24 hours period at two different airflows and at two different application rates. Each column illustrates the total amount of pesticide analysed after the test period (total recovery). The lowest dark part of each column illustrates the amount of pesticide extracted from the filter paper and the volatilisation chamber, and the upper light part illustrates the amount of pesticides volatilised. The two airflows were 0.17 and 0.67m per sec and the application rate was 0.01 mg/cm2 and 0.1 mg/cm2 for lindane and 0.015 and 0.15 mg/cm2 for MCPP methyl, corresponding to the recommended application rate and 10´ the recommended application rate. The results revealed that an increase of the airflow by a factor of 4 does not significantly increase the amount of volatilisation of the two pesticides at the two selected concentrations.
Figure 3.5
The figure illustrates the volatilisation and the total recovery of different
concentrations of lindane and MCPP methyl at two different airflows. Each column
illustrates the total amount of pesticide analysed after the test period (total recovery).
The upper light part illustrates the amount of pesticides volatilised. The application
rate was 0.01 mg/cm2 and 0.1 mg/cm2 for lindane and 0.015 and 0.15
mg/cm2 for MCPP methyl. The airflow in the chamber was 0.17 and 0.67 m/s.,
respectively (corresponding to an air exchange rate of 2 and 8 l/min). All data represent
the mean of at least two experiments (S.D. less than 10%).
Figuren illustrerer fordampningen og den totale recovery af forskellige koncentrationer af lindan og MCPP methyl ved to forskellige lufthastigheder. Hver søjle illustrerer den totale mængde pesticid, der blev analyseret efter testperiodens ophør (total "recovery"). Den lyse del illustrerer den mængde pesticid, der er fordampet. Stofferne blev tilført i koncentrationer på 0,01 og 0,1 mg/cm2 for lindan og 0,015 samt 0,15 mg/cm2 for MCPP methyl. De to undersøgte lufthastigheder var henholdsvis 0,17 og 0,67 m/s.(svarende til en luftgennemstrømningshastighed på 2 og 8 l/min). Data er et gennemsnit af mindst to forsøg (S.D. er mindre end 10%).
3.3.5 Volatilisation of lindane and MCPP methyl applied to different surfaces
Artificial surfaces
Table 3.3 illustrates the volatilisation of lindane and MCPP methyl applied to two different artificial surfaces. The pesticides were applied to a filter paper illustrating an absorbent surface and to a metal plate illustrating a non-absorbent surface. Measurements were taken during a 24 hours period. The application rate was 0.01 mg/cm2 for lindane and 0.015 mg/cm2 for MCPP methyl. The airflow in the chamber was 0.17 m per sec. The recovery in the experiments with filter paper was 91.4% for lindane and 90.3 for MCPP methyl. For the non-absorbent surface a recovery of 90.3 and 91.7% was found for lindane and MCPP methyl, respectively. All data represent the mean of at least two experiments (S.D. less than 10%).
Table 3.3
Volatilisation of lindane and MCPP methyl applied to an absorbent surface and a non-absorbent metal surface during a 24 hours period (S.D. less than 10%).
Fordampning af lindan og MCPP methyl tilført en absorberende overflade og en ikke absorberende metaloverflade i en 24 timers periode (S.D. mindre end 10%).
Hours after |
Volatilisation of
lindane |
Volatilisation of MCPP
methyl |
||
|
Applied to |
Applied to |
Applied to |
Applied to non-absorbent |
0-1 1-3 3-6 6-24 |
7.8 14.0 17.2 33.5 |
10.1 17.2 13.0 33.3 |
28.6 30.7 16.8 10.3 |
38.2 38.2 9.2 5.1 |
Total |
72.5 |
73.5 |
86.4 |
90.6 |
The results revealed that the volatilisation of each pesticide was comparable when applied to the two selected surfaces. However, even though the total amount of volatilised pesticides was the same after 24 hours it seems that the volatilisation of at least MCPP methyl was faster at the non-absorbent surface during the initial volatilisation phase. Thus, 3 hours after application 76.4% of the applied MCPP methyl was volatilised when applied to the metal surface compared with 59% when applied to the filter paper.
Soil surfaces
Table 3.4 illustrates the volatilisation of lindane and MCPP methyl applied to a typical Danish soil. Measurements were made during a 24 hours period. The application rate was 0.01 mg/cm2 for lindane and 0.015 mg/cm2 for MCPP methyl. The airflow in the chamber was 0.17 m per sec.
Table 3.4
Volatilisation of lindane and MCPP methyl applied to a typical Danish soil during a 24
hours period. All data represent the mean of at least two experiments (S.D. less than
10%).
Fordampning af lindan og MCPP methyl tilført en typisk dansk jord i en 24 timers periode. Alle data repræsenterer et gennemsnit af mindst to forsøg (S.D. mindre end 10%).
Hours after |
Volatilisation of
pesticides |
|
Lindane |
MCPP methyl |
|
0-1 1-3 3-6 6-24 |
1.7 2.6 2.0 6.7 |
3.3 3.2 2.4 7.0 |
Total |
13.0 |
15.9 |
In contrast to the artificial surfaces only a small fraction of the pesticides volatised when applied to a soil. Furthermore, it was interesting to notice that no marked difference was seen between the volatilisation of lindane and MCPP methyl when applied to the soil. At the end of the experiment 76.6 % and 64.9 % of the applied pesticides could be extracted from the soil and the chamber for lindane and MCPP methyl, respectively. Thus, the recovery in these experiments was 89.6% for lindane and 80.8% for MCPP methyl.
4. Discussion
Volatilisation plays an important role in the dispersion of pesticides in the environment. Volatilisation also leads to a rapid transport and distribution of pesticides in the atmosphere, resulting in considerable wet and dry deposition. Since volatilisation is an important factor in the fate of pesticides in the environment, it is necessary to have a method to evaluate the process.
According to Thomas et al. (1990), volatilisation can be defined as the process by which a compound evaporates in the vapour phase to the atmosphere from another environmental compartment. For pesticides and other chemicals potential volatility is related to physical/chemical parameters such as vapour pressure and Henry´s law constant of the compound. However, actual volatilisation rate will also depend on environmental conditions such as wind speed and temperature among others, and all factors that modify or attenuate the effective vapour pressure of the pesticide (Spencer, W.F. et al. 1973).
In spite of all the work that has been done in this field, up till now, measuring or assessing volatilisation is not a simple task. Many models simulating or predicting volatilisation have been published (e.g. Chen C et al. 1995; Lindhardt et al. 1994; Lindhardt and Christensen, 1994; Jansma and Linders, 1995, Smit et al., 1997, 1998). For a survey of part of the literature cf. Nolting et al., 1988. However, all the models can only be applied under certain, usually very restrictive, conditions. Therefore, the search for a better understanding of the volatilisation process and for methods accurately assessing pesticide volatility still goes on.
In the present study the volatilisation potential of pesticides is discussed and illustrated based on both physical/chemical properties of the pesticides and on experimental volatilisation experiments with the three pesticides: MCPP-P, MCPP methyl, and lindane. A laboratory model system for the direct determination of the volatilisation of pesticides from different surfaces has been developed. The purpose of this laboratory set-up is to evaluate volatilisation of pesticides relative to each other.
4.1 Estimation of volatility of pesticides based on physical/chemical parameters of the test substance
One of the most important physical/chemical properties influencing a substance’s volatility is its vapour pressure. Every chemical has a characteristic saturation vapour pressure which varies with temperature. Volatile substances, such as water, ethanol, and methanol, have comparatively high vapour pressures at room temperature (103 – 104 Pa). In contrast, pesticides have generally a comparatively low vapour pressure (approx. 10-7 – 10-2 Pa), however, many still volatilise at considerable rates (Krasel and Pestemer, 1993; Boehnke et al., 1990). The vapour pressure of lindane is measured to 5.6 x 10-3 Pa and 4.0 x 10-4 Pa for MCPP-P (at 20oC). No measured value was found for MCPP methyl (Meylan et al. 1994). Thus, judging from the measured vapour pressure lindane is expected to have a higher volatility than MCPP-P, which is in agreement with our experimental data. To make a relative comparison of all three pesticides the calculated vapour pressure was compared using a model of all 3 pesticides. The calculated values were 1.04 x 10–1 Pa for lindane, 1.1 x 10 -2 Pa for MCPP-P, and 2.5 x 10-1 Pa for MCPP methyl (all at 25oC). Even though a direct comparison between the measured and calculated values can not be made due to the use of different temperatures the relative difference between the pesticides is not changed significantly. Thus, based on these calculated vapour pressures it is expected that MCPP methyl has a higher volatility than lindane which has a higher volatilisation than MCPP-P. This is in agreement with our experimental measurements.
Another property that is often used in evaluation of the volatility of pesticides is their air-water partition coefficient, the Henry´s law constant. The Henry´s law constant is usually calculated by dividing vapour pressure (dimension Pa) by water solubility (dimension mol m-3). Consequently, uncertainties in vapour pressure values are reflected by Henry´s law constants as well. A high Henry´s law constant (> 1 Pa m3 mol-1) indicates a high volatility from aqueous solution (e.g., from moist soil). However, based on how the Henry´s law constant is calculated a substance with very low water solubility may have a rather high Henry´s law constant even if its vapour pressure is comparatively low. For the three pesticides used in the present study the calculated Henry´s law constant for lindane, mecoprop methyl and mecoprop P is 4.1, 1.03, and 2.7 ´ 10 – 3 (Pa m3 /mol), respectively. These values are based on calculated vapour pressure and a temperature of 25oC. Based on these values a much higher volatility for lindane and MCPP methyl would be expected compared with MCPP-P, which is in agreement with the present experimental measurements. However, based on Henry´s law constant a higher volatility of lindane would also be expected compared with MCPP methyl that is not in agreement with our experimental results. It may be due to the fact that the water solubility of lindane is decidedly lower than the solubility of MCPP methyl, contributing to the high Henry´s law constant. A comparable phenomenon has also been described by Walter et al. (1996) where the volatilisation of 3 different pesticides were compared (the active ingredients were not named because the study was a part of an interlab comparison). In spite of a higher Henry´s law constant of active ingredient this compound volatilised considerably less than the other two compounds which both have a higher vapour pressure, suggesting a higher volatilisation. Therefore, while these physico/chemical properties may be used to estimate relative volatility, deducing actual volatilisation behaviour from them may be erroneous. However, volatilisation of pesticides seems to be more related to the vapour pressure than to the Henry´s law constant.
4.2 Experimental measurement of volatilisation of pesticides with special reference to lindane, MCPP-P and MCPP methyl
It is evident that there are other factors besides vapour pressure and Henry´s Law constant that influence volatilisation, such as climatic parameters e.g. wind speed, temperature and humidity. Volatilisation of pesticides from natural surfaces is also influenced by their formulations, the application techniques and interactions of the substance to which it is applied (e.g. adsorption, desorption).
Looking at the practical field situation, many different factors and their interactions determine volatilisation from plant and soil surfaces. The results obtained from field experiments can fully be transferred to practical outdoor conditions. The disadvantages are, however, the high variability of results for the same compound due to many highly variable factors in field experiments. To overcome these disadvantages laboratory experiments can be helpful tools. A number of methods have been developed, ranging from very simple to high-tech designs. However, most of the experimental volatilisation measurements of pesticides in Europe are based on the German BBA test guideline.
In the present study a simple laboratory model system has been designed, in which a wide range of outdoor conditions can be simulated. The finalised method was then used to measure the volatility of selected pesticides from different surfaces and under different "climatic" conditions such as temperature, wind speed etc. The aim of the experimental set-up was to develop a simple, cost-effective and sensitive laboratory test system to assess and evaluate the volatilisation of pesticides relative to each other under different conditions.
4.2.1 The German BBA test guideline
In 1990 a BBA Guideline concerning volatilisation of pesticides was published (Nolting H-G et al.). This guideline was design as a "liberal" guideline with only a few specific demands to be met by the method applied. The intention was that after some years of experience, the methods, in use at that period, should be compared and evaluated, and it should then be decided whether a more specific guideline would have to be issued (Walter et al., 1996).
The German guideline requests details on the percentage of active ingredient volatilised 1, 3, 6, and 24 hours after application, the 24 hours value being the crucial value for the authorisation process. A few other details are requested by the BBA guideline such as: the relative humidity of the air shall be about 35% and the wind velocity shall be > 1 m/s directly above the surface. In soil experiments a standard 2.1 soil or a similar soil with maximum 1.5% organic bound carbon shall be used, and the sand quota shall be at least 70%. The water level in the soil shall be 60% of the maximum water holding capacity and must be kept at this level during the experiments.
4.2.2 Experimental measurement and evaluation of selected factors influencing on the volatilisation, and of pesticides with special focus on MCPP-P, MCPP methyl, and lindaneModel system for determination of volatilisation of chemical substances
A simple laboratory model system for the determination of the volatilisation has been used for the present study. It permits a direct determination of the volatilisation of a compound from a given surface and a mass balance can be made at the end of the study. Furthermore, different airflows, temperatures, etc. can be obtained in this experimental set-up.
The recovery and stability experiments showed that the pesticides were absorbed quantitatively by the adsorption tubes used in this test design and the total recovery was sufficient. Furthermore, the test results revealed that the substances were stable during the test period of up till 30 hours.
Design and dimension of the chamber
The BBA guideline neither requests nor suggests a specific method for assessing volatilisation. Various laboratory model systems for direct and indirect determination of volatilisation of pesticides from treated surfaces are described in the literature. Designs and dimensions of volatilisation chambers differ widely, which may be one reason for a considerable variation among the results obtained in different studies. With respect to the volatilisation chamber properties, the differences in size, experimental area, and air exchange rate are the most apparent. For practical reasons, the volatilisation is often studied in small laboratory volatilisation chambers like that used in the present study (chamber size of about 0.00008 m3) (Stork A. et al. 1994, Walter U. et al. 1996). However, some studies are also carried out using a large scale wind tunnel system with a chamber size of several m3 (Rüdel H. 1997) or under field-like conditions in a wind-tunnel /lysimeter system with a chamber size of 0.3-0.9 m3 (Stork A. et al., 1994). The volatilisation rate may also be influenced by the size of the treated area in the volatilisation chamber. Thus, when testing the volatilisation of lindane from soil in a wind tunnel under defined conditions, it was observed that from a larger soil surface (0.84m2) a lower amount of lindane (23%) volatilised than from a smaller surface (31% at 0.28m2) under otherwise identical conditions (Waymann B. and Rüdel H., 1995).
The volatilisation rate
Volatilisation of lindane and MCPP methyl applied at the recommended dose to filter paper during a 30 hour period indicates that about 28% and 8% of the applied dose was volatised during the first hour after application for MCPP methyl and lindane, respectively. After 8 hours about 80% and 50% of the applied MCPP methyl and lindane, were volatised. Only about 7% of the applied MCCP methyl volatised in the period between 8 and 30 hours after application while it was about 30% in the case of lindane. The most important objective of a volatilisation experiment according to the BBA Guideline is the determination of the volatilised quantity of the substance within 24 hours after application. This time interval seems to be reasonable when examing the experiments reported here. They document the high volatilisation rates within the first few hours after application, and the 24 hours value is measured at a time when the volatilisation process has considerably slowed down. The same pattern has also been seen in other laboratory volatilisation experiments with other pesticides (e.g. Walter et al., 1996). The results from the present experiments also show that when MCCP-P was applied to filter paper no volatilisation took place during a 24 hours period. These results are in agreement with previous investigations of the volatilisation of MCPP-P from plant surfaces and soil under laboratory conditions (Mossin J. personal communication, Danish EPA, Division of Pesticides, 1999).
Application rate
The rate of loss by volatilisation depends on the concentration on and in a given medium e.g. a soil and the concentration-vapour density relationships at the soil surface. According to Letey and Farmer (1974) there are two general mechanisms whereby pesticides move to the evaporating surface, i.e. diffusion and mass flow. Diffusion is the process by which material is transported as a result of random molecular motion caused by the molecule’s thermal energy. The random molecular motions gradually cause the molecules to become uniformly distributed in the system (Letey and Farmer, 1974). Diffusion occurs whenever a concentration gradient is present. In general the diffusion rate of a pesticide is increased with increasing concentration applied. In the present study the two pesticides were applied at a rate corresponding to the recommended application rate and 10´ the recommended application rate. When the doses were increased with a factor of 10 the amount of lindane volatising also increased, however, only with a factor of 2.4 under the present experimental conditions. In contrast, when the amount of MCPP methyl applied was increased with a factor of 10, the amount of MCPP methyl volatising increased with a factor of 8.9 illustrating the higher volatilisation potential for this compound. A comparable phenomenon has also been described in soil experiments with different application doses of lindane in a wind tunnel experiment (Waymann and Rüdel, 1995). Thus, when the application dose is increased the volatilisation is normally also increased, however, the actual increase in volatilisation is highly depended on the physical/chemical properties of the pesticides and of other environmental conditions.
Volatilisation at different airflow and humidity
Wind speed, turbulence, and relative humidity play an important role in the overall loss of pesticides by volatilisation in the field. The direct effect of increased air movement involves a more rapid removal of pesticide vapours from the soil surface, and results in an increased movement of pesticide to the soil surface (Guenzi and Beard, 1974). According to Hartley (1969) the rate of movement away from the evaporating surface is a diffusion-controlled process.
Close to the evaporating surface the air is relatively still. The vaporising substance is transported from the surface through this stagnant air layer only by molecular diffusion. Diffusion away from the surface is related to the vapour density and the molecular weight of the pesticide. The thickness of this stagnant air layer above the evaporating surface depends on the airflow rate. The studies of Farmer et al. (1972) and Igue et al. (1972) found more volatilisation of chlorinated insecticides with increasing flow rates. Waymann and Rüdel (1995) also found an increase in the volatilisation rate of lindane from both soil and plants with increasing air velocities in a wind tunnel with laminar airflow.
In the present study the volatilisation of lindane and MCPP methyl was tested at two airflows (0.17 and 0.67 m per sec.) but no significant increase in the amount of volatilisation of the two pesticides was found with increasing airflow. The airflow was measured at a distance less than 0.7 cm. from the surface which would correspond to a calculated wind speed of more than 1 m per sec at a height of 1 m above the surface (Asman, 1999). Furthermore, the airflow reached the volatilisation chamber as a turbulent flow. This increased the volatilisation of pesticides compared with a laminar airflow situation since the primary effect of wind on pesticide disappearance from foliage is assumed to be through turbulent transfer of volatilised pesticide from plant surfaces to the atmosphere (Spencer et al., 1973). Since no significant increase in the amount of volatilisation of the two pesticides was observed after an increase of the airflow by a factor of 4, it is expected that an airflow of 0.17 m per sec. is sufficient to ensure that the rate of the airflow is not the limiting factor for a high evaporation under the present experimental conditions.
Influence of the relative humidity of the air
If the relative humidity of the air is not 100%, increases in airflow will hasten the drying of the surface e.g. a soil. This indirect effect alters the soil-water content, which has an effect on the volatilisation (Guenzi and Beard, 1974).
Normally pesticides volatilise much more rapidly from wet than from dry soils because the polar water molecules are strong competitors for adsorption sites on the soil, especially to non-polar organic compounds (Dörfler et al. 1991, Petersen et al., 1994). Passing moist air over moist soil will result in little water loss. Therefore, pesticide volatilisation will continue for a long time. However, with dry air (low relative humidity), the soil dries rapidly and pesticide vapour pressure is decreased within a relatively short time. The drying effect, decreasing the volatilisation, is reversible, since remoisting the air-dry soil will increase the vapour density again to its original maximum value (Jansma and Linders, 1995).
Grass et al. (1994) measured the influence of air humidity on the volatilisation of triflualin. At a relative air humidity of 31, 49, and 78% the measured percentage of volatilised trifluralin over the first day was 66, 64, and 96%, respectively (temperature 20oC, air velocity 1.0-1.2 m/s.). In the present study a relative humidity of the air was maintained at 50% in all experiments and the test-substrate (filter paper or soil) was in equilibrium with this humidity before the pesticides were added just to ensure relative constant water content of the substrate.
Effects of different temperatures
The temperature influences the characteristic saturation vapour pressure of a given pesticide. The overall effect of an increasing temperature is an increasing volatilisation of the pesticide. This is also illustrated in the present study where an increase in the temperature from 15oC to 23oC increases the volatilisation of especially lindane.
In the field the effects of different temperatures on volatilisation of pesticides are more complex. Temperature affects the volatilisation of a given pesticide from soil by a direct influence on the vapour density of the pesticide and by temperature influences on the physical and chemical properties of the soil. Thus, temperature may influence the volatilisation of soil-incorporated pesticides through its effect on movement of the pesticide to the surface by diffusion or by mass flow in the evaporating water, or through its effect on the soil water adsorption-desorption equilibrium. Ehlers et al. (1969) reported an exponential increase in the apparent diffusion coefficient in the soil for lindane after increasing the temperature from 20oC to 40oC. However, an increase of temperature may also increase the drying rate of the soil surface. Depending on the results of the temperature effect on soil drying and the effect on vapour pressure, volatilisation will increase or decrease (Deming, 1963).
Volatilisation when applied on different surfaces
Adsorption reduces the chemical activity and, thus, also the volatilisation rate of the compound. The rate of loss by volatilisation depends on the concentration-vapour density relationships at the surface and is highly depended on the type of surface that the pesticide is applied to. It should be noted that the variation in the plants and soils used in different investigations is significant. This may be one reason for a considerable variation among the results published for many substances. To overcome these disadvantages, laboratory experiments using a standardised artificial surface can help to assess and evaluate the volatilisation of pesticides relative to each other under different conditions.
In the present volatilisation study lindane and MCPP methyl were applied to two different artificial surfaces, a filter paper illustrating an absorbent surface and a metal plate illustrating a non-absorbent surface. The results revealed that the volatilisation of each pesticide after 24 hours was comparable (about 73% for lindane and 90% for MCCP methyl of the applied doses). However, it seems that the volatilisation of at least MCPP methyl was faster at the non-absorbent surface. In contrast to the artificial surfaces only a small fraction of the applied doses volatised when the pesticides were applied to a typical Danish soil (13% for lindane and 15.9% for MCPP methyl). The results are comparable with the results found for lindane tested in a huge wind tunnel where the soil volatilisation was found to vary between 12 and 31% and volatilisation from plant varied from 52 to 62% of the initial dose after 24 hours, depending on the test conditions (Waymann and Rüdel, 1995). Furthermore, our results are also comparable to evaporative losses of lindane from plant leaf and soil surfaces after field application. Boehncke et al. (1990) observed an evaporative loss of lindane from leafs of 77 to 95% (different plant leafs) and 28% from a soil after 24 hours. Jansma and Linders, (1995) presented a review of the literature about volatilisation of selected pesticides from soil and plants after spraying. In this paper the volatilisation of lindane from plants varied from 64 to 89% in 5 different laboratory studies after a 24 hours period. From the soil surface the volatilisation of lindane varied after 24 hours from 3.1 to 22.6% of the applied dose in 8 different laboratory experiments. However, in one field experiment in USA up till 50% of the applied dose was volatilised within 24 hours.
In general the volatilisation of pesticides from soil is much smaller than from plant surfaces (e.g. Boehncke et al. 1990; Rüdel, 1997). However, significant variation is found depending on the soil used for the experiments. This is in agreement with the results found in the present study where the filter paper is more comparable with a plant surface. The volatilisation of pesticides from soil is very complex e.g. the diffusion rate of a pesticide from soil is controlled by soil bulk density, pesticide concentration, organic matter, pH, the soil water and clay contents in addition to climatic parameters like temperature, wind velocity etc. Furthermore, vertical transport is an important parameter in the evaluation of the volatilisation of pesticides from soil and occurs as a result of external forces. The pesticide is considered to be either dissolved or suspended in water, present in the vapour phase, or adsorbed on solid mineral or organic components of the soil. Mass flow of the pesticide is therefore the result of the mass flow of water and/or soil particles with which the pesticide molecule is associated. Although many factors contribute to mass transport of pesticides through soil by water, the most important factor appears to be adsorption between the pesticide and soil (Letey and Farmer 1974). Mass flow due to air movement in soil is considered negligible (Letey and Farmer, 1974).
4.3 Conclusions and suggestions for further validation work
Looking at the practical field situation, many different factors and their interactions determine volatilisation from plant and soil surfaces. The results obtained from field experiments can fully be transferred to practical outdoor conditions. The disadvantages are, however, the lack of reproducibility. To overcome these disadvantages laboratory experiments can be helpful tools.
In 1990 a German BBA guideline on assessing pesticide volatilisation was developed. This guideline was design as a "liberal" guideline with only a few specific demands. Since then a number of methods have been developed, ranging from very simple to high-tech designs.
In 1994 the volatilisation of three pesticides from plant and soil surfaces was assessed using eighteen different laboratory methods and one field method, all following the BBA guideline. With respect to the volatilisation chamber properties, the differences in size, experimental area, and air exchange rate are the most apparent. Furthermore, the height at which wind is measured varies from 0-3 cm to more than 10 cm. The aim of the present study was to see whether the different methods yielded results that were consistent among the methods and a number of terms were agreed (dose, temperature and humidity etc.). The results revealed that for all three substances tested, a considerable amount of variation among the results obtained with the different methods was observed (Walter et al. 1996).
From the scientific point of view, the inter-laboratory comparison once again proved that the problems of assessing pesticide volatilisation have not been solved yet, and that it is necessary to be very cautious when comparing volatilisation rates assessed with different methods.
In the present study a laboratory system for the direct determination of the volatilisation of pesticides from different surfaces has been developed. With the system it is possible to evaluate volatilisation of pesticides relative to each other. Taking practical aspects of testing and the demand for simple cost-effective tests into account, the main conclusions of the present study regarding the experimental set-up are as follows:
The BBA guideline on assessing pesticide volatilisation was designed as a "liberal" guideline with only a few specific demands. This guideline is a good basis, however, it should be improved and be more specific.
Design and dimension of the chamber
Design and dimension of the volatilisation chamber must be specified. The conditions of most experiments are so different that comparative considerations are difficult or impossible. An exact determination of volatilisation of pesticides can be achieved only by direct measurement of volatilisation rates using some means of trapping the vapourised pesticide. A small chamber is recommended. Such a chamber can be extracted at the end of an experiment and a mass balance can be made.
Sampling intervals
Interpreting the kinetics graphs from various volatilisation experiments with pesticides it can generally be said that the air sampling 1, 3, 6, and 24 hours after the application asked for in the German guideline seems to be reasonable. Normally, high volatilisation rates are seen within the first few hours after application, and the 24 hours values after application are measured at a time when the volatilisation process has slowed down considerably.
Airflow
According to the BBA guideline the wind velocity shall be > 1 m/s directly above the surface. However, the guideline does not exactly stress in which height the measurement should be made. When evaluating the volatilisation of a test substance from a surface the height above the surface, where the air velocity measured is very important. Thus, if the assumed wind speed at 30 cm height above a bare soil surface is 1 m/s the calculated wind speed is about 0.7 and 0.3 m/s at a height of 5 and 0.5 cm above the surface, respectively (Asman, personal communication, 1998, National Environmental Research Institute, Roskilde, Denmark). Furthermore, it is important to decide whether turbulent or laminar airflow should be used. If a small chamber with a turbulent airflow is used an airflow with a velocity of less than 1 m/s 1-5 mm above the surface might be sufficient.
Humidity of the air
The relative humidity of the air shall be about 35% according to the BBA guideline. Since the air humidity value is relatively low and not realistic for field situations, this condition should be changed. Mostly, a more realistic value of approximately 50% relative humidity should be used. However, the most important aspect is that the surface to which the pesticide is applied has the same humidity during the whole experiment.
Selected surfaces
According to the BBA guideline pesticides should be applied to two different surfaces one illustrating a leaf surface and one soil. It is important to specify the selected surface and an artificial surface e.g. a filter paper may be used to illustrate a leaf surface, however, some work has to be done to find a well-defined surface that can simulate a leaf surface. In soil experiments a German standard 2.1 soil or a similar soil is recommended in the BBA guideline. The water level in the soil must be 60% of the maximum water holding capacity and must be kept at this level during the experiments. This soil can be used as a reference soil. However, another soil, which is typical for the area of concern, should be included in the test. Furthermore, the pH value of the soil should be specified since dependency of volatilisation seems to be strong for some pesticides (Walter et al., 1996). However, data from the present study and many other studies suggest that plant (or an artificial surfaces illustrating plant surfaces) volatility is always higher than soil volatility. Therefore, it should be considered to omit the volatilisation tests with bare soil and only perform plant volatilisation tests. In most cases these results will be sufficient to assess the volatilisation behaviour of pesticides.
Application rate and technique
Several concentrations of a given pesticide should be tested and the applica tion technique must be more explicit. Application devices used in different experiment differ greatly. Everything from nozzles used in agricultural practice to modified TLC applicators to simple Hamilton syringes has been used. Furthermore, experiments must be carried out with the formulated product as well as with the active ingredient.
Temperatures
The temperature must be kept constant e.g. at 20oC or even better measured at two different temperatures e.g. 10 oC and 30oC representing different temperatures in Europe.
5. References
Baas, J (1994). Emissie van gewasbeschermingsmiddelen uit boomgaarden naar de lucht. TNO-report MW-R 94/040a, TNO –Milieu en Energie, TNO, Delft.
Boehnke A., Siebers J., and Nolting H.-G. (1990). "Investigation of the Evaporation of Selected Pesticides from Natural and Model Surfaces in Field and Laboratory", Chemosphere, 21, pp. 1109-1124.
Buser H.R. (1990). "Atrazine and Other s-Triazine Herbicides in Lakes and in Rain in Switzer land", Environmental Science & Technology, 24, pp. 1049-1058.
Chen C., Green R. E., Thomas, D.E., and Knuteson J.A. (1995). "Modeling 1,3-Dichloropropene Fumigant Volatilization with Vapour-Phase Advection in the Soil Profile", Environmental Science & Technology, 29, pp. 1816-1821.
Deming D.G. (1963). Determination of volatility losses of C14-CDAA from soil samples. Weeds 11, 91-96.
Dörfler U., Adler-Köhler R., Schneider P., Scheunert I., and Korte F. (1991). A Laboratory Model System for Determining the Volatility of Pesticides from Soil and Plant Surfaces. Chemosphere, 23, pp 485-496.
Ehlers W., Farmer W.J., Spencer W.F. and Letey J. (1969). Lindane diffusion in soils: II. Water content, bulk density, and temperature effetcs. In: Soil Sci. Soc. Amer. Proc. 33, 505-508.
Farmer W.J, Igue K, Spencer W.F. and Martin J.P. (1972). Volatility of organochlorine insecticides from soil. I. Effect of concentration, temperature, air flow rate, and vapour pressure. In: Soil Sci. Soc. Amer. Proc. 36, 443-447.
Felding G., Spliid N.H., and Asman W.A.H. (1999). Findings and effects of pesticides in air and precipitation. in press.
Gath B., Jaeschke W., Ricker I., and Zietz E. (1992). "Depositionsmonitoring von Pflanzenschutzmitteln auf dem Kleinen Feldberg. Erste Ergebnisse", Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, 44, pp. 57-66.
Gath B., Jaeschke W.,Kubiak R., Ricker I., Schmider F., and Zietz E. (1993). "Depositionsmonitoring von Pflanzenschutzmitteln: Teil 2 Süddeutscher Raum", Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, 44, pp. 57-66.
Glotfelty D.E., Majewski M.S., and Seiber J.N. (1990). "Distribution of Several Organophosphorus Insecticides and Their Oxygen Analogues in a Foggy Atmosphere", Environmental Science & Technology, 24, pp. 353-357.
Gottschild D., Siebers J., and Nolting H.-G. (1995). The volatilization and deposition of plant protection products. Strasbourg Cedex: Publishing and Documentation Service council of Europe.
Grass B., Wenclawiak B.W. and Rüdel H. (1994). Influence of air velocity, air temperature, and air humidity on the volatilisation of trifluralin from soil. In: Chemosphere, 28, no 3, 491-499
Guenzi W.D. and Beard W.E. (1974). Volatilisation of pesticides. In: Guenzi, W.D. (Ed) Pesticides in soil and water. Chapter 6, 107-122. Soil Science Society of america, Madison, Wisconsin.
Hartley G.S. (1969). "Evaporation of Pesticides", Pesticidal Formulations Research – Physical and Colloidal Chemical Abstracts, Miami Beach, Florida, USA.
Igue K, Farmer W.J., Spencer W.F. and Martin J.P. (1972). Volatility of organochlorine insecticides from soils: II Effect of relative humidity and soil water content on dieldrin volatility. In: Soil Sci. Soc. Amer. Proc., 36, 447-450.
Jansma J.W. and Linders J.B.H.J. (1995). Volatilization of pesticides from soil and plants after spraying. RIVM Report no. 679102030
Krasel G. and Pestemer W. (1993). "Pflanzenschutzmittel-Verflüchtigung von Oberflächen", Proceedings of the 8th EWRS Symposium "Quantitative approaches in weed and herbicide research and their practical application", Braunschweig, pp. 399-406.
Letey J. and farmer W.J. (1974). Movement of pesticides in soil. In: Guenzi, W.D. (Ed.) Pesticides in soil and water. Chapter 4, 67-97. Soil Science Society of America, Madison, Wisconsin.
Lindhardt B. and Christensen T.H. (1994). "Measured and Estimated Volatilisation of Naphthalene from a Sandy Soil", Chemosphere, 29, pp. 1407-1419.
Lindhardt B., Christensen T.H., and Brun A. (1994). "Volatilisation of o-Xylene from Sandy Soil", Chemosphere, 29, pp. 2625-2637.
Meylan W and Howard P. (1994). MPBPVP: Melting point, Boiling point and vapour pressure program for Microsoft Windows 3.1. Syracuse Research Corporation, Syracuse 1994.
Nations B.K. and Hallberg G.R. (1992). "Pesticides in Iowa Precipitation", Journal of Environmental Quality, vol. 21, pp. 486-492.
Nolting H.-G., Boehnke A., and Storzer W. (1988). "Zum Eintrag, Verhalten und Verbleib von Pflanzenschutzmitteln in Luft", Biologische Bundesanstalt für Land- und Forstwirtschaft, Abteilung für Pflanzenschutzmittel und Anwendungstechnik, Braunschweig, Literaturstudie.
Nolting H.-G., Siebers J., Storzer W., Wilkening A., Herrmann M., and Schlüter C. (1990). "Teil IV 6-1 Prüfung des Verflüchtigungsverhaltens und des Verbleibs von Pflanzenschutzmitteln in der Luft", in Richtlinien für die Prüfung von Pflanzenschutzmitteln im Zulassungsverfahren, Abteilung für Pflanzenschutzmittel und Anwendungstechnik der Biologischen Bundesanstalt Braunschweig, Ed. Ribbesbüttel: Saphir-Verlag.
Parker. S.P. (1984). "McGraw-Hill Dictionary of Physics". New York: McGraw-Hill Book Company.
Petersen, L.W., Rolston D.E., Moldrup P., Yamaguchi T. (1994). Volatile organic vapour diffusion and adsorption in soils. In: J. Environ. Qual., 23, 799-805.
Plimmer J.R. (1976). "Volatility", " in Herbicides – Chemistry, Degradation and Mode of Action, vol. 2, P.C. Kearney and D.D. Kaufman, Eds., 2 ed. New York, Basel: Marcel Dekker, Inc., pp. 891-933.
Rüdel H. (1997). Volatilisation of pesticides from soil and plant surfaces. Chemosphere, 35 pp 143-152.
Scharf J. and Bächmann K. (1993). "Verteilung von Pflanzenschutzmitteln in der Atmosphäre. Nah- und Ferntransportmessungen", Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, vol. 45, pp. 82-87.
Schomburg C.J., Glotfelty D.E., and Seiber J.N. (1991). "Pesticide Occurence and Distribution in Fog Collected near Monterey, California", Environmental Science & Technology, vol. 25, pp. 155-160.
Siebers J., Gottschild D., and Nolting H.-G. (1994). "Pesticides in Precipitation in Northern Germany", vol. 28, pp. 1559-1570.
Smit A.A.M.F.R., van den Berg F., and Leistra M (1997). Estimation method for the volatilization of pesticides from fallow soil. Environmental planning bureau series 2 Wageningen (The Netherlands)
Smit A.A.M.F.R., Leistra M., and van den Berg F. (1998). Estimation method for the volatilization of pesticides from plants. Environmental planning bureau series 4 Wageningen (The Netherlands)
Spencer W.F., Farmer W.J., and Cliath M.M. (1973). "Pesticide volatilization", Residue Reviews, vol. 49, pp. 1-47.
Stork A., Witte R., and Führ F. (1994). A Wind Tunnel for Measuring the Gaseous Losses of Environmental Chemicals from the soil/Plant System under Field-Like Conditions. Environ. Sci. & Polllut. Res. 1 (4) pp. 234-245
Taylor A.W. (1978). Post-application volatilization of pesticides under field conditions. Journal of the Air Pollution Control Associations vol. 28 no. 9 pp 922-927.
Walter U, Krasel G, Pestemer W. (1996). Assessing Volatilization of Pesticides: A Comparison of 18 laboratory methods and a field method. Helft 16 from BBA, Braunschweig, Germany
Waymann B. and Rüdel H. (1995). Influence of air velocity, application dose, and test area size on the volatilisation of lindane. Intern. J. Environ. Anal. Chem. vol. 58 pp 371-378.
Willis G.H., McDowell L.L., Harper L.A., Southwick L.M., Smith S. (1983). Seasonal disappearance and volatilization of toxaphene and DDT from a cotton field J. Environ. Qual. vol 12, no. 1, pp 80-85.