Impact of Regulations of Traffic Emissions on PAH Level in the Air

Summary and Conclusions

Background and implementationv
Air pollution levels and trends

Background and implementation

PAH emissions

Emissions of PAH (polycyclic aromatic hydrocarbons) originate from several sources. Traffic is an important contributor, especially in streets. A little more than half of the fuel consumption used for road transport in Denmark is made of petrol. The second important fuel is diesel oil. During the nineties severe emission standards have been introduced, both for petrol and diesel driven motor vehicles. In October 1990 legislation was introduced in Denmark which required the closed loop 3-way catalysts on all new petrol driven passenger cars. This new technology was expected to result in a significant reduction in the emission of PAH and to reduce the emission of CO, hydrocarbons and nitrogen oxides from the individual car with 70-80%. During the measuring campaign in 1992 about 10% of the petrol-driven passenger cars were equipped with catalysts. This proportion increased to about 40% during the 1996 measurements. Petrol makes up about 55% of the fuels used for road transport in Denmark. Most of the remaining part is diesel oil. The introduction of new diesel qualities for buses from July 1992 was also expected to reduce the PAH and soot emissions. The new diesel fuels have a lower distillation end point implicating a lower fuel content of hydrocarbons with high molecular weight including PAH.

Previous investigations

The Danish Environmental Protection Agency (DEPA) supported in 1992-1994 a project with the following main objectives (Nielsen et al. 1995b and c):

1. Evaluation of the contribution of traffic to PAH-emissions, including a distinction between petrol and diesel driven vehicles.
2. Determination of the PAH levels at different locations in Denmark.
3. Evaluation of the health effects of PAH and other POM (polycyclic organic matter).

Previous results

Most of the measurements were made in the period January to March 1992, i.e. before the introduction of the new diesel fuel qualities. The investigation showed that the highest levels of PAH in air were found in street air. The range was as follows: Street > city background suburbs > village > country sites. The levels of benzo(a)pyrene in city air (street: 4.4 ± 1.2 ng/m³, city background: 1.4 ± 0.6 ng/m³) appeared to be between the tolerable level of 5 ng/m³ (annual average) and the acceptable level of 0.5 ng/m³ applied as air quality criteria in the Netherlands. In Denmark it is the policy that the levels of PAH and other PAC should be as low as possible, and the outdoor air levels are regulated by means of emission limits for the various sources. The traffic emissions were the major PAH source in street air. The contribution was estimated to be 80%. The weekdays distribution was as follows: Working days: 90% and weekends: 40%. The diurnal variation showed peaks in the morning and afternoon rush hours and lowest concentrations in the night hours. The contribution from diesel vehicles was estimated to be twice the contribution from petrol cars (Nielsen 1996).

1992 to 1993 trend

The background for this project was the trend observed from 1992 to 1993 in the air levels of mutagenicity and PAH. A very strong decrease was observed in the air levels of mutagenicity, but a decrease was also observed for PAH. This may suggest that the effect of legislation in reducing the traffic emissions is more effective for PAH and other mutagenic compounds than for CO, nitrogen oxides and soot. The levels of the latter were applied as internal standards. The decreases in the levels of PAH and mutagenicity were calculated relative to these three types of compounds. However, the interpretation of the results was not unambiguous, as the meteorological conditions were different in the two sets of measurements, and the emissions of PAH and other mutagens might perhaps also have been affected by the ambient temperatures (Nielsen, 1996, Nielsen et al., 1995b and c).

Measuring programme

Measurements of 24-h levels of particulate PAC (mainly PAH) and mutagenicity were performed at a busy street in the central Copenhagen (H.C. Andersens Boulevard). At this location measurements have been performed on a daily basis of NO, gas NO_y, SO₂, CO, ozone, soot, particulate matter and inorganic elements. The traffic intensity in the street is in the magnitude of 60,000 cars each day, and the streets in the neighbourhood have also high traffic intensities. Most of the 1996 samples for PAH analyses and mutagenicity testing were collected in the winter (40%) and the spring (45%) as the major part of the 1992-1993 samples were obtained in the months January-March. The particulate phase PAC measurements include 28 PAH (3-6 rings), 2 oxy-PAH (3-4 rings), 2 S-PAC (3-4 rings) and 17 N-PAC (2-5 rings). The mutagenicity assays included the following tester strains: TA98+S9, TA98-S9 and TA98NR. In a limited number of cases a test was performed on a basic extract in addition to the total extract.
 

Air pollution levels and trend

PAC levels

The levels of PAC in the street air were lower in 1996 than in 1992. The 1996 level of benzo(a)pyrene was 2.2±0.6 ng/m³ and that of benzo(e)pyrene was 3.1±0.7 ng/m³ compared to 4.6±1.4 ng/m³ and 4.4±0.8 ng/m³ respectively in 1992. However, the results are questioning the application of benzo(a)pyrene as a reliable indicator for the air pollution with mutagenic and carcinogenic PAC as discussed later in this section. The levels of the two oxy-PAH, anthraquinone and benzanthrone, were of the same magnitude as the most abundant PAH, fluoranthene, pyrene, benzofluoranthenes, benzo(c)phenanthrene, chrysene/triphenylene, benzopyrenes, benzo(ghi)perylene and coronene. The concentrations of S- and N-PAC were about one order of magnitude lower than those of PAH. The mean mutagenic activity seen in TA98+S9 was 48±41 rev./m³. The direct mutagenic activity measured in TA98-S9 was 70% of the activity seen with metabolic activation. The level of direct acting mutagenic nitro compounds ((TA98-S9)-(TA98NR)) constituted 56% of the total level of direct acting mutagenicity, which is a typical result for air samples. The TA98+S9 mutagenic activity in the basic extracts was about 3% of the activity in the total extracts in those samples collected on days with low activity of atmospheric photochemistry. However, the photochemical processes may produce water soluble, polar indirect acting mutagens ending up in the basic extract, as the mutagenicity of this fraction increased significantly with increasing atmospheric ozone concentrations. The lowest-level of 3% is ascribed to be caused mainly by N-PAC.

PAH composition

The higher concentrations of airborne particulate matter in the 1996 samples compared to the 1992-93 seems to have samples caused a higher proportion of the lighter PAH relative to the heavier ones. The lighter PAH are distributed in the atmosphere between the gas phase and particles. The heavier PAH are only associated with particles. The particle association of the lighter PAH increases, when the amount of airborne particulate matter increases. Atmospheric chemistry of the reactive PAH (cyclopenteno(cd)pyrene, benzo(a)pyrene, perylene and anthanthrene) was another factor affecting the PAH composition in the 1996 samples relative to that in the 1992-1993 samples. Thus, the 1996 to 1992-93 ratio for the four PAH (0.42±0.05) was significantly lower than the ratio for the stable PAH, benz(a)anthracene, triphenylene, chrysene, benzo(a)-, benzo(b)-, benzo(j)- and benzo(k)fluoranthene, benzo(e)pyrene, indeno(1,2,3-cd)pyrene, benzo(ghi)perylene and coronene, (0.60±0.08) (t-test, p<0.01).

Effect of emission sources

Both the emissions from diesel cars as well as the emissions from petrol driven passenger cars were reduced in 1996 compared to 1992. The effects of the introduction of light diesel and those of catalysts for petrol passenger cars were estimated to be about 2:1. It was estimated for the 1992 samples that about 2/3 of the traffic PAH contribution originated from diesel vehicles and about 1/3 originated from petrol driven cars. The changes in the emission sources did not cause any major changes in the PAH composition in 1996 compared to 1992. The 1996 to 1992 ratios of phenanthren (0.61) and methylphenanthrenes (0.63) were very close to each other and the 1996 to 1992-93 ratios of benzo(ghi)perylene (0.64) and coronene (0.64) were not significantly different from the ratio of the other stable PAH (0.60±0.08). It is therefore concluded, that the diesel-to-petrol contribution ratio is still about 2:1 in 1996, and that the ratio of the effects of the introduction of light diesel and those of catalysts for petrol cars also must have been about 2:1. The 2:1 ratio is supported by the finding that the PAH reduction was almost three times larger than the reduction in carbon monoxide. The effect of the catalysts is expected to be the same for PAH and CO, and most of the CO in the street air originates from petrol engines. In addition, the differences between the PAH 1993 to 1992 ratio and the CO 1993 to 1992 ratio were almost the same as the differences in the corresponding 1996 to 1992 ratios suggesting that the effect on diesel emissions took place between the 1992 and 1993 measurements. The trends from 1992 to 1996 is further discussed later.

Mutagenicity and atmospheric chemistry

Atmospheric processes may affect the composition of mutagenic PAC in the polluted air masses by increasing the mutagenic activities and decreasing the benzo(a)pyrene concentrations. Therefore the health risk arising from air pollution may be underestimated if only pollution indicator concentrations are used without taking the impact of atmospheric processes into consideration. This imply that the human health risk ratio between the countryside site and the city is higher than the ratio calculated on the basis of the benzo(a)pyrene level alone. The relative mutagenicity estimated by the ratio between the content of mutagens and PAH increased strongly with the chemical age of the polluted air masses. The chemical age was estimated by the ratio between 2-nitrofluoranthene and 1-nitropyrene. The former is formed by atmospheric processes and the second one is emitted directly into the atmosphere. The increase in direct acting mutagens was larger than that of indirect mutagens. The increase in direct acting nitro-PAC and other PAC ones was the same.

Mutagenicity and long-range transport

In addition, on the 22 April 1996 the air pollution in the air at H.C. Andersens Boulevard originated from both local sources and from polluted air masses transported from the Continent. The levels of mutagenic activities increased with a factor of 5-7 compared to the mean levels of the period. The level of the traffic gases, NO, gas NO_y and CO, originating mainly from the local sources were normal (increased by a factor of 1.1-1.3). Although not comparable with the mutagenic activities, the long range transport contribution caused an increase in SO₂ (factor: 2.5), S-PAC (2.0) and benzanthrone (2.0). Most of the stable heavier PAH increased with a factor of 1.6-1.8. The increases were minor for benzo(ghi)perylene (1.4) and coronene (1.2) and close to the factor for the traffic gases. Benzo(ghi)perylene and especially coronene has a high contribution from traffic sources.

Oxy-PAH

The oxy-PAH, benzanthrone, is mainly associated with particles. Two or maybe three sources contribute to its presence in ambient air: Emission, atmospheric chemistry and perhaps resuspension. If so the identification of the last source is new, while there are conflicting data in the literature on whether atmospheric processes may be a source or not. However, considering

(1) that the summer half year level of benzanthrone is 1.5 times higher than the winter half year level,

(2) that the concentration of benzanthrone was elevated by a factor of 2.0 in the long-range transport incidence,

(3) that the relative benzanthrone content (benzanthrone/Sum PAH) is anticorrelated (r=-0.52, p<0.05) to the relative benzo(a)pyrene content (benzo(a)pyrene/Sum PAH), it is evident that a significant part of benzanthrone is formed by atmospheric processes. Resuspension of aged particles is perhaps an additional source for benzanthrone and other oxy-PAH, even though resuspension appears to be a negligible source for the presence of PAH in street air. The background for this may be, that the resuspended particles are aged and exposed to sun light and reactive atmospheric gases for a longer time. Under these circumstances the PAH may be oxidised and transformed to oxy-PAH, e.g. benzanthrone. Thus the ratio of benzanthrone to the sum of PAH (sum of benzo(e)pyrene, benzo(bjk)fluoranthene and indeno(1,2,3-cd)pyrene) increased when the amount of airborne soil dust elements, Fe, Ti, Si and Ca increased (r=0.52-0.64, p<0.05). The four soil dust elements are considered to be an indicator for the up whirling of settled particles.

Trend analysis

The variations in PAH and other mutagens were related to those of the air pollutants measured on a daily basis each year in order to limit the influence of the different meteorological conditions. The comparisons indicate that the regulations of traffic emissions induced by the environmental authorities have had a greater impact on the levels of PAH and other airborne mutagens than on the levels of nitrogen oxides, CO and soot. Thus taking all data into consideration the PAH decreased 19±12% from 1992 to 1993 and 38±8% from 1992 to 1996. The decrease in mutagenicity was 42±18% from 1992 to 1996. The decreases of the four reference components, NO, gas NO_y, CO and soot, were 3±6% from 1992 to 1993 and 18±5% from 1992 to 1996. The decreases in PAH concentrations and mutagenicity levels were significantly larger (t-test, p<0.01) than the decreases of the four reference components. The regulations of traffic emissions appear to have a greater impact on the mutagenicity levels than on the PAH levels confirming the observations in 1993. Disclosing the long-range transport incidence the decrease from 1992 to 1996 of the 7 PAH (benzo(bjk)fluoranthene, benzo(a)- and benzo(e)pyrene, benzo(ghi)perylene and coronene) was 40±7% and the decrease in the mutagenic activity in the three different tester strains were as follows: TA98+S9 59%, TA98-S9 54% and TA98NR 71%. Applying the 1992 data set where measurements of inorganic elements also were available the PAH concentrations decreased with 50±9% and the mutagenicity levels with 42±18% from 1992 to 1996. The six reference inorganic elements, S, K, V, Ni, Zn and Cu, increased in the same period with 22±16%. The comparison includes data from the long-range transport episode.

Seasonal variation

The summer to winter variation in PAC showed different patterns depending on the type of compounds and their source relations. The oxy-PAH, benzanthrone, had higher atmospheric concentrations in the summer half-year than the winter half-year. The reactive PAH, cyclopenteno(cd)-pyrene and benzo(a)pyrene showed the opposite tendency having the highest concentrations at winter-time. The photolytic degradation of benzo(a)pyrene was also confirmed by the anticorrelation between the global radiation and the ratio of benzo(a)pyrene to the PAH sum (r=-0.56, p<0.05). There were no major difference in the summer and winter levels of the stable PAH reference components, benzo(e)pyrene, benzo(bjk)fluoranthene and indeno(1,2,3-cd)pyrene.

Health risk

The most significant health risk of ambient air pollution with PAH and other mutagens is expected to be an excess lung cancer. The risk assessment made in a previous study (Nielsen et al., 1995c) was based upon estimates assuming that the PAC composition is similar in country air, street air, source emissions and in the working environment and that differences can be disregarded. For residents in Copenhagen and other heavily urbanised areas it was estimated that the air pollution with PAH and other mutagens would cause, as a maximum, five extra lung cancer cases each year among one million individuals. The risk will be lower for individuals living at country sites. The observations that atmospheric processes during the transport of PAC influence the formation of mutagenic compounds might imply that the above mentioned maximum estimate could be higher.