Chlorinated organic compounds in the troposphere over the western North Atlantic Ocean measured by aircraft

Results of 32 individual flights during 1985, 1986 and 1988 using a King Air research aircraft over the western North Atlantic Ocean and the Adirondack Mountains are presented. Measurements were made for selected trace organic compounds including α- and γ-isomers of hexachlorocyclohexane (HCH), the summed isomers of α- and γ-chlordane, dieldrin and polychlorinated biphenyls (PCBs). The concentrations of compounds at altitudes of 300–10,000 ft indicate that these compounds were well mixed in the atmosphere and that for the most part, ground-level and higher-altitude concentrations were similar in magnitude. There was a high degree of variability due to time, meteorological events and location. Concentrations over the Adirondack Mountains in 1985 were the highest measured with means of 0.198 ng m⁻³ for HCB, 0.388 ng m⁻³ for α-HCH, 0.509 ng m⁻³ for γ-HCH, 0.340 ng m⁻³ for dieldrin, 0.480 ng m⁻³ for chlordane and 0.951 ng m⁻³ for total PCBs. Concentrations over the western North Atlantic south-east of Bermuda in summer 1988 wer lower, with means of 0.093 ng m⁻³ for HCB, 0.142 ng m⁻³ for α-HCH, 0.012 ng m⁻³ for γ-HCH, 0.016 ng m⁻³ for dieldrin 0.020 ng m⁻³ for chlordane and 0.600 ng m⁻³ for PCBs. During 1988 concentrations of most of the compounds were similar in the atmosphere of the coastal zone, 50–100 km off the U.S., to those measured near Bermuda. Chlordane, and to a lesser extent dieldrin, showed a decrease in concentration away from North America.     

Measurements of atmospheric HNO3, HCl and associated species on a small network in eastern England

Measurements of gaseous HNO₃, HCl and NH₃ and particulate NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺ have been made at a small network of sites in eastern England using sampling intervals from 3 h to 7 days. Both HCl and HNO₃ are spatially rather uniform, with some variation apparently due to spatial variations in NH₃, which stoichiometrically exeeded the sum of both gaseous acids. Mean concentrations of NH₃, HCl and HNO₃ between February 1987 and January 1988 were 1.90, 0.67 and 1.01 μg m⁻³, respectively. Pollution roses revealed low NH₃ concentrations, and high associated HCl and HNO₃ with winds from the North Sea. HCl, but not HNO₃ showed an appreciable elevation in concentration on the sector NW from our site, which we speculate may be due to the large capacity of coal-fired power stations in this upwind sector. Three-hourly data have been examined for diurnal effects and its is concluded that nocturnal formation of NO₃⁻ is occurring.     

Air pollutant concentrations in Varanasi,India

This study reports the diurnal patterns in the concentrations of ozone (O₃), nitrogen dioxide (NO₂), sulphur dioxide (SO₂) and total suspended particulate matter (TSP) in the urban atmosphere of Varanasi city in India during 1989. The city was divided into five zones and three monitoring stations were selected in each zone.Ambient concentrations of NO₂ and SO₂ were maximum during winter but ozone and TSP concentrations were highest during summer. The measured maximum concentrations (2-h average) were 150 and 231 μg m⁻³ (0.078 and 0.086 ppm) for NO₂ and SO₂, respectively, for the winter season. Ozone and TSP concentrations reached a maximum of 160 (0.08 ppm) and 733 μg m⁻³, respectively, in the summer. NO₂ and SO₂ concentrations peaked in the morning and evening. Peak concentrations of O₃ occurred in the afternoon, generally between noon and 4 p.m. Maximum concentrations of O₃, NO₂, SO₂ and TSP were measured in zones I and II, and minimum in zone V.     

Pollution level and distribution of PCDD/PCDF congeners between vapor phase and particulate phase in winter air of Dalian,China

In December 2009, ambient air was sampled with active high-volume air samplers at two sites: on the roof of the No. l building of Dalian Jiaotong University and on the roof of the building of Dalian Meteorological Observatory. The concentrations and the congeners between vapor phase and particulate phase of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the air were measured. Sample analysis results showed that the concentrations of PCDD/Fs in particulate phase was higher than that in gaseous phase. The ratio of PCDD to PCDF in gaseous phase and particulate phase was lower than 0.4 in all samples. The total I-TEQ value in gaseous phase and particulate phase was 5.5 and 453.8 fg/m³ at Dalian Jiaotong University, 16.6 and 462.1 fg/m³ at Dalian Meteorological Observatory, respectively. The I-TEQ value of Dalian atmosphere was 5.5-462.1 fg/m³ which was lower than international standard, the atmospheric quality in Dalian is better.     

Characteristics of airborne particles inside southern California museums

The concentrations and chemical composition of suspended particulate matter were measured in both the fine and total size modes inside and outside five southern California museums over summer and winter periods. The seasonally averaged indoor/outdoor ratios for particulate matter mass concentrations ranged from 0.16 to 0.96 for fine particles and from 0.06 to 0.53 for coarse particles, with the lower values observed for buildings with sophisticated ventilation systems which include filters for particulate matter removal. Museums with deliberate particle filtration systems showed indoor fine particle concentrations generally averaging less than 10 μg m⁻³. One museum with no environmental control system showed indoor fine particle concentrations averaging nearly 60 μg m⁻³ in winter and coarse particle concentrations in the 30–40 μg m⁻³ range. Analyses of indoor vs outdoor concentrations of major chemical species indicated that indoor sources of organic matter may exist at all sites, but that none of the other measured species appear to have major indoor sources at the museums studied. Significant fractions of the dark-colored fine elemental (black) carbon and soil dust particles present in outdoor air are able to penetrate to the indoor atmosphere of the museums studied, and may constitute a soiling hazard to works of art displayed in museums.     

Field studies of the SO2/aqueous S(IV) equilibrium in clouds

Concentrations of S(IV) were measured in cloudwater at Great Dun Fell and compared with theoretical HSO₃⁻ assuming equilibrium between aqueous and gaseous phases in cloud. Detectable concentrations of S(IV) in the range of 1 × 10⁻⁶ to 17.2 × 10⁻⁶ mol dm⁻³ were observed only in samples which contained low H₂O₂ concentrations, generally <1 × 10⁻⁶ mol dm⁻³. Concentrations of S(IV) were below the detection limit of 1 × 10⁻⁶ mol dm⁻³ in samples which contained hign H₂O₂ levels (1 × 10⁻⁶−80 × 10⁻⁶ mol dm⁻³) confirming that either SO₂ or H₂O₂ acts as the limiting reagent in the oxidation of SO₂ in cloudwater.Equilibrium HSO₃⁻ concentrations were estimated from the measured cloudwater pH, the gas phase SO₂ concentration and the ambient temperature and found to be on average about 5 times lower than the measured S(IV) concentrations. The possible role of formaldehyde in stabilizing S(IV) in cloudwater is discussed. The kinetic data available in the literature suggest that the complexation reaction between S(IV) and HCHO is too slow to account for the observed difference between measured and calculated S(IV) concentrations over the typical lifetime of clouds in our study.S(IV) accounted for up to 10% of the SO₄²⁻ measured in stored cloudwater samples.     

On the spatial and temporal variations in atmospheric concentrations of hexachlorobenzene and hexachlorocyclohexane isomers at several locations in the province of Ontario,Canada

Hexachlorobenzene (HCB), as well as α- and γ-hexachlorocyclohexane (HCH) have been measured at seven sites in Ontario, Canada during the years 1985–1989. While two of the locations were close to a suspected source of the organochlorines, five were sites that reflected the non-urban airshed loadings which are considered to be regionally representative of the Great Lakes. At the non-urban locations, the γ-HCH (lindane) and α-HCH concentrations ranged from 15 to 190 pg m⁻³ (mean of 47 pg m⁻³) and 50 to 660 pg m⁻³ (mean of 240 pg m⁻³), respectively. The HCB concentrations varied from 20 to 310 pg m⁻³ (mean of 94 pg m⁻³). At the suspected source locations, the α:γ HCH ratio was less than unity while at locations distant from known or suspected sources, the α:γ ratio was usually greater than 7. At the non-urban locations, the highest concentrations of the α- and γ-HCH isomers were observed during the summer and the lowest concentrations were seen during the winter. The concentration of HCB, however, remained relatively constant throughout the year.     

Effects of temperature,TSP and per cent non-exchangeable material in determining the gas-particle partitioning of organic compounds

The temperature dependence of the measured, gas-particle partitioning ratio (F_T/TSP)/A_T has been examined for the case when a constant fraction x (%) of a compound is assumed to be bound within the particulate matter, and non-exchangeable with the gas phase. The parameter F_T is the total (exchangeable + non-exchangeable) measured concentration in the atmosphere (ng m⁻³), A_T is the gaseous concentration (ng m⁻³), and TSP is the level of suspended atmospheric particulate matter (μg m⁻³). It is assumed that the true thermodynamic constant K_p depends upon 1/T according to log K_p = m_p/T + b_p where m_p depends on the enthalpy of desorption of the compound of interest, b_p depends in part on other properties of the compound as well as the specific surface area of the particulate matter, and T is the temperature (Kelvin). When K_p or TSP are low, the difference between the measured quantity (F_T/TSP)/A_T and K_p can be significant even when the non-exchangeable fraction x is as low as a few per cent. This approach has been used to examine the PAH data set of Yamasaki et al. [(1982) Env. Sci. Technol.16, 189–194]. It was found that the Yamasaki data set does not allow estimates of x that are consistent with the current understanding of the temperature dependence of log K_p values. A likely reason for this result is some dependence of m_p and b_p on the exact nature of the particulate matter and atmospheric conditions such as relative humidity. It is concluded that estimates of x values for a given compound on actual particulate matter may only be possible by the direct examination of individual particulate matter samples.     

Sulphate in atmospheric particulate at five sites in Norwich (U.K.): Use of national survey archive filters

Air sample filters from a Local Authority archive have been analyzed for sulphate using XRF. This has allowed an assessment to be made of the spatial distribution of 24-h sulphate concentrations in the city of Norwich in eastern England, over a 9-month period. The overall mean sulphate concentration was 10.4 μg m⁻³, with a range of overall mean values between the sites of 7.7–12.2 μg m⁻³. This spatial variability within the city is similar to the spatial variability between cities found in another previous study. The mean SO²⁻₄/SO₂ and smoke/SO²⁻₄ ratios at the different sites are similar, but there are distinct variations in the overall mean ratios over the measurement period. There appear to be relationships between the SO²⁻₄/SO₂ ratio and temperature and humidity. The ratios are also related to wind direction, with some evidence that a proportion of the SO²⁻₄ in the urban atmosphere of Norwich originates via long-distance transport from the east.     

A characterization of the principal PM-10 species in Claremont (summer) and Long Beach (fall) during SCAQS

As part of the Southern California Air Quality Study (SCAQS), General Motors Research Laboratories made comprehensive air quality measurements during the summer of 1987 in Claremont, CA, and during the fall of 1987 in Long Beach, CA. These locations are typically in the areas of highest pollution for the respective seasons. Claremont's air quality during the summer was characterized by high concentrations of photochemically produced pollutants including ozone (O₃), nitric acid (HNO₃), particulate nitrate (NO₃⁻) and particulate organic carbon (OC). The highest concentrations of these species were experienced during the daytime sampling period (0600–1800 h) and were associated with transport from the western part of the basin. Long Beach's air quality during the fall was characterized by frequent periods of air stagnation that resulted in high concentrations of primary pollutants including PM-10, OC and elemental carbon (EC) as well as particulate NO₃⁻. Night-time levels of most constituents exceeded daytime levels due to poorer night-time dispersion conditions. At Claremont, OC and NO₃⁻ compounds account for 52% of the PM-10 while at Long Beach they account for 67% of the PM-10. On the average, there appears to be sufficient particulate ammonium (NH₄⁺) to completely neutralize the nitrate and acidic sulfates. Significant artifact OC was detected and corrected for at both sites. At Claremont the artifact accounted for 21% of the uncorrected OC mass and 13% at Long Beach. Because of NO₃⁻ and NH₄⁺ losses on Teflon filters, the measured PM-10 and fine particulate mass (FPM) had to be corrected. This correction amounted to increasing the average measured FPM and PM-10 by 17 and 12%, respectively, at Claremont, and 8 and 6%, respectively, at Long Beach.     

Spatial and temporal patterns in nitrogen dioxide concentrations in a hot desert region

This paper reports seasonal and spatial variations in the ambient air concentration of nitrogen dioxide throughout the State of Bahrain, from February to December 1992. Monitoring sites were chosen to include urban areas with high traffic density, suburban areas with low traffic density, commercial and industrial areas. Correlations between meteorological parameters and mean NO₂ concentrations were analysed, and NO₂ levels were only significantly correlated with temperature (r = 0.63). Only February, a winter month, showed a significantly lower concentration of NO₂ with an overall mean value of 23 μgm⁻³, whereas in August, a summer month, it was 33 μgm⁻³. The results revealed that in a hot region like Bahrain, NO₂ concentrations do not show significant monthly variations. Also summer-averaged NO₂ values exceeded corresponding spring and winter values. In cold regions opposite patterns were observed. Moreover, the results revealed significant spatial variations in NO₂ concentrations. In suburban areas with low traffic density, the overall mean NO₂ level was 15, with a range of 12–17 μg m⁻³, while in urban areas with high traffic density, the overall mean value was 52 with a range of 44–60 μg m⁻³. The mean NO₂ value in industrial sites with low traffic density was 21 with a range of 14–27 μg m⁻³, whereas in the same areas near major roads, it was 32 with a range of 31–32 μg m⁻³. These results indicate that automobiles exhaust are the dominant source of NO₂ in Bahrain. The highest NO₂ levels were found in roads with high traffic density, which are narrow, with several traffic lights and roundabouts, suggesting the effect of road geometry on NO₂ levels.     

Measured components in total suspended particulate matter in a Kenyan urban area

Nairobi city has a population of over 1.5 million and is growing at a rate of about 70 persons per day. Various activities in the city such as construction work, industrial processes, use of unroadworthy cars, and dust blown off unpaved roads contribute enormously to suspended particulate matter in the air. In this paper, analysis by gravimetric and energy dispersive X-ray fluorescence (EDXRF) of the suspended particulate matter in the air in the city centre, an industrial area and one residential area was carried out. The total suspended particulate matter (TSP) mean levels ranged from 69.983 to 397.903 μg m⁻³. The following components were measured from the TSP, mean values in μg m⁻³: iron 6.014–7.547, potassium 1.252–6.432, titanium 0.286–1.698, manganese 0.158–1.683, lead 0.395–1.321, bromine 0.122–0.707, zinc 0.159–0.678 and zirconium 0.017–0.245. The values of lead obtained (0.395–1.321 μg m⁻³) fall within the WHO recommendations, but compared to the values reported in some European countries, they are high. Most of the elements had low enrichment factors except for lead (104–353), bromine (429–1533) and zinc (14–79). Bromine and lead were highly correlated to the number of light vehicles (p=0.874 and 0.942, respectively). In addition the ratio of Br:Pb by weight was in the range 0.309–0.535, while the correlation factor for Br:Pb was 0.951, leading to the conclusion that both elements came from leaded gasoline.     

Wintertime measurements of aerosol acidity and trace elements in Wuhan,a city in central China

A 2-week intensive ambient aerosol study was conducted in December 1988 in Wuhan (Hubei Province), a city of nearly 2 million located on the Yangtze River in central China (P.R.C.). This is an industrial region where soft coal burning is widespread, and emission controls for vehicles and industrial facilities are minimal. The sampling site was located in one of the civic centers where residential and commercial density is highest. An Andersen dichotomous sampler was operated with Teflon membrane filters to collect fine (d_p < 2.5 μmad) and coarse (2.5 ⩽ d_p < 10 μmad) particles for total mass and element determinations. An annular denuder system (ADS) was used to collect fine fraction aerosols for analyses of ionic species including strong acidity (H⁺).The study was conducted between 18 and 30 December, which was rainless, consistently cool (3–10°C) and overcast, but without fog or acute stagnation. Fine particulate mass (PM, as μ m⁻³) averaged 139 (range 54–207); coarse PM averaged 86 (range 29–179). Trace element concentrations were also high. Crustal elements (Si, Al, Ca and Fe) were found primarily in the coarse fraction, while elements associated with combustion (S, K, Cl, Zn and Se) were enriched in the fine fraction. The concentrations of arsenic and selenium were evidence of a large source of coal burning, while vanadium levels (associated with fuel oil use) were not especially enriched.Despite the seemingly high PM loadings, ionic concentrations were not especially high. The average composition of soluble fine aerosol species (in neq m⁻³) were SO₄²⁻: 520 (range 180–980), NO₃⁻: 225 (range 50–470), Cl⁻: 215 (range 20–640), and NH₄⁺: 760 (range 280–1660). A deficit in accountable FP components (total mass compared to the total of ionic plus element masses) as well as the black appearance of collected materials indicate an abundance of carbonaceous aerosol, as high as 100 μ m⁻³. (total mass compared to the total of ionic plus element masses) as well as the black appearance of collected materials indicate an abundance of carbonaceous aerosol, as high as 100 μ m⁻³Aerosol acidity was negligible during most monitoring periods, H⁺: 14 (range 0–50 neq m⁻³, equivalent to 0–2.5 μm m⁻³ as H₂SO₄). Sulfur dioxide, measured by the West-Gaeke method for part of the study, concentrations were low. Although not directly measured, the aerosol measurments suggested that gaseous HCl (from refuse incineration) and NH₃ (animal wastes) concentrations might have been high. Higher aerosol acidity might be expected if HCl sources were more prominent and not neutralized by local ammonia or other base components.     

Characterization and source identification of respirable particulate matter in Athens,Greece

Twenty-seven samples of respirable particulate matter (RP) were collected in central Athens during the summer of 1987. The samples were analyzed for the concentration SO₄²⁻ and NO₃⁻ (collected with Teflon and nylon filters and analyzed by ion-chromatography); organic and elemental carbon (collected on quartz fiber filters and analyzed with a thermo-optical method); and 20 elements (collected on Teflon filters and analyzed by proton-induced X-ray emission spectroscopy). Simultaneously collected samples for SO₂ and HNO₃ were also evaluated (collected with annular denuders and analyzed by ion-chromatography). T The average RP concentration measured was 80.7 μg m⁻³, well above the USEPA annual standard for PM₁₀ aerosol. In addition, high levels of organic (16.9 μg m⁻³) and elemental carbon (4.2 μg m⁻³) were found. Correlations between aerosol carbon and Br, Pb, NO and NMHC (all > 0.8) confirm that gasoline and diesel powered vehicles are one of the major sources of pollution in the region. Correlations between RP and Al, Si, K, Ca and Fe also suggest that soil aerosols contribute to the high RP concentrations. Enrichment factors for the RP samples relative to the composition of soil in the Athens basin indentify major contributions from vehicles (Pb and Br) and industrial operations (S, Ti, Mn, Fe, Ni, Cu, Zn). Based on these results, a Chemical Mass Balance receptor model was applied to each of the 27 samples and the contributions to RP from soil (4.5 %), vehicles (20.3 %), steel industries (4.6. %) and cement plant emissions (3.2 %) estimated. Other major components of the RP were SO₄²⁻ (13.0 %) and organic carbon from non-vehicle and industrial sources (15.5 %).     

Determination of mutagenic activity of airborne particulates and of the benzo[α]pyrene concentrations in Athens atmosphere

The mutagenicity and benzo(α)pyrene (BαP) content of airborne particles (172 samples) from four locations in Athens was studied for 1 year starting February 1984. All the organic extracts of airborne particulate matter showed direct mutagenic activity. There was a good correlation between BαP concentration and mutagenicity in all samples. Higher values of mutagenic activity and BαP concentrations were found at the sites located in the center of the city than at the sites located in the industrial areas. The height of the sampling place has a negative effect on both measured pollution parameters. The higher percentage of biological and chemical parameters were associated with particles <3.3 μm in diameter. The mean yearly values of mutagenic activity of airborne particulates and BαP levels, were 1.9 rev m⁻³ and 2.6 ng m⁻³, respectively and can be grouped among the median, between the heavily and the lightly polluted cities of the world. Higher values of mutagenic activity were found during winter months. From the diurnal variation of mutagenic activity it was found that maximum value occurred during morning (0900–1100). The main source of mutagens seems to be the emissions from diesel powered engines and central heating.     

The removal of nitric acid to atmospheric particles during a wintertime field study

A field was conducted in Warren, MI, during the 1987/88 winter period to examined the reaction of HNO₃ with particulate salt from road deicing. Samples of gases and particles were collected on a daily basis over a 4-month period. If HNO₃ reacts with particulate salt, particles of sodium nitrate will be formed and gaseous hydrochloric acid will be released. Thus, during the periods of high salt concentrations, one would expect increases in HCl and particulate NO₃⁻ and a decrease in HNO₃. The effect was observed, although the total amount of NaNO₃ formed was not large. The increase in particulate NO₃⁻ did not appear in the large particle mode. Although more than 40% of the salt was present as very large particles (> 8 μm), most of the surface area was associated with small particles. Since the reaction occurred at the particle surface, the NO₃⁻ appeared in the small particle mode. Large-particle nitrate has been observed in association with sea salt and crustal particles and has also been attributed to reactions with HNO₃. These cases will be reconsidered based on the road salt results.     

Transported acid aerosols measured in southern Ontario

During the period 29 June 1986–9 August 1986, a field health study assessing the acute health effects of air pollutants on children was conducted at a summer girls' camp on the northern shore of Lake Erie in SW Ontario. Continuous air pollution measurements of SO₂, O₃, NO_x, particulate sulfates, light scattering, and meteorological measurements including temperature, dew point, and wind speed and direction were made. Twelve-hour integrated samples of size fractioned particles were also obtained using dichotomous samplers and Harvard impactors equipped with an ammonia denuder for subsequent hydrogen ion determination. Particulate samples were analyzed for trace elements by X-ray fluorescence and Neutron Activation, and for organic and elemental carbon by a thermal/optical technique. The measured aerosol was periodically very acidic with observed 12-h averaged H⁺ concentrations in the range < 10–560 nmoles m⁻³. The aerosol H⁺ appeared to represent the net strong acidity after H₂SO₄ reaction with NH₃(g). Average daytime concentrations were higher than night-time for aerosol H⁺, sulfate, fine mass and ozone. Prolonged episodes of atmospheric acidity, sulfate, and ozone were associated with air masses arriving at the measurement site from the west and from the southwest over Lake Erie. Sulfate concentrations measured at the lakeshore camp were more than twice those measured at inland sites during extreme pollution episodes. The concentration gradient observed with onshore flow was potentially due to enhanced deposition near the lakeshore caused by discontinuities in the meteorological fields in this region.     

北京海淀区夏季交警对多环芳烃的暴露

用个人采样装置测定了北京海淀区交警夏季多环芳烃(PAHs)暴露.以16种PAHs之和计,气态和颗粒物吸附态的平均暴露浓度分别为(1 520±759)ng/m³和(148±118)ng/m³,显著高于定点对照测定结果[气态(588±228)ng/m³,颗粒物吸附态(52±50)ng/m³].交警和对照点具有致癌作用的高环化合物总暴露浓度分别是(14.9±5.9)ng/m³和(6.7±3.6)ng/m³,主要来自颗粒物吸附态暴露.暴露量具有显著的日波动特征,主要受温度和湿度影响.气态浓度与湿度正相关,与温度负相关,颗粒物吸附态浓度则与温度和湿度看不到显著相关关系.     

Polynuclear aromatic hydrocarbon degradation by heterogeneous reactions with N2O5 on atmospheric particles

The degradation of particulate polynuclear aromatic hydrocarbons (PAH) on atmospheric soot particles in the presence of gas phase dinitrogen pentoxide (N₂O₅) was explored. Dilute diesel and wood soot particles containing PAH were reacted with∼10ppm of N₂O₅ in a 200 ℓ continuous stirred tank reactor (CSTR). To provide a stable source of particles for reaction in the CSTR, diesel or wood soot particles were injected at night into a 25 m³ Teflon outdoor chamber. The large chamber served as a reservoir for the feed aerosol, and the aerosol could then be introduced at a constant flow rate into the CSTR. PAH-N₂O₅ heterogeneous rate constants for wood soot at 15°C ranged from2 × 10⁻¹⁸to5 × 10⁻¹⁸ cm³ molecules⁻¹ s⁻¹. For diesel soot the rate constants at 16°C were higher and ranged from5 × 10⁻¹⁸to30 × 10⁻¹⁸ cm³ molecules⁻¹ s⁻¹. Comparisons with other studies suggest that sunlight is the most important factor which influences PAH decay. This is followed by ozone, NO₂, N₂O₅ and nitric acid. The rate constants of nitro-PAH formation from a parent PAH and N₂O₅ were of the order of1 × 10⁻¹⁹−1 × 10⁻¹⁸ molecules⁻¹s⁻¹. The uncertainty associated with all of these rate constants is± a factor of 3. Given, however, the small magnitude of the rate constants and the low levels of N₂O₅ present in the atmosphere, we concluded that PAH heterogeneous reactions with gas phase N₂O₅ degrade particle-bound PAH or to form nitro-PAH from PAH arenot very important. (Direct application of the specific rate constants derived in this study to ambient atmospheres should not be undertaken unless the ambient particle size distributions and chemical composition of the particles are similar to the ones reported in this study.)