Reactions of adsorbed pyrene and perylene with gaseous N2O5 under simulated atmospheric conditions

We report here results of studies concerning the reactions of two representative polycyclic aromatic hydrocarbons (PAH), pyrene and perylene, adsorbed on glass fiber filters, towards gaseous N₂O₅, NO₃ radicals, NO₂ and HNO₃ at part-per-million or lower concentrations and at ⩽ 5% relative humidity. These exposures were carried out in a passive mode in an ~ 6400-ℓ all-Teflon environmental chamber. As is the case for naphthalene vapor, neither adsorbed pyrene nor perylene reacted to any observable extent with the NO₃ radical. However, pyrene reacted rapidly with N₂O₃ to form 1-nitropyrene (1-NP) with a 60–70% yield after 50 min exposure to an initial N₂O₅ concentration of 1.5 ppm. Under these conditions, no reaction of perylene with N₂O₅ was observed. In control runs of the same duration, neither pyrene nor perylene reacted significantly ( < 0.5%) with 1.7 ppm of gaseous HNO₃ but, consistent with earlier studies, a ~ 3% yield of 3-nitroperylene (and < 0.5 % 1-NP) was formed when they were exposed to a mixture of 0.5 ppm NO₂ + 0.35 ppm HNO₃. These data show for the first time that gas phase N₂O₅ can be a strong nitrating agent for adsorbed pyrene to produce 1-nitropyrene, a powerful bacterial mutagen. Clearly, uncertainties occur in extrapolating our laboratory data to ambient atmospheres. However, when our results are combined with estimated N₂O₅ concentrations of up to ~ 15 ppb in polluted night-time atmospheres, they suggest that nitro-PAH, could be formed at significant rates in ambient air.     

Night-time tropospheric chemistry of the unsaturated alcohols (Z)-pent-2-en-1-ol and pent-1-en-3-ol: Kinetic studies of reactions of NO3 and N2O5 with stress-induced plant emissions

The night-time tropospheric chemistry of two stress-induced volatile organic compounds (VOCs), (Z)-pent-2-en-1-ol and pent-1-en-3-ol, has been studied at room temperature. Rate coefficients for reactions of the nitrate radical (NO₃) with these pentenols were measured using the discharge-flow technique. Because of the relatively low volatility of these compounds, we employed off-axis continuous-wave cavity-enhanced absorption spectroscopy for detection of NO₃ in order to be able to work in pseudo first-order conditions with the pentenols in large excess over NO₃. The rate coefficients were determined to be (1.53±0.23)×10⁻¹³ and (1.39±0.19)×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ for reactions of NO₃ with (Z)-pent-2-en-1-ol and pent-1-en-3-ol. An attempt to study the kinetics of these reactions with a relative-rate technique, using N₂O₅ as source of NO₃ resulted in significantly higher apparent rate coefficients. Performing relative-rate experiments in known excesses of NO₂ allowed us to determine the rate coefficients for the N₂O₅ reactions to be (5.0±2.8)×10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for (Z)-pent-2-en-1-ol, and (9.1±5.8)×10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for pent-1-en-3-ol. We show that these relatively slow reactions can indeed interfere with rate determinations in conventional relative-rate experiments.     

The gas-phase reaction of naphthalene with N2O5 to form nitronaphthalenes

While the formation of nitroarenes from the reaction of NO₂, containing traces of HNO₃, in air with polycyclic aromatic hydrocarbons (PAH) adsorbed on combustion generated particles is now well recognized, little is known about the gas-phase reactions of PAH. In this study, the gas-phase reactions in air of N₂O₃ with part-per-million levels of naphthalene have been studied at room temperature and atmospheric pressure in a 5800V Teflon-coated environmental chamber. The kinetic data obtained showed that in these N₂O₅-NO₃-NO₂-air mixtures studied, naphthalene did not react with the NO₃ radical at an observable rate, but that it reacted with N₂O₅ with a rate constant of ~ (2–3) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹. Significant yields of 1-nitronaphthalene and 2-nitronaphthalene ( ~ 18 and ~ 7.5%, respectively) were obtained from this reaction. The latter is a procarcinogen capable of being metabolized in animals to the carcinogen β-naphthylamine. These results and their atmospheric implications are discussed.     

Application of CALINE4 to roadside NO/NO2 transformations

The CALINE4 roadway dispersion model has been applied to concentrations of NO_x and NO₂ measured near Gandy Boulevard in Tampa, FL (USA) during May 2002. A NO_x emission factor of 0.86 gr mi⁻¹ was estimated by treating NO+NO₂ (NO_x) as a conserved species and minimizing the differences between measured and calculated NO_x concentrations. This emission factor was then used to calculate NO₂ concentrations using the NO/NO₂ transformation reactions built into CALINE4. A comparison of measured and calculated NO₂ concentrations indicates that for ambient O₃ concentrations less than 40 ppb the model under-predicts the chemical transformation of NO. The enhanced transformation of NO may be due to reactions of NO with oxidants such as peroxy radicals that are present either in the atmosphere or in vehicle exhaust.     

A calibrated source of N2O5

A calibrated source of N₂O₅ was developed for use in evaluating instruments designed to measure reactive odd-nitrogen species in the atmosphere. The desirable features of the source are the output of a very low mixing ratio of N₂O₅, the measurement of total reactive nitrogen NO_y in the source output, and the use of commercially-available detection and calibration equipment. N₂O₅ was derived from a solid crystalline sample at 192 K. Mixing ratios of N₂O₅ less than 2 ppmv (parts per million by volume) were produced in a small flow ( < 50 STPcc min⁻¹) of He carrier gas. This small flow was then diluted to obtain mixing ratios less than 50 ppbv (parts per billion by volume) in flows greater than 1 STPℓ min⁻¹. The N₂O₅ mixing ratio was determined with an accuracy of ± 15 % by using a thermal dissociator, calibrated NO/N₂ mixtures, and an NO chemiluminescence detector. The level of other reactive nitrogen species in the source output was determined with an Au catalytic converter and the NO detector.     

Analysis of NO,NO2, O3 and NOx concentrations measured at a green area of Buenos Aires City during wintertime

We analyse the air quality data measured at a green area of Buenos Aires City (Argentina) during 38 days in winter. We study the relationships between ambient concentrations of nitric oxide (NO), nitrogen dioxide (NO₂), ozone (O₃) and nitrogen oxides (NO_x=NO+NO₂). The variation of the level of oxidant (OX=O₃+NO₂) with the NO_x is obtained. It can be seen that the level of OX at a given location is made up of two contributions: one independent and another dependent on the NO_x concentration. The first one can be considered as a regional contribution, equivalent to the background O₃ concentration and the second one as a local contribution that depends on the level of primary pollution. Local oxidant sources may include direct NO₂ emissions, the reaction of NO with O₂ at high-NO_x levels, and the emission of species that promote the conversion of NO to NO₂. The final category of emissions may include the nitrous acid (HONO) that is emitted directly in vehicle exhaust. Finally, we present a diurnal variation of the local as well as regional contributions and the dependence of the last one on wind direction.     

Evidence for large average concentrations of the nitrate radical (NO3) in Western Europe from the HANSA hydrocarbon database

The nitrate radical (NO₃) was first measured in the atmosphere in the 1970s and suggestions were made that it could play a major role in oxidising many unsaturated hydrocarbons, such as those emitted from the biosphere. Analysis of the hydrocarbon mix over the North Atlantic Ocean suggested subsequently that the influence of NO₃ radical chemistry at night was even more extensive, being on a par with hydroxyl radical chemistry at some times of the year.The paper presents a detailed analysis of an extensive database of many nonmethane hydrocarbons collected at various sites around the North Sea in the mid 1990s during the HANSA project. By comparing the relative rates of oxidation of iso and normal pentane with that of toluene and benzene it clearly shows that the efficiency of NO₃ radical chemistry and hydroxyl radical chemistry over northwest Europe are similar in springtime and predicts an average nighttime NO₃ concentration of the order of 350 pptv, assuming an annual average OH concentration of 0.6×10⁶ cm⁻³. This value is very dependant on the average emission ratios of the different hydrocarbons and values between 200 and 600 pptv are possible. It is much larger than direct measurements made in Europe at the surface, but is of the same magnitude as concentrations measured recently from aircraft in the boundary layer over the northeast USA, and previously in vertical profiles by remote sounding over Europe.A simple analytical expression can be derived to calculate the NO₃ concentration at night with the only variables being ozone and the loss rate of N₂O₅, either to the ground or to aerosol surfaces. The concentrations of NO₃ calculated in this manner are similar to those derived from the analysis of the HANSA hydrocarbon database for typical conditions expected over Europe, but they are very dependant on the efficiency of the aerosol sink for N₂O₅.It is shown that NO₃ oxidation of many unsaturated hydrocarbons can indeed be more efficient than OH oxidation, especially at times of the year outside the summer season. Direct evidence for hydrocarbon oxidation by NO₃ radicals is shown by a series of peroxy radical measurements where the nighttime concentrations can be significantly higher than daytime concentrations in polluted air on occasion. Also the winter/summer (W/S) ratios of many unsaturated hydrocarbons are much lower than those expected from their removal purely by hydroxyl radical chemistry.The consequences of these findings are profound especially as satellite measurements of NO₂, a major precursor to NO₃, suggest that these high average concentrations of several hundred pptv could be widespread over most of the continents. This needs to be confirmed by direct in-situ measurement of nitrate radicals but it suggests a much larger role for NO₃ chemistry in the oxidation capacity of the atmosphere than realised hitherto.     

A numerical mesoscale model for long-term average NOx and NO2-concentration

A model for the computation of long-term average NO_x- and NO₂-concentrations is developed and applied. The model is based on the computation of average concentrations for wind-direction-wind-speed classes by numerical modelling of the NO_x and O_x (NO₂ + O₃) concentrations over a diurnal cycle with class-averaged meteorology, and empirical determination of background NO_x-, O_x-fluxes and O_x-gradients from measurements. The photostationary equilibrium between NO, NO₂ and O₃ is accounted for, together with the major physical and chemical decay-processes. A good agreement between modelled and measured yearly average NO_x- and NO₂-concentrations was obtained. The effects of policy measures were evaluated by imposing several reductions on the emission inventory.     

Atmospheric chemistry of 1-methyl-2-pyrrolidinone

Rate constants for the atmospheric reactions of 1-methyl-2-pyrrolidinone with OH radicals, NO₃ radicals and O₃ have been measured at 296±2 K and atmospheric pressure of air, and the products of the OH radical and NO₃ radical reactions investigated. Using relative rate techniques, rate constants for the gas-phase reactions of OH and NO₃ radicals with 1-methyl-2-pyrrolidinone of (2.15±0.36)×10^-11 cm³ molecule^-1 s^-1 and (1.26±0.40)×10^-13 cm³ molecule^-1 s^-1, respectively, were measured, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. An upper limit to the rate constant for the O₃ reaction of <1×10^-19 cm³ molecule^-1 s^-1 was also determined. These kinetic data lead to a calculated tropospheric lifetime of 1-methyl-2-pyrrolidinone of a few hours, with both the daytime OH radical reaction and the nighttime NO₃ radical reaction being important loss processes. Products of the OH radical and NO₃ radical reactions were analyzed by gas chromatography with flame ionization detection and combined gas chromatography–mass spectrometry. N-methylsuccinimide and (tentatively) 1-formyl-2-pyrrolidinone were identified as products of both of these reactions. The measured formation yields of N-methylsuccinimide and 1-formyl-2-pyrrolidinone were 44±12% and 41±12%, respectively, from the OH radical reaction and 59±16% and ∼4%, respectively, from the NO₃ radical reaction. Reaction mechanisms consistent with formation of these products are presented.     

Aqueous-phase nitration of phenol by N2O5 and ClNO2

Nitrophenols are present in the atmospheric gas phase and in cloud and rainwater. Their formation via aqueous-phase reactions of phenol with the nitronium ion, NO₂⁺, arising from N₂O₅ and ClNO₂ partitioning into the aqueous phase, has been proposed but not verified experimentally. Here, we demonstrate for the first time that gaseous N₂O₅ and ClNO₂ partitioning into dilute aqueous solutions of phenol yields 2- and 4-nitrophenol (and 4-nitrosophenol), but no dinitrophenol isomers. The rate of nitration does not vary significantly between 5 and 20 °C, presumably because of opposing temperature dependences in Henry's law partitioning and reaction rate coefficients. The rate coefficient for reaction of NO₂⁺ with phenol could not be directly quantified but is evidently large enough for this reaction to compete effectively with the reaction between NO₂⁺ and water and to provide a feasible route to nitrophenol production in the atmosphere.     

OH and HO2 measurements in a forested region of north-western Greece

Boundary layer concentrations of hydroxyl (OH) and hydroperoxyl (HO₂) radicals were measured at 1180 m elevation in a mountainous, forested region of north-western Greece during the AEROsols formation from BIogenic organic Carbon (AEROBIC) field campaign held in July–August 1997. In situ measurements of OH radicals were made by laser-induced fluorescence (LIF) at low pressure, exciting in the (0, 0) band of the A–X system at 308 nm. HO₂ radicals were monitored by chemical titration to OH upon the addition of NO, with subsequent detection by LIF. The instrument was calibrated regularly during the field campaign, and demonstrated a sensitivity towards OH and HO₂ of 5.2×10⁵ and 2.4×10⁶ molecule cm⁻³, respectively, for a signal integration period of 2.5 min and a signal-to-noise ratio of 1. Diurnal cycles of OH and HO₂ were measured on 10 days within a small clearing of a forest of Greek Fir (Abies Borisi-Regis). In total 4165 OH data points and 1501 HO₂ data points were collected at 30 s intervals. Noon-time OH and HO₂ concentrations were between 4–12×10⁶ and 0.4–9×10⁸ molecule cm⁻³, respectively. The performance of the instrument is evaluated, and the data are interpreted in terms of correlations with controlling variables. A significant correlation (r²=0.66) is observed between the OH concentration and the rate of photolysis of ozone, J(O¹D). However, OH persisted into the early evening when J(O¹D) had fallen to very low values, consistent with the modelling study presented in the following paper (Carslaw et al., 2001, OH and HO₂ radical chemistry in a forest region of north-western Greece. Atmospheric Environment 35, 4725–4737) that predicts a significant radical source from the ozonolysis of biogenic alkenes. Normalisation of the OH concentrations for variations in J(O¹D) revealed a bell-shaped dependence of OH upon NO_x (NO+NO₂), which peaked at [NO_x] ∼1.75 ppbv. The diurnal variation of HO₂ was found to be less correlated with J(O¹D) compared to OH.     

The dynamics of nitric acid production and the fate of nitrogen oxides

A mathematical model is used to study the fate of nitrogen oxides (NO_x) emissions and the reactions responsible for the formation of nitric acid (HNO₃). Model results indicate that the majority of the NO_x inserted into an air parcel in the Los Angeles basin is removed by dry deposition at the ground during the first 24 h of travel, and that HNO₃ is the largest single contributor to this deposition flux. A significant amount of the nitric acid is produced at night by N₂O₅ hydrolysis. Perturbation of the N₂O₅ hydrolysis rate constant within the chemical mechanism results in redistribution of the pathway by which HNO₃ is formed, but does not greatly affect the total amount of HNO₃ produced. Inclusion of NO₃-aerosol and N₂O₅-aerosol reactions does not affect the system greatly at collision efficiencies, α, of 0.001, but at α = 0.1 or α = 1.0, a great deal of nitric acid could be produced by heterogeneous chemical processes.Ability to account for the observed nitrate radical (NO₃) concentrations in the atmosphere provides a key test of the air quality modeling procedure. Predicted NO₃ concentrations compare well with those measured by Platt et al. (Geophys. Res. Lett.7, 89–92, 1980). Analysis shows that transport, deposition and emissions, as well as chemistry, are important in explaining the behavior of NO₃ in the atmosphere.     

Trend analysis of urban NO2 concentrations and the importance of direct NO2 emissions versus ozone/NOx equilibrium

The annual air quality standard of NO₂ is often exceeded in urban areas near heavy traffic locations. Despite significant decrease of NO_x emissions in 1986–2005 in the industrial and harbour area near Rotterdam, NO₂ concentrations at the urban background remain at the same level since the end of the nineties. Trend analysis of monitoring data revealed that the ozone/NO_x equilibrium is a more important factor than increasing direct NO₂ emissions by traffic. The latter has recently been identified as an additional NO₂ source due to the introduction of oxy-catalytic converters in diesel vehicles and the growing number of diesel vehicles. However, in Rotterdam over the period 1986–2005 direct NO₂ emissions by road traffic only increased 3–4%. Due to the importance of the ozone/NO_x equilibrium, it is concluded that local NO_x emissions in Rotterdam need substantial reduction to achieve lower NO₂ urban background levels. This is a relatively costly abatement strategy and, therefore, a “hotspot” approach aiming at reducing NO_x emissions by local traffic measures is more effective to meet European air quality standards.     

On the origins of night-time NO at a rural measurement site

Mixing ratios for NO and NO₂ were measured during 1980/1981 at Deuselbach, a rural site in Germany. The data are analyzed with regard to the occurrence of nocturnal NO signals and their origins. Anthropogenic influences arising from road traffic and home heating activities are identified by their dependence on wind direction. An additional non-directional component is found to exist. It shows up most frequently in summer and when it occurs, the NO mixing ratio increases with rising temperature indicating a biological origin of night-time NO. The temporal behavior of night-time NO is usually correlated with that of CO₂ but anticorrelated to that of O₃. This shows that NO is brought upwards to the air intake of the NO_x monitor from lower levels and that the source of the non-directional component of nocturnal NO is the earth's surface. The release of NO from soils is known from other work and this process is probably also responsible for the present observations. A flux estimate agrees with soil fluxes reported by other authors. The accumulation of NO in the surface air layer under stagnant conditions leads to the appearance of a morning peak of NO after sunrise when increased vertical mixing brings NO rich air up to the monitoring level. During summer, the morning peak may override the NO peak expected to occur at noon due to the photodissociation of NO₂.     

Aircraft measurements of O3, NOx,CO, VOCs,and SO2 in the Yangtze River Delta region

In this study, air pollutants, including ozone (O₃), nitrogen oxides (NO_x = NO + NO₂), carbon monoxides (CO), sulfur dioxide (SO₂), and volatile organic compounds (VOCs) measured in the Yangtze River Delta (YRD) region during several air flights between September/30 and October/11 are analyzed. This measurement provides horizontal and vertical distributions of air pollutants in the YRD region. The analysis of the result shows that the measured O₃ concentrations range from 20 to 60 ppbv. These values are generally below the US national standard (84 ppbv), suggesting that at the present, the O₃ pollutions are modest in this region. The NO_x concentrations have strong spatial and temporal variations, ranging from 3 to 40 ppbv. The SO₂ concentrations also have large spatial and temporal variations, ranging from 1 to 35 ppbv. The high concentrations of CO are measured with small variations, ranging from 3 to 7 ppmv. The concentrations of VOCs are relatively low, with the total VOC concentrations of less than 6 ppbv. The relative small VOC concentrations and the relative large NO_x concentrations suggest that the O₃ chemical formation is under a strong VOC-limited regime in the YRD region. The measured O₃ and NO_x concentrations are strongly anti-correlated, indicating that enhancement in NO_x concentrations leads to decrease in O₃ concentrations. Moreover, the O₃ concentrations are more sensitive to NO_x concentrations in the rural region than in the city region. The ratios of Δ[O₃]/Δ[NO_x] are ?2.3 and ?0.25 in the rural and in the city region, respectively. In addition, the measured NO_x and SO₂ concentrations are strongly correlated, highlighting that the NO_x and SO₂ are probably originated from same emission sources. Because SO₂ emissions are significantly originated from coal burnings, the strong correlation between SO₂ and NO_x concentrations suggests that the NO_x emission sources are mostly from coal burned sources. As a result, the future automobile increases could lead to rapid enhancements in O₃ concentrations in the YRD region.     

Kinetics of the gas-phase reactions of alkylnaphthalenes with O3, N2O5 and OH radicals at 298 ± 2 K

Rate constants for the gas-phase reactions of the OH radical with 1-methylnaphthalene and of N₂O₅ with 1- and 2-methylnaphthalene and 2,3-dimethylnaphthalene have been determined at 298 ± 2 K by use of relative rate techniques. The rate constants determined were: for the reaction of OH radicals with 1-methylnaphthalene, (5.30 ± 0.48) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; for the reaction of N₂O₅ with 1-methylnaphthalene, 2-methylnaphthalene and 2,3-dimethylnaphthalene, (3.3 ± 0.7) × 10⁻¹⁷, (4.2 ± 0.9) × 10⁻¹⁷ and (5.7 ± 1.9) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹, respectively. In addition, an upper limit to the rate constant of 1.3 × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ was measured for the reaction of O₃ with 1-methylnaphthalene at 298 ± 2 K. These data, when combined with data from previous literature, allow the atmospheric gas-phase removal processes of these alkylnaphthalenes to be quantified.     

Influence of environmental factors on net N2 and N2O production in sediment of freshwater rivers

Denitrification is an important N removal process in aquatic systems but is also implicated as a potential source of global N₂O emissions. However, the key factors controlling this process as well as N₂O emissions remain unclear. In this study, we identified the main factors that regulate the production of net N₂ and N₂O in sediments collected from rivers with a large amount of sewage input in the Taihu Lake region. Net N₂ and N₂O production were strongly associated with the addition of NO₃ ^?-N and NH₄ ⁺-N. Specifically, NO₃ ^?-N controlled net N₂ production following Michaelis–Menten kinetics. The maximum rate of net N₂ production (V _max) was 116.3 μmol N₂-N m^?2 h^?1, and the apparent half-saturation concentration (k _m) was 0.65 mg N L^?1. N₂O to N₂ ratios increased from 0.18?±?0.03 to 0.68?±?0.16 with the addition of NO₃ ^?-N, suggesting that increasing NO₃ ^?-N concentrations favored the production of N₂O more than N₂. The addition of acetate enhanced net N₂ production and N₂O to N₂ ratios, but the ratios decreased by about 59.5 % when acetate concentrations increased from 50 to 100 mg C L^?1, suggesting that the increase of N₂O to N₂ ratios had more to do with the net N₂ production rate rather than acetate addition in this experiment. The addition of Cl^? did not affect the net N₂ production rates, but significantly enhanced N₂O to N₂ ratios (the ratios increased from 0.02?±?0.00 to 0.10?±?0.00), demonstrating that the high salinity effect might have a significant regional effect on N₂O production. Our results suggest that the presence of N-enriching sewage discharges appear to stimulate N removal but also increase N₂O to N₂ ratios.     

Seasonal variation of atmospheric ammonia and particulate ammonium concentrations in the urban atmosphere of yokohama over a 5-year period

Measurements of ammonia and particulate ammonium were made in the daytime (1200–1500) at a urban site in Yokohama during the 5-year period, 1982–1986. Diurnal NH₃ concentrations showed a distinct seasonal trend with a maximum in summer. The diurnal monthly average concentrations were above 10 ppb during the late spring and summer months, while the concentrations during the winter months were between 1 and 5 ppb. The seasonal variation was found to be very similar to that of the average air temperature and showed a periodic pattern over 1 year. A good correlation was observed between diurnal NH₃ concentrations and average air temperatures during the 5-year period. The annual mean concentrations were in the range of 6.6–7.6 ppb with only a minor deviation. The diurnal monthly average concentrations of particulate NH₄⁺ were between 1 and 4 μg m⁻³ and no significant seasonal variations were seen. As a short-term study, simultaneous measurements of NH₃, HNO₃ and particulate NO₃⁻ were made. The diurnal mean concentrations of NH₃ and HNO₃ were 7.6 and 0.8 ppb, respectively. The concentration of particulate NO₃⁻ ranged from 0.3 to 6μg⁻³. Both HNO₃ and particulate NO₃⁻ concentrations were relatively low and constant. Thus, NH₃ and HNO₃ levels did not agree with the concentrations predicted from the NH₄NO₃ equilibrium constant.     

A Review of Techniques Available for Estimating Short-Term NO2 concentrations

The Environmental Protection Agency is reviewing the need for a short-term NO₂ standard based on an averaging time of three hours or less. State Implementation plans and New Source Reviews will require air quality simulation techniques capable of estimating ambient NO₂ concentrations. There is a need for multi-source (urban) models and for point source models.

A review of currently available techniques for the estimation of NO₂ concentrations resulting from NO_x point sources is presented. The available methods include simple screening techniques and refined reactive plume models. The screening techniques first use a standard gaussian dispersion model to estimate the maximum 1 hr NO_x concentration caused by the source. The second step involves estimating the fraction of this NO* concentration occurring as NO₂.

Reactive plume models numerically simulate the simultaneous effects of dispersion and chemistry on NO₂ concentrations. Organic as well as inorganic reactions are incorporated. Reactive plume models should be used, where screening techniques indicate the potential for violation of the NO₂ standard.

Current generation reactive plume models neglect the effect of turbulent concentration fluctuation on NO₂ formation and use inappropriately large dispersion coefficients to estimate plume concentrations. Approaches being developed to resolve these problems are discussed.     

Simulation of long-range transport of acidic pollutants in East Asia during the Yellow Sand event

The atmospheric chemical process was simulated using the Carbon Bond 4 (CB-4) model, the aqueous-phase chemistry in Regional Acid Deposition Model and the thermodynamic equilibrium relation of aerosols with the emission inventories of the Emission Database for Global Atmospheric Research, the database of China and South Korea and the Mesoscale Model version 2 (MM5) meteorological fields to examine the spatial distributions of the acidic pollutant concentrations in East Asia for the case of the long-lasting Yellow Sand event in April 1998. The present models simulate quite well the observed general trend and the diurnal variation of concentrations of gaseous pollutants, especially for O₃ concentration. However, the model underestimates SO₂ and NO_x concentration but overestimates O₃ concentration largely due to uncertainty in NO_x and VOC emissions. It is found that the simulated gaseous pollutants such as SO₂, NO_x, and NH₃ are not transported far away from the source regions but show significant diurnal variations of their concentrations. However, the daily variations of the concentrations are not significant due to invariant emission rates. On the other hand, concentrations of the transformed pollutants including SO₄²⁻, NH₄⁺, and NO₃⁻ are found to have significant daily variations but little diurnal variations. The model-estimated deposition indicates that dry deposition is largely contributed by gaseous pollutants while wet deposition of pollutants is mainly contributed by the transformed pollutants.