WRF-Chem simulation of East Asian air quality: Sensitivity to temporal and vertical emissions distributions

This study develops fine temporal (seasonal, day-of-week, diurnal) and vertical allocations of anthropogenic emissions for the TRACE-P inventory and evaluates their impacts on the East Asian air quality prediction using WRF-Chem simulations in July 2001 at 30-km grid spacing against available surface measurements from EANET and NEMCC. For NO₂ and SO₂, the diurnal and vertical redistributions of emissions play essential roles, while the day-of-week variation is less important. When all incorporated, WRF-Chem best simulates observations of surface NO₂ and SO₂ concentrations, while using the default emissions produces the worst result. The sensitivity is especially large over major cities and industrial areas, where surface NO₂ and SO₂ concentrations are reduced by respectively 3–7 and 6–12 ppbv when using the scaled emissions. The incorporation of all the three redistributions of emissions simulates surface O₃ concentrations higher by 4–8 ppbv at night and 2–4 ppbv in daytime over broad areas of northern, eastern and central China. To this sensitivity, the diurnal redistribution contributes more than the other two.     

Satellite derived trends in NO2 over the major global hotspot regions during the past decade and their inter-comparison

We assessed satellite derived tropospheric NO₂ distribution on a global scale and identified the major NO₂ hotspot regions. Combined GOME and SCIAMACHY measurements for the period 1996-2006 have been used to compute the trends over these regions. Our analysis shows that tropospheric NO₂ column amounts have increased over the newly and rapidly developing regions like China (11 ± 2.6%/year), south Asia (1.76 ± 1.1%/year), Middle East (2.3 ± 1%/year) and South Africa (2.4 ± 2.2%/year). Tropospheric NO₂ column amounts show some decrease over the eastern US (−2 ± 1.5%/year) and Europe (0.9 ± 2.1%/year). We found that although tropospheric NO₂ column amounts decreased over the major developed regions in the past decade, the present tropospheric NO₂ column amounts over these regions are still significantly higher than those observed over newly and rapidly developing regions (except China). Tropospheric NO₂ column amounts show some decrease over South America and Central Africa, which are major biomass burning regions in the Southern Hemisphere.     

Adjoint inverse modeling of NOx emissions over eastern China using satellite observations of NO2 vertical column densities

A four-dimensional variational data assimilation system for optimization of NO_x emissions (RC4-NO_x) was developed. A parameterized NO_x chemistry scheme was introduced into the RC4-NO_x system, and key parameters such as chemical production and loss terms of NO_x were calculated in advance using the Community Multiscale Air Quality (CMAQ) modeling system. RC4-NO_x was applied to optimize NO_x emissions over eastern China (EC) in July 1996, 1999, and 2002 using Global Ozone Monitoring Experiment (GOME) satellite observations of NO₂ vertical column densities (VCDs) and a priori emissions from the Regional Emission Inventory in Asia (REAS). After assimilation, RC4-NO_x generally reproduced the spatial distribution, regional averaged values, and time evolution of GOME NO₂ VCDs. Over EC, a priori emissions were reduced by 20% in 1996 and by 8% in 1999, whereas a posteriori emissions were almost the same as a priori emissions in 2002. A priori emissions in the Beijing region were reduced by optimization over the whole simulation period. A posteriori emissions over the Yangtze Delta were larger than a priori emissions in 2002, although they were smaller in both 1996 and 1999. As in other areas, a priori emissions over the North China Plain were reduced in 1996; but those over the eastern part of the plain were increased in 1999, and the area of increased emissions moved slightly westward in 2002. In each region, the growth rates of a posteriori emissions during both 1996–1999 and 1999–2002 became generally larger than those of a priori emissions, and the trends of a posteriori emissions became similar to those of GOME NO₂ VCDs. Our inverse modeling analysis indicates that the rate of increase of NO_x emissions over EC from 1996 to 2002 was much larger for a posteriori emissions (49%) than for a priori emissions (19%).     

In situ measurements of NO,NO2, NOy,and O3 in Dinghushan (112°E, 23°N), China during autumn 2008

Measurements of O₃, NO, NO₂, and NO_y mixing ratios were carried out at a station-Dinghushan in Guangdong province of China from Oct. 18th, 2008 to Nov. 7th, 2008. This research shows that under conditions of a strong subtropical high (temperature high, relative humidity low), on Oct. 29th, 2008 the Dinghushan station observed severe photochemical pollution. The Maximum hour average concentration of O₃ reached 128 ppbv, and the serious photochemical pollution is caused by superposition of local photochemical reaction and regional transport. The observation that NO_x ozone production efficiency (OPE) values for high O₃ pollution on Oct. 29–30th, 2008 were 10.5 and 15, which were more than the values of the city source region and lower than that of the surrounding clean areas. It means the sensitivity of O₃ generated was transitioning from VOCs limited condition to NOx-limited regime. By applying a Smog Production Model, the results show that the extent of reaction values less than 0.6 were occurred on 17 days during campaign, and 13 days for the extents of reactions more than 0.6. However, there were no data with values over 0.8, which indicates that the observation station represent a VOCs sensitive system during campaign. Analysis of the extents of reactions and wind data show that the pollution is mostly subject to a southeasterly airflow influence.     

Impacts of a strong cold front on concentrations of HONO,HCHO, O3, and NO2 in the heavy traffic urban area of Beijing

Much rain and strong winds caused by a cold front occurred in Beijing during the period of Sep. 27 to Oct. 4, 2004 and led to sharp drops in maximum and mean concentrations of HONO, HCHO, O₃, and NO₂, i.e., the maximum concentrations were reduced by 5.9, 21.3, 45.6, and 44.4 ppb, respectively, and the mean concentrations were decreased by 4.0, 5.5, 30.3, and 32.3 ppb, respectively. For daily HO_x production rates HONO photolysis was the largest contributor and over 90% contributions were from photolysis of HONO and HCHO. Large number and area percentages of soot aggregate from PM10, and high correlations between concentrations of PM10 and chemical formation of HONO suggested that heterogeneous reactions of NO₂ on surfaces of soot aggregate could be a key source of HONO in the heavy traffic areas of Beijing during the night and should be considered in air quality simulations for such areas.     

Variations of the increasing trend of tropospheric NO2 over central east China during the past decade

A numerical analysis using a regional chemical transport model (CTM) is presented in comparison with Global Ozone Monitoring Experiment (GOME) and SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) satellite NO₂ measurements over East Asia from 1996 to 2005 from a climatological perspective. Modeling results agree well with satellite retrievals in geographical distribution patterns, with systematic underestimation of the absolute values. The sharp increase in NO₂ vertical column densities (VCDs) over central east China (CEC) after the year 2000 (14.1–20.5% yr⁻¹ for the satellite observations and 10.8% yr⁻¹ for model simulations) is analyzed quantitatively over different megacity clusters. The distinct emission increase patterns are responsible for the different increase trends observed over the Beijing megacity cluster (BJ), the Yangtze Delta (YD) and other CEC regions. The growth rate of satellite measured and CMAQ-modeled NO₂ VCDs for the YD is much higher than that in other regions, with no clear seasonal variation. Apart from BJ and YD, NO₂ emissions from other regions in CEC also expand considerably.     

Assessment of air quality benefits from national air pollution control policies in China. Part II: Evaluation of air quality predictions and air quality benefits assessment

Following the meteorological evaluation in Part I, this Part II paper presents the statistical evaluation of air quality predictions by the U.S. Environmental Protection Agency (U.S. EPA)’s Community Multi-Scale Air Quality (Models-3/CMAQ) model for the four simulated months in the base year 2005. The surface predictions were evaluated using the Air Pollution Index (API) data published by the China Ministry of Environmental Protection (MEP) for 31 capital cities and daily fine particulate matter (PM_2.5, particles with aerodiameter less than or equal to 2.5 μm) observations of an individual site in Tsinghua University (THU). To overcome the shortage in surface observations, satellite data are used to assess the column predictions including tropospheric nitrogen dioxide (NO₂) column abundance and aerosol optical depth (AOD). The result shows that CMAQ gives reasonably good predictions for the air quality.The air quality improvement that would result from the targeted sulfur dioxide (SO₂) and nitrogen oxides (NO_x) emission controls in China were assessed for the objective year 2010. The results show that the emission controls can lead to significant air quality benefits. SO₂ concentrations in highly polluted areas of East China in 2010 are estimated to be decreased by 30–60% compared to the levels in the 2010 Business-As-Usual (BAU) case. The annual PM_2.5 can also decline by 3–15 μg m^?3 (4–25%) due to the lower SO₂ and sulfate concentrations. If similar controls are implemented for NO_x emissions, NO_x concentrations are estimated to decrease by 30–60% as compared with the 2010 BAU scenario. The annual mean PM_2.5 concentrations will also decline by 2–14 μg m^?3 (3–12%). In addition, the number of ozone (O₃) non-attainment areas in the northern China is projected to be much lower, with the maximum 1-h average O₃ concentrations in the summer reduced by 8–30 ppb.     

The impact of the 2005 Gulf hurricanes on pollution emissions as inferred from Ozone Monitoring Instrument (OMI) nitrogen dioxide

The impact of Hurricanes Katrina and Rita in 2005 on pollution emissions in the Gulf of Mexico region was investigated using tropospheric column amounts of nitrogen dioxide (NO₂) from the Ozone Monitoring Instrument (OMI) on the NASA Aura satellite. Around New Orleans and coastal Mississippi, we estimate that Katrina caused a 35% reduction in NO_x emissions on average in the three weeks after landfall. Hurricane Rita caused a significant reduction (20%) in NO_x emissions associated with power generation and intensive oil refining activities near the Texas/Louisiana border. We also found a 43% decrease by these two storms over the eastern Gulf of Mexico Outer Continental Shelf mainly due to the evacuation of and damage to platforms, rigs, and ports associated with oil and natural gas production.     

Analysis of emission databases for regional models

General procedures for adapting emission inventories to regional models and for studying the impact of differences in inventories on model predictions are outlined. To illustrate the methods, analysis of two inventories which are still being validated is presented. The inventories together satisfy current requirements for the NCAR regional acid deposition model (RADM). These include anthropogenic emissions of SO₂, sulfate aerosol, NO, NO₂, NH₃ and volatile organic compounds (VOC) in 10 reactivity classes, from United States and Canadian point and area sources on 80-km grid resolutions, for weekend and weekday seasonally representative days on a diurnal basis during the 1980–1982 period. Application of checking procedures, designed by our group to screen for subtle anomalies not identified at previous stages of quality assurance employed by the inventory developers, resulted in adjustments primarily to VOC emissions. Comparisons of the modified inventories, which provide an indication of uncertainties in emissions due to variations in inventory development procedures, revealed differences in the eastern United States total daily emissions to be at most on the order of 5 % for SO_x, and NO_x, 20% for VOC and 85% for NH₃. Studies of the impact of inventory differences on predictions of RADM were conducted for the 22–24 April 1981 period, which was monitored as part of the Oxidation and Scavenging Characteristics of April Rains program. Event total wet sulfate deposition differed by 10% or less while midday O₃ concentrations differed by 1% or less for individual grids over the modeling domain.     

西安市及周边地区MODIS气溶胶光学厚度与PM10浓度关系模型研究简

西安是空气污染监控和防治有代表性的西部大型城市。研究了西安市及周边地区上空气溶胶光学厚度与PM₁₀浓度的关系模型。利用2011—2012年MODIS卫星气溶胶光学厚度（AOD）遥感产品,通过数据匹配,利用地面气象观测站点的能见度数据和相对湿度数据对AOD产品进行垂直标高订正和湿度订正,2项订正显著提高了AOD和地面PM₁₀浓度的相关性,相关系数从0.36提高到0.65,按季节分类统计和订正春至冬四季的相关系数分别为0.57、0.71、0.62和0.87,夏季和冬季的订正更为有效,可用性更高,这可能由于受到不同季节气溶胶来源和特征的影响。为研究中国西部大型城市,特别是西安市空气环境监测和区域联防联控提供了一种有效方法。     

Chemical characteristics of cloud water over the Japan Sea and the Northwestern Pacific Ocean near the central part of Japan: airborne measurements

Cloud water was collected by aircraft over the Japan Sea and the Northwestern Pacific Ocean during the winter and early spring seasons. The concentrations of major ions in the cloud water were higher at cloud bases than at cloud tops. The equivalents based ratio of Na⁺/nssSO₄²⁻ and NO₃⁻/nssSO₄²⁻ at cloud bases was higher than that at cloud tops, whereas the nssCa²⁺/nssSO₄²⁻ ratio was higher at cloud tops. The concentrations of nssSO₄²⁻ were higher than those of NO₃⁻ over the Japan Sea. The ratio of NO₃⁻/nssSO₄²⁻ in the cloud water over the Pacific Ocean was higher than that over the Japan Sea. Especially, when the winter monsoon wind pressure pattern collapsed, the concentrations of NO₃⁻ were much higher than those of nssSO₄²⁻. The concentrations of peroxides in the cloud water ranged from below the detection limit to 6.2 μM, and were much lower than those in the fog water near the summit of Mt. Norikura during the summer season. Most of the samples showed the condition (NH₄⁺+nssCa²⁺)<(NO₃⁻+nssSO₄²⁻), which implies insufficient amounts of bases to neutralize the acids. Chloride loss in the cloud water was observed, and this may be caused by the deposition of HCl gas to the sea surface.     

A Survey of Nitrogen Dioxide Levels Measured Inside Mobile Homes

As part of the California Mobile Home Study, over 250 mobile homes from throughout the state were monitored for nitrogen dioxide (NO₂) concentrations. Week-long average measurements were taken with Palmes tubes in the kitchen and bedroom of each mobile home during the summer of 1984 and the winter of 1985. The study was conducted entirely by mail with the participants providing all the necessary information. Mobile homes using gas for cooking had significantly higher indoor NO₂ levels than those using electricity. Mobile homes located in the Los Angeles basin had significantly higher indoor NO₂ concentrations than did mobile homes in the rest of the state. Gas cooking, the inverse of the house volume and geographic location (as a surrogate of outdoor NO₂) were the most important variables identified by multiple linear regression.     

Overestimation of urban nitrogen dioxide by passive diffusion tubes: a comparative exposure and model study

A detailed comparative trial of passive diffusion tubes (PDT) for measurement of NO₂ in urban air has been undertaken in Edinburgh, UK. Acrylic, foil-wrapped and quartz tubes were exposed in parallel for 1-week and 4-week periods at three urban sites equipped with continuous analysers for NO, NO_x and O₃. Standard acrylic PDTs significantly overestimated NO₂ concentrations relative to chemiluminescence analysers, by an average of 27% over all sites for 1-week exposures. No significant difference was observed between standard and foil-wrapped acrylic tubes (both UV blocking). The mean ratio between quartz (UV transmitting) tubes and chemiluminescence analysers was 1.06. Quartz PDT data suggest a tendency for in situ photolysis to offset (but in a non-quantifiable way) the effect of chemical overestimation. The 4-week exposures yielded systematically lower NO₂ concentration than average NO₂ from four sequential 1-week exposures over the same period. The reduction in the apparent NO₂ sampling rate with time mostlikely arises from in situ photolysis of trapped NO₂. Hourly NO₂, NO and O₃ data for 20 1-week exposures were used as input to a numerical model of diffusion tube operation incorporating chemical reaction between co-diffusing NO and O₃ within the tube. The mean calculated overestimation of 22% for NO₂ from the PDT model simulations is close to the average difference between acrylic PDT and analyser NO₂ concentrations (24% for the same exposure periods), showing that within-tube chemistry can account for observed discrepancies in NO₂ measurement between the two techniques. Overestimation by PDT generally increased as average NO₂/NO_x ratios decreased. Accurate quantitative correction of PDT measurements is not possible. Nevertheless, PDT NO₂ concentrations were correlated with both analyser NO₂ and NO_x suggesting that acrylic PDTs retain a qualitative measure of NO₂ and NO_x variation at a particular urban location.     

Role of ammonia chemistry and coarse mode aerosols in global climatological inorganic aerosol distributions

We use an inorganic aerosol thermodynamic equilibrium model in a three-dimensional chemical transport model to understand the roles of ammonia chemistry and natural aerosols on the global distribution of aerosols. The thermodynamic equilibrium model partitions gas-phase precursors among modeled aerosol species self-consistently with ambient relative humidity and natural and anthropogenic aerosol emissions during the 1990s.Model simulations show that accounting for aerosol inorganic thermodynamic equilibrium, ammonia chemistry and dust and sea-salt aerosols improve agreement with observed SO₄, NO₃, and NH₄ aerosols especially at North American sites. This study shows that the presence of sea salt, dust aerosol and ammonia chemistry significantly increases sulfate over polluted continental regions. In all regions and seasons, representation of ammonia chemistry is required to obtain reasonable agreement between modeled and observed sulfate and nitrate concentrations. Observed and modeled correlations of sulfate and nitrate with ammonium confirm that the sulfate and nitrate are strongly coupled with ammonium. SO₄ concentrations over East China peak in winter, while North American SO₄ peaks in summer. Seasonal variations of NO₃ and SO₄ are the same in East China. In North America, the seasonal variation is much stronger for NO₃ than SO₄ and peaks in winter.Natural sea salt and dust aerosol significantly alter the regional distributions of other aerosols in three main ways. First, they increase sulfate formation by 10–70% in polluted areas. Second, they increase modeled nitrate over oceans and reduce nitrate over Northern hemisphere continents. Third, they reduce ammonium formation over oceans and increase ammonium over Northern Hemisphere continents. Comparisons of SO₄, NO₃ and NH₄ deposition between pre-industrial, present, and year 2100 scenarios show that the present NO₃ and NH₄ deposition are twice pre-industrial deposition and present SO₄ deposition is almost five times pre-industrial deposition.     

Influence of model grid resolution on NO2 vertical column densities over East Asia

NO₂ vertical column densities (VCDs) over East Asia in June and December 2007 were simulated by the Community Multi-scale Air Quality (CMAQ) version 4.7.1 using an updated and more elaborate version of the Regional Emission Inventory in Asia (REAS) version 2. The modeling system could reasonably capture observed spatiotemporal changes of NO₂ VCDs by satellite sensors, the Global Ozone Monitoring Experiment-2 (GOME-2), the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY), and the Ozone Monitoring Instrument (OMI), even at the coarsest horizontal resolution of 80 km. The CMAQ simulations were performed in a sequence of three horizontal resolutions (80 km, 40 km, and 20 km) for June and December 2007 to investigate the influence of changes of horizontal resolution on the obtained NO₂ VCDs. CMAQ-simulated NO₂ VCDs generally increased with improvements in resolution from 80 km to 40 km and then to 20 km. Increases in the CMAQ-simulated NO₂ VCDs were greater for the change from 80 km to 40 km than for those from 40 km and 20 km, in which the increases of NO₂ VCDs due to the improvement of horizontal resolution were approached convergence at the horizontal resolution of approximately 20 km. Conversely, no clear convergences in NO₂ VCDs changes were found at near Tokyo and over the East China Sea. The biases of the NO₂ VCDs simulated at a resolution of 20 km against the satellite retrievals were ～36% near Beijing (CHN1) and ～78% near Shanghai (CHN2) in summer; these errors were found to be comparable to the horizontal resolution-dependent errors, which were 18–25% at CHN1 and 44–58% at CHN2 from 80 km to 40 km. Conversely, the influence of changes of horizontal resolution in winter was relatively less compared to that in summer.

Implications: NO₂ VCDs over East Asia in June and December 2007 were simulated using CMAQ version 4.7.1 and REAS version 2. The modeling system could reasonably capture observed spatiotemporal changes of NO₂ VCDs by satellite sensors. The CMAQ simulations were performed in a sequence of three horizontal resolutions, 80, 40, and 20 km, to investigate the influence of changes of horizontal resolution on the obtained NO₂ VCDs. The results suggested that the influence of changes of horizontal resolution was larger in summer compared to that in winter. The magnitude of the influence was comparable to the biases of the NO₂ VCDs simulated at a resolution of 20 km against the satellite retrievals.     

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.     

Sources and Concentrations of Indoor Nitrogen Dioxide in Barcelona,Spain

Abstract

Sources and concentrations of indoor nitrogen dioxide (NO₂) were examined in Barcelona, Spain, during 1996– 1999. A total of 340 dwellings of infants participating in a hospital-based cohort study were selected from different areas of the city. Passive filter badges were used for indoor NO₂ measurement over 7–30 days. Dwelling inhabitants completed a questionnaire on housing characteristics and smoking habits. Data on outdoor NO₂ concentrations were available for the entire period of the study in the areas of the city where indoor concentrations were determined. Bivariate analysis was performed to investigate relationships between indoor NO₂ concentrations on one hand and outdoor NO₂ concentrations, housing, and occupant characteristics on the other. Stepwise multiple linear regression was performed with variables that were 1996 and 27.02 ppb in 1999, with the highest yearly value of 27.82 ppb in 1997. In the same time period, mean outdoor NO₂ concentration ranged between 25.26 and 25.78 ppb with a peak of 30.5 ppb in 1998. Multiple regression analysis showed that principal sources of indoor NO₂ concentrations were the use of a gas cooker, the absence of an extractor fan when cooking, and cigarette smoking. The absence of central heating was also associated with higher NO₂ concentrations. Finally, each ppb increase in outdoor NO₂ was associated with a 1% increase in indoor concentrations.     

Estimating Future NO2 Levels in the Greater Los Angeles Area by Source–Type Contribution

Two indicator pollutants, carbon monoxide (CO) for mobile source influence and sulfur dioxide (SO₂) for stationary source influence, were used to estimate source-type contributions to ambient NO₂ levels in a base year and to predict NO₂ concentrations in a future year. For a specific source-receptor pair, the so-called influence coefficient of each of three source categories (mobile sources, power plants, and other stationary sources) was determined empirically from concurrent measurements of CO and SO₂ concentrations at the receptor site and CO and SO₂ emissions from each source category in the source area. Those coefficients, which are considered time invariant, were used in conjunction with the base year and future year NO _x emission values to estimate source-type contribution to ambient NO₂ levels at seven study sites selected from the Greater Los Angeles area for both the base year period, 1974 through 1976, and the future goal year of 1987 in which the air quality standards for NO₂ are to be attained. The estimated NO₂ air quality at the seven sites is found to meet the national annual standard of 5 pphm and over 99.9% of total hours, the California 1-hr NO₂ standard of 25 pphm in 1987. The estimated power plant contributions to ambient NO₂ levels are found to be considerably smaller than those to total NO _x emissions in the area. Providing that reasonably complete air quality and emissions data are available, the present analysis method may prove to be a useful tool in evaluating source contributions to both short-term peak and long-term average NO₂ concentrations for use in control strategy development.     

Assessing multi-year changes in modeled and observed urban NOX concentrations from a dynamic model evaluation perspective

An investigation of the concentrations of nitrogen oxides (NO_X) from an air quality model and observations at monitoring sites was performed to assess the changes in NO_X levels attributable to changes in mobile emissions. This evaluation effort focused on weekday morning rush hours since urban NO_X concentrations are strongly influenced by the significant loading of emissions associated with heavy commuter traffic. On-road vehicle NO_X emissions generated by the MOBILE6 model revealed a steady decline with an overall decrease of 25% for 2002–2006. In this study, a dynamic model evaluation was undertaken that entails an assessment of the predicted concentration response of the Community Multiscale Air Quality (CMAQ) model due to changes in NO_X emissions as well as to meteorological variability spanning 3-month summer periods over five consecutive years (2002–2006) against observed concentration changes at NO_X monitoring sites located primarily in urban areas of the eastern United States. Both modeled and observed hourly NO_X concentrations exhibited maximum values that coincided with the morning peak NO_X emissions. The notable results, based on 3-h average (6–9 AM local time) NO_X concentrations, derived between the 50th and 95th percentiles of cumulative concentration distributions, revealed that modeled changes at these elevated NO_X levels generally tracked the year-to-year variations in the observed concentration changes. When summer 2002 values were used as a reference, both modeled and observed results also showed definitive decreases in weekday morning urban NO_X concentrations over this multi-year period, which can be primarily attributed to the reductions in mobile source emissions. Whereas observed NO_X concentrations have declined by about 25% over this period consistent with the decline in the modeled mobile emission sector, modeled NO_X concentration changes were close to the decreases exhibited in all (mobile + other sectors) surface NO_X emissions whose overall decline was about 15% over this multi-year period.