Day-of-Week behavior of atmospheric ozone in three U.S. cities

The weekly cycles of atmospheric ozone (O3) are of interest because they provide information about the response of O3 to changes in anthropogenic emissions from weekdays to weekends. The weekly behavior of O3 in Chicago, IL; Philadelphia, PA; and Atlanta, GA, is contrasted. In Chicago and Philadelphia, maximum 1-hr average O3 increases on weekends. In Atlanta, O3 builds up from Mondays to Fridays and declines during weekends. In all three areas, volatile organic compound (VOC)/nitrogen oxides (NOx) ratios are higher during weekends, resulting from greater than proportionate decreases in NOx relative to VOC emissions. The VOC/NOx ratios correlate with maximum 1-hr O3 concentrations in Chicago, a response consistent with a VOC-sensitive airshed. A weak correlation between O3 concentrations and VOC/NOx ratios in Philadelphia suggests the impact of transported O3, which is formed in upwind VOC-sensitive locations that may be hundreds of kilometers away. Ozone concentrations in Atlanta do not correlate with VOC/NOx ratios but with concentrations of NOx and total reactive nitrogen (NOy) carried over from the previous day. When data from 1986-1990 and 1995-1999 are compared, only small differences in the weekly behavior of O3 are observed in Chicago and Philadelphia. The day-of-week differences in O3 are amplified in the more recent period in Atlanta, a possible result of urban growth.     

Photochemical production of ozone and control strategy for Southern Taiwan

An observation-based method (OBM) is developed to evaluate the ozone (O₃) production efficiency (O₃ molecules produced per NO_x molecule consumed) and O₃ production rate (P(O₃)) during a field campaign in southern Taiwan. The method can also provide an estimate of the concentration of OH. A key step in the method is to use observed concentrations of two aromatic hydrocarbons, namely ethylbenzene and m,p-xylene, to estimate the degree of photochemical processing and amounts of photochemically consumed NO_x and NMHCs by OH. In addition, total oxidant (O₃+NO₂) instead of O₃ itself turns out to be very useful for representing ozone production in the OBM approach. The average O₃ production efficiency during the field campaign in Fall (2003) is found to be about 10.2±3.9. The relationship of P(O₃) with NO_x is examined and compared with a one-dimensional (1D) photochemical model. Values of P(O₃) derived from the OBM are slightly lower than those calculated in the 1D model. However, OH concentrations estimated by the OBM are about a factor of 2 lower than the 1D model. Fresh emissions, which affect the degree of photochemical processing appear to be a major cause of the underestimate. We have developed a three-dimensional (3D) OBM O₃ production diagram that resembles the EKMA ozone isopleth diagram to study the relationship of the total oxidant versus O₃ precursors. The 3D OBM O₃ production diagram suggests that reducing emissions of NMHCs are more effective in controlling O₃ than reducing NO_x. However, significant uncertainties remain in the OBM, and considerable more work is required to minimize these uncertainties before a definitive control strategy can be reached. The observation-based approach provides a good alternative to measuring peroxy radicals for evaluating the production of O₃ and formulating O₃ control strategy in urban and suburban environments.     

Gaseous Pollutants in St. Louis and Other Cities

During the fall of 1972, a study was undertaken to characterize gaseous air pollutants in the St. Louis metropolitan area. Information obtained in this study will be used in planning air pollution modeling studies to be carried out over the next 4 or 5 years under the Regional Air Pollution Study (RAPS) of the Environmental Protection Agency. From analyses of roadway samples, it was found that certain compounds in automotive emissions such as acetylene, carbon monoxide, o-xylene, ethylene, and 2-methylpentane occur in consistent proportions to other compounds. The concentrations of these indicator compounds in atmospheric samples were used to estimate the amounts of other hydrocarbons present that are attributable to automotive related emissions. Methane, ethane, and propane were found to originate principally from nonautomotive sources, both at St. Louis University and the St. Louis CAMP station. Similar concentrations were found in urban and non-urban areas. A comparison of ethylene-CO and propyl-ene-CO ratios in urban and non-urban areas indicates that CO can serve as an indicator of transport of urban pollutants. Sunlight irradiations of captured air samples showed increasing ozone production with increasing initial concentrations of hydrocarbons and nitrogen oxides. Substantial ozone and peroxyacetyl nitrate were formed from a total non-methane hydrocarbon concentration of only ¹/₄ ppm C.     

Trends in on-road vehicle emissions and ambient air quality in Atlanta,Georgia, USA,from the late 1990s through 2009

On-road vehicle emissions of carbon monoxide (CO), nitrogen oxides (NO_x), and volatile organic compounds (VOCs) during 1995–2009 in the Atlanta Metropolitan Statistical Area were estimated using the Motor Vehicle Emission Simulator (MOVES) model and data from the National Emissions Inventories and the State of Georgia. Statistically significant downward trends (computed using the nonparametric Theil-Sen method) in annual on-road CO, NO_x, and VOC emissions of 6.1%, 3.3%, and 6.0% per year, respectively, are noted during the 1995–2009 period despite an increase in total vehicle distance traveled. The CO and NO_x emission trends are correlated with statistically significant downward trends in ambient air concentrations of CO and NO_x in Atlanta ranging from 8.0% to 11.8% per year and from 5.8% to 8.7% per year, respectively, during similar time periods. Weather-adjusted summertime ozone concentrations in Atlanta exhibited a statistically significant declining trend of 2.3% per year during 2001–2009. Although this trend coexists with the declining trends in on-road NO_x, VOC, and CO emissions, identifying the cause of the downward trend in ozone is complicated by reductions in multiple precursors from different source sectors.

Implications:Large reductions in on-road vehicle emissions of CO and NO_x in Atlanta from the late 1990s to 2009, despite an increase in total vehicle distance traveled, contributed to a significant improvement in air quality through decreases in ambient air concentrations of CO and NO_x during this time period. Emissions reductions in motor vehicles and other source sectors resulted in these improvements and the observed declining trend in ozone concentrations over the past decade. Although these historical trends cannot be extrapolated to the future because pollutant concentration contributions due to on-road vehicle emissions will likely become an increasingly smaller fraction of the atmospheric total, they provide an indication of the benefits of past control measures.     

Modeling and direct sensitivity analysis of biogenic emissions impacts on regional ozone formation in the Mexico-U.S. border area

A spatially and temporally resolved biogenic hydrocarbon and nitrogen oxides (NOx) emissions inventory has been developed for a region along the Mexico-U.S. border area. Average daily biogenic non-methane organic gases (NMOG) emissions for the 1700 x 1000 km2 domain were estimated at 23,800 metric tons/day (62% from Mexico and 38% from the United States), and biogenic NOx was estimated at 1230 metric tons/day (54% from Mexico and 46% from the United States) for the July 18-20, 1993, ozone episode. The biogenic NMOG represented 74% of the total NMOG emissions, and biogenic NOx was 14% of the total NOx. The CIT photochemical airshed model was used to assess how biogenic emissions impact air quality. Predicted ground-level ozone increased by 5-10 ppb in most rural areas, 10-20 ppb near urban centers, and 20-30 ppb immediately downwind of the urban centers compared to simulations in which only anthropogenic emissions were used. A sensitivity analysis of predicted ozone concentration to emissions was performed using the decoupled direct method for three dimensional air quality models (DDM-3D). The highest positive sensitivity of ground-level ozone concentration to biogenic volatile organic compound (VOC) emissions (i.e., increasing biogenic VOC emissions results in increasing ozone concentrations) was predicted to be in locations with high NOx levels, (i.e., the urban areas). One urban center--Houston--was predicted to have a slight negative sensitivity to biogenic NO emissions (i.e., increasing biogenic NO emissions results in decreasing local ozone concentrations). The highest sensitivities of ozone concentrations to on-road mobile source VOC emissions, all positive, were mainly in the urban areas. The highest sensitivities of ozone concentrations to on-road mobile source NOx emissions were predicted in both urban (either positive or negative sensitivities) and rural (positive sensitivities) locations.     

Characterization of Sonic Devices Used for Cleaning Fabric Filters

ABSTRACT

Ozone reactivity scales play an important role in selecting which chemical compounds are used in products ranging from gasoline to pesticides to hairspray in California, across the United States and around the world. The California Statewide Air Pollution Research Center (SAPRC) box model that calculates ozone reactivity uses a representative urban atmosphere to predict how much additional ozone forms for each kilogram of compound emission. This representative urban atmosphere has remained constant since 1988, even though more than 25 years of emissions controls have greatly reduced ambient ozone concentrations across the United States during this time period. Here we explore the effects of updating the representative urban atmosphere used for ozone reactivity calculations from 1988 to 2010 conditions by updating the meteorology, emission rates, concentration of initial conditions, concentration of background species, and composition of volatile organic compound (VOC) profiles. Box model scenarios are explored for 39 cities across the United States to calculate the Maximum Incremental Reactivity (MIR) scale for 1,233 individual compounds and compound-mixtures. Median MIR values across the cities decreased by approximately 20.3% when model conditions were updated. The decrease is primarily due to changes in atmospheric composition ultimately attributable to emissions control programs between 1998 and 2010. Further effects were caused by changes in meteorological variables stemming from shifting seasons for peak ozone events (summer versus early fall). Lumped model species with the highest MIR values in 1988 experienced the greatest decrease in MIR values when conditions were updated to 2010. Despite the reduction in the absolute reactivity in the updated 2010 atmosphere, the relative ranking of the VOCs according to their reactivity did not change strongly compared to the original 1988 atmosphere. These findings indicate that past decisions about ozone control programs remain valid today, and the ozone reactivity scale continues to provide relevant guidance for future policy decisions even as new products are developed.

Implications: Updating the representative urban atmosphere used for the Maximum Incremental Reactivity (MIR) scale from 1988 to 2010 conditions caused the reactivity of 1223 individual compounds and combined mixtures to decrease by an average of 20.3% but the relative ranking of the VOCs was not strongly affected. This means that previous guidance about preferred chemical formulations to reduce ozone formation in cities across the United States remain valid today, and the MIR scale continues to provide relevant guidance for future policy decisions even as new products are developed.     

Rural and Urban Ozone Relationships In New York State

High ozone concentrations, often in excess of the national ambient air quality standard for photochemical oxidants, have been measured simultaneously in urban and rural areas of New York State. Average daily rural ozone concentrations were found to correlate well with daily maximum urban ozone concentrations suggesting a common source. Estimations of the quantity of ozone advectively transported into New York State are more than an order of magnitude greater than estimations of the potential photochemical generation of ozone from hydrocarbon emissions within New York State. It is suggested thai the high rural ozone levels are not primarily due to the transport of ozone and ozone precursors from olher urban areas, but are rather due to natural phenomena such as photochemical generation from naturally occurring precursors or transport of ozone from the stratosphere to the troposphere. The effectiveness of a hydrocarbon control strategy for New York State to meet the ambient air quality standard for photochemical oxidants when background levels themselves may be above the standard is questioned.     

Hydrocarbon emissions speciation in diesel and biodiesel exhausts

Diesel engine emissions are composed of a long list of organic compounds, ranging from C₂ to C₁₂₊, and coming from the hydrocarbons partially oxidized in combustion or produced by pyrolisis. Many of these are considered as ozone precursors in the atmosphere, since they can interact with nitrogen oxides to produce ozone under atmospheric conditions in the presence of sunlight. In addition to problematic ozone production, Brookes, P., and Duncan, M. [1971. Carcinogenic hydrocarbons and human cells in culture. Nature.] and Heywood, J. [1988. Internal Combustion Engine Fundamentals.Mc Graw-Hill, ISBN 0-07-1000499-8.] determined that the polycyclic aromatic hydrocarbons present in exhaust gases are dangerous to human health, being highly carcinogenic.The aim of this study was to identify by means of gas chromatography the amount of each hydrocarbon species present in the exhaust gases of diesel engines operating with different biodiesel blends. The levels of reactive and non-reactive hydrocarbons present in diesel engine exhaust gases powered by different biodiesel fuel blends were also analyzed.Detailed speciation revealed a drastic change in the nature and quantity of semi-volatile compounds when biodiesel fuels are employed, the most affected being the aromatic compounds. Both aromatic and oxygenated aromatic compounds were found in biodiesel exhaust. Finally, the conservation of species for off-side analysis and the possible influence of engine operating conditions on the chemical characterization of the semi-volatile compound phase are discussed.The use of oxygenated fuel blends shows a reduction in the Engine-Out emissions of total hydrocarbons. But the potential of the hydrocarbon emissions is more dependent on the compositions of these hydrocarbons in the Engine-Out, to the quantity; a large percent of hydrocarbons existing in the exhaust, when biodiesel blends are used, are partially burned hydrocarbons, and are interesting as they have the maximum reactivity, but with the use of pure biodiesel and diesel, the most hydrocarbons are from unburned fuel and they have a less reactivity. The best composition in the fuel, for the control of the hydrocarbon emissions reactivity, needs to be a fuel with high-saturated fatty acid content.     

Modeling toxic compounds from nitric oxide emission measurements

Determining the amount and rate of degradation of toxic pollutants in soil and groundwater is difficult and often requires invasive techniques, such as deploying extensive monitoring well networks. Even with these networks, degradation rates across entire systems cannot readily be extrapolated from the samples. When organic compounds are degraded by microbes, especially nitrifying bacteria, oxides or nitrogen (NO_x) are released to the atmosphere. Thus, the flux of nitric oxide (NO) from the soil to the lower troposphere can be used to predict the rate at which organic compounds are degraded. By characterizing and applying biogenic and anthropogenic processes in soils the rates of degradation of organic compounds. Toluene was selected as a representative of toxic aromatic compounds, since it is inherently toxic, it is a substituted benzene compound and is listed as a hazardous air pollutant under Section 12 of the Clean Air Act Amendments of 1990. Measured toluene concentrations in soil, microbial population growth and NO fluxes in chamber studies were used to develop and parameterize a numerical model based on carbon and nitrogen cycling. These measurements, in turn, were used as indicators of bioremediation of air toxic (i.e. toluene) concentrations. The model found that chemical concentration, soil microbial abundance, and NO production can be directly related to the experimental results (significant at P < 0.01) for all toluene concentrations tested. This indicates that the model may prove useful in monitoring and predicting the fate of toxic aromatic contaminants in a complex soil system. It may also be useful in predicting the release of ozone precursors, such as changes in reservoirs of hydrocarbons and oxides of nitrogen. As such, the model may be a tool for decision makers in ozone non-attainment areas.     

Impact of biogenic terpene emissions from <Emphasis Type="Italic">Brassica napus</Emphasis> on tropospheric ozone over saxony (Germany)

INTRODUCTION: The role of biogenic emissions in tropospheric ozone production is currently under discussion and major aspects are not well understood yet. This study aims towards the estimation of the influence of biogenic emissions on tropospheric ozone concentrations over Saxony in general and of biogenic emissions from brassica napus in special. MODELLING TOOLS: The studies are performed by utilizing a coupled numerical modelling system consisting of the meteorological model METRAS and the chemistry transport model MUSCAT. For the chemical part, the Euro-RADM algorithm is used. EMISSIONS: Anthropogenic and biogenic emissions are taken into account. The anthropogenic emissions are introduced by an emission inventory. Biogenic emissions, VOC and NO, are calculated within the chemical transport model MUSCAT at each time step and in each grid cell depending on land use type and on the temperature. The emissions of hydrocarbons from forest areas as well as biogenic NO especially from agricultural grounds are considered. Also terpene emissions from brassica napus fields are estimated. SIMULATION SETUP AND METEOROLOGICAL CONDITIONS: The simulations were performed over an area with an extension of 160 x 140 km2 which covers the main parts of Saxony and neighboring areas of Brandenburg, Sachsen-Anhalt and Thuringia. Summer smog with high ozone concentrations can be expected during high pressure conditions on hot summer days. Typical meteorological conditions for such cases were introduced in an conceptual way. RESULTS: It is estimated that biogenic emissions change tropospheric ozone concentrations in a noticeable way (up to 15% to 20%) and, therefore, should not be neglected in studies about tropospheric ozone. Emissions from brassica napus do have a moderate potential to enhance tropospheric ozone concentrations, but emissions are still under consideration and, therefore, results vary to a high degree. CONCLUSIONS: Summing up, the effect of brassica napus terpene emissions on ozone concentrations is noticeable, but not too pronounced. The results give a preliminary estimate on what the effect due to brassica napus emissions could be until better parameterizations can be derived from measurements.     

Uncertainty Analysis of Ozone Formation and Response to Emission Controls Using Higher-Order Sensitivities

Abstract

Understanding ozone response to its precursor emissions is crucial for effective air quality management practices. This nonlinear response is usually simulated using chemical transport models, and the modeling results are affected by uncertainties in emissions inputs. In this study, a high ozone episode in the southeastern United States is simulated using the Community Multiscale Air Quality (CMAQ) model. Uncertainties in ozone formation and response to emissions controls due to uncertainties in emission rates are quantified using the Monte Carlo method. Instead of propagating emissions uncertainties through the original CMAQ, a reduced form of CMAQ is formulated using directly calculated first- and second-order sensitivities that capture the nonlinear ozone concentration-emission responses. This modification greatly reduces the associated computational cost. Quantified uncertainties in modeled ozone concentrations and responses to various emissions controls are much less than the uncertainties in emissions inputs. Average uncertainties in modeled ozone concentrations for the Atlanta area are less than 10% (as measured by the inferred coefficient of variance [ICOV]) even when emissions uncertainties are assumed to vary between a factor of 1.5 and 2. Uncertainties in the ozone responses generally decrease with increased emission controls. Average uncertainties (ICOV) in emission-normalized ozone responses range from 4 to 22%, with the smaller being associated with controlling of the relatively certain point nitrogen oxide (NO_x) emissions and the larger resulting from controlling of the less certain mobile NO_x emissions. These small uncertainties provide confidence in the model applications, such as in performance evaluation, attainment demonstration, and control strategy development.     

Modeling an air pollution episode in northwestern United States: Identifying the effect of nitrogen oxide and volatile organic compound emission changes on air pollutants formation using direct sensitivity analysis

Air quality impacts of volatile organic compound (VOC) and nitrogen oxide (NO_x) emissions from major sources over the northwestern United States are simulated. The comprehensive nested modeling system comprises three models: Community Multiscale Air Quality (CMAQ), Weather Research and Forecasting (WRF), and Sparse Matrix Operator Kernel Emissions (SMOKE). In addition, the decoupled direct method in three dimensions (DDM-3D) is used to determine the sensitivities of pollutant concentrations to changes in precursor emissions during a severe smog episode in July of 2006. The average simulated 8-hr daily maximum O₃ concentration is 48.9 ppb, with 1-hr O₃ maxima up to 106 ppb (40 km southeast of Seattle). The average simulated PM_2.5 (particulate matter with an aerodynamic diameter <2.5 μm) concentration at the measurement sites is 9.06 μg m^?3, which is in good agreement with the observed concentration (8.06 μg m^?3). In urban areas (i.e., Seattle, Vancouver, etc.), the model predicts that, on average, a reduction of NO_x emissions is simulated to lead to an increase in average 8-hr daily maximum O₃ concentrations, and will be most prominent in Seattle (where the greatest sensitivity is??0.2 ppb per % change of mobile sources). On the other hand, decreasing NO_x emissions is simulated to decrease the 8-hr maximum O₃ concentrations in remote and forested areas. Decreased NO_x emissions are simulated to slightly increase PM_2.5 in major urban areas. In urban areas, a decrease in VOC emissions will result in a decrease of 8-hr maximum O₃ concentrations. The impact of decreased VOC emissions from biogenic, mobile, nonroad, and area sources on average 8-hr daily maximum O₃ concentrations is up to 0.05 ppb decrease per % of emission change, each. Decreased emissions of VOCs decrease average PM_2.5 concentrations in the entire modeling domain. In major cities, PM_2.5 concentrations are more sensitive to emissions of VOCs from biogenic sources than other sources of VOCs. These results can be used to interpret the effectiveness of VOC or NO_x controls over pollutant concentrations, especially for localities that may exceed National Ambient Air Quality Standards (NAAQS).

Implications: The effect of NO_x and VOC controls on ozone and PM_2.5 concentrations in the northwestern United States is examined using the decoupled direct method in three dimensions (DDM-3D) in a state-of-the-art three-dimensional chemical transport model (CMAQ). NO_x controls are predicted to increase PM_2.5 and ozone in major urban areas and decrease ozone in more remote and forested areas. VOC reductions are helpful in reducing ozone and PM_2.5 concentrations in urban areas. Biogenic VOC sources have the largest impact on O₃ and PM_2.5 concentrations.     

Application of OMI observations to a space-based indicator of NOx and VOC controls on surface ozone formation

We investigated variations in the relative sensitivity of surface ozone formation in summer to precursor species concentrations of volatile organic compounds (VOCs) and nitrogen oxides (NO_x) as inferred from the ratio of the tropospheric columns of formaldehyde to nitrogen dioxide (the “Ratio”) from the Aura Ozone Monitoring Instrument (OMI). Our modeling study suggests that ozone formation decreases with reductions in VOCs at Ratios <1 and NO_x at Ratios >2; both NO_x and VOC reductions may decrease ozone formation for Ratios between 1 and 2. Using this criteria, the OMI data indicate that ozone formation became: 1. more sensitive to NO_x over most of the United States from 2005 to 2007 because of the substantial decrease in NO_x emissions, primarily from stationary sources, and the concomitant decrease in the tropospheric column of NO₂, and 2. more sensitive to NO_x with increasing temperature, in part because emissions of highly reactive, biogenic isoprene increase with temperature, thus increasing the total VOC reactivity. In cities with relatively low isoprene emissions (e.g., Chicago), the data clearly indicate that ozone formation became more sensitive to NO_x from 2005 to 2007. In cities with relatively high isoprene emissions (e.g., Atlanta), we found that the increase in the Ratio due to decreasing NO_x emissions was not obvious as this signal was convolved with variations in the Ratio associated with the temperature dependence of isoprene emissions and, consequently, the formaldehyde concentration.     

Dynamic evaluation of a regional air quality model: Assessing the emissions-induced weekly ozone cycle

Air quality models are used to predict changes in pollutant concentrations resulting from envisioned emission control policies. Recognizing the need to assess the credibility of air quality models in a policy-relevant context, we perform a dynamic evaluation of the Community Multiscale Air Quality (CMAQ) modeling system for the “weekend ozone effect” to determine if observed changes in ozone due to weekday-to-weekend (WDWE) reductions in precursor emissions can be accurately simulated. The weekend ozone effect offers a unique opportunity for dynamic evaluation, as it is a widely documented phenomenon that has persisted since the 1970s. In many urban areas of the Unites States, higher ozone has been observed on weekends than weekdays, despite dramatically reduced emissions of ozone precursors (nitrogen oxides [NO_x] and volatile organic compounds [VOCs]) on weekends. More recent measurements, however, suggest shifts in the spatial extent or reductions in WDWE ozone differences. Using 18 years (1988–2005) of observed and modeled ozone and temperature data across the northeastern United States, we re-examine the long-term trends in the weekend effect and confounding factors that may be complicating the interpretation of this trend and explore whether CMAQ can replicate the temporal features of the observed weekend effect. The amplitudes of the weekly ozone cycle have decreased during the 18-year period in our study domain, but the year-to-year variability in weekend minus weekday (WEWD) ozone amplitudes is quite large. Inter-annual variability in meteorology appears to influence WEWD differences in ozone, as well as WEWD differences in VOC and NO_x emissions. Because of the large inter-annual variability, modeling strategies using a single episode lasting a few days or a few episodes in a given year may not capture the WEWD signal that exists over longer time periods. The CMAQ model showed skill in predicting the absolute values of ozone concentrations during the daytime. However, early morning NO_x concentrations were underestimated and ozone levels were overestimated. Also, the modeled response of ozone to WEWD differences in emissions was somewhat less than that observed. This study reveals that model performance may be improved by (1) properly estimating mobile source NO_x emissions and their temporal distributions, especially for diesel vehicles; (2) reducing the grid cell size in the lowest layer of CMAQ; and, (3) using time-dependent and more realistic boundary conditions for the CMAQ simulations.     

Comparison of Nonmethane Organic Compound Concentration Data Collected by Two Methods in Atlanta

Title I of the Clean Air Act Amendments of 1990 calls for “enhanced monitoring” of ozone, which is planned to include measurements of atmospheric non-methane organic compounds (NMOCs). NMOC concentration data gathered by two methods in Atlanta, Georgia during July and August 1990 are compared in order to assess the reliability of such measurements in an operational setting. During that period, automated gas chromatography (GO) systems (Field systems) were used to collect NMOC continuously as one-hour averages. In addition, canister samples of ambient air were collected on an intermittent schedule for quality control purposes and analyzed by laboratory GC (the Lab system). Data from the six-site network included concentrations of nitrogen oxides (NOX), carbon monoxide (CO), ozone, total NMOC (TNMOC), and 47 identified NMOCs. Regression analysis indicates that the average TNMOC concentration from the Lab system is about 50 percent higher than that from the Field system, and that the bulk of the difference is due to unidentified NMOCs recorded by the Lab system. Also, there are substantial uncertainties in predicting a single Field TNMOC concentration from a measured Lab concentration. Considering individual identified NMOCs, agreement between the systems is poor for many olefins that occur at low concentrations but may be photochemically important. Regressions of TNMOC against CO and NOX lead to the conclusion that the larger unidentified component being reported by the Lab system is not closely related to local combustion or automotive sources.     

Estimating long-term population exposure to ozone in urban areas of Europe

Tropospheric ozone concentrations regarded as harmful for human health are frequently encountered in Central Europe in summertime. Although ozone formation generally results from precursors transported over long distances, in urban areas local effects, such as reactions due to nearby emission sources, play a major role in determining ozone concentrations. Europe-wide mapping and modeling of population exposure to high ozone concentrations is subject to many uncertainties, because small-scale phenomena in urban areas can significantly change ozone levels from those of the surroundings. Currently the integrated assessment modeling of European ozone control strategies is done utilizing the results of large-scale models intended for estimating the rural background ozone levels. This paper presents an initial study on how much local nitrogen oxide (NOx) concentrations can explain variations between large-scale ozone model results and urban ozone measurements, on one hand, and between urban and nearby rural measurements, on the other. The impact of urban NOx concentrations on ozone levels was derived from chemical equations describing the ozone balance. The study investigated the applicability of the method for improving the accuracy of modeled population exposure, which is needed for efficient control strategy development. The method was tested with NOx and ozone measurements from both urban and rural areas in Switzerland and with the ozone predictions of the large-scale photochemical model currently used in designing Europe-wide control strategies for ground-level ozone. The results suggest that urban NOx levels are a significant explanatory factor in differences between urban and nearby rural ozone concentrations and that the phenomenon could be satisfactorily represented with this kind of method. Further research efforts should comprise testing of the method in more locations and analyzing the performance of more widely applicable ways of deriving the initial parameters.     

The long range transport of ozone within Europe and its control

It is widely accepted that the ozone concentrations experienced during photochemical episodes over large areas of Europe may exceed levels at which adverse environmental effects could be expected. These peak ozone concentrations can be reduced by controlling atmospheric emissions of the hydrocarbon and nitrogen oxide precursors. For ozone control to be successful over the spatial scale of Europe, long term international cooperation is required in the formulation of emission abatement strategies. A significant barrier to rapid progress has been the complexity of the processes that describe ozone formation. Highly sophisticated computer models of chemistry and transport have, up to now, been the only means to study the impact of abatement strategies. An alternative approach has been adopted here involving the development of a simplified long range transport model for ozone based on the analysis of over 60 experimental runs of a photochemical trajectory model applied to a wide range of hydrocarbon-nitrogen oxide emission combinations. Using the ozone-precursor relationship obtained, it has been possible to examine various policy options in the European context. Although taken together, three illustrative emission control scenarios reduce NO(x) and hydrocarbon emissions substantially through controls on motor vehicle exhaust, large combustion plant and solvent usage, a significant potential for photochemical ozone formation and long range transport may still remain after their implementation. The extents of precursor emission abatement that will be required, if the potential for ozone formation is to be reduced below published air quality criteria guidelines or critical levels, have been determined for each European country. The implied reductions in NO(x) and hydrocarbons relative to current levels amount to between 50 and 90%.     

The sensitivity of modeled ozone to the temporal distribution of point, area, and mobile source emissions in the eastern United States

Ozone remains one of the most recalcitrant air pollution problems in the US. Hourly emissions fields used in air quality models (AQMs) generally show less temporal variability than corresponding measurements from continuous emissions monitors (CEM) and field campaigns would imply. If emissions control scenarios to reduce emissions at peak ozone forming hours are to be assessed with AQMs, the effect of emissions' daily variability on modeled ozone must be understood. We analyzed the effects of altering all anthropogenic emissions' temporal distributions by source group on 2002 summer-long simulations of ozone using the Community Multiscale Air Quality Model (CMAQ) v4.5 and the Carbon Bond IV (CBIV) chemical mechanism with 12 km resolution. We find that when mobile source emissions were made constant over the course of a day, 8-h maximum ozone predictions changed by ±7 parts per billion by volume (ppbv) in many urban areas on days when ozone concentrations greater than 80 ppbv were simulated in the base case. Increasing the temporal variation of point sources resulted in ozone changes of +6 and −6 ppbv, but only for small areas near sources. Changing the daily cycle of mobile source emissions produces substantial changes in simulated ozone, especially in urban areas at night; results suggest that shifting the emissions of NO_x from day to night, for example in electric powered vehicles recharged at night, could have beneficial impacts on air quality.     

Long-term meteorologically independent trend analysis of ozone air quality at an urban site in the greater Houston area

The Houston-Galveston-Brazoria (HGB) area of Texas has a history of ozone exceedances and is currently classified under moderate nonattainment status for the 2008 8-hr ozone standard of 75 ppb. The HGB area is characterized by intense solar radiation, high temperature, and high humidity, which influence day-to-day variations in ozone concentrations. Long-term air quality trends independent of meteorological influence need to be constructed for ascertaining the effectiveness of air quality management in this area. The Kolmogorov-Zurbenko (KZ) filter technique, used to separate different scales of motion in a time series, is applied in the current study for maximum daily 8-hr (MDA8) ozone concentrations at an urban site (U.S. Environmental Protection Agency [EPA] Air Quality System [AQS] Site ID: 48-201-0024, Aldine) in the HGB area. This site, located within 10 miles of downtown Houston and the George Bush Intercontinental Airport, was selected for developing long-term meteorologically independent MDA8 ozone trends for the years 1990–2016. Results from this study indicate a consistent decrease in meteorologically independent MDA8 ozone between 2000 and 2016. This pattern could be partially attributed to a reduction in underlying nitrogen oxide (NO_x) emissions, particularly lowering nitrogen dioxide (NO₂) levels, and a decrease in the release of highly reactive volatile organic compounds (HRVOCs). Results also suggest solar radiation to be most strongly correlated to ozone, with temperature being the secondary meteorological control variable. Relative humidity and wind speed have tertiary influence at this site. This study observed that meteorological variability accounts for a high of 61% variability in baseline ozone (low-frequency component, sum of long-term and seasonal components), whereas 64% of the change in long-term MDA8 ozone post 2000 could be attributed to NO_x emission reduction. Long-term MDA8 ozone trend component was estimated to be decreasing at a linear rate of 0.412 ± 0.007 ppb/yr for the years 2000–2016 and 0.155 ± 0.005 ppb/yr for the overall period of 1990–2016.

Implications: The effectiveness of air emission controls can be evaluated by developing long-term air quality trends independent of meteorological influences. The KZ filter technique is a well-established method to separate an air quality time series into short-term, seasonal, and long-term components. This paper applies the KZ filter technique to MDA8 ozone data between 1990 and 2016 at an urban site in the greater Houston area and estimates the variance accounted for by the primary meteorological control variables. Estimates for linear trends of MDA8 ozone are calculated and underlying causes are investigated to provide a guidance for further investigation into air quality management of the greater Houston area.