O3 Air Quality Research Articles

Reforestation and crop land conversion impacts on future regional air quality in the Southeastern U.S.

Land in the southeastern U.S. is expected to change, e.g., given the potential demand to develop forest-to-fuel technologies or, conversely, cropification of current forests to increase food production. Possible future PM2.5 and O3 air quality for two land use/land cover change (LULCC) scenarios, reforestation and cropland conversion, are compared to a reference case scenario for the year 2050 using the Weather Research and Forecasting (WRF) and Community Multi-scale Air Quality (CMAQ) models. Changes in air quality driven by changes in climate, deposition and emissions relating to the LULCC are investigated. Reforestation in the Southeast tends to decrease the ambient O3 mixing ratio while slightly increasing summertime PM2.5 in the Southeastern U.S. Results of a climate and deposition (CD) sensitivity simulation are provided for the two alternative LULCC scenarios to isolate the impact of changing climate and deposition on PM2.5 and O3 air quality. The sensitivity results indicate that deposition and emissions changes associated with reforestation impact O3 and PM2.5 concentrations as much as, and in most cases more than, changes in meteorology. Conversion of forest to cropland in the Southeast, on the other hand, tends to increase O3 and increase PM2.5 year-round. Cropland conversion leads to increased NOX emissions and increases in the 4th highest maximum daily 8-h O3 (MDA8) of the year by up to 10ppb despite the tendency for increased deposition and decreased temperature to reduce the MDA8 mixing ratio. The results of this study show that O3 and aerosol concentrations are sensitive to reforestation and cropland conversion in the Southeast and these land use changes should be considered in air quality management plans. Further, they show the sensitivity of such calculations to land cover properties.

How emissions, climate, and land use change will impact mid-century air quality over the United States: a focus on effects at national parks

Abstract. We use a global coupled chemistry–climate–land model (CESM) to assess the integrated effect of climate, emissions and land use changes on annual surface O3 and PM2.5 in the United States with a focus on national parks (NPs) and wilderness areas, using the RCP4.5 and RCP8.5 projections. We show that, when stringent domestic emission controls are applied, air quality is predicted to improve across the US, except surface O3 over the western and central US under RCP8.5 conditions, where rising background ozone counteracts domestic emission reductions. Under the RCP4.5 scenario, surface O3 is substantially reduced (about 5 ppb), with daily maximum 8 h averages below the primary US Environmental Protection Agency (EPA) National Ambient Air Quality Standards (NAAQS) of 75 ppb (and even 65 ppb) in all the NPs. PM2.5 is significantly reduced in both scenarios (4 μg m−3; ~50%), with levels below the annual US EPA NAAQS of 12 μg m−3 across all the NPs; visibility is also improved (10–15 dv; >75 km in visibility range), although some western US parks with Class I status (40–74 % of total sites in the US) are still above the 2050 planned target level to reach the goal of natural visibility conditions by 2064. We estimate that climate-driven increases in fire activity may dominate summertime PM2.5 over the western US, potentially offsetting the large PM2.5 reductions from domestic emission controls, and keeping visibility at present-day levels in many parks. Our study indicates that anthropogenic emission patterns will be important for air quality in 2050. However, climate and land use changes alone may lead to a substantial increase in surface O3 (2–3 ppb) with important consequences for O3 air quality and ecosystem degradation at the US NPs. Our study illustrates the need to consider the effects of changes in climate, vegetation, and fires in future air quality management and planning and emission policy making.

Impacts of different plant functional types on ambient ozone predictions in the Seoul Metropolitan Areas (SMAs), Korea

Abstract. Plant functional type (PFT) distributions affect the results of biogenic emission modeling as well as O3 and particulate matter (PM) simulations using chemistry-transport models (CTMs). This paper analyzes the variations of both surface biogenic volatile organic compound (BVOC) emissions and O3 concentrations due to changes in the PFT distributions in the Seoul Metropolitan Areas, Korea. The Fifth-Generation NCAR/Pennsylvania State Meso-scale Model (MM5)/the Model of Emissions of Gases and Aerosols from Nature (MEGAN)/the Sparse Matrix Operator Kernel Emissions (SMOKE)/the Community Multiscale Air Quality (CMAQ) model simulations were implemented over the Seoul Metropolitan Areas in Korea to predict surface O3 concentrations for the period of 1 May to 31 June 2008. Starting from a performance check of CTM predictions, we consecutively assessed the effects of PFT area deviations on the MEGAN BVOC and CTM O3 predictions, and we further considered the basis of geospatial and statistical analyses. The three PFT data sets considered were (1) the Korean PFT, developed with Korea-specific vegetation database; (2) the CDP PFT, adopted from the community data portal (CDP) of US National Center for Atmospheric Research in the United States (NCAR); (3) MODIS PFT, reclassified from the NASA Terra and Aqua combined land cover products. Although the CMAQ performance check reveals that all of the three different PFT data sets are applicable choices for regulatory modeling practice, noticeable primary data (i.e., PFT and Leaf Area Index (LAI)) was observed to be missing in many geographic locations. Based on the assessed effect of such missing data on CMAQ O3 predictions, we found that this missing data can cause spatially increased bias in CMAQ O3. Thus, it must be resolved in the near future to obtain more accurate biogenic emission and chemistry transport modeling results. Comparisons of MEGAN biogenic emission results with the three different PFT data showed that broadleaf trees (BTs) are the most significant contributor, followed by needleleaf trees (NTs), shrub (SB), and herbaceous plants (HBs) to the total BVOCs. In addition, isoprene from BTs and terpene from NTs were recognized as significant primary and secondary BVOC species in terms of BVOC emissions distributions and O3-forming potentials in the study domain. A geographically weighted regression analysis with locally compensated ridge (LCR-GWR) with the different PFT data (δO3 vs. δPFTs) suggests that addition of BT, SB, and NT areas can contribute to O3 increase, whereas addition of an HB area contributes to O3 decrease in the domain. Assessment results of the simulated spatial and temporal changes of O3 distributions with the different PFT scenarios reveal that hourly and local impacts from the different PFT distributions on occasional inter-deviations of O3 are quite noticeable, reaching up to 13 ppb. The simulated maximum 1 h O3 inter-deviations between different PFT scenarios have an asymmetric diurnal distribution pattern (low in the early morning, rising during the day, peaking at 05:00 p.m., and decreasing during the night) in the study domain. Exponentially diverging hourly BVOC emissions and O3 concentrations with increasing ambient temperature suggest that the use of different PFT distribution data requires much caution when modeling (or forecasting) O3 air quality in complicated urban atmospheric conditions in terms of whether uncertainties in O3 prediction results are expected to be mild or severe.

A proposed physical mechanism for ozone-meteorology correlations using land–atmosphere coupling regimes

Correlations between surface ozone and meteorological variables exhibit a north-south gradient over the Eastern United States (US), with the ozone–temperature correlation weakening and the ozone–humidity correlation transitioning from positive to negative south of 37°N. Using 17 years of hourly August ozone, nitrogen oxide, and isoprene measurements from the Environmental Protection Agency's Air Quality System and Photochemical Assessment Measurement Stations and hourly meteorological fields from the North American Land Data Assimilation System (Phase 2), we propose that the north-south transition and widely observed ozone–humidity correlation results from a shift in the soil moisture-atmosphere coupling regime. Due to soil water limitations over the Southeast, evapotranspiration and specific humidity increase following precipitation events, and this coincides with reductions in temperature and ozone precursors. Therefore, the negative ozone–humidity correlation in the Southeast is likely a manifestation of several meteorological factors directly influencing ozone production. Surface drying, as defined by the evaporative fraction, provides a better predictor of O3 than temperature, specific humidity, or radiation for the Southeast due to its ability to retain prior precipitation information and reflect same-day atmospheric conditions relevant to O3 production. Behavior of surface fluxes and coupling may be particularly relevant for prediction of seasonal and future O3 air quality, and further investigation into the links between land–atmosphere coupling and O3 is necessary.

Are photochemical oxidant control strategies robust to the choice of chemical mechanism?

Environmental context Throughout the world there are many places where ozone levels are elevated above internationally accepted guidelines set to protect human health. Policy makers use air quality models to formulate emission control strategies to achieve these air quality goals for ozone. There are large uncertainties in these air quality models that mask the sensitivity of the model control strategies to chemical mechanism choice. Abstract Monte Carlo sampling of pre-specified parameter ranges has been used to replace a single ‘best estimate’ photochemical trajectory model run with 11 694 ‘acceptable’ model runs that are each consistent with the observations of elevated O3 during the PUMA (Pollution in the Urban Midlands Atmosphere) campaign in the UK, West Midlands during 1999. These 11 694 ‘acceptable’ parameter sets were then used for probabilistic evaluation of photochemical oxidant control strategies, based on 30% reductions in volatile organic compounds and NOx precursor emissions and on precursor emission projections to 2020. The sensitivity of single ‘best estimate’ model runs to chemical mechanism choice gave some indication of the robustness of photochemical oxidant control strategies. However, Monte Carlo parametric uncertainty analysis showed that sensitivity to mechanism choice failed to indicate the magnitudes of the likely uncertainty ranges in the O3 responses to photochemical oxidant control strategies. Furthermore, Monte Carlo uncertainty analysis showed that there may be O3 air quality disbenefits from 30% NOx emission reduction that were not apparent from ‘best estimate’ runs.

Urban air quality modeling with full O3–NOx–VOC chemistry: Implications for O3 and PM air quality in a street canyon

We examine transport and chemical transformation of reactive pollutants on an urban street using a computation fluid dynamics (CFD) model coupled with full photochemistry of reactive pollutants. An extensive comparison between simulated results and observations is conducted to evaluate the model, focusing on a field campaign occurred in Dongfeng Middle Street in Guangzhou, China. Observed CO and NO concentrations vary diurnally following traffic volumes. The model captures this observed diurnal variation and magnitudes of CO concentrations successfully. However, simulated NO concentration is three times higher than observation. This high bias is significantly reduced in the sensitivity simulation with lower NO x emissions. We find that oxidation products of O 3 photochemistry such as NO 2 and O 3 vary differently from primary pollutants, indicating important effects of photochemical reactions on their fates. The model appears to reproduce observed O 3 and NO 2 variability with time and altitude. Our analysis shows that high NO x concentrations in the urban street canyon may efficiently produce aerosol nitrate in the presence of NH 3 . Simulated inorganic NO 3 − aerosol concentration reaches up to 0.3 μg m −3 in July but increases an order of magnitude higher at lower temperature that favors partitioning of gas-phase HNO 3 to aerosol-phase, implying a serious concern for urban air quality in winter. ► We developed a coupled full chemistry-CFD model. ► The model was first applied to an observed case on an urban street canyon. ► The spatial pattern of secondary pollutants differs from the primary. ► Nitrate aerosol would be important for air quality on an urban street.

Ethanol fuel use in Brazil: air quality impacts

Brazil has a long history in the development of ethanol for use as a liquid fuel for vehicles. They have developed one of most efficient and economical systems for producing ethanol in the world. Brazil provides an example that many other countries would like to emulate. Using ethanol as a vehicle fuel has significant potential air quality impacts. This paper will review the available air quality and vehicle emissions data in Brazil, specifically focusing on vehicle related pollutants that may be impacted by the use of large quantities of ethanol in the fuel. The atmospheric concentrations of acetaldehyde (CH3CHO) and ethanol in Brazil are much higher than those in other areas of the world, while the concentrations of the single ring aromatic compounds and small carboxylic acids are more typical of observations elsewhere. Acetaldehyde and ethanol increase in vehicle emissions and nitrogen oxides (NOx) may increase when ethanol fuels are used. Both CH3CHO and NOx are very important contributors to photochemical air pollution and ozone (O3) formation. There are very significant O3 air quality problems in Brazil, most studied in the larger cities of Sao Paulo and Rio de Janeiro. These are issues that must be evaluated for other areas of the world that are considering the use of high ethanol content vehicle fuels.

U.S. Ozone Air Quality under Changing Climate and Anthropogenic Emissions

We examined future ozone (O3) air quality in the United States (U.S.) under changing climate and anthropogenic emissions worldwide by performing global climate-chemistry simulations, utilizing various combinations of present (1990s) and future (Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 2050s) climates, and present and future (2050s; IPCC SRES A2 and B1) anthropogenic emissions. The A2 climate scenario is employed here because it lies at the upper extreme of projected climate change for the 21st century. To examine the sensitivity of U.S. O3 to regional emissions increases (decreases), the IPCC SRES A2 and B1 scenarios, which have overall higher and lower O3-precursor emissions for the U.S., respectively, have been chosen. We find that climate change, by itself, significantly worsens the severity and frequency of high-O3 events ("episodes") over most locations in the U.S., with relatively small changes in average O3 air quality. These high-O3 increases due to climate change alone will erode moderately the gains made under a U.S. emissions reduction scenario (e.g., B1). The effect of climate change on high- and average-O3 increases with anthropogenic emissions. Insofar as average O3 air quality is concerned, changes in U.S. anthropogenic emissions will play the most important role in attaining (or not) near-term U.S. O3 air quality standards. However, policy makers must plan appropriately for O3 background increases due to projected increases in global CH4 abundance and non-U.S. anthropogenic emissions, as well as potential local enhancements that they could cause. These findings provide strong incentives for more-than-planned emissions reductions at locations that are currently O3-nonattainment.

Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality

Reducing methane (CH4) emissions is an attractive option for jointly addressing climate and ozone (O3) air quality goals. With multidecadal full‐chemistry transient simulations in the MOZART‐2 tropospheric chemistry model, we show that tropospheric O3 responds approximately linearly to changes in CH4 emissions over a range of anthropogenic emissions from 0–430 Tg CH4 a−1 (0.11–0.16 Tg tropospheric O3 or ∼11–15 ppt global mean surface O3 decrease per Tg a−1 CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost‐effective CH4 controls would offset the positive climate forcing from CH4 and O3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and improve O3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm−2 to climate forcing and ∼4 ppb to surface O3 in 2030 under the baseline scenario. Although the response of surface O3 to CH4 is relatively uniform spatially compared to that from other O3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O3 formation regime is NOx‐saturated. In the model, CH4 oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O3 than CH4 oxidation in the free troposphere. In NOx‐saturated regions, the surface O3 sensitivity to CH4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O3 sensitivity to CH4.

Ozone air quality and radiative forcing consequences of changes in ozone precursor emissions

[1] Changes in emissions of ozone (O3) precursors affect both air quality and climate. We first examine the sensitivity of surface O3 concentrations (O3srf) and net radiative forcing of climate (RFnet) to reductions in emissions of four precursors – nitrogen oxides (NOx), non-methane volatile organic compounds, carbon monoxide, and methane (CH4). We show that long-term CH4-induced changes in O3, known to be important for climate, are also relevant for air quality; for example, NOx reductions increase CH4, causing a long-term O3 increase that partially counteracts the direct O3 decrease. Second, we assess the radiative forcing resulting from actions to improve O3 air quality by calculating the ratio of ΔRFnet to changes in metrics of O3srf. Decreases in CH4 emissions cause the greatest RFnet decrease per unit reduction in O3srf, while NOx reductions increase RFnet. Of the available means to improve O3 air quality, therefore, CH4 abatement best reduces climate forcing.

Crop responses to ozone: uptake, modes of action, carbon assimilation and partitioning

ABSTRACTThe inhibitory effects of tropospheric O3 on crop photosynthesis, growth, and yield have been documented in numerous studies over the past 35 years. In large part, the results of this research supported governmental regulations designed to limit tropospheric O3 levels to concentrations that affected crop production at economically acceptable levels. Recent studies have brought into question the efficacy of these concentration‐based O3 standards compared with flux‐based approaches that incorporate O3 uptake along with environmental and biotic factors that influence plant responses. In addition, recent studies provide insight into the biochemical mechanisms of O3 injury to plants. Current interpretations suggest that upon entry into the leaf intercellular space O3 rapidly reacts with components of the leaf apoplast to initiate a complex set of responses involving the formation of toxic metabolites and generation of plant defence responses that constitute variably effective countermeasures. Plant species and cultivars exhibit a range of sensitivity to O3, evident as heritable characteristics, that must reflect identifiable biochemical and molecular processes that affect sensitivity to O3 injury, although their exact makeup remains unclear. Ozone clearly impairs photosynthetic processes, which might include the effects on electron transport and guard cell homeostasis as well as the better‐documented effects on carbon fixation via decreased Rubisco activity. Translocation of photosynthate could be inhibited by O3 exposure as well. Further, the influence of tropospheric O3 needs to be considered when assessing potential effects of rising concentrations of atmospheric CO2 on crop production. Advances in O3 flux modelling and improved understanding of biochemical and molecular effects of O3 on photosynthetic gas exchange and plant defence processes are leading to more complete, integrated assessments of O3 impacts on crop physiology that continue to support the rationale for maintaining or improving current O3 air quality standards as well as providing a basis for development of more O3‐tolerant crop lines.

Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions

To simulate ozone (O3) air quality in future decades over the eastern United States, a modeling system consisting of the NASA Goddard Institute for Space Studies Atmosphere‐Ocean Global Climate Model, the Pennsylvania State University/National Center for Atmospheric Research mesoscale regional climate model (MM5), and the Community Multiscale Air Quality model has been applied. Estimates of future emissions of greenhouse gases and ozone precursors are based on the A2 scenario developed by the Intergovernmental Panel on Climate Change (IPCC), one of the scenarios with the highest growth of CO2 among all IPCC scenarios. Simulation results for five summers in the 2020s, 2050s, and 2080s indicate that summertime average daily maximum 8‐hour O3 concentrations increase by 2.7, 4.2, and 5.0 ppb, respectively, as a result of regional climate change alone with respect to five summers in the 1990s. Through additional sensitivity simulations for the five summers in the 2050s the relative impact of changes in regional climate, anthropogenic emissions within the modeling domain, and changed boundary conditions approximating possible changes of global atmospheric composition was investigated. Changed boundary conditions are found to be the largest contributor to changes in predicted summertime average daily maximum 8‐hour O3 concentrations (5.0 ppb), followed by the effects of regional climate change (4.2 ppb) and the effects of increased anthropogenic emissions (1.3 ppb). However, when changes in the fourth highest summertime 8‐hour O3 concentration are considered, changes in regional climate are the most important contributor to simulated concentration changes (7.6 ppb), followed by the effect of increased anthropogenic emissions (3.9 ppb) and increased boundary conditions (2.8 ppb). Thus, while previous studies have pointed out the potentially important contribution of growing global emissions and intercontinental transport to O3 air quality in the United States for future decades, the results presented here imply that it may be equally important to consider the effects of a changing climate when planning for the future attainment of regional‐scale air quality standards such as the U.S. national ambient air quality standard that is based on the fourth highest annual daily maximum 8‐hour O3 concentration.

Variability in surface ozone background over the United States: Implications for air quality policy

The U.S. Environmental Protection Agency (EPA) presently uses a 40 ppbv background O3 level as a baseline in its O3 risk assessments. This background is defined as those concentrations that would exist in the absence of North American emissions. Lefohn et al. [2001] have argued that frequent occurrences of O3 concentrations above 50–60 ppbv at remote northern U.S. sites in spring are of stratospheric origin, challenging the EPA background estimate and implying that the current O3 standard (84 ppbv, 8‐hour average) may be unattainable. We show that a 3‐D global model of tropospheric chemistry reproduces much of the observed variability in U.S. surface O3 concentrations, including the springtime high‐O3 events, with only a minor stratospheric contribution (always <20 ppbv). We conclude that the previous interpretations of a stratospheric source for these events underestimated the role of regional and hemispheric pollution. While stratospheric intrusions might occasionally elevate surface O3 at high‐altitude sites, our results indicate that these events are rare and would not compromise the O3 air quality standard. We find that the O3 background is generally 15–35 ppbv, with some incidences of 40–50 ppbv in the west in spring at high‐elevation sites (>2 km). It declines from spring to summer and further decreases during O3 pollution episodes. The 40 ppbv background assumed by EPA thus actually underestimates the risk associated with O3 during polluted conditions. A better definition would represent background as a function of season, altitude, and total surface O3 concentration. Natural O3 levels are typically 10–25 ppbv and never exceed 40 ppbv. International controls to reduce the hemispheric pollution background would facilitate compliance with an AOT40‐type standard (cumulative exposure to O3 above 40 ppbv) in the United States.

Ozone Air Quality over North America: Part II—An Analysis of Trend Detection and Attribution Techniques

ABSTRACT Assessment of regulatory programs aimed at improving ambient O3 air quality is of considerable interest to the scientific community and to policymakers. Trend detection, the identification of statistically significant long-term changes, and attribution, linking change to specific clima-tological and anthropogenic forcings, are instrumental to this assessment. Detection and attribution are difficult because changes in pollutant concentrations of interest to policymakers may be much smaller than natural variations due to weather and climate. In addition, there are considerable differences in reported trends seemingly based on similar statistical methods and databases. Differences arise from the variety of techniques used to reduce nontrend variation in time series, including mitigating the effects of meteorology and the variety of metrics used to track changes. In this paper, we review the trend assessment techniques being used in the air pollution field and discuss their strengths and limitations in discerning and attributing changes in O3 to emission control policies.

Increasing background ozone in surface air over the United States

The long‐term trend of background O3 in surface air over the United States from 1980 to 1998 is examined using monthly probability distributions of daily maximum 8‐hour average O3 concentrations at a large ensemble of rural sites. Ozone concentrations have decreased at the high end of the probability distribution (reflecting emission controls) but have increased at the low end. The cross‐over takes place between the 30th and 50th percentiles in May–August and between the 60th and 90th percentiles during the rest of the year. The increase is statistically significant at a 5% level in spring and fall, when it is 3–5 ppbv. The maximum increase is in the Northeast. A possible explanation is an increase in the O3 background transported from outside the United States. Better understanding of the causes of the increase is needed because of its implications for meeting O3 air quality standards.

Effect of rising Asian emissions on surface ozone in the United States

The effect of increasing fossil fuel combustion in eastern Asia on surface O3 air pollution in the United States is examined with a global three‐dimensional tropospheric chemistry model. Tripling of Asian anthropogenic emissions from 1985 to 2010 is expected to increase monthly mean O3 concentrations by 2–6 ppbv in the western United States and by 1–3 ppbv in the eastern United States, the maximum effect being in April–June. This increase would more than offset the benefits of 25% domestic reductions in anthropogenic emissions of NOx and hydrocarbons in the western United States. Asian influence may be less under the stagnant conditions leading to violations of the U.S. air quality standard. Nevertheless, our results suggest that a global perspective is necessary when designing a strategy to meet regional O3 air quality objectives.

Synoptic Circulation and Summertime Ground-Level Ozone Concentrations at Vancouver, British Columbia

The Kirchhofer synoptic classification procedure is applied to both mean sea level and 500-hPa NMC gridded pressure fields for the vicinity of southwestern British Columbia. Exceedances of the Canadian 1-h Ambient O3 Air Quality Objective of 82 ppb at Port Moody, Vancouver, are associated with the coincidence of a low-level thermal trough and an upper-level ridge of high pressure. Analysis of synoptic sequences also reveals the importance of persistence in the development of elevated O3 concentrations. The application of synoptic climatology to ground-level O3 in Vancouver highlights the need for consideration of more than one atmospheric level in map-typing schemes. An extension of the basic Kirchhofer approach to permit multilevel computer-assisted map typing is advocated.

Efficacy of nitrogen oxides and hydrocarbons emissions controls in ozone attainment strategies as predicted by the Urban Airshed Model

The primary object of this paper is to provide a preliminary assessment of the effectiveness of NO x vs Volatile Organic Compounds (VOC) emissions control options in improving O3 air quality over the New York metropolitan area. To this end, we have applied the Urban Airshed Model (UAM) with the Carbon Bond IV (CB-IV) chemical mechanism utilizing the results of the Regional Oxidant Model (ROM) for the specification of initial/boundary concentrations and wind fields to the UAM. After examining the sensitivity of the predicted O3 concentrations to initial/boundary conditions and biogenic emissions, we have evaluated the impact of various hypothetical emissions reduction options on O3 air quality. Nested ROM/UAM simulations with an across-the-board reduction of 75% in the NO x and VOC emissions from sources located within the New York metropolitan area indicate that the option of VOC-only control is superior to the NO x -only control in reducing not only peak O3 levels over the entire modeling domain but also population exposure to unhealthy O3 levels. The model predicts that the combined 75% NO x and VOC control option also reduces the peak O3 concentration, but the improvement in O3 air quality is less than that predicted for the 75% VOC-only control strategy. Additional modeling analyses with different mix and levels of emissions control and meteorological conditions are needed to confirm these preliminary findings.

Ozone Control Strategies Based on the Ratio of Volatile Organic Compounds to Nitrogen Oxides

The 1990 Clean Air Act Amendments require states with O3 nonattainment areas to adopt regulations to enforce reasonable available control technologies (RACT) for NOX stationary sources by November 1992. However, if the states can demonstrate that such measures will have an adverse effect on air quality, NOX requirements may be waived. To assist the states in making this decision, the U.S. EPA is attempting to develop guidelines for the states to use in deciding whether NOX reductions will have a positive or negative impact on O3 air quality. Although NOX is a precursor of O3, at low VOC/NOX ratios, the reduction of NOX can result in increased peak O3. EPA is examining existing information on VOC/NOX ratios to develop “rules of thumb” to guide the states in their decision-making process. An examination of 6 a.m. to 9 a.m. VOC/NOX ratios at a number of sites in the eastern U.S. indicates that the ratio is highly variable from day-to-day and there is no apparent relationship between ratios measured at different sites within the same area. In addition, statistical analysis failed to identify significant relationships between the 6 a.m. to 9 a.m. VOC/NOX ratio and the maximum 1-hr. O3 within a given area. Since we know from smog chamber and modeling studies that such a relationship exists, this further invalidates the assumption that a ratio measured at a single site is representative of the ratio for the entire region. Based on this Information, we conclude that having the 6 a.m. to 9 a.m. ambient VOC/NOX ratio for a given area is insufficient information, by itself, to decide whether a VOC-alone, a NOx-alone, or a combined VOC-NOX reduction strategy is a viable or optimum O3-reduction strategy.

Ambient Ozone Concentration Patterns among Eastern U.S. Urban Areas Using Factor Analysis

Data on dally maximum ozone concentrations measured at ambient air monitoring stations operated by state and local air pollution control agencies in the Eastern United States were analyzed using principal factor analysis. Four orthogonal factors representing O3 formation potentials were derived using the statistical package SPSS; these factors accounted for over two-thirds of the variations in 1978 summer O3 levels at 21 urban-oriented stations. The analysis confirmed that O3 variations are similar among stations within defined geographical areas; this confirmation supports the widely held theory that ambient O3 formations are reglonwlde. The analysis suggested that trends analysis for determining general progress in improving O3 air quality should be based on aggregate statistics from clusters of monitors rather than from a single monitoring station within areas associated with the derived factors.