Ozone Design Values Research Articles

Is There a Formaldehyde Deficit in Emissions Inventories for Southeast Michigan?

Formaldehyde is a key Volatile Organic Compound (VOC) and ozone precursor that plays a vital role in the urban atmospheric radical budget on par with water vapor, ozone, and nitrous acid. In addition to modulating radical and ozone production, ambient formaldehyde has both carcinogenic and non-carcinogenic inhalation health effects. This study concludes that ambient formaldehyde in the Southeast Michigan (SEMI) ozone nonattainment area may be underestimated up to a factor of two or more by regional air quality models. The addition of plausible amounts of primary formaldehyde to the U.S. National Emissions Inventory based on estimated formaldehyde-to-CO emission ratios partially alleviates this modeling deficit and indicates the presence of formaldehyde concentrations above 5 ppb at a previously unsuspected location northeast of Detroit. Standard 24-h formaldehyde samples obtained during the Michigan-Ontario Ozone Source Experiment (MOOSE) verified the presence of high ambient formaldehyde concentrations at this location. Moreover, the addition of plausible amounts of primary formaldehyde to VOC emissions inventories may add more than 1 ppb of ozone to ambient air in the SEMI nonattainment area, where ozone design values exceeded the U.S. National Ambient Air Quality Standard (NAAQS) by 1–2 ppb for the 2018–2020 design value period.

Observational-based assessment of contributions to maximum ozone concentrations in the western United States

ABSTRACT Archived Ozone Design Values (ODVs) provide smoothed temporal records of maximum ozone concentrations impacting monitoring sites throughout the US. Utilizing time series of ODVs recorded at sites along the US West Coast, we separately estimate ODV contributions from US background ozone and from production driven by US anthropogenic precursor emissions. Sondes launched from Trinidad Head in northern California measure the vertical distribution of baseline ozone transported ashore from the Pacific; this profile is reflected in the increase of the US background ODV contribution with monitoring site elevation in both rural and urban areas. The ODVs that would result from US background ozone alone are small at coastal, sea level locations (average ~45 ppb), but increase with altitude; above 1 km US background ODVs can exceed 60 ppb. US background ozone contributions now constitute the majority of the maximum ODVs throughout the US west coast region, including the Los Angeles urban area, which records the country’s highest ODVs. US anthropogenic emissions presently cause enhancements of 35 to 55 ppb to the maximum ODVs in the Los Angeles area; thus, local emission controls can further reduce ozone even though the background contribution is larger. In other US west coast urban areas ODV enhancements from US anthropogenic emissions are much smaller than the US background ODV contribution. The past decrease in US anthropogenic ODV enhancements from emission controls is larger than generally realized – a factor of more than 6 from 1980 to 2020, while US background ODV contributions varied to only a small extent over those four decades. Wildfire impacts on ODVs are significant in urban areas of the Pacific Northwest, but not over the vast northern US rural region. There is an indication that agricultural emissions of nitrogen oxides in California’s Salinas Valley increase downwind maximum ODVs by 5–10 ppb. Implications: In 2020 the ozone design values (ODVs) resulting from transported background ozone alone are now larger than the ODV enhancements from US anthropogenic precursor emissions, even in the Los Angeles urban area, where the nation’s highest ODVs are recorded. The US anthropogenic ODV enhancements have been reduced by more than a factor of 6 from 1980 to 2020. The maximum US background ODV contributions have varied somewhat, but in each of the US west coast urban areas it was 60 ppb or larger in 2000. These contributions are so large that reducing maximum urban ODVs to the 70 ppb required by the 2015 ozone NAAQS is very difficult. There remains relatively little room for further reducing ODVs through domestic emission controls alone. From this perspective, degraded US ozone air quality in the western US is primarily due to the US background ozone contribution, with the US anthropogenic enhancement making a significant, but smaller contribution. Notably, the US background ODV has slowly decreased (~1 ppb decade−1; Parrish, Derwent, and Faloona 2021) since the mid-2000s; cooperative, international emission control efforts aimed at continuing or even accelerating this background ozone decrease may be an effective approach to further ODV reductions, since the US background ODV is largely due to a hemisphere-wide, transported reservoir of ozone with contributions from all northern midlatitude continents. Given the major contribution of background ozone to observed ODVs, future reviews of the ozone NAAQS will be better informed if observational-based estimates of background ODV contributions are considered, in addition to model-derived estimates upon which past reviews have solely relied.

Understanding the Paths of Surface Ozone Abatement in the Los Angeles Basin

AbstractOzone in the urban atmosphere can accumulate from photochemistry involving nitrogen oxides, carbon monoxide, and volatile organic compounds mainly emitted from fossil fuel combustions and volatile chemical products. Concentrated ozone near the earth's surface has a detrimental impact on the ecosystem and our health. The Los Angeles Basin is a classic example of an urban region with long‐standing ozone pollution, frequently in violation of its standard, designated by the US Environmental Protection Agency (EPA). Studies have reported substantial declines of ozone and their precursor concentrations over the past half century in this area due to extensive pollution controls. Since 2000, however, the ozone design value has remained the same and has not been able to fully reach the air quality standard set by the EPA despite decreases in its precursor concentrations. To understand this phenomenon, we utilize a chemical transport modeling approach to reproduce historical ozone trends and to conduct sensitivity runs relative to a 2010 baseline. Based on our analysis of the model simulations comparing the weekends and weekdays, rather than focusing only on the weekly averages, we diagnose that the Los Angeles Basin is actually going under a significant transition in photochemistry toward a lower ozone concentration. This reveals how air quality management strategies are slowly moving towards a pathway of reducing ozone concentrations in the future. Our study, however, suggests that further reductions of NOx emissions can lead to continued improvement of ozone pollution in this area.

2030 Ozone Source Attribution From Nox Emission Reduction Modeling Study

This paper describes the use of a photochemical grid modeling brute force method approach in a hypothetical study to determine the change in future year ozone design values based on reducing future year NOx emissions by a specified amount from each of the major source categories.

Air quality and source apportionment modeling of year 2017 ozone episodes in Albuquerque/Bernalillo County, New Mexico

ABSTRACT Albuquerque/Bernalillo County, New Mexico, is currently in attainment of the 2015 National Ambient Air Quality Standard (NAAQS) for ozone (70 ppb), but its ozone design values have increased in recent years. Air quality and source apportionment modeling with the Comprehensive Air Quality Model with Extensions (CAMx) was conducted for Albuquerque/Bernalillo County to develop a refined understanding of ozone source apportionment in the region, estimate ozone concentrations in the year 2025 based on projected changes in anthropogenic emissions, and evaluate the sensitivity of future ozone concentrations to various changes in local and non-local emissions. The study focused on two ozone episodes during June and July 2017 when 8-hr average ozone concentrations were greater than 70 ppb. Based on the modeling results, ozone during the June 2017 episode was found to be driven largely by contributions from non-local and regional emissions, whereas ozone during the July 2017 episode was driven more strongly by local emissions from within Albuquerque/Bernalillo County. On high ozone days, anthropogenic emissions from within Albuquerque/Bernalillo County contributed between 8% and 19% (6–14 ppb) of total ozone. Half of this local ozone contribution was from on-road mobile sources. Fire emissions contributed as much as 2 ppb of ozone on a given day. Contributions from large power plants in New Mexico were as large as 1 ppb on a given day but less than 0.5 ppb on most days. Modeled ozone concentrations in Albuquerque/Bernalillo County were also sensitive to emissions from oil and gas emissions in New Mexico. If projected emission reductions by 2025 materialize, these reductions could reduce future peak 8-hr average ozone concentrations by as much as 3–4% compared to 2017 values. Implications: The results of this study have important implications for air quality management in Albuquerque/Bernalillo County. Ozone in Albuquerque/Bernalillo County is the result of local and non-local emissions, is impacted by wildfires, and is sensitive to statewide oil and gas emissions. The magnitude of modeled contributions from anthropogenic emissions within Albuquerque/Bernalillo County is strongly influenced by meteorological conditions, transport pathways, and the presence of wildfire. This modeling is important for understanding the potential effectiveness of local emission controls in Albuquerque/Bernalillo County, and can serve as a basis for testing future regional and local emission control options.

Assessing the manageable portion of ground-level ozone in the contiguous United States

ABSTRACT Regional air quality models are widely being used to understand the spatial extent and magnitude of the ozone non-attainment problem and to design emission control strategies needed to comply with the relevant ozone standard through direct emission perturbations. In this study, we examine the manageable portion of ground-level ozone using two simulations of the Community Multiscale Air Quality (CMAQ) model for the year 2010 and a probabilistic analysis approach involving 29 years (1990–2018) of historical ozone observations. The modeling results reveal that the reduction in the peak ozone levels from total elimination of anthropogenic emissions within the model domain is around 13–21 ppb for the 90th−100th percentile range of the daily maximum 8-hr ozone concentrations across the contiguous United States (CONUS). Large reductions in the 4th highest 8-hr ozone are seen in the regions of West (interquartile range (IQR) of 17–33%), South (IQR 22–34%), Central (IQR 19–31%), Southeast (IQR 25–34%), and Northeast (IQR 24–37%). However, sites in the western portion of the domain generally show smaller reductions even when all anthropogenic emissions are removed, possibly due to the strong influence of global background ozone, including sources such as intercontinental ozone transport, stratospheric ozone intrusions, wildfires, and biogenic precursor emissions. Probabilistic estimates of the exceedances for several hypothetical thresholds of the 4th highest 8-hr ozone indicate that, in some areas, exceedances of such hypothetical thresholds may occur even with no anthropogenic emissions due to the ever-present atmospheric stochasticity and the current global tropospheric ozone burden. Implications: Because air pollution is intricately linked to adverse health effects, National Ambient Air Quality Standards (NAAQS) have been established for criteria pollutants to safeguard human health and the environment. Areas not in compliance with the relevant standards are required to develop plans and policies to reduce their air pollution levels. Regional-scale air quality models are currently being used routinely to inform policies to identify the emissions reduction required to meet and maintain the NAAQS throughout the country. This paper examines the feasibility of the 4th highest ozone, which is used to derive the ozone design value for NAAQS, complying with various current and hypothetical 8-hr ozone thresholds over CONUS based on the information embedded in 29 years of historical ozone observations and two modeling scenarios with and without anthropogenic emissions loading.

Comparison of Machine Learning and Land Use Regression for fine scale spatiotemporal estimation of ambient air pollution: Modeling ozone concentrations across the contiguous United States

BackgroundSpatial linear Land-Use Regression (LUR) is commonly used for long-term modeling of air pollution in support of exposure and epidemiological assessments. Machine Learning (ML) methods in conjunction with spatiotemporal modeling can provide more flexible exposure-relevant metrics and have been studied using different model structures. There is however a lack of comparisons of methods available within these two modeling frameworks, that can guide model/algorithm selection in air quality epidemiology. ObjectiveThe present study compares thirteen algorithms for spatial/spatiotemporal modeling applied for daily maxima of 8-hour running averages of ambient ozone concentrations at spatial resolutions corresponding to census tracts, to support estimation of annual ozone design values across the contiguous US. These algorithms were selected from nine representative categories and trained using predictors that included chemistry-transport model predictions, meteorological factors, land use and land cover, and stationary and mobile emissions. MethodsTo obtain the best predictive performance, model structures were optimized through a repeated coarse/fine grid search with expert knowledge. Six target-oriented validation strategies were used to prevent overfitting and avoid over-optimistic model evaluation results. In order to take full advantage of the power of different algorithms, we introduced tuning sample weights in spatiotemporal modeling to ensure predictive accuracy of peak concentrations, that is crucial for exposure assessments. In spatial modeling, four interpretation and visualization tools were introduced to explain predictions from different algorithms. ResultsNonlinear ML methods achieved higher prediction accuracy than linear LUR, and the improvements were more significant for spatiotemporal modeling (nearly 10%-40% decrease of predicted RMSE). By tuning the sample weights, spatiotemporal models can predict concentrations used to calculate ozone design values that are comparable or even better than spatial models (nearly 30% decrease of cross-validated RMSE). We visualized the underlying nonlinear relationships, heterogeneous associations and complex interactions from the two best performing ML algorithms, i.e., Random Forest and Extreme Gradient Boosting, and found that the complex patterns were relatively less significant with respect to model accuracy for spatial modeling. ConclusionMachine Learning can provide estimates that are actually more interpretable and practical than linear regression to improve accuracy in modeling human exposures. A careful design of hyperparameter tuning and flexible data splitting and validations is crucial to obtain reliable and stable results. Desirable/successful nonlinear models are expected to capture similar nonlinear patterns and interactions using different ML algorithms.

Estimating background contributions and US anthropogenic enhancements to maximum ozone concentrations in the northern US

Abstract. US ambient ozone concentrations have two components: US background ozone and enhancements produced from the country's anthropogenic precursor emissions. Only the enhancements effectively respond to national emission controls. We investigate the temporal evolution and spatial variability in the largest ozone concentrations, i.e., those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, within the northern tier of US states. We focus on two regions: rural western states, with only small anthropogenic precursor emissions, and the urbanized northeastern states, which include the New York City urban area, the nation's most populated. The US background ODV (i.e., the ODV remaining if US anthropogenic precursor emissions were reduced to zero) is estimated to vary from 54 to 63 ppb in the rural western states and to be smaller and nearly constant (45.8±3.0 ppb) throughout the northeastern states. These US background ODVs correspond to 65 % to 90 % of the 2015 NAAQS of 70 ppb. Over the past 2 to 3 decades US emission control efforts have decreased the US anthropogenic ODV enhancements at an approximately exponential rate, with an e-folding time constant of ∼22 years. These ODV enhancements are relatively large in the northeastern US, with state maximum ODV enhancements of ∼35–64 ppb in 2000, but are not discernible in the rural western states. The US background ODV contribution is significantly larger than the present-day ODV enhancements due to photochemical production from US anthropogenic precursor emissions in the urban as well as the rural regions investigated. Forward projections of past trends suggest that average maximum ODVs in northeastern US will drop below the NAAQS of 70 ppb by about 2021, assuming that the exponential decrease in the ODV enhancements can be maintained and the US background ODV remains constant. This estimate is much more optimistic than in the Los Angeles urban area, where a similar approach estimates the maximum ODV to reach 70 ppb in ∼2050 (Parrish et al., 2017a). The primary reason for this large difference is the significantly higher US ODV background (62.0±2.0 ppb) estimated for the Los Angeles urban area. The approach used in this work has some unquantified uncertainties that are discussed. Models can also estimate US background ODVs; some of those results are shown to correlate with the observationally based estimates derived here (r2 values for different models are ∼0.31 to 0.90), but they are on average systematically lower by 4 to 13 ppb. Further model improvement is required until their output can accurately reproduce the time series and spatial variability in observed ODVs. Ideally, the uncertainties in the model and observationally based approaches can then be reduced through additional comparisons.

Empirical Development of Ozone Isopleths: Applications to Los Angeles

Understanding quantitative relationships between ambient ozone concentrations and precursor emissions is important to policymakers and stakeholders. Such relationships are often captured as ozone isopleth diagrams developed using air quality models. Model-based approaches have limitations, including errors stemming from uncertainties in inputs and modeled processes, and can be computationally burdensome. We develop and apply an empirical method based on ozone design values calculated in the South Coast Air Basin, California, for 1975–2016 to construct ozone isopleths. The study domain is the area with the highest ozone levels in the United States that has experienced high levels of emissions control. Quadratic and log-quadratic models were constructed, and both capture the actual observations very well (R2 ∼ 0.98) and re-create the general characteristics of traditional air quality model-generated isopleths. The empirical approach benefits from being based on observations that are highly accurate (but have a low spatial coverage). Analysis shows that uncertainties are <30% near likely future control levels. Furthermore, the method shows that the nitrogen oxide (NOx)–volatile organic compound (VOC)–ozone system in the 1970s was in the region where VOC controls were most effective and then more recently moved to the region where both VOC and NOx controls are effective.

Standard and alternative procedures for projecting future ozone in the Houston area using relative reduction factors

Abstract Air quality model results are used in a relative sense as part of the process to demonstrate attainment of the U.S. ozone standard. Ozone model predictions for a future year are divided by corresponding model results for a base year to obtain a Relative Reduction Factor (RRF), and the base-year measured ozone design value (DV) is multiplied by the RRF to estimate the future-year design value (DVF). We developed alternative RRFs and DVFs that focus on the increment of local ozone production above regional background (RB). An advantage of the alternative DVFs is that measurements can be used to determine RB ozone, thereby increasing the use of measurements in the DVF estimates. We simulated ozone formation in the Houston area for May–September 2012 and 2028 using the Comprehensive Air Quality Model with Extensions and compared the standard and alternative RRFs and DVFs. If model results are used for all the RB ozone concentrations in our new equation to calculate DVFs, the alternative and standard DVFs are very similar for Houston-Galveston-Brazoria (HGB). If measurements are used for the RB ozone concentrations we combine with the base-year DVs, the alternative DVFs are noticeably larger than the standard DVFs for about half the sites, with the increase being 7 ppb at the site (Deer Park) with the largest standard and alternative DVFs. The differences arise primarily from including different days in the averaging process for the RRFs versus the base-year DVs. We evaluated model performance for the days included in the RRFs and found that the model over-predicted RB ozone (normalized mean bias (NMB) = 26%) and under-predicted the anthropogenic ozone increment (NMB = -22%) from HGB emissions and therefore can be expected to under-estimate ozone response to HGB emission controls.

A new method for assessing the efficacy of emission control strategies

Regional-scale air quality models and observations at routine air quality monitoring sites are used to determine attainment/non-attainment of the ozone air quality standard in the United States. In current regulatory applications, a regional-scale air quality model is applied for a base year and a future year with reduced emissions using the same meteorological conditions as those in the base year. Because of the stochastic nature of the atmosphere, the same meteorological conditions would not prevail in the future year. Therefore, we use multi-decadal observations to develop a new method for estimating the confidence bounds for the future ozone design value (based on the 4th highest value in the daily maximum 8-hr ozone concentration time series, DM8HR) for each emission loading scenario along with the probability of the design value exceeding a given ozone threshold concentration at all monitoring sites in the contiguous United States. To this end, we spectrally decompose the observed DM8HR ozone time series covering the period from 1981 to 2014 using the Kolmogorov-Zurbenko (KZ) filter and examine the variability in the relative strengths of the short-term variations (induced by synoptic-scale weather fluctuations; referred to as synoptic component, SY) and the long-term component (dictated by changes in emissions, seasonality and other slow-changing processes such as climate change; referred to as baseline component, BL). Results indicate that combining the projected change in the ozone baseline level with the adjusted synoptic forcing in historical ozone observations enables us to provide a probabilistic assessment of the efficacy of a selected emissions control strategy in complying with the ozone standard in future years. In addition, attainment demonstration is illustrated with a real-world application of the proposed methodology by using air quality model simulations, thereby helping build confidence in the use of regional-scale air quality models for supporting regulatory policies.

Future-Year Ozone Isopleths for South Coast, San Joaquin Valley, and Maryland

Many areas of the United States are working toward achieving the 2015 ozone National Ambient Air Quality Standard (NAAQS) attainment level. The objective of this study was to develop future-year (2030) volatile organic compounds and nitrogen oxides (VOC-NOx) isopleth diagrams of the 4th highest maximum daily 8-h average ozone design value concentrations at monitors of interest in the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) in California, and in Maryland. The simulation results showed there would be attainment of the 2015 ozone NAAQS in 2030 without further controls at the selected monitors: 27% in SoCAB, 57% in SJV, and 100% in Maryland. The SoCAB ozone isopleths developed in this study were compared with those reported in the South Coast Air Quality Management District 2016 Air Quality Management Plan. There are several differences between the two modeling studies, the results are qualitatively similar for most of the monitors in the relative amounts of additional emission reductions needed to achieve the ozone NAAQS. The results of this study provide insight into designing potential control strategies for ozone attainment in future years for areas currently in non-attainment. Additional photochemical modeling using these strategies can then provide confirmation of the effectiveness of the controls.

Ozone Design Values in Southern California's Air Basins: Temporal Evolution and U.S. Background Contribution

AbstractCalifornia's ambient ozone concentrations have two principal contributions: U.S. background ozone and enhancements produced from anthropogenic precursor emissions; only the latter effectively respond to California emission controls. From 1980 to 2015 ozone has been monitored in eight air basins in Southern California. The temporal evolution of the largest measured concentrations, i.e., those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, is described very well by an exponential decrease on top of a positive offset. We identify this offset as the ODV due to the U.S. background ozone (i.e., the concentration that would be present if U.S. anthropogenic precursor emissions were reduced to zero) and is estimated to be 62.0 ± 1.9 ppb in six of the basins. California's emission control efforts have reduced the anthropogenic ozone enhancements by a factor of ~5 since 1980. However, assuming that the current rate of exponential decrease is maintained and that U.S. background ODV remains constant, projections of the past decrease suggest that ~35 years of additional emission control efforts will be required to reach the new NAAQS of 70 ppb in the Los Angeles area. The growing predominance of U.S. background ozone contributions has shifted the maximum ozone concentrations in all air basins from later to earlier in the summer. Comparisons indicate that currently accepted model estimates of U.S. background ozone concentrations in Southern California are somewhat underestimated; thus, reducing ozone in this region to the 2015 NAAQS may be more difficult than currently expected.

Dynamic Evaluation of Photochemical Grid Model Response to Emission Changes in the South Coast Air Basin in California

This paper describes a study to evaluate the capability of a photochemical grid modeling system to predict changes in ozone concentrations in response to emission changes over a period of several years. The development of regulatory emission control plans to meet air quality standards primarily relies on modeled projections of future-year air quality, although a weight of evidence approach (which takes into account a number of factors including modeling results, model evaluation and other pertinent information such as ambient trends) is recommended and is also typically used as part of the attainment demonstration. Thus, it is important to determine if the modeling system used to project future-year quality can correctly simulate ozone responses to the projected emissions reductions. Uncertainties and errors in modeled projections can lead to erroneous estimates of emissions controls required to attain the standards. We use two existing regulatory modeling databases, employed for forecasting future-year air quality in the South Coast Air Basin (SoCAB) of California, for a number of historical years to evaluate the ability of the system to accurately simulate the observed changes in air quality over a multi-year period. The evaluation results with the older (2012) database show that the modeling system consistently under-predicts the reductions in ozone in response to emission reductions over the years. Model response improves with the newer (2016) database with good agreement at some locations, but the system still tends to under-predict ozone responses by as much as a factor of 2 in recent years for the Basin maximum ozone design value. This suggests that future-year estimates of ozone design values may be overly conservative, resulting in emission controls that are technologically challenging or very expensive to implement. The development of better emission inventories and model inputs is recommended to develop a modeling system that more accurately responds to emission changes. Future regulatory planning should include dynamic evaluation in addition to the traditional operational evaluation of the model to provide more confidence to all stakeholders that the resulting policy decisions are necessary to attain the air quality standards and to protect public health.

Air-quality implications of widespread adoption of cool roofs on ozone and particulate matter in southern California

The installation of roofing materials with increased solar reflectance (i.e., "cool roofs") can mitigate the urban heat island effect and reduce energy use. In addition, meteorological changes, along with the possibility of enhanced UV reflection from these surfaces, can have complex impacts on ozone and PM2.5 concentrations. We aim to evaluate the air-quality impacts of widespread cool-roof installations prescribed by California's Title 24 building energy efficiency standards within the heavily populated and polluted South Coast Air Basin (SoCAB). Development of a comprehensive rooftop area database and evaluation of spectral reflectance measurements of roofing materials allows us to project potential future changes in solar and UV reflectance for simulations using the Weather Research Forecast and Community Multiscale Air Quality (CMAQ) models. 2012 meteorological simulations indicate a decrease in daily maximum temperatures, daily maximum boundary layer heights, and ventilation coefficients throughout the SoCAB upon widespread installation of cool roofs. CMAQ simulations show significant increases in PM2.5 concentrations and policy-relevant design values. Changes in 8-h ozone concentrations depend on the potential change in UV reflectance, ranging from a decrease in population-weighted concentrations when UV reflectance remains unchanged to an increase when changes in UV reflectance are at an upper bound. However, 8-h policy-relevant ozone design values increase in all cases. Although the other benefits of cool roofs could outweigh small air-quality penalties, UV reflectance standards for cool roofing materials could mitigate these negative consequences. Results of this study motivate the careful consideration of future rooftop and pavement solar reflectance modification policies.

Impact of natural gas development in the Marcellus and Utica shales on regional ozone and fine particulate matter levels

The Marcellus and Utica shale formations have recently been the focus of intense natural gas development and production, increasing regional air pollutant emissions. Here we examine the effects of these emissions on regional ozone and fine particulate matter (PM2.5) levels using the chemical transport model, CAMx, and estimate the public health costs with BenMAP. Simulations were performed for three emissions scenarios for the year 2020 that span a range potential development storylines. In areas with the most gas development, the ‘Medium Emissions’ scenario, which corresponds to an intermediate level of development and widespread adoption of new equipment with lower emissions, is predicted to increase 8-hourly ozone design values by up to 2.5 ppbv and average annual PM2.5 concentrations by as much as 0.27 μg/m3. These impacts could range from as much as a factor of two higher to a factor of three lower depending on the level of development and the adoption of emission controls. Smaller impacts (e.g. 0.1–0.5 ppbv of ozone, depending on the emissions scenario) are predicted for non-attainment areas located downwind of the Marcellus region such as New York City, Philadelphia and Washington, DC. Premature deaths for the ‘Medium Emissions’ scenario are predicted to increase by 200–460 annually. The health impacts as well as the changes in ozone and PM2.5 were all driven primarily by NOx emissions.

Trends analyses of 30 years of ambient 8 hour ozone and precursor monitoring data in the South Central U.S.: progress and challenges.

In the last 30 years ambient ozone concentrations have notably decreased in the South Central U.S. Yet, current ambient ozone concentrations measured over the past three years 2013-2015 in this area of the U.S. are not meeting the U.S. 2015 8 hour ozone standard of 70 parts per billion (ppb). This paper provides an update on long-term trends analyses of ambient 8 hour ozone and ozone precursor monitoring data collected over the past 30 years (1986-2015) in four South Central U.S. cities, following up on two previously published reviews of 20 and 25 year trends for these cities. All four cities have benefitted from national ozone precursor controls put in place during the 1990s and 2000s involving cleaner vehicles (vehicle fleet turnover/replacement over time), cleaner fuels, cleaner gasoline and diesel engines, and improved inspection/maintenance programs for existing vehicles. Additional ozone precursor emission controls specific to each city are detailed in this paper. The controls have resulted in impressive ambient ozone and ambient ozone precursor concentration reductions in the four South Central U.S. cities over the past 30 years, including 31-70% ambient nitrogen oxides (NOx) concentration declines from historical peaks to the present, 43-72% volatile organic compound (VOC) concentration declines from historical peaks to the present, a related 45-76% VOC reactivity decline for a subset of VOC species from historical peaks to the present, and an 18-38 ppb reduction in city 8 hour ozone design value concentrations. A new challenge for each of the four South Central U.S. cities will be meeting the U.S. 2015 8 hour ozone standard of 70 ppb.

Assessment of Selective Reduction Strategies over Northeastern Regions of Ohio Using a Photochemical Grid Model—A Case Study

This study is primarily focused on Cleveland-Akron-Lorain nonattainment area in northeastern Ohio. The base year 2002 considered in this study witnessed one of the worst cases of ozone pollu- tion in this region with ozone values exceeding previous 8-hour ozone standards of 84 ppb. The photochemical modeling system CAMx was applied to compute relative reduction factors, which were used subsequently to scale the base year (2002) values at the ozone monitoring sites to esti- mate the ozone design values for the year 2009 in Cleveland area for different emissions reduction scenarios. The results disclose that the largest change in the 2009 ozone design values consequent to applying auxiliary emission control strategies is between 1 and 1.5 ppb, which still left certain counties in northeastern Ohio unable to demonstrate attainment. The results are important in the development of emission control strategies in today's context with instillation of even more strin- gent ozone standards and potential future increases in ozone concentrations due to climate change.

Dynamic evaluation of CMAQ part II: Evaluation of relative response factor metrics for ozone attainment demonstrations

The U.S. Environmental Protection Agency provides guidelines on the use of air quality models for projecting whether an emission reduction strategy will lead to future pollutant levels that are at or below the National Ambient Air Quality Standards (NAAQS). The EPA's guidance document for ozone attainment demonstrations recommends an attainment test for the 8-h ozone NAAQS based on using the ratio of output from “future” and “base” model simulations through the calculation of location-specific Relative Response Factors (RRF). The 2007 guidance document as well as other related studies have recommended the use of retrospective evaluation studies in order to evaluate the ability of an air quality model to represent a change in air quality (dynamic evaluation) rather than relying solely on operational evaluation of model predictions under base line conditions. Here simulations from the Community Multiscale Air Quality (CMAQ) modeling system were conducted for 2002 and 2005, a time period characterized by significant emissions reductions associated with the EPA's Nitrogen Oxides State Implementation Plan Call (NOx SIP Call) as well as mobile sources. These simulations were used to evaluate the performance of different forms of the RRF metric for projecting 2002 to 2005 against 2005 observed ozone levels. The evaluation study showed that the current form of the RRF calculation is generally well designed for predicting the future 8-hr ozone “design value” metric used for determining attainment. Specifically, the methodology of using air quality model simulations in a relative sense provided better estimates of future ozone design values than using the modeled future year simulation alone. Alternative forms of the RRF metric were found to be very similar to the current methodology in terms of evaluation metrics. However, alternative RRF metrics were sensitive to the number of days used in the calculation of the RRF. Approaches which used more days in the RRF calculation (relative to the 2007 guidance approach) had slightly higher bias and error in predicting 2005 design values compared to approaches using only a subset of high ozone days.

Past and future ozone trends in California's South Coast Air Basin: Reconciliation of ambient measurements with past and projected emission inventories

This paper updates the historic trends (1980–2010) in ambient ozone and ozone precursor concentrations in the South Coast Air Basin (SoCAB) and examines the evolution of the ozone-precursor relationship in the Basin. Whereas reductions in NOx (oxide of nitrogen) emissions have decreased nitrate and PM2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) concentrations in the Basin during the past decade, ozone levels have increased at the central basin locations since about 2005 following a reversal in the decline of volatile organic compound (VOC)/NOx ratios during the previous two decades. A chemical box model was used to simulate the effects of changes in precursor concentrations on ozone formation using day-of-week–specific initial precursor concentrations that were derived from measurements and projected to 2020 based on expected emission reductions from 2005 (−10% VOC and −50% NOx). Results show that peak ozone formation rates in 2020 will increase on weekdays by a factor of 3 relative to 2005 and will be comparable to 1995 weekday and 2005 Sunday rates. Ozone production will become precursor limited on Sundays in 2020, but with higher initial rates than 2005. Although a greater NOx reduction scenario in 2020 of −75% will result in even higher initial ozone formation rates, precursor limitation is reached quickly, leading to a further shift westward in the location of peak ozone levels. However, ozone levels will likely be lower in downwind areas where transport is more important than local production of ozone. The ambient versus emission inventory reconciliation indicates a factor of 2 underestimation of VOC emissions in 2009 relative to NOx. Other analyses suggest that there is an overall increase in VOC emissions on hot days that is not fully accounted for by emission inventory estimates. Air quality models using emission inventories that underestimate VOC emissions relative to NOx may lead to inaccurate forecasting of the consequence of emission reductions. Implications: The rate and efficiency of ozone formation and accumulation in the SoCAB is more rapid than would be indicated by air quality model simulations based on the current inventory. Projected reductions in NOx emissions without concurrent reductions in VOC emissions will likely cause ozone to increase during the next decade within central regions of the SoCAB compared with a flat or slightly declining trend in far downwind locations. Air quality statistics that are commonly used to track progress toward attainment, such as basin-wide ozone design value and standard exceedances mask these varying trends within the Basin.