Eric Zalewsky

Impact of Biogenic Emission Uncertainties on the Simulated Response of Ozone and Fine Particulate Matter to Anthropogenic Emission Reductions

ABSTRACT The role of emissions of volatile organic compounds and nitric oxide from biogenic sources is becoming increasingly important in regulatory air quality modeling as levels of anthropogenic emissions continue to decrease and stricter health-based air quality standards are being adopted. However, considerable uncertainties still exist in the current estimation methodologies for biogenic emissions. The impact of these uncertainties on ozone and fine particulate matter (PM2.5) levels for the eastern United States was studied, focusing on biogenic emissions estimates from two commonly used biogenic emission models, the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the Biogenic Emissions Inventory System (BEIS). Photochemical grid modeling simulations were performed for two scenarios: one reflecting present day conditions and the other reflecting a hypothetical future year with reductions in emissions of anthropogenic oxides of nitrogen (NOx). For ozone, the use of MEGAN emissions resulted in a higher ozone response to hypothetical anthropogenic NOx emission reductions compared with BEIS. Applying the current U.S. Environmental Protection Agency guidance on regulatory air quality modeling in conjunction with typical maximum ozone concentrations, the differences in estimated future year ozone design values (DVF) stemming from differences in biogenic emissions estimates were on the order of 4 parts per billion (ppb), corresponding to approximately 5% of the daily maximum 8-hr ozone National Ambient Air Quality Standard (NAAQS) of 75 ppb. For PM2.5, the differences were 0.1–0.25 μg/m3 in the summer total organic mass component of DVFs, corresponding to approximately 1–2% of the value of the annual PM2.5 NAAQS of 15 μg/m3. Spatial variations in the ozone and PM2.5 differences also reveal that the impacts of different biogenic emission estimates on ozone and PM2.5 levels are dependent on ambient levels of anthropogenic emissions. IMPLICATIONS The findings presented in this study demonstrate that uncertainties in biogenic emission estimates due to different emission models can have a significant effect on the model estimates of ozone and PM2.5 concentrations; specifically, the changes in these concentrations due to reductions in anthropogenic emissions considered in regulatory modeling scenarios. These results point to the need for further research aimed at improving biogenic emission estimates as well as better characterizing their dependency on environmental factors and the fate of these emissions once released into the atmosphere.

Rethinking the Assessment of Photochemical Modeling Systems in Air Quality Planning Applications

Abstract The U.S. Environmental Protection Agency provides guidelines for demonstrating that future 8-hr ozone (O3) design values will be at or below the National Ambient Air Quality Standards on the basis of the application of photochemical modeling systems to simulate the effect of emission reductions. These guidelines also require assessment of the model simulation against observations. In this study, we examined the link between the simulated relative responses to emission reductions and model performance as measured by operational evaluation metrics, a part of the model evaluation required by the guidance, which often is the cornerstone of model evaluation in many practical applications. To this end, summertime O3 concentrations were simulated with two modeling systems for both 2002 and 2009 emission conditions. One of these two modeling systems was applied with two different parameterizations for vertical mixing. Comparison of the simulated base-case 8-hr daily maximum O3 concentrations showed marked model-to-model differences of up to 20 ppb, resulting in significant differences in operational model performance measures. In contrast, only relatively minor differences were detected in the relative response of O3 concentrations to emission reductions, resulting in differences of a few ppb or less in estimated future year design values. These findings imply that operational model evaluation metrics provide little insight into the reliability of the actual model application in the regulatory setting (i.e., the estimation of relative changes). In agreement with the guidance, it is argued that more emphasis should be placed on the diagnostic evaluation of O3-precursor relationships and on the development and application of dynamic and retrospective evaluation approaches in which the response of the model to changes in meteorology and emissions is compared with observed changes. As an example, simulated relative O3 changes between 1995 and 2007 are compared against observed changes. It is suggested that such retrospective studies can serve as the starting point for targeted diagnostic studies in which individual aspects of the modeling system are evaluated and refined to improve the characterization of observed changes.

An Assessment of the Emissions Inventory Processing Systems EMS-2001 and SMOKE in Grid-Based Air Quality Models

Abstract In the United States, emission processing models such as Emissions Modeling System-2001 (EMS-2001), Emissions Preprocessor System-Version 2.5 (EPS2.5), and the Sparse Matrix Operator Kernel Emissions (SMOKE) model are currently being used to generate gridded, hourly, speciated emission inputs for urban and regional-scale photochemical models from aggregated pollutant inventories. In this study, two models, EMS-2001 and SMOKE, were applied with their default internal data sets to process a common inventory database for a high ozone (O3) episode over the eastern United States using the Carbon Bond IV (CB4) chemical speciation mechanism. A comparison of the emissions processed by these systems shows differences in all three of the major processing steps performed by the two models (i.e., in temporal allocation, spatial allocation, and chemical speciation). Results from a simulation with a photochemical model using these two sets of emissions indicate differences on the order of ±20 ppb in the predicted 1-hr daily maximum O3 concentrations. It is therefore critical to develop and implement more common and synchronized temporal, spatial, and speciation cross-reference systems such that the processes within each emissions model converge toward reasonably similar results. This would also help to increase confidence in the validity of photochemical grid model results by reducing one aspect of modeling uncertainty.

An Operational Assessment of the Application of the Relative Reduction Factors in the Demonstration of Attainment of the 8-Hr Ozone National Ambient Air Quality Standard

Abstract The U.S. Environmental Protection Agency in 1997 revised the 1-hr ozone (O3) National Ambient Air Quality Standard (NAAQS) to one based on an 8-hr average, resulting in potential nonattainment status for substantial portions of the eastern United States. The regulatory process provides for the development of a state implementation plan that includes a demonstration that the projected future O3 concentrations will be at or below the NAAQS based on photochemical modeling and analytical techniques. In this study, four photochemical modeling systems, based on two photochemical models, Community Model for Air Quality and the Comprehensive Air Quality Model with extensions, and two emissions processing models, Sparse Matrix Optimization Kernel for Emissions and Emissions Modeling System, were applied to the eastern United States, with emphasis on the northeastern Ozone Transport Region in terms of their response to oxides of nitrogen and volatile organic carbon-focused controls on the estimated design values. With the 8-hr O3 NAAQS set as a bright-line test, it was found that a given area could be termed as being in or out of attainment of the NAAQS depending upon the modeling system. This suggests the need to provide an estimate of model-to-model uncertainty in the relative reduction factor (RRF) for a better understanding of the uncertainty in projecting the status of an areas attainment. Results indicate that the model-to-model differences considered in this study introduce an uncertainty of the future estimated design value of ∼3–5 ppb.

Expected ozone benefits of reducing NOx emissions from coal-fired electricity generating units in the Eastern United States

ABSTRACT On hot summer days in the eastern United States, electricity demand rises, mainly because of increased use of air conditioning. Power plants must provide this additional energy, emitting additional pollutants when meteorological conditions are primed for poor air quality. To evaluate the impact of summertime NOx emissions from coal-fired electricity generating units (EGUs) on surface ozone formation, we performed a series of sensitivity modeling forecast scenarios utilizing EPA 2018 version 6.0 emissions (2011 base year) and CMAQ v5.0.2. Coal-fired EGU NOx emissions were adjusted to match the lowest NOx rates observed during the ozone seasons (April 1–October 31) of 2005–2012 (Scenario A), where ozone decreased by 3–4 ppb in affected areas. When compared to the highest emissions rates during the same time period (Scenario B), ozone increased ∼4–7 ppb. NOx emission rates adjusted to match the observed rates from 2011 (Scenario C) increased ozone by ∼4–5 ppb. Finally in Scenario D, the impact of additional NOx reductions was determined by assuming installation of selective catalytic reduction (SCR) controls on all units lacking postcombustion controls; this decreased ozone by an additional 2–4 ppb relative to Scenario A. Following the announcement of a stricter 8-hour ozone standard, this analysis outlines a strategy that would help bring coastal areas in the mid-Atlantic region closer to attainment, and would also provide profound benefits for upwind states where most of the regional EGU NOx originates, even if additional capital investments are not made (Scenario A). Implications: With the 8-hr maximum ozone National Ambient Air Quality Standard (NAAQS) decreasing from 75 to 70 ppb, modeling results indicate that use of postcombustion controls on coal-fired power plants in 2018 could help keep regions in attainment. By operating already existing nitrogen oxide (NOx) removal devices to their full potential, ozone could be significantly curtailed, achieving ozone reductions by up to 5 ppb in areas around the source of emission and immediately downwind. Ozone improvements are also significant (1–2 ppb) for areas affected by cross-state transport, especially Mid-Atlantic coast regions that had struggled to meet the 75 ppb standard.

An examination of the 6:00 a.m.-9:00 a.m. measurements of ozone precursors in the New York City metropolitan area.

Abstract In recent years, ambient measurements of hourly ozone precursor concentrations, namely speciated and total nonmethane organic compounds (NMOCs), have become available through the Photochemical Assessment Monitoring Stations (PAMS) program. Prior to this, NMOCs were measured in the central business district using a canister to obtain the 3-hr integrated sample for the 6:00 a.m.–9:00 a.m. period. Such sampling had been carried out annually for nearly a decade at three locations in the New York City metropolitan area. The intent of these measurements, along with measurements of the other ozone precursor, NOx, was to provide an understanding of ozone formation and the emissions loading and mix in the urban area. The analysis of NMOC and NOx measurements shows a downward trend in the case of NMOC. In addition, we compared the canister-based NMOC concentrations with data obtained from the PAMS program for the 6:00 a.m.–9:00 a.m. period. Analysis of the NMOC concentrations reveals poor spatial correlation between the various monitors, reflecting the effect of localized emissions. This suggests that NMOC measurements made at a single location cannot be viewed as representative of the entire region. On the other hand, correlations were found to be higher among the NOx monitors, indicating the commonality of emission sources and associated physical and chemical processes. Also, the magnitude of the average total NMOC measured at the continuous sampling locations (PAMS) was found to be ~50% lower than the measurements reported from the canister-based measurements. However, such large differences were not evident in the case of the NOx concentrations. It is therefore important to note that, while the NMOC to NOx ratio is spatially variant, it is also dependent upon the method by which the total NMOC are measured in an urban area. Furthermore, the ratios have been found to be correlated with neither the daily maximum ozone nor the daily average ozone concentrations measured within the urban region.On the other hand,correlations were found to be higher among the NOxmonitors, indicating the commonality of emission sources and associated physical and chemical processes. Also, the magnitude of the average total NMOC measured at the continuous sampling locations (PAMS) was found to be ~50% lower than the measurements reported from the canister-based measurements. However, such large differences were not evident in the case of the NOx concentrations. It is therefore important to note that,while the NMOC to NOx ratio is spatially variant, it is also dependent upon the method by which the total NMOC are measured in an urban area. Furthermore, the ratios have been found to be correlated with neither the daily maximum ozone nor the daily average ozone concentrations measured within the urban region. Hence, the 6:00 a.m.–9:00 a.m. ratio of NMOC to NOx concentrations cannot be used as the sole criteria to determine whether an urban area is NOx- or VOC-limited.

Integrating Observations and Modeling in Ozone Management Efforts

Many urban areas in the Eastern United States have been classified to be in non-attainment for ozone, placing a high priority on finding cost-effective emission control measures for improving ambient ozone air quality. Recognizing the complexities associated with the nation’s ozone non-attainment problem, the 1990 Clean Air Amendments mandated the use of grid-based photochemical models for evaluating emission control strategies in urban areas having a serious or higher designation. Given the influx of elevated concentrations of ozone and its precursors into the urban-scale modeling domains (regional-scale transport), many states in the Eastern U.S. were unable to demonstrate ozone attainment for urban areas in their 1994 State Implementation Plans (SIPs) submittal using the urban-scale models. The 1994 SIPs were based on the UAM-IV photochemical model (Morris et al., 1990), which is an urban-scale model that reflects the state-of-science of the late 1980’s. Systems Applications International (SAI) recently developed the UAM-V, a regional-scale ozone air quality model, which contains some new features over the UAM-IV such as multi-scale modeling capability, grid nesting, plume-in-grid (PiG) treatment for point sources, etc. (SAI, 1995). Of particular interest is this model’s treatment of subgrid-scale processes relating to the transport, transformation, and interaction of elevated plumes with the ground-level plume.

An Integrated Modeling and Observational Approach for Designing Ozone Control Strategies for the Eastern U.S.

Despite vigorous attempts to control the ozone problem during the past three decades, ozone levels in many areas over the Eastern United States continue to exceed the National Ambient Air Quality Standards. Until recently, photochemical models were applied to simulate historical ozone episodic events to examine the future ozone non-attainment problem. When the model’s ability to reproduce the observed ozone air quality was deemed acceptable, control measures needed to meet and maintain the ozone standards were evaluated using projected emissions inventories and historical episodic meteorological conditions. Since the episodic meteorological events under which the model has performed best may never occur in the future, there is an inherent uncertainty in the controls identified as required to comply with the ozone standards.

Regional Refined Grid Modeling of Acidic and Mercury Deposition over Northeastern US and the Contribution of New York Power Point Sources

The purpose of the study was to refine the grid resolution from previous regional level assessments by reducing the latest “standard” 12 km down to a 4 km grid level in a novel application of the CMAQ modeling system on an annual timescale. The application was to determine the overall acidic and mercury deposition over New York State (NYS) and the contribution of the NY power sector point sources. To that end, the latest available EPA NEI for 2011 and WRF simulated meteorological data were generated on the 4 km grid domain over the Northeastern US centered on NYS. For mercury, emissions of the elemental, oxidized and particulate species were characterized using stack test and technology based data to allow for the proper assessment of the relative contribution from EGUs and WTE facilities using species dependent wet removal factors and dry deposition velocities. The results for mercury deposition indicate very low contributions from total NYS sources, but shows the importance of both wet and dry components. The impacts of emissions outside the modeling domain were found to clearly dominate total depositions in NYS. For acidic deposition, the importance of wet deposition for sulfate is found, while for total sulfur and nitrates, dry deposition has a significant contribution. For NYS power sector, the significant contribution of dry deposition of SO2 is highlighted. The projected total wet depositions of sulfate, nitrate and mercury compare very favorably with observed levels at NADP sites.

Dynamic Evaluation of Long-Term Air Quality Model Simulations over the Northeastern U.S.

Dynamic model evaluation assesses a modeling system’s ability to reproduce changes in air quality induced by changes in meteorology and/or emissions. In this paper, we illustrate various approaches to dynamic model evaluation utilizing 18years of air quality simulations performed with the regional-scale MM5/SMOKE/CMAQ modeling system over the Northeastern U.S. for the time period 1988–2005. A comparison of observed and simulated weekly cycles in elemental carbon (EC) and organic carbon (OC) concentrations shows significant differences, indicating potential problems with the magnitude and temporal allocation of traffic-related emissions and the split between primary and secondary organic aerosols. A comparison of the observed and simulated interrelationships between temperature and ozone over the 18-year simulation period reveals that the high end of the modeled ozone concentration distribution is less influenced by interannual variability in the high end of the temperature distribution as compared to the observations.