Research Highlights

EPA scientists have primary responsibility for the continual development of CMAQ and its related models. There are currently a number of research projects underway that focus on a range of topics, from exploration of atmospheric science fundamentals to incorporation and evaluation of recent scientific breakthroughs into research versions of the CMAQ model. Eventually, this new knowledge will serve the public by being incorporated in future publicly available CMAQ releases. The research and development team welcome comments and questions about these topics and would be happy to discuss the potential for collaborations.

Research Highlights: Table of Contents

A New Aerosol Dry Deposition Model for Air Quality and Climate Modeling

Researcher: Jon Pleim

Posted: September 22, 2024

Citation: Pleim, J. E., Ran, L., Saylor, R. D., Willison, J., & Binkowski, F. S. (2022). A new aerosol dry deposition model for air quality and climate modeling.  Journal of Advances in Modeling Earth Systems, 14, e2022MS003050. https://doi. org/10.1029/2022MS003050

Aerosol dry deposition is an important sink for atmospheric particles that are a health hazard and a significant climate forcer. Uncertainties in modeling aerosol dry deposition hamper accurate predictions of air quality and climate.  The dominant removal processes (gravitational settling, Brownian diffusion, inertial impaction, and interception) are all dependant on particle size.  However, the parameterizations of some of these processes, particularly impaction and interception, are very uncertain, especially for complex vegetative canopies.  Most aerosol dry deposition models used in air quality and climate models are based on the seminal work by W. G. N. Slinn published in several papers in the 1970s and 1980s.   In recent years, advanced measurement techniques, including size-resolved eddy correlation fluxes, have enabled accurate measurements of aerosol dry deposition in a variety of landscapes.  Several recent studies have aggregated dry deposition velocity data by particle size from many published field experiments and compared to models used in air quality models.  These studies show very large discrepancies between models and measurements, especially in forested areas, that potentially have significant effects on aerosol concentration and thus, air quality and climate forcing.  In this research the aerosol dry deposition calculations for the Community Multiscale Air Quality (CMAQ) model are rederived for better agreement with the consensus of measured data.  The key innovation is the addition of a second inertial impaction term for microscale obstacles such as leaf hairs, microscale ridges, and needleleaf edge effects. The most significant effect of the new model is to increase the mass dry deposition of the accumulation mode aerosols in CMAQ by almost an order of magnitude in forested areas with lesser increases for shorter vegetation.
 

Measured aerosol dry deposition velocities from literature as functions of particle size for evergreen needleleaf forest
Measured aerosol dry deposition velocities from literature as functions of particle size for evergreen needleleaf forest. Symbols represent median values with error bars that represent estimated uncertainty, usually inter-quartile range. The lines show predictions by various models assuming u* = 0.4 m/s including the new model (new Community Multiscale Air Quality) in magenta and the recent Emerson et al. (2020) model in turquoise.

Impact of Aerosol Nitrate Photolysis on air quality   

Researcher: Golam Sarwar

Posted: July 2024

Citation:  Sarwar, G., Henderson, B.H., Hogrefe, C., Mathur, R., Gilliam, R., Callaghan, A., B., Lee, J., Carpenter, L. J.: Examining the Impact of the photolysis of aerosol nitrate over Northern Hemisphere, Science of the Total Environment, 917, 170406, 2024. https://doi.org/10.1016/j.scitotenv.2024.170406

This article shows that increasing the net lifetime of nitrogen oxides in a numerical air quality model can have a large impact on simulated ozone and reduce model bias compared to ozone observations. Nitrogen oxides are removed from the atmosphere by chemical conversion to nitric acid and partitioning to particulate nitrate. Particulate nitrate has been historically treated in models as terminal nitrogen oxide loss that is eventually removed by deposition. However, recent field and experimental studies challenge the loss assumption by suggesting that particulate nitrate is broken down by light to recreate nitrogen oxides. Field and chamber experimental studies support aerosol nitrate photolysis, and the rate of photolysis can vary by 1-3 orders of magnitude depending on the relative availability of halogens from sea salt in the aerosol-phase. We used the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of particulate nitrate photolysis on air quality over the Northern Hemisphere. Consistent with previous studies in different models, CMAQ shows that this new pathway substantially increases model large scale ozone distributions. The changes in ozone distributions improve the comparison of model ozone with (1) vertical column densities with the Ozone Monitoring Instrument retrievals, (2) surface monitors, and (3) ozonesonde measurements. The aerosol nitrate photolysis results are very promising because they specifically increase ozone in spring without large changes in summer and largely rectifies the pronounced underestimation of springtime ozone in current CMAQ simulations.

Graphical abstract depicting the role aerosol nitrate photolysis plays in the atmosphere
Depiction of aerosol nitrate photolysis in atmospheric chemistry. Reproduced from Sarwar et al. (2024).

Predictions of PFAS regional-scale atmospheric deposition and ambient air exposure

Researcher: Emma L. D'Ambro

Posted: January 2024

Citation: D'Ambro, Emma L., Benjamin N. Murphy, Jesse O. Bash, Robert C. Gilliam, Havala O.T. Pye, 2023: Predictions of PFAS regional-scale atmospheric deposition and ambient air exposure, Science of The Total Environment, Volume 902, https://doi.org/10.1016/j.scitotenv.2023.166256 

Per- and polyfluoroalkyl substances (PFAS) are a class of industrially produced compounds whose emissions to air may lead to direct inhalation of airborne PFAS or surface or ground water contamination via transport and deposition. Additional human exposure may then occur by ingestion of contaminated water, or ingestion of food sources contaminated with the contaminated water (e.g. fish). Air quality modeling techniques can be used to quantify air concentrations and deposition fluxes, which in turn can be used to estimate the magnitude of different exposure pathways. In 2021, we developed CMAQ-PFAS, a version of the Community Multiscale Air Quality (CMAQ) model that simulates the emissions, transport, and fate of PFAS. We applied CMAQ-PFAS to a case study in Eastern North Carolina, investigating a comprehensive set of PFAS air emissions reported by a chemical manufacturer and evaluating their fate in the atmosphere. In more recent work, we have stress-tested the model to see which assumptions have the largest impact on results. We found that the chemical speciation of emissions had the largest impact on total PFAS deposition (Fig. 1) and this was primarily driven by the solubility and volatility of each model species. Simplifying the chemical speciation to a single species with mole-weighted physicochemical properties can enhance or diminish the deposition by up to 250%. Changing the model horizontal resolution from 1 km to 4 km does not significantly impact results, but further decreasing resolution to 12 km increases deposition by 40% and should be a consideration in future studies using low horizontal resolution. Including incremental emission controls for this case minimally impacts the total PFAS deposition (up to 3%) since a limited number of compounds were targeted for control. The most important model inputs are realistic air emission rates in terms of both total magnitude and chemical speciation. We also compared predicted air concentrations for GenX (HFPO-DA, CAS 13252-13-6) to an inhalation screening level derived from the US EPA’s reference dose (RfD = 3 ng per kg body weight per day). We derive a value of 7.5 ng m-3 for the inhalation screening level and find that for a cumulative ~15 days throughout 2018, the model predicts GenX ambient air concentrations in the vicinity of the facility above this level. These results suggest there may be health impacts from atmospheric GenX air concentrations near the facility, and that more investigation is warranted.

Box plot showing the percent of total annual PFAS emissions deposited via wet (blue) and dry (gray scale) deposition within ~150 km of the facility, for each of the model scenarios.
The percent of total annual PFAS emissions deposited via wet (blue) and dry (gray scale) deposition within ~150 km of the facility, for each of the model scenarios.

Development and application of CRACMM version 1

Researcher: Havala O. T. Pye

Posted: September 2023

Chemical mechanisms describe the atmospheric transformations of organic and inorganic species and connect air emissions to secondary species such as ozone (O3), fine particles, and hazardous air pollutants (HAPs). The Community Regional Atmospheric Chemistry Multiphase Mechanism is being developed to strengthen connections between emissions and chemical pathways to these endpoints. The initial version of the mechanism, including approach to emissions and chemistry, is described in a manuscript by Pye et al. (2023). The development and analysis of the mechanism was facilitated by adding metadata to the SPECIATE database to identify different organic compounds so python and structure-property tools could be used to determine properties of the emissions and how they should be represented in the mechanism. Pye et al. estimate the amount of ROC mass mapped to mechanism species is 40% higher than current operational mechanisms since unspeciated compounds as well as those at lower volatility and reactivity are represented in the mechanism (see Figure below).

Anthropogenic and wood-burning ROC emissions and their relative potential hydroxyl radial reactivity (OHR), O3 formation, and OA for 2017 U.S. conditions by volatility class
Anthropogenic and wood-burning ROC emissions and their relative potential hydroxyl radial reactivity (OHR), O3 formation, and OA for 2017 U.S. conditions by volatility class. Biogenic VOCs (BVOC) are not considered here. Ozone and OA formation potentials are calculated using simple approaches. Metrics are aggregated from the individual species level to volatility classes: low volatility organic compounds (LVOC), semivolatile organic compounds (SVOC), intermediate volatility organic compounds (IVOC), and volatile organic compounds (VOC). Reproduced from Figure 9 of Pye et al. (2023).

Linking gas-phase chemistry with organic aerosol (OA) formation in CRACMM enabled the treatment of new OA precursors such as furans and phenolic compounds not previously considered in CMAQ. The attribution of aromatic SOA to phenolic vs non-phenolic routes is important as it affects how much OA is predicted overall and how it is attributed to various sources. In the case of benzene SOA, the more SOA comes from phenol vs non-phenol channels, the higher the total SOA potential of U.S. emissions (as phenol > benzene emissions) and larger attribution to sources with high phenol to benzene ratios such as wildland fires and residential wood combustion.

CRACMM has now been applied in CMAQ for both the northeast U.S. (4km horizontal resolution) and contiguous U.S. (12km horizontal resolution). Work by Place et al. (2023) demonstrates CRACMM predictions of ozone for the northeastern U.S. and finds that CRACMM shows less bias that the RACM2 mechanism upon which it was built (+2.1 vs +4.2 ppb mean bias). Place et al. attribute the changes in ozone relative to RACM2 primarily to updates in the inorganic reaction rate constants and monoterpene chemistry. CRACMM is also being used to understand the role of different anthropogenic sources in criteria pollutant formation. For example, in work by Seltzer et al. (2023), CRACMM was used to estimate the impacts of asphalt paving emissions on ozone and fine particles. On average, asphalt paving was estimated to account for 2-4% of secondary organic aerosol with episodic values potentially higher.

The “Community” aspect of CRACMM reflects the fact that scientific advances from the scientific community are an important part of the mechanism. An example of how the community contributed to CRACMM is the development of a compact, efficient isoprene chemical mechanism by Wiser et al. (2023) which is available in CMAQv5.4 as CRACMM1AMORE. The Automated Model Reduction (AMORE) technique uses graph theory to improve the representation of the complex interactions between isoprene and anthropogenic species without any meaningful change in the runtime of CRACMM. The EPA team will continue identify scientific advances from the community that are ready for incorporation in future versions of CRACMM.

More information on CRACMM

Citations:

Place, B. K., Hutzell, W. T., Appel, K. W., Farrell, S., Valin, L., Murphy, B. N., Seltzer, K. M., Sarwar, G., Allen, C., Piletic, I. R., D'Ambro, E. L., Saunders, E., Simon, H., Torres-Vasquez, A., Pleim, J., Schwantes, R. H., Coggon, M. M., Xu, L., Stockwell, W. R., and Pye, H. O. T.: Sensitivity of northeastern US surface ozone predictions to the representation of atmospheric chemistry in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMMv1.0), Atmos. Chem. Phys., 23, 9173–9190, https://doi.org/10.5194/acp-23-9173-2023, 2023.

Pye, H. O. T., Place, B. K., Murphy, B. N., Seltzer, K. M., D'Ambro, E. L., Allen, C., Piletic, I. R., Farrell, S., Schwantes, R. H., Coggon, M. M., Saunders, E., Xu, L., Sarwar, G., Hutzell, W. T., Foley, K. M., Pouliot, G., Bash, J., and Stockwell, W. R.: Linking gas, particulate, and toxic endpoints to air emissions in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM), Atmos. Chem. Phys., 23, 5043-5099, https://doi.org/10.5194/acp-23-5043-2023, 2023.

Seltzer, K. M., Rao, V., Pye, H. O. T., Murphy, B. N., Place, B. K., Khare, P., Gentner, D. R., Allen, C., Cooley, D., Mason, R., and Houyoux, M.: Anthropogenic secondary organic aerosol and ozone production from asphalt-related emissions, Environmental Science: Atmospheres, https://doi.org/10.1039/D3EA00066D, 2023.

Wiser, F., Place, B. K., Sen, S., Pye, H. O. T., Yang, B., Westervelt, D. M., Henze, D. K., Fiore, A. M., and McNeill, V. F.: AMORE-Isoprene v1.0: a new reduced mechanism for gas-phase isoprene oxidation, Geosci. Model Dev., 16, 1801-1821, https://doi.org/10.5194/gmd-16-1801-2023, 2023.

Quantifying the contributions of various precursor emissions to ozone formation

Researcher: Qian Shu

Posted: June 2023

Citation: Shu, Q., Napelenok, S. L., Hutzell, W. T., Baker, K. R., Henderson, B. H., Murphy, B. N., and Hogrefe, C.: Comparison of ozone formation attribution techniques in the northeastern United States, Geosci. Model Dev., 16, 2303–2322, https://doi.org/10.5194/gmd-16-2303-2023, 2023

In the United States, the national average ambient ozone concentration has decreased by 22 % since 1990, owing to regulations such as the Clean Air Act (CAA) on NOx and VOC emissions. Long-term space observations have also confirmed the improvement in air quality. However, many major metropolitan areas continue to exceed the ozone National Ambient Air Quality Standards (NAAQS) set by the U.S. EPA. To continue to reduce ozone levels, it is critical to develop effective emission control strategies. The effectiveness of any control strategy hinges on accurately quantifying the contributions of various precursor emissions to ozone formation. In this study, the Integrated Source Apportionment Method (ISAM) has been revised in the Community Multiscale Air Quality (CMAQ) model. We update ISAM to maximize its flexibility, particularly for ozone modeling, by providing multiple attribution options, including products inheriting attribution fully from nitrogen oxide reactants, fully from VOC reactants, equally from all reactants, or dynamically from NOx or VOC reactants based on the indicator gross production ratio of hydrogen peroxide (H2O2) to nitric acid (HNO3). The updated ISAM has been incorporated into the most recent publicly accessible version of CMAQ (v5.3.2). This study documents these ISAM updates and demonstrates their impacts on source apportionment results for ozone and its precursors. Additionally, the ISAM results are compared with the Ozone Source Apportionment Technology (OSAT) in the Comprehensive Air-quality Model with Extensions (CAMx) and the brute-force method (BF). All comparisons are performed for a demonstration domain over the northeastern US for a short summer case study (9 and 10 August 2018). General similarities among ISAM, OSAT, and BF results add credibility to the new ISAM algorithms. Despite some localized differences, all source apportionment techniques provide useful data to identify the geographical and temporal contributions of ozone and its precursors.

Characterizing variations in ambient PM2.5 concentrations

Researcher: Golam Sarwar

Posted: March 2023

Citation: Sarwar, G, Christian Hogrefe, H., Henderson, B.H., Foley, K., Mathur, R., Murphy, B., Ahmed, S., 2023. Characterizing variations in ambient PM2.5 concentrations at the U.S. Embassy in Dhaka, Bangladesh using observations and the CMAQ modeling system. Atmospheric Environment, 2023, 296, 119587. https://doi.org/10.1016/j.atmosenv.2023.119587

The U.S. Embassy has been measuring fine particle (PM2.5) concentrations in Dhaka since 2015, but comprehensive analysis and interpretation of these measurements has been lacking. In this study, we analyze ambient PM2.5 mass concentrations measured at the U.S. Embassy in Dhaka for 2016-2021 in conjunction with predictions from the Community Multiscale Air Quality (CMAQ) model over the Northern Hemisphere. We find that concentrations vary seasonally, with the highest occurring in winter and the lowest in monsoon seasons. Mean winter PM2.5 concentrations reached ~165-175 mg/m3 while monsoon concentrations remained ~30-35 mg/m3. Annual mean PM2.5 concentrations reached ~79.7-98.0 mg/m3. On an annual basis, about 50% of the days exceeded the daily PM2.5 standard of 65 mg/m3. Daily mean PM2.5 concentrations remained elevated (>65 mg/m3) for more than 80 consecutive days. The comparison of Community Multiscale Air Quality (CMAQ) model results with observed data suggests that the model can reproduce the seasonal variation of observed data but underpredicts observed PM2.5 in winter months with a normalized mean bias of 13-32%. In the model, organic aerosol is the largest component of PM2.5, of which secondary organic aerosol plays a dominant role. Model sensitivity simulations estimate that transboundary pollution has a large impact on the PM2.5concentration in Dhaka, with an annual mean contribution of ~40 mg/m3 accounting for ~50% of the annual mean observed data.

A comparison of CMAQ predicted monthly mean PM2.5 concentrations with observations at the U.S. Embassy in Dhaka. Model and observed PM2.5 concentrations are shown on the left y-axis and normalized mean bias (NMB) values are shown on the right y-axis, and the horizontal dashed line represents 0 NMB.
A comparison of CMAQ predicted monthly mean PM2.5 concentrations with observations at the U.S. Embassy in Dhaka. Model and observed PM2.5 concentrations are shown on the left y-axis and normalized mean bias (NMB) values are shown on the right y-axis, and the horizontal dashed line represents 0 NMB.

Study quantifies multi-decadal changes in inter-continentally transported ozone pollution to North America

Researcher: Rohit Mathur

Posted: September 2022

Citation: Mathur, R., Daiwen, K., Napelenok, S.L., Xing, J., Hogrefe, C., Sarwar, G., Itahashi, S., Henderson, B.H. (2022) How Have Divergent Global Emission Trends Influenced Long-Range Transported Ozone to North America? Journal of Geophysical Research: Atmospheres, e2022JD036926. https://doi.org/10.1029/2022JD036926

Several locations across the United States in noncompliance with the national standard for ground-level ozone are thought to have sizable influences from distant extra-regional emission sources or natural stratospheric ozone, which complicate the design of local emission control measures. A recent study published in the Journal of Geophysical Research quantifies the changing amounts of ozone pollution inter-continentally transported from other world regions to the United States. “Background” or amount not produced locally constitutes a larger fraction of the ozone pollution at a location as local control measures are implemented. Detailed CMAQ calculations are analyzed to explain the role of the dominant emission source regions across the Northern Hemisphere that drive the 1990–2010 changes noted in observed ground-level ozone measurements across the United States. In recent years, the contributions of open-sea shipping emissions to ozone imported to the U.S. troposphere are comparable to those from the transport of ozone attributable to East Asian emissions and could be higher in the future if commercial shipping operations were to increase in response to anticipated growth in seaborne trade. In addition to the ozone attributable to emissions from different world regions, air masses entering the North American domain have sizable contributions of natural ozone of stratospheric origin, variability in which needs improved quantification to guide background ozone assessments and policy deliberations.

Estimated contributions of different source regions to long-range transported O3 to North America during Spring for 1990 (a-f) and 2010 (g-l). All estimates are for the 0-2km average O3 mixing ratios. The source regions are defined as (1) Europe including all of Russia (EUR+RUS), (2) East and Southeast Asia (EAS+SEA), (3) South Asia (SAS; the Indian subcontinent), (4) Northern Africa and the Middle East (NAF+MDE), (5) Central America (CAM), and (6) Other (the rest of the geographic domain including the vast oceanic regions).  [Figure 9 from Mathur et al. 2022.]

Performance Evaluation in Multiscale WRF-CMAQ Simulations

Researcher: Ana Torres-Vazquez

Posted: March 2022

Citations:

Torres-Vazquez, Ana, Pleim, Jonathan, Gilliam, Robert, Pouliot, George. (2022, February 10). Performance Evaluation of the Meteorology and air Quality Conditions from Multiscale WRF-CMAQ Simulations for the Long Island Sounds Tropospheric Ozone Study (LISTOS). Journal of Geophysical Research: Atmospheres, e2021JD035890. https://doi.org/10.1029/2021JD035890

Summary: Surface-level ozone pollution across the country is closely monitored by the U.S. Environmental Protection Agency due to the threat it can pose to public health and safety. Ozone is not emitted naturally; instead, it forms when other pollutants are emitted and exposed to sunlight. Ozone pollution can be especially harmful near large urban areas like New York City (NYC) where high emissions of pollutants and local meteorology interact to produce high ozone concentrations downwind over Long Island Sound (LIS) and into Connecticut. In this study, we used a weather and chemistry model, WRF-CMAQ, to explore how surface winds over the LIS impact high ozone along the Connecticut and Long Island coastlines.

July 10 1-min wind speed (shaded) and direction (left axis) measurements (dots) and hourly 1.33 km model simulation wind speed (shaded) and direction (left axis) (squares) at ground stations (a) Bronx, NY; (b) Queens, NY; (c) Stony Brook, NY; (d) Greenwich, CT; (e) Westport, CT; (f) New Haven, CT; and (g) Madison, CT).
July 10 observed 1-min winds (dots) and hourly 1.33km model simulated winds (squares).  The left axis indicates the wind direction and the color sharing indicates the wind speed.  Modeled and measured values are shown for 7 ground stations: (a) Bronx, NY; (b) Queens, NY; (c) Stony Brook, NY; (d) Greenwich, CT; (e) Westport, CT; (f) New Haven, CT; and (g) Madison, CT).

Measurements made during a multi-agency collaborative field campaign (LISTOS) were used to test and evaluate the coupled WRF-CMAQ model at 12 km, 4 km and 1.33 km horizontal grid spacing. The evaluation suggests using higher resolutions resulted in improved surface meteorology statistics throughout the whole summer, with temperature biases seeing the biggest statistical improvements when using 1.33-km grid spacing. Case studies of two high ozone concentration events (July 10 and August 6) revealed that sound breezes and low-level jets had a critical role in transporting pollutant-rich, shallow marine air masses from the Long Island Sound inland over the Connecticut coast.

Our work highlights the importance of accurately modeling meteorological conditions so that we can adequately represent high ozone events in models. Continued work in these areas of study can help policy makers develop strategies that account for all the factors that cause ozone pollution in this area.

Secondary Organic Aerosol and Cardiorespiratory Mortality

Researcher : Havala Pye

Posted: January 2022

Citations: 

Pye, H.O.T.; Ward-Caviness, C.K.; Murphy, B.N.; Appel, K. W.; and Seltzer, K.M., Secondary organic aerosol association with cardiorespiratory disease mortality in the United States. Nat Commun 2021, 12, 7215. https://doi.org/10.1038/s41467-021-27484-1

Pye, H.O.T.; Appel, K. W.; Seltzer, K.M.; Ward-Caviness, C.K.; and Murphy, B.N., Human-Health Impacts of Controlling Secondary Air Pollution Precursors. Environ Sci Technol Lett 2022. https://doi.org/10.1021/acs.estlett.1c00798

Summary: Fine particulate matter in air, PM2.5, is associated with negative health outcomes including cardiorespiratory disease death. However, the role of individual PM2.5 components in adverse health outcomes has not been robustly demonstrated. Recent changes in ambient PM2.5 composition combined with advances in modeling of secondary organic aerosol (SOA), now allows for examining the role of SOA and its components in cardiorespiratory disease death rates in a way not previously possible. In work by Pye et al. (2021), PM2.5 constituent concentrations from the Community Multiscale Air Quality (CMAQ) modeling system along with county-level data from the Centers for Disease Control and Prevention, were used to examine the relationship between PM2.5 components and cardiorespiratory disease deaths. SOA was strongly associated with mortality independent of total PM2.5 mass, and spatial variability in SOA across the U.S. was associated with a larger increase in cardiorespiratory mortality rates than total PM2.5. On a per mass basis, SOA was associated with about a 6.5× higher rate of mortality than total PM2.5.

Since SOA was found to be an important driver of the health impacts, Pye et al. (2021) suggest strategies to further reduce health impacts of fine particles, as well as understanding how death rates have changed in the recent past, requires consideration of SOA. Using PM2.5-component specific information, follow-on work by Pye et al. (2022) examined the effectiveness of different precursor emission controls on reducing air pollution associated mortality in the U.S. Anthropogenic VOC emission reductions were predicted to be more than twice as effective as equivalent 25% reductions of SOX or NOX at reducing air pollution-associated cardiorespiratory mortality in the United States. Most (85%) of the VOC-reduction benefits were predicted to result from reduced SOA with the remainder from ozone. As VOC emissions are not trending downward as fast as SOX or NOX, future pollution control efforts could achieve greater health benefits by considering VOC emissions (e.g., from volatile chemical products and other sources).

Demonstrating the Viability of Global Weather Data for Global CMAQ

Researcher : Robert Gilliam

Posted: September 2021

Citation: Gilliam, R. C., Herwehe, J. A., Bullock, Jr, O. R., Pleim, J. E., Ran, L., Campbell, P. C., & Foroutan, H. (2021). Establishing the suitability of the Model for Prediction Across Scales for global retrospective air quality modeling. Journal of Geophysical Research: Atmospheres, 126, e2020JD033588. https://doi.org/10.1029/2020JD033588

Figure 1 Seasonal observed precipitation (PRISM, in millimeters) with estimates from the two weather models MPAS and WRF.
Figure 1 Seasonal observed precipitation (PRISM, in millimeters) with estimates from the two weather models MPAS and WRF.

Summary: Substantial progress has been made to develop an Advanced Air Quality Modeling System (AAQMS) to holistically simulate influences of global emissions and meteorology on regional and local air quality. A recent paper by Gilliam et al. (2021) documents the evaluation and the successful integration of EPA’s model updates in the Model for Prediction Across Scales (MPAS), a global meteorology model that is central to the AAQMS with CMAQ. Updates to MPAS included the implementation of four-dimensional data assimilation (Bullock et al., 2018), the implementation of the Pleim-Xiu land-surface model and atmospheric mixing routine into MPAS.

Annual simulations were conducted for a comprehensive evaluation that examined model performance over all seasons. The evaluation of MPAS focused on weather near the surface, weather aloft (up to about 12 km), radiation (a key in photochemistry), and precipitation. MPAS compared well against observations by matching or improving upon the error levels of the current regional meteorology driver for CMAQ, the Weather Research and Forecasting model (WRF). As an example, seasonal precipitation statistics indicated that MPAS has slightly lower error and bias than WRF. Figure 1 below shows the season precipitation totals that were observed (PRISM) and modeled by MPAS and WRF. Features in the PRISM observational dataset were better replicated by MPAS from the winter precipitation over the southern U.S. to summer precipitation over the western U.S. While there are certainly areas of improvement in MPAS, this evaluation lends confidence in MPAS as the meteorological driver for the AAQMS.

Evaluation of 15 years of modeled NOX across the contiguous United States

Researcher : Kristen Foley

Posted : May 2021

Citation: Toro, C., Foley, K., Simon, H., Henderson, B., Baker, K., Eyth, A., Timin, B., Appel, W., Luecken, D., Beardsley, M., Sonntag, D., Possiel, N. Evaluation of 15 years of modeled atmospheric oxidized nitrogen compounds across the contiguous United States. Elem Sci Anth (2021) 9: 1. DOI: https://doi.org/10.1525/elementa.2020.00158

This work was part of an EPA cross-agency workgroup that evaluated emissions and air quality modeling of NOX. The workgroup's findings are summarized in the following EPA White Paper: Overview of Progress and Findings from the Cross-EPA Coordination Effort for Understanding and Evaluating NOX Emissions Discrepancies (PDF)(48 pp, 2.1 MB, 11-01-2021, 454-R-21-008). U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards.

Photo of traffic on an urban road

Summary: In recent years, many published studies comparing ambient NOX and NOY concentrations to modeled or inventory values found a high bias on the order of 1.4 - 2 times observed levels. Some researchers proposed reducing mobile-source NOX emissions by 30-70% in their modeling applications. Many of these applications were based on evaluation of summer 2011 modeling to leverage the 2011 National Emissions Inventory and various summer field campaigns such as the 2011 DISCOVER-AQ Baltimore. Here, NOX estimates from annual photochemical simulations for years 2002-2016 are compared to surface network measurements of NOX and NOY to evaluate the Community Multiscale Air Quality (CMAQ) modeling system performance by U.S. region, season, and time-of-day. In addition, aircraft measurements from 2011 DISCOVER AQ are used to evaluate how emissions, chemical mechanism, and measurement uncertainty each contribute to the overall model performance. We show distinct seasonal and time-of-day patterns in NOX performance. Summertime NOx is overpredicted with bimodal peaks in bias during early morning and evening hours and persisting overnight.  The summertime morning NOX bias dropped from between 28% and 57% for earlier years (2002-2012) to between -2 and 7% for later years (2013-2016). Summer daytime NOX tends to be unbiased or underpredicted.  In winter, the evening NOX overpredictions remains, but NOX is unbiased or underpredicted overnight, in the morning, and during the day. NOX overpredictions are most pronounced in the Midwestern and Southern U.S. with Western regions having more of a tendency towards model underpredictions of NOX.  Modeled NOX performance has improved substantially over time, reflecting updates to the emissions inputs and the CMAQ air quality model.  Model performance improvements are largest for years simulated with CMAQv5.1 or later and for emissions inventory years 2014 and later, coinciding with reduced onroad NOX emissions from vehicles with newer emission control technologies and improved treatment of chemistry, deposition, and vertical mixing in CMAQ.

Normalized Mean Bias (NMB) of morning modeled NOX and observed NOX. Morning hours are 4 – 9 a.m. Local Standard Time. Version number on the y axis refers to CMAQ version. Data have been aggregated by season for each annual simulation across all monitors in the US domain.

Results

Characterizing Air Emissions, Transport, and Deposition of PFAS from a Fluoropolymer Manufacturing Facility

Researcher : Emma D'Ambro

Posted : January 2021

Citation: D'Ambro, L., Emma, Pye, H.O.T., Bash, O., Jesse, B., James, A., Efstathiou., C, Gilliam, C., Robert, Reynolds, L., Talgo, K., and Murphy, N., Benjamin. Characterizing the Air Emissions, Transport, and Deposition of Per- and Polyfluoroalkyl Substances from a Fluoropolymer Manufacturing Facility. Environmental Science & Technology (2021), 55, 2, 862-870, https://doi.org/10.1021/acs.est.0c06580

Figure 1. Well-detects of PFAS around the Chemours facility outside Fayetteville, NC, as measured by NC DEQ. Direction of prevailing wind and Cape Fear River flow are shown with arrows, locations of NC DEQ deposition sampling sites are shown with blue circles. Underlying map courtesy of NC DEQ.

Summary: Per- and polyfluoroalkyl substances (PFAS) are a class of industrially produced compounds whose emissions to air may contribute, via transport and deposition, to concentrations in surface water, ground water, and private well water in the vicinity of large point sources. Air quality modeling techniques can be used to quantify air concentrations and deposition fluxes to help parse the role of exposure pathways such as direct inhalation and ingestion via contaminated water. We apply the Community Multiscale Air Quality model (CMAQ) to a case study in Eastern North Carolina to model the PFAS emissions and transport at fine scale (1 km) from the Chemours Inc. Fayetteville-Works (Fig. 1). Reported air emissions by the facility include 53 species and their emission rates. The top compounds by mass are explicitly implemented into CMAQ-PFAS, with the remaining lower emission rate species (< 45 kg yr-1) lumped into an aggregate species. The behavior of atmospheric PFAS emissions is largely unknown due to the lack of atmospheric studies and physicochemical property measurements. We use OPERA, a physicochemical property estimation model, and an updated deposition module to estimate the air concentration and deposition of individual and total PFAS compounds from this facility. We specifically investigate compounds of public and regulatory concern, namely GenX, as well as the entire suite of emitted PFAS. Homologues are compared to investigate the role of structure on deposition and air concentration as a function of distance from the facility. We find that only a small fraction (<5%) of GenX and Total PFAS air emissions deposit within 150 km of the facility (Fig. 2). The majority of emissions then are transported to other parts of the U.S. and beyond. To our knowledge, this is the first time a comprehensive set of PFAS emissions from a manufacturing point source has been evaluated for its impact on the atmosphere. Our results are an important step in understanding the behavior of significant PFAS emissions from a point source and can be used to inform studies quantifying exposure and biological effects.

Figure 2. Emission and total deposition rates predicted by CMAQ-PFAS for the entire suite of emitted PFAS and of GenX. 4.8% of total PFAS and 2.6% of total GenX is predicted to deposit within ~150 km of the manufacturer. The rest is transported further into the U.S. and beyond.

Using Near-Field Exposure Modeling to Inform Ambient Air Emissions and Models

Researcher: Havala Pye

Posted: September 2020

Citation: Qin, M., Murphy, B.N., Isaacs, K.K., McDonald, B.C., Lu, Q., McKeen, S.A., Koval, L., Robinson, A.L., Efstathiou, C., Allen, C., Pye, H.O.T. Criteria pollutant impacts of volatile chemical products informed by near-field modeling. Nat Sustain (2020). https://doi.org/10.1038/s41893-020-00614-1

Read only link:  https://rdcu.be/b76hV

Paint Pots
Summary: Understanding the role of volatile chemical products (VCPs, see emerging applications of CMAQ) on air quality and health requires information on product usage, the fraction of product emitted to air, and the reaction pathways that result in ozone and secondary organic aerosol (SOA). In recent work by Qin et al., the High-Throughput Stochastic Human Exposure and Dose Simulation (SHEDS-HT) model was used to estimate product usage and the emissions of VOCs while the CMAQ model along with observational constraints from the CalNex field campaign were used to bound the role of VCPs in ozone and SOA formation. SHEDS-HT, literature, and top-down constraints imposed by ambient observations suggest that emissions of VOCs from VCPs are likely underestimated in the NEI for California and potentially across the United States. After remedying emission magnitudes and SOA formation potential, CMAQ indicates VCPs are responsible for ~41% of fresh SOA and ~17% of maximum daily 8-hr average ozone in summer Los Angeles. Inhalation was also estimated to be more competitive with dermal and ingestion exposure to organic compounds than previously thought.

Photolysis of Particulate Nitrate Oxidizes Sulfur Dioxide in the Atmosphere

Researcher: Golam Sarwar

Posted: August 2020

Citation: Zheng, H., Song, S., Sarwar, G., Gen, M., Wang, S., Ding, D., Chang, X., Zhang, S., Xing, J., Sun, Y., Ji, D., Chan, C. K., Gao, J., McElroy, M. B.: Contribution of particulate nitrate photolysis to heterogeneous sulfate formation for winter haze in China, Environ. Sci. Technol. Lett. 2020, publication Date: June 25, 2020, https://doi.org/10.1021/acs.estlett.0c00368

Enahncement of sulfate concentration at a particulate nitrate photolysis reate of 100 x the nitric acid photolysis rate. Note that the scale is not linear. 
Summary: Recent experimental studies suggest that particulate nitrate can photolyze and produce nitrous acid in aerosol liquid water which can subsequently oxidize sulfur dioxide into sulfate. In this study, we implemented a heterogeneous reaction in the Community Multiscale Air Quality (CMAQ) model to represent the conversion of sulfur dioxide into sulfate due to the photolysis of particulate nitrate and examined its impact on sulfate production in China. The current model without the chemistry substantially under-predicts observed atmospheric sulfate concentrations in China. Model results suggest that the reaction can increase sulfate production in China and the enhancement depends on the particulate nitrate photolysis rate used in the analysis. Reported photolysis rate of particulate nitrate vary by 1-3 orders of magnitude. The reaction can increase sulfate concentration by ~15% of the gap between the model prediction and observed concentration in winter when a particulate nitrate photolysis rate of 10 x the nitric acid photolysis rate is used. However, it increases sulfate concentration by ~65% of the gap between the model and observed concentration at a particulate nitrate photolysis rate of 100 x the nitric acid photolysis rate. Additional research is currently underway to examine the impact of the reaction on sulfate production over the Northern Hemisphere.

EPA Leads Study Synthesizing Information on Acidity of Atmospheric Particles and Clouds

Researcher: Havala Pye
Posted: May 2020

Citation: Pye, H. O. T., Nenes, A., Alexander, B., Ault, A. P., Barth, M. C., Clegg, S. L., Collett Jr., J. L., Fahey, K. M., Hennigan, C. J., Herrmann, H., Kanakidou, M., Kelly, J. T., Ku, I.-T., McNeill, V. F., Riemer, N., Schaefer, T., Shi, G., Tilgner, A., Walker, J. T., Wang, T., Weber, R., Xing, J., Zaveri, R. A., and Zuend, A.: The acidity of atmospheric particles and clouds, Atmos. Chem. Phys., 20, 4809–4888, https://doi.org/10.5194/acp-20-4809-2020, 2020.

Global distribution of fine particle and cloud droplet pH based on models and observations.

Summary: The acidity of atmospheric condensed phases, particles and clouds, has implications for particulate matter formation, deposition, and metal solubilization. Historical measurements indicate that cloud and fog droplet acidity has changed in recent decades in response to controls on emissions from human activity, while the limited trend data for suspended particles indicate acidity may be relatively constant. A new review by Pye et al. in Atmospheric Chemistry and Physics reflects the efforts of an international team to synthesize knowledge on the acidity of atmospheric particles and clouds. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale. 

As part of the activity, CMAQ as well as other models’ fine aerosol pHF (figure below) and cloudwater pHF values were presented. Since cloud and aerosol pH are an important property influencing a wide range of endpoints, improvements to how aerosol and cloud pH are represented in models could potentially enhance policy and programs informed by them. For locations with observationally constrained pH estimates, agreement between models and observations can be within 1 pH unit. However, the acceptable level of agreement required between models and observations depends on the target of a specific assessment (e.g., PM sensitivity to emissions, deposition of nutrients and acidity, metal solubility). Evaluating a model’s pH in the context of how an error in pH may propagate to an endpoint of interest could help uncover model biases that have been unidentified to date and increase the understanding of the effects of emissions, human activity, and climate change on society and the Earth system as a whole.

The article is included in the Encyclopedia of Geosciences, a collection of peer-reviewed scientific review articles on topics relevant to the geosciences, published in the open-access journals of the European Geosciences Union (EGU).

Particle pHF at the surface predicted by (a) CMAQv5.2 for Aitken + accumulation modes, (b) GEOS-Chem for bulk fine aerosol, (c) TM4-ECPL for PM1, and (d) TM4-ECPL for coarse aerosol.

EPA Study Shows Significant Ground-Level Ozone in the U.S. Mountain West Attributed to Lightning-Induced Nitrogen Oxide Emissions

Researcher: Daiwen Kang     
Posted February 2020

Citation: Kang, D., R. Mathur, G. A. Pouliot, R. C. Gilliam, and D. C. Wong, 2020: Significant ground-level ozone attributed to lightning-induced nitrogen oxides during summertime over the Mountain West States.  npj Climate and Atmospheric Science, 3, doi:10.1038/s41612-020-0108-2.

Summary: Stringent National Ambient Air Quality Standards are required to protect susceptible populations and improve public health. Accordingly, effective control measures on man-made contributions to air pollution must consider natural sources and their contributions to background pollution levels. Lightning-induced nitrogen oxides (LNOX) in the presence of sunlight, volatile organic compounds, and water vapor can be a relatively large but uncertain source of ozone (O3). Using lightning flash data to estimate LNOX emissions in the Community Multiscale Air Quality (CMAQ) model, EPA scientists demonstrated that typical summertime lightning activity across the U.S. Mountain West injects NOX emissions comparable to those from man-made sources over the region, contributing significantly to the amount of ground-level O3 in that region. This study was recently published in the Nature Partner Journal Climate and Atmospheric Science and highlights the growing need for comprehensive models, such as CMAQ, to quantify natural sources and their contributions to background O3.

Time series of the mean bias difference between the two simulations and the daily mean lightning flashes.

New Bidirectional Ammonia Flux Model in an Air Quality Model Coupled with an Agricultural Model

Researcher: Jonathan Pleim    
Posted October 2019

Citation: Pleim, J. E., Ran, L., Appel, W., Shephard, M. W., & Cady‐Pereira, K. (2019). New bidirectional ammonia flux model in an air quality model coupled with an agricultural model. Journal of Advances in Modeling Earth Systems, 11. https://doi.org/10.1029/2019MS001728

Summary:  Ammonia surface flux is bidirectional; that is, net flux can be either upward or downward. In fertilized agricultural croplands and grasslands there is usually more emission than deposition especially in midday during warmer seasons. In North America, most of the ammonia emissions are from agriculture with a significant fraction of that coming from fertilizer. In this study we developed a new bidirectional ammonia flux modeling system that has been implemented in the Community Multiscale Air Quality (CMAQ) model version 5.3, which has close linkages with the Environmental Policy Integrated Climate (EPIC) agricultural ecosystem model. Daily inputs from EPIC are used to calculate soil ammonia concentrations that are combined with air concentrations in CMAQ to calculate bidirectional surface flux. The model is evaluated against surface measurements of NH3 concentrations, NH4+ and SO42− aerosol concentrations, NH4+ wet deposition measurements, and satellite retrievals of NH3 concentrations. The evaluation shows significant improvement over the base model without bidirectional ammonia flux. Comparisons to monthly average satellite retrievals show similar spatial distribution with the highest ammonia concentrations in the Central Valley of California (CA), the Snake River valley in Idaho, and the western High Plains. In most areas the model underestimates, but in the Central Valley of CA, it generally overestimates ammonia concentration. Case study analyses indicate that modeled high fluxes of ammonia in CA are often caused by anomalous high soil ammonia loading from EPIC for particular crop types. While further improvements to parameterizations in EPIC and CMAQ are recommended, this system is a significant advance over previous ammonia bidirectional surface flux models.

Ammonia concentration from CMAQ on left and CrIS satellite retrievals on right averaged for June 2016

Anthropogenic combustion emissions of NOx facilitate oxidation and resulting PM2.5 from biogenic monoterpenes

Researcher: Havala Pye       
Posted: March 2019

Citation: Pye, H. O. T., D’Ambro, E., Lee, B., Schobesberger, S., Takeuchi, M., Zhao, Y., Lopez-Hilfiker, F., Liu, J., Shilling, J., Xing, J., Mathur, R., Middlebrook, A., Liao, J., Welti,A., Graus, M., Warneke, C., de Gouw, J., Holloway, J., Ryerson, T., Pollack, I., Thornton, J. A.: Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation. P Natl Acad Sci USA, https://doi.org/10.1073/pnas.1810774116, 2019.

Summary:  Recent work by Pye et al. (2019) shows that autoxidation pathways dictate the abundance and anthropogenic modulation of secondary organic aerosol (SOA) from monoterpenes. A key feature of the mechanistic SOA developed in the work is the low volatility of the particles, making them relatively insensitive to changes in primary organic aerosol or other organic aerosol sources via absorptive partitioning feedbacks. While reductions in NOx are predicted to lead to increasing autoxidation and potential for PM2.5, they also reduce oxidant abundance which can offset the changes in SOA yield such that NOx emission reductions have co-benefits for monoterpene SOA from first generation autoxidation pathways. Evidence for the mechanism was observed downwind of Atlanta in the summer of 2013 where autoxidation products were enhanced in the presence of elevated NO. The mechanism developed by Pye et al. indicates that about a quarter of the recent (1990-2010) decline in organic aerosol in the southeast US may be due to changes in monoterpene SOA.

Read more about monoterpene SOA development in CMAQ: Improving the representation of monoterpene chemistry leading to PM2.5 in CMAQ.

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Last updated on September 22, 2024