Research Needed to Improve Air Quality Modeling: Recommendations from a 2022 Workshop

The Coordinating Research Council (CRC) and the California Air Resources Board (CARB) co-sponsored an Air Quality Research Needs (AQRN) workshop that was held on November 8 and 9, 2022 at the University of California, Davis. The workshop attracted researchers from academia, federal and state agencies, industry, and other stakeholders and included over 60 attendees.

Many advances have been made over the years in physics and chemistry in air quality models (AQMs) that also take advantage of advances in computational resources; however, major gaps and uncertainties remain. Therefore, stakeholders were brought together to reflect on the status of AQMs and brainstorm on future research needs to further improve the models to meet the ever-growing needs and the evolving research drivers. A previous AQRN workshop was held in 2016¹. This article provides an overview of the workshop and summarizes the prioritized key research issues and future research needs.

WORKSHOP OBJECTIVES AND ORGANIZATION

The objective of the workshop was to brainstorm on key research issues related to future air quality model development needs and identify specific research needs. The workshop was organized around four themes to facilitate discussions: emissions, meteorology, chemistry, and regional and local air quality modeling.

Three speakers for each workshop theme provided an overview of the current state of science, research gaps, and initial thoughts on future research needs. This was followed by a plenary brainstorming session to identify and discuss big picture issues/needs. The breakout sessions identified specific research issues under each theme and attendees voted for the top five research issues for each theme. The breakout session participants also discussed specific research needs to address the research issues that were identified. At a final plenary session, the results of the breakout sessions were presented, discussed and voted on to determine the overall top ten research issues.

TOP RESEARCH ISSUES/NEEDS

The overall top ten research issues that were identified are shown in Table 1, and details of each of these issues are provided below.

Table 1. Top Ten Overall Research Issues Identified from the 2022 AQRN Workshop.

Issue #	Research Issue	Theme
1	Multi-component Satellite / Ground Data Assimilation	Modeling
2	Emissions Inventories on Environmental Justice-Relevant Scales	Emissions
3	Coupling + Feedbacks between Air Pollution and Meteorology	Modeling
4	Chemistry of Today and Tomorrow	Chemistry
5	Consistent Implementation of Planetary Boundary Layer (PBL) Processes in Air Quality and Meteorological Models	Meteorology
6	Measure and Speciate N-containing and Particulate Matter (PM) Emissions from Fuel Combustion	Emissions
7	Organic Aerosol (OA) Temperature Dependence	Chemistry
8	Urban Biogenic Emissions	Emissions
8 (tie)	Gas-Phase Mechanism Evaluation	Chemistry
10	Characterization of Global Scale Processes that Act as Boundary Conditions for Cities	Modeling

Research Issue 1: Multi-component Satellite / Ground Data Assimilation

AQMs have limitations in accurately predicting pollutant concentrations². Data assimilation methods that merge satellite measurements, ground-based measurements, and model predictions improve predictions of various components of air pollution³. The key research needs identified are:

• Develop methods that combine measurement data into continuous fields while still respecting conservation of mass, and chemical kinetic/thermodynamic principles;

• “Big data” to test new methods for data integration;

• Intercomparison studies to identify the most promising assimilation methods.

Research Issue 2: Emissions Inventories on Environmental Justice Relevant Scales

Environmental Justice (EJ) communities often experience degraded air quality, but our current inventories don’t have the necessary spatial scales, chemical and sectoral/compositional detail to support needed resolution for modeling in the EJ communities. The key research needs identified are:

• Reconcile spatial scales of existing inventories for EJ communities and those used for regional and regulatory modeling;

• Compare top-down versus bottom-up inventories on EJ relevant spatial scales;

• Develop appropriate surrogates for leveraging inventories from local scales to improve larger scale inventories;

• Develop spatial distribution of electric vehicles activity and future trends.

Research Issue 3: Coupling + Feedbacks between Air Pollution and Meteorology

The highest air pollution exposure events in the western US are now almost exclusively driven by wildfires⁴. AQMs struggle to predict concentrations on the scales needed to represent complex behavior of fire plumes, therefore we need to improve the fundamental abilities of coupled meteorology-air pollution models to predict fire plume behavior. The key research needs identified are:

• New coupled algorithms within weather models that are computationally efficient;

• Multiscale models with adaptive grids or meshless techniques.

Research Issue 4: Chemistry of Today and Tomorrow

Challenges faced today in understanding organic aerosol (OA) will become even greater as emissions sources evolve. Current understanding of OA is shaped by chamber experiments; however, the chamber data that are most widely used to underpin air quality models are not adequately reflective of environmental conditions and sources⁵. The key research needs for more relevant data are:

• Laboratory experiments over a wider range of conditions that improve overlap with current and future atmospheric conditions;

• Identify methods to best extrapolate from chamber conditions to the atmosphere, i.e., models;

• Further expand coverage of OA precursor emissions from traditional internal combustion engines to new fuels, volatile chemical products (VCPs), cooking, asphalt, biomass burning, and more-diverse biogenic volatile organic compounds (VOCs).

Research Issue 5: Consistent Implementation of Planetary Boundary Layer (PBL) Processes in Air Quality and Meteorological Models

Some AQMs employ PBL schemes that differ from the meteorological model, which can produce unintended errors in the air quality simulation. In meteorological models, PBL processes are constrained by continuous implicit feedback between wind and temperature profiles and turbulent parameters⁶. Without a similar constraint in the AQMs, the transport of chemical species should be applied identically to the meteorological model. The key research needs identified are:

• Develop and test new approaches such as: including the chemical species scalars in the same code responsible for meteorological PBL processes and the AQM for on-line integrated meteorology-chemistry models; and using the same PBL module in the meteorological model and the AQM for coupled models;

• For PBL models that use non-local gradient adjustment schemes or higher-order closure (i.e., 2^rd order or higher), either develop methods to include chemical tracers or assess the error incurred by approximate solutions.

• Deployment of a nation-wide, high vertical resolution, profiling network with hourly frequency for both meteorology and chemical measurements for use in model evaluation.

Research Issue 6: Measure and Speciate N-containing and PM Emissions from Fuel Combustion

Large uncertainties in NO_x and PM emissions from many in-use combustion sources exist⁷. Emissions from sources such as off-road mobile, residential equipment, and transportation with locomotive, marine, and aircraft are not well understood. This is especially true for Ammonia (NH₃) emissions from NOx control devices, which are likely to increase and are poorly quantified. The key research needs identified are:

• Speciated N-containing (Nitric Oxide or NO, Nitrogen Dioxide or NO₂, Nitrous Oxide or N₂O) and PM emissions from in-use on-road, non-road, and other fuel combustion (e.g., residential);

• Determine NO/NO₂ emission ratios and NH₃ emission factors for different combustion sources;

• Improve non-road locomotive, marine, and aircraft emissions data;

• Determine extended idling emissions and locations, fleet composition, and defeat device presence of trucks;

• Determine deterioration factors for any fuel combustion sources.

Research Issue 7: OA Temperature Dependence

Temperature strongly influences atmospheric OA concentrations, as observations show summertime PM_2.5 increases with temperature⁸. Understanding this temperature dependence and evaluating options for controlling PM_2.5 on the hottest days are aspects of aerosol chemistry that test our understanding of process and mechanisms in ways that are different from examination of average concentrations. The key research needs identified are:

• Conduct experimental studies that characterize OA temperature dependence for relevant precursors;

• Conduct ambient measurements that characterize temperature dependence of Secondary OA (SOA) together with temperature dependence of precursor emissions;

• Compare observed OA temperature dependence against AQMs.

Research Issue 8: Urban Biogenic Emissions

Urban vegetation can be a major source of VOCs and a significant contributor to ozone and secondary organic aerosol formation⁹. Lack of appropriate detail in urban land cover classifications and emission factors can cause uncertainty in urban biogenic emissions. In some cities, such as New York City, biogenic VOCs can be the main VOCs related to ozone formation. The key research needs identified are:

• Update urban speciated biogenic emission inventories;

• Better parameterization of urban specific emission factors related to stress such as drought and forest edge effects;

• Comparison of the emission parameterization of urban vegetation with natural forest or grassland ecosystems;

• Improve land cover classification and biomass density maps by leveraging existing and upcoming VPRM (Vegetation Photosynthesis and Respiration Model) inputs for CO₂ respiration.

Research Issue 8 (tied): Gas-Phase Mechanism Evaluation

Gas-phase reactions are central to understanding atmospheric oxidants, PM, and many toxic air contaminants¹⁰. There is an ongoing need for chemical mechanism maintenance and evaluation, as new information emerges, to provide confidence that air quality management strategies are well-grounded. The key research needs identified are:

• Conduct gas-phase mechanism evaluation that considers the speciation of odd-hydrogen (Hox) and Noy (sum of NOx and all oxidized atmospheric odd-nitrogen species), the amount of NOx recycled from NOy, sensitivity of ozone production to VOC/NOx ratio, and other factors such as influence of temperature;

• Investigate how uncertainties in mechanism parameters influence overall mechanism uncertainty and develop alternate mechanism realizations (e.g., “hot” and “cold” versions of the same mechanism) that support “ensemble” applications of air quality models.

Research Issue 10: Characterization of Global Scale Processes that Act as Boundary Conditions for Cities

As emissions in the US continue to decline, “background” concentrations account for an increasing fraction of the urban air pollution¹¹. Understanding the trends in future concentrations for background pollutants will be critical in planning attainment strategies. Previous studies suggest background concentrations for ozone in the U.S. range between 30 – 60 ppb depending on the location, but there can be large uncertainty in these estimates. The key research needs identified are:

• Use global scale models that use the same chemical mechanisms as regional scale models;

• Evaluate global scale models to reproduce historical trends in background concentrations, identify critical sources/mechanisms, and project those into the future.

Conclusion

The list of air quality research issues and research needs identified as part of the 2022 AQRN workshop can serve as a guide to research organizations when prioritizing their own research activities. Given limited resources available for research, we encourage different organizations to collaborate to leverage their resources to address issues that may require significant funding (e.g., any research that requires large field/laboratory experiments). The list of issues and research needs can also serve as a guide to researchers who are looking to collaborate with others on these high priority topics. Many research projects could address multiple issues simultaneously.