Assessing Compliance with Updated NAAQS PM2.5 Annual Standard: The Role of Regional and Transboundary Emissions

The recent revision of the annual National Ambient Air Quality Standard (NAAQS) for fine particulate matter (PM2.5) by the Environmental Protection Agency (EPA), from 12.0 µg/m³ to 9.0 µg/m³, highlights the urgency to address the persistent air pollution challenges that adversely affect public health across the United States. In response to this updated regulation, our study presents comprehensive analysis of both local and transboundary emission sources to assess the feasibility of adhering to this stricter limit. Our study uses the GEOS-Chem chemical transport model (version 14.3.1), to quantify PM_2.5 concentrations over North America (9.75-60° N, 60-130° W) with a horizontal resolution of 0.5° by 0.625°, driven by meteorological data from NASA’s Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2). Anthropogenic emissions data are sourced from the Community Emissions Data System (CEDS), overwritten by the 2016 National Emissions Inventory (NEI) from the EPA over the contiguous U.S. (CONUS), while biomass burning emissions are calculated with the Global Fire Assimilation System (GFAS) at a 0.1°×0.1° spatial resolution, with adjustments made for plume heights using emission-flux-weighted averages to ensure accurate representation in coarser resolution simulations. We perform simulations for the years 2021 and 2022 —years characterized by different levels of wildfire activity within CONUS—after a one-month initialization period. Four distinct GEOS-Chem simulation sets were conducted: a baseline reflecting current emission conditions (BASE), and three sensitivity scenarios—excluding anthropogenic emissions (AnthroOff), fire emissions (FireOff), and both (Least emission scenario). To evaluate the simulation, we compare the BASE simulation annual mean surface PM_2.5 concentrations with surface measurements from EPA within the study period (https://www.epa.gov/outdoor-air-quality-data/). We also present the modeled grid box exceedances of the updated 9 μg/m³ limit in each model grid box under each of the above-mentioned sensitivity scenarios. Our analysis focuses on assessing the emission sources contributing to PM_2.5 exceedances over various regions, highlighting challenges from both anthropogenic sources and natural phenomena such as wildfires, which have escalated in frequency and intensity recently due to climate change. We also investigate the impact of transboundary pollution, particularly from Canadian wildfires, which further complicates adherence to the new tightened PM_2.5 standard. In conclusion, our study provides insights into the complex dynamics between natural and anthropogenic sources affecting air quality standards and assesses the potential efficacy of various emission control scenarios. This evaluation offers policymakers, researchers, and public health officials with essential data to formulate effective strategies for improving air quality and safeguarding public health, thereby addressing the complex challenges of meeting stringent air quality regulations in a changing climate.