Effects of Biomass Burning Emissions on Air Quality Over the Continental USA: A Three-Year Comprehensive Evaluation Accounting for Sensitivities Due to Boundary Conditions and Plume Rise Height

Abstract

We report a comprehensive evaluation of the impacts of biomass burning on regional ozone and fine particulate matter (PM2.5) over the continental USA, southern Canada, and northern Mexico during 2012–2014 using the Community Multiscale Air Quality (CMAQ) chemical transport model. Inputs included the Fire INventory from National Center for Atmospheric Research (FINN) for fire emissions, Biogenic Emission Inventory System (BEIS) for biogenics, the US Environmental Protection Agency (USEPA)’s National Emissions Inventory of 2011 (NEI2011) for anthropogenic sources, and Weather Research and Forecasting (WRF) model fields for meteorology. In situ data were taken from the Texas Commission on Environmental Quality (TCEQ)’s Continuous Ambient Monitoring Stations (CAMS) and the USEPA’s Air Quality System (AQS) networks. This study has marked improvements over the previous biomass burning evaluations, which are as follows: (a) a significantly longer simulation episode; (b) use of 3-D dynamic boundary conditions; (c) grid nudging to improve meteorological fields; and (d) physically representative fire plume rise model. Observations showed ozone hot spots of 60–70 parts per billion (ppb) across the Western Mountain region and California. The model was able to reproduce these only in 2012, underpredicting in California otherwise. Monthly mean biomass impacts of 2–3 ppb, averaged over daylight hours (6:00–18:00 CST), were predicted for California and Idaho in 2012 and 2013. The largest impacts were predicted for summer 2013, adding 3 ppb in northern Mexico and southeastern Canada, and 1 ppb in Florida, New Mexico, and Colorado. For April 2014, the model predicted 1–2 ppb disparities in ozone over the southern USA; a 1–2 ppb impact in southeastern Oregon, northwestern Nevada, and southern Idaho during July 2014; and in August, up to 3 ppb changes in western California, Central Oregon, Idaho, southwestern Canada, and southern Georgia. The model was unable to accurately capture the high PM2.5 concentrations across the domain. Large monthly mean fire impacts of up to 10 μg m−3 were predicted for southeastern Canada in July 2012 and June and July 2013, and for Alabama, Georgia, Idaho, and southwestern Canada for October 2013. In June 2014, the model significantly underpredicted when the biomass impact was minimal, indicating that uncertainty in biomass emissions was not the probable cause for model-measurement error.