Dominance of Wildfires Impact on Air Quality Exceedances During the 2020 Record‐Breaking Wildfire Season in the United States

Abstract

Biomass burning (BB) emits a large quantity of aerosols and trace gases into the atmosphere, often leading to hazardous air quality and health problems (Koning, et al., 1985). In the summer of 2020, the western United States experienced a record-breaking wildfire season. A series of large wildfires, fueled by accumulated biomass, heatwaves, and dry winds, burned more than 10.2 million acres. These wildfires spread rapidly and destroyed several small towns in California, Oregon, and Washington. According to MODIS (Moderate Resolution Imaging Spectroradiometer) measured FRP (fire radiative power) from 2002 to 2020 (Figure 1a), the monthly total FRP in September 2020 (big red dot) over the contiguous United States (CONUS) is the highest in over the past 19 years and is more than twice as large as the second highest. Dense wildfire smoke also produced hazardous air quality that affected millions of people in major cities for weeks. Based on Suomi NPP VIIRS (Visible Infrared Imaging Radiometer Suit) 550 nm aerosol optical depth (AOD) measurements, the fire smoke was transported across the continent to the eastern U. S. coast via the westerlies in the middle of September (Figure 1b).Previous studies (Cascio, 2018;C. E. Reid et al., 2016) demonstrated that a strong association exists between exposure to wildfire smoke and all-cause mortality and respiratory morbidity. Strong positive associations

Influence of Wildfire on Urban Ozone: An Observationally Constrained Box Modeling Study at a Site in the Colorado Front Range

“…The identification of long-range smoke is supported by HRRR-Smoke simulations, which show that smoke during this period largely resulted from fires in Northern California (∼1500 km from Boulder, Figure S5). California experienced a historic fire season in late 2020, 16 notably including the August Complex fire that burned over 1 million acres. While NO x remained relatively constant through each period, regional O 3 and CO mixing ratios increased during the local smoke period followed by larger increases during the long-range transported smoke period.…”

“…A statistical analysis of fires between 1984 and 2018 indicate a doubling of fire events within the last half of this period and a quadrupling of the median annual area burned for the western United States . These trends have significantly impacted air quality in the last two decades, for example with well-documented increases in particulate matter. − …”

Daily Local-Level Estimates of Ambient Wildfire Smoke PM2.5 for the Contiguous US

“…Atmospheric chemical transport models (CTMs) offer one common approach to linking wildfire activity to concentrations of key pollutants and downstream human impacts, − but their use faces a number of difficulties . First, uncertainties in wildfire emission inventories have been shown to lead to substantial differences in CTM-estimated smoke PM 2.5 concentrations, with up to 20-fold differences in estimated regional wildfire-attributed PM 2.5 concentrations when different leading inventories are fed into the same transport model, although recent efforts have shown that satellite-based emission calibration can improve the simulation accuracy . Second, the complex chemistry regimes during wildfire (in particular, production of organic aerosols) and hyperlocalized meteorology are often not well-captured by CTMs, leading to concerns over their ability to correctly model the transport of wildfire smoke. − Third, parameterized heights of plume injections and the vertical allocations of emissions differ considerably across different CTMs and often compare poorly with observed injection heights, leading to discrepancies in surface PM 2.5 concentrations when compared with in situ observations. , …”

Science-based Policy Recommendations for Fine Particulate Matter in the United States

“…Wildland fires are responsible for an increasing number of days exceeding NAAQS levels across the US at PM 2.5 monitor locations ( 11 ). The majority of PM-related health impacts in the U.S. may be attributable to wildland fires in future years due to continued reductions in anthropogenic emissions combined with climate-related increases in wildland fire emissions ( 12 ).…”