Air quality in the U.S. during the 2023 wildfire season

During the summer of 2023, Canada was devastated by a series of large and uncontrolled wildfires that set the unprecedented record of over 40 million acres burned and over 120,000 people evacuated. The smoke from the Canadian wildfires reached the U.S. and negatively affected the air quality in several states. The goal of this paper is to document the U.S. air quality in 2023 within the historical and regulatory contexts; more specifically, to compare the concentrations of fine particulate matter (PM2.5) and tropospheric ozone (O3), assessed using statistics as close as possible to those recommended for comparison against the National Ambient Air Quality Standards (NAAQS), in 2023 against those in the past 20 years. We report that the year 2023 was an anomaly, meaning that the well-established trend of decreasing concentrations and improving air quality was suddenly reversed in 2023, with national-median PM2.5 and O3 concentrations exceeding \SI{33}{\micro\gram\per\cubic\metre} and 75 parts per billion (ppb), respectively. These values had not been reached since 2007 and 2012 for PM2.5 and O3, respectively. Notably, the 75th percentile for PM2.5 was the highest and the 90th percentiles was the third highest in 2023 since 2000. Out of 50 states (plus the District of Columbia), 34 and 49 experienced summer-average concentrations in 2023 that were higher than those in 2013--2022 for PM2.5 and O3, respectively. For PM2.5, the states that experienced the highest median concentrations were: California, Alaska, Oregon, and Idaho with 95, 64, 52, and \SI{50}{\micro\gram\per\cubic\metre}; for O3, they were: California, Maryland, Texas, Connecticut, and Pennsylvania with 114, 88, 88, 86, and 85 ppb. In summary, in the year 2023 the air quality in the U.S. was the worst of the past 15 and 10 years in terms of PM2.5 and O3, respectively.