O3 photochemistry in the western US: mixing wildfire smoke with urban emissions
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Ozone (O3) is a respiratory irritant and is hazardous to human health. O3 pollution is prevalent in urban areas but is also influenced by the transport of wildfire smoke. When wildfire smoke mixes with urban emissions, observed impacts on O3 are highly variable and not well understood. In this work, I investigate O3 impacts from smoke in 18 western US cities during late summer (July to September) from 2013 to 2017. To do so, I use ground-based monitoring data for O3, NOx (nitrogen oxides), and PM2.5 (particulate matter < 2.5 Âµm in diameter) from Air Quality System sites and satellite-based hazard mapping system (HMS) fire and smoke product to identify overhead smoke. I present three key findings. First, O3 and PM2.5 are elevated at nearly all sites on days influenced by smoke, with the greatest mean enhancement occurring during multiday smoke events; nitrogen oxides (NOx) are not consistently elevated across sites. Second, PM2.5 and O3 exhibit a nonlinear relationship such that O3 increases with PM2.5 at low to moderate 24-h PM2.5, peaks around 30-50 μg m-3, and declines at higher PM2.5. Third, the rate of increase of morning O3 is higher and NO/NO2 ratios are lower on smoke-influenced days, which could result from additional atmospheric oxidants in smoke. In addition, I note that while the HMS product is a useful tool for identifying smoke, O3 and PM2.5 are elevated on days before and after HMS-identified smoke events implying that a significant fraction of smoke events is not detected. These findings motivate further research, particularly with high quality NOx and photolysis measurements. I conclude with a brief description of these measurements taken at Mount Bachelor Observatory during the 2019 FIREX-AQ campaign, which provide a rich dataset to explore the complexities of O3 photochemistry in smoke.
- Atmospheric sciences