The impact of wildfire smoke on ozone production in an urban area: Insights from field observations and photochemical box modeling

This study examines the effect of wildfire smoke on ozone (O-3) production at an urban site in Bakersfield, CA. We used data from smoky and non-smoky weekdays in summer 2018. Median surface observations across the smoky and non-smoky weekdays showed that morning and afternoon O-3 concentrations were mainly affected by local photochemistry. Observed daytime median concentrations of O-3, particulate matter with diameters less than 2.5 mu m (PM2.5), and carbon monoxide (CO) were approximately 8 parts per billion (ppb), 8 mu g per cubic meter (mu g m(-3)), and 40 ppb higher, respectively, on the smoky weekdays. The observed median sum of the daily-average concentrations of volatile organic compounds (Sigma VOCs) was approximately 10 ppb greater on the smoky weekdays. Measured daytime median NOx levels were almost identical on the smoky and non-smoky weekdays, indicating that the enhancement in NOx due to smoke was negligible. We used the Framework for 0-D Atmospheric Modeling (F0AM) box model to examine the photochemical processes on the smoky and non-smoky weekdays. The maximum model-predicted instantaneous O-3 production rates (PO3) were about 18 and 9 ppb h(-1) on the smoky and non-smoky weekdays, respectively. Model sensitivity tests showed that (1) O-3 was sensitive to both NOx and VOCs on the smoky weekdays, (2) aldehydes significantly affected O-3 formation when wildfire smoke was overhead, and (3) the O-3 production regime on the non-smoky weekdays was likely NOx- saturated. Our results suggest that a combination of anthropogenic VOC and NOx reductions will be the most effective strategy for decreasing O-3 on typical non-smoky days. In contrast, due to the high VOC levels in smoke plumes, only reductions in NOx are expected to have a significant effect on lowering O-3 concentrations on typical smoky days.