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

Abstract This study examines the effect of wildfire smoke on ozone (O3) 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 O3 concentrations were mainly affected by local photochemistry. Observed daytime median concentrations of O3, particulate matter with diameters less than 2.5 μm (PM2.5), and carbon monoxide (CO) were approximately 8 parts per billion (ppb), 8 micrograms per cubic meter (μg m-3), and 40 ppb higher, respectively, on the smoky weekdays. The observed median sum of the daily-average concentrations of volatile organic compounds (ΣVOCs) 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 O3 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) O3 was sensitive to both NOx and VOCs on the smoky weekdays, (2) aldehydes significantly affected O3 formation when wildfire smoke was overhead, and (3) the O3 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 O3 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 O3 concentrations on typical smoky days.