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

Increasing trends in biomass burning emissions significantly impact air quality in North America. Enhanced mixing ratios of ozone (O 3 ) in urban areas during smoke-impacted periods occur through transport of O 3 produced within the smoke or through mixing of pyrogenic volatile organic compounds (PVOCs) with urban nitrogen oxides (NO x = NO + NO 2 ) to enhance local O 3 production. Here, we analyze a set of detailed chemical measurements, including carbon monoxide (CO), NO x , and speciated volatile organic compounds (VOCs), to evaluate the effects of smoke transported from relatively local and long-range fires on O 3 measured at a site in Boulder, Colorado, during summer 2020. Relative to the smoke-free period, CO, background O 3 , OH reactivity, and total VOCs increased during both the local and long-range smoke periods, but NO x mixing ratios remained approximately constant. These observations are consistent with transport of PVOCs (comprised primarily of oxygenates) but not NO x with the smoke and with the influence of O 3 produced within the smoke upwind of the urban area. Box-model calculations show that local O 3 production during all three periods was in the NO x -sensitive regime. Consequently, this locally produced O 3 was similar in all three periods and was relatively insensitive to the increase in PVOCs. However, calculated NO x sensitivities show that PVOCs substantially increase O 3 production in the transition and NO x -saturated (VOC-sensitive) regimes. These results suggest that (1) O 3 produced during smoke transport is the main driver for O 3 increases in NO x -sensitive urban areas and (2) smoke may cause an additional increase in local O 3 production in NO x -saturated (VOC-sensitive) urban areas. Additional detailed VOC and NO x measurements in smoke impacted urban areas are necessary to broadly quantify the effects of wildfire smoke on urban O 3 and develop effective mitigation strategies.