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

Increasing trends in biomass burning emissions significantly impact air quality in North America. Enhanced mixing ratios of ozone (O₃) in urban areas during smoke-impacted periods occur through transport of O₃ produced within the smoke or through mixing of pyrogenic volatile organic compounds (PVOCs) with urban nitrogen oxides (NO = NO + NO₂) to enhance local O₃ production. Here, we analyze a set of detailed chemical measurements, including carbon monoxide (CO), NO, and speciated volatile organic compounds (VOCs), to evaluate the effects of smoke transported from relatively local and long-range fires on O₃ measured at a site in Boulder, Colorado, during summer 2020. Relative to the smoke-free period, CO, background O₃, OH reactivity, and total VOCs increased during both the local and long-range smoke periods, but NO mixing ratios remained approximately constant. These observations are consistent with transport of PVOCs (comprised primarily of oxygenates) but not NO with the smoke and with the influence of O₃ produced within the smoke upwind of the urban area. Box-model calculations show that local O₃ production during all three periods was in the NO-sensitive regime. Consequently, this locally produced O₃ was similar in all three periods and was relatively insensitive to the increase in PVOCs. However, calculated NO sensitivities show that PVOCs substantially increase O₃ production in the transition and NO-saturated (VOC-sensitive) regimes. These results suggest that (1) O₃ produced during smoke transport is the main driver for O₃ increases in NO-sensitive urban areas and (2) smoke may cause an additional increase in local O₃ production in NO-saturated (VOC-sensitive) urban areas. Additional detailed VOC and NO measurements in smoke impacted urban areas are necessary to broadly quantify the effects of wildfire smoke on urban O₃ and develop effective mitigation strategies.