Changes in ozone and precursors during two aged wildfire smoke events in the Colorado Front Range in summer 2015

Abstract. The relative importance of wildfire smoke for air quality over the western US is expected to increase as the climate warms and anthropogenic emissions decline. We report on in situ measurements of ozone (O₃), a suite of volatile organic compounds (VOCs), and reactive oxidized nitrogen species collected during summer 2015 at the Boulder Atmospheric Observatory (BAO) in Erie, CO. Aged wildfire smoke impacted BAO during two distinct time periods during summer 2015: 6–10 July and 16–30 August. The smoke was transported from the Pacific Northwest and Canada across much of the continental US. Carbon monoxide and particulate matter increased during the smoke-impacted periods, along with peroxyacyl nitrates and several VOCs that have atmospheric lifetimes longer than the transport timescale of the smoke. During the August smoke-impacted period, nitrogen dioxide was also elevated during the morning and evening compared to the smoke-free periods. There were nine empirically defined high-O₃ days during our study period at BAO, and two of these days were smoke impacted. We examined the relationship between O₃ and temperature at BAO and found that for a given temperature, O₃ mixing ratios were greater (∼ 10 ppbv) during the smoke-impacted periods. Enhancements in O₃ during the August smoke-impacted period were also observed at two long-term monitoring sites in Colorado: Rocky Mountain National Park and the Arapahoe National Wildlife Refuge near Walden, CO. Our data provide a new case study of how aged wildfire smoke can influence atmospheric composition at an urban site, and how smoke can contribute to increased O₃ abundances across an urban–rural gradient.