Aged Wildfire Smoke and Summertime Photochemistry in the Northern Colorado Front Range

Western U.S. wildfires are major sources of ozone (O₃) precursors and aerosols. The  relative importance of wildfire smoke for air quality over the western U.S. is expected to increase as the climate warms and anthropogenic emissions continue to decline.  We report on field measurements of O₃, a suite of volatile organic compounds (VOCs), and reactive nitrogen species (NO, NO₂, peroxyacyl nitrates (PANs), nitric acid (HNO₃), and short chain alkyl nitrates) collected during July and August 2015 at the Boulder Atmospheric Observatory (BAO).  We observed aged wildfire smoke during two distinct time periods from 7-10 July and 16 – 30 August. The smoke was transported from the Pacific Northwest and Canada across much of the continental U.S. Carbon monoxide (CO) and PM_2.5 showed clear increases during the smoke-impacted periods, along with PANs and VOCs with atmospheric lifetimes longer than the transport timescale (e.g., ethyne and benzene).  During the August smoke event, NO₂ was also elevated during morning and evening hours as compared to the smoke-free time periods, potentially due to changes in nighttime nitrogen chemistry. There were a total of ten days during summer 2015 where the calculated maximum 8-hour average O₃ at BAO exceeded 70 ppbv, and four of these days were smoke-impacted. We examined the relationship between O₃ and temperature at BAO and found that, for a given temperature, O₃ was elevated by more than 5-10 ppbv during smoke-impacted time periods as compared to smoke-free time periods. This was also observed in O₃ and temperature data from 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 influences atmospheric composition in an urban site, and how smoke can contribute to increased O₃ abundances across an urban-rural gradient.