Investigating High O3 Days at the Boulder Atmospheric Observatory in Summer 2015

The Northern Colorado Front Range is currently in nonattainment of the U.S. EPA National Ambient Air Quality Standard (NAAQS) for ozone (O3). Significant recent research efforts have been devoted to investigating the underlying causes of the high O3 abundances observed in this region. A growing population and a recent boom in oil and natural gas production within the Denver-Julesberg Basin have contributed to increased anthropogenic emissions of many O3 precursors. Better understanding the contributions of emissions from different sectors in the Front Range to O3 production can help inform more effective control strategies. Here, we aim to use measurements of O3 and a suite of O3 precursors made at the Boulder Atmospheric Observatory (BAO) to investigate the causes of high O3 abundances in the Northern Colorado Front Range. Measurements spanned 6 weeks during summer 2015 (3 additional weeks of measurements were impacted by wildfire smoke) and included O3, CO, CH4, 40+ volatile organic compounds (VOCs), NO, NO2, NOy, nitric acid (HNO3), peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and methacryloyl peroxynitrate (MPAN). We define a "high O3 day" as any day in which the maximum hourly average O3 mixing ratio was greater than the 95th percentile of all daytime (10am - 4pm MDT) hourly average O3 mixing ratios in the study period. We find that high O3 days at BAO are coincident with high O3 days at routine monitoring sites throughout the Front Range. We observe a positive relationship between O3 and the calculated OH reactivity attributable to species associated with oil and natural gas production. We also find that tracers of photochemistry such as acyl peroxy nitrates (APNs) are closely correlated with O3 on high O3 days. High abundances of PPN with respect to PAN on high O3 days suggest that anthropogenic emissions of O3 precursors play a dominant role in photochemistry at BAO. We also compare high and low O3 days with respect to estimated O3 production efficiency (OPE).