Exploring the Influence of Oil and Gas Emissions on Photochemistry in the Northern Colorado Front Range During Summer 2015

The Colorado northern Front Range exceeds the National Ambient Air Quality Standard (NAAQS) for ozone (O₃). This region is experiencing strong population growth, and has substantial volatile organic compound (VOC) emissions from the production of oil and natural gas. We measured a suite of oxidized nitrogen (NO_y) species at the Boulder Atmospheric Observatory (BAO) in summer 2015 including NO, NO₂, peroxyacetyl nitrates (PANs), nitric acid (HNO₃), and short-chain alkyl nitrates. Co-located hourly VOC measurements included C₂-C₈ non-methane hydrocarbons (NMHCs), including the alkyl nitrate parent hydrocarbons, C₁-C₂ halocarbons, and several oxygenated species (methyl ethyl ketone, acetone, acetaldehyde). Using Positive Matrix Factorization (PMF) we have identified five distinct VOC mixtures. Using the source factors generated from the PMF analysis we calculated the VOC reactivity (VOCr) contributions of each source. The oil and natural gas factor dominates VOCr in the mornings. The biogenic factor, dominated by isoprene, makes a major contribution on certain afternoons. We find that the NO_y budget is largely closed during the night and morning, but a substantial un-speciated NO_y component develops after sunrise and peaks near noon. We attribute this missing NOy to un-quantified organic nitrates. We use the data to calculate a range of branching ratios for organic nitrate formation. Finally we present observationally based estimates of O₃ production efficiency for several periods of enhanced O₃ in summer 2015.