Observations of ozone, acyl peroxy nitrates, and their precursors during summer 2019 at Carlsbad Caverns National Park, New Mexico.

Abstract

Carlsbad Caverns National Park (CAVE) is located in southeastern New Mexico and is adjacent to the Permian Basin, one of the most productive oil and natural gas (O&G) production regions in the United States. Since 2018, ozone (O₃) at CAVE has frequently exceeded the 70 ppbv 8-hour National Ambient Air Quality Standard. We examine the influence of regional emissions on O₃ formation using observations of O₃, nitrogen oxides (NO_x = NO + NO₂), a suite of volatile organic compounds (VOCs), peroxyacetyl nitrate (PAN), and peroxypropionyl nitrate (PPN). Elevated O₃ and its precursors are observed when the wind is from the southeast, the direction of the Permian Basin. We identify 13 days during the July 25 to September 5, 2019 study period when the maximum daily 8-hour average (MDA8) O₃ exceeded 65 ppbv; MDA8 O₃ exceeded 70 ppbv on 5 of these days. The results of a positive matrix factorization (PMF) analysis are used to identify and attribute source contributions of VOCs and NO_x. On days when the winds are from the southeast, there are larger contributions from factors associated with primary O&G emissions; and, on high O₃ days, there is more contribution from factors associated with secondary photochemical processing of O&G emissions. The observed ratio of VOCs to NO_x is consistently high throughout the study period, consistent with NO_x-limited O₃ production. Finally, all high O₃ days coincide with elevated acyl peroxy nitrate abundances with PPN to PAN ratios > 0.15 ppbv ppbv^-1 indicating that anthropogenic VOC precursors, and often alkanes specifically, dominate the photochemistry.Implications: The results above strongly indicate NO_x-sensitive photochemistry at Carlsbad Caverns National Park indicating that reductions in NO_x emissions should drive reductions in O₃. However, the NO_x-sensitivity is largely driven by emissions of NO_x into a VOC-rich environment, and a high PPN:PAN ratio and its relationship to O₃ indicate substantial influence from alkanes in the regional photochemistry. Thus, simultaneous reductions in emissions of NO_x and non-methane VOCs from the oil and gas sector should be considered for reducing O₃ at Carlsbad Caverns National Park. Reductions in non-methane VOCs will have the added benefit of reducing formation of other secondary pollutants and air toxics.