Pollution characteristics and potential sources of Peroxyacetyl Nitrate in a petrochemical industrialized City, Northwest China

Peroxyacetyl Nitrate (CH₃C(O)O₂NO₂, PAN), a typical secondary product of photochemical reactions, is well known to be a better photochemical indicator due to the only secondary photochemical source in the troposphere. Studies on PAN pollution are sparse in northwest China, resulting in a limited understanding of photochemical pollution in recent years. Herein, the measurement of PAN, O₃, volatile organic compounds (VOCs), NO_x, other related species, and meteorological parameters were conducted from May 1 to August 31, 2022, at an urban site in Lanzhou. The mean and maximum hourly PAN concentrations were 0.55 ± 0.30 ppb (mean ± standard deviation) and 4.06 ppb, respectively. The key factors affecting PAN pollution included the precursors, temperature, and atmospheric oxidation capacity, etc. The positive correlations (episodes: R = 0.52; non- episodes: R = 0.55) between PAN and O₃ suggested that local photochemistry played a dominant role in enhancing PAN and O₃ formation. According the observation-based model, VOCs dominated the photochemical formation of PAN in Lanzhou. The key first-generation precursors were identified as alkenes and alkanes on PAN episodes, while alkenes and aromatics on non-episodes. The oxidation of acetaldehyde, photolysis and oxidation of methylglyoxal, as well as radical recycling, were the primary formation pathways of peroxyacetyl radicals (CH₃C(O)OO•, PA). Furthermore, the double-peak pattern of PAN diurnal variation was first explored in Lanzhou. The modeling results suggested that the chemical "double-peak" formation of PAN was associated with sufficient precursors and strong atmospheric oxidation capacity. Our study could provide some new insights into the formation mechanism of PAN for petrochemical industrialized city in China.