Understanding summertime peroxyacetyl nitrate (PAN) formation and its relation to aerosol pollution: insights from high-resolution measurements and modeling

Abstract

Peroxyacetyl nitrate (PAN), a key indicator of photochemical pollution, is generated similarly to ozone (O3), through reactions involving specific volatile organic compounds (VOCs) and nitrogen oxides. Notably, PAN has been observed at unexpectedly high concentrations (maximum: 3.04 ppb) during the summertime. The average daily values of PAN show a strong correlation with black carbon (BC) (R = 0.77) and O3 (R = 0.77), suggesting a close connection between summertime haze and photochemical pollution. We addressed the puzzle of summertime PAN formation and its association with aerosol pollution under high-O3 conditions in Xiamen, a coastal city in southeastern China, by analyzing continuous high-temporal-resolution data utilizing box modeling in conjunction with the Master Chemical Mechanism (MCM) model. The MCM model, with an index of agreement (IOA) value of 0.75, effectively investigates PAN formation, performing better during the clean period (R2: 0.68; slope K: 0.91) than the haze one (R2: 0.47; slope K: 0.75). Using eXtreme Gradient Boosting (XGBoost), we identified NH3, NO3-, and PM2.5 as the primary factors for simulation bias. Moreover, the net production rate of PAN becomes negative with PAN constrained, suggesting an unknown compensatory mechanism. Both relative incremental reactivity (RIR) and empirical kinetic modeling approach (EKMA) analyses indicate that PAN formation is VOC-controlled. Controlling emissions of VOCs, particularly alkenes, C5H8, and aromatics, would mitigate PAN pollution. PAN promotes OH and HO2 while inhibiting the formation of O3, RO2, NO, and NO2. This study deepens our comprehension of PAN photochemistry while also offering scientific insights for guiding future PAN pollution control strategies.