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 through a process similar to ozone (O3), involving the photochemical reactions of specific volatile organic compounds (VOCs) in the presence of nitrogen oxides. Notably, PAN has been observed at unexpectedly high concentrations (maximum: 3.04 ppb) during summertime that the daily maximum values of PAN were better correlated to black carbon (BC) (R2=0.85) than ozone (O3) (R2=0.75), suggesting that summertime haze and photochemical pollution were deeply connected. We addressed the puzzle of summertime PAN formation and its association with aerosol pollution under high ozone conditions by analyzing continuous high temporal resolution data utilizing box modeling in conjunction with the master chemical mechanism (MCM). With an IOA value of 0.75, the MCM model proves to be an ideal tool for investigating PAN photochemical formation. The model performed better during the clean period (R2: 0.6782, slope K: 0.9097) than during the haze period (R2: 0.4708, slope K: 0.7477). Through the machine learning method of XGBoost, we found that the top three factors leading to simulation bias were NH3, NO3, and PM2.5. Moreover, the net production rate of PAN becomes negative with PAN constrained, suggesting the existence of an unknown compensatory mechanism. Both RIR and EKMA analyses indicate that PAN formation in this region is VOC-controlled. Controlling emissions of VOCs, particularly alkenes, C5H8, and aromatics, would mitigate PAN pollution. RIR results also show that during the clean period, PAN is more sensitive to changes in various pollutants than during the haze period, underscoring the importance of deep emission reductions. 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.