Influence of thermal decomposition and regional transport on atmospheric peroxyacetyl nitrate (PAN) observed in a megacity in southern China

Abstract

Peroxyacetyl nitrate (PAN) is a typical secondary product of photochemical smog and is widely considered an indicator when evaluating the severity of photochemical pollution, but there have been few quantitative studies on the thermal decomposition of ambient PAN in southern China. In this study, online sampling of PAN and ozone (O3) was conducted from August to November 2020, which is the most severe photochemical pollution period of the year, in Shenzhen, a megacity in southern China. During the sampling period, two photochemical pollution periods occurred in August (EP1) and October (EP2). The mean O3 concentrations during EP1 and EP2 were very close, 36.3 and 36.9 ppb, respectively, while PAN concentrations were very different (0.66 and 1.06 ppb, respectively). The diurnal variation showed that the PAN formation rate in the initial stage of photochemical reactions (9:00–11:00) during EP1 was significantly faster than that during EP2, while there was significant consumption at noon and almost complete decomposition at night in EP1. Calculating the thermal decomposition of PAN (TPAN), we found that the average concentration of TPAN in EP1 was 8.47 ppb, approximately 9 times that in EP2 (0.94 ppb), which significantly reduced the concentration of PAN in EP1. This indicates that when using PAN as an indicator of photochemical reaction activity, the consumption of PAN due to thermal decomposition must be considered. In addition, we found that regional transport may significantly contribute to the formation of PAN by increasing the concentration of precursor VOCs, although the lifetime of PAN was only a few hours due to a large amount of thermal decomposition.