Seasonal characteristics of atmospheric peroxyacetyl nitrate (PAN) in a coastal city of Southeast China: Explanatory factors and photochemical effects

Peroxyacetyl nitrate (PAN) acting as a typical indicator of photochemical pollution can redistribute NOx and modulate O-3 production. Coupled with the observation-based model (OBM) and a generalized additive model (GAM), the intensive observation campaigns were conducted to reveal the pollution characteristics of PAN and its impact on O-3, the contributions of influencing factors to PAN formation were also quantified in this paper. The F values of GAM results reflecting the importance of the influencing factors showed that ultraviolet radiation (UV; F value = 60.64), O-x (O-x = NO2 + O-3, 57.65), and air temperature (T, 17.55) were the main contributors in the PAN pollution in spring, while the significant effects of O-x (58.45), total VOCs (TVOCs, 21.63), and T (20.46) were found in autumn. The PAN formation rate in autumn was 1.58 times higher than that in spring, relating to the intense photochemical reaction and meteorological conditions. Model simulations revealed that acetaldehyde oxidation (46 % +/- 4 %) contributed to the dominant formation pathway of PA (hence PAN), followed by methylglyoxal oxidation (28 % +/- 3 %) and radical cycling (19 % +/- 3 %). The PAN formation was highly VOC sensitive, as surplus NOx (compared with VOCs abundance) prevented NOx from being the limiting factor photochemical formation of secondary pollution. At our site, PAN promoted and inhibited O-3 formation under high and low ROx levels, respectively. The PAN promoting O-3 formation mainly occurred during the periods of 11:00-16:00 (local time) when the favourable meteorological conditions (high UV and T) stimulated the photochemical reactions to offer ROx radicals, which accounted for 17 % of the whole monitoring periods in spring and 31 % in autumn. The analysis of PAN formation mechanism and its positive or negative effect on ozone provided scientific insights into photochemical pollution mechanisms under various pollution scenarios in coastal areas.