Seasonal discrepancies and inter-relationship of peroxyacetyl nitrate (PAN), ozone, and other environmental factors in Hangzhou, East China

Abstract

Peroxyacetyl nitrate (PAN, CH3C(O)OONO2) and ozone (O3) are both important photochemical products, both of which are detrimental to air quality, human health and ecosystem. In this study, we measured atmospheric PAN, O3 and other pollutants in Hangzhou during the whole 2021 year. Observed results revealed that the average concentrations of PAN and O3 during the whole observation period were 1.59 and 27.09 ppbv, respectively. PAN concentrations declined from the weekday to weekend, while O3 presented the reverse pattern. The abundance of PAN and O3 showed different variations in all seasons. Serious photochemical pollution in spring and autumn were featured by higher PAN levels (mean of 1.74 and 1.66 ppbv); whereas those in summer and spring were characterized by higher O3 levels (mean of 32.34 and 30.98 ppbv). With the extremely higher PAN/O3 ratio of 0.10, the results suggested the high-level photochemical pollution in Hangzhou. In all seasons, the diurnal peak of PAN appeared earliest in summer could be associated with rapid decomposition rate of PAN at noon with higher temperatures. The correlation between PAN and other factors varies across all seasons due to the environmental conditions are unique in each season. Through analysis of conditional probability function (CPF) and toluene/benzene ratio, traffic sources were identified as the main factor contributing to high PAN and O3 accumulation at a regional scale. Furthermore, backward trajectory analysis revealed that marine air masses affected the air of the monitoring site in both summer and autumn; whereas continental air masses impacted it in both spring and winter. This study provides novel insights into photochemical pollution process in a coastal city, eastern China.