Formation of peroxyacetyl nitrate (PAN) and its impact on ozone production in the coastal atmosphere of Qingdao, North China.

Abstract

Peroxyacetyl nitrate (PAN), acting as a relatively long-lived reservoir for both NOx and radicals, plays a crucial role in ozone (O3) formation in the troposphere. However, its quantitative impacts on radical concentrations and O3 production were rarely studied in the coastal atmosphere. In this study, ambient concentrations of PAN, O3, and related species were simultaneously measured from October 5 to November 10, 2018 (autumn), and July 14 to August 24, 2019 (summer) at a rural coastal site in Qingdao, North China. The formation mechanism of PAN and its impact on in-situ O3 production were explored with an observation-based chemical box model. Photochemical formation of PAN and O3 was controlled by both NOx and VOCs, and acetaldehyde and methylglyoxal were the main contributors to PAN formation. However, the sensitivities of PAN to precursors were larger than that of O3 in autumn while smaller in summer, which was mainly caused by the rapid decomposition of PAN at high temperatures. Zero-out sensitivity simulation showed that PAN could either promote or inhibit the in-situ O3 formation by affecting the radical chemistry. It tended to suppress O3 production by competing with precursors and terminating radical chain reactions under low-NOx and low-ROx circumstances but enhanced O3 production by supplying RO2 radicals under conditions with sufficient NOx. This study provides some new complementary insights into the formation mechanism of PAN and its impacts on O3 production, and has implications for the formulation of control policy to mitigate regional photochemical pollution in northern China.