Impacts of precursors on peroxyacetyl nitrate (PAN) and relative formation of PAN to ozone in a southwestern megacity of China

Peroxyacetyl nitrate (PAN), an important species of photochemical smog, has received much concern for its impact on atmospheric chemistry and relation with other pollutants including ozone (O3). This study conducted PAN measurements at the CS site (urban area) and the NQ site (suburban area) of Chongqing in summer in 2015. Maximum PAN concentrations were 12.17 (mean: 2.05 ± 1.94 ppbv) and 8.53 ppbv (mean: 0.84 ± 0.89 ppbv) at the CS and NQ sites, respectively. An observation-based box model coupled with RACM2 was applied to explore PAN formation mechanisms and its key precursors. Results showed that PAN formation was limited by both nitrogen dioxides (NOX) and volatile organic compounds (VOCs) at both sites, and NOX and nitrous acid were essential PAN precursors, especially on polluted days. For VOCs, key first-generation precursors were identified as alkenes and aromatics at the CS site (aromatics and isoprene at the NQ site). On polluted days, the relative production efficiency of PAN to O3 enhanced more significantly at the NQ site (0.9 times) than at the CS site (0.4 times), which was related to formation/destruction reactivities of PAN and O3. Furthermore, this study firstly explored the quantitative impacts of precursors to the relative formation of PAN to O3, and the results indicated that NOX predominated their relative formation at both sites. For VOCs, urban alkenes and suburban aromatics contributed to elevated PAN formation relative to O3, and the increase of aromatics dominated the PAN enhancement relative to O3 during the polluted period.