Wintertime Peroxyacetyl Nitrate (PAN) Hotspot over China Observed from Space: Interpretation of CrIS Results Using Updated GEOS-Chem

Peroxyacetyl nitrate (PAN) is an important tracer of photochemistry formed through the oxidation of nonmethane volatile organic compounds (NMVOCs) in the presence of nitrogen oxides (NOx ≡ NO + NO2). We use CrIS satellite observations and GEOS-Chem simulations to identify a persistent hotspot of free tropospheric PAN over the Sichuan Basin (SCB) in China during winter. The basin topography promotes the initial vertical lifting, while the subsequent temperature inversion layer in the free troposphere (FT) traps the lifted NOx and NMVOCs, thereby facilitating PAN formation. Budget analysis for a 3-D box representing the SCB FT (825–215 hPa) shows that photochemical production accounts for 73% of PAN formation, while vertical transport from below contributes 27%. Dominant photochemical PAN formation stems from stagnant air masses enabling sufficient oxidation of precursors and low winter temperatures prolonging PAN lifetime. Horizontal transport dominates PAN removal (50%), suggesting the SCB as a winter source of PAN and NOx to downwind regions. This study highlights that meteorology and terrain drive severe winter PAN pollution in the SCB.