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 non-methane 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 in the Sichuan Basin (SCB) in China during winter. The basin’s topography promotes vertical pollutant transport, while temperature inversion accumulation of NOx and NMVOCs enhances PAN formation. Budget analysis reveals that PAN in the free troposphere predominantly originates from in situ photochemical production via the reaction of peroxyacetyl radicals (PA) with NO₂ (73%), with the remaining 27% from vertical transport driven by topographic circulation. The primary factors are stagnant air masses and reduced thermal decomposition under low temperatures. Horizontal transport accounts for 50% of PAN removal. This export positions the SCB as a key winter source of PAN and NOx for the atmosphere.