Vertical characteristics of peroxyacetyl nitrate (PAN) from a 250-m tower in northern China during September 2018

Peroxyacetyl nitrate (PAN), formed through photochemical reactions between volatile organic compounds (VOCs) and NOx, is the second-most important photochemical pollutant. Although several studies have investigated ground-level PAN in northern China, the vertical characteristics of PAN in this region have yet to be examined. In this study, measured concentrations of PAN and ozone (O-3) near the surface and at 220 m were obtained from a 250-m observation tower in Tianjin during September 2018. Results show that the mean PAN concentrations at 220 m are 5.3-19.1% higher than ground-level values during the night (20:00-7:00), which can be attributed to difference in vertical mixing and chemical processes. When the inversion layer is below 220 m, the two layers (220 m and ground level) are separated into the residual layer and inversion layer, between which turbulent exchange is weak. Thus, a vertical gradient for PAN exists during the night. After sunrise, the inversion layer breaks up, and PAN concentrations become more homogeneous due to strong daytime vertical mixing. Role of chemical processes is examined using WRF-Chem model, and it is found that the PAN family-PAN and CH3C(O)O-2 radicals-decompose quickly near the surface in the evening due to strong NO emission sources, while those at 220 m decompose more slowly and could also be produced by the nighttime oxidation between VOCs and O-3. Modeling results further show that nighttime high PAN concentrations in the residual layer can occur throughout all of northern China.