Supplementary material to "Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during cold-season haze events"

Photochemical pollution over the North China Plain (NCP) are attracting considerable concern. Peroxyacetyl nitrate (PAN) is usually viewed as the second most important photochemical pollutant featuring high mixing ratios during warm seasons. Our observations at a background site in the NCP identified high PAN concentrations even during coldseason haze events. The abrupt increasing rates of PAN by 244% and 178% over the morning hours (8:00–12:00) on 10/20 20 and 10/25, 2020 were 10.6 and 7.7 times those on clean days. The pollution days were characterized by higher temperature and humidity, accompanied by anomalous southerlies. Enhanced local photochemistry has been identified as the dominant factor that controls PAN increase in the morning at the rural site, as the time when prevailing wind turned to southerlies was too late to facilitate direct transport of PAN from the polluted urban region. By removing the effect of direct transport of PAN, we provide a quantitative assessment of net PAN chemical production rate of 0.45 ppb h-1 on the polluted morning, 25 also demonstrating the strong local photochemistry. Using observations and calculated photolysis rates, we find that oxidation of acetaldehyde by hydroxyl radical (OH) is the primary pathway of peroxyacetyl radical formation at the rural site. Acetaldehyde concentrations and production rates of HOx (HOx = OH + HO2) radical on pollution days were 2.8 and 2 times that on clean days, respectively, leading to the abrupt increase of PAN in the morning. Formaldehyde (HCHO) photolysis dominates the daytime HOx production thus contributing to fast photochemistry of PAN. Our observational results fully 30 explain the cause of rapid increase of PAN during cold days at a rural site of the NCP, as well as provide the evidence of important role of HCHO photolysis in secondary pollutants at lower nitrogen oxide emissions. This highlights the imperative to implement strict volatile organic compounds controls out of summer seasons over the NCP. 1 https://doi.org/10.5194/acp-2021-359 Preprint. Discussion started: 23 June 2021 c © Author(s) 2021. CC BY 4.0 License.