Extreme PM2.5 and nonmethane hydrocarbon (NMHC) pollution often occurs simultaneously during the winter. To study the formation mechanism of two pollution events in Chengdu from 23 December 2016 to 31 January 2017, we explored the weather conditions, chemical composition, secondary pollutant conversion, aerosol hygroscopic growth, and potential source contribution function (PSCF) during this period. During the study period, the humidity was high (67.9%), the wind speed was low (1.0 m s-1), the height of the planetary boundary layer was low (463.4 m), and the atmosphere remained stationary. The potential source regions of PM2.5 and NMHCs were locally polluted sites in the southwestern and southern regions of Chengdu, affected by the southwesterly air mass trajectories. PM2.5 and sulfur oxidation ratios (SOR), nitrogen oxidation ratios (NOR) and secondary organic aerosol formation potential (SOAP) showed a strong positive correlation. As pollution increased, the conversion from SO2, NOx and NMHCs to sulfate, nitrate and SOAs increased, resulting in an increase in the secondary aerosol concentration. As the relative humidity increases, aerosols begin to undergo rapid hygroscopic growth, which seriously affects the visibility of the atmosphere. In general, pollutant emissions, static weather, and secondary conversion, among other factors, lead to the occurrence of this persistent extreme haze pollution.
Keywords: Chengdu; Hygroscopic growth; Potential source contribution function (PSCF); Secondary aerosols; Stationary synoptic conditions.
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