Secondary inorganic aerosols and aerosol acidity at different PM pollution levels during winter haze episodes in the Sichuan Basin, China.

Wintertime fine particle (PM) pollution remains to be perplexing air quality problems in many parts of China. In this study, PM compositions and aerosol acidity at different pollution levels at an urban cite in the southwest China's Sichuan Basin were investigated during a sustained winter haze episode. Organic matter was the most abundant component of PM, followed by nitrate, sulfate and ammonium. Shares of organic aerosol in PM mass decreased with the elevated PM levels, while the enhancements of sulfate and secondary organic aerosol were much less than that of nitrate and ammonium during heavy pollution with increased ratios of nitrate to sulfate, implying a significant role of nitrate in the haze formation. Results also suggest the nighttime chemistry might contribute substantially to the formation of nitrate under severe pollutions. The daily average aerosol pH showed a decreasing trend with the elevated levels of PM, and this increased aerosl acidity was mainly due to the fast rising secondary inorganic aerosol (SIA) concentration, with the increase in hydronium ion concentration in air (H) surpassing the dilution effect of elevated aerosol liquid water content (LWC). Thermodynamic model calculations revealed that the air environment was NH-rich with total NH (NH + NH) greater than required NH, and the aerosol pH exponentially declined with the decreasing excess NH (p < 0.01). This study demonstrated that under air stagnation and NH-rich environment during winter, the raised relative humidity (RH) would lead to an increase in LWC and thereby facilitate the aqueous chemistry processes with the neutralization capacity of NH to form sulfate and nitrate, which would further increase the LWC and lower the pH. This self-amplifying SIA formation might be crucial to the severe PM pollution and haze events during winter, and therefore cutting both NO and NH emissions would benefit stopping the self-amplification.