Response of imidazole-containing particles to emission reduction policies in China: Insights from observations in a megacity in the Sichuan Basin

Imidazole (IM) particles in the atmosphere affect climate, atmospheric chemical reactions, and human health. However, research on IM particles in the Sichuan Basin (SCB), one of the areas of China affected most heavily by haze, remains very scarce. This study used single-particle aerosol mass spectrometry to investigate IM-containing particles in Chengdu, one of the megacities in the SCB, during summer and winter before and after implementation of the Three-year Action Plan to Win the Blue-Sky Defense War (BSDW). We found that IM-containing particles accounted for 1.2 %-12.0 % of all detected particles, and they highly mixed with carbonaceous components, secondary inorganic species, and organic nitrogen. From before to after the BSDW, the proportion of IM-containing particles decreased by 1.8 % in summer, but increased by 9.6 % in winter. Ammonium/amines and carbonyl compounds were closely related to IM-containing particles; the highest proportion of IM-containing particles occurred in particles mixed with amines and carbonyls. The number fraction of IM-containing particles in all seasons was higher at night than during daytime. The potential source areas of IM-containing particles showed notable narrowing after the BSDW, and the high-value areas were found distributed closer to Chengdu and its surrounding areas. In the winter before the BSDW, most IM-containing particles (> 70 %) were mixed with organic carbon (OC) particles, and the contributions of OC and mixed organic-elemental carbon (OC-EC) particles increased with aggravation of pollution, whereas OC-EC and Metal particles played a more crucial role in the winter after the BSDW.