Chemical characteristics of haze particles in Xi'an during Chinese Spring Festival: Impact of fireworks burning.

Fireworks burning releases massive fine particles and gaseous pollutants, significantly deteriorating air quality during Chinese Lunar New Year (LNY) period. To investigate the impact of the fireworks burning on the atmospheric aerosol chemistry, 1-hr time resolution of PM2.5 samples in Xi'an during the winter of 2016 including the LNY were collected and detected for inorganic ions, acidity and liquid water content (LWC) of the fine aerosols. PM2.5 during the LNY was 167±87μg/m3, two times higher than the China National Ambient Air Quality Standard (75μg/m3). K+ (28wt.% of the total ion mass) was the most abundant ion in the LNY period, followed by SO42- (25wt.%) and Cl- (18wt.%). In contrast, NO3- (34wt.%) was the most abundant species in the haze periods (hourly PM2.5>75μg/m3), followed by SO42- (29.2wt.%) and NH4+ (16.3wt.%), while SO42 - (35wt.%) was the most abundant species in the clean periods (hourly PM2.5<75μg/m3), followed by NO3- (23.1wt.%) and NH4+ (11wt.%). Being different from the acidic nature in the non-LNY periods, aerosol in the LNY period presented an alkaline nature with a pH value of 7.8±1.3. LWC during the LNY period showed a robust linear correlation with K2SO4 and KCl, suggesting that aerosol hygroscopicity was dominated by inorganic salts derived from fireworks burning. Analysis of correlations between the ratios of NO3-/SO42- and NH4+/SO42- indicated that heterogeneous reaction of HNO3 with NH3 was an important formation pathway of particulate nitrate and ammonium during the LNY period.