The impacts of firework burning at the Chinese Spring Festival on air quality: insights of tracers, source evolution and aging processes

To understand the impact of firework-burning (FW) particles on air quality and human health during the winter haze period, 39 elements, 10 water-soluble ions and 8 fractions of carbonaceous species in atmospheric PM_2.5 in Nanjing were investigated during the 2014 Chinese Spring Festival (SF). Serious regional haze pollution persisted throughout the entire sampling period, with PM_2.5 averaging at 113 ± 69 μg m⁻³ and visibility at 4.8 ± 3.2 km. The holiday effect led to almost all the chemical species decreasing during the SF, except for Al, K, Ba and Sr which were related to FW. The source contributions of coal combustion, vehicle emission and road dust decreased dramatically, whereas FW contributed to about half of the PM_2.5 during the SF period. The intensive emission of FW particles on New Year's Eve accounted for 60.1% of the PM_2.5. Fireworks also obviously modified the chemical compositions of PM_2.5, with 39.3% contributed by increased organic matter, followed by steadily increased loadings of secondary inorganic ions. The aging processes of the FW particles lasted for about 4 days reflected by the variations of Ba, Sr, NH₄⁺, NO₃^−, SO₄^2− and K⁺, characterized by heterogeneous reactions of SO₂ and NO_x on crustal materials directly from FW, the replacement of Cl^− by NO₃^− and SO₄^2−, coating of NO₃^− and SO₄^2− on soot, formation of secondary organic aerosols and metal-catalyzed formation of NO₃^− and SO₄^2− at higher relative humidity. During aging, the main contributors to the extinction coefficient shifted from elemental carbon and organic matter to ammonium sulfate. The particles raised higher cancer risk of 1.62 × 10^−6 by heavy metals (especially for Cd and As). This study provided detailed composition data and first comprehensive analysis of the aging processes of FW particles during the serious haze pollution period and their potential impact on human health