Evaluation on Impact of Spring Festival Fireworks and Firecrackers Setting-off on Yancheng City Based on a Bayesian-Optimized XGBoost Model

• • K+ and Mg2+ concentrations increased by 15.89 and 20.08 times, respectively, on Lunar New Year's Eve, serving as core water-soluble ion tracers for fireworks emissions, enabling precise source apportionment in urban air quality management. • • Inorganic elements Ba and Sr exhibited growth multiples of 127.72 and 121.26 on Lunar New Year's Eve, respectively, matching metal colorant compositions in fireworks, providing unambiguous chemical fingerprints for pollution tracing. • • Fireworks contributed 20.3% to PM2.5 on Lunar New Year's Eve, with PM2.5 concentrations reaching 4.4 times non-setting-off levels, highlighting the need for targeted emission control measures during peak periods. • • During the fifth day period, secondary conversion ions (SO4^2-, NO3^-) showed increased contribution, with NH4+ strongly correlated with SO4^2- and NO3^- (R^2 ≥ 0.85), confirming ammonium salt formation and secondary organic aerosol (OC/EC > 15) mechanisms, essential for understanding atmospheric chemical evolution.