Analysis on the Characteristics and Formation Mechanisms of Air Pollution Episodes in Tianjin during Spring Festival of 2015

To study the impact of firework discharge on air quality in Tianjin, fine particulate matter (PM2.5, aerodynamic diameter is less than or equal to 2.5 microns particulate matter) and its major chemical compositions as well as meteorological variables were measured in Tianjin from 18 Feb to 7 Mar using various instruments like the Tapered Element Oscillating Microbalance PM2.5 instruments with Filter Dynamic Measurement System, an online Rapid Collector of Fine Particles and Ion Chromatography system, an online OC/EC analyzer, a ceilometer and a microwave radiometer and so on. In this study, three air pollution episodes are selected for analysis. The results show that the implementation of policies of prohibition and restriction on firework discharge resulted in a decline in firework discharge volume and the hourly maximum value of PM2.5 concentration on the Chinese New Year's Eve of 2015 was lower than that in 2014; the set-up of areas of prohibitions and restrictions on firework discharge led to obvious spatial differences in PM2.5 concentration in Tianjin with the maximum difference of 394 μg/m³. The attenuated backscatter densities were similar everywhere from the surface to 80-m height due to the impact of fireworks discharge, suggesting identical vertical distribution of PM2.5 below 80 m. This result is consistent with previous measurements. Air pollution episode 1 (EP 1) was caused by dense discharge of fireworks, in which the main chemical species in PM2.5 are K⁺, SO₄^2-, and Cl^-. Air pollutants like SO₂ and CO evidently increased during this episode. The formation of air pollution episode 2 (EP 2) was attributed to unfavorable meteorological conditions including temperature inversion, wetness inversion, convergence and high relative humidity, which promoted the secondary transformation of SO₄^2- and NO₃^- on the surfaces of particles from firework discharge via heterogenous reactions during EP 2. The third air pollution episode (EP 3) was initiated by the widespread discharge of fireworks on the Lantern Festival and characterized by complex pollution. Collaborative growth of the secondary inorganic components and the O₃ concentration were found during this episode. In addition, regional transport also played an important role in EP 3.