Pollution Characteristics of Water-soluble Ions in PM2.5 During the Lantern Festival of 2021 in Zibo City

Abstract

Based on the online ion data, we have analyzed the cause of a PM2.5 pollution episode, which happened during the Lantern Festival in Zibo in 2021. The pollution characteristics of water-soluble ion components were analyzed, the formation mechanism of secondary inorganic ions (SNA) was discussed, and the changes in the liquid water content (LWC) and pH value of particulate matter before and after pollution were comparatively analyzed. The results showed that the pollution period before the Lantern Festival (T1) and the pollution period at night (T2) ρ(WSIIs) during the Lantern Festival were 46.83 μg·m-3 and 71.18 μg·m-3, respectively, which were 2.3 times and 3.6 times that of the cleaning period, respectively. Among them, the growth multiple of SNA during the T1 period was greater[ρ(NO3-) was 2.9 times, ρ(SO42-) was 2.8 times, and ρ(NH4+) was 2.4 times] than the growth multiple of PM2.5 (2.1 times), which showed that the increase in SNA concentration during the T1 period was the main reason for the increase in PM2.5 concentration. Furthermore, the Cl-, K+, and Mg2+ concentrations, which were 4.0, 14.8, and 16.5 times that of the cleaning period, respectively, increased significantly during the T2 period, indicating that the fireworks and firecrackers caused the rapid increase in the PM2.5 concentration during the T2 period. The LWC during the pollution period was 49.37 μg·m-3, which was 2.9 times that of the cleaning period. LWC was mainly affected by RH and NH4+ during the T1 period and was also affected by Mg2+ during the T2 period. The average pH during the pollution period in Zibo was 4.79±1.54, which was 0.14 lower than that during the cleaning period. The pH during the T1 period was affected by the combined effects of SO42- and NH4+, which made it decrease 0.53 compared to that during the cleaning period. The pH value during the T2 period may be affected by the K+, Cl-, and Mg2+ emitted from the fireworks and firecrackers, causing the pH to increase 0.65 compared to that during the cleaning period. The formation mechanism showed that SO42- was mainly generated by heterogeneous hydrolysis during the pollution episode, whereas NO3- was mainly generated by homogeneous reactions. On the whole, during the pollution episode, the increase in PM2.5 concentration before the Lantern Festival was mainly caused by the increase in SNA concentration, and the increase the night of the Lantern Festival was mainly caused by setting off fireworks and firecrackers.