[Secondary Inorganic Pollution Characteristics During Heavy Pollution Episodes of 2017 in Tianjin]

Based on monitoring data collected at the supersite of Tianjin in 2017, seven typical heavy pollution episodes were investigated. The concentrations of air pollutants and secondary inorganic transformation products were analyzed to study the secondary inorganic pollution characteristics during the heavy pollution episodes. Compared to clean weather, concentrations of NO₃^- and SO₄^2- during the heavy pollution episodes increased at rapid growth rates. These rates were obviously higher than the rate for PM_2.5 increases, which indicates that the secondary inorganic reactions had an important influence on PM_2.5 pollution during the episodes. The concentrations of PM_2.5 and SO₂ during the episodes in the latter half of the year were lower than those in the first half of the year probably because a substantial amount of coal use had been controlled. During the heavy pollution episodes, the NO₂/SO₂ values were 1.5 to 19.6, with higher values in the latter half of the year than the first half of the year suggestive of a greater influence from mobile sources. During most episodes, NO₃^- concentrations were higher than SO₄^2- concentrations, and SOR values were higher than NOR values, which shows that the secondary transformation of nitrate and sulfate both played important roles during the heavy pollution episodes. When SO₂ concentrations decreased significantly, SO₄^2- concentrations did not decrease obviously, thus indicating that besides the secondary inorganic reactions, other factors also had a large impact on the generation of sulfate.