Insight into the non-linear responses of particulate sulfate to reduced SO2 concentration: A perspective from the aqueous-phase reactions in a megacity in Northern China

Abstract

The non-linear relationship between particle sulfate and SO2 is still unclear. Continuous observations were conducted in a typical city in Northern China from 2017 to 2020. SO2 concentration continually decreased, however, the SO42− concentration and the sulfur oxidation ratio increased in 2018–2019. The major reason might be the variations of sulfate formation by the aqueous-phase reactions, especially for the O2 + TMI oxidation (oxygen catalyzed by transition-metal ions). Particle pH, SO2, and Mn concentration were the main factors affecting the O2 + TMI reaction rate. Sensitivity analysis indicated that the decrease of the O2 + TMI reaction rate caused by SO2 or Mn concentration dropping by half can only offset the increased rate caused by a 0.1 unit decrease in particle pH. Considering the actual atmospheric environment in Northern China, if the particle pH continues to drop, the slow decline of SO2 and Mn concentration may not prevent the increase in the O2 + TMI oxidation rate. In addition to meteorological parameters, the concentrations of SO42−, crustal elements, and total ammonium were the primary factors for the decrease in particle pH in the past 3 years. Therefore, the impact of pH on sulfate formation must be considered when controlling dust and ammonia emissions.