Trend in Fine Sulfate Concentrations and the Associated Secondary Formation Processes at an Urban Site in North China

Abstract

There has been a significant decline in SO2, the main precursor of sulfate, in North China over the past decade due to strict sulfur removal measures, whereas the amount of photochemical oxidants such as ozone has continued to increase in this region. In this study, we examined temporal variation in the concentrations of fine sulfate in urban Ji’nan in North China from 2008 till 2015. Over this period, the sulfate concentration decreased by –3.86 ± 2.50 µg m–3 yr–1 (–10.0% yr–1), which is slower that the rate of decrease for SO2 during the same period (–11.6% yr–1). Nevertheless, the sulfur oxidation ratio and the concentrations of ozone and calcium (an indicator of dust particles) increased over this period. An analysis of the seasonal and diurnal variations in sulfate and the related parameters in 2015 indicated that the ambient sulfate concentration was largely influenced by the amount of SO2, atmospheric oxidants, aerosol loading, and meteorological conditions. A detailed investigation of the production of sulfate in eight case studies found that the observed sulfate production rate was in the range of 1.1–10.8 µg m–3 h–1. Numerical calculations revealed that SO2 oxidation by OH and H2O2 was a major contributor to sulfate production during the daytime in warm seasons. At all times of the day in cold seasons and at nighttime in warm seasons, the heterogeneous SO2 reaction on aerosol surfaces contributed 30.1%–65.7% of the sulfate production. The increasing amount of ozone and dust particles in this region, which are associated with photochemical pollution and urban dust emissions, are responsible for the slower decrease in sulfate concentration. Therefore, photochemical smog and urban dust should receive adequate attention in order to mitigate the sulfate pollution.