Influence of nitrogen oxides on heterogeneous uptake and oxidation of sulfur dioxide on yellow sand particles

In China, large amounts of sulfur dioxide (SO2) from coal combustion have been emitted with the progression of industrial and economic activities. In addition, since the emissions of nitrogen oxides and hydrocarbons from automobile exhaust and fossil fuel combustion have been increasing, there is the possibility that ozone (O-3) and gaseous nitric acid (HNO3) would be produced through photochemical reactions. Taking into account those circumstances, the evaluation of the uptake extent of acidic gases and oxidants to mineral dust particles may provide interesting information about the transport and fate of such gases. There are some laboratory studies about the heterogeneous reactions of air pollutants on Chinese mineral dust particles. Furthermore, these reports showed that coexistent components might importantly contribute to heterogeneous uptake and/or to oxidation of SO2. On the other hand, there are a few reports about the possibility that yellow sand particles act as chemical reaction sites in the atmosphere, and as carriers of chemical products, on the basis of chemical analysis of the yellow sand particles during the long-range transport from the source to Japan. Therefore, there is the possibility that air pollutants were attached to yellow sand particles and/or reacted with the particles. In order to obtain information about influence, of moisture, NO2, and gaseous nitric acid (HNO3) on heterogeneous reaction of SO2 on Lanzhou soil particles collected in Lanzhou, China, laboratory experiments were performed in the use of a cylindrical flow reactor. We focused uptake and oxidation of SO2 in this study. The uptake of SO2 was estimated by monitoring the concentration of SO2 continuously. As a result, a high reactivity of SO2 with Lanzhou soil particles was shown. However, the uptake of SO2 was controlled in the presence of HNO3 at < 10% RH, which suggests a rapid acidification of the particle surface by HNO3 uptake. At 80%RH, the uptake of SO2 increased significantly in comparison with < 10%RH and had little influence of NO2 and HNO3. The sulfur oxidation was discussed by means of oxidation ratio of sulfur (ORS). Moisture and NO2 increased the ORS values, and a synergic effect between both components was exhibited. This result indicated the contribution Of NO2 and metal components eluted from the particles to oxidation reaction. HNO3 also contributed to acceleration of oxidation at 80%RH. It is considered that the characteristic transformation of the China loess particle surfaces by SO2 uptake might be related significantly to moisture, NO2, and HNO3.