Characteristics of air pollution in Beijing during sand-dust storm periods

In the Beijing area, March and April have the highest frequency of sand-dust weather. Floating dust, blowing sand, and dust storms, primarily from Mongolia, account for 71%, 20%, and 9% of sand-dust weather, respectively. Ambient air monitoring and analysis of recent meteorological data from Beijing sand-dust storm periods revealed that PM sub(10) mass concentrations during dust storm events remained at 1500 kg m super(-3), which is five to ten times higher than during non-dust storm periods, for fourteen hours on both April 6 and 25, 2000. During the same period, the concentrations in urban areas were comparable to those in suburban areas, while the concentrations of gaseous pollutants, such as SO sub(2), NO sub(x), NO sub(2), and O sub(3), remained at low levels, owing to strong winds. Furthermore, during sand-dust storm periods, aerosols were created that consisted not only of many coarse particles, but also of a large quantity of fine particles. The PM sub(2.5) concentration was approximately 230 kg m super(-3), accounting for 28% of the total PM sub(10) mass concentration. Crustal elements accounted for 60-70% of the chemical composition of PM sub(2.5), and sulfate and nitrate for much less, unlike the chemical composition of PM sub(2.5) on pollution days, which was primarily composed of sulfates, nitrates, and organic material. Although the very large particle specific surface area provided by dust storms would normally be conducive to heterogeneous reactions, the conversion rate from SO sub(2) to SO sub(4) super(2-) was very low, because the relative humidity, less than 30%, was not high enough.