A comparison of the physical and optical properties of anthropogenic air pollutants and mineral dust over Northwest China

Emissions of mineral dust and its mixing with anthropogenic air pollutants affect both regional and global climates. Our fieldwork in late spring 2007 (April 25-June 15) measured the physical and optical properties of dust storms mixed with local air pollutants at a rural site about 48 km southeast of central Lanzhou. Levels of air pollutants and aerosol optical properties were observed during the experiment, with concentrations of NOx (6.8 +/- 3.3 ppb, average +/- standard deviation), CO (694 +/- 486 ppb), SO2 (6.2 +/- 10 ppb), O-3 (50.7 +/- 13.1 ppb), and PM10 (172 +/- 180 mu g m(-3)), and aerosol scattering coefficient (164 +/- 89 Mm(-1); 1 Mm = 10(6) m) and absorption coefficient (11.7 +/- 6.6 Mm(-1)), all much lower than the values observed during air pollution episodes in urban areas. During a major dust storm, the mass concentration of PM10 reached 4072 mu g m(-3), approximately 21-fold higher than in non-dust storm periods. The mixing ratios of trace gases declined noticeably after a cold front passed through. The observed CO/SO2 and CO/NOx ratios during air pollution episodes were 4.2-18.3 and 13.7-80.5, respectively, compared with the corresponding ratios of 38.1-255.7 and 18.0-245.9 during non-pollution periods. Our investigations suggest that dust storms have a significant influence on air quality in areas far from their source, and this large-scale transport of dust and air pollutants produces major uncertainties in the quantification of the global effects of emissions over Northwest China.