Aerosol radiative, physical, and chemical properties in Beijing during June 1999


Beijing experiences air pollution such that the sky overhead is gray much of the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient σsp and absorption coefficient σap were measured under dry conditions (instrumental relative humidity <40%) during a 1-week intensive field sampling period in June 1999 in Beijing, China. Additional measurements included the aerosol mass size distribution, chemical composition of the aerosol mass having particle diameters less than 2.5 μm (PM2.5) as well as the chemical composition of the total suspended particulate matter. The mean (and standard deviation) for hourly averages of σsp, σap, and the single-scattering albedo ω were 488 Mm-1 (370 Mm-1), 83 Mm-1 (40 Mn-1), and 0.81 (0.08), respectively, which is significantly higher than values reported in urban regions of the United States. The relatively high values of σsp were accompanied by a daily mean value for the PM2.5 mass concentration of 136 μg m-3 (48 μg m-3), which is significantly higher than the proposed U.S. 24-hour average mean National Ambient Air Quality Standard of 65 μg m-3. The visual range during the field study, based on measurements of σsp and σap, was typically less than 6 km. For several days that did not have rain or fog, there was a clear diurnal trend in σsp, σap, and ω, with peak values in the early morning and minima that occur in the evenings. The peaks correspond to minima in ambient temperature and maxima in relative humidity. Mass size distribution measurements indicate that although ∼80% of the aerosol mass was located in the coarse particle mode (Dp > 1.0 μm), the submicron aerosol was responsible for ∼80% of the light scattering at 530 nm. The largest contribution to the PM2.5 aerosol mass was due to organic compounds, which accounted for ∼30% of the mass. The contributions of sulfate, ammonium, and nitrate to the PM2.5 mass concentration were ∼15%, 5%, and 8%, respectively. Mineral aerosol contributed ∼16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999. Copyright 2001 by the American Geophysical Union.