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

Beijing experiences air pollution such that the sky overhead is gray much o f the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient ap and absorption coeffi cient sigma (ap) were measured under dry conditions (instrumental relative humidity < 40%) during a 1-week intensive field sampling period in June 199 9 in Beijing, China. Additional measurements included the aerosol mass size distribution, chemical composition of the aerosol mass having particle dia meters less than 2.5 mum (PM2.5) as well as the chemical composition of the total suspended particulate matter. The mean (and standard deviation) for hourly averages of sigma (sp), sigma (ap), and the single-scattering albedo omega were 488 Mm(-1) (370 Mm(-1)), 83 Mm(-1) (40 Mm(-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 sigma (sp) wer e accompanied by a daily mean value for the PM2.5 mass concentration of 136 mug m(-3) (48 mug m(-3)), which is significantly higher than the proposed U.S. 24-hour average mean National Ambient Air Quality Standard of 65 mug(- 3) in. The visual range during the field study, based on measurements of si gma (sp) and sigma (ap), was typically less than 6 km. For several days tha t did not have rain or fog, there was a clear diurnal trend in sigma (sp), sigma (ap), and omega, with peak values in the early morning and minima tha t occur in the evenings. The peaks correspond to minima in ambient temperat ure and maxima in relative humidity. Mass size distribution measurements in dicate that although similar to 80% of the aerosol mass was located in the coarse particle mode (D-p > 1.0 mum), the submicron aerosol was responsible for similar to 80% of the light scattering at 530 nm. The largest contribu tion to the PM2.5 aerosol mass was due to organic compounds, which accounte d for similar to 30% of the mass. The contributions of sulfate, ammonium, a nd nitrate to the PM2.5 mass concentration were similar to 15%, 5%, and 8%, respectively. Mineral aerosol contributed similar to 16% to the PM2.5 aero sol mass. These data show that combustion-related particles rather than win d-blown dust dominated the light extinction budget during June 1999.