[1] The elemental (EC) and organic carbon (OC) fractions of ambient aerosols were determined by thermo-chemical analysis of 24-h samples collected during 2006 at 18 stations in China located at various rural, urban and remote locations. The annual mean concentration levels are found to be 0.35 ± 0.01 mg EC m À3 and 3.0 ± 0.21 mg OC m À3 for the remote background sites; 3.6 ± 0.93 mg EC m À3 and 16.1 ± 5.2 mg OC m À3 for the regional sites; and 11.2 ± 2.0 mg EC m À3 and 33.1 ± 9.6 mg OC m À3 for the urban sites, respectively. At rural sites representing regionally dispersed aerosols, levels are comparable to other locations in Asia. At all sites, both EC and OC show a consistent seasonal variation with a peak in winter, dropping in spring, reaching a minimum in summer and then increasing in autumn. The ratio of OC to EC is on the order of 3 for the urban locations, but can reach as high as 6 at the rural sites. This may partly be due to the open biomass burning in field of rural area, but may also indicate the presence of a regional background of secondary organic carbon (SOC) in China. These high ratios of OC/EC complicate the assessment of the climatic impacts of carbonaceous aerosols in China, as optical scattering from the high OC concentrations may lead to a cooling effect that counteracts the possible warming caused by EC optical absorption.

A B S T R A C T This paper reports on the analysis of 24-h aerosol data measured during 2006, at 14 monitoring sites in China. Measurements included seven-wavelength Aethalometers, thermal/optical reflectance analyses of filter samples and determination of dust aerosols. Black (elemental) carbon (BC, EC) is found to be the principal light-absorbing aerosol over many parts of China; however, the fraction of apparent light absorption attributed to dust varied from 14% in winter, to 11% in spring, to 5% in summer to 9% in autumn. Aerosol light absorption in urban areas was larger than in rural areas by factors of 2.4 in winter, 3.1 in spring and 2.5 in both summer and autumn. These differences may lead to contrasts in radiative, thermal and cloud modification effects between urban and rural areas. Absorption 'hotspots' were located in the Sichuan Basin, the provinces south of Beijing, the Pearl Delta River regions and the Guanzhong Plain. The mass absorption coefficient for aerosol BC (σ BC ) based on Aethalometer data is estimated to be 11.7 m 2 g −1 at 880 nm wavelength (λ) with inverse (λ −1 ) wavelength scaling, whereas the mass absorption coefficient for dust (σ dust ) is 1.3 m 2 g −1 on average without significant wavelength dependence.

Seasonal characteristics of PM2.5, organic carbon (OC) and elemental carbon (EC) were studied in Taiyuan from 2009 to 2010. PM2.5 samples were collected by pre-baked quartz filters with high-volume air sampler, and then OC and EC were analyzed by thermal/optical reflectance method. The annual average concentrations of PM2.5, OC and EC were 220.2, 37.4 and 19.6 µg/m 3 respectively, which were higher than those in most regions in China. Total carbonaceous aerosol (TCA) accounted for more than one third of the total PM2.5 mass. The levels of PM2.5, OC and EC were the highest in winter, followed by spring, fall and summer. OC and EC were well correlated in summer compared with other seasons, indicating the presence of other additional sources such as biomass burning in fall, coal combustion for heating in winter and dust in spring. Higher OC/EC ratios in winter might be primarily attributed to the significant increase of direct emission of OC as a result of coal and biomass combustion, and also cooling effect of carbonaceous aerosols due to low temperature and stagnated atmospheric condition. These results showed that the pollution of carbonaceous particles in Taiyuan was serious, and might be an inducing factor of dust haze, especially in winter.