Aerosol properties and their spatial and temporal variations over North China in spring 2001

Aerosol properties and their spatial and temporal variations over North China were analysed based on ground-based radiometer data for spring 2001. On the basis of the retrievals from sun/sky radiance and broad-band radiation measurements at four AERONET stations and eight first-class radiation stations over North China, the analysis comprised a detailed description of aerosol loading, size and absorption in this period. The impact of dust events on aerosol properties over the downwind region was emphasized.Heavy aerosol loading and notable temporal variation over North China were revealed by both datasets. The average aerosol optical depth at 750 nm at the eight radiation stations ranged from 0.32 in Ejinaqi to 0.68 in Beijing, with the averaged coefficient of variation being 70%. Aerosol optical depth was dominantly contributed to by dust over western China, with 68% (±5%) of aerosol optical depth at 550 nm being attributed to large dust aerosols. A dramatic increase in aerosol optical depth associated with the remarkable decrease in the Ångstrom wavelength exponent was observed in Beijing and Xianghe during the dust episode. This indicated that a huge number of large particles were emitted into the atmosphere during the dust period, with the result that the contribution to aerosol optical depth from coarse particles approached the value observed in the dust source region.

Anthropogenic pollution also frequently contributed to the high aerosol optical depth in Beijing and Xianghe, but this was characterized by fine particles, with more than 70% of aerosol optical depth at 550 nm being attributed to fine particles. Pure desert aerosol in Chinese dust source regions absorbs much less solar radiation than predicted by known aerosol models. The retrieved single-scattering albedo was around 0.98 (±0.01) and had little wavelength dependence, which is in agreement with the ground-based and satellite retrievals in other dust source regions. Contrarily, anthropogenic aerosol exhibits much stronger absorption in the urban region, with the single-scattering albedo ranging from 0.89 ± 0.04 (at 440 nm) to 0.83 ± 0.05 (at 1020 nm). Due to the large difference in the absorption between dust and anthropogenic aerosol, the consequence of the input of a large volume of dust aerosols is not only to enhance the aerosol loading but also to reduce the aerosol absorption. Retrievals in Beijing showed that the single-scattering albedo increased to about 0.90 and had little spectral dependence when anthropogenic pollution and a dust event together affected Beijing; as for the pure dust period, it ranged from 0.92 at 440 nm to 0.97 at 1020 nm. The significant decrease in the aerosol absorption due to the dust outbreak can result in a decrease in aerosol atmospheric heating efficiency; this warrants further research since the increasing trend of aerosol loading with strong absorption in China is supposed to play an active role in regional and global climate change and the hydrological cycle.