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Abstract
[1] Almost 4 years' worth of Aerosol Robotic Network (AERONET) data from Beijing are compiled with historic aerosol data obtained in a few years during the 1980s and 1990s. Aerosol properties, including aerosol optical depth (t), single scattering albedo (w) and asymmetry factor ( g), show a large day-to-day variation and a distinct seasonal pattern. Aerosol loading is at its maximum during the summer and at its minimum during the winter. Aerosol loading frequently increases gradually from the background level to tens of times that of the background level a few days later, then quickly returns to the background level because of the passage of a weather system. Aerosol optical depth at 550 nm (tau(550nm)) varies from about 0.28 in 1980 to about 0.68 in 2005, implying a significant increase in aerosol loading during the past 25 years. Historic w and g values are generally in agreement with AERONET data except that aerosol absorption during the winter is quite different. Aerosols induce significant changes in net solar radiation at the surface and at the top of the atmosphere (TOA). The aerosol direct radiative forcing (ADRF) at the surface varies from - 17.2 W m(-2) in January to - 63.5 W m(-2) in June and then to - 18.5 W m(-2) in December. ADRF at the TOA shows a similar seasonal variation to that at the surface, but the value is approximately 28 - 42% of the latter. A combination of high aerosol optical depth and strong aerosol absorption leads to significant ADRF in the atmosphere, ranging from 1.9 to 117.6 W m(-2), thus increasing atmospheric heating by 0.3 to 4.6 K/day. The long-term change in aerosol loading in China and its effects on climate and the environment deserve much attention.

DOI:
10.1029/2006JD008119

ISSN:
0148-0227