Direct aerosol radiative forcing based on measurements of aerosol radiative, chemical and physical properties in China

This study is focused on characterization of aerosol radiative, physical and chemical properties as well as modeling of direct aerosol radiative forcing in two of the most climatic important regions of China, the Yangtze delta region and the Northwest China desert region. Based on two field measurement campaigns, aerosol loadings are found to be relatively high both in the Yangtze delta region and near the Gobi desert dust source region. But the aerosol properties, and hence radiative effects are different in these two locations. Aerosol properties in the Yangtze delta region are more representative of urban aged aerosols. Organic compounds are the main chemical species accounting for about 50% of the PM_2.5 mass, and dominate the attenuation of solar radiation. A major part of the organic aerosol is believed to be from biomass burning sources. Local anthropogenic sources and longer range transport from nearby urban areas may also have important contributions to the aerosol loading in this area. Aerosol measurements near the Gobi desert source region indicate that there is significant variability in aerosol loadings and properties. Two populations of aerosols, one dominated by desert dust and the other dominated by local pollutants, are distinguished. The measurements suggest that both local pollution and dust sources influence aerosol properties, and contribute to the climate forcing of aerosols in the Gobi desert region. Based on the measurements, a Tropospheric Ultraviolet-Visible radiation model and a broad-band radiative transfer model are used to estimate the direct aerosol shortwave radiative forcing at the surface and the top of atmosphere. The modeling results show that over the Yangtze delta and the Gobi desert region the amount of photosynthetically active radiation reaching the surface are reduced by 16% and 7% as a result of the direct radiative effect of aerosols, and the mean direct aerosol radiative forcing at the top of atmosphere for total solar radiation are −12.1 and −2.5 W m−2, respectively. The results suggest that aerosols have significant influence on the radiation balance, and hence climate change in both the Yangtze delta and the Northwest desert region of China.