A Study of Aerosol Direct Radiative Effect and Its Impacts on Global Terrestrial Ecosystem Cycles

Aerosols can absorb and scatter solar radiation, thus cause the total solar radiation reaching the surface to drop and the fraction of diffuse radiation to increase, which influence the surface radiation budget. The global surface radiation with and without consideration of aerosols are calculated by the Fu-Liou atmospheric radiative transfer model based on the MODIS aerosol products, CERES cloud products and other remote sensing data. The aerosol direct radiative effect is calculated based on the two scenarios of aerosols. Our calculation showed that in 2007, aerosols decreased the global total radiation by 9.16 W m-2 on average. Large decrease generally occurred in places with high AOD. As for the diffuse radiation, aerosol-induced changes were either positive or negative. Large increase generally occurred in places with high surface albedo, while large decrease generally occurred in places with high cloud fraction. The global aerosol-induced diffuse radiation change averaged 8.17 W m-2 in 2007. The aerosol direct radiative effect causes the photosynthetic active radiation to increase, and its influences on the global carbon cycle of terrestrial ecosystem are studied by using the Community Land Model (CLM). Calculations show that the aerosol direct radiative effects caused the global averages of terrestrial gross primary productivity (GPP), net primary productivity (NPP), heterotrophic respiration (RH), autotrophic respiration (RA), and net ecosystem productivity (Reco) to increase in 2007, with significant spatial variations however. The global average changes of GPP, NPP, NEP, RA, RH and Reco in 2007 were +6.47 gC m-2, +2.23 gC m-2, +0.34 gC m-2, +4.24 gC m-2, +1.89 gC m-2, +6.13 gC m-2, respectively. Examinations of the carbon fluxes show that the aerosol direct radiative effects influence the terrestrial ecosystem carbon cycles via the following two approaches: First, the diffuse fertilization effect, i.e. more diffuse radiation absorbed by vegetation shade leaves (photosynthetic active radiation, PAR) results in higher photosynthetic rates; Second, the radiation changes lead to changes in temperature and humidity, thereby changing the rates of the plant biophysical and chemical processes.