Optical Depth And Direct Radiative Forcing Distribution Of Nitrate Aerosol In China

In this paper, the latest-published complex refractive indexes of nitrate in HITRAN 2004 database have been used to calculate the optical properties of nitrate through Mie scattering model of spherical particles and they have been compared with the optical properties of sulfate; the optical properties of nitrate aerosol are added in the improved BSTAR5C/CCSR/NCC radiative transfer model; using this model, the optical depth of nitrate aerosol in the visible bands over China in 2005 and its direct radiative forcing at the top of the atmosphere have been calculated, and their seasonal variations also have been discussed; in the end, the effect of cloud on direct radiative forcing of nitrate aerosol has been analyzed and then the following conclusions are mainly achieved:(1) The extinction, scattering, absorption coefficient and single scattering albedo in per unit volume concentration in different spectra of sulfate and nitrate aerosol are calculated. From the results a conclusion could be made that nitrate aerosol is a strong-scattering aerosol whose scattering density is a little more than sulfate aerosol's. So the radiative forcing of nitrate would be smaller when calculated with sulfate's optical properties.(2) The distribution of nitrate aerosol's optical thickness has obvious seasonal variations. The average optical thickness is the largest in winter, and smallest in summer. The distribution of optical thickness is consistent with distribution of the aerosol's column burden. Much of nitrate aerosol is located industrialized areas, which proves that the human activities has a significant impact on the distribution of nitrate aerosol's optical thickness.(3) Under clear sky conditions, the distribution of direct radiative forcing of nitrate aerosol shows a significant seasonal variation, with the maximum in spring and the minimum in summer. The distribution of radiative forcing is basically consistent with that of column burden of nitrate aerosol, but there is also sort of difference in some places. This is because apart from the column burden, the vertical distribution of aerosol, surface properties and meteorological field also have effects on radiative forcing. The radiative forcing ranges over sea in summer and autumn are significantly less than those in winter and spring because of the control weather system and the adjustment of land-sea breeze in the East Asia in different seasons.(4) Under cloudy sky conditions, the direct radiative forcing of nitrate is 2-3 times smaller than that of clear sky condition. Cloud plays a strong role in weakening the direct radiative forcing of nitrate aerosol. The greater the cloud amount is, the stronger the extinction effect is. Of course, the effect of cloud on direct radiative forcing of aerosols is also affected by other factors such as the relative position between cloud and aerosols, cloud optical depth, cloud thickness, and other various factors also have impact on direct radiative forcing of nitrate aerosol.(5) The sensitivity experiments show that solar zenith angle and surface albedo have great effects on nitrate radiative forcing. It's worth noting that when small solar zenith angle and large surface albedo exsiting, the radiative forcing of nitrate will be positive. The effects of relative position between cloud and aerosols on radiative forcing show different characteristics with different cloud optical thickness.