دانلود رایگان مقاله تعیین کمیت بازتاب پراکندگی آئروسل آلی برفراز مناطق سوزان

Abstract

Despite growing evidence of light-absorbing organic aerosols (OAs), their contribution to the Earth's radiative budget is still poorly understood. In this study we derived a new empirical relationship that binds OA single scattering albedo (SSA), which is the ratio of light scattering to extinction, with sulfate + nitrate aerosol optical depth (AOD) and applied this method to estimate OA SSA over the tropical biomass burning regions. This method includes division of the attribution of black carbon (BC) and OA absorption aerosol optical depths from the Aerosol Robotic Network (AERONET) observation and determination of the fine-mode ratio of sea-salt and dust AODs from several atmospheric chemistry models. Our best estimate of OA SSA over the tropical biomass burning regions is 0.91 at 550 nm. Uncertainties associated with observations and models permit a value range of 0.82–0.93. Furthermore, by using the estimated OA SSA and comprehensive observations including AERONET, Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR), we examined the first global estimate of sulfate + nitrate AOD through a semi-observational approach. The global mean sulfate + nitrate AOD of 0.017 is in the lower range of the values obtained from 21 models participated in AeroCom phase II. The results imply that most aerosol models as well as climate models, which commonly use OA SSA of 0.96–1.0, have so far ignored light absorption by OAs and have overestimated light scattering by sulfate + nitrate aerosols. This indicates that the actual aerosol direct radiative forcing should be less negative than currently believed.

4. Conclusion

We developed an empirical method for determining OA SSA over the tropical biomass burning regions. A simplified process flow chart for deriving OA SSA is described in Fig. 2. The basic concept of the method is that atmospheric fine particles are composed of sulfate þ nitrate, BC, OA, fdust, and fss. By using fAOD and AAOD data from the monthly ground-based AERONET observation, we formulated an equation for sulfate þ nitrate AOD as a function of OA SSA with the optically separated BC and OA AAODs from total AAOD and the measured BC SSA. We derived two sets of observationally constrained AAOD separation methods from C2012 and B2012. With regard to BC SSA, we used two measured values of 0.25 from Bond and Bergstrom (2006) and 0.19 from Magi (2009, 2011). For the fdust and fss AODs, a semi-observational approach was proposed that obtains their AOD by multiplying the fine-mode ratio of dust þ sea-salt AOD from the multi-model mean by the observed cAOD. Although the multi-model mean fine-mode ratio of dust þ sea-salt AOD was used, their contribution to sulfate þ nitrate AOD was relatively small. Therefore, the final equation, i.e., Eq. (5) or its applied version in Eq. (6), is predominantly an observationally constrained approach in which the equation to determine the sulfate þ nitrate AOD has a sole parameter (i.e., OA SSA). Our empirical method for determining OA SSA contains three independent uncertainty factors in (1) separating BC and OA AAODs from the total AAOD, (2) using model simulations for fdust and fss AODs, and (3) adopting the measured BC SSA. A quantitative comparison of these factors in determining sulfate þ nitrate AOD and OA SSA was presented in detail.