Contribution of ammonium nitrate to aerosol optical depth and direct radiative forcing by aerosols over East Asia

摘要

This study focused on the contribution of ammonium nitrate (NH₄NO₃)to aerosol optical depth (AOD) and direct radiative forcing (DRF) by aerosolsover an East Asian domain. In order to evaluate the contribution, chemistry-transport model(CTM)-estimated AOD was combined with satellite-retrieved AOD, utilizing a dataassimilation technique, over East Asia for the entire year of 2006. Using theassimilated AOD and CTM-estimated aerosol optical properties, the DRF byaerosols was estimated over East Asia via a radiative transfer model (RTM).Both assimilated AOD and estimated DRF values showed relatively goodagreements with AOD and DRF by aerosols from AERONET. Based on these results,the contributions of NH₄NO₃ to AOD and DRF by aerosols(ΦAOD and ΦDRF) were estimated for the four seasons of 2006over East Asia. Both ΦAOD and ΦDRF showed seasonalvariations over East Asia within the ranges between 4.7% (summer) and31.3% (winter) and between 4.7% (summer) and 30.7% (winter),respectively, under clear-sky conditions, showing annual averagecontributions of 15.6% and 15.3%. Under all-sky conditions,ΦDRF varied between 3.6% (summer) and 24.5% (winter), showingannual average contribution of 12.1% over East Asia. These annual averagecontributions of NH₄NO₃ to AOD and DRF are almost comparable to theannual average mass fractions of NH₄NO₃ in PM_2.5 and PM₁₀(17.0% and 14.0%, respectively). ΦAOD and ΦDRF wereeven larger in the locations where NH₃ and NO_x emission rates arestrong, such as the central East China (CEC) region and Sichuan Basin. Forexample, under clear-sky conditions, both ΦAOD and ΦDRFover the CEC region range between 6.9% (summer) and 47.9% (winter) andbetween 6.7% (summer) and 47.5% (winter), respectively. Based on thisanalysis, it was concluded that both ΦAOD and ΦDRF cannotbe ignored in East Asian air quality and radiative forcing studies,particularly during winter.