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This study focused on the contribution of ammonium nitrate (NH₄NO₃) to aerosol optical depth (AOD) and direct radiative forcing (DRF) by aerosols over an East Asian domain. In order to evaluate the contribution, chemistry-transport model (CTM)-estimated AOD was combined with satellite-retrieved AOD, utilizing a data assimilation technique, over East Asia for the entire year of 2006. Using the assimilated AOD and CTM-estimated aerosol optical properties, the DRF by aerosols was estimated over East Asia via a radiative transfer model (RTM). Both assimilated AOD and estimated DRF values showed relatively good agreements 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 2006 over East Asia. Both ΦAOD and ΦDRF showed seasonal variations over East Asia within the ranges between 4.7% (summer) and 31.3% (winter) and between 4.7% (summer) and 30.7% (winter), respectively, under clear-sky conditions, showing annual average contributions of 15.6% and 15.3%. Under all-sky conditions, ΦDRF varied between 3.6% (summer) and 24.5% (winter), showing annual average contribution of 12.1% over East Asia. These annual average contributions of NH₄NO₃ to AOD and DRF are almost comparable to the annual average mass fractions of NH₄NO₃ in PM_2.5 and PM₁₀ (17.0% and 14.0%, respectively). ΦAOD and ΦDRF were even larger in the locations where NH₃ and NO_x emission rates are strong, such as the central East China (CEC) region and Sichuan Basin. For example, under clear-sky conditions, both ΦAOD and ΦDRF over the CEC region range between 6.9% (summer) and 47.9% (winter) and between 6.7% (summer) and 47.5% (winter), respectively. Based on this analysis, it was concluded that both ΦAOD and ΦDRF cannot be ignored in East Asian air quality and radiative forcing studies, particularly during winter.