Abstract
Abstract. 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 PM2.5 and PM10 (17.0% and 14.0%, respectively). ΦAOD and ΦDRF were even larger in the locations where NH3 and NOx 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.
Highlights
Ammonium nitrate (NH4NO3) is an important particulate constituent that is mainly produced via reversible heterogeneous reaction of gaseous ammonia (NH3) and gaseous nitric acid (HNO3) (i.e., NH3(g) + HNO3(g) ↔ NH4NO3(p))
In order to estimate the contribution of NH4NO3 to aerosol optical depth (AOD) and direct radiative forcing (DRF) by aerosols in East Asia, meteorological model (MM), chemistry-transport model (CTM), and radiative transfer model (RTM) simulations were carried out sequentially in this study
In order to confirm the accuracy of the CMAQestimated particulate concentrations, a comparative analysis between CMAQ-estimated and EANET/CAWNETobserved particulate concentrations was conducted
Summary
Ammonium nitrate (NH4NO3) is an important particulate constituent that is mainly produced via reversible heterogeneous reaction of gaseous ammonia (NH3) and gaseous nitric acid (HNO3) (i.e., NH3(g) + HNO3(g) ↔ NH4NO3(p)). Since NH4NO3 is a volatile species, its formation has been observed at locations where partial pressures or mixing ratios of NH3 and HNO3 are high (i.e., typically urban and/or highly populated areas). For this reason, the NH4NO3 formation has been investigated over urban or highly polluted areas (e.g., Zheng et al, 2002; Kim et al, 2006), whereas it has been largely neglected in global air quality modeling studies (e.g., van Dorland et al, 1997; Chin et al, 2001; Takemura et al, 2002). Many previous studies have reported that the influence of NH4NO3 on aerosol optical properties (AOPs) and on direct radiative forcing (DRF) by aerosols is insignificant
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