Quantification of Aerosol Wet Deposition Using a Meteorology-Chemistry Model

Aerosols affect the radiative properties of clouds by increasing the cloud particle number concentration and the lifetime of clouds by decreasing cloud particle size. On the other hand, clouds affect aerosol production and removal that occurs through rainfall and snowfall (wet deposition). The aerosol-cloud interactions are highly uncertain although the interactions have a major impact on the global radiation budget. While this understanding is important, there remain many unanswered questions. We showed a reduction in PM_2.5 due to precipitation by observational data [1], but the details of the wet deposition process had not been discussed. Therefore, a simulation using a meteorology-chemistry model was carried out with the aim of identifying which processes are effective in wet deposition.

In this study, we used Offline Scale-Chem [2], an offline coupled meteorological model [3][4] and chemical transport model [5][6]. The initial and boundary conditions were obtained from MANL and the calculation period was from 201910010000 to 11010000. We calculated the contribution of below-cloud scavenging (washout). It is revealed that a higher contribution of the total wet deposition was due to the washout process for nitrate than for sulfate. The averaged washout contribution was 59.7% for sulfate and 74.1% for nitrate. The analysis of the contribution of the washout process to the change in the average of PM_2.5 concentrations showed that the amount of change in PM_2.5 concentrations was -4.28 μg/m³, quantitatively indicating that the washout process contributed to the reduction of PM_2.5 concentrations. The present results suggest that the deposition processes are effective in reducing PM_2.5 concentration.

[1] Fujino and Miyamoto, 2022. [2] Nakata et al., 2021. [3] Nishizawa et al., 2015. [4] Sato et al., 2015. [5] Kajino et al., 2019. [6] Kajino et al.,2021.