[A-AS07] 大気化学

[AAS07-07] Importance of Arctic dust with high ice nucleating ability in Arctic mixed-phase clouds

*河合 慶¹、松井 仁志¹、當房 豊^2,3 (1.名古屋大学、2.国立極地研究所、3.総合研究大学院大学)

キーワード：北極ダスト、氷晶核、混合相雲、全球エアロゾルモデル

The Arctic is warming more rapidly than any other regions of the world. Recent observations show that dust is emitted from ice- and vegetation-free areas (e.g., glacial outwash plains) in the Arctic region (hereafter Arctic dust), which has a remarkably high ice nucleating ability under conditions relevant for the formation of mixed-phase clouds (consisting of both supercooled water droplets and ice crystals), compared with desert dust such as Asian dust and Saharan dust, because of the presence of certain organic matter. However, the impacts of Arctic dust with high ice nucleating ability on the number concentrations of ice nucleating particles (INPs) and radiative balance in the Arctic are not well understood. In this study, we incorporate an observation-based ice-nucleation parameterization indicating the high ice nucleating ability of Arctic dust into the global aerosol-climate model CAM-ATRAS. Arctic dust is mostly emitted in summer and fall, when the Arctic surface temperatures are high and snow coverage is low, and distributed in the Arctic lower troposphere (>60°N and >700 hPa). If Arctic dust has the same ice nucleating ability as desert dust, it does not act efficiently as INPs in the Arctic lower troposphere in summer and fall because the ice nucleating ability of desert dust is low at temperatures warmer than about −15°C. However, when considering the high ice nucleating ability of Arctic dust, INP observations at various locations in the Arctic region are generally better reproduced; the number concentrations of Arctic and total dust INPs in the Arctic lower troposphere during summer and fall increase by more than a factor of 100; and almost all of the total dust INPs are contributed from Arctic dust. This substantial increase in Arctic dust INPs is caused by the higher ice nucleating ability of Arctic dust than desert dust at temperatures between −20°C and −5°C, which is typical of the Arctic lower troposphere in summer and fall. The increase in Arctic dust INPs enhances the annual-mean effects of total dust INPs on net cloud radiative forcing at the top of the atmosphere and net downwelling radiative flux at the surface in the Arctic by 0.03 and 0.04 W m⁻², respectively. These values should be further investigated because these estimates are influenced by uncertainties in many factors (e.g., the spatial distributions and concentrations of dust, INPs, and cloud ice and water). Our results show that Arctic dust with the observed high ice nucleating ability plays a dominant role in the number concentrations of Arctic and total dust INPs in the Arctic lower troposphere during summer and fall. This study therefore demonstrates the importance of considering an ice-nucleation parameterization suitable for Arctic dust when simulating INPs in the Arctic, and their impacts on aerosol-cloud interactions in the Arctic need to be more accurately estimated.