[A-AS06] Atmospheric Chemistry

[AAS06-08] Seasonal variation of wet deposition fluxes of black carbon in East Asia: long-term measurements

*Tatsuhiro Mori¹, Kazuhiko Miura¹, Sho Ohata², Nobuhiro Moteki², Makoto Koike², Yutaka Kondo³, Kazunori Nakagome⁴, Masanori Yoshikawa⁵, Aya Iwasaki⁶ (1.Tokyo University of Science, 2.Graduate School of Science, The University of Tokyo, 3.National Institute of Polar Research, 4.Nagano Environmental Conservation Research Institute, 5.Fukui Prefectural Institute of Public Health, 6.Okinawa Prefectural Institute of Health and Environment)

Keywords:Black carbon, Wet deposition, Field observation, East Asia

Quantitative understanding of wet removal process of black carbon (BC) is important because it controls temporal and spatial distribution of BC in atmosphere. Majority of BC-containing particles in a planetary boundary layer are mainly active as cloud condensation nuclei and removed by precipitation. To understand the wet removal process of BC, it is essential to measure BC mass concentration in air (M_BC) and in rainwater (C_BC) for a long-term period, simultaneously. In this study, a simultaneous measurement of M_BC and C_BC was made at Cape Hedo in Okinawa prefecture during 2010-2016, at Cape Echizen in Fukui prefecture during 2012-2016, and at Happo in Nagano prefecture during 2012-2016. Seasonal variations of both mass concentrations and the wet deposition fluxes (F_BC) of BC were presented.

M_BC was measured by a Continuous Soot Monitoring System (COSMOS). Rainwater samples were collected on a daily basis by using an automated wet-only sampler and C_BC was measured by consisting of a pneumatic nebulizer (Marin-5) and a single-particle soot photometer (SP2). The monthly averaged M_BC and C_BC at Cape Hedo showed a marked seasonal variation, which was highest in spring (0.25±0.11 μg m^-3 and 66.1±70.0 μg L^-1, respectively) and lowest in summer (0.06±0.03 μg m^-3 and 5.2±4.2 μg L^-1, respectively). The high M_BC and C_BC in spring were associated with transport of air masses from the Asian continent by northwesterly winds. At Cape Echizen and Happo, both monthly averaged M_BC also showed a distinct seasonal variation, which was highest in spring (0.31±0.05 μg m^-3 and 0.17±0.08 μg m^-3, respectively). On the other hand, both monthly averaged C_BC were highest in winter (19.6±14.1 μg L^-1 and 14.4±10.8 μg L^-1, respectively). The high C_BC in winter were not always associated with M_BC in a planetary boundary layer. F_BC, estimated as the product of C_BC and precipitation, also showed a distinct seasonal variation. The monthly F_BC at Cape Hedo was highest in spring (11.8 mg m^-2 month^-1) and about 80% of annual F_BC occurred in this season on average. Monthly F_BC at Cape Echizen and Happo were highest in winter (4.08 mg m^-2 month^-1 and 2.05 mg m^-2 month^-1, respectively) and about 40-50% of annual F_BC occurred in this season on average. These F_BC observed in this study were very useful data to varify F_BC calculated by climate models.