. Strong black carbon absorption enhancement attributed to secondary organic aerosol formation in Seoul

Abstract
Black carbon (BC), an aerosol produced through incomplete combustion, acts as a short-term climate forcer through its direct absorption of solar radiation and through its indirect impacts on clouds. A significant portion of BC is of anthropogenic origin and originates in polluted urban environments, where co-emitted species impact both its lifetime and optical properties. However, the impacts of co-emitted species on BC's radiative effects and removal efficiency are still highly uncertain. Observations of BC mass concentration, size, coating thickness, and hygroscopicity of associated internally mixed materials were made with a dual humidified single particle soot photometer (HD-SP2) during the May/June 2016 KORUS-AQ aircraft campaign in South Korea. These data were used to investigate the evolution of BC in a polluted megacity (the Seoul Metropolitan Area). Systematic, repeated aircraft measurements over a 6-week period at different times of the day indicate significant diurnal variability in both BC mass concentrations and coating thickness. Secondary organic aerosol formation significantly enhanced BC absorption over a period of ~1 day, as reflected by both SP2-constraints on BC-containing aerosol properties and fine- mode aerosol absorption measured with a PSAP. We estimated absorption enhancement increased by a factor of 2 over 24 hours (based on air-mass photochemical age), while the hygroscopicity of materials internally mixed with BC remained relatively constant. These observations provide important atmospheric constraints on the role of co-emitted species for BC's radiative effects and lifetime in East Asia, the most important anthropogenic source region for BC.