Diurnal and seasonal characteristics of the optical properties and direct radiative forcing of different aerosol components in Seoul megacity.

The temporal variations (diurnal and seasonal) of the optical properties and direct aerosol radiative forcing (DARF) of different aerosol components (water-soluble, insoluble, black carbon (BC), and sea-salt) were analyzed using the hourly resolution data (PM2.5) measured at an urban site in Seoul, Korea during 2010, based on a modeling approach. In general, the water-soluble component was predominant over all other components (with a higher concentration) in terms of its impact on the optical properties (except for absorbing BC) and DARF. The annual mean aerosol optical depth (AOD, τ) at 500nm for the water-soluble component was 0.38±0.07 (0.06±0.01 for BC). The forcing at the surface (DARFSFC) and top of the atmosphere (DARFTOA), and in the atmosphere (DARFATM) for most aerosol components (except for BC) during the daytime were highest in spring and lowest in late fall or early winter. The maximum DARFSFC occurred in the morning during most seasons (except for the water-soluble components showing peaks in the afternoon or noon in summer, fall, or winter), while the maximum DARFTOA occurred in the morning during spring and/or winter and in the afternoon during summer and/or fall. The estimated DARFSFC and DARFATM of the water-soluble component were in the range of -49 to -84Wm-2 and +10 to +22Wm-2, respectively. The DARFSFC and DARFATM of BC were -26 to -39Wm-2 and +32 to +51Wm-2, respectively, showing highest in summer and lowest in spring, with morning peaks regardless of the season. This positive DARFATM of BC in this study area accounted for approximately 64% of the total atmospheric aerosol forcing due to strong radiative absorption, thus increasing atmospheric heating by 2.9±1.2Kday-1 (heating rate efficiency of 39K day-1τ-1) and then causing further atmospheric warming.