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, tau) at 500 nm for the water-soluble component was 0.38 +/- 0.07 (0.06 +/- 0.01 for BC). The forcing at the surface (DARF(SFC)) and top of the atmosphere (DARF(TOA)), and in the atmosphere (DARF(ATM)) for most aerosol components (except for BC) during the daytime were highest in spring and lowest in late fall or early winter. The maximum DARF(SFC) 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 DARF(TOA) occurred in the morning during spring and/or winter and in the afternoon during summer and/or fall. The estimated DARF(SFC) and DARF(ATM) of the water-soluble component were in the range of -49 to -84 W m(-2) and +10 to +22 W m(-2), respectively. The DARF(SFC) and DARF(ATM) of BC were -26 to -39 W m(-2) and +32 to +51 W m(-2), respectively, showing highest in summer and lowest in spring, with morning peaks regardless of the season. This positive DARF(ATM) 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 +/- 12 K day(-1) (heating rate efficiency of 39 K day(-1) tau(-1)) and then causing further atmospheric warming. (C) 2017 Elsevier B.V. All rights reserved.

Temporal variations of optical properties of urban aerosol in Seoul were estimated by the Optical Properties of Aerosols and Clouds (OPAC) model, based on hourly aerosol sampling data in Seoul during the year of 2010. These optical properties were then used to calculate direct radiative forcing during the study period. The optical properties and direct radiative forcing of aerosol were calculated separately for four chemical components such as water-soluble, insoluble, black carbon (BC), and sea-salt aerosols. Overall, the coefficients of absorption, scattering, and extinction, as well as aerosol optical depth (AOD) for water-soluble component predominated over three other aerosol components, except for the absorption coefficient of BC. In the urban environment (Seoul), the contribution of AOD (0.10 similar to 0.12) for the sum of OC and BC to total AODs ranged from 23% (spring) to 31% (winter). The diurnal variation of AOD for each component was high in the morning and low in the late afternoon during the most of seasons, but the high AODs at 14: 00 and 15: 00 LST in summer and fall, respectively. The direct negative radiative forcing of most chemical components (especially, NO3 -of water-soluble) was highest in January and lowest in September. Conversely, the positive radiative forcing of BC was highest in November and lowest in August due to the distribution pattern of BC concentration.