An Aerosol Climatology at Kyoto: Observed Local Radiative Forcing and Columnar Optical Properties

Abstract

In order to evaluate the radiative effect of the atmospheric aerosol at Kyoto, Japan, surface solar irradiance and columnar aerosol optical properties were observed in the period between September 1998 and December 2001. The aerosol optical thickness, which is an indicator of the columnar mass burden and the overall radiative effect of the aerosol, was on average 0.27 at a wavelength of 500 nm. Springtime aerosol optical thickness was generally higher primarily because of ?yellow dust? from the Asian continent. The Ångström exponent had values ranging from 0.5 to 2.8, with an average value of 1.64, and was found to be low in periods during which the aerosol optical thickness was high. As a first step toward calculating the local climate impact of the atmospheric aerosol at Kyoto, the clear-sky direct radiative forcing is considered in this paper. For an evaluation of the surface aerosol radiative forcing, observed total surface fluxes measured by a pyranometer are subtracted from modeled surface fluxes derived from a non-aerosol-laden atmosphere. From the obtained relationship between the aerosol optical thickness and the surface aerosol radiative forcing, it is concluded that there is a high variability in the physical and chemical characteristics of the aerosol at this location. The surface radiative forcing efficiency, which is the radiative forcing for unit optical thickness, was ?85.4 W m?2 on average at Kyoto. This observed value is very similar to recently observed surface aerosol radiative forcings during the Indian Ocean Experiment (INDOEX) and the Asia Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaigns. The aerosol radiative forcing at the top of the atmosphere (TOA) was calculated from measured in situ aerosol optical properties and retrieved properties from a comparison of measured and simulated ground solar irradiances. While employing average aerosol optical properties at Kyoto, comprehensive cooling at the TOA was found. It was also found that the difference between the surface and the TOA aerosol radiative forcings produced a large amount of atmospheric heating because of the relatively low single-scattering albedo. Surface forcing was about 3 times as high as the TOA forcing at Kyoto. The main conclusion of this study is that the aerosol over Kyoto significantly alters the atmospheric energy budget. The aerosol is highly variable in terms of its optical properties, so that a highly nonlinear relationship exists between the surface radiative forcing and the aerosol optical thickness.