Measurements of Optical Properties of Aerosols by Cavity Ring-Down Spectroscopy in Mega-City Tokyo, Japan

Aerosol particles have an important role in radiation balance in the atmosphere by scattering and absorbing incident light. Therefore, accurate determination of the optical properties of atmospheric aerosols is essential. There are two components to aerosol optical extinction: scattering and absorption. Extinction coefficients of atmospheric aerosol have commonly been determined by measuring scattering coefficients using nephelometer (Neph) and absorption coefficients using an aethalometer or particle/soot absorption photometer (PSAP). Both the aethalometer and PSAP rely on collection of the aerosol particles onto a filter substrate followed by a measurement of light reflectance and/or transmittance. Because of the temporal variations of the conversion parameter, these techniques may have relatively large uncertainties. Extinction coefficients can also be measured using cavity ring-down spectroscopy (CRDS). In this work, simultaneous measurements of extinction, scattering, and absorption coefficients of the ambient aerosols were performed in central Tokyo from 14 August to 2 September 2007 using a newly developed CRDS instrument, Neph and PSAP, respectively. Gas phase ozone concentrations were also measured using a commercial instrument. Extinction coefficients measured using CRDS were compared with those determined from the sum of scattering and absorption coefficients measured by nephelometer and PSAP. By comparing the single scattering albedo (SSA) calculated by three different combinations from extinction, scattering, and absorption, it is suggested that the PSAP overestimates absorption coefficients by a factor of 2. By taking the overestimation of the PSAP into account, the extinction coefficients measured by CRDS were in excellent agreement with those for Neph+PSAP with the ratio of (Neph+PSAP)/CRDS = 1.02, except for the data on days when high concentrations of ozone (more than 130 ppbv) were observed. For the data when high concentrations of ozone were observed, extinction coefficients for Neph+PSAP were 15-30 % larger than those for CRDS. Using the corrected data, SSA values during summertime in central Tokyo are estimated.