Simultaneous retrievals of column ozone and aerosol optical properties from direct and diffuse solar irradiance measurements

Abstract

[1] A retrieval technique has been developed to simultaneously determine column ozone amounts and aerosol optical properties using surface observations of solar ultraviolet direct normal and diffuse horizontal irradiance from a multifilter rotating shadowband radiometer. The retrieval consists of a Bayesian scheme involving a tropospheric ultraviolet radiative transfer model. The technique was tested using cloud-free observations collected during a Mexico City Metropolitan Area air pollution field campaign from April to May 2003. Retrieval results compared favorably to those of independent techniques, including ozone amounts from a direct-Sun method, Langleyderived aerosol optical depths, and aerosol single-scattering albedos from a direct-todiffuse irradiance ratio technique. Further comparisons were performed between the measurements and model simulations when using the retrieval results as inputs, both from the proposed technique and the combined independent methods. Simulations using the results of the new method were found to agree with the observations within the assumed limits of measurement and model uncertainty. It is anticipated that the technique will be applied across a 33-site network of radiometers maintained by the U.S. Department of Agriculture UV-B Monitoring and Research Program for development of aerosol climatologies and for providing ground validation for satellite measurements.

Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area

“…However, this is not a problem because, for λ<415 nm, the albedo is very low for almost all surfaces (exception for snow and ice) and minor mischaracterizations of albedo induce negligible error in retrieved 0,λ . Including all sources of error, the uncertainty of the 0,λ values are about ±0.03 for all wavelengths Goering et al, 2005). This estimate does not seem to apply to the 615-nm wavelength channel because the 0,λ values often exceeded 1.0.…”

“…The actinic fluxes from the SR were inverted to find 0,λ using a different method, based on the TUV4.4 model. Goering et al (2005) and Michalsky and Kiedron (2008) describe the use of this model applied to surface irradiance measurements to find 0,λ from either UV-MFRSR measurements or Rotating Shadowband Spectrometer measurements. The method we employ here is slightly different than theirs because we use the TUV4.4 with actinic flux, not irradiance, to find 0,λ .…”

“…This good agreement, coming from two different instruments with independent calibrations, bolsters the view that the 0,λ values are reasonable. Second, 0,λ derived from co-located UV-MFRSR measurements (Goering et al, 2005) is shown for a wavelength of 368 nm, as well as the error bars (±0.03) associated with this retrieval. Agreement to within the retrieval error is evident for the 0,λ derived from the SR and UV-MFRSR.…”

“…The columnar absorption is calculated as the columnar abundance multiplied by the appropriate absorption coefficient, a function of wavelength. The raw τ ext,λ values also were corrected from ozone absorption in the Chappius and Hartley-Higgins bands (Goody and Yung, 1989), assuming a columnar ozone value of 275 DU, a climatological value for the atmospheric column in April 2003 obtained from the satellite borne Total Ozone Mapping Spectrometer (TOMS) instrument (http://jwocky.gsfc.nasa.gov). Over all cases considered here, maximum optical thicknesses for these absorbing species are 1.67, 0.03, and 0.04, for ozone (303 nm), NO 2 (400 nm), and SO 2 (300 nm), respectively.…”

“…Plots of τ ext,λ determined from Eq. (4) overlaid by MFRSR-derived τ ext,λ values, as well as an additional value from the UV-MFRSR at 368 nm (Goering et al, 2005), indicate that theÅngström relationship is satisfied well between the wavelengths of 368 nm and 870 nm. This in turn suggests that the extrapolation of the extinction to 300 nm will be reasonably good, although optical thickness measurements at 300 nm would be necessary to confirm this assertion.…”