Comparison of in situ and columnar aerosol spectral measurements during TexAQS-GoMACCS 2006: testing parameterizations for estimating aerosol fine mode properties

Abstract

During the 2006 Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS 2006), the optical, chemical and microphysical properties of atmospheric aerosols were measured on multiple mobile platforms and at ground based stations. In situ measurements of the aerosol light extinction coefficient (&sigma;_ep) were performed by two multi-wavelength cavity ring-down (CRD) instruments, one located on board the NOAA R/V <i>Ronald H. Brown</i> (RHB) and the other located at the University of Houston, Moody Tower (UHMT). An AERONET sunphotometer was also located at the UHMT to measure the columnar aerosol optical depth (AOD). The &sigma;_ep data were used to extract the extinction Ångström exponent (&aring;_ep), a measure of the wavelength dependence of &sigma;_ep. There was general agreement between the &aring;_ep (and to a lesser degree &sigma;_ep) measurements by the two spatially separated CRD instruments during multi-day periods, suggesting a regional scale consistency of the sampled aerosols. Two spectral models are applied to the &sigma;_ep and AOD data to extract the fine mode fraction of extinction (η) and the fine mode effective radius (<i>R</i>_eff,f). These two parameters are robust measures of the fine mode contribution to total extinction and the fine mode size distribution, respectively. The results of the analysis are compared to <i>R</i>_eff,f values extracted using AERONET V2 retrievals and calculated from in situ particle size measurements on the RHB and at UHMT. During a time period when fine mode aerosols dominated the extinction over a large area extending from Houston/Galveston Bay and out into the Gulf of Mexico, the various methods for obtaining <i>R</i>_eff,f agree qualitatively (showing the same temporal trend) and quantitatively (pooled standard deviation = 28 nm).