Comparison of aerosol optical depth inferred from surface measurements with that determined by Sun photometry for cloud‐free conditions at a continental U.S. site

Abstract

Abstract. Evaluation of the forcing of climate by aerosol scattering of shortwave radiation in cloud-free conditions (direct aerosol forcing) requires knowledge of aerosol optical properties on relevant spatial and temporal scales. It is convenient to measure these properties at the surface. However, before these measurements can be used to quantitatively estimate direct climate forcing, it is necessary to determine the extent to which these properties are representative of the entire atmospheric column. In this paper we compare aerosol optical depth (AOD) determined by Sun photometry at the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site in north central Oklahoma for several cloud-free days with estimates of AOD based on two methods. First, the aerosol extinction measured at the surface (taken as the sum of the aerosol scattering and absorption coefficients at instrumental relative humidity of-20%) is multiplied by the mixing height determined from temperature profiles from radiosonde measurements. Even under conditions of vigorous midday mixing this approach underestimates AOD by as much as 70% using dry aerosol measurements and by roughly 40% when hygroscopic growth of aerosol under ambient relative humidity is taken into account. This discrepancy is attributed primarily to underestimation of aerosol column extinction, as confirmed by examination of normalized aerosol backscatter profiles obtained from micropulse lidar (MPL), which show substantial contributions of aerosol loading above the atmospheric boundary layer. The second approach uses MPL profiles of normalized aerosol backscatter to estimate the vertical profile of aerosol extinction using surface values. The resulting AOD's are on average 30% less than measured values. This discrepancy is attributed to hygroscopic growth of aerosols in the atmospheric column. The results show that at the SGP site even under conditions of vigorous mixing in the atmospheric boundary layer the aerosol optical depth cannot be estimated with surface measurements of aerosol extinction unless information on the vertical profile of aerosol extinction is taken into account.

Retrieval of High-Resolution Atmospheric Particulate Matter Concentrations from Satellite-Based Aerosol Optical Thickness over the Pearl River Delta Area, China

“…The variations of water vapor could lead to the swelling or condensation of particles, and consequently the change of the microstructure and composition of the particles, which affects the relationship between AOT and PM. Vertical profile of aerosols could also introduce errors in the relationship, because of the significant contribution of aerosols above the boundary layer to extinction, especially when weak atmospheric convection happens [48]. Therefore, information on the aerosol vertical profiles from ground-based lidars and space-borne lidars, such as the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), could be applied into the retrieval of PM from satellite data.…”

Comparison of surface and column measurements of aerosol scattering properties over the western North Atlantic Ocean at Bermuda

“…However they did find at their site that the fraction of small particles was larger near the surface relative to higher altitudes. Bergin et al (2000) concluded that attempts to predict column AOD from surface measurements, both with and without associated lidar measurements, underestimated the measured column AOD by between 30 and 70 %. Slater and Dibb (2004) found, in rural New England, that there was only a moderate correlation of surface and column aerosol properties.…”

“…These measurements are often assumed to be representative of the overlying aerosol column, or at least of the aerosol boundary layer. Several experiments have been conducted to characterize the connection between surface and column aerosol properties (Bergin et al, 2000;Andrews et al, 2004;Slater and Dibb, 2004;Sheridan et al, 2012;Quinn et al, 2004). Most of these experiments were at continental locations including Oklahoma (Bergin et al, 2000;Andrews et al, 2004), rural New England (Slater and Dibb, 2004), central Illinois (Sheridan et al, 2012), and Big Bend, Texas (Hand et al, 2004).…”

Classifying aerosol type using in situ surface spectral aerosol optical properties

“…SAE represents the wavelength dependence of scattering and varies inversely with particle size, so that small values of SAE indicate larger aerosol particles (e.g., dust and sea salt), and large values of SAE indicate relatively smaller aerosol particles (Schuster et al, 2006;Bergin et al, 2000, and references therein). AAE represents the wavelength dependence of absorption and depends on the composition of absorbing aerosols, such that aerosol materials have a unique range of AAE values Bergstrom et al, 2002Bergstrom et al, , 2007.…”