BNL-66601-00/04-Rev. 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

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. 1 . I n t r o d u c t i o n The decrease in planetary absorption of shortwave radiation due to scattering by anthropogenic aerosols during clear-sky conditions, termed direct aerosol radiative forcing, is estimated to be roughly I W m-z on a global annual average [Churlson et al., 1992; Kiehl and BriegLeb, 1993; Intergovernmental Panel on Climute Change, I9951 and may be as great as 50 W rn~: locally and instantaneously near source regions [Schwartz, 19961. In large part because of the patchy nature of aerosol forcing on both temporal and spatial scales, as well as the general lack of knowledge of aerosol radiative properties, the uncertainty in estimates of global mean direct aerosol radiative forcing is at least ‘Environmental Chemistry Division, Brookhaven NatIonal Laboratory, Upton, New York. 2 Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado. ’ Now at Georgia Institute of Technology, School of Civil and Environmental Engineering and School of Earth and Atmospheric Sciences, Atlanta, Georgia. ’ Science Systems and Applications, NASA Goddard Space Flight Center, Greenbelt, Maryland. Copyright 2000 by the American Geophysical Union. Paper number l999JD900454 Ol48-0227/00/~999JD900454$09.00 a factor of two [Penner et al., 1994; IPCC, 1995; Schwartz and Andreae, 1996; Quinn et al., 199G]. The key aerosol property governing direct shortwave radiative forcing is aerosol optical depth (AOD) (integral of the aerosol extinction coefficient with height; throughout the paper the term AOD refers to the vertical optical depth, i.e., air mass equal to one) which is a measure of the aerosol loading, or “extensive” aerosol property [Ogren et uZ., 19961. Pertinent “intensive” aerosol properties are single scattering aibedo (fraction of aerosol extinction that is due to scattering versus absorption), and upscatter fraction (fraction of light scattered into the upward hemisphere) [Charlson et al. 1992; Haywood and Shine, 1995; Boucher and Anderson, 1995; Nemesure et al., 1995; Schwartz 19961. Estimates of direct radiative forcing have generally relied on surface measurements of aerosol radiative properties [Charkon et al., 1992; IPCC, I9951 under the assumption that aerosol properties over the entire tropospheric column are similar to those at the surface. However, it has yet to be shown on a systematic basis that aerosol optical properties at the surface are suitably representative of the integrated column properties to justify this assumption. Several recent studies have raised concern over the determination of AOD by Sun photometry [Kate et LIZ., 1997; Halthore et ul., 19981. Model estimates of the broadband direct normal solar irradiance during cloud-f?ee conditions based on AOD values determined from Sun photometry agree with measured values, but the same AOD’s lead to an overestimation