[1] The lidar (extinction-to-backscatter) ratios at 0.55 and 1.02 mm and the spectral lidar, extinction, and backscatter ratios of climatically relevant aerosol species are computed on the basis of selected retrievals of aerosol properties from 26 Aerosol Robotic Network (AERONET) sites across the globe. The values, obtained indirectly from sky radiance and solar transmittance measurements, agree very well with values from direct observations. Low mean values of the lidar ratio, S a , at 0.55 mm for maritime (27 sr) aerosols and desert dust (42 sr) are clearly distinguishable from biomass burning (60 sr) and urban/industrial pollution (71 sr). The effects of nonsphericity of mineral dust are shown, demonstrating that particle shape must be taken into account in any spaceborne lidar inversion scheme. A new aerosol model representing pollution over Southeast Asia is introduced since lidar (58 sr), color lidar, and extinction ratios in this region are distinct from those over other urban/industrial centers, owing to a greater number of large particles relative to fine particles. This discrimination promises improved estimates of regional climate forcing by aerosols containing black carbon and is expected to be of utility to climate modeling and remote sensing communities. The observed variability of the lidar parameters, combined with current validated aerosol data products from Moderate Resolution Imaging Spectroradiometer (MODIS), will afford improved accuracy in the inversion of spaceborne lidar data over both land and ocean.Citation: Cattrall, C., J. Reagan, K. Thome, and O. Dubovik (2005), Variability of aerosol and spectral lidar and backscatter and extinction ratios of key aerosol types derived from selected Aerosol Robotic Network locations,

Prior laboratory studies of Trichodesmium have shown a high iron requirement that is consistent with the biochemical demand for iron in the enzyme nitrogenase. Summer delivery of iron, in the form of Saharan dust, may provide an explanation for Trichodesmium blooms observed in offshore waters of the West Florida shelf over the last 50 yr. During ecology and oceanography of harmful algal blooms (ECOHAB) field studies, background iron levels (0.1-0.5 nmol kg Ϫ1 ) were found at the surface during periods of minimal dust delivery (May 2000 and October 1999). In contrast, total dissolved iron concentrations on the order of ϳ16 nmol kg Ϫ1 were measured at the West Florida shelf-break after a July 1999 Saharan dust event that was identified by advanced very high resolution radiometer (AVHRR) imagery, ground-based radiometers, air mass analysis, and aerosol samples (dust and non-sea-salt nitrate) collected throughout South Florida. The Trichodesmium response following this July dust event was a 100-fold increase over background biomass, reaching a surface stock of ϳ20 colonies L Ϫ1. Surface dissolved concentrations of both inorganic and organic phosphorus decreased below detectable limits during this bloom. Dissolved organic nitrogen concentrations associated with the bloom (15-20 M) were 3-4-fold greater than background and much larger than ambient NO concentrations (Ͻ0.5 mol kg Ϫ1 ). If all dissolved organic Ϫ 3 nitrogen (DON) is converted to urea and ammonium, this organic nitrogen could have supported the red tide of Ͼ20 g chl L Ϫ1 of the toxic dinoflagellate, Gymnodinium breve, found along the West Florida coast during October 1999.

[1] The single-scattering albedo and phase function of African mineral dust are retrieved at 14 wavelengths across the visible spectrum from ground-based measurements of the aerosol optical thickness and the sky radiance taken in the solar principal plane. The retrieval algorithm employs the radiative transfer equation to solve by iteration for these properties that best reproduce the observed sky radiance, and is therefore independent of particle shape. The estimated error in the retrieved single-scattering albedo is less than 0.02 due to the precision of the solar-reflectance-based calibration of the radiometer. The phase function retrieved at 860 nm is robust under simulations of expected experimental errors and may be used to characterize aerosol scattering at the directly measured scattering angles (i.e., Â 155°). The phase function retrieved at 443 nm, however, is too sensitive to such errors to confidently describe the angular scattering at blue wavelengths. The single-scattering albedo displays a spectral shape expected of iron-bearing minerals but is much higher than climate models have assumed, indicating that wind-blown mineral dust cools Earth more than is generally believed. The method may be applied to any combination of airborne and ground-based measurements over the ocean and can complement more involved ground-based retrievals through its insensitivity to particle shape and ability to retrieve aerosol properties at relatively small aerosol optical depths.INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1640 Global Change: Remote sensing; 1694 Global Change: Instruments and techniques; 4275 Oceanography: General: Remote sensing and electromagnetic processes (0689); KEYWORDS: aerosol, mineral dust, absorption, sky radiance, scattering, optical properties Citation: Cattrall, C., K. L. Carder, and H. R. Gordon, Columnar aerosol single-scattering albedo and phase function retrieved from sky radiance over the ocean: Measurements of Saharan dust,