Abstract. The Angstrom wavelength exponent ct, which is the slope of the logarithm of aerosol optical depth (xa) versus the logarithm of wavelength ()•), is commonly used to characterize the wavelength dependence of xa and to provide some basic information on the aerosol size distribution. This parameter is frequently computed from the spectral measurements of both ground-based sunphotometers and from satellite and aircraft remote sensing retrievals. However

Abstract. Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented. IntroductionMan is altering the aerosol environment through land cover change, combustion of fossil fuels, and the introduction of particulate and gas species to the atmosphere. Each perturbation has some impact on the local aerosol environment. How much aerosol man is contributing to the atmosphere is not •øUniversity of New Mexico, Albuquerque, New Mexico.•qnstituto de Pesquisas Espaciais, Sao Jose dos Campos, San Paolo, Brazil.•2National Oceanic and Atmospheric Administration, Silver Spring, Maryland.•3Scripps Institute of Oceanography, La Jolla, California.•4Department of Applied Science, Brookhaven National Laboratory, Upton, New York.•SNow at Naval Research Laboratory, Washington, D.C.•6Ben Gurion University of the Negev, Sede Boker, Israel.•7CARTEL, Universit6 de Sherbrooke, Sherbrooke, Quebec, Canada.•sSAIC-GSC, Beltsville, Maryland, and NASA GSFC, Greenbelt, The simplest, and, in principle, the most accurate and easy to maintain monitoring systems are ground based. Aerosol optical depth is the single most comprehensive variable to remotely assess the aerosol burden in the atmosphere from groundbased instruments. This variable is used in local investigations to characterize aerosols, assess atmospheric pollution, and make atmospheric corrections to satellite remotely sensed data. It is for these reasons that a record of aerosol optical depth spanning most of the twentieth century has been measured from Sun photometers. The vast majority are site specific, short-term investigations with little relevance for seasonal, annual, or long-term trend analysis, however a few multiyear spatial studies have contributed to our knowledge and experience (Table 1). The following section reviews these investigations, past and present, which significantly addressed long-term measurements over widely distributed locations or provided a significant contribution that allowed development of a network for long-term photometric aerosol observations. The earliest systematic results come from the Smithsonian Institution solar observatories. Roosen e! al. [1973] computed extinction coefficients from 13 widely separated sites during the first half of the twentieth century using spectrobolometer observations by the Astrophysical Observatory of the Smithsonian Institution. They concluded the aerosol burden did not 12,067

[1] The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths (t f and t c ) from the spectral shape of the total aerosol optical depth (t a = t f + t c ). This purely optical technique avoids intermediate computations of the PSD and yields a direct optical output that is commensurate in complexity with the spectral information content of t a . The separation into t f and t c is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of t a . Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that t c variation is coherent with photographic evidence of thin cloud events and that t f variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of t f and t c are wellcorrelated, if weakly biased, relative to formal inversions of combined solar extinction and sky radiance data. The spectral inversion technique permitted a closer scrutiny of a standard (temporally based) cloud-screening algorithm. Perturbations of monthly or longer-term statistics associated with passive or active shortcomings of operational cloud screening were inferred to be small to occasionally moderate over a sampling of cases. Diurnal illustrations were given where it was clear that such shortcomings can have a significant impact on the interpretation of specific events; (1) commission errors in t f due to the exclusion of excessively high-frequency fine mode events and (2) omission errors in t c due to the inclusion of insufficiently high-frequency thin homogeneous cloud events.

Abstract. The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August-September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun-sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths ('c,), aerosol size distributions, and singlescattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured % and the total column water vapor or precipitable water vapor (