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

Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. We report the results of a comprehensive optical and physical characterization of ambient aerosol in 5 key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2-5 years. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 years. Aerosol found over the tropical Pacific Ocean (at 3 sites between 20°S and 20°N), still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of τ a (500 nm)=0.07, mode value at τ am =0.06 and standard deviation of 0.02 to 0.05. Average Ångström exponent ranged from 0.3 to 0.7 characterizes the wavelength dependence of the optical thickness. Over the tropical to subtropical Atlantic (2 stations at 7°S and 32°N) the optical thickness is significantly higher: τ a (500 nm)=0.14 and τ am =0.10 due to the frequent presence of dust, smoke and urban/industrial aerosol. For both oceans the atmospheric column aerosol is characterized by a bimodal log-normal size distribution with a fine mode at effective radius R eff =0.11±0.01 µm and coarse mode at R eff =2.1±0.3 µm. A review of the published 150 historical ship measurements from the last three decades shows that τ am was around 0.07 to 0.12 in general agreement with the present finding. The information should be useful as a test-bed for aerosol global models and aerosol representation in global climate models. With global human population expansion and industrialization, these measurements can serve in the 21st century as a basis to assess decadal changes in the aerosol concentration, properties and radiative forcing of climate.

Results from a network of sun photometers spread throughout the globe will be discussed.

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