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
