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

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Two coccolithophore blooms in the Gulf of Maine were studied in 1988 and 1989. Each bloom was about 50,000 km2 in area and confined to the top 20 m of the water column. Maximal cell concentrations were 2,000 cells ml−1 and coccolith densities of 3 × 105 ml−1 were observed. The coccolith : cell ratio was highest in the bloom center (region of most intense reflectance) and lowest at the bloom periphery, an indication of varying organic vs. inorganic C production. Chlorophyll concentrations were generally low within the bloom and no relation could be observed between major nutrients and coccolithophore abundance. Backscattered light was profoundly affected by coccolith density and was slightly wavelength‐dependent. We calculated total backscattering as well as backscattering (bb) caused exclusively by coccoliths and derived the algorithm relating coccolith density to backscattering. Although cells were efficient light absorbers, coccoliths showed negligible light absorption. Diffuse attenuation was lowest in the green and blue‐green part of the visible spectrum. At the center of the bloom, coccoliths contributed >75% of the backscattering signal and > 50% of the beam attenuation signal. The most accurate way to estimate coccolith concentrations via remote sensing is to measure water‐leaving radiance in the green wavebands.

Ocean colour is recognised as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS); and spectrally-resolved water-leaving radiances (or remote-sensing reflectances) in the visible domain, and chlorophyll-a concentration are identified as required ECV products. Time series of the products at the global scale and at high spatial resolution, derived from ocean-colour data, are key to studying the dynamics of phytoplankton at seasonal and inter-annual scales; their role in marine biogeochemistry; the global carbon cycle; the modulation of how phytoplankton distribute solar-induced heat in the upper layers of the ocean; and the response of the marine ecosystem to climate variability and change. However, generating a long time series of these products from ocean-colour data is not a trivial task: algorithms that are best suited for climate studies have to be selected from a number that are available for atmospheric correction of the satellite signal and for retrieval of chlorophyll-a concentration; since satellites have a finite life span, data from multiple sensors have to be merged to create a single time series, and any uncorrected inter-sensor biases could introduce artefacts in the series, e.g., different sensors monitor radiances at different wavebands such that producing a consistent time series of reflectances is not straightforward. Another requirement is that the products have to be validated against in situ observations. Furthermore, the uncertainties in the products have to be quantified, ideally on a pixel-by-pixel basis, to facilitate applications and interpretations that are consistent with the quality of the data. This paper outlines an approach that was adopted for generating an ocean-colour time series for climate studies, using data from the MERIS (MEdium spectral Resolution Imaging Spectrometer) sensor of the European Space Agency; the SeaWiFS (Sea-viewing Wide-Field-of-view Sensor) and MODIS-Aqua (Moderate-resolution Imaging Spectroradiometer-Aqua) sensors from the National Aeronautics and Space Administration (USA); and VIIRS (Visible and Infrared Imaging Radiometer Suite) from the National Oceanic and Atmospheric Administration (USA). The time series now covers the period from late 1997 to end of 2018. To ensure that the products meet, as well as possible, the requirements of the user community, marine-ecosystem modellers, and remote-sensing scientists were consulted at the outset on their immediate and longer-term requirements as well as on their expectations of ocean-colour data for use in climate research. Taking the user requirements into account, a series of objective criteria were established, against which available algorithms for processing ocean-colour data were evaluated and ranked. The algorithms that performed best with respect to the climate user requirements were selected to process data from the satellite sensors. Remote-sensing reflectance data from MODIS-Aqua, MERIS, and VIIRS were band-shifted to match the wavebands of SeaWiFS. Overlapping data were u...

[1] Micropulse lidar (MPL) systems were used to measure aerosol properties during the Indian Ocean Experiment (INDOEX) 1999 field phase. Measurements were made from two platforms: the NOAA ship R/V Ronald H. Brown, and the Kaashidhoo Climate Observatory (KCO) in the Maldives. Sun photometers were used to provide aerosol optical depths (AOD) needed to calibrate the MPL. This study focuses on the height distribution and optical properties (at 523 nm) of aerosols observed during the campaign. The height of the highest aerosols (top height) was calculated and found to be below 4 km for most of the cruise. The marine boundary layer (MBL) top was calculated and found to be less than 1 km. MPL results were combined with air mass trajectories, radiosonde profiles of temperature and humidity, and aerosol concentration and optical measurements. Humidity varied from approximately 80% near the surface to 50% near the top height during the entire cruise. The average value and standard deviation of aerosol optical parameters were determined for characteristic air mass regimes. Marine aerosols in the absence of any continental influence were found to have an AOD of 0.05Ϯ0.03, an extinction-to-backscatter ratio (S ratio) of 33Ϯ6 sr, and peak extinction values around 0.05 km Ϫ1 (near the MBL top). The marine results are shown to be in agreement with previously measured and expected values. Polluted marine areas over the Indian Ocean, influenced by continental aerosols, had AOD values in excess of 0.2, S ratios well above 40 sr, and peak extinction values approximately 0.20 km Ϫ1 (near the MBL top). The polluted marine results are shown to be similar to previously published values for continental aerosols. Comparisons between MPL derived extinction near the ship (75 m) and extinction calculated at ship level using scattering measured by a nephelometer and absorption using a particle soot absorption photometer were conducted. The comparisons indicated that the MPL algorithm (using a constant S ratio throughout the lower troposphere) calculates extinction near the surface in agreement with the ship-level measurements only when the MBL aerosols are well mixed with aerosols above. Finally, a review of the MPL extinction profiles showed that the model of aerosol vertical extinction developed during an earlier INDOEX field campaign (at the Maldives) did not correctly describe the true vertical distribution over the greater Indian Ocean region. Using the average extinction profile and AOD obtained during marine conditions, a new model of aerosol vertical extinction was determined for marine atmospheres over the Indian Ocean. A new model of aerosol vertical extinction for polluted marine atmospheres was also developed using the average extinction profile and AOD obtained during marine conditions influenced by continental aerosols.