How well do aerosol retrievals from satellites and representation in global circulation models match ground-based AERONET aerosol statistics?

Abstract

Statistics from sky/sunphotometers at AERONET sites throughout theworld provide the background for a comparison of monthly or seasonallyaveraged aerosol optical depths to retrievals by operational satellitesand to representations in global models. Available data-sets, however,rarely relate to the same year(s). With strong year-to-year variationseven for monthly averaged aerosol optical depths and open issueson sampling biases and regional representation by local measurementsonly larger discrepancies are investigated. Aerosol optical depthsretrievals of five different satellites and five different globalmodels are compared. Quantitative accurate satellite retrievals overland remain a challenge and even their relative difference cannotprovide clear answers on regional representation. Model predictedaerosol optical depth averages are usually smaller than AERONET.The behavior of models is further explored on a component basis.For sulfate, dust, carbon and sea-salt optical depths, mass and assumedaerosol sizes are compared. For the conversion of the column (dry)component mass in optical depth in models, assumptions for componentaerosol size and aerosol humidification are critical. Statisticalcomparisons to ground-based monitoring will be more useful, if temporaldifferences are removed. This requires data from the same time-periodand the use of sampling screens, to accommodate less frequent measurements.For the understanding of regional representation by local measurements,satellite data play a key role. Necessities to validate criticalaerosol assumptions in models or satellite retrievals require field-experiments that focus on individual aerosol components plus continuedand additional monitoring (e.g. AERONET) at sites, where a particularaerosol component dominates.