A study of the impact of aerosols on the energy budget of the atmosphere using satellite measurements of climatic parameters and computer models

Abstract

A deterministic spectral shortwave radiative transfer model was used forthe computation of the Earth's atmospheric radiation budget, based on hightemporal and spatial resolution satellite data of aerosols and atmospheric climaticparameters from the Moderate Resolution Imaging Spectroradiometer(MODIS) sensor.The study focused on the evaluation of the aerosol direct radiative effect(DRE) on the radiation budget components. Due to the high spatialand temporal variability of aerosols, the DRE, which constitutes a crucialcomponent of the overall eect of aerosols on climate, is thus also highlyvariable.The aerosol direct eect on the tropospheric ozone photolysis rate, J(O1D),was also examined, being a dominant sink of tropospheric ozone. We notethat tropospheric ozone contributes to the global greenhouse eect. Thus,J(O1D) is an important climatic parameter, which needs to be studied usingmodelling approaches, due to the scarcity of measuring stations, andbecause it takes place primarily below 330 nm, a spectral region where theaerosol eect is a key operating factor.The aerosol direct eect on potential evaporation was also assessed. Potentialevaporation equals actual evaporation in shallow lakes, and constitutesa crucial parameter of the hydrological cycle. The aerosol DRE decreasespotential evaporation by decreasing the solar radiation reaching the Earth'ssurface.The model runs were performed for the period 2000{2010 over severalsites in Greece, which are characterised by high aerosol loads, with uniquecharacteristics in terms of seasonal variation and origin. Two research stationsin Crete (HCMR/AERONET and Finokalia), were selected due to theappropriateness of the island for studying Saharan dust episodes, which arefrequent in the wider Eastern Mediterranean, and the availability of ground{based data for both model supplementary input and validation. The modelwas also run over four lakes in Central Greece, which constitute the mainwater supply reservoirs of the city of Athens, for the evaluation of the aerosol eect on potential evaporation.MODIS Level 2 data of aerosols, clouds and atmospheric parameters wereanalysed and processed, and used as input to the model. These data areavailable since 2000, on a daily basis and at 10km10km and 5km5kmspatial resolution. The model takes into account all physical parameters andprocesses that aect signicantly the solar radiation transfer. The aerosolDRE is determined at the Earth's surface, within the atmosphere and at thetop of the atmosphere.The model output downwelling shortwave radiation was successfully validatedagainst ground{based measurements at the HCMR and Finokalia stationsand at the four lakes in Central Greece. The model output J(O1D) wassuccessfully validated against Finokalia station measurements. The analysisof the aerosol DRE on the model radiation budget, J(O1D) and potentialevaporation was performed on an instantaneous/daily mean, seasonal andinter{annual basis. Dust event eects were also quantied, and trends duringthe period examined were assessed and evaluated in terms of correspondingtrends and eects of operating factors, including aerosols, clouds and totalozone.Results show a decreasing trend in aerosols and the corresponding DREover all sites examined. Changes in the radiation budget components, however,are also controlled by other factors; an increase in cloud fraction overHCMR station counterbalanced the eect that the DRE reduction wouldhave caused. Similarly, although the DRE on J(O1D) has decreased, J(O1D)has not increased as was expected, due to an increase in total atmosphericozone. The presence of aerosols reduces potential evaporation by about 0.5mm on a mean daily basis, reaching up to 2 mm in summer. However, adecreasing trend in the aerosol load and DRE was found over all lakes duringthe period 2001{2010.Depending on the availability of model input data, the methodology developedin this study is applicable to any region of specic interest over theglobe.