Effect of water vapor on the determination of aerosol direct radiative effect based on the AERONET fluxes

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Huttunen, J., Arola, A., Myhre, G., Lindfors, A. V., Mielonen, T., Mikkonen, S., Schafer, J. S., Tripathi, S. N., Wild, M., Komppula, M., Lehtinen, K. E. J.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Aerosol Robotic Network
• Online Access: https://doi.org/10.5194/acp-14-6103-2014; https://www.atmos-chem-phys.net/14/6103/2014/

The aerosol direct radiative effect (ADRE) is defined as the change in the solar radiation flux, F , due to aerosol scattering and absorption. The difficulty in determining ADRE stems mainly from the need to estimate F without aerosols, F 0 , with either radiative transfer modeling and knowledge of the atmospheric state, or regression analysis of radiation data down to zero aerosol optical depth (AOD), if only F and AOD are observed. This paper examines the regression analysis method by using modeled surface data products provided by the Aerosol Robotic Network (AERONET). We extrapolated F 0 by two functions: a straight linear line and an exponential nonlinear decay. The exponential decay regression is expected to give a better estimation of ADRE with a few percent larger extrapolated F 0 than the linear regression. We found that, contrary to the expectation, in most cases the linear regression gives better results than the nonlinear. In such cases the extrapolated F 0 represents an unrealistically low water vapor column (WVC), resulting in underestimation of attenuation caused by the water vapor, and hence too large F 0 and overestimation of the magnitude of ADRE. The nonlinear ADRE is generally 40–50% larger in magnitude than the linear ADRE due to the extrapolated F 0 difference. Since for a majority of locations, AOD and WVC have a positive correlation, the extrapolated F 0 with the nonlinear regression fit represents an unrealistically low WVC, and hence too large F 0 . The systematic underestimation of F 0 with the linear regression is compensated by the positive correlation between AOD and water vapor, providing the better result.