ALBERT

Description: Scattering and absorption by aerosols during EUCAARI-LONGREX: can airborne measurements and models agree? Atmospheric Chemistry and Physics Discussions, 11, 18487-18525, 2011 Author(s): E. J. Highwood, M. J. Northway, G. R. McMeeking, W. T. Morgan, D. Liu, S. Osborne, K. Bower, H. Coe, C. Ryder, and P. Williams Scattering and absorption by aerosol in anthropogenically perturbed air masses over Europe has been measured using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM) on 14 flights during the EUCAARI-LONGREX campaign in May 2008. The geographical and temporal variations of the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably over the data set from 0.86 to near unity. Dry aerosol optical depths ranged from 0.03 to 0.24. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Very good agreement (to within 30 %) can be achieved for scattering, but the modelling of absorption is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon. Agreement with the measured absorption can only be achieved if organic carbon is assumed to be only weakly absorbing. Hygroscopic growth curves derived from the wet nephelometer indicate moderate water uptake by the aerosol with a campaign mean f (RH) value (change in scattering) of 1.3 at 80 % relative humidity. This value is consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer (AMS), which are primarily mixed organics and nitrate and some sulphate. As expected the effect of humidity is to raise the single scattering albedo, and to increase the aerosol optical depth. This study represents an important new body of data regarding European aerosol amounts, composition and optical properties and additionally demonstrates the importance of airborne measurements of black carbon mass and aerosol hygroscopicity.