Airborne in situ characterization of dry aerosol optical properties in a multisource influenced marine region

In situ data from the 2nd Aerosol Characterization Experiment (ACE-2) were used to describe the aerosol optical properties in a marine environment perturbed by continental sources, such as the outflow of European aerosol and dust outbreaks from North Africa. The data consist of airborne measurements made with an integrating nephelometer and absorption photometer. The cases investigated in the ACE-2 experiment included vertical profiles flown in dusty, polluted and clean airmasses. While it was possible to describe the extensive optical properties (scattering and absorption coefficients) for the clean cases in a more general way, the polluted and dusty cases showed much variability from case to case. The intensive properties (single-scattering albedo and angstrom ngstrom exponent) showed little variability with height, in agreement with other studies. A key result in this paper is the strong relationship between single-scattering albedo and transport time for the polluted cases. Calculations of aerosol optical depth (AOD) made using in situ measurements showed that in the marine-dominated airmasses the contribution from the free troposphere (FT) to the total columnar AOD is not negligible. In the polluted cases, the marine boundary layer (MBL) dominated the contribution to the total AOD, when this was topped by a clean FT. In the cases when pollution outbreak in the MBL was accompanied by a dusty FT, most of the columnar AOD was due to the dust layer. Assessment of the horizontal variability of the aerosol field, achieved by employing the dry scattering coefficient as proxy data, showed that there was variability in the aerosol concentration at scales below the airmass scale in which the aerosol was embedded. This shows that the concept that the aerosol concentration varies horizontally at the same scale as the airmass can lead to significant errors in the aerosol modelling schemes, in particular on the estimates of the impact of aerosols on the Earth's radiative balance.