1Environment Canada, Toronto, Ontario, M3H5T4, Canada
2ETH, Zurich, Switzerland
3Scripps Institute of Oceanography, University of California, San Diego, 92093, USA
4University of Utah, Salt Lake City, Utah, 84112-0110, USA
5Max Planck Institute for Biogeochemistry, Jena, Germany
6Aerodyne Research, Inc., Billerica, MA 01821-397, USA
Abstract. Atmospheric cooling from the effect of anthropogenic carbonaceous aerosol particles on the reflectivity of sunlight by water clouds remains an uncertainty for climate prediction. Airborne measurements of aerosol and cloud properties as well as light extinction were made below, in and above stratocumulus over the Northwest Atlantic Ocean on consecutive days. On the first day, the history of the below-cloud fine particle aerosol was marine and the fine particle sulphate and organic carbon mass concentrations measured at cloud base were 2.4 μg m−3 and 0.9 μg m−3, respectively. On the second day, the below-cloud aerosol was continentally influenced and the fine particle sulphate and organic carbon mass concentrations were 2.3 μg m−3 and 2.6 μg m−3, respectively. Correspondingly, the number concentrations of aerosol particles below cloud were approximately a factor of two higher on the second day, while the below-cloud size distributions were similar on both days. The cloud droplet number concentrations (CDNC) on the second day were approximately three times higher than the CDNC measured on the first day, and the vertically integrated cloud light extinction measurements indicate a 6% increase in the cloud albedo principally due to the increase in the carbonaceous components on the second day. Locally, this albedo increase translates to a daytime radiative cooling of ~12 W m−2. This result provides observational evidence that the role of anthropogenic carbonaceous components in the cloud albedo effect can be much larger than that of anthropogenic sulphate, as some global simulations have indicated.