Influence of relative humidity upon pollution and dust during ACE-Asia: Size distributions and implications for optical properties

An extensive set of aerosol physical and optical measurements was taken over the waters east of Asia during the Aerosol Characterization Experiment‐Asia (ACE‐Asia) project in the spring of 2001. Dust storms upwind of the study area combined with intense pollution plumes from coastal cities yielded an opportunity to examine both types of aerosol, in isolation and as they interacted. Scattering calculated from aerosol size distributions measured with an optical particle counter agreed well with simultaneous nephelometer measurements. We periodically heated sample air to evaporate sulfates and organic material. The change in volume upon heating agreed well with simultaneous measurements of aerosol composition. This volatile material was distributed on dust in rough proportion to surface area. Here we use the particle size and composition data to improve estimates of scattering at ambient humidity and to examine the effects of mixing on the optical properties of both pollution and dust aerosols. The presence of dust results in uptake of soluble and condensible species onto its surface and thereby reduces the mass scattering efficiency of the pollution aerosol by 50% and suppresses the change in scattering due to relative humidity (f(RH)) by up to 35%.