Characterization of aerosols cloud residuals using a coupled counterflow virtual impactor and aerosol mass spectrometer during the 2010 Whistler Aerosol and Cloud Study (WACS)

Mountain sites offer a unique opportunity for ground-based cloud sampling and simultaneous measurements of clouds, aerosols, and trace gases to further our understanding of cloud processes. During the Whistler Aerosol and Cloud Study (WACS) in summer 2010, two sites on Whistler Mountain were instrumented for measurements of gases, clouds, and aerosols. The lower altitude site was below the mountain tree-line and was selected to characterize cloud-base aerosol, often dominated by biogenic aerosol in spring. The Whistler sites, in general, were very clean with minimal influence from anthropogenic sources. At the higher elevation site, (Whistler Peak), routine measurements of trace gases and aerosol chemistry were augmented to include cloud microphysics instrumentation and a ground-based counter-flow virtual impactor (CVI) for collection of cloud drop residuals. The cloud residuals were sampled and characterized by several instruments downstream including a C-ToF-Aerosol Mass Spectrometer, a Single Particle Mass Spectrometer, an Ulta-High Sensitivity Aerosol Spectrometer, a Condensation Particle Counter, and a Soot Photometer. Cloud water was also collected at both sites with bulk cloud water samplers and analyzed for inorganic and organic species. The atmospheric PM1 aerosol was dominated by the organic fraction, and the relative amount of organics to total aerosol mass decreased throughout multi-hour cloud events whereas the relative amounts of sulfate increased. A comparison of size distributions derived from the AMS indicates that organics were mostly associated with nitrate, whereas sulphate was found in larger sizes suggesting the influence of different sources or processes. Evidence of cloud processing is shown by the size distributions of the cloud residuals and post cloud particles which were observed to extend to larger sizes compared to the size distributions of the pre-cloud particles. This paper will present an overview of the evolution of the chemical and physical properties of the atmospheric aerosol through an examination of the pre-cloud, cloud residuals, and post-cloud aerosol with a focus on the AMS spectra and the cloud microphysics. This discussion will address how the pre-cloud aerosol chemistry relates to the chemical properties of the cloud residual aerosol with particular emphasis on the organic composition and relative amount of oxygenation of both.