Laboratory Measurements of the Scavenging of Interstitial Aerosol in Polluted Clouds

In 1881, John Aitken wrote “I have said that if there was no dust there would be no fogs, clouds, nor mists...” [1]. In the intervening 135 years, those words have been reaffirmed and quantified. Aerosol particles (i.e. dust) play a critical role in determining many cloud properties, ranging from the ways in which clouds interact with solar and terrestrial radiation to when and where precipitation develops. In turn, clouds are the primary removal mechanism for aerosol particles smaller than a few micrometers and larger than a few nanometers [2]. The rate at which clouds clean the atmosphere is receiving renewed attention because of the feedback between aerosol and cloud droplet concentrations which determines the cloud's probability for precipitation and thus its lifetime. Marine stratocumulus, forming in the outflow from polluted conditions over continents, can transform themselves from a closed cellular structure to an open one by reducing the aerosol concentration [3]. That transformation has dramatic effects on their radiative and hydrologic impact.Using a new mixing cloud chamber (called the Pi Chamber because of the 3.14 cubic meter internal volume), we can create and sustain clouds almost indefinitely, which enables us to measure steady state cloud properties with a high degree of confidence. In this case, we injected NaCl aerosol from a constant output atomizer into the chamber in which we had created an unstable temperature gradient. The unstable gradient with a wet top and bottom surface promotes cloud formation through Rayleigh-Benard convection and subsequent mixing of air parcels having different temperatures and water vapor concentrations. At steady state, the cloud droplet number concentration was ~ 300 cm^-3, with a mean diameter of 10 to 15 micrometers. The concentration of interstitial aerosol was approximately 10⁴ cm^-3. When the aerosol source is turned off, the cloud decays over four to five hours. Measurement of the distribution of interstitial aerosol as a function of time is then used to derive characteristic decay times as a function of aerosol size. Our data suggest that the lifetime of most interstitial particles in the accumulation mode is governed by cloud activation – particles are removed from the Pi Chamber when they activate and settle out of the chamber as cloud droplets. As cloud droplets are removed, these interstitial particles activate until the initially polluted cloud cleans itself and all particulates are removed from the chamber. Our data also indicate that smaller particles, with dry diameters in the range 10 to 20 nm, are not activated, but are instead removed through diffusion, enhanced by the fact that droplets are moving relative to the suspended aerosol [4]. Implications for these results for cloud and aerosol systems in the atmosphere will be discussed.

1. Aitken, J., 1882. On Dust, Fogs, and Clouds. Proc. Royal Soc. Edinburgh 11, 14–18. doi:10.1017/S0370164600046666

2. Pierce, J.R., Croft, B., Kodros, J.K., D’Andrea, S.D., Martin, R.V., 2015. The importance of interstitial particle scavenging by cloud droplets in shaping the remote aerosol size distribution and global aerosol-climate effects. Atmos. Chem. Phys. 15, 6147–6158. doi:10.5194/acp-15-6147-2015

3. Goren, T., Rosenfeld, D., 2015. Extensive closed cell marine stratocumulus downwind of Europe—A large aerosol cloud mediated radiative effect or forcing? J. Geophys. Res. Atmos. 120, 2015JD023176. doi:10.1002/2015JD023176

4. Pruppacher, H., Klett, J., 1997. Microphysics of clouds and precipitation, 2^nd ed., Chpt. 17.4, Kluwer.