Laboratory Measurements of Aerosol Scavenging in a Cloudy, Turbulent Environment

 
Poster PDF

Authors

Will H. Cantrell — Michigan Technological University
Gregory Kinney — Michigan Technological University
Abu Sayeed Md Shawon — Los Alamos National Laboratory
Claudio Mazzoleni — Michigan Technological University
Raymond A Shaw — Michigan Technological University

Category

Microphysics (cloud, aerosol and/or precipitation)

Description

How are aerosol particles removed from the atmosphere? Those with diameters greater than a few microns have an appreciable negative vertical velocity and can be removed by settling. Particles smaller than few tens of nanometers have a high diffusivity and can be removed via diffusion. However, the in-between range of a few tenths of a micron has a small settling velocity and a low diffusivity, which produces a bottleneck in removal. As a consequence, this size range is frequently called the “accumulation mode” because particles in it accumulate in the atmosphere. The primary removal mechanism for the accumulation mode is cloud processing, because those particles readily serve as cloud condensation nuclei. The probability that an aerosol particle activates is traditionally understood to depend upon size and chemical composition, described in Köhler theory. Recent measurements, both laboratory (Chandrakar et al., 2017) and field (Verheggen et al., 2007), have shown that size and chemical composition may not be enough to predict which particles will activate. Turbulent fluctuations in the scalar fields which couple aerosols and cloud droplets (i.e. temperature and water vapor) must be considered. Measurements from Michigan Tech's turbulent mixing chamber (the Pi Chamber), show that, in the presence of turbulent fluctuations, the correspondence between size and activation is not clearly defined. We create steady-state, turbulent cloud conditions and compare the distribution of interstitial aerosol to the distribution of cloud droplet residuals. (The residuals are measured using a pumped counterflow virtual impactor.) That comparison shows that some aerosol particles are just as likely to remain as interstitial as they are to be activated, a result of fluctuations in the saturation ratio. We will discuss these results in the context of aerosol size and chemical composition, strength of the turbulence, and saturation ratio in the cloud. Chandrakar, K. K., Cantrell, W., Ciochetto, D., Karki, S., Kinney, G., & Shaw, R. A. (2017). Aerosol removal and cloud collapse accelerated by supersaturation fluctuations in turbulence. Geophys. Res. Lett., 44(9), 4359-4367. Verheggen, B., Cozic, J., Weingartner, E., Bower, K., Mertes, S., Connolly, P., Gallagher, M., Flynn, M., & Baltensperger, U. (2007). Aerosol partitioning between the interstitial and the condensed phase in mixed‐phase clouds. J. Geophys. Res., 112(D23), doi:10.1029/2007JD008714