Laboratory Measurements of Cloud Scavenging of Interstitial Aerosol by Activation in a Turbulent Environment

Aerosol particles and cloud droplets are inextricably intertwined in Earth's atmosphere. Every cloud droplet forms on an aerosol particle while clouds are the primary removal mechanism for aerosol particles with diameters in the range ~ 50 to 1000 nm – particles which are too big to be efficiently removed via diffusion and too small to have an appreciable settling velocity. The probability that an aerosol particle becomes a cloud droplet (or is activated) 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) also need to 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 no longer sharp. In our experiments, the chemical composition of all aerosol is the same, so the diameters of the particles determine the critical supersaturations required for activation. 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 (CVI).) 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. The comparison of cloud droplet residuals and interstitials also allows us to place bounds on the quasi-steady state supersaturation in the chamber, by identifying the diameter at which particles no longer appear in the droplet residuals, but do appear in the interstitials. Complementary measurements of temperature and water vapor concentration support those general observations.

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