Experimental Assessment of Collection Efficiency of Submicron Aerosol Particles by Cloud Droplets Using the New MIT-CFC

The interplay between aerosol particles and water droplets in the atmosphere, especially in clouds, influences both aerosol and cloud properties. The major uncertainty in our understanding of climate arises in the indirect effect of aerosol and their ability to impact cloud formation and consequently alter the global radiative balance. The collision between a water droplet and aerosol particles that results in coalescence is termed “collection” or “coagulation”. The coagulation process is considered to be an important mechanism for removing aerosol particles from the atmosphere, and it can also influence cloud lifetime, precipitation formation and the global radiation budget. Coagulation can induce ice nucleation via contact freezing at temperatures below 0°C. Contact freezing is currently the least understood ice nucleation mechanism and can be potentially important for mixed-phase cloud formation.

Although there have been extensive experimental and theoretical effort to better understand the aerosol collection process by water droplets using different instruments and mathematical expressions, most of these studies focus on the drizzle and rain drops (diameter ~ up to a few millimeter) while very few studies used cloud droplets (diameter ~ tens of micron). Well-constrained parameters necessary to describe particle collection efficiency by cloud droplets for use in models have not been available due the lack of ability to observe coagulation on a single-particle basis in a well-controlled environment.

In this work the collection efficiency of spherical submicron particles by cloud droplets have been examined using the new Massachusetts Institute of Technology - Contact Freezing Chamber (MIT-CFC). A stream of 30-micron cloud droplets fall freely into the chamber and collide with aerosol particles. The coagulated particles are sent to the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for chemical composition analysis. PALMS is a single-particle mass spectrometer and gives information on the size and the chemical composition of each particle. Coagulated particles from the MIT-CFC have mass spectral signatures from both the aerosol particles and the droplet residuals, while the droplet residual contains no signature of the aerosol particles. This is the first time coagulation has been seen on a single-particle basis. The coagulation efficiency of different aerosol sizes and concentrations under atmospherically relevant temperatures and different relative humidity conditions will be presented in this work. These measurements are a key step before contact freezing will be examining with the chamber.