Instruments such as single particle mass spectrometers are used to determine aerosol properties (e.g., size, optical properties and chemical composition) in situ and in real time. We also create artificial clouds in small chambers which mimic atmospheric conditions in the laboratory and field for experiments with precise control.
We have found that both field and laboratory studies are required to meet our goal of understanding the effect of particles on the Earth’s radiative balance. Field studies are used to determine how particles are formed and evolve in different regions – often with their own unique chemistry. Our group uses mountaintop stations (the Jungfraujoch High Altitude Research Station, located in the Swiss Alps, is shown above) and research aircraft (the NASA WB-57F, shown below) to gain access to ice-containing clouds which are only common at high latitude and/or altitude. We perform laboratory experiments to understand the fundamental processes involved in particle formation and evolution and how they interact with water vapor to form droplets and ice crystals.
Clays and other mineral dusts play a significant role in cloud formation because they are prevalent in the atmosphere and serve as condensation and ice nuclei. It is therefore important to understand and quantify the conditions under which such particles affect cloud formation. We generate mineral dust aerosols in the laboratory and observe how well they initiate droplet formation under various supersaturation conditions. With our setup, we can generate aerosols via agitating dry powder or by atomizing a powder–water slurry. We then use a differential mobility analyzer (DMA) to choose a specific particle size (based on the particles’ electrical mobility). Using a condensation particle counter (CPC), we measure the total concentration of particles in the size-selected aerosol stream, and in parallel, we pass the stream into a cloud condensation nuclei counter (CCNc) to determine what fraction of the aerosol particles form droplets. Since we can vary the supersaturation conditions in the CCNc, we can thereby determine the critical supersaturation—the supersaturation at which the particles form droplets—for various aerosol types and sizes.
Not all analysis is conducted on laboratory surrogates or in the field. A recent collaboration with Sarah Das at WHOI concerns analysis of particles trapped in ice. Graduate student Matt Osman is analyzing past particle records contained in Greenland’s ice sheet.
Another of our group’s goals is to understand the nucleation of ice. We have worked with Olaf Stetzer and Ulrike Lohmann at the Swiss Federal Institute of Technology and Droplet Measurement Technologies of Boulder, CO in developing a small ice nucleation chamber. The team working on this project, from MIT and DMT, is shown below. We will bring this chamber, known as the SPectrometer for Ice Nuclei (SPIN), to the AIDA facility in Germany in July, 2012 (please see Field Studies).