The research team was focusing on understanding the impact of aerosols on clouds as clouds have a significant influence over climate. Low-level clouds reflect incoming sunlight back into space, which aids in cooling the planet. Anything that disturbs these low-level clouds impacts our climate. Human-produced pollution releases airborne particles into the atmosphere and can act as a seed for forming a cloud droplet; clouds that form in polluted airmasses are likely to be more reflective than those without pollution. The study showed that some gas phase compounds more more likely to stick to particles, which in turn made them ‘soapier’ and aided their ability to form cloud droplets.
Clouds are formed when water vapour condenses on cloud condensation nuclei (CCN), which are atmospheric particulates. Variations in the concentrations of CCN affect cloud properties, which in turn affect global and regional climate. Organic matter makes up a significant percentage of aerosol mass in the troposphere (the lowest portion of Earth’s atmosphere); the amount of organic matter can strongly influence the activity and concentration of CCN.
The researchers produced evidence that methylglyoxal and acetaldehyde, two atmospheric trace gases, can increase aerosol CCN activity. This occurs even if the trace gases do not contribute organic mass to the aerosol particles. The team generated the aerosol particles in the lab and then exposed them to methylglyoxal and/or acetaldehyde in the lab’s aerosol reaction chamber for up to five hours. Afterwards, the particles were tested for their ability to form cloud droplets using a cloud chamber.
The results indicated that gas-phase surfactants may enhance the activity of atmospheric CCN. The researchers concluded that “volatile organics in the atmosphere may act as a reservoir of surfactants that can be taken up by aerosol particles and augment their CCN activity.”
The image shows the GEOS-CHEM (top) and NASA-GMI (bottom) global climate models; the researchers showed that predictions of cloud droplet numbers could increase up to 20 percent due to organic gas adsorption.
Neha Sareen, Allison N. Schwier, Terry L. Lathem, Athanasios Nenes, and V. Faye McNeill. Surfactants from the gas phase may promote cloud droplet formation. PNAS, February 4, 2013 DOI: 10.1073/pnas.1204838110
Image courtesy of Columbia University School of Engineering and Applied Science)