Chamber Studies Uncover New Pathways for Atmospheric Aerosol Growth

Smith, J., University of California, Irvine

Aerosol Processes

Aerosol Processes

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Global modeling results showing potential impact of new organic growth mechanism on cloud condensation nuclei (CCN) populations,


Global modeling results showing potential impact of new organic growth mechanism on cloud condensation nuclei (CCN) populations,

Science

The first few hours following the formation of a new atmospheric aerosol particle are a competition between the growth of the particle due to the condensation of low volatility “sticky” gases and the loss of the particle by collisions with other aerosol particles. New observations from the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN in Geneva, Switzerland, have recently shed light on two new growth pathways: growth by uptake of clusters containing acids and bases and growth from the uptake of low-volatility gases formed from organic vapors such as those emitted from trees.

Impact

The ability to accurately predict the growth of newly formed atmospheric particles by these new mechanisms will allow modelers to more accurately assess whether these particles can grow fast enough to become the seed for cloud droplets. Global models that incorporate these new measurements of the growth due to organic condensation (see figure) predict that this process may contribute to as much as a 50% increase in the number of cloud condensation nuclei, thus potentially impacting global climate and precipitation patterns.

Summary

Two new laboratory studies from CLOUD have uncovered new pathways by which nascent aerosol particles grow to sizes that may impact clouds and climate. These articles, published recently in the journals Nature Communications and Nature, are the result of a collaboration between ASR researchers and US and European partners. In the first study, small acid-base clusters, which are not generally accounted for in the measurement of sulfuric acid vapor, were found to participate in the growth process, leading to enhanced growth rates beyond that predicted by the uptake of simple acid-base complexes. The second study focuses on the potential role of purely organic compounds to the growth of new particles. That study showed that organic vapors that drive initial growth have extremely low volatilities. As the particles increase in size, subsequent growth is primarily due to more abundant organic vapors of slightly higher volatility. A particle growth model was presented that quantitatively reproduces the measurements performed at CLOUD. Furthermore, a parameterization of the first steps of growth was implemented in a global aerosol model, in which it was found that concentrations of atmospheric cloud concentration nuclei can change by up to 50% in comparison with previously assumed growth rate parameterizations.