Small Particle Growth and Aging at SGP

15 April 2019 - 15 June 2020

Lead Scientist: Don Collins

Observatory: SGP

Among the ways atmospheric aerosols affect climate is through their influence on the reflectivity and lifetime of clouds. But that influence is limited to only those particles larger than about 50 to 100 nm on which cloud droplets can form at typical atmospheric supersaturations. Many of those cloud condensation nuclei (CCN) originate as ~1 nm nucleated particles that must grow to that threshold size before being lost to coagulation with larger particles. This project is designed to provide an improved understanding of the rates, effects, and controls of the growth and atmospheric processing of small particles that can become CCN through condensational growth and oxidation. The primary experimental tool to be used is the Captive Aerosol Growth and Evolution (CAGE) chamber, which, unlike traditional Teflon chambers, is designed to be operated outside at the location to be studied. The cylindrical chamber is constructed mostly of highly light transmitting FEP Teflon so that the UV and visible spectral intensity inside approaches that just outside. A sheet of gas-permeable expanded PTFE (ePTFE) Teflon seals one end of the cylindrical enclosure. Efficient exchange of water vapor and trace gases across that ePTFE membrane maintains near-ambient gas composition inside. Particles initially of the same size and chemical composition will be injected into and trapped within the chamber where they will be altered in a way and at a rate similar to that if they were in the surrounding environment. Those particles will be intermittently sampled and their size and properties measured over periods of several hours to more than a day. Two identical CAGE chambers will be used for this project, with one serving as a control and either the particle properties or gas phase composition in the other perturbed in some way. Observed differences in the rates at which particles change in the two will be used to assess sensitivities to the composition of the particles and to the concentrations of one or more trace gases.

The experiments will be conducted at the Atmospheric Radiation Measurement Climate Research Facility (ACRF) Southern Great Plains (SGP) site in rural Oklahoma during 2 month periods in years 1 and 2 of the 3-year project. SGP is the optimal site for this work because i) it is extensively instrumented and characterized, ii) new particle formation and growth events are common from spring through fall and can provide context for the chamber results, and iii) relevant trace gas concentrations vary little over time relative to more populated and/or polluted areas where local emissions sources are more important. Results from the 1st year experiments will guide changes in approach and foci for those in the 2nd year. Results from both sets of experiments will be analyzed and published and also used to test, improve, and/or develop model parameterizations.