CAREER: Size Resolved Measurement of Cloud Condensation Nuclei Concentration and Properties

Texas A&M Research Foundation, College Station TX

Investigators

Abstract

This grant supports the design and fabrication of a novel instrument for studying the atmospheric aerosols that serve as condensation nuclei for the formation of cloud droplets. The instrument consists of a differential mobility analyzer (DMA), which separates the aerosol particles into a narrow range of sizes, followed by a cloud condensation nucleus separator (CCNS), which separates the more hygroscopic particles from the less hygroscopic ones. It is the hygroscopic particles that can serve as condensation nuclei. The CCNS consists of two concentric cylinders, between which an electric field is applied. The annular region between the cylinders is basically a thermal gradient diffusion chamber. The inner and outer walls are maintained at different temperatures and at water saturation by means of wet filter paper, so that the space between them is supersaturated. The particles are charged when they leave the DMA, and the field applied to the CCNS causes them to migrate slowly towards the inner wall as they move with the air through the chamber. The electrical mobility of the particles is reduced as they grow by condensation. Hence the particles that grow large have less chance of migrating to the inner wall than those that remain small. From the initial population of near-equal sized particles that enter the CCNS, the less hygroscopic ones are thus deposited on the inner wall, and those that serve as CCN at the applied supersaturation pass through. The two-stage instrument therefore makes it possible to study the nucleating properties of aerosol particles in different size ranges. As a further step in the analysis, the chemical composition of the CCN that exit the instrument may be determined using a single-particle mass spectrometer. Results of the research are relevant to the basic physics of cloud formation and to the question of aerosol effects on the solar reflectivity of clouds. The educational component of this CAREER award consists of a summer course for undergraduate students on techniques, applications, and practical aspects of meteorological and atmospheric chemical measurements. The culmination of the course is a week-long field campaign in Marina, CA, centered on airborne observations with the research facilities of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS).