Crystallization and Deliquescence of Atmospheric Particles

Harvard University, Cambridge MA

Investigators

Abstract

The phase transitions of ambient particles will be studied in the field using a 1-by-3 Tandem Differential Mobility Analyzer with differential programming of relative humidity ("1-by-3 TDMA"). The study will take place during June 2007 at the DOE Atmospheric Radiation Measurement (ARM) site in the Southern Great Plains (SGP) near Billings, Oklahoma. Three testable hypotheses will be evaluated initially. The hypotheses are based upon the results of laboratory studies, which to date are untested for atmospheric particles. Hypothesis 1: Aerosol deliquescence and crystallization events will be observed most often on those days for which the degree of chemical neutralization is high. Why? Laboratory experiments show that acidity reduces crystallization relative humidity (CRH). Moreover, laboratory results also show that sufficiently acidic particles are fully hygroscopic (nondeliquescent). Hypothesis #2: Aerosol CRH's will be shifted higher on those days for which there is a high prevalence of metals in the accumulation mode. In contrast, the deliquescence relative humidities (DRH) will remain unchanged. The reason for this is that metals and their oxides are suspected heterogeneous nuclei, which increase CRH without affect DRH. Higher CRH values imply that crystalline particles occur more frequently in the atmosphere. Hypothesis #3: The fraction of particles undergoing phase transitions will increase in tandem with the fraction of particles identified as "more hygroscopic" in TDMA measurements. This is because the "less hygroscopic" particles are believed to be composed of soot and other non-dissolving organic molecules, neither of which undergo RH-regulated phase transitions. This project will provide the first demonstration of a novel technique to observe and quantify phase transitions of atmospheric aerosols outside the laboratory. The data obtained will help determine the importance of the deliquescence state of aerosols in the radiative balance of the atmosphere. The project will provide research opportunities of two graduate students.