Observations of condensed phase liquid water and water mediated partitioning during the Southern Oxidant and Aerosol Study (Invited Presentation)

Condensed phase liquid water plays an important role in tropospheric chemistry by providing a matrix that mediates partitioning of water-soluble gases, which can subsequently form sulfate and organic aerosol via aqueous phase chemical reactions. The principle variables that control the amount of condensed phase liquid water are dry aerosol volume, ambient relative humidity, and particle hygroscopicity. Although the potential of condensed phase liquid water to facilitate secondary organic aerosol formation via reversible and irreversible processes is well established in laboratory studies, the extent to which this occurs in the atmosphere is unknown. Here we report in situ measurements of aerosol water content using the semi-volatile differential mobility analyzer (SVDMA), an instrument and technique developed for this study. Measurements were conducted June 1 – July 15, 2013, during the Southern Oxidant and Aerosol Study (SOAS) at the Centreville, Alabama research site. The SVDMA measures volume distributions of ambient atmospheric aerosols in three states: unperturbed, dried, and dried then re-humidified. Unperturbed measurements characterize the aerosol distribution at ambient conditions; for dry spectra, the sample is routed through a cold trap (dT = -30 K), and for re-humidified spectra the sample are brought back to ambient humidity using a Nafion membrane. Differences between unperturbed and dry volume concentrations provide the sum of water and semi-volatile compounds that have evaporated during the drying process; differences between unperturbed and re-humidified aerosol volumes quantify composition changes due to compounds that are irreversibly lost upon drying. Results indicate that liquid water is an important contributor to ambient aerosol volume during the early morning hours when relative humidity is highest. Overall aerosol water content can be characterized by kappa ~ 0.2 to 0.3, which is consistent with a mix of hygroscopic organic and inorganic compounds. A diel cycle in kappa is observed, suggesting that temperature, gas-phase chemistry, and relative humidity can lead to changes in particle composition on short time scales. No statistically significant differences between the unperturbed and the dried then re-humidified aerosol volumes is observed, indicating either that water-mediated partitioning of compounds is reversible or that the effect is smaller than the measurement uncertainty. We anticipate that these data will help constrain models that quantify aerosol volume and water content.