Molar Mass, Surface Tension and Droplet Growth Kinetics of Marine Organics from Measurements of CCN Activity R. H. Moore 1, E. D. Ingall 2, A. Sorooshian 3, A. Nenes 1,2 1 School of Chemical & Biomolecular Engineering, 2 School of Earth & Atmospheric Sciences Georgia Institute of Technology, Atlanta, GA 30332, 3 Department of Chemical Engineering, California Institute of Technology, Pasadena, CA Earth & Atmospheric Sciences Graduate Student Symposium November 2, 2007

Motivation Size-resolved chemical composition of marine aerosol at Mace Head, IR, during (a) low-biological activity and (b) high- biological activity. O’Dowd et al., Nature, 431, WIOC WSOC Organic matter is transferred to the aerosol phase via sea spray. Small particles (< 1 μm) are significantly enriched in organic matter. (O’Dowd et al., 2004; Oppo et al., 1999) This enrichment will affect the CCN properties of these small particles. Need to characterize CCN-relevant properties.

Approach Size-resolved chemical composition of marine aerosol at Mace Head, IR, during (a) low-biological activity and (b) high- biological activity. O’Dowd et al., Nature, 431, WIOC WSOC Can analyze aerosol produced from seawater However, typical dissolved organic matter (DOM) concentrations are a few ppm, salt concentrations are ~35,000 ppm. Need to concentrate organics to measure their influence Newly-developed combined electrodialysis / reverse osmosis technique to enrich the organics.

Estuarine Sample: Location: mouth of Ogeechee River ~200 L collected at 2 m depth Higher concentration of dissolved organic matter (DOM) Seawater Sampling Savannah Gulfstream Sample: Location: open ocean ~200 L collected at 84 m depth Lower concentration of dissolved organic matter (DOM)

Electrodialysis / Reverse Osmosis Method 200-L Sample ~35,000 ppm Salt Salt Water Reverse Osmosis Electro- dialysis Solid, Freeze- Dried DOM sample The electrodialysis (ED) and reverse osmosis (RO) systems are cycled in 2 steps: Step 1: ED removes salts, diluting the sample Step 2: RO removes water, reconcentrating the sample Solute Composition: C org, initial = ~3x10 -5 wt% C org, final = >30 wt% Vetter et al., Sep. Purif. Tech., in review

RO Unit Electrodialysis Stack Electrodialysis / Reverse Osmosis Method

CCN Activity Surface Tension Effect adjusted by adding (NH 4 ) 2 SO 4

Köhler Theory Analysis Rearranging to solve for the molar volume… What we want! EmpiricallyFitted Constants WSOC and Inorganic Composition ~1 for Organics Padro et al., Atmos. Chem. Phys. Discuss. 7(2): ; Asa-Awuku et al., Atmos. Chem. Phys. Discuss. 7(2):

Inferring Organic Molar Mass and Surface Tension from CCN Activation Experiments Calculate molar mass of organic Calculate surface tension and carbon conc. at activation for each salted sample s crit / D p pair : CCN activity data of salted sample & composition + Initially, assume surface tension of water CCN activity data of sample & composition + Parameterize surface tension vs. carbon conc. using Szyszkowski-Langmuir Iterate until molar masses converge Estuarine M = 4340 ± 266 g/mol Gulfstream M = 4370 ± 1070 g/mol Plot surface tension versus carbon concentration and compare with direct measurements. Will They Match?

Inferring Organic Molar Mass and Surface Tension from CCN Activation Experiments Moore et al., Geophys. Res. Lett., in preparation

Generalized Methodology: 1.Measure CCN activity of pure sample and sample with salt added at several supersaturations 2.Compute ω’s for samples at each supersaturation 3.Determine organic molar mass from Köhler Theory Analysis of pure sample, initially assuming surface tension of water 4.Use this computed molar mass and ω’s of salted sample to infer surface tension depression as a function of carbon concentration. 5.Use inferred surface tension equation to iterate over Steps 3-4 to yield organic molar mass and surface tension. Inferring Organic Molar Mass and Surface Tension from CCN Activation Experiments

Droplet Growth

Summary Köhler Theory Analysis is able to provide a reasonable estimate of the dissolved organic molar mass, which is consistent for both the Gulfstream and Estuarine samples. Using an independent mixture containing greater salt fraction, the surface tension can be inferred, with excellent agreement to the directly measured values. While the organics present are surface-active, they do not inhibit droplet growth kinetics.

Acknowledgements C. Hennigan and R. Weber for use of their TOC Analyzer S. Balachandran and A. Russell for of their the Ion Chromatograph The crew of the R/V Savannah, C. Jackson, E. M. Perdue and P. Pfromm for assistance with sample collection and processing DOE GCEP Graduate Research Fellowship NSF CAREER Award NSF Grants OCE and OCE

Summary Köhler Theory Analysis is able to provide a reasonable estimate of the dissolved organic molar mass, which is consistent for both the Gulfstream and Estuarine samples. Using an independent mixture containing greater salt fraction, the surface tension can be inferred, with excellent agreement to the directly measured values. While the organics present are surface-active, they do not inhibit droplet growth kinetics.

CCN Measurements from SMCA Nenes and Medina, Aerosol Sci. Tech., in review