Molar Mass, Surface Tension and Droplet Growth Kinetics of Marine Organics from

Measurements of CCN Activity

R. H. Moore1, E. D. Ingall2, A. Sorooshian3, A. Nenes1,2

1School of Chemical & Biomolecular Engineering, 2School of Earth & Atmospheric SciencesGeorgia Institute of Technology, Atlanta, GA 30332, 3Department of Chemical Engineering,

California Institute of Technology, Pasadena, CA 91125

2007 Earth & Atmospheric SciencesGraduate 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, 2004.

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, 2004.

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:Corg, initial = ~3x10-5 wt%Corg, final = >30 wt%

Vetter et al., Sep. Purif. Tech., in review

RO

Unit

Electrodialysis Stack

Electrodialysis / Reverse Osmosis Method

CCN ActivitySurface Tension

Effect

adjusted by adding(NH4)2SO4

23

pc ds

Köhler Theory Analysis

23

21

21

127

256

p

i iii

i

w

w

w

wc d

M

MRT

Ms

23

pc ds

Rearranging to solve for the molar volume…

organiciii

i

i

w

w

organicorganic

organic

organic

MRT

M

M

2332

127256

What we want!What we want!

EmpiricallyEmpiricallyFittedFitted

ConstantsConstants

iii

iii m

m

WSOC and WSOC and Inorganic Inorganic

CompositionComposition

~1 for Organics~1 for Organics

Padro et al., Atmos. Chem. Phys. Discuss. 7(2): 3805-3836; Asa-Awuku et al., Atmos. Chem. Phys. Discuss. 7(2): 3589-3627

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 scrit / Dp 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

Ite

rate

unt

il m

olar

mas

ses

conv

erge

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.

3/1

232

1

27

256

organic

organic

w

w

organiciii

i

i

organic

organicorganicorganic

M

RT

M

M

M

3

33

.,827

Asd

C csoluteavgorganicact

organiciii

i

i

w

w

organicorganic

organic

organic

MRT

M

M

2332

127256

23

pc ds

CTwater 1ln

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 OCE0425624 and OCE0526178

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