Molar Mass, Surface Tension and Droplet Growth Kinetics of Marine Organics from Measurements of CCN...
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Transcript of Molar Mass, Surface Tension and Droplet Growth Kinetics of Marine Organics from Measurements of CCN...
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