Atmos. Chem. Phys. Discuss., 8, 16789-16817, 2008
© Author(s) 2008. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Aerosol hygroscopicity in the marine atmosphere: a closure study using high-resolution, size-resolved AMS and multiple-RH DASH-SP data
S. P. Hersey, A. Sorooshian, S. M. Murphy, R. C. Flagan, and J. H. Seinfeld
Departments of Chemical Engineering and Environmental Science and Engineering, Caltech, 1200 E. California Blvd, Pasadena, CA, 91125, USA

Abstract. We have conducted the first closure study to couple high-resolution aerosol mass spectrometer (AMS) composition data with size-resolved, multiple-RH, high-time-resolution hygroscopic growth factor (GF) measurements from the differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP). These data were collected off the coast of Central California during seven of the 16 flights carried out during the MASE-II field campaign in July 2007. Two of the seven flights were conducted in airmasses that originated over the continental United States. These flights exhibited elevated organic volume fractions (VForganic=0.46±0.22, as opposed to 0.24±0.18 for all other flights), corresponding to significantly suppressed GFs at high RH (1.61±0.14 at 92% RH, as compared with 1.91±0.07 for all other flights), more moderate GF suppression at intermediate RH (1.53±0.10 at 85%, compared with 1.58±0.08 for all other flights, and no measurable GF suppression at low RH (1.31±0.06 at 74%, compared with 1.31±0.07 for all other flights). Organic loadings were slightly elevated in above-cloud aerosols, as compared with below-cloud aerosols, and corresponded to a similar trend of significantly suppressed GF at high RH, but more moderate impacts at lower values of RH. A hygroscopic closure based on a volume-weighted mixing rule provided excellent agreement with DASH-SP measurements (R2=0.79). Minimization of root mean square error between observations and predictions indicated mission-averaged organic GFs of 1.20, 1.43, and 1.46 at 74, 85, and 92% RH, respectively. These values agree with previously reported values for water-soluble organics such as dicarboxylic and multifunctional acids, and correspond to a highly oxidized, presumably water-soluble, organic fraction (O:C=0.92±0.33). Finally, a backward stepwise linear regression revealed that, other than RH, the most important predictor for GF is VForganic, indicating that a simple emperical model relating GF, RH, and the relative abundance of organic material can provide accurate predictions of hygroscopic growth in the marine atmosphere.

Citation: Hersey, S. P., Sorooshian, A., Murphy, S. M., Flagan, R. C., and Seinfeld, J. H.: Aerosol hygroscopicity in the marine atmosphere: a closure study using high-resolution, size-resolved AMS and multiple-RH DASH-SP data, Atmos. Chem. Phys. Discuss., 8, 16789-16817, doi:10.5194/acpd-8-16789-2008, 2008.
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