Aerosol hygroscopicity in the marine atmosphere: a closure study using high-time-resolution, multiple-RH DASH-SP and size-resolved C-ToF-AMS 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, Pasadena, CA 91125, USA
Abstract. We have conducted the first airborne hygroscopic growth closure study to
utilize data from an Aerodyne compact Time-of-Flight Aerosol Mass
Spectrometer (C-ToF-AMS) coupled 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 characterized by
continental origin. These flights exhibited elevated organic volume fractions
(VForganic=0.56±0.19, as opposed to 0.39±0.20 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 good agreement with DASH-SP
measurements (R2=0.78). Minimization of root mean square error between
observations and predictions indicated mission-averaged organic GFs of 1.22,
1.45, and 1.48 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 (mission-averaged 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 (R2=0.77) of hygroscopic
growth for the California coast.
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-time-resolution, multiple-RH DASH-SP and size-resolved C-ToF-AMS data, Atmos. Chem. Phys., 9, 2543-2554, doi:10.5194/acp-9-2543-2009, 2009.