Aerosol hygroscopicity in the marine atmosphere: a closure study using high-time-resolution, multiple-RH DASH-SP and size-resolved C-ToF-AMS data Departments of Chemical Engineering and Environmental Science and Engineering, Caltech, Pasadena, CA 91125, USA
Received: 14 Jul 2008 – Published in Atmos. Chem. Phys. Discuss.: 04 Sep 2008 – Published: 07 Apr 2009Abstract. 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.