Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements H. Wex1, M. D. Petters2, C. M. Carrico2, E. Hallbauer1, A. Massling1,*, G. R. McMeeking2,**, L. Poulain1, Z. Wu1, S. M. Kreidenweis2, and F. Stratmann1 1Institute for Tropospheric Research, Leipzig, Germany 2Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA *now at: National Environmental Research Institute, Aarhus University, Roskilde, Denmark **now at: Centre for Atmospheric Science, University of Manchester, Manchester, UK
Abstract. Secondary Organic Aerosols (SOA) studied in previous laboratory experiments
generally showed only slight hygroscopic growth, but a much better activity
as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic
growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud
Interaction Simulator), using a portable generator that produced SOA
particles from the ozonolysis of α-pinene, and adding butanol or butanol and
water vapor during some of the experiments. The light scattering signal of
dry SOA-particles was measured by the LACIS optical particle spectrometer
and was used to derive a refractive index for SOA of 1.45. LACIS also
measured the hygroscopic growth of SOA particles up to 99.6% relative
humidity (RH), and a CCN counter was used to measure the particle
activation. SOA-particles were CCN active with critical diameters of e.g.
100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively.
But only slight hygroscopic growth with hygroscopic growth factors ≤1.05
was observed at RH<98% RH. At RH>98%, the hygroscopic growth
increased stronger than would be expected if a constant hygroscopicity
parameter for the particle/droplet solution was assumed. An increase of the
hygroscopicity parameter by a factor of 4–6 was observed in the RH-range
from below 90% to 99.6%, and this increase continued for increasingly
diluted particle solutions for activating particles. This explains an
observation already made in the past: that the relation between critical
super-saturation and dry diameter for activation is steeper than what would
be expected for a constant value of the hygroscopicity. Combining
measurements of hygroscopic growth and activation, it was found that the
surface tension that has to be assumed to interpret the measurements
consistently is greater than 55 mN/m, possibly close to that of pure water,
depending on the different SOA-types produced, and therefore only in part
accounts for the discrepancy between hygroscopic growth and CCN activity
observed for SOA particles in the past.
Citation: Wex, H., Petters, M. D., Carrico, C. M., Hallbauer, E., Massling, A., McMeeking, G. R., Poulain, L., Wu, Z., Kreidenweis, S. M., and Stratmann, F.: Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements, Atmos. Chem. Phys., 9, 3987-3997, doi:10.5194/acp-9-3987-2009, 2009.