Abstract
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.
Highlights
The atmospheric aerosol affects the Earth’s climate by influencing the radiation budget directly by scattering and absorbing incoming solar radiation (Pilinis et al, 1995) and indirectly by affecting cloud microphysical properties (Twomey, 1977), cloud formation (Petters et al, 2006), and cloud lifetime (Albrecht, 1989)
Secondary Organic Aerosols (SOA) is comprised of many different compounds, with the exact composition depending on the precursor and oxidant, measured hygroscopic growth factors are surprisingly similar for different systems (Virkkula et al, 1999; Saathoff et al, 2003; Baltensperger et al, 2005; VanReken et al, 2005; Varutbangkul et al, 2006; Prenni et al, 2007; Engelhart et al, 2008)
Typical SOA diameter growth factors are 1.1 (+/−0.05) at relative humidities (RH) ∼90%. This contrasts with measurements of the ability of SOA particles to serve as cloud condensation nuclei (CCN) (Huff Hartz et al, 2005; VanReken et al, 2005; King et al, 2007; Prenni et al, 2007; Duplissy et al, 2008; Engelhart et al, 2008), which is far greater than their hygroscopic growth factors would suggest from CCN modeling (Prenni et al, 2007)
Summary
The atmospheric aerosol affects the Earth’s climate by influencing the radiation budget directly by scattering and absorbing incoming solar radiation (direct aerosol effect) (Pilinis et al, 1995) and indirectly by affecting cloud microphysical properties (Twomey, 1977), cloud formation (Petters et al, 2006), and cloud lifetime (Albrecht, 1989) (indirect aerosol effects). SOA is comprised of many different compounds, with the exact composition depending on the precursor and oxidant, measured hygroscopic growth factors are surprisingly similar for different systems (Virkkula et al, 1999; Saathoff et al, 2003; Baltensperger et al, 2005; VanReken et al, 2005; Varutbangkul et al, 2006; Prenni et al, 2007; Engelhart et al, 2008). Previous studies were limited to hygroscopic growth measurements at RH
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