Abstract
Abstract. A newly developed ice nucleation experimental set up was used to investigate the heterogeneous ice nucleation properties of three Saharan and one Spanish dust particle samples. It was observed that the spread in the onset relative humidities with respect to ice (RHi) for Saharan dust particles varied from 104% to 110%, whereas for the Spanish dust from 106% to 110%. The elemental composition analysis shows a prominent Ca feature in the Spanish dust sample which could potentially explain the differences in nucleation threshold. Although the spread in the onset RHi for the three Saharan dust samples were in agreement, the active fractions and nucleation time-lags calculated at various temperature and RHi conditions were found to differ. This could be due to the subtle variation in the elemental composition of the dust samples, and surface irregularities like steps, cracks, cavities etc. A combination of classical nucleation theory and active site theory is used to understand the importance of these surface irregularities on the nucleability parameter, contact angle that is widely used in ice cloud modeling. These calculations show that the surface irregularities can reduce the contact angle by approximately 10 degrees.
Highlights
Ice clouds constitute the largest source of uncertainty in predicting the Earths’ climate behavior according to last Intergovernmental Panel on Climate Change 2007 report (Forster et al 2007)
Four different heterogeneous ice nucleation mechanisms are hypothesized: deposition nucleation, condensation freezing, immersion freezing, and contact freezing
Ice nucleation properties of mineral dust particles collected from the Sahara and Spain were determined using a Thermal Gradient Diffusion Chamber (TGDC)
Summary
Ice clouds constitute the largest source of uncertainty in predicting the Earths’ climate behavior according to last Intergovernmental Panel on Climate Change 2007 report (Forster et al 2007). The dominant ice formation mechanism, for temperature below −38 ◦ C, is homogeneous nucleation. Kulkarni of the supercooled water droplets and aqueous aerosol particles, with the nucleation rate increasing for colder temperatures (Jeffrey and Austin, 1997; Pruppacher and Klett, 1997; P&K97). Four different heterogeneous ice nucleation mechanisms are hypothesized: deposition nucleation (direct deposition of water vapor onto the surface of the IN), condensation freezing (freezing of the condensate formed on the surface of the IN), immersion freezing (freezing initiated by the IN located within the droplet), and contact freezing (freezing occurs the moment IN comes in contact with a supercooled water droplet or aerosol solution droplets). Koop et al (2000), and Jeffrey and Austin (1997) and references therein showed good agreement between homogeneous ice nucleation observations and theories. While observational and theoretical results for homogeneous nucleation can be compared with good agreement, theoretical treatments of heterogeneous nucleation that involve IN surfaces are very difficult to relate to observations (Cantrell and Heymsfield, 2005)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
