An aerosol chamber investigation of the heterogeneous ice nucleating potential of refractory nanoparticles
Nanoparticles of iron oxide (crystalline and amor-phous), silicon oxide and magnesium oxide were investi-gated for their propensity to nucleate ice over the temper-ature range 180–250 K, using the AIDA chamber in Karl-sruhe, Germany. All samples were observed to initiate ice formation via the deposition mode at threshold ice super-saturations (RHi thresh) ranging from 105% to 140% for temperatures below 220 K. Approximately 10% of amorphous Fe 2 O 3 particles (modal diameter = 30 nm) generated in situ from a photochemical aerosol reactor, led to ice nucleation at RHi thresh = 140% at an initial chamber temperature of 182 K. Quantitative analysis using a singular hypothesis treatment provided a fitted function [n s (190K) = 10 (3.33⇥s ice)+8.16 ] for the variation in ice-active surface site density (n s :m 2) with ice saturation (s ice) for Fe 2 O 3 nanoparticles. This was im-plemented in an aerosol-cloud model to determine a pre-dicted deposition (mass accommodation) coefficient for wa-ter vapour on ice of 0.1 at temperatures appropriate for the upper atmosphere. Classical nucleation theory was used to determine representative contact angles (✓) for the different particle compositions. For the in situ generated Fe 2 O 3 parti-cles, a slight inverse temperature dependence was observed with ✓ = 10.5 at 182 K, decreasing to 9.0 at 200 K (com-pared with 10.2 and 11.4 respectively for the SiO 2 and MgO particle samples at the higher temperature). These observations indicate that such refractory nanopar-ticles are relatively efficient materials for the nucleation of ice under the conditions studied in the chamber which cor-respond to cirrus cloud formation in the upper troposphere. The results also show that Fe 2 O 3 particles do not act as Correspondence to: J. M. C. Plane (email@example.com) ice nuclei under conditions pertinent for tropospheric mixed phase clouds, which necessarily form above ⇠233 K. At the lower temperatures (<150 K) where noctilucent clouds form during summer months in the high latitude mesosphere, higher contact angles would be expected, which may reduce the effectiveness of these particles as ice nuclei in this part of the atmosphere.