Enhanced ice nucleation efficiency of microcline immersed in dilute NH3 and NH -containing solutions

Content Provider	Semantic Scholar
Author	Kumar, Anand Marcolli, Claudia Luo, B. Peter, Thomas
Copyright Year	2018
Abstract	Potassium containing feldspars (K-feldspars) have been considered key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities = 0.9 – 1.0, were investigated by means of a differential scanning calorimeter (DSC). The 10 measured heterogeneous ice nucleation onset temperatures, deviate strongly from ∆ , the values calculated from the water-activity-based approach (where ∆ ∆ with a constant offset ∆ with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH -salts (molalities <~ 1 mol kg-1 corresponding to >~ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water 1 and 5.5 K above ∆ , revealing NH3 and NH to significantly enhance the IN of the microcline surface. 15 Conversely, more concentrated NH3 and NH solutions show a depression of the onset temperature below ∆ by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH as cation exhibit nucleation temperatures ∆ even at very small solute concentrations. In all these cases, the fraction of water volume frozen heterogeneously displays a decrease as solute concentration increases. This deviation from ∆ const. indicates specific chemical interactions between particular 20 solutes and the microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K+ available on the microcline surface with externally added cations (e.g.NH ). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH3 molecules hydrogen bonded on the microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen 25 fraction in dilute NH3 and NH solutions. Moreover, we show that aging of microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near neutral solutions since most of it recovers when aged particles are re-suspended in pure water. In contrast, exposure to severe acidity (pH <~ 1.2) or alkalinity (pH >~ 11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for ice nucleation on airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and NH3/NH -free 30 conditions, to a 5-K enhancement during condensation freezing when microcline particles experience high humidity ( >~ 0.96) at warm (252 257 K) and NH3/NH -rich conditions. Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-46 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 22 January 2018 c © Author(s) 2018. CC BY 4.0 License.
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