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Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions. Part I - The K-feldspar Microcline Enhanced ice nucleation efficiency of microcline immersed in dilute NH3 and NH44-containing solutions
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kumar, Arun Marcolli, Claudia Peter, Thomas |
| Copyright Year | 2018 |
| Abstract | Potassium containing feldspars (K-feldspars) have been considered key mineral dusts for ice nucleation (IN) in mixedphase 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 10 of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aaww = 0.9 – 1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, TThet(aaww) deviate strongly from TThet wwhet(aaww), the values calculated from the water-activitybased approach (where TThet wwhet(aaww) = TTmelt(aaww + ∆aaww) with a constant offset ∆aaww with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4-salts (molalities <~ 1 mol kg-1 corresponding to aaww >~ 0.96), we find IN 15 temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (TThet(aaww = 1)) and 5.5 K above TThet wwhet(aaww), revealing NH3 and NH4 to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH3 and NH4 solutions show a depression of the onset temperature below TThet wwhet(aaww) 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 NH4 as cation exhibit nucleation temperatures TThet(aaww) < TThet wwhet(aaww) even at very small solute concentrations. 20 In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from ∆aaww = const. indicates specific chemical interactions between particular 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. NH4). 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 25 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 fraction in dilute NH3 and NH4 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 30 irreversibly. Implications for IN on airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions, to a 5-K enhancement during condensation freezing when microcline particles experience high humidity (aaww >~ 0.96) at warm (252 257 K) and NH3/NH4-rich conditions. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.atmos-chem-phys-discuss.net/acp-2018-46/acp-2018-46-AR2.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
