资源环境科技发展态势分析平台

Potassium-containing feldspars (K-feldspars) have been considered as 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)(2)SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities a(w) = 0.9-1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, T-het (a(w)), deviate strongly from T-het(Delta awhet) (a(w)), the values calculated from the water-activity-based approach (where T-het(Delta awhet) (a(w)) = T-melt (a(w) + Delta a(w)(het)) with a constant offset Delta a(w)(het) with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4+ salts (molalities less than or similar to 1 mol kg(-1) corresponding to a(w) greater than or similar to 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (T-het (a(w) = 1)) and 5.5 K above T-het(Delta awhet) (a(w)), 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 T-het(Delta awhet) (a(w)) 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 T-het (a(w)) < T-het(Delta awhet) (a(w)) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Delta a(w)(het) = 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 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 resuspended in pure water. In contrast, exposure to severe acidity (pH less than or similar to 1.

2) or alkalinity (pH greater than or similar to 11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in 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 (a(w)greater than or similar to 0.96) at warm (252-257 K) and NH3/NH4+-rich conditions.