Heterogeneous freezing of droplets with immersed mineral dust particles – measurements and parameterization

摘要

During the measurement campaign FROST (FReezing Of duST), LACIS(Leipzig Aerosol Cloud Interaction Simulator) was used toinvestigate the immersion freezing behavior of size selected, coatedand uncoated Arizona Test Dust (ATD) particles with a mobilitydiameter of 300 nm. Particles were coated with succinic acid(C₄H₆O₄), sulfuric acid (H₂SO₄) and ammonium sulfate((NH₄)₂SO₄). Ice fractions at mixed-phase cloudtemperatures ranging from 233.15 K to 239.15 K (±0.60 K) weredetermined for all types of particles. In this temperature range,pure ATD particles and those coated with C₄H₆O₄ or smallamounts of H₂SO₄ were found to be the most efficient icenuclei (IN). ATD particles coated with (NH₄)₂SO₄ were themost inefficient IN. Since the supercooled droplets were highlydiluted before freezing occurred, a freezing point suppression dueto the soluble material on the particles (and therefore in thedroplets) cannot explain this observation. Therefore, it isreasonable to assume that the coatings lead to particle surfacealterations which cause the differences in the IN abilities. Twodifferent theoretical approaches based on the stochastic and thesingular hypotheses were applied to clarify and parameterize thefreezing behavior of the particles investigated. Both approachesdescribe the experimentally determined results, yielding parametersthat can subsequently be used to compare our results to those fromother studies. However, we cannot clarify at the current state whichof the two approaches correctly describes the investigated immersionfreezing process. But both approaches confirm the assumption thatthe coatings lead to particle surface modifications lowering thenucleation efficiency. The stochastic approach interprets thereduction in nucleation rate from coating as primarily due to anincrease in the thermodynamic barrier for ice formation (i.e.,changes in interfacial free energies). The singular approachinterprets the reduction as resulting from a reduced surface densityof active sites.