Changes in ambient atmospheric relative humidity (RH) and temperature can cause phase transitions of atmospheric particulate. Salt crystal nucleation and growth from an aqueous solution upon evaporation (efflorescence) often occurs at a lower RH than the reverse process (deliquescence). This hysteresis effect is due to the energy barrier of forming a new phase. In some instances, nucleation is completely inhibited and efflorescence does not occur even under extremely dry conditions. Instead, an amorphous non-crystalline phase forms. For example, microdroplets of NH4NO3 do not effloresce homogeneously even upon decreasing the RH to &lt;1%. The mechanism for the inhibition of NH4NO3 efflorescence is unclear but has been suggested to arise from NH3 and HNO3 evaporation from the aqueous phase. In contrast to the homogeneous case, NH4NO3 efflorescence has been observed when a solid particle is immersed in a droplet prior to decreasing RH [Han et al. J. Geophys. Res. 2002; Mikhailov et al. Atm. Chem. Phys. 2004]. However, heterogeneous efflorescence induced by contact with an externally located particle (i.e. contact efflorescence) has not been probed. Presented here are laboratory experiments of contact efflorescence of NH4NO3 microdroplets, an atmospherically relevant compound that does not homogeneously effloresce. Experiments were performed in a long working-distance optical trap where levitated microdroplets were monitored in situ to determine size, phase and morphology. Collisions were tracked and quantified by imaging scattered laser light. Using soluble heterogeneous nuclei relevant to e.g. biomass burning (K2SO4) and sea spray aerosol (NaCl), NH4NO3 effloresced as high as ~50% RH upon a single collision. While efflorescence initiated by an immersed particle can only be studied at one characteristic efflorescence RH, contact efflorescence does not suffer from this limitation. Phase transformations are initiated after lowering the RH, so a range of RH from zero to the deliquescence RH can be studied. Thus, the influence of NH3 and HNO3 evaporation as a function of time was probed by equilibrating the droplet at low RH for different lengths of time prior to contact. Contact efflorescence has only recently been proposed as a pathway for the formation of solid particulate in the atmosphere, buts its relevance remains unclear. The results presented here will be discussed in this context, with emphasis on areas of potential relevance and future research.

Contact-induced efflorescence of amorphous inorganic microparticles

249th National Meeting (2015)

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