Depositional ice nucleation onto crystalline hydrated NaCl particles: a new mechanism for ice formation in the troposphere

Abstract

Abstract. Sea-salt aerosol (SSA) particles are ubiquitous in the marine boundary layer and over coastal areas. Therefore SSA have ability to directly and indirectly affect the Earth's radiation balance. The influence SSA have on climate is related to their water uptake and ice nucleation characteristics. In this study, optical microscopy coupled with Raman spectroscopy was used to detect the formation of a crystalline NaCl hydrate that could form under atmospheric conditions. NaCl (s) particles (∼1 to 10 µm in diameter) deliquesced at 75.7 ± 2.5 % RH which agrees well with values previously established in the literature. NaCl (aq) particles effloresced to a mixture of hydrated and non-hydrated particles at temperatures between 236 and 252 K. The aqueous particles effloresced into the non-hydrated form at temperatures warmer than 252 K. At temperatures colder than 236 K all particles effloresced into the hydrated form. The deliquescence relative humidities (DRH) of hydrated NaCl (s) particles ranged from 76.6 to 93.2 % RH. Based on the measured DRH and efflorescence relative humidities (ERH), we estimate crystalline NaCl particles could be in the hydrated form 40-80 % of the time in the troposphere. Additionally, the ice nucleating abilities of NaCl (s) and hydrated NaCl (s) were determined at temperatures ranging from 221 to 238 K. Here, depositional ice nucleation is defined as the onset of ice nucleation and represents the conditions at which the first particle on the substrate nucleated ice. Thus the values reported here represent the lower limit of depositional ice nucleation. NaCl (s) particles depositionally nucleated ice at an average S ice value of 1.11 ± 0.07. Hydrated NaCl (s) particles depositionally nucleated ice at an average S ice value of 1.02 ± 0.04. When a mixture of hydrated and anhydrous NaCl (s) particles was present in the same sample, ice preferentially nucleated on the hydrated particles 100 % of the time. While both types of particles are efficient ice nuclei, hydrated NaCl (s) particles are better ice nuclei than NaCl (s) particles.

Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles

“…The OH-bending modes of water are centered at 1644 cm 1 and 1664 cm 1 , in agreement with previous studies. 5,42,43 Wise et al also reported both the OH-stretching and OH-bending regions of NaCl·2H 2 O at 244 K but the Raman peaks in the OH-stretching region were not resolved in great detail. In addition to the OH-stretching and OH-bending modes reported before, we have observed the librational modes of hydrated NaCl in the low-frequency region (from 200 cm 1 to 800 cm 1 ).…”

“…The latter can serve as an efficient ice nucleating particle and might be essential for the microphysics of high altitude marine clouds. 4,5,7,8 According to the bulk phase diagram of NaCl/water, anhydrous NaCl is the thermodynamically stable phase formed upon efflorescence of solution above 273.3 K. 9,10 Below this peritectic temperature, the dihydrate form is more stable. However, measurements performed with submicron droplets have shown that sodium chloride dihydrate does not form at alexei.kiselev@kit.edu temperatures above 253 K. 11 A similar observation was made by Koop et al, who studied the efflorescence and deliquescence behavior of micron-sized droplets of NaCl solution and found that NaCl dihydrate was only formed via heterogeneous nucleation on available surfaces of ice.…”

“…1 The phase state of sea spray particles controls their physical and chemical properties, for example, in interaction with ozone 2 or in heterogeneous nucleation of ice. [3][4][5] Although SSA has a complex composition, 6 the phase transition of NaCl can play an important role in the final state and morphology of ambient aerosol particles. NaCl aerosol can exist either as aqueous solution droplets or in one of the two crystalline forms: anhydrous NaCl and NaCl dihydrate (NaCl·2H 2 O).…”

“…Wise et al 5 probed the efflorescence of micron size (1 µm−10 µm) droplets of NaCl solution deposited onto a hydrophobic quartz substrate in a setup combining an environmental cell and a Raman microscope. The authors observed a partitioning between crystallized anhydrous NaCl and NaCl dihydrate in the temperature range from 236 K to 252 K, with no dihydrate forming above 252 K. In this temperature regime, a linear relationship between the fraction of NaCl dihydrate and temperature was proposed by Wise et al In contrast, Wagner et al used the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber to investigate the temperature-dependent partitioning between the two crystalline phases of NaCl and observed no formation of sodium chloride dihydrate above 235 K. 19 The strong increase in dihydrate formation was found at the temperature 13 K lower than in the study of Wise et al, which was attributed to the smaller size of the solute droplets (between 1 and 2 µm in diameter), and to a possible influence of a heterogeneous surface aiding the nucleation of NaCl dihydrate.…”

Feldspar minerals as efficient deposition ice nuclei

“…The ability to participate in electrostatic interactions may then be added as a characteristic of a good IN, beyond the ability to bond simply with hydrogen (Pruppacher and Klett, 1997). Indeed, it may be for this reason that soluble salts, such as ammonium sulfate and sodium chloride, have been shown to show deposition IN abilities, provided that the temperature is below their eutectic Wise et al, 2012).…”