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

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 Sice value of 1.11± 0.07. Hydrated NaCl(s) particles depositionally nucleated ice at an average Sice 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.