A continuous flow diffusion chamber study of sea salt particles acting as cloud nuclei: deliquescence and ice nucleation

Abstract

Phase changes of sea salt particles alter their physical and chemical properties, which is significant for Earth's chemistry and energy budget. In this study, a continuous flow diffusion chamber is used to investigate deliquescence, homogeneous and heterogeneous ice nucleation between 242 K and 215 K, of four salts: pure NaCl, pure MgCl 2 , synthetic sea water salt, and salt distilled from sampled sea water. Anhydrous particles, aqueous droplets and ice particles were discriminated using a polarisation-sensitive optical particle counter coupled with a machine learning analysis technique. The measured onset deliquescence relative humidities agree with previous studies, where sea water salts deliquescence at lower humidities than pure NaCl. Deliquesced salt droplets homogenously freeze when the relative humidity reaches a sufficiently high value at temperatures below 233 K. From 224 K and below, deposition nucleation freezing on a fraction of NaCl particles was observed at humidities lower than the deliquescence relative humidity. At these low temperatures, otherwise unactivated salt particles deliquesced at the expected deliquescence point, followed by homogeneous freezing at temperatures as low as 215 K. Thus, the observed sea salt particles exhibit a triad of temperature-dependent behaviours. First, they act as cloud condensation particles (CCNs) > 233 K, second they can be homogeneous freezing nuclei (HFNs) < 233 K and finally they act as ice nucleating particles (INPs) for heterogeneous nucleation <224 K.

Solid Ammonium Nitrate Aerosols as Efficient Ice Nucleating Particles at Cirrus Temperatures

“…Homogeneous freezing at temperatures ≥230.5 K must have been preceded by the deliquescence of the initially crystalline AN/AS particles. The particle growth associated with the uptake of water during deliquescence can be detected in the time series of the OPC3 count rates in individual size channels (Kong et al, 2018). Figure 6b shows the time series of particle counts in the two lowermost OPC3 size channels (ch1 and ch2) and in two upper particle size bins (ch6 and ch7) during the INKA humidity scan at 238.5 K corresponding to measurement (1) in Figure 6a.…”

Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes

“…7a the stability of the PINCii chamber background is illustrated, and in Fig. 7b initial tests of the conditions for water droplet breakthrough are shown to be in line with other existing CFDCs (Stetzer et al, 2008;Garimella et al, 2016;Kong et al, 2018). During HyICE2018 data for instrument intercomparison were also collected using multiple existing INP measurement platforms (PINC, FRIDGE, SPIN, etc.).…”

“…An important implication of such findings is that common atmospheric surfactants, such as VOCs, can promote and/or hinder water uptake, changing the hydrophilicity of atmospheric nanoparticles. Additional, laboratory investigations focused on how soluble salts may nucleate ice and whether or not at below-eutectic temperatures sea salts might act as ice-nucleating particles rather than deliquesce as CCN (Kong et al, 2018;Wagner et al, 2018;Castarède and Thomson, 2018). All of these processes may affect cloud evolution and lifetime and thereby impact fundamental environmental processes like the water cycle and radiative balance.…”

A Link between the Ice Nucleation Activity of Sea Spray Aerosol and the Biogeochemistry of Seawater

“…Crevices and pores on the particle surface can promote liquid water 20 condensation that subsequently freezes (David et al, 2019). Ice nucleation can also occur when the soluble components of an internally mixed particle deliquesce below water saturation, exposing insoluble INPs to liquid water (Khvorostyanov et al, 2004;Kong et al, 2018).…”