Observation of sea‐salt fraction in sub‐100 nm diameter particles at Cape Grim

Abstract

AbstractVolatility‐hygroscopicity tandem differential mobility analyzer measurements were used to infer the composition of sub‐100 nm diameter Southern Ocean marine aerosols at Cape Grim in November and December 2007. This study focuses on a short‐lived high sea spray aerosol (SSA) event on 7–8 December with two externally mixed modes in the Hygroscopic Growth Factor (HGF) distributions (90% relative humidity (RH)), one at HGF > 2 and another at HGF~1.5. The particles with HGF > 2 displayed a deliquescent transition at 73–75% RH and were nonvolatile up to 280°C, which identified them as SSA particles with a large inorganic sea‐salt fraction. SSA HGFs were 3–13% below those for pure sea‐salt particles, indicating an organic volume fraction (OVF) of up to 11–46%. Observed high inorganic fractions in sub‐100 nm SSA is contrary to similar, earlier studies. HGFs increased with decreasing particle diameter over the range 16–97 nm, suggesting a decreased OVF, again contrary to earlier studies. SSA comprised up to 69% of the sub‐100 nm particle number, corresponding to concentrations of 110–290 cm−3. Air mass back trajectories indicate that SSA particles were produced 1500 km, 20–40 h upwind of Cape Grim. Transmission electron microscopy (TEM) and X‐ray spectrometry measurements of sub‐100 nm aerosols collected from the same location, and at the same time, displayed a distinct lack of sea salt. Results herein highlight the potential for biases in TEM analysis of the chemical composition of marine aerosols.