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

Abstract

Volatility-hygroscopicity tandem differential mobility analyzer measurements were used to infer the composition of sub-100nm 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-100nm particle number, corresponding to concentrations of 110-290 cm-3. Air mass back trajectories indicate that SSA particles were produced 1500km, 20-40 h upwind of Cape Grim. Transmission electron microscopy (TEM) and X-ray spectrometry measurements of sub-100nm 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.