Role of the Volatile Fraction of Marine Aerosol on its Hygroscopic Properties

Abstract

The hygroscopic growth factor (HGF) of 85 and 20 nm particles marine aerosols was measured during January 2006 for a three-week period within the frame of the MAP (Marine Aerosol Production) winter campaign, using the TDMA technique. The results were compared to aerosol produced in a simulation tank by bubbling air through sea water sampled at the coastal site of Mace Head during the campaign, and through synthetic sea water (exempt of organic substances). This simulation was assimilated to primary production. The 85 nm HGF observed in the atmosphere during clean marine sectors were lower than the ones measured from the bubbling processes: the sea salt HGF mode is slightly lower and there is an additional 1.5 HGF mode on the atmospheric aerosol. This would indicate that either the sea water sampled near Mace Head was not as rich in hydrophobic matter as further up wind or that secondary processes have occurred during transport. The role of the volatile fraction of the aerosols was then studied by gently heating the particles at 90°C (without particle size change) and measuring the subsequent HGF change, with a combination of Volatility and Hygroscopicity TDMA (i.e., the VHTDMA). We observed that the volatilization of less than 10% by diameter of the particles lead to an inhibition of the 1.5 GF mode for 85 nm particles but not for 20 nm particles. This result would indicate that secondary condensing processes, implying volatile substances, would have influenced the 85 nm particles. These results only apply to low biological activity periods.