Laboratory investigations into the effect of marine organic material on the sea-salt aerosol generated by bubble bursting

Abstract

A simple aerosol generation rig has been used to investigate the effect of marine organic material on the flux and inorganic particulate enrichment of marine aerosol. Aerosol was generated in the laboratory from UK coastal seawater samples collected on a monthly basis for 1 year, starting February 1988. Initial aerosol generation rates in the laboratory apparatus were typically between 0.3 and 1.0 ml min−1, dropping to a baseline value of around 0.1 ml min−1 after a prolonged period of bubbling. Initial aerosol generation rates showed no significant seasonal trend, whereas the total amount of aerosol generated before the rate fell to the baseline reached a plateau at around 60 ml from March to October, a factor of six greater than earlier in the year. Dissolved organic carbon (DOC) and surfactant activity have been measured in seawater from February 1988 to June 1990. The measurements show evidence of seasonal cycles for both determinands. Surfactant activity (expressed in equivalent units of the non-ionic model surfactant Triton X-100) shows a broad peak in the late autumn (2.5-3.2 mg l−1). Peak DOC concentrations were seen in early summer (1.9-3.4 mg Cl−1). It was also found that the surfactant-to-DOC ratio varies seasonally, which suggests that the DOC pool changes through the year. Neither initial aerosol generation rate nor total aerosol generated can be correlated with surfactant activity or DOC, although high surfactant levels can dramatically reduce both. Particulate organic carbon (POC) is highly variable (0.1-14 mg Cl−1), the ratio of POC to particulate loading being more useful. This ratio ranged from 15 to 4000 mg C g−1. Peak values may indicate biological production. Laboratory experiments investigating the aerosol enrichment of 3, 5 and 10 μm diameter synthetic silica particles were carried out at various seawater particulate loadings and surfactant activities. The enrichment factor (EF) of the particles was found to increase approximately linearly with surfactant activity. Particulate loading increases produced only a small increase in the uptake of silica particles in the aerosol, with EF remaining essentially constant. Varying the particle size appeared to have no effect on EF.