FTIR measurements of functional groups and organic mass in aerosol samples over the Caribbean

An aircraft-based aerosol concentrator consisting of three virtual impactors in series has been used to collect particles for Fourier transform infrared (FTIR) transmission spectroscopy on stretched Teflon filters. Submicron atmospheric aerosol samples were collected near St. Croix aboard the National Center for Atmospheric Research C-130 aircraft during the passing efficiency of a low-turbulence inlet project. FTIR identified sulfate, ammonium, silicate, and organic functional groups, and a four-solvent rinsing procedure separated these functional groups into fractions of increasing hygroscopicity. The Caribbean aerosols contained 0.01–1.6 μg m−3 of Si, indicative of African mineral dust at all altitudes, with higher Al/Ca ratios in the boundary layer. Organic compounds were present in high and low dust conditions with estimated total organic mass varying from 0.8–4 μg m−3 during the project. Aliphatic carbon and carbonyl carbon were quantified at concentrations of up to 4 and 0.8 μg m−3, respectively, and were summed to calculate organic carbon and organic mass (OM) concentrations. Sequential solvent rinsing with repeated FTIR analysis revealed that 60–90% of OM in the free troposphere was hygroscopic, with hydrophobic organic compounds accounting for 20–50% of OM in the marine boundary layer.