Black carbon concentrations and sources in the marine boundary layer of the tropical Atlantic Ocean using four methodologies

Abstract

Abstract. Black carbon (BC) is the highly carbonaceous byproduct of biomass burning and fossil fuel combustion with a composition ranging from thermally stable soot to less recalcitrant charcoal. Atmospheric particulate matter samples across the tropical Atlantic Ocean were quantified for BC using four different methods: chemothermal oxidation at 375 °C (CTO-375), pyrene fluorescence loss, thermal optical transmittance, and optical transmission attenuation. The highest BC concentrations were detected in the Caribbean Sea and off the African coast, with a regional average of 0.6 μg m−3 for both. The lowest average concentrations were measured off the coast of South America at 0.2 to 0.3 μg m−3. The thermally-based CTO-375 method generally detected lower BC concentrations than the other three methods. The ratio of soot-like BC, as defined by the CTO-375 method, relative to the broader BC combustion continuum, as defined by the pyrene fluorescence loss, was <1 for all regions except for the Caribbean, supporting that charcoal was an important fraction of the aerosol BC. Regions impacted by biomass burning emissions should utilize multiple methods to better apportion the BC concentrations and sources.