Black carbon concentrations across the tropical Atlantic boundary layer using three methods
Abstract
24 particulate black carbon (BC) samples were quantified using three methods: a chemo-thermal oxidation at 375°C (CTO-375), a thermal optical transition method (Sunset Method), and pyrene fluorescence loss (PFL). BC samples were taken using a high-volume air sampler aboard the R/V Endeavor during the summer of 2010 in the Equatorial Atlantic Ocean in order to sample the aeolian "hotspot" plume of aerosols that extend from the African continent. Models have shown that annual elemental carbon (EC) deposition fluxes in this area could be as high as 25μg cm-2 a-1, which may be a significant contribution to the overall carbon budget as well as climate forcing simulations. Expected BC concentrations for this area, based on a global BC inventory using the MOGUNTIA global transport model, range between 0.01-1μg/m3 depending on season. The CTO-375 processed samples were run on an IRMS to get the total organic carbon (TOC), BC concentrations, and δ13C isotope ratios. BC was detected in every sample and concentrations ranged between 0.16-9.97μg/m3. BC concentrations were highest off the coast of the US and Caribbean islands but were lower (0.16-0.26μg/m3) in the African plume sampled between 1-5°N and 21-29°W. BC comprised between 13-81% of the TOC pool and δ13C isotopes between the TOC and BC for each sample differed by no more than 3‰. The TOC and BC pools average was -28.9‰ and -28.1‰ respectively, suggesting a C4 plant origin. A different isotope trend was evident for samples 16-19 (African plume). Their δ13C average was -23.1‰ for TOC, suggesting a mostly marine origin of the organic carbon or a mix of marine with C3 and C4 plants. This supports the observed lower BC/TOC ratio in these samples (13-50%). EC concentrations determined by the Sunset Method ranged between 0-0.32μg/m3, with EC being detected in only 8 of the 24 samples. In general, EC was found above the detection limit near the United States coastline or in the "hotspot" plume off Africa. The detected EC comprised between 1-22% of the OC pool (with one possible outlier detecting >50% of the OC pool) where the highest EC/OC ratios occurred near the US coast. The PFL method complements the other two approaches by measuring the entire proposed BC continuum (soot, charcoal, char, and partially charred biomass) rather than being biased to soot, such as in the CTO-375 method. BC values ranged between 0.07-2.49μg/m3 where the highest values were found within the African plume and off the coast of the Caribbean islands. BC/TOC values were between 4.2%-74% and agreed most closely to the CTO-375 values suggesting that soot is the main form of BC. The Sunset Method predicted the lowest concentration of EC in all samples, with the highest concentration of 0.32μg/m3. This method was the only technique to not detect EC in every sample, suggesting that our samples may require a longer sampling time. Although CTO-375 and PFL methods resulted in comparable results, the PFL-based results suggested elevated BC concentrations in the African plume as we had expected. Since BC/EC is an operational definition, multiple methods offer the greatest opportunity to evaluate true concentrations which are required for proper model verification. In conclusion, BC is present in high concentration in the remote Eq-Atlantic Ocean boundary layer, suggesting a strong climate forcing capability in this study area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A43I..04P
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles