Variability of Organic and Sulfate Concentrations in the North Atlantic

Georges Saliba1, Chia-Li Chen2, Savannah Lewis1, Lynn M Russell1, Patricia Quinn3, Timothy S Bates4 and Michael Behrenfeld5, (1)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (2)University of California Riverside, Riverside, CA, United States, (3)Atmospheric Chemistry Group & TPOS 2020 project, University of Washington/JISAO & NOAA/PMEL, Seattle, WA 98115, Seattle, WA, United States, (4)NOAA PMEL, Seattle, United States, (5)Oregon State University, Corvallis, OR, United States
Organic and sulfate-containing particles contribute the majority of aerosol number concentrations in the marine boundary layer (MBL). These particles are often large enough to activate and become cloud droplets and are therefore important for improving our understanding of cloud formation over the oceans. However, the link between seawater ecosystems and chemical composition of aerosol is still not clear. In this study, we use results from four North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) campaigns to investigate the variability in chemical composition of particles in the MBL using an aerosol mass spectrometer (AMS). For all campaigns, 17% to 65% of the variability in sulfate concentrations is explained by black carbon (BC) and fragment 60 (f60, a tracer for biomass burning emissions) suggesting a large contribution of anthropogenic sulfate. In contrast, BC and f60 explain only 22% to 36% of the variability of AMS organics. Moreover, AMS organics correlated strongly with Fourier Transform Infrared (FTIR) carboxylic acid group mass concentrations (an indicator of secondary processes) for NAAMES 2 (biomass climax) and NAAMES 3 (biomass decline transition). The springtime timing suggests local oceanic sources provide the volatile organic compounds (VOCs) to contribute to secondary particles, but transported VOCs may also contribute. Single-particle mass spectra showed that the fraction of internally mixed sulfate and organic particles was lower for NAAMES 1 and NAAMES 4 (8% and 9%, respectively) than for NAAMES 2 and NAAMES 3 (14% and 19%, respectively), also consistent with a larger contribution of secondary organic components in more productive seasons. The higher fraction of internally mixed particles for NAAMES 2 and NAAMES 3 compared to NAAMES 1 and NAAMES 4 is consistent with size distribution measurements that show overlapping particle modes for sulfate and organics during NAAMES 2 and NAAMES 3, but these modes were separate for NAAMES 1 and NAAMES 4. NAAMES measurements suggest that periods of elevated ocean productivity are associated with significant secondary organic formation. However, further research is needed to identify the source and character of the VOCs in order to provide a direct link between ocean ecosystems and secondary organic particles.