Chemical composition and sources of coastal marine aerosol particles during the 2008 VOCALS-REx campaign
The chemical composition of aerosol particles ( Dp ≤ 1.5 μm) was measured over the southeast Pacific Ocean during the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-Rex) between 16 October and 15 November 2008 using the US Department of Energy (DOE) G-1 aircraft. The objective of these flights was to gain an understanding of the sources and evolution of these aerosols, and of how they interact with the marine stratus cloud layer that prevails in this region of the globe. Our measurements showed that the marine boundary layer (MBL) aerosol mass was dominated by non-sea-salt SO 42−, followed by Na +, Cl −, Org (total organics), NH 4+, and NO 3−, in decreasing order of importance; CH 3SO 3− (MSA), Ca 2+, and K + rarely exceeded their limits of detection. Aerosols were strongly acidic with a NH 4+ to SO 42− equivalents ratio typically < 0.3. Sea-salt aerosol (SSA) particles, represented by NaCl, exhibited Cl − deficits caused by both HNO 3 and H 2SO 4, but for the most part were externally mixed with particles, mainly SO 42−. SSA contributed only a small fraction of the total accumulation mode particle number concentration. It was inferred that all aerosol species (except SSA) were of predominantly continental origin because of their strong land-to-sea concentration gradient. Comparison of relative changes in median values suggests that (1) an oceanic source of NH 3 is present between 72° W and 76° W, (2) additional organic aerosols from biomass burns or biogenic precursors were emitted from coastal regions south of 31° S, with possible cloud processing, and (3) free tropospheric (FT) contributions to MBL gas and aerosol concentrations were negligible. The very low levels of CH 3SO 3− observed as well as the correlation between SO 42− and NO 3− (which is thought primarily anthropogenic) suggest a limited contribution of DMS to SO 42− aerosol production during VOCALS.