Chemical composition and sources of coastal marine aerosol particles during the 2008 VOCALS-REx campaign

摘要

The chemical composition of aerosol particles (D_p ≤ 1.5 μm) was measured over the southeast Pacific ocean during the VOCALS-REx experiment between 16~October and 15 November 2008 using the US DOE G-1 aircraft. The objective of these flights was to gain an understanding of the sources and evolution of these aerosols, and how they interacted 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 SO₄2?, followed by Na+, Cl?, Org, NH₄+, and NO₃?, in decreasing order of importance; CH₃SO₃?1 (MSA), Ca2+, and K+ rarely exceeded their limits of detection of ~0.05 and ~0.15 μg m?3 for anions and cations, respectively. The aerosols were strongly acidic as the NH₄+ to SO₄2? equivalence ratio was typically ? deficits caused by both HNO₃ and H₂SO₄, and were externally mixed with SO₄2? particles as the AMS detected no NO₃? whilst uptake of HNO₃ occurred only on SSA particles. The SSA loading as a function of wind speed agreed with that calculated from published relationships, and contributed only a small fraction of the total accumulation mode particle number. Vertical distribution of MBL SSA particles (D_p ≤ ~1.5 μm) was uniform, suggesting a very limited dilution from entrainment of free tropospheric (FT) air. It was inferred that because all of the aerosol species (except SSA) exhibited a strong land-to-sea gradient, they were of continental origin. Comparison of relative changes in median values using LOWESS fits as proxies suggests that (1) an oceanic source of NH₃ is present between 72° W and 76° W, and (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) FT contributions to MBL gas and aerosols were negligible. Positive Matrix Factorization analysis of organic aerosol mass spectra obtained with the AMS showed an HOA on 28 October 2008 but not on 6 November 2008 that we attribute to a more extensive cloud processing on the later date. A highly oxidized OOA factor resembling fulvic acid was found associated with anthropogenic and biogenic sources as well as long range transported biomass burn plumes in the FT air. A sulfur-containing OOA factor identified as MSA was strongly correlated with SO₄2?, hence anthropogenic. The very low levels of CH₃SO₃? observed suggest a limited contribution of DMS to SO₄2? aerosols production during VOCALS.