Atmospheric sulfur cycling in the southeastern Pacific – longitudinal distribution, vertical profile, and diel variability observed during VOCALS-REx

Abstract

Dimethylsulfide (DMS) emitted from the ocean is a biogenic precursor gas for sulfur dioxide (SO₂) and non-sea-salt sulfate aerosols (SO₄^2&minus;). During the VAMOS-Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in 2008, multiple instrumented platforms were deployed in the Southeastern Pacific (SEP) off the coast of Chile and Peru to study the linkage between aerosols and stratocumulus clouds. We present here observations from the NOAA Ship <i>Ronald H. Brown</i> and the NSF/NCAR C-130 aircraft along ~20° S from the coast (70° W) to a remote marine atmosphere (85° W). While SO₄^2&minus; and SO₂ concentrations were distinctly elevated above background levels in the coastal marine boundary layer (MBL) due to anthropogenic influence (~800 and 80 pptv, respectively), their concentrations rapidly decreased west of 78° W (~100 and 25 pptv). In the remote region, entrainment from the free troposphere (FT) increased MBL SO₂ burden at a rate of 0.05 &plusmn; 0.02 μmoles m^&minus;2 day^&minus;1 and diluted MBL SO₄² burden at a rate of 0.5 &plusmn; 0.3 μmoles m^&minus;2 day^&minus;1, while the sea-to-air DMS flux (3.8 &plusmn; 0.4 μmoles m^&minus;2 day^&minus;1) remained the predominant source of sulfur mass to the MBL. In-cloud oxidation was found to be the most important mechanism for SO₂ removal and in situ SO₄^2&minus; production. Surface SO₄^2&minus; concentration in the remote MBL displayed pronounced diel variability, increasing rapidly in the first few hours after sunset and decaying for the rest of the day. We theorize that the increase in SO₄^2&minus; was due to nighttime recoupling of the MBL that mixed down cloud-processed air, while decoupling and sporadic precipitation scavenging were responsible for the daytime decline in SO₄^2&minus;.