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

Abstract

Abstract. Dimethylsulfide (DMS) emitted from the ocean is a biogenic precursor gas for sulfur dioxide (SO2) and non-sea-salt sulfate aerosols (SO42). 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 Ronald H. Brown and the NSF/NCAR C-130 aircraft along ~20° S from the coast (70° W) to a remote marine region (85° W). While SO42− 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 offshore (~100and 25 pptv). Compared to the "mass" entrainment fluxes of SO42− and SO2 from the free troposphere (0.5 ± 0.3 and 0.3 ± 0.2 μmoles m−2 day−1), the sea-to-air DMS flux (3.8 ± 0.1 μmoles m−2 day−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 SO42− production. Surface SO42− loading in the remote region displayed pronounced diel variability, increasing rapidly in the first few hours after sunset and then decaying for the rest of the time. We theorize that the increase in SO42− 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 SO42−.