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 (SO 2 ) and non-sea-salt sulfate aerosols (SO 2− 4 ). During the VAMOSOcean-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 atmosphere (85 • W). While SO 2− 4 and SO 2 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 SO 2 burden at a rate of 0.05 ± 0.02 µmoles m −2 day −1 and diluted MBL SO 2 4 burden at a rate of 0.5 ± 0.3 µmoles m −2 day −1 , while the sea-to-air DMS flux (3.8 ± 0.4 µmoles m −2 day −1 ) remained the preCorrespondence to: B. J. Huebert (huebert@hawaii.edu) dominant source of sulfur mass to the MBL. In-cloud oxidation was found to be the most important mechanism for SO 2 removal and in situ SO 2− 4 production. Surface SO 2− 4 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 SO 2− 4 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 SO 2− 4 .

Impact of the modal aerosol scheme GLOMAP-mode on aerosol forcing in the Hadley Centre Global Environmental Model

“…Figure 5 shows schematically how each aerosol scheme responds to an increase in DMS flux. Both observations and modelling have shown that the dominant source of sulphate is through aqueous-phase oxidation in the presence of stratocumulus clouds (Andreae et al, 1999;O'Dowd et al, 1999;Yang et al, 2011;Woodhouse et al, 2013). The CLASSIC scheme, which does not simulate aerosol number and assumes a fixed modal distribution for sulphate aerosol, is only able to respond to an increase in sulphur by directly increasing the CCN number (label "a" in Fig.…”

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

“…Allen et al (2011) and Kleinman et al (2012) found that both carbon monoxide (a continental tracer) and sulfate aerosol mass also were higher in the boundary layer near shore than offshore. While sulfate aerosol can be produced from plankton-derived dimethylsulfide or DMS, Yang et al (2011a) showed that DMS actually tended to be lower near shore than offshore. These facts, together with the backtrajectories presented by Bretherton et al (2010), , and below, substantiate the hypothesis that the enhanced accumulation-mode particles near shore are derived from continental and not marine sources.…”

Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem

“…Here we briefly describe the observations that are employed in this modeling study. Detailed descriptions of the instruments can be found elsewhere (e.g., Allen et al, 2011;Wood et al, 2011a;Yang et al, 2011).…”