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 atmosphere (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 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 ± 0.02 μmoles m−2 day−1 and diluted MBL SO42 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 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− 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 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−.
Highlights
The ocean is the largest source of natural reduced sulfur gas to the atmosphere, most of which is in the form of dimethylsulfide (DMS), a volatile organic compound produced from phytoplankton
Lidar measurements showed that the average velocity variance in the marine boundary layer (MBL) was ∼50 % lower during the daytime, with mixed layer height (MLH) only reaching 60∼70 % of Zi in the remote region, generally below the stratocumulus cloud bottom but above the lifting condensation level (LCL)
In order to evaluate the effects of anthropogenic sulfur inputs to the atmosphere, we must first understand the natural background cycles
Summary
The ocean is the largest source of natural reduced sulfur gas to the atmosphere, most of which is in the form of dimethylsulfide (DMS), a volatile organic compound produced from phytoplankton. M. Yang et al.: Atmospheric sulfur cycling in the southeastern Pacific and removed from the marine boundary layer (MBL) through deposition to the ocean surface and oxidative reactions in the gas and aqueous phase. Gas phase oxidation of SO2 by OH and subsequent reactions with water vapor yield sulfuric acid vapor (H2SO4(g)), which usually condenses upon preexisting aerosol surfaces and increases non-sea-salt sulfate aerosol (SO24−) mass. In a remote marine environment where CCN represent a substantial fraction of the aerosol number, a gap at 70∼80 nm in diameter in the number distribution is often observed, which has been coined the “Hoppel minimum” (Hoppel et al, 1986) Aerosols smaller than this minimum are unactivated, while larger aerosols have been activated and grown through cloud processing into the accumulation mode (0.1∼1μm). Following up on the DMS budget presented in Yang et al (2009), we will examine the diel budgets of SO2 and SO24−
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