Biogenic sulfur emissions and aerosols over the tropical South Atlantic: 3. Atmospheric dimethylsulfide, aerosols and cloud condensation nuclei

Abstract

We measured dimethylsulfide in air (DMSa) and the number concentration, size distribution, and chemical composition of atmospheric aerosols, including the concentration of cloud condensation nuclei (CCN), during February‐March 1991 over the tropical South Atlantic along 19°S (F/S Meteor, cruise 15/3). Aerosol number/size distributions were determined with a laser‐optical particle counter, condensation nuclei (CN) concentrations with a TSI 3020, and cloud condensation nuclei (CCN) with a Hudson‐type supersaturation chamber. Aerosol samples were collected on two‐stage stacked filters and analyzed by ion chromatography for soluble ion concentrations. Black carbon in aerosols was measured by visible light absorption and used to identify and eliminate periods with anthropogenic pollution from the data set. Meteorological analysis shows that most of the air masses sampled had spent extended periods over remote marine areas in the tropical and subtropical region. DMSa was closely correlated with the sea‐to‐ air DMS flux calculated from DMS concentrations in seawater and meteorological data. Sea salt made the largest contribution to aerosol mass and volume but provided only a small fraction of the aerosol number concentration. The submicron aerosol had a mean composition close to ammonium bisulfate, with the addition of some methanesulfonate. Aerosol (CN and CCN) number and non‐sea‐salt sulfate concentrations were significantly correlated with DMS concentration and flux. This suggests that DMS oxidation followed by aerosol nucleation and growth in the marine boundary layer is an important, if not dominating, source of CN and possibly CCN. The degree of correlation between DMS and particle concentrations in the marine boundary layer may be strongly influenced by the different time scales of the processes regulating these concentrations. Our results provide strong support for several aspects of the CLAW hypothesis, which proposes the existence of a feedback loop linking DMS emission from marine plankton to sulfate aerosol and global climate.

Smoking Rain Clouds over the Amazon

“…Over the blue ocean, a wide tail of large drops occurs already at cloud base (Fig. 4A), apparently because of the large sea salt aerosols (16) that are present in the trade wind BL in sufficient quantities to induce such an effect (17). The surface wind during that flight was ϳ8 m s Ϫ1 , just enough to produce white caps and sea spray.…”

Tropospheric Sulfate Aerosol Formation Via Ion-Ion Recombination

“…2 The photooxidation of DMS is the dominant sulfate source in the remote marine environment. 3,4 However, the number of CCN associated with DMS emissions remains uncertain. Homogeneous nucleation of sulfuric acid vapor generated from DMS has been proposed as a source of new particles, as happens in other parts of the atmosphere.…”

“…5 This mechanism tends to be highly localized in its effects, 6,7 whereas measurements at remote marine sites suggest a more ubiquitous and steady background population of sulfuric acid-based aerosols with concentrations in the range of 100-1000/cm 3 . 3,4,8,9 It is also known that homogeneous nucleation of sulfuric acid is not generally favored in the MBL-except in isolated locales-because of pre-existing particles, which scavenge free acid vapor and prevent the build-up of concentrations sufficient for nucleation under reasonable conditions. 10 Another source of boundary layer aerosols is associated with entrainment from the free troposphere.…”

Re-visiting the CLAW hypothesis

“…[17][18][19][20][21][22][23] These findings, however, lacked the spatial and temporal coverage necessary to resolve whether this DMS-CCN coupling is widespread and, therefore, relevant for global climate processes. Other works suggested that globally, and especially in the northern hemisphere, most of the atmospheric aerosols and CCN do not come from DMS oxidation but from continental sources.…”