Chemistry of Cloud Formation
in early development
The following selections of abstracts may provide a glimpse of how an idea is investigated and moves towards an accepted explanation for one of the phenomena we observe.
Preliminary Investigation of the Role that DMS (Dimethyl Sulfide) and Cloud Cycles Play in the Formation of the Aerosol Size Distribution.
Hoppel, W. A. ; Fitzgerald, J. W. ; Frick, G. M. ; Larson, R. E. ; Wattle, B. J.
NAVAL RESEARCH LAB WASHINGTON DC
A series of experiments designed to study the production of new particulate matter by photolysis of dimethyl sulfide (DMS) and the effect that nonprecipitating clouds, have on the aerosol size distributions were carried out in Calspan Corporation's 600 cum environmental chamber during January and February 1986. The results show that DMS, the most abundant natural source of sulfur, is photooxidized to some product of low volatility that can form new particles by homogeneous nucleation or condense on existing aerosols causing them to grow. To explain these observations, a theoretical study of the nucleation properties of methane sulfonic acid (MSA) was undertaken. The nucleation thresholds, calculated using thermodynamic data for MSA, show that at 70% RH, and MSA concentration of only 0.006 ppb will result in a supersaturated environment in which MSA will condense on preexisting particles larger than 0.02 micron radius. If the MSA concentrations increase to 30 ppb, then spontaneous formation of MSA solution droplets occurs by homogeneous binary nucleation. Simulations of the evolution of the size distribution observed for the DMS irradiation experiments with a dynamic aerosol model that includes the effects of coagulation, growth by condensation, and deposition to the walls of the chamber, yield results that are in excellent agreement with the observed evolution.
source: Defense Technical Information Center 29 JUL 1987
Dimethyl sulfide and cloud condensation nucleus correlations in the northeast Pacific Ocean
Hegg, Dean A.; Ferek, Ronald J.; Hobbs, Peter V.; Radke, Lawrence F.
(U of Washington, Seattle)
A regression analysis on measurements of cloud condensation nucleus (CCN) and dimethyl sulfide (DMS) concentrations in the boundary layer over the northeastern Pacific Ocean shows these two parameters to be highly correlated. This complements and supports coherent seasonal trend data on DMS and CCN concentrations obtained at Cape Grim, Tasmania. The correlation between CCN and DMS at both northern and southern hemispheric remote marine sites, provides empirical support for the DMS-cloud-climate hypothesis. A possible nonlinear relationship between DMS and size-segregated CCN concentrations is suggested by the data.
source: Journal of Geophysical Research (ISSN 0148-0227), vol. 96, July 20, 1991, p. 13,189-13,191. Research supported by Quest for Truth Foundation.
Dimethylsulfide (DMS) in the Bering Sea and Adjacent Waters:
In-situ and Satellite Observations
Oceanic dimethylsulfide (DMS) is the major natural source of sulfur to the atmosphere (Bates et al., 1992). As a volatile odiferous sulfur compound, DMS serves as an olfactory attractant for sea birds (Nevitt et al., 1995). In the atmosphere DMS is oxidized to produce aerosol particles, which affect the acid-base chemistry of the atmosphere (Charlson and Rodhe, 1982) and the radiative properties of marine stratus clouds (Shaw 1983, 1987; Charlson et al., 1987; Andreae and Crutzen, 1997). An increase in the DMS flux has the potential to increase the number of atmospheric aerosol particles and the cloud drop number concentration. An increase in the number of small cloud drops suppresses precipitation leading to longer-lived clouds (Albrecht, 1989; Lohmann and Feichter, 1997). This effectively increases the cloud cover, which leads to more radiation reflected back to space.
The source of atmospheric DMS is the surface ocean. The production of DMS and its precursor, dimethylsulphoniopropionate (DMSP) is confined largely to a few classes of marine phytoplankton, specifically the Dinophyceae and the Prymnesiophycease, which include coccolithopheres (Keller et al., 1989). Blooms of the cocolithophore, Emiliania huxleyi, in the Gulf of Maine and Northeast Atlantic produced concentrations of DMS and DMSP that were as much as an order of magnitude higher than in the surrounding waters (Matrai and Keller, 1993; Malin et al., 1993). Although actively growing cells release only small quantities of DMSP and DMS, DMS is produced during cell senescence (Nguyen et al., 1988; Turner et al., 1988), zooplankton grazing (Dacey and Wakeham, 1986; Wolfe and Steinke, 1996), and the interaction of bacterioplankton (Kiene and Bates, 1990; Bates et al., 1994; Ledyard and Dacey, 1994) and viral pathogens (Malin et al., 1998; Hill et al., 1998).
Since the production and seawater consumption of DMS are highly species specific and dependent upon the bacteria, phytoplankton, and zooplankton communities, the concentration of DMS will be strongly influenced by the physical processes controlling water mass interactions and nutrient availability (Turner et al., 1996). This includes the input of atmospheric nutrients. Coccolithophore blooms have been shown to occur in seasonally stratified waters after the spring diatom bloom has depleted inorganic nutrient levels (Holligan, 1987). It has also been suggested that iron limitation might lead to coccolithophore dominated populations in regions such as the Gulf of Alaska (Martin et al., 1989).
The first true color images over the Bering Sea after the launch of SeaWiFS in August 1997 showed the presence of an extensive coccolithophore bloom. This was the first reported occurrence of a coccolithophore bloom in the Bering Sea, and it has since been associated with anomalous oceanographic and atmospheric conditions. The bloom has reoccurred annually since 1997.
DMS was measured in the Bering Sea in the spring of 1981 as part of the PROBES (Processes and Resources of the Bering Sea Shelf) program (Barnard et al., 1984). DMS concentrations in seawater were strongly correlated with the cell density of the haptophyte, Phaeocystis poucheti and ranged from 1 to 17 nM with a mean of 3 nM. DMS measurements were also made in the Bering Sea on a ship-of-opportunity in September 1985 by Bates and co-workers (1987). Concentrations ranged from 0.5 to 20 nM with a mean of 6 nM. Since DMS is biologically produced, the concentrations in the subarctic regions are very seasonally dependent (Bates et al., 1987).
When microscopic plankton in the shallow sea encounter sunlit skies, the upshot is a cloud-forming compound called dimethyl sulfide (DMS). Although researchers had long suspected a link between DMS production, ocean biota, sunlight, and cloud formation, the exact connection was unknown. Oceanographers in Spain have now figured out that the relationship is a negative feedback loop (Science 2007, 315, 506). DMS is the largest natural source of atmospheric sulfur and a precursor to cloud-forming aerosols. By using oceanographic data from around the world, Sergio M. Vallina and Rafel Simó at the Institute for Ocean Sciences, Barcelona, found that during sunny periods, ocean biota release more DMS, which in turn brings about cloud formation and a consequent reduction of DMS production.
source: Chemical & Engineering News, 29 Jan. 2007
Strong Relationship Between DMS and the Solar Radiation Dose over the Global Surface Ocean
Sergio M. Vallina and Rafel Simó
Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain.
Marine biogenic dimethylsulfide (DMS) is the main natural source of tropospheric sulfur, which may play a key role in cloud formation and albedo over the remote ocean. Through a global data analysis, we found that DMS concentrations are highly positively correlated with the solar radiation dose in the upper mixed layer of the open ocean, irrespective of latitude, plankton biomass, or temperature. This is a necessary condition for the feasibility of a negative feedback in which light-attenuating DMS emissions are in turn driven by the light dose received by the pelagic ecosystem.
Source: Abstract: Science 26 January 2007:
Vol. 315. no. 5811, pp. 506 - 508