Chemical enhancement of CO₂ exchange in natural waters

Abstract

Exchange of CO2 under low turbulence conditions and high pH can be enhanced by hydration reactions of CO, with hydroxide ions and water molecules in the boundary layer. A series of field experiments was performed on several lakes, including alkaline closed-basin lakes, using enclosures (helmets) to study the enhancement process in nature. In addition, the enhancement of CO, exchange was studied in laboratory experiments with freshwater and seawater. The results of the experiments are compared with published theoretical calculations. Within the experimental uncertainties and shortcomings of the chemical enhancement models, reasonable agreement was observed between experimental and theoretical results for seawater. The experiments indicate, in accordance with theory, that chemical enhancement has a minor effect on air-sea gas exchange of CO, under average oceanic turbulence conditions. However, for the equatorial CO, source regions, with high temperatures and low winds, the calculated CO, enhancement amounts to 4-8% of the total exchange. The observations on lakes show poorer agreement with models, which is attributed to experimental uncertainty and poor characterization of the chemistry of the lake waters. The experiments show that chemical enhancement of CO, is ubiquitous for the alkaline closed-basin lakes with enhancements of up to a factor of three.Air-water gas transfer for slightly soluble gases is retarded in the frst several hundred microns of the liquid (Liss 1983). Slightly soluble gases arc defined as those that have Ostwald solubility coefficients (0) of < 10, where p is the volum,e of gas at temperature T and partial pressure p dissolved per unit volume of water (thus ,G' is dimensionless). In the simplest gas transfer model-the stagnant boundary layer model-transfer through the boundary layer occurs by molecular diffusion while above and below the liquid boundary layer transport occurs by much faster turbulent processes (Liss and Slater 1974). The gas transfer velocity (k)(cm h-l) through the stagnant boundary layer can thus be defined as k = 3,600 D z-l.(1) D is the molecular diffusion coefficient of the gas (cm* s-l), 3,600 is the conversion from hours to seconds, and z is the stagnant boundary layer thickness (cm) (Fig. 1). Chemically reactive gases can bypass the rate-limiting molecular diffusion step by reacting in the boundary layer. Although theory and several experimental studies indicate that the stagnant film model is conceptually incorrect, the principle of chemical enhancement remains unchanged, irrespective of gas transfer model. CO2 gas can undergo two hydration reactions in water:CO2 + OH-= HC03-.i AcknowledgmentsWe thank Rachel Oxburgh and Rob No11 for assisting in the experiments on the closed-basin lakes. Kitack Lee performed the alkalinity analyses of the seawater samples. Cathy Costa and Ed Harrison supplied wind-speed data for the equatorial Pacific enhancement exercise. Suggestions by two anonymous reviewers and Bernard Boudreau were most helpful.This research was suppo...

Eos, Transactions, American Geophysical Union Volume 88, Number 12, 20 March 2007

“…The surface water of the lake was spiked with SF 6 in order to determine the rate at which a chemically inert gas escaped to the atmosphere. The result was that the escape rate of SF 6 was the same as that for a nearby freshwater reservoir and consistent with expectation based on the wind speed over the lake [Wanninkhof et al, 1987].…”

“…Further, while experiments Wanninkhof and Knox, 1996] using Mono Lake water did yield a signifi cant CO 2 enhancement over chemically inert gases at low agitation rates, the difference was nowhere near enough to explain the observed 14 C buildup. This led to the conclusion either that CO 2 exchange in Mono Lake was governed by some unusual factor unique to the lake, or that someone had disposed of enriched 14 C in the lake .…”

Diurnal sampling reveals significant variation in CO2 emission from a tropical productive lake

“…where α is the factor for chemical enhancement of diffusion (Wanninkhof and Knox, 1996) and k is the coefficient of gas exchange for CO 2 at a given temperature. k was calculated from k 600 for low wind speeds (Equation 4) (Cole and Caraco, 1998) and from Schmidt numbers ratio (Equation 5) (Jähne et al, 1987): (4)…”

Estimation of bubble-mediated air–sea gas exchange from concurrent DMS and CO&lt;sub&gt;2&lt;/sub&gt; transfer velocities at intermediate–high wind speeds

“…There is a chemical enhancement of the CO 2 flux due to ionisation at the sea surface (Hoover and Berkshire, 1969). The effect on k CO 2 has been estimated to be up to about 8 % at a wind speed of 4-6 m s −1 (Wanninkhof and Knox, 1996), which amounts to a maximum impact of a few cm h −1 . By neglecting these effects we have slightly overestimated k w and k bub,CO 2 , but the magnitude of these corrections would be small relative to the environmental scatter or measurement uncertainty.…”