Radon piston velocity

The effect of wind velocity on (a) thin-film thickness and (b) piston velocity. The solid line represents results obtained from measurements made in wind tunnels. In situ measurements were made from distributions of the naturally occurring radioisotopes of carbon and radon. Source From (a) Broecker, W. S., and T.-H. Peng (1982). Tracers in the Sea. Lamont-Doherty Geological Observatory, p. 128, and (b) Bigg, G. R. (1996). The Oceans and Climate. Cambridge University Press, p. 85. 

These authors used the global average piston velocity determined by Broecker and Peng (391 by the radon deficit method, 2.8 m/day. The Othmer-Thakar relationship was used to calculate the diffusivity of DMS, which has never been determined experimentally. Since the calculated diffusivity for DMS (1.2 x 10-5 cm2/s) is similar to that calculated for radon, the radon deficit piston velocity was assumed to apply to DMS without correction. The DMS concentrations used in this study were based on more than 600 surface ocean samples from a variety of environments. The global area weighted concentration used for the calculation was 102.4 ng S/l, resulting in a flux of 39 x 1012 g S/yr. 

We can attempt to apply the same type of model to the H2S data, however there are two additional unknown factors involved. First, we do not have a measurement of the sea surface concentrations of H2S. Second, the piston velocity of H2S is enhanced by a chemical enrichment factor which, in laboratory studies, increases the transfer rate over that expected for the unionized species alone. Balls and Liss (5Q) demonstrated that at seawater pH the HS- present in solution contributes significantly to the total transport of H S across the interface. Since the degree of enrichment is not known under field conditions, we have assumed (as an upper limit) that the transfer occurs as if all of the labile sulfide (including HS ana weakly complexed sulfide) was present as H2S. In this case, the piston velocity of H2S would be the same as that of Radon for a given wind velocity, with a small correction (a factor of 1.14) for the estimated diffusivity difference. If we then specify the piston velocity and OH concentration we could calculate the concentration of H2S in the surface waters. Using the input conditions from model run B from Figure 4a (OH = 5 x 106 molecules/cm3, Vd = 3.1 m/day) yields a sea surface sulfide concentration of approximately 0.1 nM. Figure S illustrates the diurnal profile of atmospheric H2S which results from these calculations. 

Figure 4. Box model results compared to Caribbean transect data ((6), solid symbols). Units are m/day for Vp and 106 molecule/cm3 for midday maximum OH. (a) Runs using piston velocities obtained from the radon deficit (V Rn = 3.1) and SFg lake study (VpSF6 = 2.2) wind speed relationships. Midday maximum OH concentrations (shown on plot) were adjusted to give mean DMS levels in agreement with the shipboard data, (b) Model runs with lower piston velocities and lower OH showing less diurnal variation. Conditions used were (a) Vp = 1.7, OH = 8.0 (b) Vp = 1.1, OH = 5.0 (c) Vp = 0.6, OH = 3.0.

A number of other functions have been proposed that relate piston velocity to wind speed. These are shown along with the Liss and Merlivat [59] relationship in Fig. 5. On the basis of TTO data from measurements in the Equatorial Atlantic using the radon deficiency method, Smethie et al. [62] proposed a linear relationship between wind speed and gas exchange. Wanninkhof [26] proposed a quadratic function and noted the effect of using an average wind speed when the dependence of exchange on wind speed is nonlinear. The relationship is for steady winds or short-term winds and was determined from the data for bomb 002 and the wind speed distribution around the global mean wind speed. The model predictions of Deacon [28], also shown in Fig. 5, are appropriate only to low wind speeds. 

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