Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

Abstract

As carbon monoxide within the oceanic surface layer is produced by solar radiation, diluted by mixing, consumed by biota, and outgassed to the atmosphere, it exhibits a diurnal cycle. The effect of dilution and mixing on this cycle is examined using a simple model for production and consumption coupled to three different mixed layer models. The magnitude and timing of the peak concentration, the magnitude of the average concentration, and the air-sea flux are considered. The models are run through a range of heating and wind stress and compared to experimental data reported by Kettle [1994]. The key to the dynamics is the relative size of four length scales; Dmix, the depth to which mixing occurs over the consumption time; L, the length scale over which production occurs; L out, the depth to which the mixed layer is ventilated over the consumption time; and Leorap , the depth to which the diurnal production can maintain a concentration in equilibrium with the atmosphere. If Dmi x >> L, the actual model parameterization can be important. If the mixed layer is maintained by turbulent diffusion, D mi x can be substantially less than the mixed layer depth. If the mixed layer is parameterized as a homogeneous slab, D mi x is equivalent to the mixed layer depth. If Dmi x > Lout, production is balanced by consumption rather than outgassing. The ratio between D mi x and Leonap determines whether the ocean is a source or a sink for CO. The main thermocline depth H sets an upper limit for D mi x and hence D mix/L , D mix/Lout, and D mix/Lcomp. The models are run to simulate a single day of observations. The mixing parameterization is shown to be very important, with a model which mixes using smallscale diffusion producing markedly larger surface concentrations than models which homogenize the mixed layer completely and instantaneously. Paper number 96JC00463. 0148-0227/96/96JC-00463509.00 forcing. If it is O(1 d-•) or smaller, the maximum concentrations are seen at the end of the day.In the presence of mixing, however, the picture becomes more complicated. The importance of the optical properties of the water, the timing of the maximum concentration, and the partition between gas exchange and consumption as sink terms can all depend on the mixing. This paper uses the diurnal cycle of carbon monoxide (CO) in order to attack the following four questions:1. When is dilution important in determining the surface concentration and flux of a photochemically produced trace species such as CO?2. How sensitive is the diurnal cycle to the exact parameterization of the optics, consumption, and outgassing, and how does such sensitivity depend on the mixing? 3. How sensitive are the results to different parameterizations of the dilution? One question of particular interest is the difference between slab models, which homogenize the mixed layer instantaneously and completely, and eddy viscosity models which require some time for the mixed layer to become homogenized. 4. When can a tracer such as CO be used to improve understanding of the...

The diel cycle of carbonyl sulfide in marine surface waters: Field study results and a simple model

“…Our data indicate that the variations in the diurnal pattern of sea surface COS are related to the combined effects of both UV light and wind-induced vertical mixing, with lowest COS concentrations accompanied by high wind speeds and low light levels. Downward mixing at high wind speeds would tend to reduce both sea surface concentration and diel amplitude of dissolved COS (Ferek and Andreae, 1984;Ulshrfer et al, 1995), as has been suggested by several modeling studies of COS (Doney et aL, 1995;Najjar et al, 1995) and CO (Gnanadesikan, 1996), which like COS is produced photochemically in sea surface water. Overall, our observations are consistent with the idea that photoproduction, hydrolysis removal, and wind-induced downward mixing are key factors controlling the biogeochemical cycling of sea surface COS (Andreae and Ferek, 1992;Ferek and Andreae, 1984;Ulshrfer et al, 1996;Ulsh6fer et al, 1995).…”

“…Our kinetic box model considers the effect of vertical mixing implicitly by adjusting the in situ photoproduction constant to the prevailing conditions. A potential improvement would be to link our kinetic model to a physical model, which accounts for the diel cycle of turbulent transport within the upper mixed layer of the ocean (Doney et al, 1995;Gnanadesikan, 1996;Kantha and Clayson, 1994).…”

Biological consumption of carbon monoxide in Delaware Bay, NW Atlantic and Beaufort Sea

“…As a short-lived photoproduct, CO is a key species for modeling the couplings among optics, photochemistry, biology, mixing dynamics and gas exchange in the upper ocean (Kettle 1994, Doney et al 1995, Gnanadesikan 1996, Najjar et al 2003. Moreover, photoproduction of CO itself is important, since CO is the second most abundant carbon-containing product of CDOM photochemistry, with a yield higher than the sum of all the known photochemically produced organic compounds (Mopper et al 1991).…”

Open-ocean carbon monoxide photoproduction

“…First evidence for a predominantly photochemical CO source in seawater came from observations of a pronounced diurnal cycle in sea-surface CO concentrations. These observations were later explained by the interplay between a strong photoproduction term (Wilson et al, 1970;Redden, 1982;Bullister et al, 1982;Gammon and Kelly, 1990;Zuo and Jones, 1995;Zafiriou et al, 2003) and fast removal by microbial oxidation Seiler, 1980, 1982;Zafiriou et al, 2003), although air-sea gas exchange Erickson, 1989;Bates et al, 1995;Zuo and Stubbins et al, 2006) and downward mixing (Kettle, 1994(Kettle, , 2005Doney et al, 1995;Najjar et al, 1995;Gnanadesikan, 1996;Johnson and Bates, 1996) also may be important. These interactions between biogeochemical and physical processes lead to complex spatial and temporal patterns in CO cycling that still pose a challenge today.…”

“…Current best estimates of the combined strength of the dissolved organic carbon sink associated with DOM photodegradation are in the order of 10-30% of global oceanic primary production (Miller and Moran, 1997;Mopper and Kieber, 2000), clearly relevant on Global scales. Furthermore, CO has also emerged as a key tracer for use in testing and tuning models of other mixed-layer processes, including photochemistry, ocean optics, radiative flux, mixing and air-sea gas exchange (Kettle, 1994(Kettle, , 2005Doney et al, 1995;Najjar et al, 1995;Gnanadesikan, 1996;Johnson and Bates, 1996). Hence, improved modelling and quantification of marine CO photoproduction will contribute to our understanding of a variety of interconnected marine biogeochemical processes.…”