No abstract

Photochemical reactions may affect the photic zone, the surface sunlit layer of freshwaters, oceans, and estuarine regimes where fresh and saltwaters begin to interact. Hydrosphere photochemistry influences the world's oceans (about 70% of the Earth's surface), and the illuminated portion of the hydrosphere is especially active and diverse. 'Ransport, material exchanges, and biology (including aquatic primary pro-3 S A Environ. Sci.Technol.* The authors, ivlto also served as ihe orpni:f>!,s ~~~~i i t m i r i w ~! / t h c ,VAT0 Advanced Research Insriruw. ore (clockwise from u p p r 1c.h): Hod Zika. Riduird Zepp. Jucques Joussor-Duhien, and Oliver C. Z~!firiou.

[1] Studies to characterize sources and sinks of carbon monoxide (CO) in the mixed layer were carried out at sites covering large regions of the north and south Pacific. Apparent quantum yield spectra for the photochemical production of CO from colored dissolved organic matter were measured, as were first-order net microbial CO consumption rate constants. Contrary to initial expectations, neither photoproduction nor biooxidation parameters exhibited strong regional variations, except that in the Southern Ocean CO biooxidation rate coefficients were very low. Global ''blue-water'' CO flux terms derived from the data (in Tg carbon from CO per year, CO-C a À1 ) are: photochemical source, 50 (estimated range, 30-70), microbial sink, 32 (estimated range, 10-60) and total CO sink (microbial plus gas exchange), 38 (estimated range, 13-60). Considering uncertainties and extrapolation biases, these independently estimated source and sink terms are thus in, or close to, balance at $40 (range of overlap, 30-60) Tg CO-C a À1 . The simplest interpretation of this balance is that no major net sources or sinks (i.e., light-independent production, photoproduction at >450 nm) remain undiscovered, though considerable uncertainty in actual process rates remains. These CO fluxes are, however, very much smaller than some recently estimated values. The origins and implications of these discrepancies are discussed, and the coastal budget term is approximated.

Quantification of photochemical O 2 uptake provides a measure of total chemical photooxidation of dissolved organic matter (DOM). Here we study this process and present estimates suggesting that photooxidation has the potential to significantly modify marine DOM pools, complementing or exceeding oxidation via coupled chemicalbiological pathways. We measured apparent quantum yields (AQYs) of photobleaching, O 2 uptake, and H 2 O 2 production in several coastal marine samples and in dilutions of a tropical estuarine water with oligotrophic seawater. O 2 -loss AQYs varied little among samples or with dilution but decreased linearly from 1.2 ϫ 10 Ϫ3 at 300 nm to 0.3 ϫ 10 Ϫ3 at 400 nm and dropped about threefold to near-constant values with increasing absorbed light dose. H 2 O 2 production, about 45% of O 2 uptake, showed similar dependencies, whereas singlet oxygen (O 2 ( 1 ⌬ g )) reactions contributed less than 1% of O 2 uptake for typical coastal water. Implications of these findings for photochemical O 2 , H 2 O 2 , and DOM cycling are discussed.Modeling the dose-dependence of O 2 loss and photobleaching at 310 nm required three DOM pools. In the simplest case, about 90% is a weakly absorbing, low-AQY pool of DOM admixed with two similar-sized pools of more photochemically reactive DOM. This result suggests that rigorously extrapolating laboratory data to the environment requires detailed mapping of dose-wavelength-photobleaching AQY surfaces.Action spectra and DOM flux estimates for coastal photooxidative chemistry were derived. Site-specific potential rates are comparable to available in situ data. Globally, the DOM photolysis capacity appears to be larger than estimated coastal DOM inputs, especially in tropical and temperate areas, including areas with maximal DOM inputs.

It is generally accepted that methyl iodide is a major contributor to the flux of iodine from the ocean to the atmosphere, but its sources in the ocean are largely unknown, an exception being production by certain species of coastal macrophytic algae. Preliminary results reported here indicate production of methyl iodide in filtered seawater that has been irradiated with either sunlight or an artificial light source having a spectral output closely approximating to sunlight at sea level. The production was enhanced when the water samples were deoxygenated and by the addition of iodide to this water. The results appear to be consistent with a mechanism involving reaction between photochemically produced methyl radicals and iodine atoms. It is estimated that the process is capable of making a significant contribution to global iodine fluxes.