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.

No abstract

The production of singlet oxygen by thiazine dye photosensitization, as measured by the rate of photooxidation of tryptophan, was found to be very sensitive to changes of pH in the range 5-9. For methylene blue in aerated solutions, the production of 'O,* is approximately five times more efficient in basic than in acidic medium. This was shown to be related to the pK's of the triplet dyes, by evaluating the yields of '02* from the lifetimes and the quenching rate constants for the two ionic species of sensitizer triplets measured by laser flash photolysis. Changes in the quenching rate constants of the thiazine triplet states can be correlated with the triplet energies.

No abstract