THE LONG-WAVELENGTH PHOTOCHEMISTRY OF CARBON-DISULFIDE

The photochemistry of carbon disulfide was examined over a concentrati on range that pertains to tropospheric conditions. At low mixing ratio s (67.79, 282.4, and 485.4 parts per billion by volume (ppbv)) in air, photochemical oxidation is very slow and carbonyl sulfide (OCS) produ ct was not observed. Upper limits were established for the quantum yie ld based on a practical OCS quantification limit of 10 ppbv (Phi OCS < 0.0022, 0.00079, and 0.00045, respectively). Solar photolysis at a mi xing ratio of 282.4 ppbv gave Phi OCS < 0.00065. At mixing ratios betw een 13 and 223.6 parts per million by volume (ppmv), Phi OCS increases linearly with PCS2 from 0.001 to 0.01. The Phi CS2, Phi OCS, and Phi CO measured did not depend significantly on the light intensity employ ed, which argues against the involvement of two-photon processes. At h igh PCS2 (above 3 torr) Phi CS2, Phi OCS, and Phi CO were independent of PCS2. This behavior resembles that observed for photopolymerization of CS2 in the absence of oxygen. Carbon monoxide, observed at mixing ratios as low as 72.0 ppmv, forms by photodegradation of (CS2)(x) poly mer that has previously photoincorporated oxygen. Photopolymerization under a nitrogen atmosphere was also first-order in CS2 with quantum y ields of 0.0027, 0.0036, and 0.0063 at 74.83, 145.1, and 297.2 ppmv, r espectively. Both photopolymerization and photooxidation appear to cea se below about 1 ppmv. This is the behavior expected from a bimolecula r reaction involving a long-lived excited state (CS2') that is necess ary for particle growth Regardless of the mechanism of photooxidation, the observed quantum yields are too small to compete with hydroxyl-in itiated oxidation of CS2 at tropospheric mixing ratios.