The long‐wavelength photochemistry of carbon disulfide

Abstract

The photochemistry of carbon disulfide was examined over a concentration range that pertains to tropospheric conditions. At low mixing ratios (67.79, 282.4, and 485.4 parts per billion by volume (ppbv)) in air, photochemical oxidation is very slow and carbonyl sulfide (OCS) product was not observed. Upper limits were established for the quantum yield based on a practical OCS quantification limit of 10 ppbv (ΦOCS<0.0022, 0.00079, and 0.00045, respectively). Solar photolysis at a mixing ratio of 282.4 ppbv gave ΦOCS<0.00065. At mixing ratios between 13 and 223.6 parts per million by volume (ppmv), ΦOCS increases linearly with PCS2 from 0.001 to 0.01. The ΦCS2, ΦOCS, and ΦCO measured did not depend significantly on the light intensity employed, which argues against the involvement of two‐photon processes. At high PCS2 (above 3 torr) ΦCS2, ΦOCS, and ΦCO 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 polymer that has previously photoincorporated oxygen. Photopolymerization under a nitrogen atmosphere was also first‐order in CS2 with quantum yields of 0.0027, 0.0036, and 0.0063 at 74.83, 145.1, and 297.2 ppmv, respectively. Both photopolymerization and photooxidation appear to cease below about 1 ppmv. This is the behavior expected from a bimolecular reaction involving a long‐lived excited state (CS2*′) that is necessary for particle growth. Regardless of the mechanism of photooxidation, the observed quantum yields are too small to compete with hydroxyl‐initiated oxidation of CS2 at tropospheric mixing ratios.