The Ozonolysis of Methylated Selenide Compounds in the Atmosphere: Isotopes, Kinetics, Products, and Mechanisms.

Selenium (Se) is an essential micronutrient with an important atmospheric component in its biogeochemical cycle. In this cycle, phytoplankton form volatile organic Se species, such as dimethyl selenide (CH 3 SeCH 3 ) and dimethyl diselenide (CH 3 SeSeCH 3 ), which are emitted into the atmosphere. To predict the atmospheric fate of these methylated Se compounds, we investigated their ozonolysis reaction. We used proton-transfer-reaction time-of-flight mass spectrometry to quantify atmospheric Se and its isotopes, and used this method in kinetic and product studies. The ozonolysis of CH 3 SeCH 3 proceeded with a rate constant of (7.4 ± 2.2) × 10 -17 cm 3 molec -1 s -1 at 26 ± 1 °C with an activation energy of 50 ± 14 kJ mol -1 forming dimethyl selenoxide (CH 3 Se(O)CH 3 ). Comparatively, CH 3 SeSeCH 3 reacted with O 3 at (2.6 ± 0.9) × 10 -17 cm 3 molec -1 s -1 at 27 ± 1 °C with an activation energy of 56 ± 5 kJ mol -1 forming methylselinic acid (CH 3 Se(O)OH). At 20 ppbv of O 3 , the atmospheric lifetimes of CH 3 SeCH 3 and CH 3 SeSeCH 3 are 7.6 and 22 h, respectively. The Se oxidation products were confirmed by synthesis and can serve as new atmospheric tracers of methylated Se compounds. Overall, we measured Se isotopes in real time and determined the rate constants, activation energies, and oxidation products. These mechanisms can now be used to determine the quantitative atmospheric fate of Se toward O 3 , and thus its distribution within a changing climate.