Photochemical Production of Methyl Halides with Guaiacol as the Precursor.

Methyl halides are crucial trace greenhouse gases in the atmosphere, playing a significant role in global climate change and the atmospheric environment. This study investigated the photochemical production of methyl halides in an artificial seawater system using guaiacol as a precursor through laboratory simulation experiments. The influences of various environmental factors, including illumination time, radiation wavebands, illumination intensity, concentrations of guaiacol and halide ions (X⁻), Fe³⁺, salinity, dissolved oxygen (DO), and pH value on the photochemical production of methyl halides were examined. We demonstrated that increased illumination intensity and duration promote the photochemical production of methyl halides, with a notable enhancement under UV-B radiation. Guaiacol and halide ions were identified as key precursors, and their high concentrations facilitated the formation of methyl halides. Additionally, different types of halide ions exhibited a competitive relationship in producing methyl halides. The study found that an increase in pH inhibited the photochemical formation of CH₃I due to the reaction between OH⁻ and ·CH₃. Dissolved oxygen was found to inhibit the photochemical formation of CH₃I while promoting the formation of CH₃Cl. Conversely, an appropriate concentration of Fe³⁺ enhanced the photochemical production of methyl halides. Field observations indicated a high photochemical production of methyl halides in the natural waters near Qingdao's coastal area, likely due to the high concentration of dissolved organic matter (DOM), which supports photochemical reactions. Furthermore, the photochemical production of methyl halides in natural seawater was significantly higher than in dark conditions, underscoring the importance of illumination in promoting these photochemical processes in seawater. [ABSTRACT FROM AUTHOR]