Wavelength-dependence of the photolysis of diiodomethane in seawater

The wavelength-dependence of CH2I2 photolysis was studied in the laboratory using natural seawater. Irradiations were carried out using a 1 kW Xe lamp equipped with a monochromator, and halocarbon concentrations were determined by purge-and-trap gas chromatography/mass spectrometry. The absolute quantum yields for CH2I2 photodissociation were determined between 290 and 365 nm, and varied between 0.27 and 0.39. Our calculations show that photolysis of CH2I2 under natural sunlight conditions occurs predominantly between 300 and 350 nm. In seawater, calculated CH2I2 photolysis rates decrease quickly with depth, with 90% attenuation occurring in the top 10-25 m depending on the water clarity. A simple photochemical box model of a shallow equatorial mixed layer showed that most of the CH2I2 is photolysed before reaching the sea surface, where this compound is depleted with respect to the underlying water column.