Laser-induced fluorescence instrument for the detection of IO
High abundances of iodine monoxide (IO) are known to exist and to participate in local photochemistry of the marine boundary layer. IO participates in the catalytic destruction of ozone and the sequestration of atmospheric gaseous mercury through bromine photochemistry. IO and higher-order iodine oxides are involved in the formation of new particles in coastal marine environments. IO has also been shown to affect the oxidizing capacity of the troposphere by altering the partitioning of NO2/NO and HO2/HO and by activating chlorine and bromine in sea salt aerosols. In the stratosphere, these same processes can lead to enhanced ozone loss rates. Detailed photochemical models that include iodine photochemistry, however, are hampered by the lack of observational data. The distribution of IO in vertical, horizontal, and temporal coordinates is largely unknown, so the impact of IO on global photochemistry cannot be predicted. The resolution of these important scientific issues requires an in situ IO instrument.
A ground-based instrument for the in situ detection of IO by laser-induced fluorescence (LIF) has been recently developed in the Anderson Group. IO radicals are excited at 445 nm to the ví = 2 of the excited-electronic state, and fluorescence is observed from the same vibrational state. The 445 nm light is produced by an all solid-state, high repetition rate Nd:YAG-pumped Ti:Sapphire laser system. The IO instrument is calibrated by two independent techniques: chemical titration and absorption via cavity ringdown spectroscopy. The detection limit of the instrument was determined to be 0.1 ppt in 5 sec integration. A test deployment to Nahant, MA took place in August 2007, and future deployments of the ground-based IO instrument are being planned. This new instrument will, in time, be deployable in airborne campaigns providing longer-term contributions to research in the free troposphere and stratosphere.