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.

Ground-based instrumentation for the in situ detection of IO by laser-induced fluorescence (LIF) is described. IO radicals are excited at 445 nm to the v' = 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 design, calibration, operation, and performance of the prototype instrument are described, and the results from the first field deployment to Nahant, MA in August 2007 are reported as well. This new instrument will, in time, be deployable in airborne campaigns providing longer-term contributions to research in the free troposphere and stratosphere.

Finally, the first rotationally resolved ultraviolet absorption cross sections for the 2⁰₀4¹₀ vibrational band of the A¹A₂-X¹A ₁ electronic transition of formaldehyde (HCHO) are reported. Accurate rotationally resolved cross sections are of significant importance for the development of in situ HCHO LIF instruments and for atmospheric monitoring. HCHO is a prime tracer for convective events, and in situ measurements of HCHO will be complementary to the observational database of IO.