Pulsewidth dependence of ozone interference in the laser fluorescence measurement of OH in the atmosphere

Study of the Formation Dynamics of OH from the Photolysis of O3 by Ultrashort Laser Pulses

We report the first experimental detection of the laser-induced formation dynamics of OH under normal pressure, which was observed using the pump-probe scheme with two 250 fs laser pulses at 282.2 nm. The first laser photolyzes O₃ to form OH, and the second laser excites OH to induce fluorescence at 308 nm. The fluorescence cannot be detected with a single beam because the femtosecond pulses have already passed through the sample before the formation of OH from their own photolysis. A photochemical dynamic model was built, and the experimental results are in good agreement with the model. This method can also be used for other OH-related dynamic measurements, which can effectively avoid laser-generated interference from the photolysis of O₃.

Atmospheric field measurements of the hydroxyl radical using laser-induced fluorescence spectroscopy

The hydroxyl radical, OH, is the most important cleansing agent in the Earth's atmosphere, removing the majority of trace gases by oxidation, including greenhouse gases and CFC replacements. It is intimately involved in the chemistry that generates photochemical smog, which includes many substances harmful to health, such as ozone and particulate matter. In this review, the technique of laser-induced fluorescence for the detection of OH in the atmosphere is described, using as an example the fluorescence assay by gas expansion (FAGE) instrument developed at the University of Leeds. The comparison of measured OH concentrations at a given field site with those calculated by an atmospheric model, which is a mathematical representation of the underlying chemistry, provides one of the best methods to test whether the key chemical and physical processes are understood. Examples are given for field measurements made in clean and polluted environments.

Pressure dependence of ozone interference in the laser fluorescence measuremens of OH in the atmosphere

Experimental results are presented on the pressure dependence of ozone interference. Good agreement was observed between these results and relevant theoretical considerations. It is pointed out that chemical modulation can lead to sizable amounts of spurious signal in the presence of ozone dissociation.

Stratospheric ozone and hydroxyl radical measurements by balloon-borne lidar

A balloon-borne lidar system for the measurement of ozone and hydroxyl radical in the stratosphere has been constructed and flown by Goddard Space Flight Center. On this flight ozone concentration at altitudes between 20 and 37 km were determined with vertical resolution of approximately 0.5 km. In addition, horizontally resolved ozone measurements with 0.15-km resolution were obtained over a 2-km range. The temporal variation of the hydroxyl radical concentration was measured in the 34-37-km altitude region, ranging from 40 parts/trillion shortly after noon to approximately 5 parts/trillion 2 h after sunset. Improvements in the system are proposed to increase the sensitivity of the instrument below the parts/trillion level, permitting hydroxyl determinations in the 20-30-km altitude range.