AN OVERVIEW OF THE TROPOSPHERIC OH PHOTOCHEMISTRY EXPERIMENT, FRITZ-PEAK IDAHO-HILL, COLORADO, FALL 1993

`An extensive study of tropospheric photochemical trace species, inclu ding the hydroxyl radical OH was conducted in the Colorado Rocky Mount ains from early August to early October 1993. Many of the parameters t hat influence the abundance of OH in the atmosphere, including j(O-3 a nd NO2), formaldehyde, nonmethane hydrocarbons, NOx, ozone, H2O, aeros ols, and others, were measured simultaneously with OH. Instrumentation for measurement of OH included a long-path spectroscopic technique an d several in situ techniques: ion-assisted mass spectrometry, laser-in duced resonance fluorescence, and a liquid scrubber/liquid chromatogra phic method. Instrument comparisons between long-path and in situ meas urements were conducted for OH, O-3, CH2O, NO2, and H2O, and these mea surements provide a valuable insight into the chemistry of the region. The complete data set provided a robust set of inputs to a zero-dimen sional model for exploring OH photochemistry. Independently measured O H concentrations were in agreement within one standard error much of t he time, thus giving confidence that hydroxyl is being measured correc tly. OH concentrations were typically low during clean continental air flow with concentrations rarely above 4 x 10(6) cm(-3). During occasio ns of polluted airflow from the Denver-Boulder metropolitan areas, val ues of OH rose as high as 1.2 x 10(7) cm(-3). The nearly complete suit e of trace gases measured simultaneously with the hydroxyl radical sev erely constrained the models and provided numerous opportunities to co mpare relations between species (e.g. OH versus NOx, OH/HO2, RO(2) ver sus NOx). Modeled hydroxyl results using the complete species suite we re about a factor of 1.5 higher than measured OH concentrations sugges ting that the photochemistry may not be well understood. The campaign data provide new insights into the chemistry of the lower troposphere. This paper provides an overview of the campaign and brief description s of results from individual experiments. Details are provided in the accompanying papers in this issue.