GROUND-BASED COMPARISON OF NO2, H2O, AND O-3 MEASURED BY LONG-PATH AND IN-SITU TECHNIQUES DURING THE 1993 TROPOSPHERIC OH PHOTOCHEMISTRY EXPERIMENT

One of the major objectives of the 1993 Tropospheric OH Photochemistry Experiment (August 8 to October 4, 1993) was to compare atmospheric m easurements of the important trace gas species NO2, O-3, and H2O by in situ techniques and by long-path differential absorption spectroscopy . These compounds were measured over a 20.6 km folded optical path fro m Fritz Peak Observatory to Caribou Mine where a retroreflector array is located. In situ measurements were made at Idaho Hill, 0.7 km north west of Caribou Mine and 25 km west of Boulder, Colorado. NO2 was dete rmined at Idaho Hill by photolysis followed by NO chemiluminescence. O -3 and H2O were measured at this site using calibrated commercial inst ruments. Using fractional differences ([long path - Idaho Hill]/Idaho Hill) as a measure of agreement, overall agreement for ozone and water are +3+/-7% and -4+/-17%, respectively. For NO2 the comparison is com plicated by the influence of strong sources and large ambient variabil ity during the measurements. During times of clean westerly flow, frac tional differences of +30+/-110% were observed. Further analysis of th e data to evaluate these biases show that for mixing ratios greater th an 300 pptv (well above the lo detection limit of 30-45 pptv) the two methods differ by no more than 10%, a value well within the combined u ncertainties of the measurement techniques.