INTERCOMPARISON OF TROPOSPHERIC OH AND ANCILLARY TRACE GAS MEASUREMENTS AT FRITZ-PEAK-OBSERVATORY, COLORADO

The determination of the concentration of OH in the Earth's tropospher e is of fundamental importance to an understanding of the chemistry of the lower atmosphere. Although many experiments to measure OH concent ration have been performed in recent years, very few operate at sensit ivities necessary to measure the extremely low amount of OH in the cle an troposphere (0.1-0.2 parts per trillion by volume at summertime loc al noon). This paper describes an informal intercomparison campaign he ld at Fritz Peak, Colorado, in summer 1991 to intercompare the OH conc entrations determined from a spectroscopic instrument and an in situ c hemical conversion instrument, both with sensitivities at or below 5 x 10(5) molecules cm(-3) Ancillary measurements including those of O-3, CO, NO, NO2, NOy, H2O, SO2, aerosols, solar flux, and meteorological parameters were also performed to test photochemical theories of OH fo rmation. These measurements also provided a means for comparing air ma sses at the long path and in situ sites. The intercomparison was very successful with measured values of OH concentration in agreement withi n one standard error much of the time. OH concentrations were typicall y low, rarely above 4 x 10(6) cm(-3), with only slow growth during the morning hours, indicating the possible presence of scavenger species. Model results suggest higher than measured OH concentrations or the p resence of scavenger species.