AN INTERCOMPARISON OF SPECTROSCOPIC LASER LONG-PATH AND ION-ASSISTED IN-SITU MEASUREMENTS OF HYDROXYL CONCENTRATIONS DURING THE TROPOSPHERIC OH PHOTOCHEMISTRY EXPERIMENT, FALL 1993

Hydroxyl plays a central role in the chemistry of the troposphere. Sin ce the importance of OH was first recognized in the early 1970s, many experiments have attempted to measure OH, but very few have been succe ssful, and much controversy has surrounded the measurements made [Cros ley, this issue]. Thus, intercomparison of OH measurements is a requir ement for community acceptance of derived OH concentrations, and accur ate measurements of OH are essential to provide robust modeling of tro pospheric chemistry since models typically allow OH concentrations to be determined within the model for lack of measurements. The Troposphe ric OH Photochemistry Experiment held in the Colorado Rocky Mountains in fall 1993 brought together four instruments that to measure OH. In order to make a proper comparison, a number of other trace species and relevant jvalues were measured simultaneously with hydroxyl to provid e a photochemical understanding of the measurements and an understandi ng of the airflow in the mountainous region in which this work was con ducted. This region provided opportunities to measure both clean and p olluted air over a wide dynamic range of species that affect the OH co ncentration. The measurements extended over a long enough period of ti me that a large quantity of data were collected under a variety of atm ospheric conditions, thus allowing for a meaningful comparison of tech niques. The long-path spectroscopic laser system and the in situ selec ted ion chemical ionization mass spectrometry system determined OH con centrations for 4 weeks simultaneously. Generally, the derived concent rations from the two instruments agreed within the 2 sigma error bars over half the observation time. One quarter of the time, OH difference s could be explained by different air masses between the long path and in situ site which contained different concentrations of trace specie s that affect OH abundance. Approximately one quarter of the data disa greed outside the observational errors with no obvious explanation. Th e long-path experiment typically gave concentrations about 20% higher than the ion-assisted experiment. This could be caused by inaccuracies in the absolute calibrations of the two instruments, or by trace spec ies variations along the long path that differed from those at the in situ site, Generally, agreement between ion-assisted and long-path OH measurements was good, showing that measurements of hydroxyl in the tr oposphere can be performed with good accuracy on a routine basis.