SELECTED-ION CHEMICAL-IONIZATION MASS-SPECTROMETRIC MEASUREMENT OF OH

Ion-assisted OH measurements are presented from two different standpoi nts. First, instrument operation and calibration are discussed along w ith recent improvements in measurement techniques. A chemical model of inlet titration chemistry in both clean and polluted air is also incl uded in this section. The results of OH measurements using the present technique are presented for the entire tropospheric OH photochemistry campaign. Hydroxyl radical concentrations of 2-4 x 10(6) molecules cm (-3) were typical at noon for clean, clear days. In polluted air masse s, OH concentrations up to 8 x 10(6) molecules cm(-3) were observed. V ariation of OH with changing and differing air masses is clearly shown and discussed.