The past few years have seen the development of a number of methods for direct measurement of tropospheric OH (special issue of Journal of Atmospheric Science, October 1995). Two of these methods, a long-path absorption (LPA) instrument (Mount, 1992) and a chemical ionization mass spectrometry (CIMS) instrument (Eisele and Tanner, 1991) were intercompared formally at a mountain site in Colorado during the Tropospheric OH Photochemistry Experiment (TOHPE). Under well-mixed atmospheric conditions where the local OH measurement from CIMS could be compared to the long-path average from LPA, the intercomparison demonstrated a good correlation between the two instruments down to concentrations of less than 1 x 106 molecules/cm3, with no significant bias (Crosley, 1997).
A number of ancillary chemical measurements were made during TOHPE that McKeen et al. (1997) used to compare the observed OH concentrations to values computed from a standard photochemical model. The model overestimated OH concentrations by a factor of 1.3 on average. It captured 48% of the variance in the CIMS instrument, although much of that variance was driven by the diurnal cycle. It was not correlated with the LPA instrument, which may reflect the nonlocal nature of the latter measurement.
The model overestimate of OH in TOHPE is consistent with other model measurement comparisons conducted at continental sites (Poppe et al, 1995; Thompson, 1995; George et al, 1999). As discussed by McKeen et al. (1997), possible causes include inadequate model representation of hydrocarbon chemistry or of uptake of HOx by aerosols. Eisele et al. (1996) conducted a model-measurement comparison using the CIMS instrument at Mauna Loa Observatory, Hawaii (3.4 km altitude); they found good agreement when subsiding motions brought free tropospheric air to the site but a factor of 2 model overestimate under upslope flow, supporting the view that biogenic hydrocarbons may provide important sinks for OH. Frost et al. (1999) found a median model overestimate of 32% in simulation of aircraft observations for clean marine air.
An important aspect of these model-measurement comparisons has been to examine the ability of models to reproduce the dependence of OH concentrations on chemical and meteorological variables. Poppe et al. (1995) found that their model could capture successfully the observed correlations of OH concentrations with UV intensity, temperature, humidity, and CO concentration. Measurements in TOHPE showed OH concentrations increasing with increasing NO, up to about 2 ppbv NO, and then decreasing, consistent with model calculations of N0X versus hydrocarbon-limited chemistry (Eisele et al., 1997; McKeen et al., 1997).
Aircraft measurements of OH and H02 concentrations in the upper troposphere have been reported by Brune et al. (1998, 1999) and Wennberg et al. (1998). The measured 0H/H02 ratios and their variances agree with model values to within the uncertainties of the relevant rate constants, implying a good understanding of the cycling of H0X (Jaegle et al., 2000). The observed H0X concentrations are often several times lower than would be predicted solely from the 0(1D) + H20 source (R4) and support the presence of other primary H0X sources in the upper troposphere including acetone, peroxides, and aldehydes.
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