FIGURE 11.51 Correlation between measurements made by the mass spectrometry-derivatization technique and long-path UV absorption in rural Colorado for lower NO^ conditions (adapted from Mount et al., 1997a).
(Mount et al., 1997a). The slope of the line is 0.82 + 0.06; i.e., the mass spectrometer point measurements were about 20% lower than the UV measurements over a path length of 10.3 km. About 25% of the data were different by amounts outside the experimental errors. Such discrepancies may be due to comparing distance-averaged to point values and/or to calibration inaccuracies.
An intercomparison of the mass spectrometer method with an LIF instrument, however, was not as good. While the slope of the plot of LIF versus the MS measurements was 0.73, the r value was only 0.26, in part due to poor laser performance in the LIF instrument during the studies (Mather et al., 1997).
Extensive intercomparisons using the radiocarbon technique have not been carried out. Campbell et al. (1995) compared measurements using the radiocarbon technique to those from an LIF instrument (Chan et al., 1990). The values obtained were frequently near the detection limits of the instruments, but despite that, were reasonably well correlated (r2 = 0.74). However, the slope of a plot of the radiocarbon versus LIF absolute concentrations was 2.9, i.e., there was a difference of about a factor of three.
In short, given the challenges associated with measuring OH, the disagreement between the various methods is not surprising and the discrepancies appear to be improving as the methods are developed further.
(2) HO2 and R02 There are three approaches that are used to measure H02 and/or R02: (f) conversion of H02 and/or ROz to OH and measurement of the latter using techniques already described, (2) a chemical amplifier method, and (3) matrix isolation ESR.
Conversion of HO2 to OH. H02 can be measured by conversion into OH by its fast reaction with NO,
H02 + NO -> OH 4- NOz, followed by measurement of the OH by one of the methods described in the preceding section. For example, LIF detection of OH generated by reacting H02 with NO has been used to measure H02 at both remote and urban sites (Hard et al., 1984, 1992a).
Another approach combines the mass spectrometric derivatization approach with chemical amplification (Reiner et al., 1997, 1998). In this instrument, HOz and R02 are converted to OH through the reactions in the chemical amplifier approach discussed below, and the OH is then converted to H2S04 by reaction with S02 and measured by chemical ionization mass spectrometry using N0^(HN03) clusters as described earlier. In this case, the use of isotopically labeled S02 is not necessary, since the ambient H2S04 concentration is much smaller than that of the peroxy radicals.
FIGURE 11.50 Correlation between OH measurements made by DOAS and by LIF in a rural area in Germany in August 1994. The data indicated by asterisks were measurements made when the wind was from a particular direction suggesting it might contain unrecognized OH sources affecting the long-path DOAS measurements (adapted from Hofzumahaus et al., 1998).
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