P

Because H02 radical concentrations in the troposphere are typically about two orders of magnitude larger than those of OH, the contribution of ambient OH to the signal does not present a problem.

Chemical amplifier method. Another approach, known as the chemical amplifier method, pioneered by Cantrell and Stedman (Cantrell and Stedman, 1982; Cantrell et al., 1984) has been used extensively to measure the combination of H02 and R02 (although the latter is not necessarily with 100% efficiency; vide infra). This method involves the conversion of H02 to OH in a chain reaction with a length of ~100-200. Figure If.52 is a schematic diagram of one such instrument (Cantrell et al., 1993). Air containing HOz, ROz, OH, and other species is sampled into the instrument, where it is mixed with NO, typically at ~ 3 ppm, and CO, at about 7-10% of the total flow. H02 reacts with NO as above to generate OH. In the presence of large concentrations of CO, HOz is regenerated:

Thus, a chain reaction is set up in which H02 converts NO to N02 and is subsequently regenerated by the OH + CO reaction. The NOz is measured using techniques such as those described earlier; in the case of the system in Fig. If.52, the luminol chemilumines-cence technique is used. Termination of the chain occurs via reactions such as

OH + NO + M -> HONO + M, H02 + NOz + M -> H02N02 + M,

H02 -» Wall loss. The H02 concentration is given by

H02 Molecule

CO N2

FIGURE 11.52 Schematic diagram of chemical amplifier apparatus for measurement of H02 and R02 (adapted from Cantrell et al., 1993).

CO N2

FIGURE 11.52 Schematic diagram of chemical amplifier apparatus for measurement of H02 and R02 (adapted from Cantrell et al., 1993).

where the chain length is defined as the number of N02 molecules formed per initial H02 radical.

In addition to H02, organic peroxy free radicals are also measured, although not necessarily with 100% efficiency. For example, if CH302 is also present, the following reactions occur:

CH302 + NO -» CH30 + N02, CH30 + 02 -> H02 + HCHO.

The H02 then reacts as above in a chain reaction. While CH302 forms H02 in a straightforward series of reactions, larger ROz radicals may not. For example, as discussed in Chapter 6, a significant fraction of the reactions of larger R02 radicals with NO generates stable organic nitrates, R0N02, rather than RO + N02. In addition, larger alkoxy radicals may not solely undergo reaction with 02 to generate H02; indeed, as seen in Chapter 6, this is a minor path for some organic peroxy radicals, where decomposition and/or isomer-ization may predominate. As a result, the chemical amplifier measures H02 and some weighted fraction of R02 radicals.

For example, Cantrell and co-workers (1993) estimate the efficiency of conversion of simple alkyl peroxy radicals to vary from 0.93 for CH3CH202 to 0.47 for (CH3)2C02, and it may be even less for larger alkyl peroxy radicals. This may be the reason that in some intercomparison studies, the matrix isolation-ESR technique (vide infra), which measures the sum of R02, gives some higher concentrations for some individual measurements than the chemical amplifier method (e.g., Zenker et al., 1998).

Calibration has been carried out using known H02/R02 sources such as the thermal decomposition of PAN or H202 (e.g., Cantrell et al., 1993), photolysis of H202 or water vapor (e.g., Schultz et al., 1995), and the photolysis of CH3t in the presence of 02 (e.g., Clemitshaw et al., f 997). This in effect allows the chain length to be determined so that peroxy radical concentrations can be derived from the increase in N02 as given above. However, there appear to be some factors affecting the sensitivity that are not well understood. For example, the chain length has been shown to be sensitive to the concentration of water vapor in air in at least one instrument, for reasons that are not clear (Mihele and Hastie, 1998).

Matrix isolation-electron spin resonance. A third method used to measure H02 and R02 is matrix isolation with ESR (see earlier description of matrix isolation). Because H02 and R02 have distinct ESR signals, they can be differentiated (Mihelcic et al., 1985, 1990, 1993). For example, Fig. 11.53, part A, shows the ESR spectrum obtained when approximately

FIGURE 11.53 Matrix isolation-ESR measurement of N02 (680 ppt), NO, (5.2 ppt), H02 (10 ppt), and ER02 (5 ppt) in Schavinsland, Germany, in August 1990 (adapted from Mihelcic et al., 1993).

8 L of air in rural Germany was trapped in a polycrys-talline matrix of D20 at 77 K (Mihelcic et al., 1993). Spectrum b shows the ESR spectrum of N02; it can be seen that most of the observed ESR signals are due to NOz, calculated from reference spectra to be present at a concentration of 0.68 ppb in this sample. Spectrum c is the difference between spectra a and b, magnified by a factor of five. Spectra d, e, and f are those of N03, H02, and R02, respectively, and their sum is shown by the heavy line through spectrum c. Clearly, the signals in spectrum c reflect contributions from these three radicals, at concentrations of 5.2 ppt N03, 10 ppt H02, and 5 ppt R02 in this particular sample. Detection limits for this method are 5 ppt for H02 and R02, respectively (Mihelcic et al., 1993).

Fewer intercomparison studies have been carried out for peroxy radicals than for OH. Two chemical amplification methods were compared during a measurement campaign in Brittany, France (Cantrell et al., 1996). Although the measurements tended to track one another, there is more scatter than might be expected, given the similar nature of the instruments. For example, a plot of the data from one instrument against those from the second had a slope of 0.7f but a correlation coefficient of only r = 0.36. In another study (Zenker et al., 1998), comparison of three chemical amplifier techniques to matrix isolation-ESR gave agreement to within 25% for two of the chemical amplifier methods and the ESR approach. The third chemical amplifier technique gave on average values that were about 65% of the matrix isolation-ESR values.

Measurements using the chemical amplifier technique were also carried out at the same time as the mass spectrometer derivatization method was used, with titration of the H02 to OH (Cantrell et al., 1997a). The chemical amplifier values were a factor of 2-3 times higher than those measured using the mass spectrometer approach, possibly because the latter measured H02 whereas the former measured H02 and some weighted fraction of ROz. Finally, comparison of chemical amplifier measurements to those using matrix isolation-ESR (Volz-Thomas et al., 1995; cited by Cantrell et al., 1997b) shows agreement within about 40% for clean or moderately polluted air masses. For more heavily polluted air, the chemical amplifier was systematically lower, suggesting that there were significant concentrations of larger R02 radicals to which the chemical amplifier was less sensitive.

Typical tropospheric concentrations. Figure 11.54 shows the diurnal variation of average typical peroxy radical concentrations made using the chemical amplifier technique in Cape Grim, Tasmania, and Mace Head, Ireland (Carpenter et al., 1997). As is the case for OH, HOz and R02 typically peak around noon, when photolysis is maximum, and are much smaller at night, particularly in low-NOx environments where there is little nighttime NO, (e.g., Monks et al., 1996; Carslaw et al., 1997a; Stevens et al., 1997). Peak concentrations are in the 10x-109 cm-3 range in remote areas (e.g., Carpenter et al., 1997; Fischer et al., 1998), with higher concentrations in polluted areas. For example, in downtown Denver, peak concentrations of 3 X 109 radicals cm"3 have been measured (Hu and Sted-man, 1995).

FIGURE 11.54 Diurnal profile of average (H02 + R02) concentrations measured at Cape Grim, Tasmania (•), and at Mace Head, Ireland (■), under clean air conditions using a chemical amplification technique. (Adapted from Carpenter et al., 1997.)

a Measured spectrum a Measured spectrum

N02 Reference spectrum

N02 Reference spectrum

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