Peroxy radicals measurement methods

In summaty, HOz and ROz radical concentrations are substantially greater than those of OH, typically by several orders of magnitude. There are several different approaches to measuring these peroxy radicals, and the results from these are in overall agreement as to the magnitude of the concentrations and their diurnal variation. However, there have not been a significant number of intercomparison studies of these methods, so evaluation of the absolute accuracies will require further work. 

Spectroscopic Methods. HO and the other peroxy radicals have characteristic absorptions due to various molecular processes. In principle, these spectroscopic features could be used to determine atmospheric concentrations of peroxy radicals. The discussion of spectroscopic techniques in the measurement of peroxy radicals is divided into descriptions of specific spectral regions. General issues related to the use of spectroscopy for quantitative analysis are presented next. 

The initial rates in Figures 1 to 6 were normally linear over the period required to measure them, and the effect of hydroperoxide on rate is therefore probably small in the concentration ranges used. However, this problem is being investigated in more detail. From the induction period in Figure 7, determined by the method described by Hammond and his coworkers (4), a stoichiometric factor, n, the number of peroxy radicals reacting with each zinc salt molecule, can be calculated. Values... 

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 ROz, gives some higher concentrations for some individual measurements than the chemical amplifier method (e.g., Zenker et al., 1998). 

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 0.71 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... 

Reiner, T., M. Hanke, and F. Arnold, Atmospheric Peroxy Radical Measurements by Ion Molecule Reaction-Mass Spectrometry A Novel Analytical Method Using Amplifying Chemical Conversion to Sulfuric Acid, J. Geophys. Res., 102, 1311-1326 (1997). 

The discussion that follows is divided into two sections. The Spectroscopic Methods section includes those measurement techniques that involve the interaction of a photon with a peroxy radical. The Chemical Conversion Methods section describes the measurement of another molecule or radical to which a peroxy radical has been converted. 

Chemical Amplification. The measurement of a small electrical signal is often accomplished by amplification to a larger, more easily measured one. This technique of amplification can also be applied to chemical systems. For peroxy radicals, Cantrell and Stedman (117) proposed, as a possible technique, the chemical conversion of peroxy radicals to N02 with amplification (i.e., more than one N02 per peroxy radical). This method has also been used for laboratory studies of H02 reactions on aqueous aerosols (21). The following chemical scheme was proposed as the basis of the instrument ... 

Table I. Summary of Peroxy Radical Measurement Methods...

Several potential peroxy radical measurement techniques exist in the realm of atmospheric chemistry studies, although most have been used only in the laboratory. The techniques are summarized in Table I. Possibly, some laboratory methods could be applied to atmospheric measurements. The database for ambient peroxy radical concentrations in the troposphere and stratosphere is meager. Much of the available stratospheric data yield concentrations of H02 higher than those calculated with computer models. The reasons for this systematic difference are not known. In the troposphere, more measurements are called for in conjunction with other related species such as ozone, NO, NOjNo2 andjcv It wiH also t>e appropriate to develop multiple methods, and, when they have reached maturity, to perform intercomparison studies. 

At this time, it should be pointed out that the iodometric method applied in our experiments was not sensitive to tertiary peroxides. Therefore, our measurements are not expected to include peroxides formed by self-termination of tertiary peroxy radicals. 

Because the luminol detection system is also sensitive to NO2, chemical amplification methods have been attempted to further decrease detection limits for PANs below the pptv range for trace-level measurements. With this approach, the PANs are thermally decomposed to NO2 in the presence of large amounts of NO (6 ppm) and CO (8%). Thermal decomposition of the PANs yields peroxy radicals which initiate a free-radical chain oxidation of NO to NO2, producing several NO2 molecules (approximately 180 (20) for each PAN decomposed. This technique has been used as a gas chromatography detector to achieve ultratrace detection limits without sample preconcentration. The detector exhibits a slightly nonlinear response relative to conventional BCD, attributed to the nonlinear response of the luminol reaction in the presence of NO at 6 ppm. 

Conceptually, the simplest method of measuring of kt is to generate a relatively high concentration of peroxy radicals and follow their disappearance by a suitable spectrometric method such as ESR or UV. Some of the techniques by which peroxy radicals may be generated in high concentrations are ... 

The discovery of the rapid reactions of organic peroxy radicals with NO3 and the recognition of their importance in tropospheric chemistry has been a significant achievement within this project. Similarly the development of a method for the detection of ROx has provided a large number of measurements of ROx to be made in the planetary boundary layer and a few to be made in the free troposphere (Izana, Tenerife). This has allowed box models of HO2 and RO2 chemistry to be developed and the inadequacies of our current understanding to be pointed out. In conclusion the aims and objectives of LACTOZ have been well served by the achievements of this project. 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

Novel method for the measurement of gas phase peroxy radical absorption spectra, J. Phys. Chem. 96 (1992) 982. 

DSC and oxygen uptake experiments have been used to measure the oxidative stability of gamma-irradiated ethylene-propylene elastomers [4], The oxidative irradiation environment generated peroxy radicals that were involved in the air-degraded samples. The specific heat capacity dependences on temperature determined for the two methods of irradiation were dissimilar. 

Since the expression ESR application to the study of molecular motion easily reminds of the so-called spin label and spin probe methods, these topics are included in the present article the related problems will be considered in Sect. 9 and 10. However, any trapped free radical must be a label for the study of molecular motion when the temperature dependences of the spectral parameters are adequately measured. The discussion described in Sect. 6 is an example of the study using peroxy radicals as labels. The discussion in Sect. 4 is another example of ESR application to molecular motion study using the spectral intensity. Examples of molecular motion based on the changes of spectral parameters of trapped free radicals are presented in Sects. 5 and 8. All this should show how the trapped radicals can be used as direct labels in order to apply ESR without substantially modifying the investigated materials. 

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