Differential optical absorption spectrometry DOAS

The reaction with BrO, however, is potentially important under some conditions. As discussed in Chapter 11, UV-visible differential optical absorption spectrometry (DOAS) has been used to measure BrO under these conditions (e.g., see Hausmann et al., 1993 Platt and Hausmann, 1994 and Tuckermann et al., 1997). Concentrations up to about 30 ppt have been measured. The rate constant for the BrO-DMS reaction is 2.6 X 10 13 cm3 molecule-1 s-1 at 298 K (Bedjanian et al., 1996) and gives with essentially unit yield DMSO and a bromine atom (Barnes et al., 1993 Bedjanian et al., 1996) ... 

Pitts et al. (1985) first used differential optical absorption spectrometry (DOAS) to establish unequivocally that N02 injected into a mobile home forms HONO. Interestingly, the dependence of the rate of HONO generation on the N02 concentration was similar to that measured in laboratory systems, consistent with production in, or on, a thin film of water adsorbed on surfaces. A number of studies have confirmed that the behavior is similar to that in laboratory systems i.e., the rate of production of HONO increases with N02 and with relative humidity. Indoor levels of HONO as high as 8 ppb as a 24-h average and 40 ppb as a 6-h... 

The Frankfort LPA instrument (51-53) departs from both of these instruments in two principal ways it achieves the necessary path length within a 6-m folded-path cell, and it rapidly scans a narrow-band frequency-doubled dye laser across the spectral region of interest (the Qi(2) line group) in a process sometimes called differential optical absorption spectrometry (DOAS). The scanning rate is sufficient to ensure that the observed air volume is chemically and physically stationary during each scan (the baseline standard deviation is less than 2 x 10-4 for a 0.2-ms scan). The laser output is actively feedback-stabilized to provide a flat spectral baseline, and a detection limit better than 10"5 in optical density has been claimed. A summary of published LPA configurations is given in Table II. 

The typical atmospheric concentrations of OH and HO2 radicals in the daytime are 10 and 10 molecules cm ( 0.1 pptv and a few pptv), respectively. The detection techniques whose accuracy is thought to be satisfactory for their measurements are laser-induced fluorescence (LIF) at low pressure which is called Fluorescence Assay by Gas Expansion (FACE), and chemical ionization mass spectrometry (CIMS), and they have recently been used widely. In either of the methods, directly measured is OH, and HO2 is measured by converting to OH utilizing the reaction of HO2 -I- NO —> OH -I- NO2 (reaction (7.10)) adding NO to the atmospheric samples just before the introduction to the detector. Other than these methods, differential optical absorption spectroscopy (DOAS) is also used for the measurements of OH in fields and smog chambers. 

Towards this goal, there have been extensive studies that have compared PTR-MS measurements of atmospheric VOCs with those obtained by other atmospheric analytical techniques, such as GC-MS [17], GC-FID [21-26], atmospheric pressure chemical ionization mass spectrometry (AP-CIMS) [27], differential optical absorption spectroscopy (DOAS) [24,28] and Fourier transform infrared (FTIR) spectroscopy [17,29], in addition to offline sampling methods coupled to GC analysis [30-34], These studies have shown that PTR-MS is capable of accurately measuring concentrations of VOCs providing that there is no contribution to the miz of interest in a mass spectram by interfering species. If other compounds are present in the atmospheric sample which can lead to ions (protonated parent compounds, cluster ions or fragment ion species) at the nominal mJz of the protonated VOC of interest, then the lack of specificity associated with PTR-MS requires that the actual identity of the compound still needs to be confirmed by other analytical techniques, such as GC-MS. 

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