FIGURE 11.40 Percentage of total carbonyl compounds due to each of the straight-chain aldehydes in the Los Angeles area (adapted from Grosjean et al., 1996a).
1 2 3 4 5 6 7 8 9 1011121314 Aldehyde straight chain carbon number
1 2 3 4 5 6 7 8 9 1011121314 Aldehyde straight chain carbon number ethanol in the ~0.1- to 1-ppb range in rural areas (Goldan et al., 1995; Leibroek and Slemr, 1997; Riemer et al., 1998). In areas where ethanol is used as a fuel, the concentrations can be much higher. For example, in Porto Alegre, Brazil, ethanol concentrations up to 68 ppb have been measured (Grosjean et al., 1998b). Higher alcohols such as n-butanol have also been reported to be present in air in smaller concentrations; for example, Riemer et al. (1998) reported concentrations of ~55 ppt at a rural site in the southeastern United States.
As discussed in Chapter 6.J.1, there are also biogenic emissions of multifunctional alcohols, which are treated in that section.
(3) Carboxylic acids The smallest carboxylic acid, formic acid, can be measured using infrared spectroscopy (Table 11.2), since it has characteristic absorption bands. As discussed earlier and seen in Fig. 11.33b, mass spectrometry with chemical ionization using SiF5~ also revealed HCOOH in an indoor environment (Huey et al., 1998). However, since the sensitivity in these initial studies was about two orders of magnitude less than that for HN03, the detection limit may be about the same as that for FTIR and TDLS. Formic and acetic acids have been monitored continuously from aircraft (Chapman et al., 1995) and their surface flux determined by eddy correlation (Shaw et al., f998) using atmospheric pressure ionization mass spectrometry. Detection limits are about 30 ppt.
Gas-phase carboxylic acids have been sampled using mist chambers (e.g., Andreae et al., 1987; Talbot et al., 1988), condensates (Dawson and Farmer, 1988), filters coated with alkaline compounds such as KOH, NaOH, K2C03, and Na2C03 (e.g., Grosjean et al., 1990; Nolte et al., 1997), and denuders coated with NaOH (Keene et al., 1989). The acid anions are then separated and detected using ion chromatography. It should be noted that interferences have been reported for some of these methods. For example, the conversion of formaldehyde to formic acid and PAN to acetate on alkaline filters has been observed (Andreae et al., f 987; Keene et al., 1989; Grosjean and Parmar, 1990), and with some ion chromatography columns, coelution of several anions can be a problem (e.g., see Jaffrezo et al., 1998). The results of one intercomparison study (Keene et al., 1989) suggest that artifacts in these measurement methods occur episodically and that care should be taken in their application.
A promising method involves derivatization by reaction with pentafluorobenzyl bromide (Chien et al., 1998). Carboxylic acids (RC(O)OH) react to form the esters, RC(0)0CH2C6F5, which can be measured by
GC-MS. This method has the advantage of increased sensitivity and selectivity.
Formic and acetic acids are found primarily (>98%) in the gas phase (e.g., Andreae et al., 1987; Talbot et al., 1988). Concentrations of gas-phase HCOOH and CH3COOH in rural areas are typically ~0.3-3 and ~ 0.5-2 ppb, respectively (Andreae et al., 1987; Talbot et al., 1988; Dawson and Farmer, 1988; Sanhueza et al., 1996; Nolte et al., 1997; Granby et al., 1997a,b), although higher concentrations, up to 32 ppb for CH3COOH, have been observed in wood-burning areas (Gaffney et al., f997). In urban areas, HCOOH and CH3COOH concentrations are about the same, typically in the range of ~ 1—10 ppb (e.g., see Tuazon et al., 1981; Dawson and Farmer, 1988; Grosjean, 1990; Grosjean et al., 1990; Khare et al., 1997; Granby et al., 1997a; Nolte et al., 1997; and Gaffney et al., 1997).
Multifunctional acids containing a carbonyl group such as pyruvic acid [CH3C(0)C00H] are typically measured using the derivatization techniques used for aldehydes and ketones, such as the DNPH method (e.g., see Lee et al., 1995).
g. PAN, Other Peroxynitrates, and Alkyl Nitrates
The formation and fate of peroxyacyl nitrates, RC(0)00N02, were discussed in Chapter 6.1. These compounds are almost universally measured using gas chromatography with electron capture detection (GC-ECD), although a luminol chemiluminescence detector has also been used in which PAN is thermally decomposed to NOz at the end of the column and the N02 measured (Burkhardt et al., 1988; Blanchard et al., 1990; Gaffney et al., f998). In polluted atmospheres where the concentrations are higher, FTIR has also been used (Table 11.2). For a summary of methods, see reviews by Gaffney et al. (1989) and Kleindienst (1994).
Of the peroxyacyl nitrates, the most prevalent compound is peroxyacetyl nitrate, R = CH3, with perox-ypropionyl nitrate (PPN, R = C2H5) typically being present at the next highest concentration. Because they are formed in the VOC-NOx photochemical cycles, the highest levels of PAN are often seen downwind of urban areas rather than in the center. For example, in the Los Angeles area, some of the highest concentrations have been measured at a mountain site about 35 km northeast of Los Angeles (Grosjean et al., f993a, f996b).
Peak concentrations of PAN in or downwind of major urban areas during periods of high photochemical activity can reach levels as high as ~35 ppb (e.g., see Tuazon et al., 1981; Tanner et al., 1988; Grosjean et al., 1993a, 1996b; Altshuller, 1993; Williams et al., 1993; Kleindienst, 1994; Suppan et al., 1998; and Gaffney et al., 1998, 1999). In rural areas, peak concen trations up to about a ppb are typical (e.g., see Corkum et al., 1986; Andersson-Skold et al., 1993; Ridley et al., 1990a; Gaffney et al., 1993, 1997; Hastie et al., 1996; Nouaime et al., 1998; and Roberts et al., 1998). However, in remote areas where NO and NOx levels are small (vide supra), a few tens of ppt to ca. several hundred ppt are common (e.g., see Rudolph et al. 1987; Singh et al., 1990; Ridley et al., 1990b, 1998 Perros, 1994; Talbot et al., f994; Heikes et al., 1996 Beine et al., f 996, f997; Solberg et al., 1997; and Singh et al., 1998).
As might be expected from the chemistry common to the formation of 03 and PAN, the two are often highly correlated both temporally and geographically. Figure 11.41, for example, shows the median values for the diurnal variation of ozone and PAN at one site in Athens, Greece, during meteorological conditions conducive to photochemical smog formation (Suppan et al., 1998). Both increase about 9 a.m. and continue at relatively high levels until late in the afternoon.
Smaller concentrations of higher members of the series have also been observed. In and downwind of polluted urban areas, peak concentrations of PPN of ~ 0.4-4 ppb have been reported (e.g., see Grosjean et al., 1993a, f996b; and Williams et al., 1993), whereas the concentrations in rural areas are about an order of magnitude smaller (e.g., Ridley et al., 1990a). For example, typical average PPN levels in the southeastern United States have been reported to be ~50 ppt, with the PPN/PAN ratio being about 0.15 in air masses impacted by anthropogenic emissions (Nouaime et al., 1998; Roberts et al., 1998). Peroxy-n-butyryl nitrate (n-C3H7C(0)00N02) and peroxymethacroyl nitrate (MPAN, CH2=C(CH3)C(0)00N02) have been measured at concentrations up to ~l-2 ppb (Williams et al., 1993; Grosjean et al., 1993a, f993b; Gaffney et al., 1999). Since MPAN is an oxidation product of isoprene
FIGURE tt.41 Diurnal variation of median concentrations of PAN and 03 at a monitoring site in Athens, Greece, during periods of high photochemical activity (adapted from Suppan et al., 1998).
(see Chapter 6), it is found primarily in forested areas having significant isoprene emissions. For example, in rural areas near Nashville, Tennessee, the average concentration was about 30 ppt (Nouaime et al., 1998), with a typical MPAN/PAN ratio of 0.10-0.17 (Roberts et al., 1998).
Simple alkyl nitrates are also commonly measured using GC-ECD, usually with preconcentration either by cryotrapping or using a solid sorbent (e.g., Atlas and Schauffler, 1991; Ridley et al., 1997). Another approach is GC with an NO detector as described earlier (e.g., Flocke et al., f998). In this approach, the compounds are converted to NO over a catalyst as they emerge from the GC column, and the NO measured by its chemiluminescence reaction with 03.
A number of alkyl nitrates have been observed in the troposphere, including methyl nitrate and ethyl nitrate, as well as all of the isomers of the higher alkyl nitrates up to C5 (e.g., see Buhr et al., 1990; Ridley et al., 1990a; O'Brien et al., f 995; and Flocke et al., 1998). Although the specific isomers were not identified, the C6-Cx alkyl nitrates have also been measured (O'Brien et al., 1995; Flocke et al., 1998). A summary of the measurements through about f998 is found in Flocke et al. (f 998).
Of the simple alkyl nitrates, methyl nitrate is present in the highest concentration. For example, in measurements made in Schauinsland, a rural area in Germany, concentrations of CH30N02 up to 216 ppt were measured. The median value, however, was only 19 ppt (Flocke et al., 1998). In the same studies, the median concentrations for ethyl nitrate, «-propyl nitrate, 2-pro-pyl nitrate, and 1-butyl nitrate were 9, 3, 12, and 2 ppt, respectively. The sum of the Cj-Cf, alkyl nitrates averaged 120 ppt, which is only ~3% of the NOy. Similarly, in rural Ontario, Canada, 17 different organic nitrates were identified in air, but their sum was only 0.5-3% of NOy (O'Brien et al., 1995). in aircraft measurements over the Pacific Ocean near Hawaii, average values for methyl nitrate near the surface were ~6 ppt and the sum of C,-C5 alkyl nitrates was <5% of the total NO>( (Ridley et al., 1997).
in short, a variety of alkyl nitrates are present in air, but at relatively small concentrations compared to the peroxyacyl nitrates and to NOv.
h. H2 O2 and Organic Peroxides
There are a variety of methods for collecting and measuring H202 and organic peroxides in air. H202 is especially water soluble and hence partitions between the gas phase and clouds and fogs (e.g., Macdonald et al., 1995). While the collection techniques for air versus clouds and fogs are different, the analytical techniques are the same.
Collection of air for peroxide analysis has been accomplished using a number of approaches, including mist chamber sampling, diffusion scrubbers (e.g., see Dasgupta et al., 1988), impingers, and cryogenic trapping (e.g., see Sakugawa and Kaplan, 1987). Artifacts have been observed with many of the sampling systems. For example, Sakugawa and Kaplan (1987) reported that H202 collected by impingers was higher than by cryotrapping and attributed this to generation of H202 by aqueous-phase reactions in the bubbler. Indeed, the generation of H202 in water when 03 is bubbled into it has been observed (e.g., Zika and Saltzman, 1982; Heikes, 1984). On the other hand, artifact formation of H202 and hydroxymethyl hydroperoxide during cryogenic trapping of air was reported by Staffelbach et al. (1995, 1996) and attributed to reactions of alkenes with 03 in the traps. In addition, H202 is a sufficiently "sticky" molecule that it is readily lost to surfaces in sampling systems, so that such surfaces prior to scrubbing into solution must be minimized (e.g., Lee et al., 1991, 1993). Differences in collection efficiencies for different hydroperoxides must also be taken into account (e.g., de Serves and Ross, 1993).
Various techniques for measuring peroxides in air are reviewed by Gunz and Hoffmann (f990) and Sakugawa et al. (1990). H202 can be measured spectroscop-ically by FTIR and by TDLS (e.g., Slemr et al., 1986), although the FTIR detection limit is too high to be of value except for relatively rare, extremely large concentrations (see Table 11.2). More common are derivatiza-tion methods, and of these, one using /7-hydroxyphen-ylacetic acid (POPHA) has been applied extensively to ambient air.
The POPHA method is based on the oxidation of horseradish peroxidase in the +3 state to its +5 state (Lazrus et al., 1985, 1986; Kok et al., 1986). This oxidized form is then reduced by electron transfer from POPHA, generating the POPHA free radical. The POPHA free radicals self-react to produce the dimer, which, upon excitation at 320 nm, fluoresces at 405 nm. The overall reaction is
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