Mass (amu)

figure 11.33 Typical chemical ionization mass spectra taken using (a) HS04~ as the chemical ionization reagent or (b) SiF5~ as the CI reagent. Note the change to a logarithmic scale in (b) above ~ 2000 counts per second (CPS) (adapted from Mauldin et al., 1998; and Huey et al., 1998).

there is not a large data base of measurements in a variety of locations and types of air masses that is believed to accurately portray its concentrations, particularly in remote atmospheres. However, the inter-comparison studies described by Hering et al. (1988) suggest that concentrations in polluted urban areas are as high as 25 ppb for 4- to 6-h averages, levels that have also been measured over much shorter time periods using FTIR (Biermann et al., 1988). Similar ppb levels (up to ~5 ppb) have been measured near Vienna, Austria, in the summer (Piringer et al., 1997).

A brief informal intercomparison study of the mass spectrometry methods with a nylon filter pack method near Boulder, Colorado, gave average levels of 0.38-1.6 ppb when the wind carried air from the direction of the greater metropolitan Denver urban area and 0.f4-0.56 ppb when the wind was downslope and westerly, where there are fewer emissions sources; previous filter pack measurements at this site gave concentrations ranging from a few ppt, characteristic of remote regions, to several ppb, characteristic of polluted urban areas (Fehsenfeld et al., 1998).

Measurements of HN03 in the marine boundary layer are typically of the order of tens to hundreds of ppt. For example, Heikes et al. (f 996) reported average concentrations of 160 ppt, with a range from 30 to 280 ppt. In the middle and upper troposphere, concentrations of ~ 100-400 ppt have been reported (e.g., Singh et al., 1998).

(6) N03 As discussed earlier, the nitrate radical can be measured using visible spectroscopy and its absorption bands, particularly the one at 662 nm. As a result, visible absorption spectroscopy has been the method of measurement used most extensively for N03. As discussed shortly, a matrix isolation technique has also been applied with success in some studies.

Noxon et al. (1978) were the first to report the detection of N03 and to estimate its column abundance in the atmosphere, using its absorption at 662 nm and the moon as the light source. Their initial hypothesis was that most of the N03 was in the stratosphere. However, Noxon et al. (1980) subsequently showed using the moon as the light source, or alternatively a surface-based lamp with a 10-km path length at the Fritz Peak Observatory in Colorado, that N03 was also present in the troposphere at concentrations up to a few hundred ppt. About the same time, N03 was also detected and measured in the polluted troposphere by Piatt et al. (f980b). Since then, there have been a number of measurements of its column abundance and concentrations at specific locations in the troposphere (e.g., see Piatt, 1994; and Plane and Smith, 1995), all of which are at night or at sunset or sunrise due to the rapid photolysis of N03 during the day.

Vertical profiles, and in particular the amounts of tropospheric N03, have been extracted from measurements of the column abundance as a function of solar zenith angle at sunrise using either the moon or scattered sky light as the light source (e.g., Smith and Solomon, 1990; Smith et al., 1993; Weaver et al., 1996; Aliwell and Jones, 1996a, 1996b, 1998). As the sun rises, the column abundance of N03 decreases due to photolysis. During the night, the sun is sufficiently below the horizon that the atmosphere is in darkness throughout the stratosphere and troposphere. As it rises to a solar zenith angle of ~ 97°, altitudes down to 40 km are exposed to direct sunlight, and by the time the solar zenith angle is 93°, only the region below 10 km is not exposed to direct sunlight. Because the photolysis of N03 is so fast, under these conditions any signal remaining must be attributable to tropospheric

N03 (e.g., Smith and Solomon, 1990; Weaver et al., 1996; Aliwell and Jones, 1998).

Using this approach in various locations, it has been shown that the relative contributions of stratospheric and tropospheric N03 vary considerably. For example, in the Antarctic in spring, essentially all of the N03 was in the stratosphere (Smith et al., 1993), whereas at Fritz Peak, Colorado, in the summer, about equal amounts were in the troposphere and stratosphere (Weaver et al., 1996). Assuming that this tropospheric N03 was in a 1-km-thick bounday layer, the average N03 radical concentration in this layer at sunrise was about 20 ppt. Aliwell and Jones (1998) using a similar approach at Cambridge, England, suggest the average concentration could be as high as 89 ppt.

Surface measurements of N03 have been made using folded light paths and a light source such as a Xe arc lamp (Piatt, 1994; Plane and Smith, 1995). Concentrations as high as ~350 ppt have been observed in polluted urban areas (Piatt et al., 1980b, 1981; Piatt and Janssen, 1995), although many times even in polluted areas, the concentrations are below 20 ppt (Biermann et al., 1988). This likely reflects the balance between sources and sinks. For example, since N03 reacts rapidly with NO, significant concentrations of N03 will not be observed close to NO emission sources.

A second technique, matrix isolation-electron spin resonance (ESR), described earlier for NOz measurements, has also been used to measure N03 in the atmosphere (Mihelcic et al., 1985, 1990, 1993). Because there is a large concentration of N02 in air compared to other paramagnetic species, this dominates the spectra. However, the contribution of N02 can be subtracted using a reference spectrum and the residual then matched using the simultaneous fit of other contributing species (Mihelcic et al., 1990) to derive the contributions of species such as N03, H02, and R02. A sample spectrum containing contributions from all of these species is shown later (Fig. 11.53) in the discussion of HOz and R02 measurements. The detection limit of this method for N03 is 3 ppt (Mihelcic et al., 1993). The disadvantage is that it is not a "real-time" method, and as with any sampling of free radicals, care must be taken not to destroy the radicals before they are trapped and/or measured.

Typical tropospheric concentrations. Studies carried out in a remote region, at Izana de Tenerife in the Canary Islands, showed average nighttime N03 concentrations in clean air from the mid-Atlantic to be ~8 ppt, with a maximum of ~20 ppt (Carslaw et al., 1997b), and the concentrations are often below the detection limits (e.g., <3 ppt at Loop Head, Ireland; Piatt and Janssen, 1995).

In rural-suburban areas, concentrations between these two have been observed. For example, in central California peak concentrations were typically about 30 ppt, with a maximum value of ~80 ppt (Smith et al.,

1995), and at Kap Arkona in the Baltic Sea, the average nighttime concentrations were 8 ppt (Heintz et al.,

1996). However, even in such rural-suburban areas, high concentrations can occur, e.g., 280 ppt at Deuselbach, Germany (Piatt et al., 1981; Piatt and Janssen, 1995).

In short, the concentration of N03 in the troposphere can vary from very small, low-ppt concentrations to several hundred ppt, depending on the particular air mass. As discussed in Chapters 7 and 10, at typical tropospheric levels, it is believed to play a major role in nighttime chemistry, in some cases rivaling daytime OH for the net oxidation of certain organics, particularly alkenes (e.g., see Aliwell and Jones, 1998) as well as certain gaseous PAH.

(7) HONO Because of its importance as an OH source by photolysis at dawn, particularly in polluted areas, there have been a number of measurements reported for HONO. The two methods used most commonly have been DOAS and denuder methods. In addition, diffusion scrubber and photofragmenta-tion—laser-induced fluorescence methods have been developed, although they have not seen widespread use.

DOAS. As discussed earlier, HONO has been measured at the earth's surface at a number of sites around the world by DOAS using its characteristic absorption bands in the 340- to 380-nm region. Because this is a spectroscopic method, it has high selectivity as well as sensitivity. The disadvantage is that it requires a unique set of equipment and, more importantly, skilled experimentalists to accurately extract the HONO signal from others present in air, particularly that of N02, which is essentially always present simultaneously.

Denuders. The principle behind denuders has been discussed earlier with respect to HN03. Several types of wall coatings have been used for trapping HONO, including Na2C03 (Ferm and Sjodin, 1985) and a triple denuder coated first with tetrachloromercurate to remove S02 and HN03, followed by two Na2C03-coated tubes (Febo et al., 1993; Febo and Perrino, 1995; Febo et al., 1996). A wet diffusion denuder using NaOH as the absorbing agent has also been described (Simon and Dasgupta, 1995). The nitrite ion is then extracted and measured, most commonly using ion chromatography but, in some cases, using colorimetric methods (e.g., Ferm and Sjodin, 1985).

Figure 11.34 shows one set of measurements of HONO made in Milan, Italy, using DOAS and a denuder method, respectively (Febo et al., 1996). In this

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