Info

FIGURE 11.34 HONO measurements at Milan, Italy, using DOAS or denuders, respectively, in March 1994 (adapted from Febo et al., 1996).

particular study, the DOAS and denuder methods are in reasonably good agreement, although this is not always the case.

As is the case for denuder methods in general, artifacts are possible and this is particularly the case for nitrite, which can be oxidized relatively easily to nitrate. Positive artifacts due to high concentrations of N02 and PAN can occur, and S02 can also interfere (Perrino et al., 1990). For example, HONO has been reported during the day using denuders at levels much higher than would be expected given its rapid photolysis, suggesting the presence of significant positive artifacts (e.g., see Lammel and Cape, 1996).

Diffusion scrubber. In one such approach, air containing the HONO flows past a microporous membrane through which a flow of water is pumped. HONO (and other gases) diffuse through the membrane and are "scrubbed" into the water and measured by ion chromatography (Vecera and Dasgupta, 1991). A second approach involves reducing the absorbed nitrite ion to NO using ascorbic acid and then measuring the NO by chemiluminescence (Kanda and Taira, 1990; Harrison et al., 1996). Since there is often significant NO present, HONO must be determined using difference methods.

Photofragmentation-laser-induced fluorescence. In an approach similar to that which has been applied to HN03, HONO can be photolyzed to generate OH and the OH measured using laser-induced fluorescence (Rodgers and Davis, 1989):

HONO + hv{355 nm) -» OH(X2II), » = » + NO, (13) OH(X2n),»= o + hv(282 nm) -> OH(A22), '= i

™ OH(A2X),' = 0, (14) OH(A2S),■'.() -» OH(X2n) + hv{309 nm). (15)

The detection of OH and the uncertainties associated with it are discussed in detail shortly. The detection limit is a few tens of ppt for a measurement time of f5 min (Rodgers and Davis, 1989).

Atmospheric pressure ionization mass spectrometry has also been used to meaure HONO in indoor air enviroments (Spicer et al., 1993a) and outdoors from a research aircraft (Berkowitz et al., 1998). In this case, HONO does not have a parent peak in either positive or negative ion modes, so that adduct formation must be used to form a parent ion. In this case, CC14 was introduced into the corona discharge region, forming Cl~, which forms an adduct with HONO. Peaks at m/e 82 and 84 corresponding to the 35C1~ and 37C1~ adducts are observed in the negative ion mode and their colli-sionally induced fragmentation to m/e 46 followed.

Typical surface concentrations of HONO. Because it is so readily photolyzed, HONO builds up at night and its concentrations rapidly drop at dawn. Figure 11.35 summarizes the range of measured tropospheric HONO concentrations and their average values in remote to polluted urban regions (Lammel and Cape, 1996). The highest concentrations are generally found in polluted areas where there are higher concentrations of precursor N02 (see Chapter 7). Concentrations up to 10 ppb have been observed in Milan, Italy (Febo et al., 1996), and the Los Angeles area (Piatt et al., 1980a; Harris et al., 1982; Atkinson et al., 1986; Winer and Biermann, 1994). Peak values of several ppb have been observed in many other urban locations, including, for example, Ispra, Italy (Andrés-Hernández et al., 1996), and Göteborg, Sweden (Sjödin, 1988).

(8) NH3 A number of measurement techniques have been used for ammonia, including a spectroscopic method, denuder methods, and filter packs.

Photofragmentation-laser-induced fluorescence (PD-LIF). This spectroscopic method is based on the photofragmentation of NH3 in a two-photon process using 193-nm radiation, followed by laser-induced fluorescence of the NH fragment (Schendel et al., f990). The processes are as follows:

NH3 + 2M193 nm) -> NH(b1 £ +),.•-= o + H2 or2H,

NH(c'n),..' = » -> NH(a'A),"= o + hv{325 nm). (18)

Selectivity is obtained by tuning the NH excitation laser to a specific rotational transition and following specific fluorescence transitions. Detection limits of ~5 ppt for

FIGURE 11.34 HONO measurements at Milan, Italy, using DOAS or denuders, respectively, in March 1994 (adapted from Febo et al., 1996).

March

March

11. ANALYTICAL METHODS FOR GASES ANL) PARTICLES IN THE ATMOSPHERE

Los Angeles area

Lubbock, Texas

Chicago Cologne Mainz

Gothenburg

Milan

Mt. Mitchell, NC, USA San Gabriel Mtns., CA: day San Gabriel Mtns., CA: night Sierra Nevada Mtns., CA, USA Claremont, CA, USA Riverside, CA, USA

Jülich, Germany

Braunschweig, Germany: Summer Braunschweig, Germany: Winter

SE England

Near Rome, Italy

Ispra, N. Italy Claro, Switzerland

Alert, Canadian Arctic SW Desert, Utah, USA Savanna, Venezuela Loop Head, Ireland

FIGURE 11.35 Range of tropospheric HONO concentrations observed in remote to polluted areas. The averages are shown by the vertical bars. Note the scale is logarithmic (adapted from Lammel and Cape, 1996).

Urban

Was this article helpful?

0 0

Post a comment