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average have been reported in normal, in-use buildings and homes (Febo and Perrino, 1991; Spengler et al., 1993; Weschler et al., 1994).

The ratio of HONO to N02 indoors can be quite large, up to ~0.15 (e.g., Febo and Perrino, 1991; Brauer et al., 1990, 1993; Spengler et al., 1993). This can be compared to typical values of a few percent outdoors (see Chapter 11). High levels of HONO (up to ~ 30 ppb) have also been measurd in automobiles in use in polluted urban areas, and again, the ratio of HONO to N02 was quite large, ~0.4, compared to 0.02-0.03 measured outdoors in the same study (Febo and Perrino, 1995).

Figure 15.5, for example, shows one set of measurements of N02, HONO, and NO after injection of N02 into a home used for research purposes (Spicer et al., 1993). As N02 decays, HONO is formed. Small amounts of NO are also generated, as has been observed in laboratory studies (e.g., Spicer et al., 1989). Delayed release of HONO was observed, suggesting that HONO was adsorbed in part on the surfaces and was subsequently released to the gas phase. For example, when the house was purged with outside air, the HONO levels dropped; however, on closing up the house again, the levels increased immediately to a few ppb. Degassing of HONO from the interior surfaces of automobiles has also been reported (Febo and Perrino, 1995). Interestingly, in measurements made in a commercial office building, indoor HONO concentrations were observed not to be enhanced under conditions of high indoor 03, suggesting that dissolved HONO in a surface film of water is readily oxidized to nitrate by 03 (Weschler et al., 1994).

The generation of NO in Fig. 15.5 was attributed by Spicer and co-workers to a reaction of gaseous N02

Time

FIGURE 15.5 Concentrations of N02, HONO, and NO after injection of N02 into a research house (adapted from Spicer et al., 1993).

Time with adsorbed HONO:

The same process was hypothesized to explain some time periods in a commercial office building when indoor NO actually exceeded outdoor NO (Weschler et al., 1994).

As is the case in laboratory systems, equivalent amounts of HN03 are not observed as might be expected from the stoichiometry of reaction (f), likely due to HN03 remaining on the surface after formation and/or being taken up by surfaces. For example, Spicer et al. (1993) used atmospheric pressure ionization mass spectrometry (see Chapter 11.A.2), which could measure HN03 with a sensitivity of 50 ppt, during the N02 decay and HONO formation in the research home. HN03 was only observed, and in small concentrations (< 1 ppb), when an unvented space heater was operating, and it decayed rapidly in the absence of sources. Similarly, Salmon et al. (1990) measured HN03 indoors and outdoors at five museums in the Los Angeles area and found that indoor concentrations were less than ~40% of those outdoors, and typically about 10%. The accumulation of nitrate on indoor surfaces in a commercial building has been reported by Weschler and Shields (1996a) and attributed to the formation and uptake of HN03 via reactions of N03 (see later) and/or oxidation of nitrite (i.e., adsorbed HONO) in an aqueous surface film.

Subsequently, it was shown that HONO is also directly emitted by gas stoves (Pitts et al., 1989). For example, Fig. 15.6 shows the concentrations of N02 and HONO measured using DOAS when two top burners of a new, residential gas kitchen stove were turned on in a mobile home with both the central ventilation and air conditioning running. N02 from the gas stove emissions reached almost 300 ppb, and HONO about

Time (minutes)

FIGURE 15.6 N02 and HONO measured in a mobile home with two burners of a kitchen stove on; the above-range exhaust was turned on at the time shown (adapted from Pitts et al., 1989).

Time (minutes)

FIGURE 15.5 Concentrations of N02, HONO, and NO after injection of N02 into a research house (adapted from Spicer et al., 1993).

FIGURE 15.6 N02 and HONO measured in a mobile home with two burners of a kitchen stove on; the above-range exhaust was turned on at the time shown (adapted from Pitts et al., 1989).

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