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FIGURE 15.3 Indoor concentrations of N02 measured in ice skating rinks using different types of power for the ice resurfacers. The median and the 25th and 75th percentiles are shown in each case (adapted from Brauer et al., 1997).

In the absence of such sources of NOx, indoor and outdoor concentrations are quite similar (e.g., Weschler et al., 1994), since removal of NO and N02 indoors, e.g., on surfaces, is relatively slow. However, as discussed shortly, although the surface reaction of N02 is relatively slow, it is still of interest since it generates nitrous acid (HONO). Different surfaces found inside homes have been found to have different removal rates for N02. Figure 15.4, for example, shows measured rates of removal of N02 by a number of common household materials (Spicer et al., 1989). Large variations in removal rate (and hence the formation of products such as NO and HONO; see later) are evident, varying from negligible for plastic storm windows to quite large for wallboard.

In short, there is a variety of evidence that there are higher levels of N02 indoors when combustion sources are present and that the concentrations generated indoors can be quite substantial in some circumstances. One word of caution is in order, however, particularly with regard to earlier measurements of N02. As discussed in the following section, significant concentrations of HONO are generated both by a heterogeneous reaction of N02 on surfaces and by direct emissions from combustion sources, in some measurement methods used for N02, HONO is also detected and hence reported as N02. This is particularly true for the 03 chemiluminescence method and for electrochemical sensors (e.g., Spicer et al., 1994), so that N02 reported using these techniques should be regarded as upper limits to its concentrations. This problem can be circumvented through the use of denuders to remove HONO prior to sampling into the instrument.

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