FIGURE 11.27 Ratio of the sum of (NO + N02 + PAN + HN03 + particulate nitrate) measured individually to total measured NO,, at three sites in the United States (Whitetop Mountain, Tennessee; Bondville, Illinois; and Scotia, Pennsylvania) and one in Canada (Egbert, Ontario). The bars represent the range of results and the mid-range lines the median values (adapted from Parrish et al, 1993).

As discussed by Parrish et al. (1993), the Bondville HNO3 data may be artificially low, so that the apparent "missing NOy" of 25% may be an overestimate. Given the difficulty in measuring individual components of NOy such as HN03 (vide infra) at the very low levels found in the atmosphere, these data suggest that extent of the "missing NO " is relatively small on average.

However, note the wide range of total NO>( that could be accounted for in Fig. If.27. As discussed by Parrish et al. (f 993), values above 100% must be due to systematic errors whereas those below 100% may reflect either systematic errors or true "missing NO ."

Figure 11.28 shows similar data for measurements made at Idaho Hill, Colorado, in the fall of 1993 (Williams et al., 1997). Measurements were made of NO, N02, PAN, PPN, HNO3, and particulate nitrate, as well as total NOv. Two sets of meteorological conditions were encountered, one where the wind was downslope and from the west where there were few sources nearby, and one where the wind was upslope, carrying pollutants from urban areas to the east. Figure f 1.28a shows that for upslope air masses from the east with relatively fresh emissions, the sum of the measured compounds accounts, within experimental error, for the total NO . The average ratio of ENO /total NOy was 1.06 + 0.15. On the other hand, during periods with cleaner, downslope air from the west (which has also had more time to react), the sum of the individual compounds frequently does not add up to the total measured NO>( (Fig. 11.28b). The deficit ranges from 0 to 50% of the measured total NO...

The average contributions of the various oxides of nitrogen to NO for the two conditions are shown in Fig. If.29 (Williams et al., 1997). The mean total NO^ measured under the more polluted conditions was 4.3 + 2.4 ppb, with essentially all of it accounted for by the measured individual compounds. Under the cleaner

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