FIGURE 11.50 Correlation between OH measurements made by DOAS and by LIF in a rural area in Germany in August 1994. The data indicated by asterisks were measurements made when the wind was from a particular direction suggesting it might contain unrecognized OH sources affecting the long-path DOAS measurements (adapted from Hofzumahaus et al., 1998).

be due to unrecognized OH sources that affected the long-path DOAS measurements more than the point measurements made by LIF. Exclusion of those data points improves the correlation (r = 0.90) and the slope of the line is 1.09 + 0.04 with an intercept within experimental error of zero.

Similarly, agreement between UV absorption and the mass spectrometer technique is quite good. Figure 11.51 shows a plot of OH concentrations measured using the mass spectrometry derivatization technique compared to those measured using long-path UV absorption in a rural area in Colorado for clear days with NOx below 450 ppt measured by an in situ technique and NOz below 500 ppt averaged over the long path

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