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The staff report discusses the accuracy and precision of
chemiluminescent methods for measuring NO2. 
Accurate measurement methods for NO2 are available
(Ryerson et al., 2000, JGR, vol 105, pp. 26447-26461;
Thornton et al., 2000, Analytical Chemistry, vol 72,
pp. 528-539), but these are methods are not routinely used in
ambient air monitoring networks. I recommend inclusion of these
newer measurement methods in Chapter 3 of the technical support
document.

It is known (e.g., Winer et al., 1974, ES&T, 
vol 8, pp. 1118-1121) that chemiluminescent analyzers respond not
only to NO2 but also to other nitrogen-containing air pollutants
including peroxyacyl nitrates. This could be a significant source
of positive bias in ambient NO2 measurements. PANs are one
example, but are not the only class of pollutants for which
interferences with the NO2 measurement may occur. Nitrous and
nitric acids may also get reported as NO2, assuming they are not
lost in air sampler inlet lines.

This could be important in comparing clinical studies where the
exposure is presumably to pure & well-characterized levels of NO2,
and the real atmosphere where exposures include not only NO2 but
also other pollutants that may have different types and/or
severity of health effects.

In summary, I disagree with the characterization of routine NO2
measurements as being accurate, in the sense of being free of
significant interferences. This issue may be deserving of further
consideration.