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surement uncertainties that is arduous but effective. It uses formal, rigorous, and unbiased inter- and intramethod comparisons of techniques and instruments. The most instructive of those instrument intercomparisons have the following features:

• Several different techniques are used to measure the same species.

• Insofar as possible, measurements are made at the same place and time and under typical operating conditions.

• Accuracy and precision estimates are stated in advance of the study.

• Atmospheric samples are ''spiked'' with known amounts of species that are potential artifacts. Where possible, samples of ambient air as well as synthetic air are spiked.

• All investigators prepare their results independently and separately from the others.

• Investigators jointly (or through an independent party) compile separate results and assess the state of agreement or disagreement.

• Results and conclusions are published in a peer-reviewed journal.

• The process is repeated occasionally.

There have been several field studies devoted specifically to the assessment of instrument reliability, as opposed to obtaining data to answer a geophysical question (Hoell et al., 1987a,b; Hering et al., 1988; Fehsenfeld et al., 1987, 1990). Because these intercomparisons provide the only objective assessment of instrument capability, the scientific community's knowledge of the accuracy of current instrumentation depends heavily on these results. The instruments and techniques available for measurement of NOy' VOCs, and oxidant species; the basic operating principles of these devices; and highlights of the tests done thus far to determine instrument reliability are summarized below.

Measurement Techniques for Oxides of Nitrogen and Their Oxidation Products

The reactive oxides of nitrogen in the atmosphere are largely nitric oxide (NO) and nitrogen dioxide (NO2), known together as NOx. During the daytime, there is a rapid interconversion of NO and NO2. One important byproduct of this interconversion is the photochemical production of ozone in the troposphere. In addition, NOx is converted to a variety of other organic and inorganic nitrogen species. These are the compounds that make up the reactive nitrogen family, NOy (NOx + organic nitrates + inorganic nitrates).

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