An informal intercomparison of NOy measurement techniques was conducted from June 13 to July 22, 1994, at a site in Hendersonville, Tennessee, near Nashville. The intercomparison involved five research institutions: Brookhaven National Laboratory, Environmental Science and Engineering, Georgia Institute of Technology, NOAA/Aeronomy Laboratory, and Tennessee Valley Authority. The NOy measurement techniques relied on the reduction of NOy species to NO followed by detection of NO using O3-chemiluminescence. The NOy methods used either the Au-catalyzed conversion of NOy to NO in the presence of CO or H2 or the reduction of NOy to NO on a heated molybdenum oxide surface. Other measurements included O3, NOx, PAN and other organic peroxycarboxylic nitric anhydrides, HNO3 and particulate nitrate, and meterological parameters. The intercomparison consisted of six weeks of ambient air sampling with instruments and inlet systems normally used by the groups for field measurements. In addition, periodic challenges to the instruments (spike tests) were conducted with known levels of NO, NO2, NPN, HNO3 and NH3. The NOy levels were typically large and highly variable, ranging from 2 ppbv to about 100 ppbv, and for much of the time was composed mostly of NOx from nearby sources. The spike tests results and ambient air results were consistent only when NOx was a substantial fraction of NOy. Inconsistency with ambient air data and the other spike test results is largely attributed to imprecision in the spike results due to the high and variable NOy background. For the ambient air data, a high degree of correlation was found with the different data sets. Of the seven NOy instrument/converters deployed at the site, two (one Au and one Mo) showed evidence of some loss of conversion efficiency. This occurred when the more oxidized NOy species (e.g., HNO3) were in relatively high abundance, as shown by analysis of one period of intense photochemical activity. For five of the instruments, no significant differences were found in the effectiveness of NOy conversion at these levels of NOy with either Au or Mo converters. Within the estimated uncertainty limits there was agreement between the sum of the separately measured NOy species and the NOy measured by the five of the seven techniques. These results indicate that NOy can be measured reliably in urban and suburban environments with existing instrumentation. ¿ 1998 American Geophysical Union