Intercomparison of ground‐based NO_<i>y</i> measurement techniques

Abstract

An informal intercomparison of NO y 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 NO y measurement techniques relied on the reduction of NO y species to NO followed by detection of NO using O 3 ‐chemiluminescence. The NO y methods used either the Au‐catalyzed conversion of NO y to NO in the presence of CO or H 2 or the reduction of NO y to NO on a heated molybdenum oxide surface. Other measurements included O 3 , NO x , PAN and other organic peroxycarboxylic nitric anhydrides, HNO 3 and particulate nitrate, and meteorological 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, NO 2 , NPN, HNO 3 and NH 3 . The NO y 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 NO x from nearby sources. The spike tests results and ambient air results were consistent only when NO x was a substantial fraction of NO y . 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 NO y background. For the ambient air data, a high degree of correlation was found with the different data sets. Of the seven NO y 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 NO y species (e.g., HNO 3 ) 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 NO y conversion at these levels of NO y with either Au or Mo converters. Within the estimated uncertainty limits there was agreement between the sum of the separately measured NO y species and the NO y measured by the five of the seven techniques. These results indicate that NO y can be measured reliably in urban and suburban environments with existing instrumentation.