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Atmospheric modelers and development engineers need accurate measures of NO2 emissions from motor vehicles. Due to the oxidative reaction of oxygen with NO, these measurements (typically taken from a bag sample) can be inaccurate if care is not taken to minimize the sample residence time in the bags. This reaction occurs slowly at low NO concentrations, however, at higher NO concentrations the reaction can rapidly speed up (for example, 50 ppm NO will experience a 10% concentration reduction in 6.5 minutes). This report explores the factors contributing to this artifact for emissions test cells. Estimates of the error in NO2 emission rate measurements for several scenarios are presented. Additionally, kinetic expressions of the reaction rate are shown to be fairly accurate for our test conditions, but should not be used in general without verification of the non-existence of competing, hindering or accelerating species within the sample bag.

Dynamic single range analyzers are designed to cover the wide range of concentrations that once required multiple ranges. The use of single range analyzers is attractive because they can significantly reduce installation costs, gas cylinder charges, and facility storage space. The new technology relies on expansion of the digital dynamic range of the analyzer combined with the availability of a high accuracy gas divider with a 500 to one dilution ratio and a large number of cut points. A series of four MEXA 7000 series bag bench analyzers manufactured by Horiba Instruments, Inc., were evaluated to compare multiple range operation with single range operation. This report describes the operation of these analyzers and summarizes the evaluation methods and results. The evaluation verified the ability of the analyzers to operate in single range mode.

The new generation of emission analyzers, MEXA 7000 series, manufactured by Horiba Instruments, Incorporated, operate as single range instruments. The wide operational range of the new analyzers requires a new calibration/linearization tool, a new gas divider capable of providing more calibration points over a wider dynamic operational range. The operation of the new gas divider (Horiba GDC-703), based on mass flow control technology, provides 40 cut points with a minimum step size of 1/500, or 0.2%, of the calibration gas value. This paper describes the new Horiba gas divider, GDC-700 series. The evaluation proves the gas divider's viability for use in engine and vehicle emissions test laboratories. Extensive sets of data were collected to provide an evaluation of the gas divider over a wide range of operation for measurement linearity, repeatability, and accuracy.