MEASUREMENT OF NO2 AND HNO3 IN DIESEL EXHAUST-GAS BY TUNABLE DIODE-LASER ABSORPTION SPECTROMETRY

A tunable diode laser absorption spectrometer (TDLAS) has been used to make real-time measurements of NO//2 and HNO//3 in diesel engine exhaust. Teflon filters in the sample line protected the optics from exhaust particulates. Nitric acid and NO//2 were measured in the range of 40-800 ppbv and 1-30 ppmv, respectively. The mixing ratio of HNO//3 was found to decrease as engine load increased and to increase with engine speed. The HNO//3 levels exhibited slow response (10-30 min) both for changes in engine operating condition and when controlled spikes of HNO//3 were added to the diluted exhaust gas flow. NO//2 concentrations reached steady values within 5 min of changes in operating conditions. The data indicate that the observed HNO//3 is present in the exhaust stream immediately after dilution and is not produced by subsequent reactions of NO//x. There was no correlation between the variations in particulate nitrate and the gas-phase HNO//3 with load and engine speed.