A fast-response chemical ionization mass spectrometer for in situ measurements of HNO3 in the upper troposphere and lower stratosphere

A chemical ionization mass spectrometer instrument has been developed for i n situ measurements of nitric acid (HNO3) in the upper troposphere and lowe r stratosphere from the NASA WB-57 aircraft. Fast and sensitive measurement s of HNO3 are achieved by using a low surface area heated Teflon sampling i nlet and detection techniques that employ ion-molecule reactions. Sensitivi ty to HNO3 is determined in flight by adding HNO3 from a calibrated HNO3 pe rmeation source into the sample air flow, and instrument background is meas ured by displacing ambient air from the sampling inlet with a flow of dry n itrogen in the sampling inlet. Instrument temperatures, pressures, and gas flows are controlled in flight to maintain a constant detection sensitivity in a changing ambient operating environment. The initial performance of th is new instrument is evaluated using HNO3 and ozone data obtained between 6 and 19 km during the 1999 NASA Atmospheric Chemistry of Combustion Emissio ns Near the Tropopause mission. The data reveal a sensitivity of 0.73 Hz/pp tv, a detection limit of 30 pptv for 10 s integration times, and a fast tim e response (< 1 s). Further reductions in the background HNO3 signal will i mprove performance. (C) 2000 American Institute of Physics. [S0034-6748(00) 01710-X].