A chemical ionization mass spectrometer (CIMS) from the Air Force Research Laboratory at Hanscom AFB was used to measure levels of HNO3 in the exhaust stream of the F100 jet engine both in a test cell at NASA Glenn Research Center (GRC) at simulated altitudes 3--17 km and in flight behind F-16 aircraft at 9--11 km altitude. The HNO3 concentrations have been expressed as emission indices (EI) by normalizing to engine-produced CO2 measured by GRC instruments in the test cell and NASA-Langley Research Center instruments in flight. The test cell data showed that EI(HNO3) is independent of fuel sulfur level, and is highly dependent upon altitude for altitudes at which most commercial aircraft fly (below 12 km), mainly because the emission is a strong function of combustor inlet temperature, which is lower at high altitudes. EI(HNO3) at military power was 0.003 g NO2 kg-1 fuel at low altitude (~3 km), and increased to 0.02 g kg-1 for altitudes above 11 km. Operating the engine at 85% maximum turbine speed increased the figures above by a factor of about 2.5. At engine idle, EI(HNO3) was typically 0.14 g kg-1. The decrease in EI(HNO3) with combustor inlet temperature is attributed to the temperature dependence of the NO2 + OH reaction producing HNO3. Comparing HNO3 emission indices with those measured at GRC for NO and NOx allowed us to determine the fraction of NO2 which is converted into HNO3 in the combustion process as a function of inlet temperature. Conversion of NO2 to HNO3 is 0.9% at military power for altitudes ≥11 km, and twice that at 85% maximum turbine speed. The conversion fraction changes dramatically in the 8--11 km altitude range, and is only 0.02% at 3 km altitude, for military power, or 0.2% at 85% maximum turbine speed. At engine idle, this fraction was about 8% at all altitudes. The accuracy of the (average) figures quoted above is ¿25%. The in situ data, obtained on 10 flights during the NASA Subsonic Near-Interaction Field (SNIF) experiment, are much less definitive but are consistent with the test cell results.