Upper Troposphere and Lower Stratosphere Measurements of NO and NO2 from High-Altitude Aircraft

Accurate measurements of nitric oxide (NO) and nitrogen dioxide (NO2) in the upper troposphere / lower stratosphere region (UTLS) are required for understanding radical chemistry in the stratosphere and its impact on ozone, as well as for quantifying sources NOx (the sum of NO + NO2) in the UT region (e.g. lightning) and interpretation of satellite remote sensing columns of NO2. In the stratosphere, NOx catalytically destroys ozone. In addition, it can react with halogens to create reservoir species such as ClNO2 and ClONO2. Therefore, understanding the concentrations and chemistry of NOx in the stratosphere are critical for predicting the extent of ozone loss in the ozone layer. However, there is a dearth of measurements of these species above 15 km altitude. Additionally, many measurements in the upper part of the free troposphere in the 8-12 km region show substantial disagreement with photochemical steady-state calculations. The net result is significant uncertainties on the NOx concentrations in the upper troposphere and lower stratosphere.

Here, we present recent measurements of NO and NO2 made up to 19 km in the UTLS on board a high-altitude aircraft. These measurements utilize the recently-developed NOAA laser-induced fluorescence instrument that makes direct measurements of NO. This instrument has excellent precision and a sub-ppt detection limit in 1 second so is ideal for measuring very low mixing ratios of NO. In this work, we have added a second channel and a custom-built photolysis cell to convert to NO2 to NO using a 395 nm LED. What is novel about this photolysis inlet is that it is located in a cold pylon extending outside the aircraft. This ensures that the air sample remains at very cold ambient temperatures until after the NO2 photolysis, thereby minimizing any thermal decomposition of NOy species which might otherwise cause an artifact in the measurements. Our measurements show very low NO2/NO ratios in this UTLS region, in good agreement with photochemical steady state calculations.