High-Alitutde Aircraft Measurements of NO and NO2 in the Upper Troposphere and Lower Stratosphere over South-East Asia

Nitric oxide (NO) and nitrogen dioxide (NO₂) are important trace gases in the upper troposphere and lower stratosphere (UTLS).  In the upper troposphere, NO is primarily formed by lightning and can react with other radicals to form a suite of reactive nitrogen (NO_y) species and produce ozone.  In the lower stratosphere, NO_x can be responsible for catalytic ozone destruction and can form halogen reservoirs such as ClNO₂ and ClONO₂.  Accurate measurements of NOx species are therefore important for understanding and diagnosing UTLS chemical regimes.  Further, understanding NO₂ concentrations in the stratosphere and upper troposphere is critical for accurate satellite retrievals of tropospheric NO₂. 

In-situ measurements of NO_x species above 12-15 km are infrequent and challenging.  Where there are measurements of NO_x species up to about 12 km, NO mixing ratios tend to be reasonably well-reproduced using global models.  However, NO₂ measurements frequently agree well with models and photostationary state calculations up to about 8 km, but at higher altitudes routinely are significantly higher than expected relative to the measured NO.  Proposed sources of this discrepancy include measurement artifacts affecting the in-situ measurements, inaccuracies in rate constants at low temperatures, and missing chemistry.  The net result is significant uncertainty in the NO₂ concentrations in the UTLS.

Here we present results from the recent high-altitude aircraft campaign Asian Summer Monsoon Chemistry and CLimate Impact Project (ACCLIP) over southeast Asia in summer 2022.  We show measurements from the recently-developed NOAA NO laser-induced fluorescence instrument up to altitudes of almost 20 km.  This instrument makes direct NO measurements with precision and accuracy sufficient for measurements below one ppt with one second of integration, making it ideal for aircraft campaigns.  In order to measure NO₂ with this instrument, we have developed a photolysis inlet which converts NO₂ to NO using an LED centered at 395 nm.  What is novel about this NO₂ conversion is that it is done in an unpressurized pylon of the aircraft and thus the sample remains at the UTLS ambient temperatures until after photolysis.  This thus significantly reduces the potential interferences from NO_y species that can thermally decompose to NO₂ at room temperature, eliminating a potential source of major artifact.  Our data show good agreement with photostationary state calculations performed for these flights.