by Frank Flocke (NCAR/ACD)
The Community Airborne Research Instrumentation (CARI) Group at NCAR supports some of the “basic” chemical tracer measurements on the NSF/NCAR aircraft. Any investigator leading a study and using our aircraft can request these instruments. Our instruments fly frequently because the chemical tracers are used in many different ways to provide scientists with information abut the type of air mass they are sampling.
For DC3 some of our measurements are very important for the success of the mission. CARI provides measurements of Ozone (O3), Nitrogen oxides (NOx), Carbon Monoxide (CO), Carbon Dioxide (CO2) and Methane (CH4).
Nitrogen oxides are of central importance for DC3, since one of the ways it is produced is by lightning inside thunderstorms. The energy released in a lightning strike is large enough to split the Nitrogen and Oxygen molecules in the air into atoms, some of which recombine to produce Nitrogen Monoxide, NO. In the atmosphere, NO reacts with ozone to form Nitrogen Dioxide, NO2. We measure both of these trace gases once every second on the GV. The sum of NO and NO2 is called NOx. One of the goals of DC3 is to better quantify the amount of NOx produced by lightning and its role in the chemistry of the upper atmosphere.
As described elsewhere in this blog (and also on the DC3 home page), thunderstorms act somewhat like a giant vacuum, because the physics driving the storm is bringing air from close to the Earth’s surface up to the top of the troposphere in a relatively short period of time (10s of minutes). Human activities on the ground also release NOx into the atmosphere (NOx tends to be higher near urban centers since much of it comes from transportation and power generation) and so can forest or agricultural fires. If this NOx is pumped up into the upper atmosphere by a thunderstorm passing overhead, we use the other chemical measurement to quantify how much is coming from lightning and how much was transported up from the surface. Combustion processes in engines and fires always also produce CO together with the NOx, but lightning does not produce CO. Also, the CO2 mixing ratio at the surface is almost always different from the CO2 up high. CO2 can also be used to identify aircraft contrails, which also contain NOx emitted from the jet engines. This way we can use the chemical tracers together to calculate the relative amounts of NOx in the upper troposphere coming from lightning and from human activity.
The NASA DC-8 often samples the air near the surface simultaneously with the NSF/NCAR GV sampling the upper air outflow from the storms. This is very important for “matching” the inflow and outflow air, but it also requires the instruments on both aircraft to perform in the same way and cross-calibrate them when possible. Flights together in close formation are planned to make sure all measurements compare well and can be used as one unified data set. More on that later.
There are other chemical tracers measured on both aircraft, which we can use to make even more detailed assessments of tracers and their origin, but that is material for yet another blog post down the line.