2011-03-01: The development of a nitrogen dioxide sonde

KNMI has developed a working NO2 sonde. The NO2 sonde can be applied in an operational network and also be used for satellite and AQ-model validation. The sonde is attached to a small meteorological balloon and measures a tropospheric NO2 profile. The NO2 sonde has a vertical resolution of 5m and a measurement range between 1 and 100 ppbv.
The sonde uses the chemiluminescent reaction of NO2 in an aqueous luminol solution. Chemiluminescent is an exothermic reaction, but instead of heat, light is emitted. The NO2-luminol reaction produces faint blue/purple light (at about 425 nm), which is detected by an array of silicon photodiodes. The luminol solution is optimized to be specific to NO2 only.
The instrument is housed in a polystyrene (PS) foam box. PS is light in weight and a good insulator. The inside of the PS box is painted black, so ambient light is absorbed and can not disturb the measurement. To have an extra light barrier the instrument is placed in a black cardboard box. A radio transmitter (Vaisala RS92 radiosonde) is attached to the outside of the PS box, and sends the measurements back to the radio receiver.

Design of the instrument:
Figure 1 shows a schematic diagram of the design of the instrument.
Figure 1
A luminol reservoir is shown on the right. The liquid pump moves the luminol solution to the reaction vessel. The used liquid pump, is a small piezoelectric diaphragm pump (Bartels microComponents mp6). The Teflon air pump, which is taken from an ozone sonde, forces the ambient air into the reaction vessel with a flow of 4 ml/s. The tubes connected to the air pump are also made of Teflon. The gas is leaving the Teflon tube by two holes of 1mm diameter. The NO2 in the gas flow reacts with the luminol solution, which emits blue/violet light at a wavelength of 425 nm. The emitted light is detected by an array of silicon photodiodes (the seeing photodiodes). An aluminum mirror is placed on the opposite site to reflect the emitted photons back to the seeing photodiodes. An identical array of photodiodes, blind photodiodes, is mounted behind the aluminum foil to measure – and correct for – electrical effects. The secondary function of the gas flow is to force the luminol solution out of the reaction vessel back to the luminol reservoir, so luminol is recycled continuously. The recycling of luminol is necessary to reduce acidification of the system by carbon dioxide (CO2).

NO2 profiles:
During the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring Instruments (CINDI) in June/July 2009, six NO2 sondes were launched and measured six NO2 profiles. Figure 2 shows the six profiles that were measured. The first NO2 sonde was launched on 18 June 2009, Figure 2a. This profile shows a maximum NO2 concentration around 930 m altitude and a clear top of the boundary layer at 2 km. Its vertical resolution is higher than that of the other sondes, because it had a smaller vertical velocity. The profile of 23 June 2009 (Figure 2b), shows at an altitude of 4.8 km a peak in NO2 concentration. This peak could have been caused by a layer of airplane exhaust. The profile of 30 June 2009 (Figure 2c) shows a second NO2 layer (1-3 km) above the boundary layer (0 – 1 km). The origin of this “second layer” could not be determined from meteorological measurements at Cabauw.
Figure 2: Six vertical NO2 profiles were measured during CINDI June/July 2009. On the x-axis the NO2 volume mixing ratio (ppbv) and on the y-axis the altitude (km). The dashed lines indicate the boundary layer height above Cabauw around 10:30 UT, measured by the KNMI Ceilometer. The blue lines indicate the outside air temperature during the sonde flight.

More information:
In December 2010, an article about the NO2 sonde is published in Atmospheric Measurement Techniques (AMT). More detailed information about the NO2 sonde can be found here.