A unique NO2-sensor that combines a high accuracy with a small size.
NO2 profile measurements
Nitrogen dioxide (NO2) is a reddish-brown poisonous gas that is emitted primarily by humans (anthropogenic emissions) after which it enters the atmosphere. NO2 is a product of fossil fuel combustion inside combustion engines of motorized vehicles and aircraft, but industry and power plants also play an important role.
The total amount of Nitrogen dioxide in the atmosphere is measured from space daily by satellite instruments such as the Ozone Monitoring Instrument (OMI) and Global Ozone Monitoring Experiment-2 (GOME-2). The NO2 total columns derived from satellite measurements are often divided in a tropospheric amount, the amount of NO2 from the surface up to ~10 kilometre, and a stratospheric amount, the amount of NO2 above ~10 kilometre. To make this distinction between the lower and the upper part of the atmosphere, an assumption has to be made about the shape of the vertical NO2 profile in the troposphere. This assumption is based on global chemical transport model calculations, which unfortunately have a large margin of error. These large errors in the assumed NO2 vertical profile shape cause a large uncertainty in satellite derivations of about 30-40 % of the tropospheric amount.
Satellite validation
Given its large margin of error, the validity of the assumed Nitrogen dioxide vertical profile shape in the troposphere, thus the quality of the retrieved tropospheric column amount of NO2 has to be checked, a process called validation. The validation of satellite observations can be done by performing independent measurements of the vertical NO2 profile shape. Measurement of the vertical NO2 profile is accompanied by a large number of challenges regarding the availability of suitable instruments and measurement platforms. From the ground, only passive measurement techniques can be deployed like MAX-DOAS, which has a poor vertical resolution, or NO2-LIDAR, which is very expensive. Commercial monitors can actively sample NO2, but in addition to being pricey, they have to be mounted on board an aircraft to measure vertical profiles. Aircraft themselves produce NO2 in flight, which can influence the measurements and, in addition, both their horizontal and vertical speed are too high to accurately capture the NO2 variability. The most suitable platform for the vertical measurement of trace gases is a meteorological balloon filled with Helium or Hydrogen gas. Because these gases are lighter than air, they can lift a payload of up to 1500 grams at a controlled speed of 5 meters per second. The only problem is that to date, no Nitrogen dioxide sensors are available that meet the requirements to be deployed when attached to a weather balloon. This application requires a sensor that combines a high sensitivity and accuracy with a small weight and size, that can be operated under the harsh conditions at high altitudes. To fill this gap both in scientific knowledge and the market, the KNMI has developed the NO2-sonde, a small, relatively cheap and lightweight sensor that can measure the vertical profile of Nitrogen dioxide to a height of about 15 kilometres. Since the speed at which the balloon rises is 5 metres per second, NO2-sonde measurements have a resolution of 5 meters (one measurement every 5 metres).
The NO2-sonde
The KNMI NO2-sonde is a unique NO2-sensor whose design meets many difficult requirements. It is lightweight (450 grams), relatively cheap (< € 1000,- per flight), energy efficient (< 2 Watt), can be operated in a difficult environment (-50 ° C, rain, low pressure), and it has a spatiotemporal resolution of 5 metre per second. It is a stand-alone measurement device that can be operated from a weather balloon while measuring how much Nitrogen dioxide is present in the atmosphere and at what height. For the detection of NO2, the NO2-sonde uses a chemical reaction called chemiluminescence. The detector is filled with a solution containing a chemical named Luminol, which releases energy in the form light (luminescence) when it comes into contact with NO2 in ambient air. The colour of the light released in this particular reaction is violet/blue and has a wavelength of about 425 nanometre. From this chemical reaction, the concentration of Nitrogen dioxide can easily be derived since it is directly proportional to the amount of light that is produced. The light released by the reaction between Luminol and NO2 is detected by photodiodes that convert light into an electrical current, which is amplified and converted to an output signal in Volt. The absolute uniqueness of the NO2-sonde is underlined by the fact that in April 2017, for its development, a patent has been granted to the KNMI by Het Octrooicentrum Nederland. In addition to being suitable for ballon launched, the NO2 sensor has also been deployed on drones and bikes.