The NO2 camera

Air quality
The NO2 camera is a new, unique instrument which flows down from the ALTIUS mission. Just like ALTIUS, it takes images of a scene (e.g. smokestack plumes, city skylines) at several wavelengths of interest which are absorbed by a harmful by-product of every combustion process (engines, heating): NO2. The camera’s ultimate goal is the monitoring of NO2 emissions produced by industrial sites, as well as ambient levels in the urban environment. By doing so, the NO2 camera could bridge the gap between satellite-borne instruments covering the Earth’s surface with a coarse resolution (several km²), and the sparse network of official air quality monitoring stations.

Bridging a gap in air quality monitoring

Belgian citizens are exposed to much higher levels of NO2 than other European citizens.

The reasons are well-known:

  • large industrial sites (especially oil refineries)
  • a dense road network contribute to high background levels of air pollution.

Recent public-supported experiments have shed new light on how variable the NO2 levels can be. For the average citizen, neither the low resolution satellite maps nor the sparse air quality stations provide meaningful data about their direct exposure to the harmful NO2.

What is needed is:

  1. more in-situ sensors
  2. new instruments capable of producing an increased spatio-temporal resolution

A new instrument concept – when space heritage meets ground-based applications

Recognizing that the instrumental concept of ALTIUS could be optimized for the remote sensing of NO2 on ground, BIRA-IASB has developed a prototype of a NO2 camera, i.e. an instrument capable of imaging the NO2 field above large point sources such as cities.

By inserting a tunable spectral filter inside a classical optical setup (lenses, detector), one can take snapshots of a scene at the wavelengths which are effectively absorbed by NO2 (these are found in the blue part of the natural light spectrum). By analyzing the contrast of images acquired at several wavelengths, one can retrieve the abundance of NO2 encountered by the photons on their way through the atmosphere.

The obvious benefits of this concept are:

  1. a broad field of view
  2. a relatively high data rate enabled by a fully-electronic control chain

With this instrument, actual emission fluxes of smokestacks can be estimated (see introductory image) and compared with reported values. Another important application is the high-resolution mapping of the pollution above a city (Figure 3), a step towards a full street-level pollution map.

Figure 1: Modelled spatial distribution of NO2 in Antwerp region. Credit: maps.atmosys.eu.
Figure 1: Modelled spatial distribution of NO2 in Antwerp region. Credit: maps.atmosys.eu.
Figure 2: The NO2 camera in operations at Antwerp's harbour in February 2018.
Figure 2: The NO2 camera in operations at Antwerp's harbour in February 2018.
Figure 3: NO2 field above Brussels, as observed with the NO2 camera from KMI-IRM rooftop in the direction of the city center. The large building in the right side is the Sint Elizabeth Hospital in Uccle.
Figure 3: NO2 field above Brussels, as observed with the NO2 camera from KMI-IRM rooftop in the direction of the city center. The large building in the right side is the Sint Elizabeth Hospital in Uccle.