What is NO2?
Nitrogen dioxide (NO2) is a member of the Nitrogen oxides group (NOx = NO + NO2), which plays a key role in tropospheric chemistry regulating the level of ozone and therefore maintaining the oxidizing capacity in the troposphere.
NO2 is an extremely reactive gas (lifetime of a couple of hours) which interacts with tiny particles in the atmosphere. High amount of NO2 concentration in the troposphere leads to the formation of the reddish brown smog that hangs over most of the very large cities in the world.
How is atmospheric NO2 produced?
In the troposphere (i.e. low part of the atmosphere where we live), NOx are created by burning fossil fuels. Traffic, power plants, heavy industry and oil refineries are the biggest emitters of NO2 (Jaegle et al., 2005; Martin et al., 2003). Netherlands produce 2.2% of European NOx emissions; Germany 10%, Belgium 1.8% and Great Britain 10% (Kuenen et al., 2011).
At the global scale, there are still some debates about the exact estimates of NOx sources and sinks (Lerdau et al., 2000): current average numbers are about 30% of NOx comes from fossil fuel combustion with ~86% released into the boundary layer due to surface processes. Complementary major sources are biomass burning (19%), microbial release from soil (32%) and lightning.
Indeed, did you know that “a typical thunderstorm flash produces 15 (2–40)×1025 NO molecules per flash, equivalent to 250 mol NOx or 3.5 kg of N mass per flash with uncertainty factor from 0.13 to 2.7″? (Schumann and Huntrieser 2007).
Why shall we observe atmospheric NO2?
Since NO2 has a very short lifetime in the atmosphere and interacts with many other components, it contributes to changes in the atmosphere chemistry. Therefore, the reasons to improve our knowledge of the global distributions of NOx are numerous:
- Exposure to NO2 leads to adverse health impacts;
- The chemical budget of tropospheric O3 (ozone), which leads as well to adverse health effects for humans and stress the vegetation, is largely determined by the concentration of NOx (Jacob et al., 1996). The knowledge of tropospheric ozone distribution and its budget is strongly limited by lack of NO and NO2 observations in the troposphere. Moreover, because quantification of O3 – Ozone is very challenging, tropospheric NO2 products are a real asset;
- NOx are the precursors of (ammonium) nitrate, which is an important component of particulate matter and is also harmful for the health of people;
- NOx contribute to acidification and eutrophication of soils and surface waters. They also play a role in the formation of acid rain. Indeed, within a couple of hours or days, NOx is converted to nitric acid and nitrates, which are subsequently removed by rain and dry deposition.
- Finally, NOx indirectly affect the global climate by affecting OH, and therefore modifying the levels of the greenhouse gases (O3 and CH4 – Methane) (Shindell et al., 2009).
A typical NO2 satellite map?
The map above depicts the concentration of NO2 in the troposphere as retrieved from the Dutch-Finnish OMI satellite measurements (on-board the American NASA EOS-Aura platform). This map is obtained from a multi-year average and shows the main regions contributing to the release of NOx in the low part of the atmosphere (cf. green and red colours).
Green areas over Central Africa are likely related to biomass burning activities.
Furthermore, ship emissions can be observed over seas and oceans (see the light blue – cyan tracks)
Some reference NO2 satellite missions / products?
The DOMINO (Derivation of OMI tropospheric NO2) (Boersma et al., 2011) product contains worldwide concentrations of NO2 in the troposphere derived from OMI. This product is used by a large number of air quality studies (e.g. Curier et al., 2014; Reuter et al., 2014; Ding et al., 2015). The successor will be the QA4ECV product (project PI: Dr. K. Folkert Boersma).
Other NO2 products are available from:
- ESA past mission SCIAMACHY, on-board ESA ENVISAT, early morning (10:00), total & tropospheric NO2 (also available on TEMIS website)
- ESA past mission GOME , on-board ERS-2, total and tropospheric NO2 (also available on TEMIS website)
- ESA past mission MIPAS, on-board ESA ENVISAT, early morning (10:00), stratospheric NO2, early morning
- European (new-generation meteorology program, EUMETSAT) current mission GOME2, on-board MetOp series (A, B & C), total & tropospheric NO2, early morning (also available on TEMIS website)
- Canadian current mission ACE-FTS, on-board SciSat-1, NO2