Parts of this WebPage are extracted from Chimot, J., Global mapping of atmospheric composition from space – Retrieving aerosol height and tropospheric NO2 from OMI, PhD book, Delft University of Technology (TU Delft), The Royal Netherlands Meteorological Institute (KNMI), July 2018.
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. Nitrogen oxides NOx are a family of molecules built upon nitrogen and oxygen atoms, the most abundant elements in the atmosphere. Their relative abundance is very low, less than one part per million. Nevertheless, scientific interest is driven by their crucial influence on the tropospheric part where humans live.
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?
The main NOx sources are of two types, all related to combustion processes:
- Thermal: at temperature above 2000 K, dioxygen molecules thermolyze (e.g. in combustion chambers, biomass burning or with lightning) leading to a sub- sequent reaction between oxygen atoms and nitrogen N2 molecules:
O2 + (heat) ←→ 2O
O+N2 ←→NO+N
N+O2 ←→NO+O
- Fuel: oxydation of nitrogen-bearing fuels such as coal and oil releases the nitrogen N bound as a free radical.
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. In particular, NO2 enhances the effect of allergens, bronchial reactivity and increases admissions for respiratory disease (Sunyer et al., 1997);
- 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 the semi-volatile (ammonium) nitrate NH4NO3, 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 have complex and diverse effects on climate. On the one hand, they lead to a warming effect due to the greenhouse nature of O3 (Ramanathan et al., 1985). On the other hand, they also lead to a cooling effect, notably via the secondary aerosol formation. Moreover, high concentration of NOx leads to high abundance of OH reducing then the levels of the greenhouse gases O3 and CH4 – Methane (Shindell et al., 2009). Overall, the average resulting impact of NOx is an indirect cooling effect on our climate, but at the detriment of a poor air quality (Shindell et al., 2009).
As mentioned above, NOx play a central role in the production of tropospheric O3. This occurs via the photolysis of NO2 by sunlight at wavelengths ≤ 420 nm:
NO2 +hν→NO+O(3P)
O(3P)+O2 +M→O3 +M
M is any other molecule. After this photostationnary equilibrium, NO and O3 are usually consumed to form NO2 again:
NO+O3 +M→NO2 +O2
However reactions above do not lead to a net ozone production. Alternative reactions involving the hydroperoxyl radical HO2 allow the conversion of NO into NO2 without oxidizing O3:
HO2 +NO→NO2 +OH
The availability of HO2 guarantees the extra-production of O3. The production mechanisms of HO2 are usually ensured by the presence of CO – Carbon monoxide and the incomplete combustion of hydrocarbons and/or the oxidation of volatile organic compounds (VOCs). Other multiple stage reactions also occur when NO molecules encounter CO – Carbon monoxide and O2, CH4 – Methane and O2 or non-methane hydrocarbons. For all these reactions, NO molecules evolve to NO2 without interacting with O3.
The regime of tropospheric O3 can be then limited by a reduction in NOx as well as CO or hydrocarbon, or a decrease of sunlight (e.g. in wintertime). Unfortunately, these trace gases are very abundant in city centres (Sillman et al., 1990). It also critically depends on the vertical distribution of NOx , small amount of NOx ending up in the free troposphere has a much longer lifetime and therefore a much stronger ozone production potential.
A significant part of the complexity of tropospheric chemistry is also related to the free radical OH which governs the oxidation of our air, and therefore subsequent transformation of chemical species either naturally released by biomass or issues from anthropogenic activities. OH has an average lifetime of 1 second and is sometimes called “detergent of the atmosphere” due to its strong oxidizer capability. OH is a particularly reactive specie and an important sink of the green-house gas CH4 – Methane via the following slow reaction (Burrows et al., 2011):
OH + CH4 → CH3 + H2O,
and also reacts with CO – Carbon monoxide and other hydrocarbons leading to the production of HO2 :
OH+CO+O2 →CO2 +HO2.
NOx does not only trigger tropospheric O3 production but also influences OH abundance. Indeed, O3 is the primary source of OH in the troposphere in the presence of water vapour and sunlight with wavelengths ≤ 320 nm.
NO can also create OH by itself, in presence of HO2. Therefore, NO catalyses oxidation by OH which can be seen as a way to prevent toxic levels of CO – Carbon monoxide and hydrocarbon concentrations in the atmosphere. However, at high NOx concentration levels, NO2 removes OH by forming HNO3 leading then to the formation of semi-volatile ammonium nitrate aerosols in the presence of ammonia NH3.
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).
DOMINO and QA4ECV include both stratospheric and tropospheric NO2 products & are based on the OMI measurements acquired in the mid-day (about 1:45 pm). They can be downloaded on the TEMIS website.
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
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