Frequently asked questions on OMI tropospheric nitrogen dioxide measurements

On the significance of the measurements

  • Does the red colour signal smog at the surface?
    • No, since OMI does not measure the concentration at the surface directly. Instead, OMI measures the total amount of nitrogen dioxide (NO2) in a column up to about 10 km.

    • However, research has shown that high NO2 column amounts in OMI measurements (red colours) usually coincide with high concentration levels at the surface. This concentration is usually not as high that you can it moderate or heavy smog, but it does still indicates an elevated level of air pollution. The OMI measurements clearly show that The Netherlands suffers from a significantly higher level of air pollution by NO2. See also the answers to the next questions.

    Note that smog does not exist only of NO2. Other important ingredients are aerosols, ozone, sulphur dioxide and hydro carbons.

  • The OMI map shows a red colour over The Netherlands but according to surface measurements of the Dutch National Institute of Public Health and the Environment (RIVM) it just minor smog. What causes the difference?
    The surface measurements from RIVM are related to the European norms for smog. Those norms are related to a peak level surface concentration. The threshold level is an average concentration of 200 microgram/m3 NO2 during one hour. The peak levels should not exceed this threshold more than 18 hours per year. Peak levels concentrations are most of the time well below the threshold values and therefore one speaks of minor smog.
    OMI does not measure the concentration at the surface directly. Instead, OMI measures the total amount of nitrogen dioxide (NO2) in a column up to about 10 km. For this column amount there are no direct European norms. The relation between column amounts as measured by satellites and surface concentrations is complicated. Therefore, a red coloured area in the OMI map cannot be related directly to European norms and no conclusion with respect to exceeding some norm can be drawn from the OMI maps.
    The colours in the OMI maps are chosen in such a way that you can clearly see the difference between different areas. Column amounts of 20 x 10 power 15 molecules per cm2 and higher all have same red-purple colour.

  • Can OMI measurements be related to ground level concentrations, like those being measured by the Dutch National Institute of Public Health and the Environment (RIVM)?
    The largest contribution to the measured NO2 column amount is located in the boundary layer. This is the bottom layer of the atmosphere in which air is mixed quite rapidly and in which the NO2 concentrations are relatively constant. The height of boundary layer varies between a few hundred meters till a few kilometres. The height varies during the day and depends on the weather and season of the year. A rough estimate of the NO2 surface concentration can be calculated by dividing the column amount with the height of the boundary layer.
    A study, using a chemical model of the atmosphere, showed that NO2 measurements by satellites are consistent with surface measurements and that the red-purple coloureds area roughly have surface concentration of 40 microgram/m3 NO2. This value is similar to the European threshold norm for the year-average concentration.

  • What is the trend in NO2 air pollution in The Netherlands?
    The European threshold norm for the year-average NO2 concentration was exceeded only along busy main roads and occasionally in urban city centres during 2003 and 2004. The year-average NO2 concentration has been decreasing on average with 2% per year during the last 10 years. (Source: MNP/RIVM). See graphical representation.

on the colours and numbers of the maps

  • What is the meaning of the colours and numbers?
    OMI measures the total amount of nitrogen dioxide in the troposphere. The troposphere is the lower part of the atmosphere, from the surface till around 10 km. The unit is the number of NO2 molecules within a small, high column from 1 cm times 1 cm horizontally, from the surface till 10 km. Column amounts of 20 x 10 power 15 molecules per cm2 and higher all have same red-purple colour.

  • Why aren't there measurements during rush hours?
    The EOS Aura satellite, of which OMI is part of, circles around the world in a so-called polar orbit, which enables daily global measurements. The satellite passes over an area each day at about the same time, which is around one o'clock in the afternoon. So OMI does not measure during rush hours.
    However, the lifetime NO2 in the boundary layer is typical one hour at the surface till a few hours when transported to higher altitudes by wind and turbulence. So part of NO2 produced during the rush hour is still around at the time OMI passes over. Therefore, OMI will usually measure a large part of all the NO2 produced during the whole morning and the early hours of the afternoon. Because OMI measures over a wide area, successive orbits often overlap above The Netherlands, giving us two measurements in the afternoon 100 minutes apart (see also the next question).

  • Some areas seem to overlap. Why is that?
    OMI measures not only the directly below the satellite, but also over a wide area to the side, covering about 2600 km at the ground. At higher latitudes, like in The Netherlands, those wide views of successive orbits overlap, so we often have two measurements in the afternoon above The Netherlands some 100 minutes apart.
    The 2600 km wide area that OMI covers at the ground is divided in 60 measurement areas (ground pixels). Those ground pixels are the smallest direct below the satellite (13 km long x 24 km wide) and they are become wider towards the outer edges. If both measurements are good (i.e. there were no clouds blocking OMI view at that moment), the measurement with the smallest ground pixel is plotted on top. Otherwise, the only usable measurement is plotted.

  • Other questions

    • Who and where is Nitrogen Dioxide formed?
      Nitrogen oxide (NOx) is the generic name for a group of highly reactive gasses consisting of nitrogen (N) and oxygen (O). Nitrogen oxides are formed at high temperatures when N2 and O2 react in the air. These high temperatures exist for example in gasoline and kerosene engines, in industrial processes, in burning of natural material (biomass) and in lightning. In The Netherlands, the largest contribution to the emission of NOx is made by traffic (65%). Other important sources are industry and production of electricity using fossil fuels.

    • What are health risks?
      Nitrogen dioxide is contributing to air pollution. Other important air pollutants are aerosols, ozon, sulphur dioxide and hydrocarbons. For an overview of the way NOx influences human life and the environment, see this information page from the American Environmental Protection Agency (EPA).

    • On Sundays there seems much less air pollution in The Netherlands by nitrogen dioxide than on other days. Why is that and where did the NO2 disappear to?
      The lifetime of nitrogen dioxide is rather short. In the atmosphere, NO2 is being converted to HNO3 and N2O5. These gasses are easily dissolved in rain drops (acid rain) and rain down to the surface. The lifetime of NO2 varies from one hour at the surface to several days high in the atmosphere (when NOS is being transported by wind and turbulence to higher altidues). In the OMI maps we can clearly see that NO2 is being transported over hundreds of kilometres. On Sunday, most NO2 produced on Saturday has already disappeared and since there is not much traffic during the morning, much less NO2 is being produced.

    • Which areas have on average less air pollution?
      Considering air pollution by NO2, this can be seen on the year averaged maps based on measurement made by SCIAMACHY. This Dutch- German-Belgium instrument satellite instrument is part of ESA's ENVISAT mission and has been in action a few years longer than OMI (since 2002). The year average map can be found at the TEMIS web site.
      The measurements of OMI and SCIAMACHY show that The Netherlands have on average a higher NO2 air pollution than most other parts of Europe.
      Note that NO2 is an important contributor to air pollution but not the only one. Other important ingredients are aerosols, ozone, sulphur dioxide and hydro carbons.

    • What else is OMI doing?
      OMI is one of four instruments on-board NASA's EOS-Aura satellite, launched on 15 July 2004. The aim of the Aura satellite is to observe atmospheric chemistry, and specifically to monitor the ozone layer, the Earth's climate and air pollution. OMI measures ozone, several other trace gases (nitrogen dioxide, aerosols (fine dust particles), sulphur dioxide, chloride dioxide, bromine oxide, formaldehyde) as well as clouds and ultraviolet radiation at the surface. All those gases can be monitored worldwide on a daily basis and at a city size scale, thanks to OMI's wide observing viewing angle and small pixels.