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edited Mar 3, 2018 at 20:36

You can see the responses from the creators of OMI for similar questions here:

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.

You can see the responses from the creators of OMI for similar questions here:

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.

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created Mar 3, 2018 at 20:26

f.thorpe ♦

The OMI tropospheric NO2 retrieval algorithm does not provide surface concentrations of NO2, but rather a column density. So, it isn't appropriate to directly correspond those OMI retrievals to health standards that are based on ambient concentrations at the surface. There have been many attempts to correlate surface concentrations of NO2 to tropospheric NO2, but the correlation is fair at best. Furthermore, the tropospheric column inherently changes depending on elevation, since the amount of atmosphere in the tropospheric column decreases as the surface elevation increases. There are also several assumptions that go into the retrieval that create a large amount of error (e.g. stratospheric column subtraction, a priori profiles, surface albedo, etc.).

As I discuss in this post, it is possible to distinguish clean, polluted, and highly polluted tropospheric NO2 columns. However, the exact thresholds are specific to location and sometimes season as well, since the retrieval algorithm responds differently depending on parameter assumptions. In general, if you are seeing values over ~5x10^15 molec/cm^2, you can be sure there is real NO2 pollution. Anything over 1x10^16 molec/cm^2 is highly polluted. The higher the values are, the more confidence there is in the retrieval.