The variety of mechanisms for the creation and destruction of NO_x means that the concentrations of NO_x in the lower troposphere in industrialized areas and near biomass burning sites are very variable.

As a result of the short lifetime of NO_x in the lower troposphere (several hours) observations of boundary layer NO_x contain important information about emissions. Thus well-equipped ground measuring stations routinely monitor NO₂ concentrations. NO can be measured by chemiluminescence. NO₂ is first converted to NO, either on a catalyst or by photolysis. NO₂ can also be measured by long path DOAS and more recently by Lidar.

However, the local variability means that it is difficult to measure the concentrations of NO_x in a global context.

• There are practical limits on the number of point measurements of NO_x that can be made at ground level or from aircraft.
• The high variability of NO_x levels requires a very good spatial and temporal resolution in observations, but, unfortunately, these are not available either on the land, or on the ocean or high in the atmosphere.

Advantages of remote sensing from space

Remote sensing of NO₂ from space offers global coverage combined with a reasonable spatial and temporal resolution of measured data. The entire surface of the Earth can be covered within a few days.

The spatial resolution of these measurements is of the order of 30x60 km for the SCIAMACHY instrument, for example.