Basic information on NO2, Ozone and absorbing aerosol index


Nitrogen dioxide (NO2) belongs to a group of highly reactive gases known as nitrogen oxides (NOx). The main man-made sources of NO2 are burning of fossil fuels (coal, oil, gas), traffic, biomass burning, and some industrial processes. NO2  can act  as a precursor gas for the formation of surface-level ozone pollution, it also contributes to the formation of acid rain and photochemical smog. High NO2 concentrations are also unhealthy for humans e.g. causing respiratory problems.  Hence, NOis an air pollutant.  Natural sources of NO2 include  e.g. lightning, but the contribution is only minor compared to the anthropogenic emissions. 

Satellites that measure radiation at UV/VIS part of the spectrum can be used to observe NO2. There are essentially two parameters that are retrieved from satellite measurements: total column NO2 and Tropospheric column NO2.  The residence time of NO2 in the lower Troposphere is short, typically about few hours, and therefore there is a direct link between satellite-based NO2 columns and actual NOx emission strengths from anthropogenic sources. 

The Tropospheric NO2 column can be used to study

  • where the locations of high emissions are (e.g. cities, industrial areas). NO2 is overall a good proxy for air quality.
  • how emissions /air quality have changed over time (months, years) globally or at specific location. NO2 can also be analysed together with aerosol optical depth (AOD). Aerosols (particulate matter) are another important air pollutant causing e.g. many health problems for humans. 
  • how large (forest, grass) fires affect local air quality.

In AC SAF, Tropospheric NO2 observations are provided as Level 2 NRT and Offline products as well as Level 3  gridded global Data Record, disseminated via DLR ATMOS data service. 


Tropospheric Ozone  is a major air pollutant and a greenhouse gas.  Tropospheric ozone is a so called secondary pollutant, since it is not emitted directly into the atmosphere but formed in the atmosphere when precursor gases such as NOx, react with solar (UV)  radiation. It is very harmful for crop production and human health.  Tropospheric O3 has also a warming effect to the climate.  The tropospheric  provided for tropical region, extending from 20S up to 20N. This is because  the retrieval is done with a convective cloud differential (CCD) algorithm, where the stratospheric ozone column is estimated as the column above high reaching convective clouds. The tropospheric column is achieved from subtracting the stratospheric part from the total column. 

The tropospheric ozone can be used to study

  • Long-term changes in O3 in the tropical regions
  • Analysing O3 changes with NO2 data record, that is an important precursor for O3


Absorbing Aerosol Index (AAI)  indicates the presence of elevated absorbing aerosols in the troposphere like desert dust, smoke, and volcanic ash. AAI separates the spectral contrast at two ultraviolet (UV) wavelengths caused by absorbing aerosols from that of other effects, including molecular Rayleigh scattering, surface reflection, gaseous absorption and aerosol and cloud scattering. AAI is a unitless quantity, positive values of AAI indicating elevated amount of aerosols present in the atmosphere. It should be noted, however, that AAI values depend on several factors: aerosol loading, layer height, and type. This should be considered when comparing e.g. different episodes.  

The AAI observations can be used to study 

  • Transport of smoke from fires, combined e.g. with IASI CO
  • Dust storms
  • Dispersion of ash from volcanic episodes
  • Air quality events where absorbing aerosols are present

From GOME-2 observations AAI is provided as Offline Level 2 data, disseminated via Finnish Meteorological Institute data service. There are two kind of products available, "AAI" and "AAI from PMDs", that differ by their spatial resolution.  The Spatial resolution of "AAI" is equal to the spatial resolution of the GOME-2 instruments: 40 km x 40 km for Metop-A and 80 km x 40 km for Metop-B. The spatial resolution of AAI from the Polarisation Measurement Detectors (PMDs) is equal to the spatial resolution of the small field-of-view PMD detectors: 5 km x 40 km for Metop-A and 10 km x 40 km for Metop-B. 

Last modified: Monday, 21 October 2019, 1:32 PM