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Basic information on NO₂ and H₂O

Nitrogen dioxide (NO₂) belongs to a group of highly reactive gases known as nitrogen oxides (NO_x). 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, NO₂ is 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 NO_x 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) available at CM SAF. Aerosols (particulate matter) are another important air pollutant causing e.g. many health problems for humans. 
• how large (forest, grass) fires (e.g. in Africa) affect local air quality

Water vapour (H₂O) is a key component of the Earth's atmosphere since it has strong effect on Earth's radiative balance. It is expected that with warming climate the water vapour content in the atmosphere will rise much faster than the amount of total precipitation. Water vapour contributes to the formation of clouds and precipitation, and hence it plays a major role in both meteorological phenomena and climate.  Water vapour also affects the atmospheric composition via the growth of aerosol particles and chemistry e.g. related to the formation of so called hydroxyl radical.

Satellites provide observations on total column water vapour content. Satellites offer a unique opportunity to study spatial and temporal variability of H₂O. Satellite-based H₂O can be used to study

• global scale variation of H₂O, also e.g. combined with other radiation-related parameters