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, 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 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 CO to study for example the difference in combustion efficiency for various processes.
- how large (forest, grass) fires (e.g. in Africa) affect local air quality
As NO2 is also measured with GOME-2 (on Metop), one could compare the difference between NO2 as measured with TROPOMI and GOME-2. GOME-2 data is accessible through the AC SAF data.
Carbonmonoxide (CO) is produced during incomplete combustion. Similarly to NO2, the main man-made sources of CO are burning of fossil fuels (coal, oil, gas), traffic, biomass burning, and some industrial processes. CO itself is an air pollutant, but -more importantly- it is a precursor of ozone, a much more harmful air pollutant. Moreover, CO is the main sink of OH which is the cleansing agent in the atmosphere. As such, CO affects the lifetime of gases such as methane which is an important greenhouse gas.
Satellites that measure radiation in the SWIR part of the spectrum can be used to observe CO. The quantity provided by such measurements is the total column CO. As the lifetime of CO in the atmosphere is a few weeks to a few months, CO is a very useful tracer of pollution transport in the atmosphere.
The total CO column can be used to study :
- where the locations of high emissions are (e.g. cities, industrial areas). CO can also be used as a proxy for air quality
- how pollution is transported
- the effect of large fires (e.g. Africa, Amazone, USA, Canada, India, …) on air quality