Nitrogen dioxide (NO2) is a key component of urban air pollution. The nitrogen oxides ("NOx" of which NO2 is one component) are emitted from any combustion process. Coal- and gas-fired power plants and vehicles constitute the major anthropogenic (human-produced) sources. Forest fires and lightning are natural sources of NO2, but globally it is clear that anthropogenic sources dominate.
High levels of NO2 are significant as they are associated with: 1) haze that reduces visibility; 2) irritation of the eyes, nose, throat, and lungs; 3) acid rain; 4) reduced terrestrial plant growth; 5) oxygen-depleting algal blooms; and 6) corrosion of building materials.
Because of the importance of trace pollutants such as NO2 for air quality and human health around the world, instruments have been developed and installed on research satellites to measure NO2 on Earth. Collaborative teams from space agencies around the world build the sensors and retrieval algorithms to make these space-based images available to the world scientific and policy-making communities.
This animation shows a time series of NO2 measured from such satellites from October 2004 to December 2009. These particular images were captured by the Dutch-Finnish Ozone Monitoring Instrument (OMI) on NASA's Earth Observing System AURA satellite. One of the objectives of the AURA mission is to measure trace pollutants such as NO2, ozone, carbon monoxide, and aerosols by their interaction with light of visible and non-visible wavelengths. These data have been converted to false-color images so the viewer can "see" hot spots in NO2 concentration around the globe.
NO2 is a pollutant with a relatively short atmospheric lifetime, so it does not get transported far from its source. Thus, these satellite images provide a direct indication of where NO2 sources are located. In remote, unpolluted regions of the globe, NO2 will be uniformly low.
Several striking observations can be made from this global view of the NO2 air pollutant.
First, the dominant NO2 source regions correspond primarily to areas with high population and large industry, for example, eastern China, northern Europe, and the eastern United States. Because most electricity is produced by burning fossil fuels in power plants that emit NO2, these hot spots are strongly correlated with electricity usage and fuel sources.
Second, urban centers show up as smaller hot spots. High vehicle traffic leads to elevated NO2. For example, Mexico City, Tokyo, and Los Angeles are all clearly marked by their NO2 plume. If one zooms in to California, the north-south transit corridor of the I-5 freeway is actually visible from space via its NO2 signature.
Third, a clear seasonal pattern is apparent. In the northern hemisphere winter, peak NO2 is much higher in all the northern hemisphere hot spots. This is attributed both to heavier use of combustion power plants for wintertime home heating, as well as the fact that NO2 stays in the air longer in the winter. The atmospheric lifetime of NO2 is driven primarily by reactions initiated by sunlight. With less sunlight in the wintertime, reactions that break down NO2 are not easily initiated, and the NO2 is removed more slowly from the atmosphere. Urban areas report NO2 air quality standard "exceedance" events, when the level of this pollutant is higher than deemed safe by environmental agencies. This happens most frequently during the cold winter months.
Similarly, one sees hot spots over urban centers in the southern hemisphere during their local winter (June - September). In addition, some years see spread out elevated regional NO2 over southern Brazil and sub-Saharan Africa, typically peaking in September. This is the signature of "biomass burning," when large swathes of forest burn in those regions. Some seasonal burning is apparent in northern Australia as well. These are significant contributions to southern hemisphere NO2 but are dwarfed in comparison to northern hemisphere industry.
Finally, we can look for evidence of a long-term trend in regional NO2 emissions. Comparing December 2009 to December 2004, there is higher regional NO2 in China compared to North America or Northern Europe in 2009. This correlates with China's booming economy in recent years. In December of 2004 the relative amounts of NO2 over these regions was more consistent. This can be attributed both to economic activity as well as to effective policy efforts to reduce NOx emissions in some countries.
These satellite datasets provide a rich basis for interpretation of urban/rural, seasonal, and decadal (long-term) trends in air quality.
Boersma, K.F., H.J. Eskes, J.P. Veefkind, E.J. Brinksma, R.J. van der A, M. Sneep, G.H.J. van den Oord, P.F. Levelt, P. Stammes, J.F. Gleason and E.J. Bucsela, Near-real time retrieval of tropospheric NO2 from OMI, Atm. Chem. Phys., 2013-2128, sref:1680-7