Modern satellite instruments—like S-5P/TROPOMI (ESA), active since October 2017—measure spectra of backscattered solar UV and visible radiation with high spatial resolution (7×3.5 km²). From these spectra, we can derive the atmospheric columns (vertically integrated concentrations) of several trace gases. Here, we show a map of the average tropospheric NO₂ column along the Mississippi River, derived from the satellite observations of S-5P/TROPOMI between December 2017 and October 2018 (data-processing and gridding by Christian Borger and Steffen Beirle, Max-Planck-Institute for Chemistry, Mainz, Germany; data source: S-5P/TROPOMI from ESA/Copernicus).

Together, NO and NO₂ are referred to as NO_x, which is emitted from many human activities involving combustion, for example power plants, industrial processes, or car engines. Natural processes like lightning, biomass burning, or soil emissions also produce NO_x. However, in over-densely populated regions, anthropogenic sources usually strongly dominate the observed levels. NO₂ affects human health and is involved in many atmospheric chemical reactions like, for example, the formation of ozone smog.

One important advantage of satellite observations is that they reveal spatial patterns of trace-gas distributions. Since the atmospheric lifetime of NO_x is rather short (usually a few hours), high atmospheric NO_x levels are often found close to strong emission sources like power plants or cities. Thus, from trace-gas maps derived from S-5P/TROPOMI observations, many of these emission sources can be identified and quantified.

The NO₂ map of the Mississippi region shows many such NO_x hot spots. This is to be expected, because rivers usually attract humans and lead to the foundation of settlements, harbors, and marketplaces. Most of these places have high traffic density, which then causes high NO_x pollution levels—especially in city centers. Power plants are also often located near rivers for cooling purposes, and industrial activities benefit from being located next to rivers because of the cheap transport of goods.

All of these sources are responsible for the high NO₂ concentrations along the Mississippi River, as seen in the map from S-5P/TROPOMI. Here, it is also interesting to note that—especially in the southern part of the Mississippi—large areas to the west and east of the river show much smaller NO₂ levels, indicating regions with less human activity.

For comparison with other regions on Earth, maps of the average distribution of NO₂ along the River Nile and Yangtze River are shown (please note the different color scales). Many pollution sources are identifiable along these rivers too, but, compared to the Mississippi region, the pollution levels are much higher. For the River Nile, the high NO₂ values are located in the delta and in a narrow section along the river.