Massive plumes of the gas nitrogen dioxide can be seen hovering in the troposphere (low atmosphere) above China. Nitrogen dioxide is a by-product of fossil fuel combustion. The gas acts as a pollutant and also promotes the formation of low-level ozone, which is harmful to human health. Most major industrial areas have high concentrations of nitrogen dioxide in the surrounding atmosphere, though China's was the worst in October 2010. This image was generated by accumulating all of the data taken during the month of October by the GOME-2 sensor on board the MetOp-A satellite flown by the European Space Agency. NOAA and ESA openly share data from their satellites to help improve the observing and understanding of the Earth's environment.