The global economic cycle and satellite-derived NO2 trends over shipping lanes

In recent years, space-borne spectrometers have been used to detect shipping emissions of nitrogen oxides. Driven by economic growth, these emissions have been increasing for several decades, yet in few studies it has been attempted to detect trends in ship emitted NO2 from space. Here a method is presented that enhances the shipping signal in satellite measurements of NO2, which makes it possible to detect non-linear trends on a monthly to yearly basis. The method removes variations in NO2measurements over shipping lanes that are not related to shipping and that obscure shipping trends. With this method we could detect non-linear trends in NO2over major shipping lanes in the Mediterranean Sea, the Red Sea, the Indian Ocean and the South Chinese Sea. The shipping signal displays a large increase of 62–109% between 2003 and the summer of 2008 and a sharp decline of 12–36% afterwards, corresponding to the global economic recession of 2008-2009. These two trends are detected over all four shipping lanes by several space-borne spectrometers. Because of high correlations between satellite data mutually and between satellite data, shipping statistics and international trade volumes, we conclude that the detected trends are caused by actual changes in shipping emissions. This study therefore shows that it is possible to detect short-term economic fluctuations in satellite measurements of NO2 over major shipping lanes.