In recent years, the role of ship emissions has attracted increasing attention. Recent inventories estimate that they contribute significantly to anthropogenic emissions of nitrogen oxides, with a large impact on composition and chemistry of the marine boundary layer. Nevertheless, the number of observational studies is still rather small. Here we present the first detection of ship tracks in NO2 maps derived from satellite data. We have used the congested track connecting Sri Lanka to Indonesia to estimate the corresponding ship emissions of nitrogen oxides to 23 (10--73) Gg /yr. In addition, we were able to derive the mean lifetime of boundary layer NOx to be 3.7 (1.9--6.0) hours in the respective region. Our estimates are in good agreement with ship emission inventories (about 22--54 Gg /yr) as well as the short lifetimes (of down to 2 hours at daytime) as recently modeled for ship plumes.