Estimation of the lifetime of nitrogen oxides over India using SCIAMACHY observations

The NOx lifetime is needed to determine NOx source strength and allow for source apportionment studies. Satellite observations of tropospheric column NO2 may be used to estimate the NOx lifetime by analysing tropospheric NO2 decay in the outflow region. In this study, we used Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) observations of tropospheric NO2 to quantify the lifetime of NOx, ‘τNOx ’, over the Indian region. We analysed a tropospheric NO2 plume that originated over the city of Mumbai and its downwind evolution over the Arabian Sea as a function of distance (decay curve). The observed pattern of decay curve is fitted with the exponential decay fit model, which gives an e-folding distance of 107 km. The fit result yields a mean lifetime of NOx of about τNOx = 10.2 h based on SCIAMACHY observations. In addition, we quantified τNOx using CHIMERE chemical mechanisms and compared the result with the satellite-based estimation. The estimated τNOx for India, based on CHIMERE simulation, is about 11.9 h, comparable to the τNOx calculated by satellite observations.