Temporal variation of urban NO x concentration in India during the past decade as observed from space

Abstract

Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) tropospheric NO2 retrievals (average during March–April–May 2003–2006) were used to locate the major urban sources of NO2 in India. To show that tropospheric NO2 measurements from satellites can provide useful information on day-to-day changes in urban NO2 concentration, we compared 2-year (October 2004–October 2006) records of surface air NO2 concentrations obtained at one of the measurement sites in Delhi to retrievals of tropospheric NO2 columns from Ozone Monitoring Instrument (OMI) observations. The quality of correlation between satellite observations at the urban scale and surface measurements (45%) provided confidence to investigate the temporal changes in the local emission over major urban locations. Combined Global Ozone Monitoring Experiment (GOME) and SCIAMACHY measurements for the period March 1996–December 2007 were used to compute the trends over the selected urban locations. A statistical analysis was applied to derive trends for this period over these locations. The time series of the monthly NO2 columns for these 11 years were fitted with a linear function superposed on an annual seasonal cycle. We found a statistically significant increase in tropospheric NO2 levels over the major urban cities in India. Notable urban locations of positive trends were found to be the cities where the urban population is growing rapidly.