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.

Acknowledgements

We thank the Director, IITM, for encouragement during the progress of the work. Thanks are also due to www.temis.nl for providing tropospheric NO₂ data and CPCB, New Delhi, for surface air NO₂ data. P.S.K. received finance through the ‘SPATRAM-MIGE Polar Project’, funded by the Portuguese Science Foundation, FCT.