Application of satellite observations for identifying regions of dominant sources of nitrogen oxides over the Indian Subcontinent

We used SCIAMACHY (10:00 LT) and OMI (13:30 LT) tropospheric NO₂ columns to study diurnal and seasonal patterns in NO₂ concentrations over India. Using characteristics of seasonal variability in tropospheric NO₂ columns, we present a simple methodology to identify the dominant NOx source category for specific regions in India. Regions where the dominant source category is classified as biomass burning are found generally to agree with the ATSR fire count distribution. Relating OMI NO₂ columns to surface NOx emission, we find that biomass burning emission account for an average flux of 1.55×10¹¹ molecules cm⁻²s⁻¹ during the peak burning period. Furthermore, extrapolating this estimated flux to the total burned area for the year 2005, biomass burning is estimated to account for 72 Gg of N emissions. Additional analysis of fire events in Northeast India shows a marked increase in TES retrieved O₃ concentrations, suggesting significant photochemical ozone formation during the peak biomass burning period. Regions where the dominant source type was categorized as anthropogenic are in good agreement with the distribution of major industrial regions and urban centers in India. Tropospheric NO₂ columns over these anthropogenic source regions increased by 3.8% per year between 2003 and 2011, which is consistent with the growth in oil and coal consumption in India. The OMI-derived surface NO2 mixing ratios are indirectly validated with the surface in situ measurements (correlation r=0.85, n=88) obtained from the air quality monitoring network in Delhi during August 2010 to January 2011. Most of the OMI-derived surface NO₂ values agree with surface-based measurements, supporting the direct utility of OMI observation for emission estimates. Finally, we use OMI NO₂ columns to estimate NOx emissions for selected large cites and major thermal power plants in India and compare these estimates with the INTEX-B and EDGAR emission inventory. We find that, for a few locations, OMI-derived emission show fair agreement; however, for many locations, NOx emissions differ from INTEX-B and EDGAR inventories.