Inter-annual variations in satellite observations of nitrogen dioxide and formaldehyde over India

Abstract

Nitrogen dioxide (NO 2 ) and formaldehyde (HCHO) are important reactive trace gases due to their role regulating the oxidation capacity of the troposphere through ozone (O 3 ) production and because they are pollutants causing health hazards. Over the last two decades, satellite and ground observations have reported a clear increase in the tropospheric HCHO and NO 2 over India, with larger increases over the urban regions due to an increase in anthropogenic emissions. We compare observations from four different satellites for a period covering 1995–2013 over India. Some differences are observed in the calculated growth trends between the different satellites for both HCHO and NO 2 , although an increasing trend is seen by all the satellites. The mean growth rate calculated for the HCHO vertical column density (VCD) is 1.51 ± 0.44% yr −1 , while tropospheric NO 2 VCDs are calculated to calculated to grow faster at 2.20 ± 0.73% yr −1 , indicating that NO x emissions are increasing faster than the emissions for volatile organic compounds (VOCs). The faster increase in NO x as compared to VOCs is however expected considering the large fraction of natural VOCs. The ratio of HCHO/NO 2 , which is an indicator of the relative sensitivity of surface ozone to emissions of nitrogen oxides (NO x = NO + NO 2 ) and VOCs, is also studied. The satellite-derived ratios indicate that over most of India, O 3 production is limited by NO x , although in urban regions and over the Indo-Gangetic Plain, the O 3 production is VOC limited, especially during the winter season. We also compare the modeled HCHO and NO 2 VCDs from the ECHAM5-HAMMOZ model using the RETRO emission inventory and the results indicate that the VOC emissions in the model are overestimated leading to larger modeled HCHO VCDs.