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

Nitrogen dioxide (NO2) 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 NO2 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 NO2, 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 NO2 VCDs are calculated to calculated to grow faster at 2.20 +/- 0.73% yr(-1), indicating that NOx emissions are increasing faster than the emissions for volatile organic compounds (VOCs). The faster increase in NO as compared to VOCs is however expected considering the large fraction of natural VOCs. The ratio of HCHO/NO2, which is an indicator of the relative sensitivity of surface ozone to emissions of nitrogen oxides (NOx = NO + NO2) and VOCs, is also studied. The satellite-derived ratios indicate that over most of India, O-3 production is limited by NOx, 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 NO2 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. (C) 2015 Elsevier Ltd. All rights reserved.