Simultaneous Observations of Nitrogen Dioxide, Formaldehyde and Ozone in the Indo-Gangetic Plain

This study reports the concentrations of nitrogen dioxide (NO2) and formaldehyde (HCHO), retrieved using the Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique and collocated observations of surface ozone (O-3) conducted over the Indo-Gangetic Plain (IGP) during the 2014 monsoon period as part of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX). The average daytime NO2 mixing ratio was 0.81 +/- 0.20 ppbv (parts per billion by volume) (range: 0.08-6.06 ppbv). NO2 was observed to decrease during the morning between 06:00 and 09:00 local time and then stabilise for the rest of the day. The average daytime HCHO mixing ratio was 1.93 +/- 0.60 ppbv (range: 0.32-8.81 ppbv). Unlike NO2, HCHO, driven by daytime photochemical formation from hydrocarbon precursors, increased during the early morning. The average O-3 mixing ratio was 30.0 +/- 13.0 ppbv (range: 2.7-81.9 ppbv) during the daytime and 22.5 +/- 10.2 ppbv (range: 1-63 ppbv) during the nighttime. Analyses of the back trajectories indicatedfound that the NO2 mixing ratios during CAIPEEX-2014 were affected by long-range transport from thermal power plants situated about 110 km to the south but the HCHO mixing ratios and O-3 production were influenced by local emissions. These observations suggest that in rural IGP, ozone concentrations are affected by local emission rather than by long-range transport.