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 (O3) 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 O3 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 O3 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.