What is driving the diurnal variation in tropospheric NO2 columns over a cluster of high emission thermal power plants in India?

We investigated the diurnal variation of tropospheric nitrogen dioxide (NO2) over a cluster of high emission thermal power plants (HETPPs) in India using two products from satellite-based UV/Visible spectrometers together with a chemistry-transport-model. The different overpass times of the satellite spectrometers: SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) (1030 h LT) and Ozone Monitoring Instrument (OMI) (1330 h LT)), enable detection of changes in NO2 columns at two different times of the day, providing some insight on its diurnal variation. Observations show elevated tropospheric NO2 columns in the afternoon compared to the morning in the vicinity of the HETPPs (~65% higher), contrary to the expected decrease due to enhanced photochemical loss of NO2 in the afternoon and increased concentrations in the morning due to peak-hour vehicular emissions. The observed diurnal variability was simulated using a regional atmospheric composition model and the deviations in nitrogen oxide (NO), NO2, ozone (O3) and hydroperoxy radical (HO2) between 1030 h and 1330 h within the planetary boundary layer (PBL) were examined. We also examined the vertical and horizontal accumulated tendencies of NO2 (advection) and wind profiles directly over the emission. The results show that the vertical variability of chemical loss processes, driven by a change in vertical mixing of NO2 within the PBL and variability in advection (driven by winds) leads to a net increase in the afternoon NO2 columns over the HETPPs, even if we assume flat emissions throughout the day. These results suggest that meteorology along with PBL evolution affects the diurnal evolution of NO2 columnar abundance over locations with high emissions in India.