Mobile DOAS (Differential Optical Absorption Spectroscopy) circular measurements of tropospheric nitrogen dioxide (NO₂) were performed on a number of days in 2012 and 2014–2016 around St.Petersburg. These observations figured out an evolution of urban pollution plume, released from the megacity. The HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectories) model was configured to simulate the observed NO₂ dispersion, taking into account the municipal inventory database of urban nitrogen oxides (NO_x = NO+ NO₂) emission. The simulation results were found consistent with the data of mobile measurements, allowing to fit an estimate of integral NO_x emission rate from St.Petersburg (about 60 kt year⁻¹ on the average). Coincident satellite measurements the Aura Ozone Monitoring Instrument (OMI) of tropospheric NO₂ mostly agree with the data of simulation on the days of mobile observations. In general, presented results demonstrate the capabilities of joint interpretation of disparate tropospheric NO₂ data in the vicinity of a megacity.