We present mobile differential optical absorption spectroscopy (DOAS) zenith-sky observations of tropospheric NO2 from field experiments carried out at St Petersburg (Russia) on a number of days during May-October of 2009-2012. We conducted a detailed analysis of our measurements on a closed route around the entire city that took place on 14 August 2012. We used the data from these circular observations to derive our top-down estimate of total NOx emission from St Petersburg on that day, i.e. (12.6 +/- 2.4) x 10(24) molecules s(-1). This value, if assumed to be constant throughout the year, would correspond to a total emission rate of (31 +/- 6) kt/year (calculated on condition that all NOx was NO2). Our estimate is half of that reported in a published official inventory of St Petersburg (63 kt/year). Possible reasons for that discrepancy and an outlook for further improvements are considered and discussed further.