Abstract: Of the data through diurnal observation using a high building in Yanta District, South Xi’an in March 2013, analysis was done for vertical variation of ρ(NO_x) in the surface layer atmosphere and its influential factors. Results show that during the observation period, the daily mean of ρ(NO_x) was 0.100 toand 0.120 mg•m^-3 and did not vary much from day to day; ρ(NO_x) increased with the elevation and peaked at 53-65m, but the trend varied temporally; at ≤29 m it peaked during the period of 14:00 – 16:00, and bottomed  during the period of 22:00 – 00:00; at 35-65 m, it kept rising during the day time, peaked at 20:00, dropped to the lowest at 22:00 and then turned slowly upwards again; and at 71-83 m, it dropped to the bottom during the period of 22:00 to 00:00 and bounced back rapidly. It was held that ρ(NO_x)varied under the integrated impact of anthropogenic emissions, atmospheric stability, weather conditions, urban canopy and other meteorological elements and was negatively related to air pressure and wind speed, and positively to air temperature.

Key words: nitrogen oxide, vertical distribution, diurnal variation, meteorological element, Xi’an city