While high concentration of pre-existing particles tends to inhibit atmospheric new particle formation (NPF), the severely polluted megacities around the world are becoming hot spots of NPF. Measurements of particle number-size distributions using scanning mobility particle sizer (SMPS) showed that the particle bursts observed on 82% of all observation days, indicating that a frequent particle formation from gaseous precursors can occur albeit the relatively high concentrations of pre-existing particles in an urban environment, Kanpur, India. During the NPF events, the number concentration of Aitken-mode particles constituted more than 50% of the total particle mass. Further, using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), we examined the evolution of chemical features in sub-micron particles during these events. It is worth mentioning that AMS can not detect particles less than 40 nm diameter and therefore it is not possible to assess the chemistry driving NPF. Our results indicate that the oxygenated organic aerosols constituted the largest fraction of sub-micron particles (nearly 77%). The tracer of HOA, m/z 57 (C4H9+) ion, also showed the significantly enhanced signal intensity during all NPF event days. Moreover, the higher fraction of organic ion, m/z 44 (CO2+), suggested the presence of less-volatile, highly oxidized OAs (LV-OOAs) during the NPF event days, indicating that the growth of new particles was mainly driven by the condensation of low-volatility organic species. The significantly enhanced signal intensity of nitrogen-containing organic ions (i.e. CHN+, CH4N+, C2H4N+, C3H8N+, C5H12N+) in the sub-micron aerosols during NPF events compared with non-NPF events further suggested that amines might have played an important role in these events. Thus, our findings underline the significance of nitrogen-containing compounds in secondary aerosol formation processes in severely polluted urban environments.