Chemically resolved gas- and particle-phase measurements capable of constraining new particle formation (NPF) remain scarce in South Asia. Here, we investigate the precursors driving NPF in a coastal megacity and examine why only a subset of nucleation events evolves into sustained particle growth. We present the first comprehensive 20-day field study in a Bay of Bengal coastal megacity, integrating aerosol microphysical, chemical, gas-phase, and VOC measurements with positive matrix factorization (PMF). Photochemically driven formation of 10–30 nm particles occurred between 07:00 and 11:00 IST, whereas sustained growth into the accumulation mode occurred only 2 of 20 days, with growth rates of 4.53 and 3.43 nm h⁻¹. The particle formation rate, condensation sink (CS), and coagulation sink did not differ between growth and non-growth days, indicating that neither formation intensity nor scavenging distinguished the two regimes. Instead, sustained growth occurred when northerly continental air masses prevailed, whereas growth ceased when a strong sea breeze shifted transport to marine air influenced by power-plant emissions. PMF resolved six factors, including an NPF factor dominated by 10–30 nm particles that was associated with biogenic VOCs. Its invariance between weekdays and weekends, and negative correlation with vehicular traffic factor, indicates that NPF was driven primarily by photo-oxidation of biogenic precursors, rather than by anthropogenic activity. These findings suggest that, in coastal cities, precursor availability and air-mass origin may exert a stronger influence on particle growth than CS, although long-term observations are needed to establish its broader applicability.