Atmospheric new particle formation (NPF) supplies up to half of global cloud condensation nuclei, yet the growth of sub-15 nm nanoparticles—the stage most vulnerable to scavenging—remains poorly constrained, largely because the volatility of oxygenated organic molecules (OOMs) is highly uncertain. Using a purpose-built laminar flow reactor that isolates particle–particle coagulation from OOM condensation, we show that six of seven widely used OOM volatility parameterizations substantially overestimate nanoparticle growth rates, with the largest bias under the high-NOx conditions. A recent parameterization constrained by ambient organic aerosol volatility reproduces our laboratory observations across diverse OOM precursors, seed sizes (3–5 nm), and NOx regimes. By applying this laboratory-validated framework to NPF events at Lake Tai, China, in summer 2023, OOM and H₂SO₄ condensation explain about 53 % of the observed 3–15 nm growth rate, leaving a residual that persists even at the upper bound of measurement uncertainty. Together, our laboratory experiments and field observations provide strong evidence that particle coagulation and vapour condensation alone cannot account for ambient nanoparticle growth, revealing a clear gap in our understanding of this process. This gap may point to additional processes, not yet accounted for in current frameworks, that could contribute to nanoparticle growth in polluted atmospheres, or it may reflect uncertainties in other parameters.