Thermodynamic origin of atmospheric new particle formation: Theory for stable nucleating pairs

Atmospheric new particle formation (NPF) has been observed across virtually all tropospheric environments, yet the meteorological conditions that favor or suppress it have resisted clear theoretical resolution. Here we show that, for neutral chemistries in which a strongly bound molecular pair constitutes the critical cluster, the thermodynamic precondition for NPF is set by a single quantity: the effective binding free energy of that pair, [Formula: see text], which captures the competing influences of temperature and relative humidity in a closed-form analytical framework. Applied to the sulfuric acid-dimethylamine system in the boundary layer, the theory reproduces observed clustering across laboratory experiments and field campaigns spanning a polluted megacity, a remote high-altitude site, and an ultra-pristine boreal peatland, all without kinetic fitting parameters; precursor concentrations and the condensation sink enter as separable factors that govern the subsequent production rate. The framework reveals an optimal [Formula: see text] that maximizes cluster production, providing a thermodynamic basis for why NPF peaks at midday, favors spring over other seasons, occurs under heat wave conditions despite temperatures approaching [Formula: see text]40 ^°C, and can even arise on rare evenings in near-pristine conditions. More broadly, the theory exposes the inherent relationship between atmospheric conditions and the earliest steps of particle formation, offering a general interpretive tool that complements and extends what observations and simulations alone cannot provide.