Role of Low-Volatility Oxygenated Organic Molecules in Urban Aerosol Nucleation in Houston, Texas

Atmospheric new particle formation (NPF) is an extremely complex chemical process that produces aerosols directly from gas phase species, impacting air quality, human health, and climate. Observations have shown frequent NPF in polluted urban areas even with high preexisting aerosols, but it is unclear how low-volatility oxygenated organics contribute to urban aerosol nucleation. Urban NPF studies in Chinese megacities show that urban nucleation takes place from sulfuric acid and dimethylamine, yet the measured aerosol nucleation rates cannot be explained with sulfuric acid and dimethylamine alone. There are abundant oxygenated organic molecules (OOM) in Chinese megacities, but these OOMs primarily form from oxidation reactions of aromatic compounds and the majority of OOMs contain nitrates and the volatilities of OOMs are not sufficiently low enough to be able to nucleate. To understand the role of low-volatility OOMs in urban aerosol nucleation, we conducted comprehensive measurements of NPF precursors in Houston, the 4th most populated and polluted urban site in the United States. Our observations, together with numerical parameterizations constrained by the in-situ measured chemical precursors and based on the algorithms provided by the chamber experiments, show that rapid nucleation and growth of freshly formed clusters can be explained by the measured sulfuric acid, base, and low-volatility OOMs (with saturation vapor concentrations in the low or extremely volatility ranges). The chemical composition analysis of OOMs, together with F0AM box model simulations incorporated with the Master Chemical Mechanism, show that under the urban Houston conditions, OOMs form oxidation from both biogenic and anthropogenic VOCs, and autoxidation and dimerization of organic peroxides are not suppressed by NOx. Our findings thus contrast with previous urban studies mostly made in Chinese megacities, demonstrating the distinctively different roles of organics in urban aerosol nucleation under different urban settings due to different emission profiles and chemical compositions of air pollutants. Given rapid global urbanization and increasing emissions of emerging chemical pollutants in the United States and Europe, this multicomponent nucleation process will be crucial for mitigating air pollution in the evolving urban climate.