1. Process-level understanding of new particle formation in wintertime Beijing was obtained based on measurement performed by state-of-the-art instruments. 2. The analysis of sulfuric acid cluster composition and budget showed that sulfuric acid-base clustering initiated new particle formation. 3. Condensable organic vapors were characterized and demonstrated to have a crucial influence on the growth of newly formed particles.

A source of missing sulfate production associated with high-level fine-particle pollution in the megacities of China is believed to stem from the oxidation of a notable fraction of sulfur dioxide (SO 2 ) by nitrogen dioxide (NO 2 ) in aqueous aerosol environments, suggesting that an unknown reaction pathway exists for aqueous sulfur oxidation. At weakly acidic aerosols, the dissolved SO 2 mainly exists in the form of HSO 3 − , whereas at neutral aerosols, SO 3 2− becomes the main form. Herein, by using both ab initio molecular metadynamics simulations and high-level quantum mechanical calculations, we show a hitherto unreported chemical mechanism for the formation of sulfate through the reaction between HSO 3 − /SO 3 2− anions at the surface/in the interior of a water nanodroplet and gas-phase NO 2 molecules. For weakly acidic aerosols, contrary to the conventional high-barrier electron-transfer pathway in the gas phase, HSO 3 − at the water nanodroplet surface can transfer an electron to NO 2 with a low free-energy barrier of 4.7 kcal/mol through a water bridge. For neutral aerosols, the electron-transfer pathway between SO 3 2− in the interior of the water nanodroplet and NO 2 needs to overcome a lower free-energy barrier of 3.6 kcal/mol to form SO 3 − , with the assistance of the hydrogen-bonding network of water molecules. This new reaction pathway for the sulfate formation from HSO 3 − /SO 3 2− via water nanodroplets and gaseous NO 2 provides a new perspective on the growth of haze particles from pre-existing aqueous aerosols and suggests that new control strategies are needed to address haze pollution.

We present a combined molecular dynamics (MD) and classical nucleation theory (CNT) approach to address many issues regarding the nucleation of inorganic aerosols. By taking parameters from MD simulations, we find the CNT predicts fairly reasonable free-energy profiles for the hygroscopic nucleation of aerosols. Moreover, we find that the ionization of sulfates can play a key role in stabilizing aqueous clusters and that both the size of the critical nucleus and the nucleation barrier can be significantly lowered by the H2SO4 and NH4HSO4, whereas the effect of NH3 on nucleation is negligible. NH4HSO4 provides stronger enhancement effect to aerosol formation than H2SO4. In view of the consistency between the theoretical prediction and experimental observation, the combination of MD simulation and CNT appears to be a valuable approach to gain deeper understanding of how aerosol nucleation is affected by different chemical species.

Based on ab initio simulations, we show that dimethylamine molecules can also promote the conversion of atmospheric SO2 to sulfate.