Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Sulfuric acid (SA)‐dimethylamine (DMA)‐H₂O cluster formation has been proven to be responsible for a significant part of new particle formation (NPF) in a Chinese megacity. However, the possible involvement of common atmospheric acids in the subsequent growth of SA‐DMA clusters remains elusive. We simulated formation and growth of clusters using atmospheric relevant concentrations of SA, DMA, and trifluoroacetic acid (TFA), a commonly observed atmospheric perfluorocarboxylic acid, using Density Functional Theory combined with Atmospheric Cluster Dynamics Code. The presence of TFA leads to complex cluster formation routes and an enhancement of NPF rates by up to 2.3 ([TFA] = 5.0 × 10⁶ molecules cm⁻³, [SA] = 1.0 × 10⁶ molecules cm⁻³, and [DMA] = 1.5 × 10⁹ molecules cm⁻³). The agreement of (SA)₁·(DMA)_1‐2·(TFA)₁ concentrations between simulations and ambient measurements during NPF events validates model predictions and implies that perfluorocarboxylic acids could potentially boost atmospheric SA‐DMA NPF rates.

Significance: New particle formation (NPF) is an important global phenomenon, contributing nearly half of the cloud condensation nuclei in nature. Today, NPF is believed to be mainly promoted by low-volatile species formed in atmosphere. Herein, we show that in certain cases, the formation of low-volatile species could undermine NPF. Specifically, we identify previously unreported catalytic reactions between alcohols and SO ₃ which yield low-volatile organic sulfates. Rather than being a promoter to NPF, the low-volatile organic sulfates can compete for consuming SO ₃ , thereby disfavoring H ₂ SO ₄ formation. Such unexpected quenching effects on NPF are most likely to occur in dry and polluted regions with abundant alcohols, illustrating the importance in understanding the interplay between nucleation precursor formation and subsequent NPF.