Mixing Ratio and Relative Humidity Govern Dimethylamine Uptake on Ammonium Sulfate-Glyoxylic Acid Aerosols: Kinetics and Brown Carbon Formation

The ammonia-amine exchange reaction serves as a key driver of atmospheric aerosol aging, yet its modulation by secondary organic aerosols (SOA), particularly those containing organic acids, remains poorly understood. We investigated the uptake of dimethylamine (DMA) by internally mixed ammonium sulfate and glyoxylic acid (GA) particles under varying relative humidity (RH) and mixing ratios. Our results revealed that DMA uptake generally increased with RH, but the dependence on GA proportion varied distinctly with humidity. Specifically, under dry conditions, uptake increased monotonically with GA molar fractions, whereas under humid conditions, increasing GA molar fractions initially suppressed uptake followed by an enhancement. This kinetic behavior was attributed to liquid-liquid phase separation (LLPS) during particle aging, which produced an aqueous salt-rich phase containing highly oxygenated multifunctional organic acids (HOMAs), with saturated N-heterocyclic compounds enriched at its surface. These low-volatility reaction products, formed preferentially under dry conditions, significantly amplified both amine uptake and aerosol mass. Furthermore, they induced aerosol light absorption in the near-ultraviolet region, indicating the presence of brown carbon. These results underscore the critical role of organic acids in amine-mediated aging processes, highlighting a pathway that significantly impacts aerosol optical properties and regional air quality.