Detection of Sulfuric Acid - Dimethyl Amine Nucleation Process using Ambient Pressure Proton Transfer Reaction Mass Spectrometry (AP-PTRMS)

JUN ZHENG, Nanjing University of Information Science and Technology

     Abstract Number: 32
     Working Group: Aerosol Chemistry

Abstract
Globally, nucleation processes contribute about half of the aerosol number concentration in the atmosphere. The sulfuric acid – dimethyl amine (SA-DMA) nucleation system is found to be the dominating nucleation mechanism in relatively polluted environments both theoretically and empirically. Based on observations using the nitrate-CIMS, the critical step of a nucleation process was believed to be the formation of a SA-dimer, which can be stabilized by a DMA, leading to enhanced nucleation rates. However, thermodynamic calculations demonstrated that SA_n·DMA_m clusters, where n = m, were the most stable clusters, and the formation of a SA·DMA was proposed to be the critical step during the SA-DMA nucleation process, although SA_n·DMA_m clusters have not been directly observed during field measurements. These discrepancies between observations and theories were likely due to the working principle of a nitrate-CIMS. When SA and its clusters were detected by a nitrate-CIMS, a nitrate anion (NO₃^-) takes a proton from a SA molecule or attaches to the SA-containing clusters. This can decrease the acidity of SA clusters, which may alter their original composition by losing a ligand (e.g., water, ammonia, and/or DMA). Here, we developed a new technique, AP-PTRMS, to detect the major clusters formed by a SA-DMA nucleation system. Water clusters of hydronium ions (H₃O⁺·(H₂O)_n=1,2,3…)) were used as the primary reactant ions and DMA was then protonated by H₃O⁺·(H₂O)_n to form DMA·H⁺, which can readily attach to a SA_n·DMA_m cluster to complete the ionization process. Using a flow-tube reactor and in-situ generated SA gas, the most abundant SA_n·DMA_m clusters detected were SA·DMA followed by SA₂·DMA₂ and SA₃·DMA₃. These results were further verified by a kinetic model simulation based on SA_n·DMA_m condensation/evaporation rates. This new CIMS technique can be used to better reveal the true ambient nucleation processes and may also be used to measure the ambient SA concentration.