Size-Resolved Chemical Composition of Sub-20 nm Particles from Methanesulfonic Acid Reactions with Methylamine and Ammonia

Particle formation in the atmosphere from gas-phase precursors has been observed around the world; however, our fundamental understanding of the key species responsible and mechanisms involved remains uncertain. Recent laboratory studies demonstrated that acid-base reactions involving methanesulfonic acid (CH3SO3H, MSA) and small alkylamines may contribute significantly to new particle formation. To date, most of the investigations have been focused on particle number concentration and size distribution measurements in combination with quantum chemistry predictions of the most stable clusters. Here, we present the first measurements of the size-resolved chemical composition of sub-20 nm particles generated in a custom flow reactor from the reaction of MSA with methylamine (MA) in the presence or absence of NH3 using thermal desorption chemical ionization mass spectrometry (TDCIMS). A novel design of the TDCIMS inlet was evaluated, and the measurement of the chemical composition of particles was extended down to 5 nm in diameter, the smallest size yet reported for this method. MSA-MA particles with diameters smaller than 9 nm were found to be more acidic, with an acid/base molar ratio of 1.8 +/- 0.4 (1 sigma) for 5 nm particles compared to the larger particles which were neutral. A similar acid/base molar ratio trend was observed when NH3 was added to the MSA-MA combination. In the MSA-MA-NH3 system, the MA/NH3 molar ratio was higher than 1 (up to 2.6) for all particle sizes despite the much larger concentration of NH3 in the gas phase (the gas-phase MA/NH3 ratio was , similar to 0.23), indicating that MA is a key component in particle formation from this system. The potential reasons for this based on previous calculations of small clusters in this system are discussed.