New particle formation from gas-to-particle conversion represents a dominant source of atmospheric particles and affects radiative forcing, climate and human health. The species involved in new particle formation and the underlying mechanisms remain uncertain. Although sulfuric acid is commonly recognized as driving new particle formation, increasing evidence suggests the involvement of other species. Here we study particle formation and growth from methanesulfonic acid, trimethylamine and water at reaction times from 2.3 to 32 s where particles are 2-10 nm in diameter using a newly designed and tested flow system. The flow system has multiple inlets to facilitate changing the mixing sequence of gaseous precursors. The relative humidity and precursor concentrations, as well as the mixing sequence, are varied to explore their effects on particle formation and growth in order to provide insight into the important mechanistic steps. We show that water is involved in the formation of initial clusters, greatly enhancing their formation as well as growth into detectable size ranges. A kinetics box model is developed that quantitatively reproduces the experimental data under various conditions. Although the proposed scheme is not definitive, it suggests that incorporating such mechanisms into atmospheric models may be feasible in the near future.

Atmospheric particles are notorious for their effects on human health and visibility and are known to influence climate. Though sulfuric acid and ammonia/amines are recognized as main contributors to new particle formation (NPF), models and observations have indicated that other species may be involved. It has been shown that nucleation from methanesulfonic acid (MSA) and amines, which is enhanced with added water, can also contribute to NPF. While organics are ubiquitous in air and likely to be involved in NPF by stabilizing small clusters for further growth, their effects on the MSA-amine system are not known. This work investigates the effect of oxalic acid (OxA) on NPF from the reaction of MSA and methylamine (MA) at 1 atm and 294 K in the presence and absence of water vapor using an aerosol flow reactor. OxA and MA do not efficiently form particles even in the presence of water, but NPF is enhanced when adding MSA to OxA-MA with and without water. The addition of OxA to MSA-MA mixtures yields a modest NPF enhancement, whereas the addition of OxA to MSA-MA-HO has no effect. Possible reasons for these effects are discussed.

Atmospheric particles influence visibility, health and climate but the mechanisms of their formation from initial clusters and their growth to detectable particles remain largely unknown. Previous studies show that reactions of methanesulfonic acid (MSA) with ammonia and amines form particles, a process which is enhanced by water. We report here results from a combined experimental-theoretical investigation of the effect of oxalic acid (OxA) on particle formation and growth from the reaction of MSA with trimethylamine (TMA) in the absence and presence of water. The gas phase reactants were mixed in an aerosol flow reactor (1 atm, 294 K). Particle number concentrations and size distributions were measured as a function of reaction time from 0.8-12 s. The interaction of OxA with TMA with and without water does not lead to significant particle formation. When OxA is present during the reaction of MSA with TMA, there is little change (≤2 times more) in the particle number concentration but particles are larger compared to the base case of MSA with TMA alone. However, the presence of water with MSA and TMA overwhelms the effect of OxA so that OxA has no significant impact on particle number concentration or size. Results of these experiments suggest the MSA hydrate is important for particle formation and growth of the four component OxA-MSA-TMA-HO system. These results are compared to earlier studies of the effect of OxA on the MSA-methylamine reaction and interpreted based on theoretically calculated properties of small clusters of the components.