Formation of atmospheric molecular clusters consisting of methanesulfonic acid and sulfuric acid: Insights from flow tube experiments and cluster dynamics simulations

Abstract

In coastal regions and ocean areas, methanesulfonic acid (MSA; CH3SO3H) is present in considerable concentrations in the gas-phase and aerosols. It has been shown that MSA could contribute to growth and possibly form initial molecular cluster, which may lead to aerosol formation. However, quantitative concentrations and thermodynamic properties of MSA and sulfuric acid (SA; H2SO4) in the presence of water (W; H2O) remain largely uncertain. In this study, flow tube reactor was used to investigate the effects of each reactant on new particle formation (NPF) in a multi-component system consisting of MSA, SA, and W. Particles were measured for different combinations of reactants. It showed that a different order for reactant addition led to different experimental results, where the added MSA vapor to the SA-W binary system presented an obvious bimodal structure, for ternary system with SA added to the MSA-W, the similar bimodal phenomenon was not observed. The composition of clusters in the air flow was further analyzed by the commercial Atmospheric Pressure Interface Time-of-Flight Mass Spectrometer (APi-TOF-MS, Tofwerk AG), which is equipped with a homemade chemical ionization (CI) source, mass peaks corresponding to clusters that contain smaller MSA or SA molecules were clearly observed, indicating that these clusters are exist and stable. In addition, quantum chemistry calculation-based evaporation rate values were applied in a cluster dynamics model to yield formation rates of 2.6 × 102 cm−3 s−1 and cluster concentrations under different simulation conditions. This study could provide some insight into how acids interact in the atmosphere.