Reactions of formic acid with protonated water clusters: Implications of cluster growth in the atmosphere

Obtaining direct and detailed chemical information about the initial stages of nucleation continues to be a challenging task, but it is necessary for improved understanding of the growth process of particles in the atmosphere. To elucidate the role of organic molecules, specifically formic acid, in nucleation and particle growth, the present work uses a flow tube reactor to study formic acid-water interactions using water cluster distributions of 2-30 water molecules. The reactions studied herein are compared to methanol-water interactions to provide evidence of a change to the stable clathrate-like structure of the H+(H2O)(21) magic cluster. Intensity ratios of the form R = I[H+(H2O)(n)]/ I[H+(H2O)(n+1)] are used to identify prominent clusters in spectra containing pure water, methanol-water mixed clusters, and formic acid-water mixed clusters. Relative concentration flow rates of 12.5, 25, 50, and 125.5 standard cubic centimeters per minute (sccm) of formic acid were added to a pure water distribution, and the resultant spectra were compared to those from methanol relative concentrations of 25, 50, and 100 sccm. The formic acid-water mixed cluster distributions observed in these reactions do not contain a prominent peak in the intensity ratio graphs, indicating a disruption in the clathrate structure of the 21-molecule cluster while a prominent peak for 21-molecule cluster is observed in the methanol system. Also observed was an increase in intensity with increased cluster size, which indicates that formic acid-water mixed clusters could serve as a prenucleation embryo for the nucleation of water molecules and a possible particle growth pathway.