Investigation of Formic Acid involved Molecular Clusters with Atmospheric Acidic, Basic and Neutral Species

Abstract

New particle formation (NPF) is regarded as a main source of tropospheric aerosols. Formic acid (FA) is investigated as a source of NPF by studying its dimer, trimer and tetramer clusters with atmospheric nucleating species like H2O, H2S, MeNH2 and H2O2 employing quantum chemical methods. The structural characteristics, nature of interactions, interaction energies, harmonic vibrational frequencies, thermochemistry at ambient and tropospheric conditions are investigated employing DFT and MP2 methods. Acidic (H2S), basic (MeNH2) and neutral (H2O) nucleating precursors are chosen to study the effect of nature of these species on NPF. Results show that clusters of formic acid with MeNH2 have more stabilising effect than clusters with H2O, H2S and H2O2. The FA-MeNH2 clusters are formed spontaneously at ambient conditions while for others (H2O, H2O2) although the clusters are stable at ambient conditions, yet have positive binding free energy that becomes negative at tropospheric conditions. The cluster formation leads to significant red shifts and increased IR intensities for conventional hydrogen bond donor bond stretching vibrations relative to its monomeric units. For all the nucleating species, stability increases with the growth of clusters.