Writing and Applying a Monte Carlo Simulation Code to Examine Aerosol Vapor Pressure - an Important Factor in Climate Modeling

Aerosol nucleation is an important factor in climate change; however, the process of aerosol formation involving atmospheric pollutants is not well understood. The purpose of this study was to examine whether the Kelvin equation (KE) and Classical Nucleation Theory (CNT) can accurately describe the vapor pressure of the nano-droplets involved in aerosol nucleation. A mathematical comparison demonstrates that the vapor pressure of nanodroplets predicted by KE and CNT differ by a factor of e. A Metropolis Monte Carlo code was written in Fortran. Monte Carlo simulations in the canonical ensemble for a Lennard-Jones system were used to probe how temperature, total number of atoms, and system size affects the size of a nano-droplet in equilibrium with the surrounding vapor. As expected, varying these three system parameters yields different equilibrium cluster sizes. Somewhat unexpectedly, the simulations indicate that the equilibrium cluster sizes consisting of about 100 to 300 particles are well described by the predictions of both KE and CNT (with linear fits yielding R > 0.99). This study is important since experimental measurements are very challenging for small clusters sizes—conditions important for atmospheric nucleation.