A Monte Carlo approach for determining cluster evaporation rates from concentration measurements

Abstract. Evaporation rates of small negatively charged sulfuric acid–ammonia clusters are determined by combining detailed cluster formation simulations with cluster distributions measured in the CLOUD experiment at CERN. The analysis is performed by varying the evaporation rates with Markov chain Monte Carlo (MCMC), running cluster formation simulations with each new set of evaporation rates and comparing the obtained cluster distributions to the measurements. In a second set of simulations, the fragmentation of clusters in the mass spectrometer due to energetic collisions is studied by treating also the fragmentation probabilities as unknown parameters and varying them with MCMC. This second set of simulations results in a better fit to the experimental data, suggesting that a large fraction of the observed HSO₄⁻ and HSO₄⁻ ⋅ H₂SO₄ signals may result from fragmentation of larger clusters, most importantly the HSO₄⁻ ⋅ (H₂SO₄)₂ trimer.