Investigation of nucleation kinetics in H2SO4 vapor through modeling of gas phase kinetics coupled with particle dynamics

Real-time monitoring of aerosol particle formation from sulfuric acid vapor at elevated concentrations and temperatures

In the present study, time-resolved aerosol particle formation from sulfuric acid vapor is examined with special attention to the stabilization of molecular clusters in the early phase of unary nucleation. An important factor governing this process is the amount of condensable acid vapor. Here it is produced from fast gas-phase reactions in a batch-type reaction cell for which we introduce modifications enabling real-time monitoring. The key component for size- and time-resolved detection of ultrafine particles is a new 1 nm-SMPS. With this new tool at hand, the effect of varying the precursor concentration over two orders of magnitude is investigated. We demonstrate the ability to tune between different growth scenarios as indicated by the size-resolved particle traces which exhibit a transition from sigmoidal over quasi-stationary to peak-like shape. The second key parameter relevant for nucleation studies is the temperature-dependent cluster evaporation. Due to a temperature rise during the mixing stage of the experiment, evaporation is strongly promoted in the early phase. Therefore, the present study extends the T-range used in, e.g., smog chambers. We investigate this temperature effect in a kinetic simulation and can successfully combine simulated and measured data for validating theoretical evaporation rates obtained from DLPNO-CCSD(T₀)-calculations.

Neutral Sulfuric Acid-Water Clustering Rates: Bridging the Gap between Molecular Simulation and Experiment

The role of sulfuric acid during atmospheric new particle formation is an ongoing topic of discussion. In this work, we provide quantitative experimental constraints for quantum chemically calculated evaporation rates for the smallest H₂SO₄-H₂O clusters, characterizing the mechanism governing nucleation on a kinetic, single-molecule level. We compare experimental particle size distributions resulting from a highly supersaturated homogeneous H₂SO₄ gas phase with the results from kinetic simulations employing quantum chemically derived decomposition rates of electrically neutral H₂SO₄ molecular clusters up to the pentamer at a large range of relative humidities. By using high H₂SO₄ concentrations, we circumvent the uncertainties concerning contaminants and competing reactions present in studies at atmospheric conditions. We show good agreement between molecular simulation and experimental measurements and provide the first evaluation of theoretical predictions of the stabilization provided by water molecules.