The initial work has revolved around understanding the chemical composition of FNPs. State-of-the-art quantum chemical methods can be applied to study clusters up to roughly 8 molecules. Hence, to study FNPs containing up to potentially 50 molecules we need to lower the applied level of theory, while not introducing a large error in our calculations. In addition, the more molecules we need to handle, the more complicated it is to identify the molecular assembly, which is lowest in free energy and thereby most stable.
We have developed a new configurational sampling procedure that accurately can sample the complex configurational space of FNPs. This implies that we much more reliably can identify the lowest free energy cluster structures. Unfortunately, we found that none of the existing semi-empirical methods had adequate accuracy for our target purpose. So, we have re-parameterized a new semi-empirical quantum chemical method, that can be applied to study atmospheric molecular clusters (AMC-xTB) and freshly nucleated particles (FNP-xTB). Hence, now we for the first time have an accurate methodology that can directly follow the formation of FNPs all the way from single molecules to 2 nm sizes.
We have applied the newly identified sampling protocols and methods to study different sulfuric acid (SA) – base compositions of FNPs, with the bases being ammonia (AM), methylamine (MA), dimethylamine (DMA) and trimethylamine (TMA). We found that for small clusters (up to 4 acid-base pairs) the base molecule is very important and determines the nucleation properties. Hence, the formation follows the basicity of the base. For larger clusters (above 10 acid-base pairs) we found that the free energy per acid-base pair begins to level out, indicating that we are reaching a regime where the clusters behave as bulk. Here,
the basicity of the clustering base is less pronounced and hydrogen bond capacity of the base begins to contribute substantially. Overall, our results show that based on the properties of the clusters, we can disentangle the nucleation regime from the growth regime and have been able to determine the actual cluster-to-particle transition point in these systems. This cluster-to-particle transition point is found to coincide with the emergence of the first solvated ions in the cluster, i.e. the first fully coordinated molecule. We propose to define the onset of FNPs as this exact cluster-to-particle transition point.
