Abstract. In atmospheric sulfuric-acid-driven particle formation, bases are able to stabilize the initial molecular clusters and thus enhance particle formation. The enhancing potential of a stabilizing base is affected by different factors, such as the basicity and abundance. Here we use weak (ammonia), medium strong (dimethylamine) and very strong (guanidine) bases as representative atmospheric base compounds, and we systematically investigate their ability to stabilize sulfuric acid clusters. Using quantum chemistry, we study proton transfer as well as intermolecular interactions and symmetry in clusters, of which the former is directly related to the base strength and the latter to the structural effects. Based on the theoretical cluster stabilities and cluster population kinetics modeling, we provide molecular-level mechanisms of cluster growth and show that in electrically neutral particle formation, guanidine can dominate formation events even at relatively low concentrations. However, when ions are involved, charge effects can also stabilize small clusters for weaker bases. In this case the atmospheric abundance of the bases becomes more important, and thus ammonia is likely to play a key role. The theoretical findings are validated by cluster distribution experiments, as well as comparisons to previously reported particle formation rates, showing a good agreement.

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<p><strong>Abstract.</strong> In atmospheric sulfuric acid-driven particle formation, bases are able to stabilize the initial molecular clusters, and thus enhance particle formation. The enhancing potential of a stabilizing base is affected by different factors, such as the basicity and abundance. Here we use weak (ammonia), medium strong (dimethylamine) and very strong (guanidine) bases as representative atmospheric base compounds, and systematically investigate their ability to stabilize sulfuric acid clusters. Using quantum chemistry, we study proton transfer as well as intermolecular interactions and symmetry in clusters, of which the former is directly related to the base strength and the latter to the structural effects. Based on the theoretical cluster stabilities and cluster population kinetics modeling, we provide molecular-level mechanisms of cluster growth and show that in electrically neutral particle formation, guanidine can dominate formation events even at relatively low concentrations. However, when ions are involved, charge effects can stabilize small clusters also for weaker bases. In this case the atmospheric abundance of the bases becomes more important, and thus ammonia is likely to play a key role. The theoretical findings are validated by cluster distribution experiments, as well as comparisons to previously reported particle formation rates, showing a good agreement.</p>

Abstract. Sulfuric acid and dimethylamine vapours in the atmosphere can form molecular clusters, which participate in new particle formation events. In this work, we have produced, measured, and identified clusters of sulfuric acid and dimethylamine using an electrospray ionizer coupled with a planar-differential mobility analyser, connected to an atmospheric pressure interface time-of-flight mass spectrometer (ESI–DMA–APi-TOF MS). This set-up is suitable for evaluating the extent of fragmentation of the charged clusters inside the instrument. We evaluated the fragmentation of 11 negatively charged clusters both experimentally and using a statistical model based on quantum chemical data. The results allowed us to quantify the fragmentation of the studied clusters and to reconstruct the mass spectrum by removing the artifacts due to the fragmentation.

The experiments in this study were conducted using a set-up consisting of a combination of ElectroSpray Ionization, planar Differential Mobility Analyser and Atmospheric Pressure interface Time Of Flight mass spectrometer (ESI-DMA-APi-TOF MS). The main text includes a schematic representation of the set-up (Fig. 1). The APi-TOF was operated in negative mode, and all data analysis were performed using tofTools; a set of Matlab-based programs developed at the University of Helsinki.We were investigating clusters of sulfuric acid and dimethylamine. A 100 mM/100 mM dimethylamine/sulfuric acid solution in methanol and water with a ratio of 4:1 v:v was used to produce negatively charged clusters. The solution was charged by applying a negative voltage by the ESI, and then electrosprayed via a pressure gradient applied to the solution to allow it to pass through a silica capillary (30 Bm tip, 75 Bm i.d., 360 Bm o.d., non-coated, SilicaTip TM, PicoTip TM EMITTER, NewObjective). Separation of the charged clusters was achieved using a planar DMA P5 (SEADM), with the sheath flow blower operating at 18000 rpm (for further information on this DMA see (Amo-González and Pérez, 2018). The carrier gas used in the sheath flow was nitrogen (N2) gas. The outlet of the DMA was connected to an electrometer (SEADM, model LYNX-E11) with an inlet flow rate of 1 L/min and to the APi-TOF (Tofwerk AG, Thun, Switzerland) with an inlet flow rate of 0.8 L/min. The APi part consists of three chambers, which are connected to a scroll pump and a turbo pump which allows for a successive pressure drop across the chambers reaching a pressure of about 10-6 mbar towards the third (final) chamber.