Atmospheric Cluster Dynamics Code: a flexible method for solution of the birth-death equations

Abstract

Abstract.The Atmospheric Cluster Dynamics Code (ACDC) is presented and explored. This program was created to study the first steps of atmospheric new particle formation by examining the formation of molecular clusters from atmospherically relevant molecules. The program models the cluster kinetics by explicit solution of the birth-death equations, using an efficient computer script for their generation and the MATLAB ode15s routine for their solution. Through the use of evaporation rate coefficients derived from formation free energies calculated by quantum chemical methods for clusters containing dimethylamine or ammonia and sulphuric acid, we have explored the effect of changing various parameters at atmospherically relevant monomer concentrations. We have included in our model clusters with 0-4 base molecules and 0-4 sulfuric acid molecules for which we have commensurable quantum chemical data. The tests demonstrate that large effects can be seen for even small changes in different parameters, due to the non-linearity of the system. In particular, changing the temperature had a significant impact on the steady-state concentrations of all clusters, while the boundary effects (allowing clusters to grow to sizes beyond the largest cluster that the code keeps track of, or forbidding such processes), coagulation sink terms, non-monomer collisions, sticking probabilities and monomer concentrations did not show as large effects under the conditions studied. Removal of coagulation sink terms prevented the system from reaching the steady state when all the initial cluster concentrations were set to the default value of 1 m −3 , which is probably an effect caused by studying only relatively small cluster sizes.

New particle formation and growth from methanesulfonic acid, trimethylamine and water

“…69,70 The addition of a cluster or molecule to a pre-existing cluster is typically taken to occur at the gas-collision rate. The evaporation rate is obtained from the free energy difference between the product cluster and the reacting species/clusters that formed it, calculated using a quantum chemical approach.…”

“…In a similar vein, H 2 SO 4 and DMA have been shown to form an initial H 2 SO 4 ÁDMA cluster which grows by stepwise addition of the acid and base to generate clusters of increasing size with 1 : 1 stoichiometry. 28,69,70 Addition of DMA to dimethylaminium salt clusters of MSA was observed to be fastest when there was an excess acid available in the cluster to be neutralized. 80 This is similar to the case of dry conditions proposed in this study (pathway I in Fig.…”

Hydration of the methanesulfonate–ammonia/amine complex and its atmospheric implications

“…30,92 As shown in Fig. 4, for where the evaporation of (MSA À )(TMA) is even less than MSA À , and the gap is enhanced with more water.…”

Growth of atmospheric clusters involving cluster–cluster collisions: comparison of different growth rate methods

“…We simulated the time evolution of cluster concentrations in a one-component system using the Atmospheric Cluster Dynamics Code (ACDC; McGrath et al, 2012;Olenius et al, 2014). The model included the production of monomers, all the possible collision and evaporation processes between different clusters, and the losses of clusters due to an external sink.…”

“…Collision rates between clusters were obtained from Eq. (6) with C e = 0, and cluster evaporation rates were calculated from the Gibbs free energies of formation of the clusters (e.g., Ortega et al, 2012). The external losses were assumed to depend on the cluster size according to (Lehtinen et al, 2007) …”