Table 1 Treatment of different molecular cluster dynamics processes in the standard approach and in the explicit approach developed in this work. The listed potential effects of the standard simplifications are general examples, and the effects can be more diverse over various atmospheric conditions.
From: Role of gas–molecular cluster–aerosol dynamics in atmospheric new-particle formation
Dynamic process | Standard approach | Explicit approach | Potential effects of standard simplifications | Environments and cluster chemistries that are expected to be especially affected |
|---|---|---|---|---|
Time evolution of cluster concentrations | Clusters are assumed to be in instantaneous steady state at every model time step | The time evolution is explicitly simulated | Dependent on the time evolution of ambient conditions; overprediction of formation rate likely at increasing vapor concentrations, underprediction at decreasing concentrations18 | Conditions with long time scales of cluster formation, such as low vapor concentrations |
Effect of cluster formation on vapor concentrations | Clustering does not affect vapor concentrations (vapors may be reduced according to the particle formation rate, but the molecules bound in cluster phase are omitted) | Time evolution of vapor concentrations during clustering is explicitly simulated | Overprediction of vapor concentrations and thereby particle formation rate | Strongly clustering compounds, low vapor concentrations |
Sizes and compositions of newly-formed particles | All new particles formed by a given chemical mechanism are assumed to have the same molecular composition | All new particles growing out of the cluster regime through different cluster–molecule or cluster–cluster collisions are placed in the aerosol size bin that corresponds to the size of the collision product | Underprediction of sizes of new particles; inaccuracies in composition | Chemistries involving cluster–cluster coagulation, multi-component clustering including molecules and clusters of different sizes |
Cluster scavenging by aerosol particles | Effect of a given sink on formation rate can be included, but scavenged clusters are lost from the cluster–aerosol systema (Scavenging is omitted or the sink is independent of aerosol concentrations)b | Cluster–aerosol collisions are included and the collision products are distributed to the aerosol bins | Underprediction of aerosol growth due to omission of scavenged clustersa (Inaccuracies in formation rate)b | Strongly clustering compounds for which a significant amount of vapor is bound in cluster phasea (Environments with significant and/or varying sinks)b |
Aerosol evaporation beyond the smallest size covered by the aerosol model | Shrunken aerosols are removed from the particle spectrum, and the material is either lost or transferred to gas phase | Evaporated aerosols are transferred to the cluster regime | Underprediction of cluster concentrations and formation rate | Decreasing vapor concentrations with small particles that contain volatile compounds |
