Dynamics of atmospheric nucleation mode particles: A time scale analysis

A theoretical framework was constructed by which one can estimate the relative role of different processes in thedynamics of atmospheric nucleation mode particles. The framework relies on 14 timescales that describe (1) changes inthe total nuclei number concentration, (2) changes in the mean diameter of the nucleation mode and (3) concentrationsof low-volatile vapours responsible for the growth of nuclei. The magnitude of the derived timescales can be calculatedrelatively easily from the available measurement or modelling data. Application to the lower-troposphere revealed thatunder most conditions removal of nuclei is dominated by their coagulation with larger background particles and thatthis process competes very strongly with growth of nuclei to sizes relevant to atmospheric chemistry and physics.Withsome exceptions, self-coagulation of nuclei was shown to be of marginal importance compared with their growth bycondensation and their removal by coagulation. Finally, by comparing predictions based on relevant timescales withthose obtained from detailed numerical simulations, quantitative criteria were derived concerning (1) when one mayneglect self-coagulation of nuclei when looking at nucleation mode dynamics and (2) when the whole nucleation modecan be neglected because of its eventual removal. These criteria are extremely useful for atmospheric modellers whoneed to simplify their models as much as possible. From the modelling point of view, other processes requiring furtherattention are the introduction of new nuclei into the system and decrease in nuclei number concentration due to dilutionof the air