The study deepens our understanding of particle formation in varying atmospheric con-ditions. The model simulations concentrate on microscale processes while the long-term observations describe as well synoptic scale conditions.
The probability of new particle formation events is presented as a function of mean mi-croscale variables at the top of the ABL. The most essential variables are condensation sink and temporal change of temperature. The values of variables in the upper ABL have been derived from surface measurements which can, to a large degree, explain the consistency with earlier studies. The presented calculations are based on several simplifications, e.g. constant concentration of dry particles in an ascending air parcel and a non-existent entrainment zone above the ABL. Lagrange type box model sim-ulations would have allowed the description of particle dynamics in an ascending air parcel. Furthermore, 1D or 3D model simulations would have also given more reliable meteorological profiles.
The observations at Tumbarumba field station highlight the importance of spatially varying conditions in new particle formation. In the studied case latent heat flux far from the measurement station most probably determines the probability of new particle formation. The significance of air mass history in synoptic scale should be noticed when, e.g. results of column model simulations are evaluated and analysed.
The theoretical model study verifies that temporal or spatial variation in temperature and vapour concentrations can increase the mean binary water-sulphuric acid nucle-ation rate by orders of magnitude. The effect of this varinucle-ation has been parametrised with a correction factor. Under atmospheric conditions, however, the factor is signifi-cant only in limited cases. The factor is largest at high temperature and low humidity while these conditions inhibit binary H2O–H2SO4 nucleation. Even if binary nucle-ation could reach a significant level locally or momentarily, the varinucle-ation cannot ex-plain the orders of magnitude difference between observed and theoretical nucleation rates. Therefore, the sub-grid scale variation can be ignored in most of nucleation calculations in large scale models.
The further developed column models SOSA and MALTE succeed to produce reliable
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The column model simulations highlight especially the importance of atmospheric mixing and local micrometeorological conditions in aerosol studies – the conclusions on possible nucleation mechanisms in the ABL are based on vertical vapour and par-ticle profiles. Several studies have suggested that new parpar-ticle formation take place near the ABL top. Therefore, the role of large eddies and mixing between the ABL and the free troposphere should receive greater attention. A representation for large eddies in MALTE should be considered in future. This could enable studies on the significance of dilution in new particle formation that are more reliable than with the present model version. Measured vertical profiles of precursor vapours and particle number distributions would allow evaluation of model results.
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