Analysis of the growth of nucleation mode particles observed in Boreal forest

Abstract

Aerosol formation and subsequent particle growth in the ambient air have been frequently observed at a boreal forest site, Southern Finland. During a 1-year period we have observed significant aerosol formation on more than 50 days. Submicron aerosol size distributions have been measured using differential mobility analyzing technique. On several days, the growth of nucleation mode particles to Aitken mode has been observed to take place within ~ 10 h subsequent to their formation at a rate of a few nm/h. 10 days of the data, representing the particle formation days with a clear subsequent growth, were selected for a closer examination. The growth of nucleation mode particles was analysed with an integral aerosol dynamics model. The model includes photochemical formation of a condensable compound in the gas phase, condensation and particle coagulation. The observed particle growth of the nucleation mode particles can be explained by continuum/transition regime condensation theory. The model gives the gas-phase concentration of the condensing species as a function of time. Since the actual condensing species has not been experimentally identified, the molecular properties of the condensing compound were varied in the model. This sensitivity study revealed that the molecular properties have only a small effect on the daily maximum concentration of the condensing species. We also studied the sensitivity of the results to variations in the source term of the condensable species and to variations in temperature and relative humidity. Accounting for the results of the sensitivity studies, we conclude that the maximum daily concentration of gas-phase, condensing molecules achieved at the site was greater than 107 cm-3, while their saturation vapor density is below 105 cm-3.