Features of the multimodal aerosol size distribution depending on the air mass origin in the Baltic region

The purpose of this research is to study the dependence of the characteristics of the atmospheric aerosol size distribution (SD), especially its modal structure on the type and origin of the air mass using multivariate analysis. A large amount of the data of measurement campaigns of different duration, which were held all over Estonia from 1992 to 2001, was collected and systematized. The data were collected using the original electrical aerosol spectrometer, designed at the Institute of Environmental Physics of the University of Tartu. The dependence of aerosol particle size spectra on the air mass origin can be analyzed by means of air mass trajectories. The measurement data were classified according to the air mass origin taking into account of the starting point, climatic conditions and different kinds of pollution sources the air mass trajectory overpasses. The particle size mean spectra over all measurement campaigns were calculated for all selected air mass types. These mean spectra curves are quite smooth, so that their modal structure is visually not clearly seen. Therefore, their modal structure was identified by means of the factor analysis for most of the separated air masses. The retained factors were interpreted as the lognormally distributed components (modes) of the particle size spectrum. With the help of graphs that show the factor loadings as the functions of particle size, the modal diameter and the width of these modes were roughly assessed. Then, using the iterative least squares method, the parameters of the lognormal components were specified to obtain the best fit of the measured spectrum with the sum of lognormal components.