Convective transport of particulate matter in an Apennine valley: 2. Time variations in the columnar aerosol mass content and vertical profiles of aerosol mass concentration

Multispectral sun-photometric measurements of aerosol optical depth were performed at five stations located at different altitudes from about 500 to 2165 m amsl in the Leo valley in the Apennines (Italy), on some clear-sky autumn days of 1981 and 1982. These measurements showed that the aerosol optical depth greatly increased during the morning at all stations due to the convective transport of particulate matter induced by the local upslope breezes. To evaluate the corresponding variations of the vertical mass loading of particulate matter, two continental and rural aerosol extinction models were determined: the first provides the spectral series of both volume and mass aerosol extinction coefficients at nine visible and near-infrared wavelengths for 27 aerosol particle size-distribution curves of Junge-type with values of the Ångström exponent α varying between 0.15 and 1.99; the second is based on linear combinations of three monomodal particle size-distribution curves determined to give form to a nuclei mode, an accumulation mode and a coarse particle mode, respectively, whose particle number concentrations were defined to fit the values of α characterising the spectral series of aerosol optical depth. aerosol extinction models were used to obtain estimates of the columnar aerosol mass content at the five stations, finding values mostly ranging between 0.03 and 0.12 g m−2 at the two lower stations, 0.02 and 0.08 g m−2 at the two intermediate stations and between 0.01 and 0.03 g m−2 at the high-altitude Mt. Cimone station. The corresponding hourly mean percentage variations of the vertical aerosol mass loading were found to vary between a few percentages and nearly 100%, with median values ranging from 14% to 29% at the various stations. From these estimates, evaluations of the aerosol mass concentration were obtained within the four layers, varying from 15 to 160 μg m−3 in the first layer, from 15 to 95 μg m−3 in the second, from 10 to 60 μg m−3 in the third and from 5 to 20 μg m−3 in the fourth. Correspondingly, the hourly mean percentage variations were found to range between −26% and +124% within the four layers, with median values gradually increasing from +4% to +20%, passing from the first to the fourth layer.