European Commission, Joint Research Centre, Institute for Environment and Sustainability, I-21020 Ispra (VA), Italy
Abstract. Aerosol physical properties were measured at the Monte Cimone Observatory (Italy) from 1 June till 6 July 2000. The measurement site is located in the transition zone between continental boundary layer and the free troposphere (FT), at the border between the Mediterranean area and Central Europe, and is exposed to a variety of air masses. Sub-micrometer number size distributions, aerosol hygroscopicity at 90% RH, refractory size distribution at 270°C and black carbon mass were continuously measured. Number size distributions and hygroscopic properties indicate that the site is exposed to aged continental air masses, however during daytime it is also affected by upslope winds. The mixing of this transported polluted boundary layer air masses with relatively clean FT air leads to frequent nucleation events around local noon.
Night-time size distributions including fine and coarse fractions for each air mass episode have been parameterized by a 3-modal lognormal distribution. Number and volume concentrations in the sub-micrometer modes are strongly affected by the air mass origin, with highest levels in NW-European air masses, versus very clean air in the ''Arctic'' episode. During the dust episode, the coarse mode is clearly enhanced.
The observed hygroscopic behavior of the aerosol is consistent with the chemical composition described by Putaud et al. (2004a), but no closure could be made because the hygroscopic properties of the water-soluble organic matter is not known. The data suggest that WSOM is slightly-to-moderately hygroscopic, and that this property may well depend on the air mass origin and history.
Although externally mixing is observed in all air masses, the occurrence of ''less'' hygroscopic particles has mostly such a low occurrence rate that the average growth factor distribution mostly appears as a single mode. This is not the case for the dust episode, where the external mixing between less hygroscopic and more hygroscopic particles is very prominent, and indicating clearly the occurrence of a dust accumulation mode, extending down to 50 nm particles, along with an anthropogenic pollution mode.
The presented physical measurements finally allow us to provide a partitioning of the sub-µm aerosol in four non-overlapping fractions (soluble + volatile, non-soluble + volatile, refractory + non-BC, BC) which can be roughly associated with separate groups of chemical compounds (ions, organic matter, dust, BC).
For what concerns the relative contributions of the fractions, all air masses except the free-tropospheric (FT) and Dust Episodes show a similar composition within the uncertainty of the data. The latter two have a significantly higher refractory fraction, which in the FT air mass is attributed to carbonaceous particles, and in the dust episode to a sub-µm accumulation mode of dust.