| 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 the 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-μm number size
distributions, aerosol hygroscopicity near 90% RH, refractory size
distribution at 270°C and equivalent 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-μm modes are
strongly affected by the air mass origin, with highest levels in NW-European
air masses, versus very clean, free tropospheric air coming from the
N-European sector. During a brief but distinct 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. (2004), but no
closure between known chemical composition and measured hygroscopicity could
be made because the hygroscopic properties of the water-soluble organic
matter (WSOM) are not known. The data suggest that WSOM is
slightly-to-moderately hygroscopic (hygroscopic growth factor GF at 90%
relative humidity between 1.05 and 1.51), and that this property may well
depend on the air mass origin and history.
External mixing of aerosol particles is observed in all air masses through
the occurrence of two hygroscopicity modes (average GF of 1.22 and 1.37,
respectively). However, the presence of "less" hygroscopic particles has
mostly such a low occurrence rate that the average growth factor
distribution for each air mass sector actually 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-black carbon, black
carbon) which can be associated with separate groups of chemical compounds
determined with chemical-analytical techniques (ions, non-water soluble
organic matter, dust, elemental carbon). All air masses except the
free-tropospheric N-European and Dust episodes show a similar composition
within the uncertainty of the data (53%, 37%, 5% and 5%
respectively for the four defined fractions). Compared to these sectors, the
dust episode shows a clearly enhanced refractory-non-BC fraction (17%),
attributed to dust in the accumulation mode, whereas for the very clean
N-EUR sector, the total refractory fraction is 25%, of which 13%
non-BC and 12% BC. |