Physical aerosol properties and their relation to air mass origin at Monte Cimone (Italy) during the first MINATROC campaign
[摘要] Aerosol physical properties were measured at the Monte Cimone Observatory(Italy) from 1 June till 6 July 2000. The measurement site is located inthe transition zone between the continental boundary layer and the freetroposphere (FT), at the border between the Mediterranean area and CentralEurope, and is exposed to a variety of air masses. Sub-μm number sizedistributions, aerosol hygroscopicity near 90% RH, refractory sizedistribution at 270°C and equivalent black carbon mass were continuouslymeasured. Number size distributions and hygroscopic properties indicate thatthe site is exposed to aged continental air masses, however during daytimeit is also affected by upslope winds. The mixing of this transportedpolluted boundary layer air masses with relatively clean FT air leads tofrequent nucleation events around local noon.
Night-time size distributions, including fine and coarse fractions for eachair mass episode, have been parameterized by a 3-modal lognormaldistribution. Number and volume concentrations in the sub-μm modes arestrongly affected by the air mass origin, with highest levels in NW-Europeanair masses, versus very clean, free tropospheric air coming from theN-European sector. During a brief but distinct dust episode, the coarse modeis clearly enhanced.The observed hygroscopic behavior of the aerosol is consistent with thechemical composition described by Putaud et al. (2004), but noclosure between known chemical composition and measured hygroscopicity couldbe made because the hygroscopic properties of the water-soluble organicmatter (WSOM) are not known. The data suggest that WSOM isslightly-to-moderately hygroscopic (hygroscopic growth factor GF at 90%relative humidity between 1.05 and 1.51), and that this property may welldepend on the air mass origin and history.External mixing of aerosol particles is observed in all air masses throughthe occurrence of two hygroscopicity modes (average GF of 1.22 and 1.37,respectively). However, the presence of "less" hygroscopic particles hasmostly such a low occurrence rate that the average growth factordistribution for each air mass sector actually appears as a single mode.This is not the case for the dust episode, where the external mixing betweenless hygroscopic and more hygroscopic particles is very prominent, andindicating clearly the occurrence of a dust accumulation mode, extendingdown to 50 nm particles, along with an anthropogenic pollution mode.The presented physical measurements finally allow us to provide apartitioning of the sub-μm aerosol in four non-overlapping fractions(soluble/volatile, non-soluble/volatile, refractory/non-black carbon, blackcarbon) which can be associated with separate groups of chemical compoundsdetermined with chemical-analytical techniques (ions, non-water solubleorganic matter, dust, elemental carbon). All air masses except thefree-tropospheric N-European and Dust episodes show a similar compositionwithin the uncertainty of the data (53%, 37%, 5% and 5%respectively for the four defined fractions). Compared to these sectors, thedust episode shows a clearly enhanced refractory-non-BC fraction (17%),attributed to dust in the accumulation mode, whereas for the very cleanN-EUR sector, the total refractory fraction is 25%, of which 13%non-BC and 12% BC.
