Study of organic and inorganic size-segregated chemical composition of different types of tropospheric aerosol.

Tropospheric aerosol influences several important chemical and physical processes in the troposphere, due to its high variability in terms of origin, chemical composition, size-distribution and spatial distribution. In fact local and regional, biogenic and anthropogenic, primary and secondary sources as well as long-range transport affect aerosol properties over independent regions and layers. For these reasons organic and inorganic chemical composition at the different size ranges must be well characterized in order to understand and properly described in models the physical and chemical processes occurring in the troposphere. The present thesis provides a detailed description of factors controlling size-segregated inorganic and organic chemical properties of different types of tropospheric aerosol. Size segregated aerosol samples were collected during the experiments performed in 2006 within the European Project MAP (Marine Aerosol Production Project, http://macehead.nuigalway.ie/map/), and during three intensive campaigns performed in 2007 and 2008 within the Italian project MIUR-AEROCLOUDS (Study of the Direct and Indirect Aerosol Effects on Climate). A chemical characterization of both water soluble and water insoluble organic fractions and water insoluble inorganic ions was accomplished. In the framework of the Project MAP, main primary and secondary marine aerosol chemical components were investigated under clean marine conditions with relation to seasonal oceanic biological activity. Typical marine aerosol features were observed. Non sea salt sulphate (nss-SO4=) and ammonium (NH4+) dominated the submicrometer inorganic fraction, nitrate (NO3-) and sea salt are mainly distributed in the supermicrometer fraction but sea salt largely controlled the chemical composition of the supermicrometer particles. Water soluble (WSOM) and water insoluble (WIOM) organic matter mostly distributed in submicrometer particles with a large contribution. Primary organic matter (OM) resulted mostly insoluble and mainly produced by bubble bursting processes at the sea surface, as shown by Bubble Bursting experiments performed during MAP and thus indicating the prevalent secondary origin of WSOM. The overall results clearly evidenced that concentration of the main secondary components (nss-SO4=, NH4+, WSOM, MSA) are driven by seasonality of oceanic biological productivity. Furthermore, a new secondary component, alkylammonium salts, has been detected in marine aerosol samples. Dimethyl- (DMA+) and diethyl- (DEA+) ammonium, resulted the most abundant organic species, second only to MSA, detected in fine particles. These two species accumulate in submicrometer marine aerosol via acid-base reaction between gaseous biogenic amines and acidic sulphates. DMA+ and DEA+ show in analogy to the other biogenic secondary components, a variation in concentration clearly driven by biological activity. This result point to a potential important new source of marine SOA and atmospheric nitrogen at the global scale with a seasonal variation connected to the oceanic biological productivity. In the framework of the project AEROCLOUDS, night-time and daytime size segregated aerosol samples were collected concurrently at the urban site of Torre Sarca (Milan), at the urban background site of Ispra (VA), at the rural site of San Pietro Capofiume (Bologna), at the mountain site of Mt. Cimone (2165 m a.s.l.) and at the urban background site of Montelibretti (Rome), during three typical patterns of the aerosol climatology in Italy: • Aerosol accumulation in summer anticyclonic conditions (14 to18 July 2007); • Aerosol accumulation in winter anticyclonic conditions (6 to 10 February 2008); • Saharan dust long-range transport in spring (23 to 27 May 2007). At the stations located within the Po basin (Milan, Ispra and San Pietro Capofiume), the main chemical aerosol components exhibited typical seasonal features observed in other highly populated and industrialized European areas. Organic compounds and nitrate maximises in winter. Sulphate exhibits higher background levels and a higher relative contribution in summer as its formation is related to photochemical activity. Sulphate and nitrate mainly occur as ammonium salts in the fine fraction as suggested by ionic balance. The winter maximum of organics, nitrate and ammonium mass concentration results in an increase in particulate matter levels due to different mechanisms which during stable atmospheric conditions occur favouring gas to particles conversion of semi-volatile components such as ammonium nitrate and organics. The enhanced nucleation and condensation together with higher emission of fine particles accounts for a net increase of fine aerosol mass with respect to coarse mass as suggested by higher PM1/PM10 mass ratio in winter. Furthermore different diurnal cycle of boundary layer between wintertime and summertime induces differences in diurnal variation of aerosol concentration and chemical composition because in winter, when daytime mixing is very limited in height and time, the diurnal variability in aerosol mass concentration and chemical composition is less pronounced than in summer. The night-time-daytime WSOC/TC ratio values during wintertime and summertime highlighted that higher photochemical activity enhances atmospheric oxidation processes converting biogenic and anthropogenic emissions of organic compounds into a more water soluble form and, therefore, secondary organic aerosol (SOA) formation. At the mountain site of Mt. Cimone, differently from the Po valley stations, the daytime aerosol concentrations showed a clear seasonal pattern with the highest concentration in summer and the lowest in winter. This trend likely reflected the higher efficiency of vertical air masses transport during warmer months suggesting that daytime valley breezes and convective processes might transport polluted air masses from the regional boundary layer to the mountain site. Conversely, wintertime and night-time observations were less affected by influences from the PBL and are representative of the free tropospheric European baseline as suggested by fine aerosol chemical composition rich in sulphate and water-soluble organic compounds and poor in fine particulate nitrate and water insoluble carbonaceous compounds. The impact of dust transport resulted in the increase in bulk aerosol mass concentration during dust transport, in the increased concentration of water soluble calcium in the coarse fraction and in the decrease in PM1/PM10 mass ratio values at all stations with respect to regional background conditions. These changes are all attributable to enrichment into coarse particles of crustal material of Saharan origin. Furthermore we observed two chemical features indicating aerosol ageing during transport from North-African regions: the increase of WSOC contribution to TC and the shift of nitrate size distribution in the coarse range at Mt. Cimone showing that mineral dust occurred at synoptic scale. The impact of Saharan dust transport strongly affected total mass and chemistry of background aerosol at Mt. Cimone, while in urban and polluted sites at lower elevation, the impact on aerosol chemistry was much less pronounced even if specific markers such as calcium revealed that transport occurred.