Chemical characterization of atmospheric aerosols from natural and anthropogenic sources in the Mediterranean area

Abstract

The Mediterranean Sea basin constitutes a semi-enclosed area where atmospheric particles originating from natural and anthropogenic continental sources and gas-to-particle conversion processes are present at all times. The area is, in fact, located to the south of highly populated European countries characterized by industrial, semi-industrial, and rural economies, and to the north of Africa, which includes the Sahara desert. Detailed wind trajectory analysis reported in previous research studies show that more than 60% of air masses crossing the Mediterranean originate from the north-northwest sector, containing particles emitted or derived from industrial and urban sources, whereas 13–16% of air masses coming from the Sahara region carrying predominantly mineral dust. The transport of Saharan dust occurs mostly during the spring and summer seasons and causes sporadic crustal aerosol pulses to the Mediterranean area. On the other hand, aerosol scavenging by precipitation during the rainy season (from October to May) reduces aerosol concentrations. Summer is also characterized by low inversion layers and strong sunlight conditions, causing photochemical smog. Moreover, forest fires, which occur during the summer months in the Mediterranean region and in North Africa, increase black carbon and fine particle emissions. In this frame, it is clear enough that specific meteorological conditions result in high temporal variability of aerosol concentrations. There is strong evidence on the relationship between short-term and long-term exposure to atmospheric particles, with adverse health effects. Therefore, the study on atmospheric Particulate Matter (PM) (solid or liquid particles dispersed in the atmosphere which may persist for long times to undergo transport and diffusion phenomena), and the relative chemical composition of the two particle size fractions PM2.5, (aerodynamic diameter ≤ 2.5 μm) and PM10 (aerodynamic diameter ≤ 10 μm), is essential to evaluate the effect of the PM on human health and environment. The present work of thesis developed during the Ph.D. is focused on the chemical characterization of aerosol in the Mediterranean area through a monitoring program which has foreseen a number of oceanographic campaigns performed in the Mediterranean sea onboard the CNR-research vessel in the framework of the ongoing MEDOCEANOR measurements program as well as long-term measurements carried out on-land, specifically at the high altitude GAW observatory “Monte Curcio” of the CNR-IIA (1780 m a.s.l.), located on the Sila massif, Southern Italy, and thus able to intercept long-range transport air masses and across a number of monitoring sites (i.e., coastal, urban, rural sites etc.) distributed in the south of Italy as part of the I-AMICA regional network. The concentration of aerosol size fractions and its chemical composition performed at permanent ground-based stations as well as during oceanographic measurement campaigns have been analyzed in order to assess a spatially and temporally consistent measurement data across Mediterranean basin, and to investigate the main natural and anthropogenic sources affecting the air quality using source apportionment techniques. The seasonal oceanographic campaigns developed along different routes in the western sector of the Mediterranean Sea basin, and aimed to study the influence of natural and anthropogenic sources of PM and associated levels of pollutants. Chemical analysis assisted by the receptor models, identified, in particular, six main sources: crustal, volcanic, biomass burning, marine spray, industrial and vehicular traffic. The carbonaceous content in the PM sampled in Monte Curcio station shows seasonal trends for Organic Carbon (OC) and Elemental Carbon (EC) in both PM size fractions. The concentrations during the warm season are higher than those observed during the cold season and the annual levels of EC and OC were lower than those observed at the other four monitoring sites as part of the regional network “I-AMICA” distributed in southern Italy (Capo Granitola, Lamezia Terme, Lecce; Naples) due to different environmental conditions (eg, coastal/marine, suburban and urban) characterizing these sampling sites compared to “Monte Curcio” remote site. In particular, both OC and EC average concentrations were minimal at Monte Curcio and increased in the following order: remote < coastal/marine < suburban < urban (i.e., Monte Curcio < Capo Granitola < Lamezia Terme < Lecce < Naples). The Secondary Organic Carbon (SOC) was mainly present in PM2.5 at all sites, and higher SOC/OC ratios were observed at the urban and suburban site. Indeed, the yearly average SOC in Monte Curcio station has been estimated as 52% of OC in PM2.5 and representing, on average, the major mass contributes to PM2.5 during the cold season. Furthermore, the receptor models used shown differences among the possible sources of carbonaceous aerosol between different seasons. The cold season was characterized by aerosol mainly coming from the long-range transport, while during the warm season it is influenced by local and regional sources. In the following Chapters, the results have been presented and discussed.