Evidence for ships emissions in the Central Mediterranean Sea from aerosol chemical analyses at the island of Lampedusa
1Department of Chemistry, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino (Florence), Italy
2ENEA, Laboratory for Earth Observations and Analyses, 92010, Lampedusa, Italy
3ENEA Laboratory for Earth Observations and Analyses, 00123, S. Maria di Galeria, Roma, Italy
4ENEA, Laboratory for Earth Observations and Analyses, 90141, Palermo, Italy
5Department of Physics, University of Florence and I.N.F.N., Sez. Florence, Via Sansone 1, 50019 Sesto Fiorentino (Florence), Italy
Abstract. Measurements of aerosol chemical composition made on the island of Lampedusa, south of the Sicily channel, during years 2004–2008, are used to identify the influence of ship emissions on aerosol particles in the Central Mediterranean. Evidence of ship emissions influence is found in 17% of the daily samples. Aerosol samples influenced by ships are characterized by elevated Ni and V soluble fraction (about 80% for aerosol from ships, versus about 40 % for crustal particles), high V and Ni to Si ratios, and values of Vsol>6 ng m−3. Back trajectories analysis on the selected events show that air masses prevalently come from the Sicily channel, where an intense ship traffic occurs.
Vsol, Nisol, and non-sea salt SO42− (nssSO42−) show a marked seasonal behaviour, with an evident summer maximum. Such a pattern can be explained by several processes: (i) increased photochemical activity in summer, leading to a faster production of secondary aerosols, mainly nssSO42−, from the oxidation of SO2 in the ship plume; (ii) stronger marine boundary layer (MBL) stability in summer, leading to higher concentration of emitted compounds in the lowest atmospheric layers; (iii) more frequent meteorological conditions leading to consecutive days with trajectories from the Sicily channel in summer.
A very intense event in spring 2008 was studied in detail, also using size segregated chemical measurements. These data show that elements arising from heavy oil combustion (V, Ni, Al, Fe) are distributed in the sub-micrometric fraction of the aerosol, and the metals are present as free metals, carbonates, oxides hydrates or labile complex with organic ligands, so that they are dissolved in mild condition (HNO3, pH1.5).
Data suggest a characteristic nssSO42−/V ratio in the range 200–400 for ship emission aerosols in summer at Lampedusa. By using the value of 200 a lower limit for the ship contribution to total sulphates is estimated. Ship emissions account, as a summer average, at least for 1.2 μg m−3, representing about 30% of the total nssSO42−, 3.9% of PM10, 8% of PM2.5, and 11% of PM1. Within the used dataset, sulphate from ship emissions reached the peak value of 6.1 μg m−3 on 26 June 2008, when it contributed by 47% to nssSO42−, and by 15% to PM10.