Physical, optical and chemical properties of atmospheric aerosols in the western Mediterranean continental background

Abstract

[cat] Els aerosols atmosfèrics han jugat un paper crucial en el desenvolupament de l’atmosfera terrestre. Sense les partícules atmosfèriques la pluja no hauria existit i el clima seria molt diferent. No obstant això, les emissions antropogèniques han canviat de forma significativa la composició química dels aerosols atmosfèrics degut a l’emissió de partícules i de gasos precursors. A més a més, nombrosos estudis han demostrat que els aerosols atmosfèrics, tant naturals com antropogènics, no només influencien el clima de la Terra, sinò que també provoquen efectes perjudicials per la salut humana, els ecosistemes i la qualitat de l’aire. En aquesta tesi doctoral es presenta un estudi detallat de les propietats físiques, òptiques i químiques dels aerosols de fons continental mitjançant les mesures realitzades a l’estació del Montsec (MSC, 1570 m s.n.m.). Aquesta estació de muntanya està situada a l’oest del Mediterrani, es va establir durant el desenvolupament d’aquesta tesi i forma part de la xarxa GAW (Global Atmosphere Watch) des de 2014. Les propietats dels aerosols (massa (PM), nombre (N), absorció (σap), dispersió (σsp) i composició química) mesurades al MSC van presentar magnituds comparables a les obtingudes a altres llocs remots d’Europa, una vegada eliminada la influència de les intrusions de pols sahariana. Per tant, la designació de fons continental és aplicable per a l’estació del MSC. Tanmateix, algunes diferències entre les propietats mesurades al MSC i les mesurades a altres estacions remotes europees van revelar que els processos de formació de noves partícules (angl., new particle formation, NPF) com a font de partícules són molt importants a la regió oest del Mediterrani. També van destacar que els processos industrials i de combustió tenen una menor contribució a la matèria particulada (angl., particulate matter, PM) ambiental, mentre que les emissions biogèniques i dels vaixells tenen un major impacte. A més a més, van corroborar que el transport de llarga distància de la pols del Sàhara és la font natural de partícules més important en aquesta regió, i van revelar que la pols de resuspensió regional té un impacte considerable.

[eng] Atmospheric aerosols have played a key role in the development of the Earth’s atmosphere. Without atmospheric particles rainfall would be nonexistent and the climate would be very different. However, anthropogenic emissions have changed the chemical composition of atmospheric aerosols significantly through emissions of particles and precursor gases, particularly since the industrial revolution. Moreover, numerous studies have demonstrated that atmospheric aerosols, both natural and anthropogenic, do not only influence Earth’s climate, but also have adverse effects on human health, ecosystems, as well as on air quality. The global distribution of the aerosols is very uneven; therefore atmospheric particles have very different concentrations and composition depending on the region. In southern Europe, and in particular in the western Mediterranean region, the meteorological conditions with higher temperatures and lower precipitation, together with the abrupt topography around the Mediterranean Sea, hinder aerosol dispersion and prevent atmospheric wet-scavenging processes. Moreover, Mediterranean region is highly influenced by both anthropogenic emissions from shipping, power generation, industrial processes and road traffic, among others sources, and natural emissions from African dust outbreaks, soil resuspension, wildfires, sea spray, and vegetation. All these particular and complex atmospheric dynamics result in higher concentrations of aerosols around the Mediterranean Basin than in northern Europe. A detailed study of the physical, optical and chemical properties of the continental background aerosols measured at Montsec (MSC) monitoring station is presented in this work. The MSC mountain site (1570 m a.s.l.) was established during the development of this thesis; it is located in the western Mediterranean and became a member of the GAW (Global Atmosphere Watch) network in 2014. Aerosol properties (particle mass (PM), particle number (N), absorption (σap), scattering (σsp) and chemical composition) from MSC were found to be comparable in magnitude to those from other remote sites in Europe, when removing African dust outbreaks influence. This suggests that a continental background designation for MSC site is applicable. Nevertheless, some differences between MSC and other European remote sites highlight the importance of new particle formation (NPF) processes as a source of particles, the lower contribution of combustion and industrial processes, the greater impact of shipping emissions, and the higher influence of biogenic emissions. Moreover, these differences corroborate the important contribution of long-range transport of mineral dust and reveal a high impact of regional dust resuspension in the western Mediterranean region. Continental background aerosols in this region are affected by natural and anthropogenic emissions which results in a PM1 (PM of diameter less than 1 µm) comprised of (in decreasing concentrations) organic matter, sulfate, ammonium, mineral matter, nitrate, elemental carbon, sea salt and trace elements. The foremost PM1-10 (PM of diameter between 1 and 10 µm) constituent is mineral matter, followed by organic matter, nitrate, sulfate, sea salt, ammonium and elemental carbon. Comparison of σap measurements with those of elemental carbon revealed an average mass absorption cross section (MAC) at MSC of 9.1 m-2 g-1 . A clear seasonal variation is observed for the majority of the aerosol parameters measured at MSC, with the highest values in summer and the lowest in winter, except for nitrate, in agreement with previous studies in the western Mediterranean. The reduced human activity at the weekend is reflected in the concentrations of PM1 and BC with a delay of one day (minimum on Sunday and Monday), which confirms that MSC is located at a sufficient distance from direct anthropogenic emissions, although it is still affected by them. The intra-day variation of PMx, BC, σsp and PM1 major inorganic components (sulfate, nitrate, ammonium and chloride) shows poorly defined daily patterns in summer but clearer in winter, whereas concentrations of N and submicron organic aerosol show marked diurnal cycles throughout the year and regardless of the air mass origin, with a clear increase around midday.