Abstract
Abstract. Aerosol light scattering, absorption and particulate matter (PM) concentrations were measured at Montseny, a regional background site in the Western Mediterranean Basin (WMB) which is part of the European Supersite for Atmospheric Aerosol Research (EUSAAR). Off line analyses of 24 h PM filters collected with Hi-Vol instruments were performed for the determination of the main chemical components of PM. Mean scattering and hemispheric backscattering coefficients (@ 635 nm) were 26.6±23.2 Mm−1 and 4.3±2.7 Mm−1, respectively and the mean aerosol absorption coefficient (@ 637 nm) was 2.8±2.2 Mm−1. Mean values of Single Scattering Albedo (SSA) and Ångström exponent (å) (calculated from 450 nm to 635 nm) at MSY were 0.90±0.05 and 1.3±0.5 respectively. A clear relationship was observed between the PM1/PM10 and PM2.5/PM10 ratios as a function of the calculated Ångström exponents. Mass scattering cross sections (MSC) for fine mass and sulfate at 635 nm were 2.8±0.5 m2 g−1 and 11.8±2.2 m2 g−1, respectively, while the mean aerosol absorption cross section (MAC) was 10.4±2.0 m2 g−1. The variability in aerosol optical properties in the WMB were largely explained by the origin and ageing of air masses over the measurement site. The MAC values appear dependent of particles aging: similar to the expected absorption cross-section for fresh emissions under Atlantic Advection episodes and higher under aerosol pollution episodes. The analysis of the Ångström exponent as a function of the origin the air masses revealed that polluted winter anticyclonic conditions and summer recirculation scenarios typical of the WMB led to an increase of fine particles in the atmosphere (å = 1.5±0.1) while the aerosol optical properties under Atlantic Advection episodes and Saharan dust outbreaks were clearly dominated by coarser particles (å = 1.0±0.4). The sea breeze played an important role in transporting pollutants from the developed WMB coastlines towards inland rural areas, changing the optical properties of aerosols. Aerosol scattering and backscattering coefficients increased by around 40 % in the afternoon when the sea breeze was fully developed while the absorption coefficient increased by more than 100 % as a consequence of the increase in the equivalent black carbon concentration (EBC) observed at MSY under sea breeze circulation.
Highlights
The Mediterranean Basin is a very complex area where orography and atmospheric dynamics coupled with a large variety of aerosol sources give rise to a complex mixture of atmospheric particulate matter (PM)
Mean values of aerosol scattering, backscattering and absorption coefficients at the regional background station selected for this study (Montseny, NE Spain) were quite low compared with values reported in literature in more industrialized areas or cities around the Mediterranean Basin
An interesting feature of all these measured parameters was that these were driven by the sea breeze, which develops in the late morning/afternoon transporting polluted air masses from the highly urbanized/indutrialized coastline toward remote areas inland
Summary
The Mediterranean Basin is a very complex area where orography and atmospheric dynamics coupled with a large variety of aerosol sources give rise to a complex mixture of atmospheric particulate matter (PM). The particles in the atmosphere affect the Earth’s climate by cooling or heating the atmosphere depending on their scattering and absorbing properties with respect to the solar and terrestrial radiation. The magnitude of the current aerosol effect on climate is very poorly defined given that aerosols are present in the atmosphere in a huge variety of sizes, shapes, chemical composition, refractive index, etc. Fine particles with aerodynamic diameter lower than 1 μm (PM1) are highly effective in scattering and absorbing solar radiation depending on their chemical composition.
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