MBAS (Methylene Blue Active Substances) and LAS (Linear Alkylbenzene Sulphonates) in Mediterranean coastal aerosols: Sources and transport processes

Methylene Blue Active Substances (MBAS) and Linear Alkylbenzene Sulphonates (LAS) concentrations, together with organic carbon and ions were measured in atmospheric coastal aerosols in the NW Mediterranean Basin. Previous studies have suggested that the presence of surfactants in coastal aerosols may result in vegetation damage without specifically detecting or quantifying these surfactants. Coastal aerosols were collected at a remote site (Porquerolles Island-Var, France) and at a more anthropised site (San Rossore National Park-Tuscany, Italy). The chemical data were interpreted according to a comprehensive local meteorological analysis aiming to decipher the airborne source and transport processes of these classes of compounds. The LAS concentration (anthropogenic surfactants) was measured in the samples using LC-MS/MS, a specific analytical method. The values were compared with the MBAS concentration, determined by a non-specific analytical method. At Porquerolles, the MBAS concentration (103 ± 93 ng m-3) in the summer samples was significantly higher than in the winter samples. In contrast, LAS concentrations were rarely greater than in the blank filters. At San Rossore, the mean annual MBAS concentration (887 ± 473 ng m-3 in PM10) contributed about 10% to the total atmospheric particulate organic matter. LAS mean concentration in these same aerosol samples was 11.5 ± 10.5 ng m-3. A similar MBAS (529 ± 454 ng m-3) - LAS (7.1 ± 4.1 ng m-3 LAS) ratio of ∼75 was measured in the fine (PM2.5) aerosol fraction. No linear correlation was found between MBAS and LAS concentrations. At San Rossore site the variation of LAS concentrations was studied on a daily basis over a year. The LAS concentrations in the coarse fraction (PM10-2.5) were higher during strong sea storm conditions, characterized by strong air flow coming from the sea sector. These events, occurring with more intensity in winter, promoted the formation of primary marine aerosols containing LAS from the sea surface microlayer. In contrast to LAS, MBAS concentrations in the coarse fraction peaked in summer. Therefore, different sources are suggested for the two classes of compounds. In summer, increased MBAS concentration could be an indicator of phytoplanktonic blows enriching the sea surface microlayer with biogenic surfactants. In winter, some spikes in MBAS concentrations in the coarse fraction could be related to LAS concentrations and sea storm events. Moreover, in the fine fraction MBAS have a large concentration maxima in winter, corresponding to air masses coming from polluted continental areas. It is concluded that MBAS concentration in the fine or the coarse fraction is not an appropriate surrogate measurement of LAS concentration in aerosols because of the significant contribution from other sources to the MBAS concentration.