Impact of North America on the aerosol composition in the North Atlantic free troposphere
Atmospheric Chemistry and Physics Discussions
In the AEROATLAN project we study the composition of aerosols collected over ~&thinsp;5 years at Izaña Observatory (located at ~&thinsp;2400&thinsp;m&thinsp;a.s.l. in Tenerife, the Canary Islands) under the prevailing westerly airflows typical of the North Atlantic free troposphere at subtropical and mid-latitudes. Mass concentrations of sub10-µm aerosols (PM<sub>10</sub>) carried by westerly winds to Izaña, after transatlantic transport, are typically within the range 1.2 and
... the range 1.2 and 4.2&thinsp;µg&thinsp;m<sup>&minus;3</sup> (20th and 80th percentiles). The main contributors to background levels of aerosols (PM<sub>10</sub> within the 1st&ndash;50th percentiles&thinsp;=&thinsp;0.15&ndash;2.54&thinsp;µg&thinsp;m<sup>&minus;3</sup>) are North American dust (53&thinsp;%), non-sea-salt-SO<sub>4</sub><sup>=</sup> (14&thinsp;%) and organic matter (18&thinsp;%). High PM<sub>10</sub> events (75th&ndash;95th percentiles&thinsp;≈&thinsp;4.0&ndash;9.0&thinsp;µg&thinsp;m<sup>&minus;3</sup>) and are prompted by dust (56&thinsp;%), organic matter (24&thinsp;%) and nss-SO<sub>4</sub><sup>=</sup> (9&thinsp;%). These aerosol components experience a seasonal evolution explained by (i) their spatial distribution in North America and (ii) the seasonal shift of the North American outflow, which migrates from low latitudes in winter (~&thinsp;32º&thinsp;N, January&ndash;March) to high latitudes in summer (~&thinsp;52º&thinsp;N, August&ndash;September). The westerlies carry maximum loads of nss-sulphate, ammonium and organic matter in spring (March&ndash;May), of North American dust from mid-winter to mid-spring (February&ndash;May) and of elemental carbon in summer (August&ndash;September). Our results suggest that a significant fraction of organic aerosols may be linked to sources other than combustion (e.g. biogenic); further studies are necessary for this topic. The present study evidences how long-term evolution of the aerosol composition in the North Atlantic free troposphere will be influenced by air quality policies and the use of soils (potential dust emitter) in North America.