Impact of North America on the aerosol composition in the North Atlantic free troposphere

In the AEROATLAN project we study the composition of aerosols collected over similar to 5 years at Izana Observatory (located at similar to 2400 m a.s.l. in Tenerife, the Canary Islands) under the prevailing westerly airflows typical of the North Atlantic free troposphere at subtropical latitudes and midlatitudes. Mass concentrations of sub-10 mu m aerosols (PM10) carried by westerly winds to Izana, after transatlantic transport, are typically within the range 1.2 and 4.2 mu g m(-3) (20th and 80th percentiles). The main contributors to background levels of aerosols (PM10 within the 1st-50th percentiles D 0.15-2.54 mu g m(-3)) are North American dust (53 %), non-sea-salt sulfate (14 %) and organic matter (18 %). High PM10 events (75th-95th percentiles approximate to 4.0-9.0 mu g m(-3)) are prompted by dust (56 %), organic matter (24 %) and non-sea-salt sulfate (9 %). 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 (similar to 32 degrees N, January-March) to high latitudes in summer (similar to 52 degrees N, August-September). The westerlies carry maximum loads of non-sea-salt sulfate, ammonium and organic matter in spring (March-May), of North American dust from midwinter to mid-spring (February-May) and of elemental carbon in summer (August-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 suggests that 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.