Aerosol characterization in the Subtropical Eastern North Atlantic region derived from long-term AERONET measurements

Abstract. A comprehensive characterization of atmospheric aerosols in the Subtropical Eastern North Atlantic has been carried out using long-term ground-based Aerosol Robotic NETwork (AERONET) photometric observations from a unique network made up of four stations strategically located from the sea level to 3555 m height on the island of Tenerife over the period 2005–2020. This site is can be considered a sentinel of the passage of airmass going to Europe from Africa and therefore the aerosol characterization performed here adds important information to analyse their evolution during the path toward Northern Europe. Two of these stations (Santa Cruz de Tenerife –SCO- at sea level and La Laguna –LLO- at 580 m asl) are located within the Marine Boundary Layer 5 (MBL) and the other two (Izana –IZO- at 2373 m asl and Teide Peak –TPO- at 3555 m asl) are high mountain stations within the Free Troposphere (FT). Monthly climatology of Aerosol Optical Depth (AOD), Angstrom Exponent (AE), aerosol concentration, size distribution, and aerosol optical properties has been obtained for the MBL and FT. Quite consistent measurements at the four sites have been used to categorise the main atmospheric scenarios confirming the predominance of the alternating background to dust-loaded Saharan air mass conditions seasonally affecting the sites as a result of the seasonal dust transport over the Subtropical North Atlantic. Background conditions prevail in the MBL and FT most of the year while dust-laden conditions dominate in July and August. The MBL under background conditions appears as a well-mixed layer with low aerosol concentration (volume concentration, VolCon, ranging from 0.02±0.01 to 0.04±0.02 μm3 · μm−2) with a predominance of coarse mode marine aerosols (effective radius, Re ff, changing from 1.60±0.19 to 1.91±0.34 μm) and fine mode fraction < 0.35. The clean FT has been characterised by remarkably low aerosol loading and a predominant impact of fine mode aerosols throughout the year (V f / V t with a maximum value of 0.93±0.13) with an average Re ff of 0.16±0.02 μm. However, under dust-laden conditions, we observe the predominance of coarse mode aerosols, mainly in summer, with maximum VolCon values of 0.26±0.23 μm3 · μm−2 for MBL and 0.16±0.12 (0.06±0.05) μm3 · μm−2 for IZO (TPO) and a similar and quite consistent fine mode fraction of 0.120.03 in the vertical within MBL and FT. Similarities in micro-physical and optical intensive aerosol properties confirm the Saharan Air Layer (SAL) as a well-mixed layer in terms of the particulate composition. An estimation of the difference in the aerosol loading in the 1-km layer between IZO and TPO (in terms of VolCon and AOD) is performed in this study, showing that aerosol loading at IZO is double that of TPO but with similar fine mode fraction, effective radius and optical intensive properties. The long-term trend analysis at SCO shows a negative significant trend in the fine AOD mode between 2005 and 2020 (-1.8±0.5) ·10−5 yr−1, which might be linked to the large reduction of oil refining SO2 emissions at SCO refinery in 2012.