Aerosol characterisation in the subtropical eastern North Atlantic region using long-term AERONET measurements

Abstract

Abstract. A comprehensive characterisation of atmospheric aerosols in the subtropical eastern North Atlantic has been carried out using long-term ground-based Aerosol Robotic NETwork (AERONET) photometric observations over the period 2005–2020 from a unique network made up of four stations strategically located from sea level to 3555 m on the island of Tenerife. This site can be considered a sentinel for the passage of airmasses going to Europe from Africa, and therefore the aerosol characterisation performed here adds important information for analysing their evolution during their path toward Northern Europe. Two of these stations (Santa Cruz de Tenerife – SCO – at sea level and La Laguna – LLO – at 580 m a.s.l.) are located within the marine atmospheric boundary layer (MABL), and the other two (Izaña – IZO – at 2373 m a.s.l. and Teide Peak – TPO – at 3555 m a.s.l.) are high mountain stations within the free troposphere (FT). Monthly climatology of the aerosol optical depth (AOD), Ångström exponent (AE), aerosol concentration, size distribution and aerosol optical properties has been obtained for the MABL and FT. Measurements that are quite consistent across the four sites have been used to categorise the main atmospheric scenarios, and these measurements confirm an alternation between predominant background conditions and predominant dust-loaded Saharan air mass conditions caused by seasonal dust transport over the subtropical North Atlantic. Background conditions prevail in the MABL and FT for most of the year, while dust-laden conditions dominate in July and August. The MABL under background conditions appears as a well-mixed layer with a low aerosol concentration (the volume concentration, VolCon, ranges from 0.02 ± 0.01 to 0.04 ± 0.02 µm3 µm−2), a predominance of coarse-mode marine aerosols (the effective radius, Reff, changes from 1.60 ± 0.19 to 1.91 ± 0.34 µm), and a volume contribution of the fine-mode fraction Vf/Vt <0.35. The clean FT is characterised by remarkably low aerosol loading and a predominant impact of fine-mode aerosols throughout the year (Vf/Vt has a maximum value of 0.93 ± 0.13), with an average Reff of 0.16 ± 0.02 µm. However, under dust-laden conditions and mainly in summer, we observe a predominance of coarse-mode aerosols with maximum VolCon values of 0.26 ± 0.23 µm3 µm−2 for the MABL 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 0.12 ± 0.03 in the vertical within the MABL and FT. Similarities in micro-physical and optical intensive aerosol properties confirm that the Saharan Air Layer (SAL) is 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, and this shows that aerosol loading at IZO is double that at TPO, but they have similar fine-mode fractions, effective radii and intensive optical properties. The long-term trend analysis at SCO shows a significant negative trend in the fine-mode AOD between 2005 and 2020 (−1.8 ± 0.5) × 10−5 yr−1, which might be linked to the large reduction in oil-refining SO2 emissions from the SCO refinery in 2012.

Performance and systematic uncertainties of CTA-North in conditions of reduced atmospheric transmission

“…The default spring-summer seasonal dependence of the U.S Standard Atmosphere is chosen for tropospheric aerosols for layers from 2 to 10 km, and the background stratospheric aerosol profile for layers above 10 km. The model gives a total aerosol optical depth of ≈ 0.16 at 550 nm, which is in agreement with previous studies of aerosol profiles over the North Atlantic [39][40][41][42] and a relative difference of only 1.43% compared to the official atmospheric model for CTA-N used in sim_telarray.…”

LIME: Lunar Irradiance Model of ESA, a new tool for the absolute radiometric calibration using the Moon

“…Izaña also experiences 243 clear sky days per year (Toledano et al, 2018), which is critical because even thin high clouds significantly perturb the Langley calibration. Regarding the aerosol climatology, dominant background conditions are expected at both sites, with more than 69% of the time under pristine conditions (AOD<0.1 and Ångström Exponent>0.75), as shown in Barreto et al (2022). Only Saharan mineral dust episodes (about 20% of days) affect these high-altitude locations, mainly in summer (July and August).…”

The Langley ratio method, a new approach for transferring photometer calibration from direct sun measurements

“…Aerosol characterization at the site is dominated by freetroposphere conditions throughout the year, with remarkably low aerosol loading (AOD 500 nm of 0.03 or lower and average Ångström exponent, AE 440−870 nm , values of 1.01) and a predominant impact of fine-mode aerosols (Barreto et al, 2022b). Dust-laden conditions are predominantly observed in summer, with AOD 500 nm of 0.15 and AE 440−870 nm of 0.54 (Barreto et al, 2022b).…”

“…Aerosol characterization at the site is dominated by freetroposphere conditions throughout the year, with remarkably low aerosol loading (AOD 500 nm of 0.03 or lower and average Ångström exponent, AE 440−870 nm , values of 1.01) and a predominant impact of fine-mode aerosols (Barreto et al, 2022b). Dust-laden conditions are predominantly observed in summer, with AOD 500 nm of 0.15 and AE 440−870 nm of 0.54 (Barreto et al, 2022b). The predominance of extremely clean atmosphere throughout the year makes IZO a calibration site for GAW-PFR and AERONET-Cimel networks, providing an advantage when comparing the two instruments since it eliminates, to a great extent, the errors caused by turbidity or atmospheric instability.…”