Thirty Years of Atmospheric Extinction from Telescopes of the North Atlantic Canary Archipelago

Abstract

his study examines 30 years of atmospheric extinction, τ, obtained from both stellar and solar telescope measurements, at ~2.4 km MSL, from the North Atlantic Canary Archipelago?an island chain located at approximately 28°N, around 100 km from the west coast of Africa. Data from three AERONET monitors, located at varying heights on one of the main islands, were also used, although these are only available over a shorter (<10 yr) period. The Canary Archipelago is regularly affected by dust intrusions into the local atmosphere as they intersect one of the primary export pathways of mineral dust from the Sahara. The τ of ?baseline? and ?dust influenced? conditions were statistically distinguished by fitting normal-gamma mixture distributions to the observations using Markov chain Monte Carlo methods, and then the seasonal and long-term characteristics of these data were examined. The telescope data show that baseline conditions are usually stable at τ < 0.1 (except during periods influenced by volcanic aerosols) and indicate the existence of a low-amplitude seasonal variation. During dust-influenced conditions, τ regularly reaches values of a factor of 2?6 times higher than normal. The majority of dust intrusions take place during the months of July and August, when they may occur 44 ± 15% of the time, predominantly at high altitudes (with ~94.3 ± 1.6% of intrusions occurring ≥ 2.4 km), whereas during the months of November?May, dust intrusions occur far less frequently (~19 ± 7%) and are more common at lower altitudes?with intrusions at <2.4 km comprising ~ 79.5 ± 3.2% of all outbreaks. Year-to-year variations in the frequency of dust-influenced conditions (of ~9%) were found but no long-term trend over the observed 30-yr period.