Aerosol physical and optical properties and their relationship to aerosol composition in the free troposphere at Izaña, Tenerife, Canary Islands, during July 1995

Aerosol physical, optical, and chemical properties were measured at 2360 m above sea level over the Canary Islands during July 1995. Five aerosol size modes were observed. Nucleation aerosols <13 nm (geometric diameter) and a mode at 20 to 30 nm reflected local sources. A mode at 50 to 60 nm (likely from the upper atmosphere), another between 100 to 200 nm (pollutants), and mineral dust reflected distant sources. Green light scattering (adjusted to 1013 mbar and 0°C) ranged from below detection to >160 Mm −1 at relative humidities <30%. The lowest scattering was in air masses from the northwest whose aerosols were mostly pollutants. Air masses with North African mineral dust had the highest scattering. The mass scattering efficiency for dust was 0.50±0.10 m 2 g −1 compared to 0.61 m 2 g −1 at Barbados. Estimates of monthly mean scattering indicate dust accounts for 42 to 70% of total scattering. Soluble K + in aerosols was strongly correlated with total scattering during both dusty and nondusty periods regardless of source. The back‐ to total scattering ratio was essentially constant during the dusty and nondusty sampling periods, 0.138±0.013 and 0.144±0.005. Ångström exponents ranged from 1.0 to 2.1 during the nondusty period and −0.12 to 0.72 during the dusty period. The estimated single scattering albedo appeared higher during the dusty period than during the nondusty period, suggesting mineral dust to be more cooling at 550 nm than pollutants.