Long-term Variability of Aerosol Optical Properties at Mauna Loa

We investigated the variability of the aerosol scattering (sigma(sp); 1974-2015) and absorption (sigma(ap); 2000-2015) coefficients at the Mauna Loa Observatory using surface in situ measurements. Although sigma(sp) decreased during the morning (1.85 +/- 3.43 Mm(-1) at 550 nm, 8-11 local standard time [LST]), it increased during the afternoon (3.72 +/- 7.63 Mm(-1) at 550 nm, 14-17 LST) due to the development of thermally induced boundary layer winds. No distinct diurnal variation was observed in sigma(ap). The obvious increase in sigma(sp) and sigma(ap) during the spring under free troposphere conditions (8-11 LST) is attributed to long-range-transported aerosols from Asia, especially dust and pollution aerosols from Northeast Asia and biomass burning aerosols from Southeast Asia. Accordingly, sigma(sp) increased from 1974 till 2015 (at 1.89% year(-1)), whereas no significant trend was noted for either sigma(sp) or sigma(ap) from 2000 till 2015. An increasing trend for sigma(sp) prevailed in air masses originating in Northeast Asia (+0.51 Mm(-1) decade(-1)).