Long-term Variability of Aerosol Optical Properties at Mauna Loa

ABSTRACT We investigated the variability of aerosol scattering (σsp; 1974-2015) and absorption (σap; 2000-2015) coefficients at the Mauna Loa Observatory from surface in-situ measurements. σsp increased in the afternoon hours (3.72 ± 7.63 Mm-1 at 550 nm, 14–17 local standard time (LST)) due to the development of thermally induced boundary layer winds, but decreased during the morning hours (1.85 ± 3.43 Mm-1 at 550 nm, 8–11 LST). No distinct diurnal variation was observed in σap. The distinct increase in σsp and σap during the spring under free tropospheric conditions (8–11 LST) is attributed to long-range transported aerosols from Asia, and especially dust and pollution aerosols from Northeast Asia, and biomass burning aerosols from Southeast Asia. Accordingly, σsp increased during the period of 1974–2015 (i.e., 1.89 %/year), whereas no significant trend was noted in both σsp and σap during the period of 2000–2015. An increasing trend for σsp was predominant for the air mass which originated from Northeast Asia (+0.51 Mm-1/decade).