Abstract
ABSTRACT We investigated the variability of the aerosol scattering (σsp; 1974–2015) and absorption (σap; 2000–2015) coefficients at the Mauna Loa Observatory using surface in situ measurements. Although σsp 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 σap. The obvious increase in σsp and σap 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, σsp increased from 1974 till 2015 (at 1.89% year–1), whereas no significant trend was noted for either σsp or σap from 2000 till 2015. An increasing trend for σsp prevailed in air masses originating in Northeast Asia (+0.51 Mm–1 decade–1).
Highlights
Optical and radiative properties of atmospheric aerosols depend on their chemical compositions, shapes, and particle size distributions (Haywood and Ramaswamy, 1998; Delene and Ogren, 2002; Jacobson, 2002)
Elevated Rn-222 concentration—which represents how recently the air mass was in contact with the land surface (Chambers et al, 2011, 2013)—is usually associated with the upslope wind which develops along the ridge of the mountain during the afternoon hours, whereas the upslope wind is responsible for the influx of the planetary boundary layer (PBL) aerosols to Mauna Loa Observatory (MLO) (Ryan et al, 1997)
The transport of scattering-dominant maritime aerosols from PBL to MLO by the aforementioned upslope wind is thought to be the reason for the elevated single-scattering albedo (SSA) in the afternoon, since MLO is located on the Big Island of Hawaii, where no particular industrial activities are held (DBEDT, 2019)
Summary
Optical and radiative properties of atmospheric aerosols depend on their chemical compositions, shapes, and particle size distributions (Haywood and Ramaswamy, 1998; Delene and Ogren, 2002; Jacobson, 2002). These properties exhibit high spatial and temporal variations because of the relatively short lifetime and uneven geographical distribution related to emissions, chemical processes in the atmosphere, and weather patterns (Delene and Ogren, 2002; Andrews et al, 2011; Boucher et al, 2013; Collaud Coen et al, 2013; Park et al, 2019). To investigate the FT background of aerosol characteristics at MLO, it is necessary to deconvolute the influences from LRT and the local PBL
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