Diurnal and longer‐term temporal changes in the composition of atmospheric particles at Mauna Loa, Hawaii

Abstract

The collection of midtropospheric atmospheric particulate matter during upslope and downslope mountain winds was accomplished by using two types of sector controllers between February 1979 and April 1982 at Mauna Loa Observatory (MLO) on the island of Hawaii. Initially, a simple time‐of‐day system was used for sampling. This was replaced in May 1980 by a digital system which monitors meteorological conditions, aerosol data, as well as time of day to differentiate between upslope and downslope winds. Particles derived from the marine environment were found to be much lower in concentration during downslope winds than upslope winds (15 ng/m³, σ_g = 4.0 versus 270 ng/m³, σ_g = 2.5). The mean ratio was 14 with simple time‐of‐day control and 21 with the improved and more discriminating digital sector controller. Crustal‐derived particles were 2.4 times lower in concentration in downslope winds. The presence of particles of Hawaiian basalt was determined by noting its quite different chemical composition as compared with dust transported from continents, over long distances into the area. Local basalts appear to be a minor source of crustal particles with concentrations of local basalt at MLO estimated to be 15 ng/m³ (σ_g = 3.3) in downslope winds versus 87 ng/m³ (σ_g = 2.6) in samples collected under upslope conditions. After correction for this local crustal material, the upslope/downslope crustal dust ratio still remained high (2.2) and exhibited strong seasonal variations, being much lower between mid‐February to July (1.2) when high concentrations of dust are transported from Asian deserts to Hawaii, than during the remainder of the year (3.2). Upslope sulfate concentrations averaged 1.7 times higher than downslope conditions at MLO (530 ng/m³, σ_g = 1.9, versus 340 ng/m³, σ_g = 2.0).