Dependence of Backscattering Coefficients of Atmospheric Particles on their Concentration and Constitution under Dry and Humid Conditions at Southwestern Japanese Coast in Spring

The dependence of atmospheric particles’ backscattering coefficients (BSCs) on their concentration and constitution in the near surface air at southwestern Japanese coast in the spring of 2011–2015 was investigated. The BSCs were measured with a ceilometer at the wavelength 905 ± 5 nm. The size-segregated concentrations of particles larger than 0.3 µm were measured with optical particle counters. The constitution of particles in each of eight episodes, i.e., the portions of mineral dust, sea salt, sulfate and soot particles, was obtained with individual particle analysis by using an electron microscope. There was a close correlation (R 2 = 0.76) between the BSCs and the volume concentrations of the particles when the relative humidity (RH) was lower than ~70%, regardless of the difference in particle constitutions. In contrast, the BSCs normalized with aerosol concentrations differed largely even at similar concentrations of particles when the RH was larger than 70%. The BSCs of particles dominated by marine origin (sea salt) could be several times larger than those dominated by land origin (dust and soot) at 90% RH, as the latter might be slightly larger than the BSCs under the dry conditions. On the other hand, the difference of volume-size distributions of particles likely also made the BSCs largely different from each other under the humid conditions. These results indicate that the concentration of atmospheric particles was the key parameter to determine the BSCs under dry conditions, while the BSCs under humid conditions were closely dependent on the content of deliquescent components and the size distributions of the particles.