Application of lidar depolarization measurement in the atmospheric boundary layer: Effects of dust and sea-salt particles

We intensively observed the atmospheric boundary layer with a polarization lidar, a Sun photometer, and a high-volume sampler at a coastal area of Tokyo Bay. The purpose of the observation is to investigate a phenomenon discovered in the past summer: relatively high depolarization ratio events (≈10% at peak) in the lower atmosphere associated with sea breeze. From the chemical analyses of the simultaneously sampled aerosols, we found that the depolarization ratio might be related to crystallized sea salt and dust particles. A boundary structure was clearly revealed by the depolarization ratio in the lower atmosphere, which might correspond to the mixed layer (the internal boundary layer) or the sea breeze in which crystallized sea salt and/or dust particles were diffused. We also presented the first numerical calculation on the depolarization ratio of the cubic particles to apply crystallized sea-salt (NaCl) particles by the dipole discrete approximation (DDA) method: the calculation yields 8-22% of depolarization ratio for the effective size larger than 0.8 μm at the investigated wavelength (532 nm).