Free tropospheric aerosol backscatter, depolarization ratio, and relative humidity measured with the Raman lidar at Nagoya in 1994–1997: contributions of aerosols from the Asian Continent and the Pacific Ocean
Introduction
Free tropospheric aerosol particles are frequently transported long distances and can affect remote areas. The transport of mineral dust particles called Kosa (yellow sand) in Japanese, which originate from the arid and semi-arid lands of the Asian Continent, to the North Pacific region has been studied for over 20 yr (Duce et al., 1980; Merrill et al., 1989). In addition to these natural aerosols, there is concern over the influence of anthropogenic emissions from Asian industrial regions on the tropospheric chemistry in the Pacific region (Dentener et al., 1996; Hoell et al., 1997; Hatakeyama et al., 1997). However, most of the studies of the influences of particle sources in Asia on the remote free troposphere have been confined to the case studies, and there have been few statistical assessments of the contributions of the sources on it.
Several aircraft measurement campaigns have provided information on the physical and chemical properties of free tropospheric aerosol particles in the North Pacific region (e.g., Iwasaka et al., 1988; Ikegami et al., 1993; Pueschel et al., 1994). The seasonal variation of vertical profiles of aerosol extinction coefficient over Tsukuba in Japan was measured by Sasano (1996) using a Mie scattering lidar.
Raman lidar has been developed to remotely measure the vertical profiles of humidity and aerosol optical properties simultaneously (Whiteman et al., 1992; Ansmann et al., 1992; Shibata et al., 1996a, Shibata et al., 1996b; Sakai et al., 1997). Since aerosol properties such as size, phase, shape and chemical composition critically depend on the ambient relative humidity (Orr et al., 1958; Hänel, 1976; Tang, 1996), it is important to measure aerosol properties with the relative humidity in their natural state. Ferrare et al. (1998) derived the aerosol hygroscopic growth factor, real refractive index and single scattering albedo by using the aerosol extinction and backscattering profiles measured with the Raman lidar in conjunction with the aerosol size distributions measured with the optical particle counter onboard an aircraft.
In addition to aerosol backscattering and extinction coefficients obtainable with the lidar, depolarization ratio is also useful parameter for the study of aerosol microphysics because deviations from zero can indicate particle nonsphericity (e.g., Bohren and Huffman, 1983). The particle shape critically controls the particle's optical properties (e.g., Koepke and Hess, 1988), and hence affects radiative processes as well as the vertical distribution of the particles (Lacis and Mischenko, 1995).
The purpose of this study is to investigate the seasonal and altitude characteristics of the aerosol backscattering and the depolarization ratio in relation to the relative humidity in the free troposphere on the basis of Raman lidar observations at Nagoya, Japan (35.1°N, 137.0°E) from March 1994 to February 1997. Backward trajectory analyses were performed to investigate the relation between the aerosol optical properties and the transport pathways from the source areas.
Section snippets
Observations of the Raman lidar
The Raman lidar system is on the campus of Nagoya University (35.1°N, 137.0°E) in an urban area located 20 km north of the Ise Bay and 70 km inland from the Pacific Ocean. Specifications of the lidar system are listed in Table 1. More details are given by Shibata et al. (1996a). Three wavelengths of an Nd : YAG laser are vertically transmitted into the atmosphere and the backscattered light from atmospheric gases and aerosol particles is collected with a Cassegrain telescope. The light is
Vertical profiles of backscattering ratio, aerosol depolarization ratio and humidity
The four vertical profiles shown in Fig. 1 represent the seasonal characteristics of the aerosol optical properties and the humidity measured with the Raman lidar on 21 April, 23 July, 11 September and 23 December in 1994. The humidity and temperature profiles obtained with the radiosonde (launched 30 m away from the lidar at 22 : 31 JST on 10 September) are also shown in Fig. 1c. Fig. 2 shows the 5-day isentropic backward trajectories arriving at the lidar site at the altitudes of 3 and 6 km
Depolarization ratio as a function of relative humidity and chemical composition of aerosols
The chemical composition of typical aerosol particles can be classified into three types according to the particle shape and water-solubility (Fig. 6). The three types are as follows. (a) Water-insoluble particles (e.g., mineral dust): these particles are mostly nonspherical and hence indicate high δa values independent of RH. (b) Water-soluble droplets (e.g., H2SO4/H2O solution): these particles do not change phases with changes in tropospheric condition and always indicate near zero
Conclusion
The observed aerosol optical properties in relation to the relative humidity and the transport pathways indicated the following characteristics according to season and altitude.(1) The vertically integrated aerosol backscattering coefficients showed an annual maximum with sharp rises in the spring (March–May) in the 2–4 km and 4–8 km ranges. The three months mean in this season was larger than the annual mean by 1.6–2.2 times in the 2–4 km range and by 1.6–1.8 times in the 4–8 km range. The second
Acknowledgements
This research was supported by Japan Ministry of Education, Science, Sports and Culture (Grant-in-Aid for Creative Fundamental Research, Studies of Global Environmental Change with special reference to Asia and Pacific Regions, led by Professors Tamura, S., University of Tokyo, Matsuno, T., Hokkaido University, and Tanaka, M., Tohoku University; Grant number: 02NP0101, 03NP0201, 04NP0201, and 05NP0201).
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