Optical properties of tropospheric aerosols based on measurements of lidar, sun-photometer, and visibility at Chung-Li (25°N, 121°E)
Introduction
Various types of aerosol affect significantly the air quality, visibility and radiation budget of earth's atmosphere. These effects strongly depend on properties of aerosol particles. For example, aerosols can directly affect the climate by scattering and absorption of solar and terrestrial radiation. These effects depend strongly on the particle size and their optical properties (Kaufman et al., 2002; Charlson et al., 1992). Various aerosols also affect cloud formation and thereby affect the radiation indirectly (Twomey, 1991; Kaufman et al., 2002). Sizes of aerosols and their surface properties are important for their effectiveness as cloud condensation nuclei (CCN). However, characterization of various aerosols types is difficult despite their importance for understanding the environmental and climate effects.
Local pollution, biomass burning, and dust are major aerosols sources which have been investigated at several regions (Eck et al., 1999; Smirnov et al., 2002; Kaufman et al., 2002). Eck et al. (1999) used aerosol robotic network (AERONET) sun-photometer to derive the optical thickness and particle sizes of dust, biomass burnings, and urban aerosols. Their results showed biomass burning and urban aerosols with high optical thicknesses by the dominance of the accumulation mode (<0.6 μm) in the aerosol size distribution. Smirnov et al. (2002) also used AERONET data to investigate the optical properties of aerosols. They analyzed the diurnal cycle for most major urban/industrial areas within the network and found a large optical thickness for aerosols from biomass burning in Zambia, where the spring aerosols showed about 20% increase in optical thickness compared with background aerosols. Kaufman et al. (2002) reviewed climate impacts for various types of tropospheric aerosols based on AERONET, in situ measurements, and moderate resolution imaging spectroradiometer (MODIS) satellite data.
Aerosols are now widely monitored by using lidars, which are especially useful to provide vertical profiles of backscattering coefficient and depolarization ratio. The former may compare with sun-photometer measurements to derive particle size (Murayama et al., 1999; Wandinger et al., 1995) and later is useful to characterize aerosols shapes (Sassen, 2000). Lidar is also useful for long-term and short-term observations. Lidar network in Europe and Asia have been established to meet these needs (Murayama et al., 2001; Ansmann et al., 2003; Moorthy et al., 2003; Balis et al., 2004).
We have measured the tropospheric aerosols by using a lidar system located at Chung-Li (25°N, 121°E) from February 2002 to May 2004. Aerosols coming from Northern China and Southeast Asia are dominant during the spring. These two types of aerosols sometimes mix at different height regions. They may also mix with local pollutants. Therefore, characterizing the optical properties of various types of aerosols is of primary importance in understanding their environmental and climate effects.
The objective of this paper is to report characteristics of aerosols, including seasonal variation, optical thickness, and size distribution derived from lidar, sun-photometer, satellite images, and trajectory studies. We will describe the experimental and measurements methods in Section 2. The observation results and discussion are given in Section 3, and finally the conclusion in Section 4.
Section snippets
Lidar system
The lidar system used in the present study consisted of a Nd:YAG laser at a wavelength of 532 nm. The receiver consists of a 20 cm Schmidt–Cassegrain telescope. Signals are measured by photomultipliers and analyzed by multi-channel analyzers. Details about lidar research can be found in Nee et al. (1998) and Chen et al. (2002).
The lidar measures height profile of backscattered signal from aerosols, during clear sky, which are converted into backscattering ratio (R). The aerosols backscattering
Background aerosols
To understand the background aerosols distribution, vertical profiles of lidar backscattering ratio in the height region 1–30 km for the period 2002–2004 have been derived after screening the data for clouds by choosing backscatter ratio and depolarization ratio smaller than the values of 5 and 0.1, respectively. Fig. 1 shows the mean height profile of backscattering ratio, R, for this period. It is evident from this figure that the largest loading occurs below 5 km. The magnitude of
Conclusion
Aerosols originated from a combination of anthropogenic, biomass burning and dust sources have been investigated by lidar, sun-photometer, and visibility data. The sources of aerosols were investigated by satellite and back trajectory studies. The optical properties in terms of lidar backscattering ratio, depolarization ratio and optical thickness are determined and related to their sources. The larger aerosols loading occurred in the spring and with heights below 5 km primarily brought by
Acknowledgements
This research was supported by the National Science Council of ROC by Grant NSC92-2111-M-008-007, NSC93-2111-M-008-006 and the Environmental Protection Agency of Taiwan through a contract EPA-91-U1L1-02-109. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa.gov/ready.html) used in this publication and the Center for Space and Remote Sensing Research (CSRSR) for the
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