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The spatial-temporal distribution of dust aerosol is important in climate model and ecological environment. An observation experiment of the aerosol vertical distribution in the low troposphere was made using the micro-pulse lidar system from Sept. 2008 to Aug. 2009 at the oasis city Kashgar, China, which is near the major dust source area of the Taklimakan desert. The monthly averaged temporal variation of aerosol extinction profiles are given in the paper. The profile of aerosol extinction coefficient suggested that the dust aerosol could be vertically transported from the ground level to the higher altitude of above 5 km around the source region, and the temporal distribution showed that the dust aerosol layer of a few hundred meters thick appeared in the seasons of early spring and summer near the ground surface.
©2013 Optical Society of America
1. Introduction
The aerosol distribution in the atmospheric boundary layer is a critical problem in evaluating the effect of aerosol on climate changes and in making environmental protection policy. Some scientists have pointed that the uncertainty in total anthropogenic radiative forcing is dominated by the uncertainty in aerosol radiative forcing [1], so the evaluation of aerosol effects on climate must take into account high spatial and temporal variation of aerosol. Up to now, the ground-based lidar technology is one of the most effective and direct detection methods to remotely sense the small aerosol particles in the atmosphere due to their enhanced scattering at short wavelengths. After the Micro-Pulse Lidar (MPL) system was developed, it was soon used to measure the aerosol backscattering/extinction coefficient profiles owing to its full-time, care-free, portable and eye-safe advantages [2–4]. In order to study the origination and the transportation of dust aerosols in East Asia, a lot of observational sites were built and continuous measurements were carried out in China, Korea and Japan [5–7]. Most of the measurement experiments have been done east from the Taklimakan desert, while few of the observations were made at the west edge of the Gobi or the desert. In our research, a nine-month field test of aerosol monitoring was carried out using the MPL system at Kashgar for the first time, the reliable analysis of aerosol vertical structure and its monthly variation will be given in the following.
2. MPL atmospheric observations
The Kashgar field observation station is located in Sule County, Xinjiang Uygur Autonomous Region of China, which is about 180 km west from the edge of the Taklimakan desert, the largest desert and the main dust source region of China. There are no industrial manufactories and only a thinly scattered population around the observation station, so the aerosol extinction is mainly caused by the dust and influenced little by other types of aerosols.
The MPL system with a 0.16 m aperture is equipped with a Diode Pumped Solid State (DPSS) Nd: YAG laser as the transmitter source, which transmits 532 nm pulsed laser beam into the atmosphere at a repetition rate of 2500 Hz. And data acquisition is achieved by a photon-counting Channel Photomultiplier (CPMT) with a narrow field-of-view (FOV) telescope receiver and a narrow bandwidth controlled interference filter. The detailed specification is listed in Table 1 .
Table 1. Specification of the MPL System at Kashgar
The elastic lidar equation can be written as
and the aerosol extinction coefficient could be calculated in terms of the MPL backscattering signal according to the Fernald inversion algorithm [8].Where is the range-corrected return signal, is the raw measured signal at range , which is the range from lidar to the aerosol particles and can be simply defined as the mean distance of range gate from lidar; C is the lidar system constant; is the calibration range, where the aerosol extinction coefficient is normally regarded as a known quantity; and are respectively the extinction coefficients of the atmospheric aerosols and molecules; accordingly, and are the backscatter coefficients for aerosols and molecules; is the extinction-to-backscatter ratio for aerosols, and it is assumed to be not changing with range from lidar; the corresponding ratio for the molecular scattering is the constant .Using the above equation, we numerically iterate between adjacent data points in successive steps that can move either out or in from the assigned calibration range. The vertical profiles of aerosol extinction or backscattering coefficient are retrieved from the return signal of the MPL observing data. A fixed extinction-to-backscatter ratio was usually assumed for determining the aerosol extinction from the backscattering cross section, and the sensitivity analysis of different backscatter-to-extinction ratios in inversion algorithms had been widely discussed by many scientists [9–11]. The lidar ratios of Asian dust observed with a high-spectral-resolution lidar and a combined Raman elastic-backscatter lidar varied from 42 to 55 sr in most cases with a mean of 51 sr [12]. For simplicity, theis assumed as 50 sr in our analysis.
However, for MPL, the laser power is sometimes not strong enough to reach an adequate altitude where it is aerosol-free, especially in bad weather days. In this case, the extinction coefficient profiles analyzed by the Fernald method have some errors due to the assumed initial extinction values at the highest point. In order to resolve this problem, a modified Fernald method is introduced based on the analysis of lidar equation, which selects aerosol backscatter ratio at a reference point in data processing [13].
The data in bad weather days (cloudy day or surface visibility less than 5km) are not counted in our analyses, when the backscattering signals are influenced heavily by the low cloud or the strong dust, which leads to a large error in retrieving the extinction coefficient when using the MPL data. A summary of the observable days and the average visibilities at the field observation station during the period of the experiment is listed in Table 2 . For comparison, the floating-dust and flying-dust days, which are defined as surface visibility less than 10 km according to the grade of sand and dust storm weather of China, in the observable days were also recorded in the table.
Table 2. Summary of the Measurement Days and Surface Visibilities
3. Results and discussion
The following figures Fig. 1(a) –Fig. 1(i) show the monthly averaged vertical profiles of aerosol extinction coefficients with their standard deviations, retrieved by the MPL data collected from Sept. 2008 to Aug. 2009 except for the months of December, January and February, during which the MPL system was in maintenance.
Fig. 1 The monthly averaged vertical profile of aerosol extinction coefficient. Error bars are the standard deviations computed from vertical bins of each profile. (a) March; (b) April; (c) May; (d) June; (e) July; (f) August; (g) September; (h) October; (i) November.
The vertical structure of the aerosol spatial distribution is illustrated from the monthly-averaged extinction coefficient profiles in the above figures. Obviously, two kinds of aerosol spatial distribution are classified. Firstly, the aerosol extinction coefficient nearly monotonously decreases from the surface layer to the higher altitude, and this case appears in the months of March, April, May, September and October. Secondly, in the months of June, July, August and November, the aerosol extinction coefficient shows a gradual decrease below about 2 km and an increase tendency in the region of 2 km to 5 km as the altitude ascends. The reason maybe lies in that the thermal convection induced by the summer hot air transports the dust aerosols from the ground to the sky, which leads to the appearance of some maximal values at high altitudes. While in winter, the aerosol extinction coefficients in the upper atmosphere over 2 km increase because of the local residents activity, such as coal burning for heating in a cold weather, which possibly results in a large number of black carbon (BC) aerosols emitted around the observation station or transported from those oases located in the margins of the desert. As for this point, a few scientists have come to a similar conclusion [14].
The spatial-temporal distribution of the aerosol extinction coefficients in the whole observation period is illustrated in Fig. 2 , and the vertical structure and the time evolution are graphically behaved in the diagram. The thick aerosol layer is clearly marked with the contour lines near the ground surface in March and April, when the sandstorm usually sweep through the large parts of China if there was a strong airflow to the southeast, and the minimum value of the aerosol extinction coefficients appears in June and October. The aerosol extinction coefficient in the Taklimakan region shows its descending following its ascending in a year, and the tendency has also been confirmed by some other scientists, who analyzed the Aerosol Optical Depth (AOD) retrieved indirectly from the solar radiation data or measured directly by the sun photometer [15–17].
In order to evaluate the weighting effect of aerosol extinction below the height of 6 km on the extinction of the total atmospheric layer, the AOD at 532 nm was obtained by integrating the extinction coefficients along the lidar pointing path from the surface to 6 km altitude. The month-averaged AODs measured with MPL and the comparison with the results measured by a sun photometer (SP) are shown in Fig. 3 , the SP is used to measure the vertical column AOD at the wavelength of 550nm in the same observational site and on the same day in August and September. The AODs and the relative errors are listed in Table 3 . Considering the measurement errors of the two instruments, the AOD data obtained from the MPL and the SP could be regarded as equal if the relative error 15% could be accepted. So, a conclusion could be drawn that the local aerosol extinction effect is mainly caused by the aerosol layer below the 6 km in the atmosphere.
Table 3. Comparison of the AODs Measured by the Sun Photometer and the MPL
4. Summary
The spatial-temporal variations of aerosol extinction coefficient profiles are given firstly measured by the ground-based MPL in Kashgar nearby Taklimakan desert. Although the preliminary conclusion is made on the basis of an observation program lasting just a time span of less than one year, the data near dust sources are still very valuable for model evaluations and for further studies. It is also illustrated that the local aerosol distributions are influenced by the Taklimakan desert especially below the height of 1000m where the aerosol extinction coefficients are almost more than 0.1. Besides, another important phenomenon is there exists a break term between May and June, when the aerosol extinction coefficients are small in the vertical direction from the surface layer to the high altitude. Further research could be focused on combining MPL measurements with other surface aerosol observing instruments and meteorological data in the lower and upper air for a sufficient long period.
Acknowledgments
The authors are very grateful to the reviewers for valuable comments, and their advice helps us improve this paper for publication.
References and links
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