Aerosol Extraction in the Taklimakan Desert

Transcription

1 Atmospheric Pollution Research 6 (2015) Atm spheric Pollution Research Validation of aerosol optical depth and climatology of aerosol vertical distribution in the Taklimakan Desert Xuemei Zong 1, Xiangao Xia 1,2, Huizheng Che 3 1 Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 10029, China 2 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing , China 3 Key Laboratory for Atmospheric Chemistry (LAC), Institute of Atmospheric Composition Chinese Academy of Meteorological Sciences (CAMS), China Meteorological Administration (CMA), Beijing , China ABSTRACT Based on ground based sun photometer remote sensing of aerosol optical depth (AOD) at Tazhong, a site located at the center of the Taklimakan Desert in 2007 and 2008, AOD retrieved from Cloud Aerosol and Infrared Pathfinder Satellite Observations (CALIOP) data were validated. Six years vertical profiles of aerosol extinction coefficient in the Taklimakan Desert were then analyzed. A good agreement between ground based and CALIOP remote sensing AOD data was derived, with the correlation coefficient being CALIOP slightly underestimated AOD that is likely due to lower lidar ratio of dust than the real value in the CALIOP aerosol algorithm. Pronounced inter annual and seasonal variations of vertical profiles were revealed by the CALIOP retrievals. The height of dust aerosol layer can reach 4 5 km, which is more pronounced in spring and summer. Larger and smaller extinction values were observed in spring (March, April and May) and in later autumn (October and November), respectively. Dominant contribution of dust was clearly shown by the vertical profiles of color ratio (CR) and particle depolarization ratio (PDR). Keywords: Dust aerosol, aerosol vertical profile, CALIOP Corresponding Author: Xuemei Zong : : : zongxm@mail.iap.ac.cn doi: /APR Article History: Received: 29 May 2014 Revised: 04 September 2014 Accepted: 19 September Introduction Dust aerosol, as one major type of aerosols, strongly influence the transfer of radiant energy and the spatial distribution of latent heating through the atmosphere, thereby influencing the weather and climate (Kaufman et al., 2002; Lohmann and Feichter, 2005). The radiative impact of dust aerosols have ever been evaluated by all kinds of data with radiative transfer models (Highwood et al., 2003; Wang et al., 2006; Huang et al., 2009; Xia and Zong, 2009). The vertical distribution of dust aerosols is one of the critical parameters in the assessment of the dust radiative effect. Elevated layer of dust over bright surfaces can result in significant radiative forcing that depends on the dust loading as well as its properties (Carlson and Benjamin, 1980; Zhu et al., 2007). Large uncertainties are still associated with dust aerosols climatic effects due to very limited understanding of substantial spatial and temporal variability of dust aerosol physical and chemical properties although we do our best to measure aerosols properties (Tegen et al., 1996; Carslaw et al., 2010; IPCC, 2013). Much attention has been paid to characterize aerosol optical properties based on remote sensing technique in recent two decades. A few observation networks have been set up, such as Aerosol Robotic Network (AERONET, Holben et al., 2001), Aerosol/Radiation Observation Network (SKYNET, Uchiyama et al., 2005), European Aerosol Research Lidar Network (EARLINET, Pappalardo et al., 2014), Global Atmosphere Watch Programme (WMO/GAW, 2003), and China Aerosol Remote Sensing Network (CARSNET, Che et al., 2009). Satellite remote sensing technology is joined with ground based remote sensing to provide better spatial coverage (Cavalieri et al., 2010). For example, Moderate Resolution Imaging Spectrometer (MODIS, Remer et al., 2005) used for aerosol optical depth (AOD) retrievals, Multi angle Imaging Spectrometer (MISR, Kahn et al., 2005) was a useful supplement for MODIS AOD by different retrieval methods, Ozone Monitoring Instrument (OMI, Torres et al., 2007) obtained absorbing aerosol properties, and Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP, Winker et al., 2009; Winker et al., 2013) used for the vertical distribution of aerosols. Aerosol optical properties at the global scale are provided by these satellite sensors. The Cloud Aerosol and Infrared Pathfinder Satellite Observations (CALIPSO) satellite was developed jointly by NASA and the French space agency CNES to provide new capabilities to observe the vertical distribution of clouds and aerosols from space. CALIPSO satellite was launched (Vaughan et al., 2004; Winker et al., 2009) in April 2006 as a component of the US National Aeronautics and Space Administration (NASA) Afternoon Constellation (Stephens et al., 2002), or A Train. Its primary payload, CALIOP, is a dual wavelength (532 nm and nm), polarization sensitive backscatter lidar and is designed to provide vertical profiling information of clouds and aerosols. The CALIOP data are widely used to characterize seasonal variation of aerosol profiles at the regional and global scale (Liu et al., 2008a; Xia and Zong, 2009; Author(s) This work is distributed under the Creative Commons Attribution 3.0 License.

2 Zong et al. Atmospheric Pollution Research (APR) 240 Winker et al., 2013), to validate model simulations (Yu et al., 2010), to study long range transport of dust and biomass burning aerosols (Huang et al., 2010). The Taklimakan Desert is the largest desert over China, having an area of km 2. It is bounded by the Kunlun Mountains to the south, the Pamir Mountains and Tian Shan (ancient Mount Imeon) to the west and north, and the Gobi Desert to the east. The Desert fills the Tarim Basin of far Western China. The basin is cut off from the effects of the Asian monsoon, and surrounding mountains block Arctic storms. As a result, the Taklimakan Desert receives little precipitation. Marching sand dunes, some reaching a height of 200 meters, cover most of the desert floor, providing ample material for dust storms that blow eastward over China and even reach the Pacific Ocean (Huang et al., 2008). Understanding of dust optical properties in this Desert is still urgently required to further our understanding of atmospheric and climatic effects of dust aerosols. The first step of this paper is to validate CALIOP AOD products using ground based sunphotometer remote sensing data at Tazhong. Then the seasonal variation of dust aerosol profiles over Taklimakan Desert was analyzed. 2. Data The main data used in this paper is the CALIOP Level 2 aerosol profile products (CAL_LID_L2_05kmAPro Prov V3 01) from June 2006 to October We used two parameters, CAD_Score (from 20 to 100) and Ext_QC_flag (0, 1, 18, 16), to ensure good quality of aerosol products used in the analysis. The negative value of CAD_Score means the feature is aerosol. The larger the absolute value, the more confident the feature. CAD_Score [ 100] means complete confidence of aerosol. In CALIOP retrieval, the layer detection is quite accurate. Some false positives are still found beneath optically thick layers, can generally be identified by their very low CAD_Scores ( CAD_Score 20). So CAD_Score (from 20 to 100) can be confident of aerosol layer. On the other hand, the broad range can increase sample numbers which is important for statistics. Ext_QC_flag of 0 means unconstrained retrieval such as initial lidar ratio unchanged during solution process. Ext_QC_flag of 1 means constrained retrieval. Ext_QC_flag of 16 means layer being analyzed has been identified by the feature finder as being totally attenuating (i.e, opaque). Ext_QC_flag of 18 represents an unconstrained retrieval for which the lidar ratio was reduced to prevent divergence, and for which the feature finder has indicated that the layer is opaque (Winker et al., 2013). More information about the CALIOP aerosol products can be found here (U.S. NASA, 2014). Aerosol extinction coefficient was used in the analysis, additionally, aerosol color ratio (CR, the ratio of the backscatter coefficient of particles at the wavelength nm to that at 532 nm) and aerosol particle depolarization ratio (PDR, the ratio of perpendicular and parallel backscatter coefficient of particles at the wavelength 532 nm) were also used in the analysis. PDR is widely used to distinguish spherical or non spherical particle. The higher PDR means pronounced non spherical particles like dust. The CR usually shows the size distribution of particles, something like Angstrom exponent. The higher CR means size of particle is larger. Both of them are often used to be as criteria to separate different aerosol types. Cavalieri et al. (2010) have considered PDR>0.1 for dust and PDR<0.1 for biomass burning aerosols or a mixture of both. Omar et al. (2009) used PDR>0.2 as a criterion of dust from other aerosol types. A Cimel 318 sun photometer was installed at Tazhong (39 00', 83 40', m), the center of the Taklimakan Desert, from 2004 and has been running at this site continuously (Che et al., 2013). The sun photometer makes direct spectral solar radiation measurements within a 1.2 full field of view around 15 minutes at 4 normal bands (440, 675, 870, and nm), 3 polarization bands at 870 nm and 1 water vapor band at 940 nm. Measurements at 440, 675, 870, and nm were used to calculate the aerosol optical depth (AOD) (Holben et al., 1998; Eck et al., 2005). The AOD uncertainty is estimated to be (Eck et al., 1999). 3. Validation of Aerosol Optical Depth in Taklimakan For comparison, The CALIOP data were collocated with sun photometer data if the distance between the track of the CALIOP and Tazhong was not larger than 50 km. Sunphotometer AOD data measured within the CALIOP overpass time of 30 minutes were used in the collocation. Sunphotometer AOD in 532 nm was calculated by interpolation to match the CALIOP AOD of 532 nm. The AOD was calculated at some available wavelengths (440 nm, 670 nm, 870 nm and nm) by this formula (Angstrom, 1929): τ (1) Here, τ λ is AOD at wavelength of λ, α is Angstrom wavelength exponent and β is Angstrom atmospheric turbidity coefficient. The parameters α and β can be fitted by AOD of four wavelengths and then AOD at 532 nm is calculated by this formula. The comparison of AOD at Tazhong between surface and satellite is shown in Figure 1. There are 21 samples in two years. The result shows that the correlation coefficient is 0.95, indicating that CALIOP AODs are in good linear agreement with sunphotometer AODs. The standard deviations of CALIOP AOD are also drawn as error bars in Figure 1. The mean standard deviation is Except two AOD values greater than 0.80, CALIOP AOD is close or slightly smaller than the sunphotometer AOD. Slight underestimation of AOD by CALIOP revealed here is consistent with the result by Schuster et al. (2012) who compared AERONET AOD data at 147 sites with CALIOP AODs. Underestimation of dust AOD retrieved by CALIOP is likely indicative of lower lidar ratio of dust in the CALIOP aerosol algorithm than the real value. CALIOP uses a lidar ratio of 40 sr for dust (532 nm wavelength), which is based upon discrete dipole approximation calculations (Kalashnikova and Sokolik, 2002; Omar et al., 2009). However, the variability of lidar ratios reported for long range dust transport towards Europe and North America is large, and ranges from 30 to 80 sr (Mattis et al., 2002; Mona et al., 2006; Liu et al., 2008b). To be more specific, the lidar ratio should be provided in the CALIOP aerosol extinction profile retrieval algorithm, which is one of the large error sources associated with the CALIOP aerosol extinction profile products (Winker et al., 2013). Figure 1. Comparison between CALIOP AOD and sunphotometer AOD by CIMEL 318 in 2007 and 2008 (MSD is the Mean Standard Deviation).

3 Zong et al. Atmospheric Pollution Research (APR) 241 Two years (2007, 2008) sunphotometer AOD data within satellite overpass time of 30 minutes were used to calculate monthly averaged AOD values, which are compared with corresponding CALIOP AOD data at pixels not far away from Tazhong by 50 km. Figure 2 shows monthly averaged AOD values and their standard deviations from sunphotometer and CALIOP aerosol products. Both of them show similar seasonal variation, i.e., larger AOD in spring and summer and lower AOD in autumn and winter. The maximum monthly AODs are observed in spring (March, April and May). The mean values in spring are 0.90 for sunphotometer and 0.82 for CALIOP. Quite large summer AODs (June, July and August) were also observed, ranging from 0.60 to This indicates that frequent dust storms also occur in summer, which is supported by data analysis of surface meteorological observation data (Zhou, 2001). Analysis of CALIOP aerosol profiles and Multi angle Imaging Spectrometer Radiometer (MISR) AOD data showed that dust aerosols of Taklimakan Desert in summer could be transported to the Tibetan Plateau (Huang et al., 2007; Xia et al., 2008). intensively. Especially, in spring, it is a period of circulation adjustment. The cold and warm weather changes in turn and the strong synoptic system moves rapidly. That leads to higher wind velocities and more frequency of high winds than other seasons (Zhou, 2013). The aerosol layer reaches about 4 5 km in the daytime in spring that is generally larger than that in other seasons. The height of the aerosol layer shows large inter annual variation. The aerosol layer top height in summer can also reach about 3 4 km, which is favorable for long range transport to the Tibetan Plateau (Huang et al., 2007; Xia et al., 2008). Furthermore, the height in the daytime is slightly higher than that of the nighttime. The smallest aerosol extinction coefficient was observed in later autumn (October and November) and the top height of the aerosol layer was less than 2 km. Furthermore, the height of the aerosol layer in the daytime is slightly larger than that in the nighttime. 4. Climatology of Aerosol Vertical Distribution In order to have a good understanding of aerosol profiles in the Taklimakan Desert, Regional mean profiles of CALIOP extinction coefficient, CR and PDR at daytime and nighttime between latitude 36 N 42 N and longitude 75 E 90 E were used to reveal seasonal and inter annual variation of aerosol vertical distribution. The monthly average vertical profile of extinction coefficient at 532 nm from June 2006 to October 2011 in the Taklimakan Desert is shown in Figure 3. Pronounced inter annual variation of dust aerosol profile is clearly shown. The largest aerosol extinction coefficient occurs in spring (March, April and May) as a result of most frequent dust (including blowing sand and sandstorm) cases among four seasons. To be more specific, the inter annual change and secular trend of blowing sand and sandstorm are consistent with the variations of high wind (Zhou, 2001). Chen et al. (1995) and Li and Chen (1999) pointed out that the critical wind velocity of dust emission in the Taklimakan Desert is about 6 m/s at the boundary layer. The number of days with high wind (wind velocity 17.2 m/s) can reach 5 40 days in the East and South of the Taklimakan Desert and often happen in spring and summer Figure 2. Monthly average values between CALIOP AOD and Observed AOD by sunphotometer (MSDs is the mean standard deviation of sunphotometer and MSDc is that of CALIOP). Figure 3. Monthly profiles of aerosol extinction coefficient (km 1 ) in the Taklimakan Desert from June 2006 to October 2011.

4 Zong et al. Atmospheric Pollution Research (APR) 242 Figure 4 and Figure 5 show monthly averaged profiles of CR andpdr.thehighercrvalues(above0.6)andhigherpdr(above 0.2) were observed in the Taklimakan Desert all year round. That clearlyindicatesthetypeofaerosolislargedesertdust.thevalues ofcrandpdrin thedaytimearelarger than innighttime.inthe Taklimakan Desert, the wind velocity in daytime is higher than in nighttime(kong,2008).somelargerdustparticlessettledownon thesurfaceatnightandsmallerparticleshavesmallerpdrsothat CR and PDR in daytime are larger than in nighttime. In general, larger values occur in the lower layer, but in the daytime a few largervaluesalsoappearintheupperlayer.thisislikelybecause strong vertical convection can bring some dust particles to the upperlayerinthedaytime. Figure4.Monthlyprofilesofcolorratio(CR)intheTaklimakanDesertfromJune2006toOctober2011. Figure5.Monthlyparticledepolarizationratio(PDR)intheTaklimakanDesertfromJune2006toOctober2011.

5 Zong et al. Atmospheric Pollution Research (APR) Conclusions CALIOP AOD and aerosol profiles in the Taklimakan Desert were detailed in this study. Major conclusions are as follows: By comparing CALIOP AOD with sunphotometer AOD at Tazhong station, we found that these two data sets were in good agreement, with a correlation coefficient of It should also be noted that CALIOP slightly underestimated AOD, which was likely because relatively lower lidar ratio was used in the CALIOP algorithm than the real value. Pronounced seasonal and inter annual variations of aerosol profiles were revealed. The height of the aerosol layer can reach 4 5 km, the maximum height, in spring. Aerosol layer in October and November did not exceed 2 km. Analysis of CR and PDR clearly indicated that dust aerosols were the dominant aerosol types in the Taklimakan Desert. Acknowledgments The CALIOP aerosol data are available the website of NASA ( calipso.larc.nasa.gov/). 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