Study of Aerosol Optical Properties Based on Ground Measurements over Sichuan Basin, China

The characteristics of aerosol optical depth (AOD) and Ångström exponent as well as the relationship between the AOD and particulate matter (PM 10), were measured and analyzed at the Chengdu station over the Sichuan Basin in China from February 2007 to December 2009. High monthly AODs were observed in March, August and December, while a low value was observed in October. Monthly variations in Ångström exponent were opposite to the AODs in March and August. The averaged PM 10 showed a significantly seasonal variation with a peak in winter. There is a complicated (not linear) positive correlation between total AOD and PM 10 near the surface. Three typical cases under the conditions of dust and haze were studied, and the results showed that the AODs on the dust days were largest while minimum AODs occurred on haze days. On the contrast, the Ångström exponent distributions among three weather conditions were opposite to the AODs. The 3-day back-trajectory analysis indicated that the origin of the air masses largely affected the aerosol optical properties over the Sichuan Basin.


INTRODUCTION
Atmospheric aerosols affect the earth's climate (Charlson et al., 1992) in two ways: direct radiative forcing by scattering and absorbing solar radiation and indirect effects on the lifetime of clouds and precipitations.Aerosols can also serve as cloud condensation nuclei (CCN) or ice nuclei (IN) (Hansen et al., 1997;Hansen et al., 2000).Aerosol optical properties research is indispensable in the aerosol radiative effect study (Aoki and Fujiyoshi, 2003).
There are many significant researches concerning the optical properties of aerosols in global (Remer et al., 2008) and regional scales (Eck et al., 2005).Because of heavy aerosol loading from natural and anthropogenic activities in China, there are numerous studies concerning the optical properties of several regions, including the northeast, the northwest, the Yangtze River Region and the South of China (Wang et al., 2001;Li et al., 2003a;Xia et al., 2005;Cheng et al., 2008;Che et al., 2009a;Wang et al., 2010;Che et al., 2011;He et al., 2012a;Wu et al., 2012;Che et al., 2013).
Sichuan Basin is located in the southwest of China, which is influenced by the East Asian and Indian monsoon.Luo et al. (2000) have retrieved and analysed the spatial distribution and decadal changes of the AOD over China from 1961 to 1990 and found that there were two maximum centres of Sichuan Basin and South Xin Jiang Basin near 100°E latitude.Li et al. investigated the characteristics of AOD distributions and variations over Sichuan Basin from MODIS data and found that the highest AOD occurred in the spring because of the dust events (Li et al., 2003b).However, there are few studies about the column aerosol optical properties from ground-based measurements over the Sichuan Basin.The primary aim of this paper is to investigate the column aerosol optical properties (i.e., AOD and Ånsgström exponent) by measurements of CE-318 sunphotometer over Chengdu, where the characteristics of aerosols have some degree of representativeness for the urban area of the Sichuan Basin (Li et al., 2003b;Zhang et al., 2008;Lin et al., 2012).Results of this paper will benefit the validation of satellite retrievals and improvement of aerosol modelling over the Sichuan Basin in the future.

Site Description
A Cimel CE-318 sun-photometer was installed on the roof of a tall building next to the Chengdu Meteorological Administration (104°02′20′′E, 30°39′16′′N, 587.0 m) in 2007 and is still running at present.This site is situated in the middle of the Sichuan Basin and to the east of the Qinghai-Tibet Plateau.There are no tall buildings around the platform, avoiding the influence of local terrain and severe local pollution on the data collections.The aerosol optical properties at this site can be representative of the urban areas of the Sichuan Basin.

Instrument
The CE-318 sun-photometer, manufactured by the CIMEL Electronique Company, France, is a multi-channel, automatic, sun-and-sky scanning radiometer that measures not only direct solar irradiance (sun direct channel) but also the sky radiance (sky scattering channel).This type of sun-photometer has eight channels: 1020 nm, 870 nm, 670 nm, and 440 nm bands, three 870 nm polarization channels and a 940 nm channel for water vapour measurements (Holben et al., 1998).Measurements through the sun direct channel at these bands are used to calculate the aerosol optical depth (AOD).Observations by sky scattering channel including almucantar (ALM) and principal plane (PPL) scenario measurements, have sky diffuse irradiance data at four bands of 440 nm, 670 nm, 870 nm, 1020 nm, which can be used for aerosol optical property retrievals such as single scattering albedo, size distribution, refractive index, asymmetry factor, phase function, etc. (Dubovik et al., 2000).The sun-photometer at the Chengdu site is calibrated annually to verify the accuracy and reliability of the data.The calibration procedure has been described by Che et al. (2009b).
In addition, PM 10 data were acquired from Cao Tang substation of the Chengdu Environmental Monitoring Center, which is not far from the CE-318 sun-photometer site.PM 10 is measured by Tapered Element Oscillating Microbalance (TEOM) instruments (Thermo Fisher Scientific Inc., Franklin, MA; Model 1400a), with a precision of 1.0 µg/m 3 .

Data and Methodology
The sun-photometer measurements over Chengdu from Feb 1st, 2007, to Dec 31st, 2009, were used in this study.The AOD data were calculated with the ASTPwin software (Cimel Ltd.Co.) for the Level 1.0 AOD (raw results without cloud screening), the Level 1.5 AOD (cloud-screened AOD based on the work of Smirnov et al., (2000)) and the Ångström exponent between 440 and 870 nm.Daily and monthly mean values of the AOD, Ångström exponent and PM 10 were calculated.During daily average calculation, only those data ≥ 10 times per day were selected for the statistics (Table 1).Finally, there were 3276 observations obtained during the full measurement period.Four seasons are divided into spring (March to May), summer (June to August), autumn (September to November) and winter (December to February) to investigate the seasonal variations of aerosol optical properties in this study.Linear correlation is used to investigate the relationship between the AOD and the PM 10 .

Monthly and Seasonal Variations of AOD
Figs. 1(a) and 1(b) show the monthly and seasonal variations of the AOD with standard deviations at Chengdu station, respectively.The peak values of AOD appeared in March, August and December.The maximum AOD values at 1020 nm, 870 nm, 670 nm and 440 nm in March were 0.55 ± 0.13, 0.64 ± 0.16, 0.85 ± 0.22, 1.29 ± 0.38, respectively (Fig. 1(a)).The AOD values in August at these four wavelengths were 0.42 ± 0.15, 0.53 ± 0.20, 0.76 ± 0.29 and 1.19 ± 0.38, respectively.The AOD values in December were 0.44 ± 0.11, 0.54 ± 0.14, 0.75 ± 0.22 and 1.16 ± 0.36, respectively, which were lower than the values for March and August.On the other hand, the minimum value of AOD appeared in October.The values at the 4 wavelengths above were 0.19 ± 0.06, 0.24 ± 0.08, 0.32 ± 0.12 and 0.50 ± 0.19, respectively.Overall, the values of the monthly averaged AOD at each wavelength in Chengdu were generally quite large, which is consistent with conclusions from previous studies (Luo et al., 2000;Yang et al., 2008).There are two potential reasons for the high AOD values over the Basin.One is the topography of the Sichuan Basin (Zhang et al., 2009).The surface wind speed is low all-year around the Basin.There are high relative humidity and frequent occurrence of calm wind, which lead to a long residence time for aerosol particles and are not conductive to aerosol diffusion.The other reason is due to the large population of Sichuan Province, which is more than 80 million from 2010 according to the sixth national census.Increasing human activities could be one of major factors for the large AOD over the Sichuan Basin.
The mean AOD in autumn is clearly lower than those in other seasons (Wang et al., 2011) with values of 0.27 ± 0.06, 0.32 ± 0.07, 0.43 ± 0.09 and 0.68 ± 0.15 at 1020 nm, 870 nm, 670 nm and 440 nm, respectively (Fig. 1(b)).Large AOD values appeared in the winter and spring and ranged from 0.45 to 0.47, from 0.53 to 0.55, from 0.72 to 0.74 and from 1.07 to 1.13 for each wavelength, respectively.In spring, the Sichuan Basin region could be affected by dust storms from North China (Gong et al., 2003), which could contribute high AOD over the Basin.While in winter, the cold air has weak effect on the Sichuan Basin, aerosol particles from local pollution sources could not diffuse well.Additionally, there is substantial water vapour suspended in the air due to the fog in the winter morning.These factors  contributed to the high AOD in the winter.The high AODs in the summer may be caused by the effect of subtropical anticyclone, which affects most parts of China.The average surface wind speed all over the whole basin is relatively low under the control of the subtropical anticyclone in summer, which is not conducive to the local aerosol transports to other regions.In addition, the relative humidity during summer (Table 2) are large, which could benefit the hygroscopic growth of particles.High temperature also can cause the increase of the secondary organic aerosol particles (Kroll et al., 2008).All of these factors above may be related to a high AOD in summer.In autumn, the change of atmospheric circulation led to autumn precipitation in west China.(Li et al. 2012).Precipitation is beneficial to the diffusion and removal of pollutants in the atmosphere.

Variations in the Monthly and Seasonal Ångström Exponent
Figs. 2(a) and 2(b) show the variation of monthly and seasonal averaged Ångström exponents and the standard deviations at the Sichuan Basin.All of the monthly and seasonal averaged values of the Ångström exponent were more than 0.90, suggesting that the aerosol particle size is small and that the proportion of fine particles is larger than that of coarse particles.The minimum value of the Ångström exponent occurred in April (0.94 ± 0.31), and the maximum value is 1.24 ± 0.16 in November.The fluctuation in the Ångström exponent is not obvious and demonstrated that there is little variation in the aerosol effective radius over the Sichuan Basin.
Low Ångström exponents over the Sichuan Basin all appeared in spring, at approximately 0.96 ± 0.03, while high Ångström exponents appeared primarily in summer (July and August) at 1.21 ± 0.13 and 1.21 ± 0.20, respectively.High values also occurred at the end of autumn (November) at 1.24 ± 0.16.
Combined with the variations in AOD, the main reason for high AOD but low Ångström exponent in spring is the long-distance transport of dust particles from the north and northwest of China (Wang et al., 2011;Gong et al., 2003).The fine particles remained in the atmosphere with long suspension times, which maybe caused high Ångström exponent in summer.During the period of autumn rain in West China (September to November), the relative humidity is high (Table 2), which enhances the hygroscopic growth and thus reduces the Angstrom exponent in autumn.

Relationship between the AOD and Ångström Exponent
Fig. 3 shows the scatter-plot of AOD at 440 nm and the Ångström exponent based on all 3726 measurements during the observation period.Evidently, the relationship between the AOD and Ångström exponent is not a simple linear but rather complicated.The open circles with X error bars mean the averaged AOD for ranges of the Ångström exponent (0.00-0.30, 0.30-0.60,0.60-0.90,0.90-1.20 and 1.20-1.50).
Except for the range of 0.00-0.30, the AOD at 440 nm increased with an increasing Ångström exponent, which illustrated that fine particles were responsible for the high AOD over the Sichuan Basin.He et al. (2012b) pointed out that the wind speed and direction could affect aerosol optical properties.Fig. 4 shows the AOD and Ångström exponent variation depending on surface wind speed at Chengdu.In this study, the surface wind speed is classified by 8 bins of ≤ 2.0, 2.0-2.5, 2.5-3.0,3.0-3.5,3.5-4.0,4.0-4.5, 4.5-5.0 and ≥ 5.0 m/s.In general, the AOD (e.g., τ 440 nm ) decreases with wind speed increasing especial during wind speed less than 4.0 m/s.This probably indicates the wind effect on aerosol particles diffusing.Similar to AOD, the Ångström exponent also decrease with the wind speed increasing, which suggests aerosol particle size becomes larger depending on the wind speed.This can be speculated that large particles could be emitted to the atmosphere or transported from far away by strong surface wind.However, all these speculations should be studied deeply in future.

Relationship between AOD and PM 10
The relationship between AOD at 440 nm and PM 10 is studied to analyse whether chemical composition could influence the optical properties of atmospheric aerosol.The relationship between the monthly AOD at 440 nm and the monthly averaged concentration of PM 10 is investigated using a linear fit method.As Fig. 5 show, it is evident that there is a positive correlation between the AOD at 440 nm and the PM 10 under some circumstances.The correlation coefficient between the AOD at 440 nm and the PM 10 is 0.43 based on 147 samples, which indicated that the AOD at the Sichuan Basin is related not only to the concentration but also to the chemical and other physical characteristics of aerosol particles (Tao et al., 2011(Tao et al., , 2013)).Fig. 6 shows the monthly variation of the AOD, PM 10 and relative humidity <=2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.04.0-4.AOD 440nm or RH Fig. 6.Comparison of monthly variations between AOD at 440 nm and the concentration of PM 10 with the variation of relative humidity.(Li et al., 2005).Values from February to May and from October to December showed the same trend as the concentration of PM 10 .This type of positive correlation suggested that PM 10 most likely contributed to the AOD.However, a negative correlation also existed for the rest of the year.This phenomenon could probably reflect the hygroscopic growth of aerosol particles, especially in summer and early autumn (June-September), when the precipitation and relative humidity is high during this period.AOD reflect the aerosol column extinction characteristic while the PM 10 reflects the "dry" aerosol particles' concentration because there is a heating system to eliminate the moisture when the measurement is carried out.This could be one of the reasons which cause the different variation of AOD and PM 10 in summer and early autumn.
The aerosol chemical compositions have effect on the optical properties.This effect is complicated and not characterised by a simple linear relationship.The AOD data represented the integration of the extinction coefficient of the whole atmospheric column while the PM 10 data just reflected the aerosol concentration of the near surface layer.Thus, more studies concerning the vertical distribution of aerosol particles are needed in future work.

Comparison of Aerosol Optical Properties under Dust and Haze Conditions
Three typical cases of "dust", "haze", and "mixture of dust and haze" were selected according to variations in the aerosol optical properties and the PM 10 as well as the ground meteorological data.In this study, the "dust" case means the aerosol particles of the Sichuan Basin were mainly affected by dust events from deserts of Northwest China.The "haze" means the horizontal visibility is less than 10 km and the relative humidity is less than 90% at the same time.It is caused by severe pollution dominated by local emission.The case of "mixture of dust and haze" is representative for a day affected both by dust event and local pollution.The daily average concentrations of PM 10 of the three case days are 668 µg/m 3 (April 25, 2009), 140 µg/m 3 (March 22, 2009) and 103 µg/m 3 (November, 22, 2009), respectively.One can see that the AOD under dusty conditions (Fig. 7(a)) was obviously higher than that under hazy conditions (Fig. 7(c)).Under the dusty condition, the daily averaged AODs were 1.70 ± 0.04, 1.75 ± 0.04, 1.81 ± 0.04 and 1.90 ± 0.04 at 1020 nm, 870 nm, 670 nm and 440 nm, respectively.However, the AODs were only 0.24 ± 0.06, 0.31 ± 0.09, 0.50 ± 0.13 and 0.82 ± 0.19, respectively, on the hazy day.The AODs on the day with the mixture of dust and haze (Fig. 7(b)) were intermediate between the other two conditions, at approximately 0.75 ± 0.07, 0.88 ± 0.08, 1.12 ± 0.11 and 1.54 ± 0.13, respectively.The Ångström exponent is about 0.12 ± 0.02, 0.83 ± 0.05 and 1.43 ± 0.10 on the dusty day, the day with a mixture of dust and haze and the hazy day, respectively.These distributions suggested that coarse particles dominated the size distribution on the dusty day, while the fraction of small particles is large on the hazy day.On the day with a mixture of dust and haze, the Ångström exponent is intermediate between the other two weather conditions, indicating the effect of both coarse and fine particles.
The 3-day back-trajectory analyses on elevations of 100 m, 500 m and 1000 m AGL (Above Ground Level) were used to study the aerosol sources under different weather conditions using the HYSPLIT mode of the NOAA (Draxler   and Rolph, 2003).There were different transport paths for days with dust, haze and a mixture of dust and haze.Fig. 8(a) illustrates the paths of air mass motion for the dusty day.Both the air masses of 500m and 1000 m above ground level on April 25 th , 2009 passed through the Gobi and deserts of North China, which are considered to be the major sources of Asian dust in spring (Gong et al., 2003;Zhang et al., 2003).Fig. 8(b) shows the air masses on the day with a mixture of dust and haze.Air masses at the 1000 m level were from the Qinghai-Tibet Plateau, where has few human activity.Air masses at the level of 500 m originated from Xinjiang China and went through the Badain Jaran and Tengger Deserts in North China, which could carry substantial quantities of dust particles over Sichuan Basin.Although air mass path at the level of 100 m is similar to that at the level of 500 m, however the air mass just moved along the height of 100 m above ground level which could be affected by the anthropogenic aerosol emission easily.Thus the mean AOD at 440 nm is about 1.54 and the Ångström exponent is about 0.83 due to the mixture of dust and anthropogenic particles.
The back-trajectory analyses on the hazy day are shown in Fig. 8(c).All air masses at the levels of 100 m, 500 m and 1000 m came from East Europe and passed through western Siberia,the Junggar Basin and Qinghai-Tibet Plateau of China where are all regarded as clean regions.Despite the air masses from clean regions, the daily averaged AOD at 440 nm is still as high as approximately 0.82 with the average Ångström exponent about 1.42.This is mainly due to the effect of fine particles emitted from local anthropogenic activities of the Sichuan Basin.

SUMMARY AND DISCUSSION
AOD over the Sichuan Basin was generally high based on nearly 3 years of continuous measurements.AOD variation shows obvious seasonal characteristics.High AOD values appeared in winter and spring while the low AOD values appeared in summer and autumn.Low Ångström exponents appeared in spring while high Ångström exponents appeared primarily in summer.Dust events from North China could contribute to the high AOD and low Ångström exponents in spring over the Basin.Anthropogenic activities have large effect on the aerosol optical properties in other seasons.
The relationship between the AOD and Ångström exponent over Sichuan Basin is not simplely linear but rather complicated.The AOD increased with an increasing Ångström exponent, which illustrated that fine particles were responsible for the high AOD over this region.
There is a positive correlation between the AOD and the PM 10 .However, the correlation coefficient did not reach the 95% significance level, which indicated that the AOD at the Basin is related not only to the concentration but also to the chemical and other physical characteristics of aerosol particles.The mechanisms connecting the chemical composition and optical properties of aerosols require further investigation.
Through the analysis of the HYSPLIT 3-day back trajectory, aerosol optical properties over the Sichuan Basin can be affected obviously by different air masses.Frequent dust event in spring and local anthropogenic activities throughout whole year around are the major aerosol sources over the Sichuan Basin.
This work is financially supported by grants from the Project (41005086, 41005021 and 41130104) supported by NSFC, the Special Scientific Research Funds for Environment Protection Common wealth Section (201009001), the National Key Project of Basic Research (2014CB441201 & 2011CB403401), CAMS Basis Research Project (2012Y02), the research talent fund of Chengdu university of information technology (J201112).

Table 2 .
Monthly precipitation and relative humidity during 2007 and 2009.