- Email: [email protected]
Variation characteristics of atmospheric aerosol optical depths and visibility in North China during 1980–1994
Atmospheric Environment 34 (2000) 603}609
Variation characteristics of atmospheric aerosol optical depths and visibility in North China during 1980}1994 Qiu Jinhuan*, Yang Liquan Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China Received 15 January 1998; received in revised form 26 February 1999; accepted 2 March 1999
Abstract Using a method developed by Qiu (Qiu, J., 1998. A method to determine atmospheric aerosol optical depth using total direct solar radiation. J. Atmos. Sci. 55, 734}758), 0.75 lm aerosol optical depths at "ve meteorological observatories in north China during 1980}1994 are retrieved from global direct solar radiation, and variation characteristics of the depths and visibility are analyzed. These observatories are located in the cities of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. It is found that during 1980}1994 the aerosol optical depths show an increasing trend at all "ve sites. During winter the trend is stronger. In winter at Beijing and Wulumuqi, the depth increased by a factor of about two in 15 years. Pollution caused due to the burning of fossil fuel is the main cause of the change. In spring at Geermu the depth is larger and its increase is the quickest among the four seasons, mainly due to desert dust events. The Pinatubo volcanic eruption in 1991 had a signi"cant in#uence on the aerosol optical depth. The yearly averaged depths over "ve sites in 1992 after the eruption increased by 0.068 to 0.212, compared to those in 1990, while from 1992 to 1994 they generally show a decreasing trend. In some cities such as Zhengzhou and Geermu, both visibility and optical depth show an increasing trend during 1980}1994, a possible reason for this is that the aerosol particle vertical distribution shifts up in the troposphere. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are larger, which may be because of the growth of aerosol particles growing in the moist summer. Apart from Geermu, at the other four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin, visibility in summer is about twice larger than that in winter, but the di!erence between depths is small, implying the turbid lower troposphere in winter and the larger extinction coe$cient in the upper troposphere during summer. ( 1999 Elsevier Science Ltd. All rights reserved. Keywords: Direct solar radiation; Visibility; Aerosol; Optical depth; Pinatubo e!ect
1. Introduction Atmospheric aerosol is of great importance to research on the environment and climate changes, and atmospheric correction in the case of space-borne remote sensing. It also plays an important role in the formation process of clouds and fogs, a!ects the budget of the Earth's radiation, and a!ects ozone through the heterogeneous chemical reactions on its surface. Accordingly, volcanic aerosol is regarded as an important perturbation of the Earth}atmosphere system. The eruptions of Mexican El. Chichon in April of 1982 and Philippine
* Corresponding author.
Pinatubo in June of 1991 are two of the most violent ones in this century. Therefore, to study the variation features of atmospheric aerosol contents during the periods around the eruptions is particularly signi"cant. Atmospheric aerosol optical depth is an important physical parameter for indicating atmospheric turbidity and aerosol content, and it is also a crucial factor in determining the aerosol radiance climatic e!ect. This paper emphasizes particularly the long-term variation characteristics of atmospheric aerosol optical depth in China. A narrow-band sunphotometer is currently a common and e!ective ground-based means to detect the aerosol optical depth (Diermendjian, 1980; Quenzel, 1970; Shaw et al., 1973; King et al., 1978; Nakajima et al., 1983; Qiu et al., 1985; Qiu and Sun, 1994). But now there are a few of
1352-2310/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 7 3 - 9
604
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
these kinds of stations having sunphotometer observations in the world, while many meteorological stations only have long-term regular observations of the global direct solar radiation detected by a pyrheliometer. Qiu (1998) developed a method to derive 0.75 lm aerosol optical depth from the wide-band direct solar radiation. By using this method, we derived the aerosol optical depths in 12 meteorological observatories during 1980}1994, and analyzed their variation characteristics.
2. Meteorological observatory, observation instrument and data Fig. 1 shows "ve "rst-class meteorological observatories discussed in this paper, which are located in the cities of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. Apart from Geermu, they are big or medium cities. Wulumuqi and Geermu are located on the Qing-Tibet Plateau, having heights of 918 and 2807 m above sea level, respectively. The height above sea level at other sites are less than 142 m. After 1988, the Chinese-made DFY-3 or TBS pyrheliometer was used in these observatories. The pyrheliometer has a typical spectral response of 0.3}4 lm, view "eld angle of 3321@ or 4324@ and yearly sensitivity stability of $1%. Before 1989, the pyrheliometers made in USSR or modeled on USSRs were used. From 1980 the greatest direct short solar radiation and the corresponding time, mostly between Beijing time 12:00 and 13:00, have been recorded at the "rst class stations in China. We only select the radiation data detected in the cloudless conditions to avoid cloud in#uence. To retrieve 0.75 lm aerosol optical depth from the direct solar radiation, the total column ozone amount and water vapor amount must be determined (Qiu, 1998). In this paper, the total ozone amounts above Beijing are
selected from monthly mean Dobson observation data there. The ozone amounts at the rest of the observatories come from monthly mean data in 1990 derived from Nimbus-7 TOMS observations. According to the study presented by Qiu (1998), a 30% ozone amount error can lead to an aerosol optical depth error of less than 0.0021. So the e!ect of ozone amount error is small. The water vapor amount was determined from local ground water vapor pressure, using an empirical expression suggested by Yang and Qiu (1996).
3. Results and discussion As shown in numerical simulations and 1267 sets of comparative experiments presented by Qiu (1998), the error of the 0.75 lm aerosol optical depth retrieved from total direct solar radiation can generally be less than 10%, and the average result of a lot of optical depth solutions can have much better accuracy. In this paper, seasonally or year-averaged aerosol optical depth with a lot of statistical samples is emphatically analyzed, and so its error being less than 10% is estimated. Total column aerosol optical depth and surface visibility are two important parameters indicating atmospheric column aerosol and surface aerosol contents. The following is to analyze variation characteristics of 0.75 lm aerosol optical depths and visibility at the "ve observatories according to Figs. 2}6 and Table 1. In these "gures, (a) shows yearly averaged optical depth and visibility from 1980 to 1994; (b) shows seasonally averaged depth; and (c) shows total monthly averaged depth and visibility during 1980}1994. Here dq and dR are annual changes of aerosol optical depth and visibility during 1980}1994, respectively, and r is the correlation coe$cient between the depth and visibility. In Figs. 2}6a, trend lines of the depth (solid) and visibility (dotted) are also shown. Visibility data at Beijing time 14:00 in the clear days are used. In addition, January, February and December are regarded as `Wintera; March, April and May `Springa; June, July and August `Summera; September, October and November `Autumna. Table 1 lists mean aerosol optical depth (q ) and an! nual change of the season-averaged optical depth (dq) during 1980}1994, di!erence (*q) of yearly averaged optical depth in 1992 to that in 1990. At "rst, our analysis is devoted to variation trends of aerosol optical depths and visibility at the "ve observatories. Then, Pinatubo and El. Chichon e!ects on the depths, and tropospheric aerosol pollution e!ect on the depths and visibility are studied, respectively. 3.1. Variation trends
Fig. 1. Map of "ve meteorological observatories in China.
As shown in Figs. 2}6a, as far as the yearly averaged optical depths are concerned, at all "ve observatories
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
605
Fig. 2. Variation characteristics of aerosol optical depth and visibility over Wulumuqi during 1980}1994. (a) Year-averaged optical depth and visibility; (b) Season-averaged optical depth; (c) Total month-averaged optical depth.
Fig. 3. Same as in Fig. 2 except that the observations are made over Geermu.
they have an increasing trend during 1980}1994. The yearly increasing rate changes from 0.0009 to 0.0168. Especially in Beijing, the rate is up to 0.0168. Surface visibility is a measure of horizontal optical depth and aerosol pollution, and its decrease is responsible for an increase of the depth. Again as shown in Figs. 2}6a, at Wulumuqi, Harbin and Beijing observatories, it is normal that visibility has a decreasing trend, and the depth has an increasing trend. It is noted that in Beijing, although the depth has the strongest increasing trend, decreasing trend of visibility (dR"!0.031 km) is not obvious. It is more interesting that at Zhengzhou and Geermu both visibility and optical depth have an in-
creasing trend during 1980}1994 (see Fig. 3a and Fig. 6a). An explanation of the result is that the aerosol particle vertical distribution in the troposphere shifts up, probably owing to the uplifting trend of factory chimneys there since 1980. It can also be found from Figs. 2}6a that the correlation coe$cient r between year-averaged aerosol optical depth and visibility is negative for all observatories, changing from !0.202 (Harbin) to !0.584 (Geermu). The anti-correlation implies that when visibility is lower, the optical depth is usually larger. It would be pointed out that the correlation in Harbin is weak, and so to use visibility for indicating total optical depth can lead to a larger error. In addition, there is anti-
606
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Fig. 4. Same as in Fig. 2 except that the observations are made over Harbin.
correlation between the depth and visibility at Geermu and Zhengzhou, but they both have an increasing trend during 1980}1994 (as far as the variation trend is concerned, they are positively correlated). The trait can be reasonable considering that there are di!erent aerosol pollution sources in the lower troposphere, upper troposphere and stratosphere. Furthermore, di!erent variation trends of optical depths for four seasons are analyzed from Figs. 2}6b and Table 1. As shown in Table 1, for all "ve observatories and four seasons the optical depths have an increasing trend (positive change rate) except for summer at Wulumuqi,
Fig. 5. Same as in Fig. 2 except that the observations are made over Beijing.
Geermu and Harbin. The strongest increasing trend of optical depth usually occurs during winter, which is the heating period in north China, especially at Beijing, the yearly increasing rate of the depth is up to 0.0254. Only at Geermu, near some desert areas, the strongest increasing trend occurs in spring. As shown in Fig. 5b, the optical depth at Beijing in the winter of the early 1990s is about twice as large as that of the early 1980s, especially after 1988, it increases very markedly. As a result, the optical depth of winter before 1989 is obviously smaller than that of other seasons, but since 1990 the di!erence among depths is small.
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Fig. 6. Same as in Fig. 2 except that the observations are made over Zhengzhou.
In summer at Wulumuqi, Geermu and Harbin the depths have a decreasing trend. 3.2. Pinatubo and El. Chichon ewects The eruptions of El. Chichon in 1982 and Pinatubo 1991 are two of the most violent ones in this century. Because total column aerosol optical depth contains tropospheric and stratospheric aerosol contributions, a quantitative analysis of the eruption e!ect on the depth is di$cult with only the depth data. As shown in Figs. 2}6, aerosol pollution in the troposphere is lighter at Geermu, and so total optical depth can more quantitat-
607
ively re#ect the eruption e!ect. Next, mainly based on a qualitative analysis, the e!ect is studied. It can be seen from Figs. 2}6 and Table 1 that there is a signi"cant e!ect of Pinatubo and El. Chichon eruptions on the optical depths. As shown in Figs. 2}6a, at all "ve observatories, during 1990}1994 there is a maximum value of year-averaged optical depth around 1992 or 1991, and from 1992 to 1994, the depth has a reducing trend. The season-averaged depth has usually the same variation trait for all four seasons (see Figs. 2}6b). Di!erence of the depth in 1992 after the Pinatubo eruption with that in 1990 before the eruption is 0.148, 0.101, 0.068, 0.212 and 0.194 at Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou (see Table 1), respectively, showing an evident depth increase. Because year-averaged visibility in 1992 is less than that in 1990 for all observatories, surface aerosol pollution would have some contribution to the depth increase, especially at Beijing and Zhengzhou. According to lidar measurements presented by Yang and Qiu (1998), the stratospheric aerosol depth in 1992 is 0.094 larger than that in 1990. Using these results, the depth increase of about 0.1, caused by Pinatubo eruption, is estimated for four observatories of Wulumuqi, Geermu, Beijing and Zhengzhou, located over the latitude region of 34.7}43.83N. During 1981}1984, a depth peak also happened in 1982 or 1983. At Beijing, the yearly averaged depth in 1982, when El. Chichon eruption broke out, is 0.19 larger than that in 1981. Visibility in 1981 and 1983 are almost the same, but the depth in 1983 is 0.15 larger than that in 1981. At Geermu, from 1981 to 1983 both the depth and visibility increase. The depth in 1983 increased by an amount of 0.051, compared with that in 1981. In addition, some contradictory results can be found in Figs. 2}6a. For example, as shown in Fig. 2a, from 1990 to 1991 both the depth and visibility at Wulumuqi decrease, which cannot re#ect the Pinatubo e!ect. But if there is a signi"cant decrease of the middle or upper tropospheric aerosol extinction coe$cient from 1990 to 1991, the contradiction can be understood. A further study for this is needed. 3.3. Tropospheric aerosol pollution ewect A stratospheric volcanic aerosol layer can remain for a long time (about four years). Therefore, variation of monthly or seasonally averaged stratospheric aerosol depth would be small, and a large monthly or seasonal variation of the total depth is mainly dependent on tropospheric aerosol pollution. In China, fossil fuel (especially coal) is most important for industrial and living applications, and its burning is usually the main source of the tropospheric aerosol pollution, especially in winter in northern China (the heating period).
608
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Table 1 Mean aerosol optical depth (q ) and annual change of the depth (dq) during 1980}1994, di!erence (*q) of yearly averaged optical depths ! in 1992 and 1990 dq Site
Winter
Spring
Summer
Autumn
q !
*q
Wulumuqi Geermu Harbin Beijing Zhengzhou
0.0130 0.0016 0.0104 0.0254 0.0086
0.0065 0.0022 0.0016 0.0089 0.0064
!0.0021 !0.0004 !0.0052 0.0134 0.0079
0.0029 0.0016 0.0025 0.0121 0.0058
0.316 0.181 0.299 0.489 0.463
0.148 0.101 0.068 0.212 0.194
In north China, desert dust is another important aerosol pollution source. Main deserts are located in Xingjiang and the Inner Mongolia provinces, which are the main origins of Asian dust storms. Especially in spring, a lot of desert particles can be blown by strong winds and be transported over most areas of north China towards the east or southeast track in the height range of about 2}7 km (Qiu and Sun, 1994; Yang et al., 1991; Iwasaka et al., 1983). It can be seen in Figs. 2}6 and Table 1 that the pollution due to the burning of fossil fuel and desert dust events has an important e!ect on the optical depth and visibility in north China. Figs. 2}6c show the total monthly averaged depth and visibility during 1980}1994. As shown in Fig. 2c, at Wulumuqi in northwest China, visibility is very low in winter. From January to June it greatly increases, and then from June to December decreases. The visibility (+38 km) in June is about thrice larger than that (+13 km) in January or December. The depth from June to September is relatively small, being about 0.27, and in the months of winter and spring it is '0.32, while in February and March it is '0.4. Clearly, in winter at Wulumuqi the lower atmosphere is very turbid, mainly owing to coal burning during the heating period, and in summer and early autumn the tropospheric atmosphere is clearer. In February and March, the larger optical depth may be caused by both serious coal-burning pollution and desert dust events. Again observing Fig. 2a, just in winter the depth has the strongest increasing trend during 1980}1994. Geermu is a small town located on the Qing-Tibet plateau between Taklamagan desert in Xingjiang province and the Inner Mongolia desert. As shown in Fig. 3c, visibility is more than 28 km for all months, indicating a light surface aerosol pollution, and the depth is relatively small except for the months from April to August. But in spring there is lower visibility and larger optical depth. Again observing Fig. 3b, the depth in spring is about 0.14 larger than that in winter or autumn for any year during 1980}1994. It is estimated that the increasing value of
0.14 is mainly caused by desert dust events. In addition, just in spring the yearly increasing rate of the depth is largest. In summer at Geermu, the larger optical depth may be due to tropospheric aerosol particles growing in a high humidity. At Harbin of northeast China, visibility has similar month variation characteristic as at Wulumuqi. As shown in Fig. 4c, visibility is very low in winter (+10 km). From January to July it linearly increases and then from July to December it decreases. Visibility (+30 km) in July is about thrice larger than that in January or December. The optical depth has a di!erent monthly variation trait. It is smaller in autumn, being about 0.22. Visibility in summer is about twice larger than that in winter, but di!erence between optical depths of the two seasons is small. Therefore, in winter at Harbin the lower atmosphere is also turbid, but the upper troposphere is clearer. In summer, the lower atmosphere is clear, but there is the larger aerosol extinction coe$cient in the middle or upper troposphere, probably caused by aerosol particles growing in the humid summer. Considering the statistical average for 1980}1994, visibility in Beijing during September or August is largest, while in other months it changes between 14 and 16.5 km (see Fig. 5c). The optical depths in winter in Beijing are much less than those in spring and summer during 1980}1988. But as analyzed above, because of the quicker increase of the depth during winter, in the early 1990s its di!erence with other seasons is small. In all four seasons at Beijing there is an evident increasing trend of the seasonally averaged depths, and since 1990 they have been all larger than 0.45, showing very serious aerosol pollution. In addition, during spring when an Asian dust storm happens the depth is larger. As shown in Fig. 6c, at Zhengzhou there is a positive correlation between total monthly-averaged optical depth and visibility. In winter visibility and optical depth are relatively smaller, and in summer they are larger. In January visibility is low, down to 11 km, showing the turbid lower atmosphere in winter. The largest optical depth occurs in spring. For example, the depth in May is
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
about twice as large as that in January. And for almost all years from 1980 to 1994, the seasonally averaged depth in spring is the largest (see Fig. 6b). This implies that the spring desert dust events may have an important e!ect on the depth. As shown in Table 1, mean values of the 0.75 lm aerosol optical depths during 1980}1994 are 0.316, 0.181, 0.299, 0.489 and 0.463 at "ve observatories of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou, respectively. Clearly, at Beijing and Zhengzhou aerosol pollution is very serious. At Geermu located on the Qing-Tibet plateau, the depth is smaller, being about one-third of that in Beijing.
4. Conclusions (1) In north China during 1980}1994, the total column aerosol optical depth has an increasing trend. The yearly increasing rate of the yearly averaged optical depth changes between 0.0009 and 0.0168 for the "ve meteorological observatories of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. Especially at Beijing, the rate is up to 0.0168. Except for summer at Wulumuqi, Geermu and Harbin, the seasonally averaged optical depths have also an increasing trend for all four seasons and "ve sites. The strongest increasing trend of the depth is usually during winter. In winter at Beijing and Wulumuqi, in 15 years the depth increased by a factor of about two. Fossil fuel burning Pollution due to the burning of fossil fuel is the main source of the change. In spring at Geermu the depth is larger and its increase is the quickest among the four seasons, mainly due to the desert dust. (2) The eruptions of El. Chichon in 1982 and Pinatubo in 1991 had important in#uences on aerosol optical depth in north China, especially the latter eruption. Yearly averaged depths over "ve sites obviously increased in 1992 after the Pinatubo eruption by 0.068}0.212, compared to those in 1990, and from 1992 to 1994, the depths show a decreasing trend. (3) Over the two big cities of Beijing and Zhenzhou, aerosol pollution is serious. The total averaged aerosol optical depths during 1980}1994 are 0.489 and 0.463, respectively. At Beijing after 1989 the depths are all larger than 0.5 for all seasons. At Geermu located on QingTibet plateau, the depth is smaller, being about one-third of that in Beijing. (4) At all "ve sites, optical depths in spring are larger, probably owing to frequent desert dust events. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are also larger, which may be because of aerosol particles growing in the moist summer. Apart from Geermu, at the other four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin,
609
visibility in summer is about thrice larger than that in winter, but di!erences between the depths are small, implying the turbid lower troposphere in winter and the larger extinction coe$cient in the upper troposphere during summer. (5) In some sites such as Zhengzhou and Geermu, during 1980}1994 both the aerosol optical depth and visibility show increasing trends. A possible reason for this is that the aerosol particle vertical distribution in the troposphere moves up.
Acknowledgements This research was supported by National Natural Science Foundation of China.
References Diermendjian, D., 1980. A survey of light-scattering technigue used in the remote monitoring of atmospheric aerosol. Reviews of Geophysics and Space Physics 18, 341}360. Iwasaka, Y., Minoura, H., Nagaya, K., 1983. The transport and spatial scale of Asian dust-storm clouds. Tellus 35, 189}196. King, M.D., Byrne, D.M., Herman, B.M., Reagan, J.A., 1978. Aerosol size distributions obtained by inversion of spectral optical depth measurements. Journal of Atmospheric Science 35, 2153}2167. Nakajima, T., Tanaka, M., Yamauchi, T., 1983. Retrieval of the optical properties of aerosol from aureole and extinction data. Applied Optics 22, 2951}2959. Qiu, J., Sun, J., 1994. Optically remote sensing of the dust storm and the analysis. Chinese Journal of Atmospheric Science 18, 1}10. Qiu, J., Wang, H., Zhou, X., Lu, D., 1985. Experimental study of remote sensing of atmospheric aerosol size distribution by combined solar extinction and forward scattering method. Advances in Atmospheric Science 2, 307}315. Quenzel, H., 1970. Determination of size distribution of atmospheric aerosol particles from spectral solar radiation measurements. Journal Geophysical Research 75, 2915}2921. Shaw, G.E., Reagan, J.A., Herman, B.M., 1973. Investigations of atmospheric extinction using direct solar radiation measurements made with a multiple wavelength radiometer. Journal of Applied Meteorology 12, 374}380. Yang, D., Xu, X., Wen, Y., 1991. A case study on sandstorm. Acta Meteorologica Sinica 5, 150}159. Yang, J., Qiu, J., 1996. The empirical expressions of the relation between precipitable water and ground water vapor pressure for some areas in China. Chinese Journal of Atmospheric Science 20, 620}625. Yang, L., Qiu, J., 1998. E!ect of volcanic aerosol on ozone change trends over Beijing. Chinese Journal of Atmospheric Science 22, 754}761.
Variation characteristics of atmospheric aerosol optical depths and visibility in North China during 1980}1994 Qiu Jinhuan*, Yang Liquan Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China Received 15 January 1998; received in revised form 26 February 1999; accepted 2 March 1999
Abstract Using a method developed by Qiu (Qiu, J., 1998. A method to determine atmospheric aerosol optical depth using total direct solar radiation. J. Atmos. Sci. 55, 734}758), 0.75 lm aerosol optical depths at "ve meteorological observatories in north China during 1980}1994 are retrieved from global direct solar radiation, and variation characteristics of the depths and visibility are analyzed. These observatories are located in the cities of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. It is found that during 1980}1994 the aerosol optical depths show an increasing trend at all "ve sites. During winter the trend is stronger. In winter at Beijing and Wulumuqi, the depth increased by a factor of about two in 15 years. Pollution caused due to the burning of fossil fuel is the main cause of the change. In spring at Geermu the depth is larger and its increase is the quickest among the four seasons, mainly due to desert dust events. The Pinatubo volcanic eruption in 1991 had a signi"cant in#uence on the aerosol optical depth. The yearly averaged depths over "ve sites in 1992 after the eruption increased by 0.068 to 0.212, compared to those in 1990, while from 1992 to 1994 they generally show a decreasing trend. In some cities such as Zhengzhou and Geermu, both visibility and optical depth show an increasing trend during 1980}1994, a possible reason for this is that the aerosol particle vertical distribution shifts up in the troposphere. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are larger, which may be because of the growth of aerosol particles growing in the moist summer. Apart from Geermu, at the other four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin, visibility in summer is about twice larger than that in winter, but the di!erence between depths is small, implying the turbid lower troposphere in winter and the larger extinction coe$cient in the upper troposphere during summer. ( 1999 Elsevier Science Ltd. All rights reserved. Keywords: Direct solar radiation; Visibility; Aerosol; Optical depth; Pinatubo e!ect
1. Introduction Atmospheric aerosol is of great importance to research on the environment and climate changes, and atmospheric correction in the case of space-borne remote sensing. It also plays an important role in the formation process of clouds and fogs, a!ects the budget of the Earth's radiation, and a!ects ozone through the heterogeneous chemical reactions on its surface. Accordingly, volcanic aerosol is regarded as an important perturbation of the Earth}atmosphere system. The eruptions of Mexican El. Chichon in April of 1982 and Philippine
* Corresponding author.
Pinatubo in June of 1991 are two of the most violent ones in this century. Therefore, to study the variation features of atmospheric aerosol contents during the periods around the eruptions is particularly signi"cant. Atmospheric aerosol optical depth is an important physical parameter for indicating atmospheric turbidity and aerosol content, and it is also a crucial factor in determining the aerosol radiance climatic e!ect. This paper emphasizes particularly the long-term variation characteristics of atmospheric aerosol optical depth in China. A narrow-band sunphotometer is currently a common and e!ective ground-based means to detect the aerosol optical depth (Diermendjian, 1980; Quenzel, 1970; Shaw et al., 1973; King et al., 1978; Nakajima et al., 1983; Qiu et al., 1985; Qiu and Sun, 1994). But now there are a few of
1352-2310/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 7 3 - 9
604
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
these kinds of stations having sunphotometer observations in the world, while many meteorological stations only have long-term regular observations of the global direct solar radiation detected by a pyrheliometer. Qiu (1998) developed a method to derive 0.75 lm aerosol optical depth from the wide-band direct solar radiation. By using this method, we derived the aerosol optical depths in 12 meteorological observatories during 1980}1994, and analyzed their variation characteristics.
2. Meteorological observatory, observation instrument and data Fig. 1 shows "ve "rst-class meteorological observatories discussed in this paper, which are located in the cities of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. Apart from Geermu, they are big or medium cities. Wulumuqi and Geermu are located on the Qing-Tibet Plateau, having heights of 918 and 2807 m above sea level, respectively. The height above sea level at other sites are less than 142 m. After 1988, the Chinese-made DFY-3 or TBS pyrheliometer was used in these observatories. The pyrheliometer has a typical spectral response of 0.3}4 lm, view "eld angle of 3321@ or 4324@ and yearly sensitivity stability of $1%. Before 1989, the pyrheliometers made in USSR or modeled on USSRs were used. From 1980 the greatest direct short solar radiation and the corresponding time, mostly between Beijing time 12:00 and 13:00, have been recorded at the "rst class stations in China. We only select the radiation data detected in the cloudless conditions to avoid cloud in#uence. To retrieve 0.75 lm aerosol optical depth from the direct solar radiation, the total column ozone amount and water vapor amount must be determined (Qiu, 1998). In this paper, the total ozone amounts above Beijing are
selected from monthly mean Dobson observation data there. The ozone amounts at the rest of the observatories come from monthly mean data in 1990 derived from Nimbus-7 TOMS observations. According to the study presented by Qiu (1998), a 30% ozone amount error can lead to an aerosol optical depth error of less than 0.0021. So the e!ect of ozone amount error is small. The water vapor amount was determined from local ground water vapor pressure, using an empirical expression suggested by Yang and Qiu (1996).
3. Results and discussion As shown in numerical simulations and 1267 sets of comparative experiments presented by Qiu (1998), the error of the 0.75 lm aerosol optical depth retrieved from total direct solar radiation can generally be less than 10%, and the average result of a lot of optical depth solutions can have much better accuracy. In this paper, seasonally or year-averaged aerosol optical depth with a lot of statistical samples is emphatically analyzed, and so its error being less than 10% is estimated. Total column aerosol optical depth and surface visibility are two important parameters indicating atmospheric column aerosol and surface aerosol contents. The following is to analyze variation characteristics of 0.75 lm aerosol optical depths and visibility at the "ve observatories according to Figs. 2}6 and Table 1. In these "gures, (a) shows yearly averaged optical depth and visibility from 1980 to 1994; (b) shows seasonally averaged depth; and (c) shows total monthly averaged depth and visibility during 1980}1994. Here dq and dR are annual changes of aerosol optical depth and visibility during 1980}1994, respectively, and r is the correlation coe$cient between the depth and visibility. In Figs. 2}6a, trend lines of the depth (solid) and visibility (dotted) are also shown. Visibility data at Beijing time 14:00 in the clear days are used. In addition, January, February and December are regarded as `Wintera; March, April and May `Springa; June, July and August `Summera; September, October and November `Autumna. Table 1 lists mean aerosol optical depth (q ) and an! nual change of the season-averaged optical depth (dq) during 1980}1994, di!erence (*q) of yearly averaged optical depth in 1992 to that in 1990. At "rst, our analysis is devoted to variation trends of aerosol optical depths and visibility at the "ve observatories. Then, Pinatubo and El. Chichon e!ects on the depths, and tropospheric aerosol pollution e!ect on the depths and visibility are studied, respectively. 3.1. Variation trends
Fig. 1. Map of "ve meteorological observatories in China.
As shown in Figs. 2}6a, as far as the yearly averaged optical depths are concerned, at all "ve observatories
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
605
Fig. 2. Variation characteristics of aerosol optical depth and visibility over Wulumuqi during 1980}1994. (a) Year-averaged optical depth and visibility; (b) Season-averaged optical depth; (c) Total month-averaged optical depth.
Fig. 3. Same as in Fig. 2 except that the observations are made over Geermu.
they have an increasing trend during 1980}1994. The yearly increasing rate changes from 0.0009 to 0.0168. Especially in Beijing, the rate is up to 0.0168. Surface visibility is a measure of horizontal optical depth and aerosol pollution, and its decrease is responsible for an increase of the depth. Again as shown in Figs. 2}6a, at Wulumuqi, Harbin and Beijing observatories, it is normal that visibility has a decreasing trend, and the depth has an increasing trend. It is noted that in Beijing, although the depth has the strongest increasing trend, decreasing trend of visibility (dR"!0.031 km) is not obvious. It is more interesting that at Zhengzhou and Geermu both visibility and optical depth have an in-
creasing trend during 1980}1994 (see Fig. 3a and Fig. 6a). An explanation of the result is that the aerosol particle vertical distribution in the troposphere shifts up, probably owing to the uplifting trend of factory chimneys there since 1980. It can also be found from Figs. 2}6a that the correlation coe$cient r between year-averaged aerosol optical depth and visibility is negative for all observatories, changing from !0.202 (Harbin) to !0.584 (Geermu). The anti-correlation implies that when visibility is lower, the optical depth is usually larger. It would be pointed out that the correlation in Harbin is weak, and so to use visibility for indicating total optical depth can lead to a larger error. In addition, there is anti-
606
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Fig. 4. Same as in Fig. 2 except that the observations are made over Harbin.
correlation between the depth and visibility at Geermu and Zhengzhou, but they both have an increasing trend during 1980}1994 (as far as the variation trend is concerned, they are positively correlated). The trait can be reasonable considering that there are di!erent aerosol pollution sources in the lower troposphere, upper troposphere and stratosphere. Furthermore, di!erent variation trends of optical depths for four seasons are analyzed from Figs. 2}6b and Table 1. As shown in Table 1, for all "ve observatories and four seasons the optical depths have an increasing trend (positive change rate) except for summer at Wulumuqi,
Fig. 5. Same as in Fig. 2 except that the observations are made over Beijing.
Geermu and Harbin. The strongest increasing trend of optical depth usually occurs during winter, which is the heating period in north China, especially at Beijing, the yearly increasing rate of the depth is up to 0.0254. Only at Geermu, near some desert areas, the strongest increasing trend occurs in spring. As shown in Fig. 5b, the optical depth at Beijing in the winter of the early 1990s is about twice as large as that of the early 1980s, especially after 1988, it increases very markedly. As a result, the optical depth of winter before 1989 is obviously smaller than that of other seasons, but since 1990 the di!erence among depths is small.
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Fig. 6. Same as in Fig. 2 except that the observations are made over Zhengzhou.
In summer at Wulumuqi, Geermu and Harbin the depths have a decreasing trend. 3.2. Pinatubo and El. Chichon ewects The eruptions of El. Chichon in 1982 and Pinatubo 1991 are two of the most violent ones in this century. Because total column aerosol optical depth contains tropospheric and stratospheric aerosol contributions, a quantitative analysis of the eruption e!ect on the depth is di$cult with only the depth data. As shown in Figs. 2}6, aerosol pollution in the troposphere is lighter at Geermu, and so total optical depth can more quantitat-
607
ively re#ect the eruption e!ect. Next, mainly based on a qualitative analysis, the e!ect is studied. It can be seen from Figs. 2}6 and Table 1 that there is a signi"cant e!ect of Pinatubo and El. Chichon eruptions on the optical depths. As shown in Figs. 2}6a, at all "ve observatories, during 1990}1994 there is a maximum value of year-averaged optical depth around 1992 or 1991, and from 1992 to 1994, the depth has a reducing trend. The season-averaged depth has usually the same variation trait for all four seasons (see Figs. 2}6b). Di!erence of the depth in 1992 after the Pinatubo eruption with that in 1990 before the eruption is 0.148, 0.101, 0.068, 0.212 and 0.194 at Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou (see Table 1), respectively, showing an evident depth increase. Because year-averaged visibility in 1992 is less than that in 1990 for all observatories, surface aerosol pollution would have some contribution to the depth increase, especially at Beijing and Zhengzhou. According to lidar measurements presented by Yang and Qiu (1998), the stratospheric aerosol depth in 1992 is 0.094 larger than that in 1990. Using these results, the depth increase of about 0.1, caused by Pinatubo eruption, is estimated for four observatories of Wulumuqi, Geermu, Beijing and Zhengzhou, located over the latitude region of 34.7}43.83N. During 1981}1984, a depth peak also happened in 1982 or 1983. At Beijing, the yearly averaged depth in 1982, when El. Chichon eruption broke out, is 0.19 larger than that in 1981. Visibility in 1981 and 1983 are almost the same, but the depth in 1983 is 0.15 larger than that in 1981. At Geermu, from 1981 to 1983 both the depth and visibility increase. The depth in 1983 increased by an amount of 0.051, compared with that in 1981. In addition, some contradictory results can be found in Figs. 2}6a. For example, as shown in Fig. 2a, from 1990 to 1991 both the depth and visibility at Wulumuqi decrease, which cannot re#ect the Pinatubo e!ect. But if there is a signi"cant decrease of the middle or upper tropospheric aerosol extinction coe$cient from 1990 to 1991, the contradiction can be understood. A further study for this is needed. 3.3. Tropospheric aerosol pollution ewect A stratospheric volcanic aerosol layer can remain for a long time (about four years). Therefore, variation of monthly or seasonally averaged stratospheric aerosol depth would be small, and a large monthly or seasonal variation of the total depth is mainly dependent on tropospheric aerosol pollution. In China, fossil fuel (especially coal) is most important for industrial and living applications, and its burning is usually the main source of the tropospheric aerosol pollution, especially in winter in northern China (the heating period).
608
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
Table 1 Mean aerosol optical depth (q ) and annual change of the depth (dq) during 1980}1994, di!erence (*q) of yearly averaged optical depths ! in 1992 and 1990 dq Site
Winter
Spring
Summer
Autumn
q !
*q
Wulumuqi Geermu Harbin Beijing Zhengzhou
0.0130 0.0016 0.0104 0.0254 0.0086
0.0065 0.0022 0.0016 0.0089 0.0064
!0.0021 !0.0004 !0.0052 0.0134 0.0079
0.0029 0.0016 0.0025 0.0121 0.0058
0.316 0.181 0.299 0.489 0.463
0.148 0.101 0.068 0.212 0.194
In north China, desert dust is another important aerosol pollution source. Main deserts are located in Xingjiang and the Inner Mongolia provinces, which are the main origins of Asian dust storms. Especially in spring, a lot of desert particles can be blown by strong winds and be transported over most areas of north China towards the east or southeast track in the height range of about 2}7 km (Qiu and Sun, 1994; Yang et al., 1991; Iwasaka et al., 1983). It can be seen in Figs. 2}6 and Table 1 that the pollution due to the burning of fossil fuel and desert dust events has an important e!ect on the optical depth and visibility in north China. Figs. 2}6c show the total monthly averaged depth and visibility during 1980}1994. As shown in Fig. 2c, at Wulumuqi in northwest China, visibility is very low in winter. From January to June it greatly increases, and then from June to December decreases. The visibility (+38 km) in June is about thrice larger than that (+13 km) in January or December. The depth from June to September is relatively small, being about 0.27, and in the months of winter and spring it is '0.32, while in February and March it is '0.4. Clearly, in winter at Wulumuqi the lower atmosphere is very turbid, mainly owing to coal burning during the heating period, and in summer and early autumn the tropospheric atmosphere is clearer. In February and March, the larger optical depth may be caused by both serious coal-burning pollution and desert dust events. Again observing Fig. 2a, just in winter the depth has the strongest increasing trend during 1980}1994. Geermu is a small town located on the Qing-Tibet plateau between Taklamagan desert in Xingjiang province and the Inner Mongolia desert. As shown in Fig. 3c, visibility is more than 28 km for all months, indicating a light surface aerosol pollution, and the depth is relatively small except for the months from April to August. But in spring there is lower visibility and larger optical depth. Again observing Fig. 3b, the depth in spring is about 0.14 larger than that in winter or autumn for any year during 1980}1994. It is estimated that the increasing value of
0.14 is mainly caused by desert dust events. In addition, just in spring the yearly increasing rate of the depth is largest. In summer at Geermu, the larger optical depth may be due to tropospheric aerosol particles growing in a high humidity. At Harbin of northeast China, visibility has similar month variation characteristic as at Wulumuqi. As shown in Fig. 4c, visibility is very low in winter (+10 km). From January to July it linearly increases and then from July to December it decreases. Visibility (+30 km) in July is about thrice larger than that in January or December. The optical depth has a di!erent monthly variation trait. It is smaller in autumn, being about 0.22. Visibility in summer is about twice larger than that in winter, but di!erence between optical depths of the two seasons is small. Therefore, in winter at Harbin the lower atmosphere is also turbid, but the upper troposphere is clearer. In summer, the lower atmosphere is clear, but there is the larger aerosol extinction coe$cient in the middle or upper troposphere, probably caused by aerosol particles growing in the humid summer. Considering the statistical average for 1980}1994, visibility in Beijing during September or August is largest, while in other months it changes between 14 and 16.5 km (see Fig. 5c). The optical depths in winter in Beijing are much less than those in spring and summer during 1980}1988. But as analyzed above, because of the quicker increase of the depth during winter, in the early 1990s its di!erence with other seasons is small. In all four seasons at Beijing there is an evident increasing trend of the seasonally averaged depths, and since 1990 they have been all larger than 0.45, showing very serious aerosol pollution. In addition, during spring when an Asian dust storm happens the depth is larger. As shown in Fig. 6c, at Zhengzhou there is a positive correlation between total monthly-averaged optical depth and visibility. In winter visibility and optical depth are relatively smaller, and in summer they are larger. In January visibility is low, down to 11 km, showing the turbid lower atmosphere in winter. The largest optical depth occurs in spring. For example, the depth in May is
Q. Jinhuan, Y. Liquan / Atmospheric Environment 34 (2000) 603}609
about twice as large as that in January. And for almost all years from 1980 to 1994, the seasonally averaged depth in spring is the largest (see Fig. 6b). This implies that the spring desert dust events may have an important e!ect on the depth. As shown in Table 1, mean values of the 0.75 lm aerosol optical depths during 1980}1994 are 0.316, 0.181, 0.299, 0.489 and 0.463 at "ve observatories of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou, respectively. Clearly, at Beijing and Zhengzhou aerosol pollution is very serious. At Geermu located on the Qing-Tibet plateau, the depth is smaller, being about one-third of that in Beijing.
4. Conclusions (1) In north China during 1980}1994, the total column aerosol optical depth has an increasing trend. The yearly increasing rate of the yearly averaged optical depth changes between 0.0009 and 0.0168 for the "ve meteorological observatories of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. Especially at Beijing, the rate is up to 0.0168. Except for summer at Wulumuqi, Geermu and Harbin, the seasonally averaged optical depths have also an increasing trend for all four seasons and "ve sites. The strongest increasing trend of the depth is usually during winter. In winter at Beijing and Wulumuqi, in 15 years the depth increased by a factor of about two. Fossil fuel burning Pollution due to the burning of fossil fuel is the main source of the change. In spring at Geermu the depth is larger and its increase is the quickest among the four seasons, mainly due to the desert dust. (2) The eruptions of El. Chichon in 1982 and Pinatubo in 1991 had important in#uences on aerosol optical depth in north China, especially the latter eruption. Yearly averaged depths over "ve sites obviously increased in 1992 after the Pinatubo eruption by 0.068}0.212, compared to those in 1990, and from 1992 to 1994, the depths show a decreasing trend. (3) Over the two big cities of Beijing and Zhenzhou, aerosol pollution is serious. The total averaged aerosol optical depths during 1980}1994 are 0.489 and 0.463, respectively. At Beijing after 1989 the depths are all larger than 0.5 for all seasons. At Geermu located on QingTibet plateau, the depth is smaller, being about one-third of that in Beijing. (4) At all "ve sites, optical depths in spring are larger, probably owing to frequent desert dust events. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are also larger, which may be because of aerosol particles growing in the moist summer. Apart from Geermu, at the other four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin,
609
visibility in summer is about thrice larger than that in winter, but di!erences between the depths are small, implying the turbid lower troposphere in winter and the larger extinction coe$cient in the upper troposphere during summer. (5) In some sites such as Zhengzhou and Geermu, during 1980}1994 both the aerosol optical depth and visibility show increasing trends. A possible reason for this is that the aerosol particle vertical distribution in the troposphere moves up.
Acknowledgements This research was supported by National Natural Science Foundation of China.
References Diermendjian, D., 1980. A survey of light-scattering technigue used in the remote monitoring of atmospheric aerosol. Reviews of Geophysics and Space Physics 18, 341}360. Iwasaka, Y., Minoura, H., Nagaya, K., 1983. The transport and spatial scale of Asian dust-storm clouds. Tellus 35, 189}196. King, M.D., Byrne, D.M., Herman, B.M., Reagan, J.A., 1978. Aerosol size distributions obtained by inversion of spectral optical depth measurements. Journal of Atmospheric Science 35, 2153}2167. Nakajima, T., Tanaka, M., Yamauchi, T., 1983. Retrieval of the optical properties of aerosol from aureole and extinction data. Applied Optics 22, 2951}2959. Qiu, J., Sun, J., 1994. Optically remote sensing of the dust storm and the analysis. Chinese Journal of Atmospheric Science 18, 1}10. Qiu, J., Wang, H., Zhou, X., Lu, D., 1985. Experimental study of remote sensing of atmospheric aerosol size distribution by combined solar extinction and forward scattering method. Advances in Atmospheric Science 2, 307}315. Quenzel, H., 1970. Determination of size distribution of atmospheric aerosol particles from spectral solar radiation measurements. Journal Geophysical Research 75, 2915}2921. Shaw, G.E., Reagan, J.A., Herman, B.M., 1973. Investigations of atmospheric extinction using direct solar radiation measurements made with a multiple wavelength radiometer. Journal of Applied Meteorology 12, 374}380. Yang, D., Xu, X., Wen, Y., 1991. A case study on sandstorm. Acta Meteorologica Sinica 5, 150}159. Yang, J., Qiu, J., 1996. The empirical expressions of the relation between precipitable water and ground water vapor pressure for some areas in China. Chinese Journal of Atmospheric Science 20, 620}625. Yang, L., Qiu, J., 1998. E!ect of volcanic aerosol on ozone change trends over Beijing. Chinese Journal of Atmospheric Science 22, 754}761.