Horizontal visibility trends in China

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L24706, doi: /2007gl031450, 2007 Horizontal visibility trends in China Huizheng Che, 1 Xiaoye Zhang, 1 Yang Li, 2 Zijiang Zhou, 3 and John J. Qu 4 Received 1 August 2007; revised 9 October 2007; accepted 7 November 2007; published 27 December [1] Trends in Chinese horizontal visibility, the frequency of visibility >19 km, and haziness for the period between 1981 and 2005 were evaluated based on data for daily horizontal visibility. Annual means were calculated for each station and for China as a whole. Linear regression analysis was used to characterize long-term annual trends in these variables. Over the past 25 years, there has been a significant decrease in horizontal visibility ( 2.1 km per decade from 1990 to 2005) and the frequency of visibility >19 km ( 3.5% per decade) but a significant increase in the 75th percentile annual extinction coefficients (25% per 25 year). According to rapid increase of total energy consumption, the consistent spatial and temporal variations of visibility and haze support the speculation that increased aerosol loadings were responsible for the observed decreases in horizontal visibility over much of East China. Citation: Che, H., X. Zhang, Y. Li, Z. Zhou, and J. J. Qu (2007), Horizontal visibility trends in China , Geophys. Res. Lett., 34, L24706, doi: /2007gl Introduction 1 Key Laboratory of Atmospheric Chemistry, Centre for Atmosphere Watch and Services, Chinese Academy of Meteorological Sciences, Beijing, China. 2 Atmospheric Observation Technology Center, Chinese Meteorological Administration, Beijing, China. 3 National Meteorological Information Center, Chinese Meteorological Administration, Beijing, China. 4 EastFIRE Lab, College of Science, George Mason University, Fairfax, Virginia, USA. Copyright 2007 by the American Geophysical Union /07/2007GL [2] Horizontal visibility in the daytime is defined as the distance at which a large dark object can just be discerned against a light sky [Horvath, 1981]. In a non-polluted atmosphere, a natural visual range would be nearly km in different areas [Environmental Protection Agency, 2001]. Atmospheric pollution can cause a decrease in the atmospheric visibility. Therefore, visibility is of major interest in air pollution studies and climatology [Charlson et al., 1968; White and Roberts, 1977; Trijonis, 1979; Naegele and Sellers, 1981; Malm and Kreidenweis, 1997]. [3] The spatial and temporal trends, as well as manmade influences on atmospheric visibility have attracted considerable attention from researchers worldwide as part of scientific and regulatory studies [Elridge, 1966; Munn, 1973; Husar et al., 1979; Ferman et al., 1981; Husar et al., 1981; Sloane, 1982a, 1982b; Trijonis, 1982; Husar and Wilson, 1993; Lee and Sequeira, 2001]. Sloane [1982a, 1982b] has examined the methods to determine trends in visibility due to air pollution. Sloane [1982a, 1982b] has also examined in depth the effect of meteorology on visibility trends and the extraction of valid, air quality-related conclusions from these data. Detailed long term studies of visibility and haze within the United States have also been carried out by Husar et al. [1979] and Schichtel et al. [2001]. Lee [1994] and Doyle and Dorling [2002] have examined long term UK visibility measurements and found trends within these data that correlate with changing fuel consumption and changing meteorological conditions. Ghim et al. et al. [2005] analyzed and discussed temporal trends and spatial distributions of visibility measured by visibility makers from over 60 stations in Korea between 1980 and Air pollutants should have played a role in decreased variation of visibility in Korea. Husar et al [2000] evaluated the global continental haze pattern based on daily average visibility data at 7000 surface weather stations for [4] However, such coordinated research is absent in China. Few previous studies have analyzed long-term visibility observations and the effects of major air pollutants on local visibility [Fan et al., 2005]. Therefore, this work attempts to better investigate the long-term visibility variations for the last 25 years in China. 2. Data and Method [5] A Chinese daily dataset from CMA/National Meteorological Center that includes 6-hour visibilities recorded by human observers at 682 synoptic weather stations in Mainland China during the period between 1957 and 2005 was initially obtained. However, only 615 stations data with continuous records from more than 20 years after 1981 is presented within this paper due to different methods in data collection methods before Well-trained observers measure the visual range using reference objects, such as buildings and mountains, in different directions at known distances from the observer. The data uncertainty is 0.1 km according to visibility observation guideline of China Meteorological Administration which was promulgated in Annual data of total energy consumption are from National Bureau of Statistics of China ( gov.cn/). [6] A quality control procedure for visibility was applied before analysis based on the work by Doyle and Dorling [2002]. In brief, the basic rules are introduced here: (1) only 6:00 (GMT) values are selected to represent the daily regional visibility levels; (2) visibility measurements were screened for natural events such as precipitation, dust, fog, mist and gale by using the present weather code; and (3) visibility measurements were screened when the relative humidity is higher than 90%. [7] Three methods of studying the historical trends in visibility were examined for each station: (1) trends of annual and seasonal means of visibilities; (2) identification of the frequency of visibility >19 km: the percentage of observations which exceed 19 km [Gomez and Smith, 1987; Lee, 1983]; and (3) relative humidity (RH) corrected ex- L of5

2 Figure 1. (a) Annual mean of frequency of visibility >19 km (between 1981 and 2005). (b) Trend maps for the 75th percentile RH corrected extinction coefficient b ext (km 1 ). (c) Trends in annual mean visibility, station trend indicators with circles around them are significant at the 95% confidence level. (d) Trends in annual mean frequency of visibility >19 km. tinction coefficient: A relative correction factor is applied to compensate for the effect of relative humidity (RH) when yielding the dry extinction coefficient [Husar and Holloway, 1984]. The formula is as the following: 8 RH 30% 0:85 ð 30% < RH 40% ðrh 30% *0:05 þ 0:85 ð 40% < RH 50% ðrh 40% *0:05 þ 0:90 ð ðrh 50% *0:05 þ 0:95 ð 50% < RH 60% >< b ext ¼ 60% < RH 70% ðrh 60% *0:05 þ 1:00 ð 70% < RH 75% ðrh 70% *0:03 þ 1:05 ð 75% < RH 80% ðrh 75% *0:04 þ 1:20 ð 80% < RH 85% ðrh 80% *0:05 þ 1:40 ð >: 85% < RH 90% ðrh 85% *0:29 þ 1:65 ð where the b ext is the dry extinction coefficient, RH is the relative humidity, and b w ext is the wet extinction coefficient (uncorrected extinction coefficient) which is calculated via the Koschmieder relationship b w ext = K/V [Koschmieder, 1926]. Here, K is set as 1.9 in accordance with Griffing [1980] and Ozkaynak et al. [1985]. V denotes the observed visibility with unit of km 1. [8] Mean values have been calculated for each year as an annual mean: Spring (March May), summer (June August), fall (September November), and winter (December February) means were then be examined to identify either increasing, decreasing or a combination of trends over time. Long-term linear regression trends for each station were computed by using a linear regression model at the 95% confidence level. Finally, we calculated average visibilities for all chosen stations as a whole to study the variations in these variables. 3. Results Analysis 3.1. Spatial Variations of Annual Mean Visibility [9] Frequencies of visibility >19 km were calculated for each station for each year of data available. Figure 1a shows the 615 stations distribution of annual frequency of visibility >19 km in China during 1981 to It can be seen that most western and northeastern sites of China experienced visibility >19 km (>70%) during the time from 1981 to However, the annual frequencies of visibility >19 km were never more than 30% at most sites in the Huabei area, the middle-low area of the Yangtze River, some parts of the northeastern regions where the population is large and the economy is developing very rapidly. The annual frequency of visibility >19 km being lower than 50% at some sites in northwest China is mainly due to the 2of5

3 Figure 2. All-China time series of annual averages of (a) visibility (km), (b) frequency of visibility >19 km (percent), and (c) the 75th and 90th percentile annual extinction coefficients and total energy consumption for , averaged over the whole of China. contribution of sand and dust storms [Mahowald et al., 2007]. [10] Figure 1b plots the map of haze patterns in China from 1981 to The highest extinction coefficients are recorded in the Sichuan Basin in south-central China and the middle region of Liaoning Province in northeast China with the extinction levels exceeding 0.2 km 1 (10 km visual range). The haze is probably attributable to anthropogenic emissions and poor ventilation. Another area of elevated extinction coefficients is found over the Tarim Basin in northwestern China. The high values are found to be higher than This arid region is dominated by frequent springtime dust storms [Wang et al., 2003]. Throughout the region of eastern China, elevated extinction levels (larger than 0.14) are recorded stretching from the northeast to the southwest, mostly in areas of high population density and high pollutant emission. In addition, decreasing trends in the eastern China global solar radiation and possible sunshine duration in the 1980s and early 1990s have been found by Che et al. [2005]. In order to fully establish the causality of the observed reductions of visibility, additional work needs to be focused on the trends of specific aerosol chemical species, and effects of relevant meteorological variables. [11] There are 71% of stations showed a decreasing trend in visibility, and the correlations with time for 372 of the 615 stations with declining trends were significant at the 95% confidence level (Figure 1c). It can be seen from Figure 1c that most stations in the eastern half of China, where the anthropogenic activities are very frequent, that is, to the east of 105 E, showed decreasing trends in visibility. In contrast, there are some stations in northern and western China with few anthropogenic activities (e.g. Qinghai-Tibet Plateau) which showed increases in viability. [12] Frequencies of visibility >19 km experienced decreasing trends at most sites of China (62%) during the last 25 years (Figure 1d). Decreases in frequency are more obvious in the regions of the Jing-Jin-Ji Megalopolis, the Yangtze River and Pearl River Deltas, the middle area of the Yellow River, the industrial area of northeastern China, in which fairly well reflected aerosol emissions might play an important role in causing larger decreases in occurrences of visibility >19 km Temporal Variations of Annual and Seasonal Mean Visibilities, Annual Averaged Frequencies of Visibility and >19 km, and Extinction Coefficients [13] Figure 2a plots the temporal variation of visibility for all of China between 1981 and There was no 3of5

4 Table 1. Summary Statistics for Annual and Seasonal Averaged Visibilities a Annual Spring Summer Fall Winter Mean, km SD Trend, km/10 years R P < < < < < a SD, standard deviation; the trends are for the time period; R 2, R-square coefficient; P, the level of significance. significant linear trend in the annually-averaged visibility in the 1980s. The annual mean visual range was more than 27.0 km and then it decreased relatively steadily after 1990 with a significant decreasing trend approximately 2.1 km per decade. The decrease is faster after 2000 and the annual mean visual range is even lower than 25.0 km from Mean visibility in summer (29.0 km) is the best, while visibility in winter (24.5 km) is the worst. Visibility in the fall is better than in the spring. There are also significant decreasing trends for every season after 1990 (Table 1). The trends are 1.7, 2.8, 2.1, and 1.3 km/decade, respectively. Before 1990, the visibility trends in the summer, fall, and winter were not obvious. But the visibility trend in the spring showed an increasing about 1.1 km/decade with a significance level of 95%. Much attention should be paid to this phenomenon in future. [14] The annual averaged frequencies of visibility and >19 km decreases from 1981 to 2005 significantly (Figure 2b). Visibility >19 km was experienced around 73% of the time in the 1980s and 71% during the 1990s and lower than 69% after It decreases slowly before 1990, steadily in 1990s, and accelerated after The linear regression trend is about 3.5%/decade with a confidence level larger than 99% which indicates a large decrease about 8.8% in the frequency of visibility >19 km in China over the 25 year time period. [15] The annual dry extinction coefficient - b ext, and wet extinction coefficient - b w ext trends over China for the 25-yr period between are presented in Figure 2c. The trends were computed for the 75th and 90th percentile using data from all 615 stations of China. As shown, over all of China, there was a 23% increase in the 90th percentile b ext over the 25-yr period, and a 25% increase of the 75th percentile. While, if the relative humidity correction factor is not taken account of, there was a 25% increase in the 90th percentile b w ext over the 25-yr period, and a 29% increase of the 75th percentile. Since a large part of the light scattering aerosol particles over China are emitted from fossil fuel combustion [Wang and Shi, 2000], the trend of b ext is compared to the total energy consumption trends in Figure 2c. It could be found very coincidently that the b ext increases with the total energy consumption. An unusual feature of the total energy consumption data is the sharp increase in The frequency of China s haze episodes (90th percentile of b ext ) increased at an even faster rate (6.8%). 4. Conclusion [16] The annually averaged frequencies of visibility >19 km exhibited a significant declining trend of 3.5% per decade over China between In fact, over 62% of the stations in China have shown decreasing trends in the annually-averaged frequencies of visibility >19 km. The frequencies of visibility >19 km lower than 30% are mostly located in the eastern regions that have large populations and rapidly developing economies. [17] Most stations (71%) especially in the eastern part areas in China showed decreasing trends in the annuallyaveraged visibility. There was no obvious variation in annually averaged visibility during 1980s but a significant decrease in the annually-averaged visibility ( 2.1 km per decade) over China from 1990 to [18] The 75th and 90th percentile annual extinction coefficients over China for the 25-yr period of between 1981 and 2005 increased 25% and 23%, respectively. Since the frequency of China haze episodes increased with the total energy consumption very consistently, we speculate that the increasing fossil fuel combustion has likely affected the magnitude and variability of visibility over much of China, especially in the eastern part of the country. However, more rigorous studies are still needed to verify this notion. [19] Acknowledgments. This work is financially supported by grants from National Key Project of Basic Research 2006CB and 2006CB The data used in this study were obtained from CMA/ National Meteorological Center and National Bureau of Statistics of China, which are highly acknowledged. And we also would like to thank the two anonymous reviewers for their constructive comments. References Charlson, R. J., N. C. Ahlquist, and H. Horvath (1968), On the generality of correlation of atmospheric aerosol mass concentration and light scatter, Atmos. Environ., 2, Che, H. Z., G. Y. Shi, X. Y. Zhang, R. Arimoto, J. Q. Zhao, L. Xu, B. 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