Study of ozone weekend effect in Shanghai

Transcription

1 Science in China Series D: Earth Sciences 2008 SCIENCE IN CHINA PRESS Springer earth.scichina.com Study of ozone weekend effect in Shanghai TANG WenYuan 1, ZHAO ChunSheng 1, GENG FuHai 1,2, PENG Li 2, ZHOU GuangQiang 2, GAO Wei 2, XU JianMing 2 & TIE XueXi 2,3 1 Department of Atmospheric Science, School of Physics, Peking University, Beijing , China; 2 Atmospheric Chemistry Laboratory of Shanghai Meteorological Bureau, Shanghai , China; 3 Atmospheric Chemistry Division, NCAR, Boulder CO 80301, USA Analysis of observed ozone data in 2006 from five monitoring sites (Xujiahui, Chongming, Baoshan, Pudong, Jinshan) in Shanghai reveals that ozone (O 3 ) concentrations in Xujiahui are higher at weekends than those on weekdays, despite the fact that emissions of ozone precursor substances, such as oxides of nitrogen (NO x ), carbon monoxide (CO) and volatile organic compounds (VOCs) are lower at weekends than those on weekdays. The possible chemical cause of ozone weekend effect is that NO 2 /NO ratio increases at weekends by 25.61% compared with those on weekdays. In addition, because of an average 12.13% reduction in NO x (NO + NO 2 ) in the early morning (05:00-09:00) at weekends compared with that on weekdays, the ozone inhibition period ends 0.5 h earlier at weekends resulting in the longer duration of ozone accumulation and the higher ozone production rate. The rate of ozone production is a function of VOCs and NO x in the atmosphere. VOCs/NO x ratio in Xujiahui is 4.55 at weekends, and 4.37 on weekdays, belonging to the NO x -limited. The increasing VOCs/NO x ratio at weekends leads to ozone enhancement from 73 ppbv to 80 ppbv, which are consistent with ozone weekend effect in Xujiahui. Furthermore, combining with MICAPS cloud amount data, the fact that ozone weekend effect in Xujiahui weakens gradually along with the increasing of cloud amount indicates that ozone photochemical production leads to ozone weekend effect in Xujiahui of Shanghai. Shanghai, ozone, photochemical pollution, weekend effect 1 Introduction As the main atmospheric photochemical production, ground-level ozone is an importance pollutant with impact on climate and environment [1]. Ground-level ozone is formed in the air as a result of complex photochemical reactions of nitrogen oxides (NO x ), carbon monoxide (CO), and various volatile organic compounds (VOCs), which are mostly due to anthropogenic activities. Study the change of ground-level ozone concentration can contribute to improving urban air quality and realizing the effect of tropospheric ozone on the environment, agriculture and climate [2 4]. In recent years, considerable work has been done on the subject of tropospheric ozone s influence on the climate and environment by the support of national natural science foundation [5 10], especially analyses of the trends evolution of ozone in the lower troposphere in the Yangtze River [11 14]. Ozone weekend effect implies that a weekly pattern of ozone concentrations are higher at weekends than those on weekdays, despite the fact that ozone precursors are lower at weekends compared with those on weekdays. Since the late 1970s, ozone weekend effect has been observed in the northeastern cities of America (New York [15], Washington D.C. and Baltimore [16,17] ), the Los Angeles area [18,19], San Francisco [20], Tucson [21], Atlanta [22] and certain metropolitan areas, such as Cana- Received November 25, 2007; accepted May 13, 2008 doi: /s Corresponding author ( tang_sunflower@sina.com) Supported by the National Natural Science Foundation of China (Grant Nos and ), and the Special Research Project of China Meteorological Bureau Sci China Ser D-Earth Sci Sep vol. 51 no

2 dian [23] and Europe [24,25]. The most important thing is that city ozone weekend effect provides observed facts for the simulation models in assessing the sensitivity of ozone to precursor emissions (VOC and NO x ) [26] and helps to identify the most effective ozone control strategies [27], for example, NO x eliminate strategies in VOC-limited region may not attain the goal of reducing ozone. At present, there is no definitive reason of city ozone weekend effect that could be consented because of its different pattern presenting in different locations. Ground-level ozone concentration is a photochemical production, and it s controlled by a serial of complex physical and chemical processes in relation to precursor emission, local meteorological condition and photochemical reactions. In addition, Ground-level ozone is affected by city economical structure, the pattern of citizens activities and the emission of local pollutants. The California Air resources Board (CARB) outlined the potential causes of the city ozone weekend effect : A reduction in NO x at weekends that restrains ozone generating in the NO x -limited region; NO x emission delay at weekends that facilitates ozone production; increasing sunlight that reaches ground as the result of the reduction in the amount of soot in the air; change in the emission strength and emission time at weekends. Up to day, there have been few studies on the city ozone weekend effect in China. This paper concentrates on the ozone weekend effect in Shanghai. The observed data of ozone, NO x, CO, VOCs in 2006 is obtained from five monitoring sites (Chongming, Baoshan, Pudong, Jinshan, and Xujiahui). The five monitoring sites in this study are arranged in different function districts (Figure 1). Downtown Xujiahui is the business and human activities center where encounters serious traffic problem and air pollution problem. Baoshan and Pudong are defined as near-urban districts. Chongming is far from city center with less air pollution and Jinshan is an industrial region. Ozone is measured using an EC 9810 Ozone Analyzer and a UV photometer which accurately and reliably measures ozone concentrations in ambient air. This instrument has set to zero automatically and includes an optional external valve manifold and external calibration sources. The instrument meets the technical specifications for US EPA (Environmental Protection Agency). Filter is replaced once every two weeks and calibration is made every 6 months. The ozone concentration is recorded every 5 seconds. Ozone data are attained in all of five sites whereas CO, NO 2, NO and VOCs data are only available for station in Xujiahui. 1-h average value of CO, NO 2 and NO from 1 May, 2006 to 31 December, 2006 are used. 101 kinds of VOCs including aromatics, low molecular weight alkanes, alkenes and alkynes are available from 1 January, 2006 to 9 November, Only the days when Shanghai experienced high concentration ozone are used in this paper. For selected high ozone days, year-average values of daily 1-hour maximums are used as the thresholds. Those days when daily 1-hour ozone concentrations are above the thresholds are defined as high ozone day. The thresholds of the five sites are: Chongming >46.4 ppbv (10 9 ), Baoshan >40.6 ppbv, Pudong 34.5 ppbv, Xujiahui >26.6 ppbv, Jinshan >48.6 ppbv. The sum of high ozone day is 147 days in Chongming; 154 days in Baoshan; 164 days in Pudong; 162 days in Xujiahui and 167 days in Jinshan. The ozone isopleth diagram for Shanghai is generated by using OZIPR (Ozone Isopleth Plotting Package Research) in this paper. The OZIPR is a research-oriented version of EPA s OZIPP (Ozone Isopleth Plotting Package) computer modeling program. The OZIPR and OZIPP programs employ a trajectory-based air quality simulation model in conjunction with the Empirical Kinetics Modeling Approach (EKMA) to relate ozone concentrations levels of organic and nitrogen oxide (NO x ) emissions. OZIPR simulates complex chemical and 2 Data and methods Figure 1 The location of ozone monitoring sites in Shanghai. TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no

3 physical processes of the lower atmosphere through a trajectory model. The physical representation is a well-mixed column of air extending from the ground to the top of the mixed layer. This idealized air column moves with the wind (along the wind trajectory), but cannot expand horizontally. Emissions from the surface are included as the air column passes over different emission sources, and air from above the column is mixed in as the inversion rises during the day [28]. 3 Results and discussion Figure 2 summarizes the corresponding weekly cycles of ozone for each sites (Chongming, Baoshan, Pudong, Xujiahui, and Jinshan). The height of each bar is difference from daily 1-hour ozone maximum to year average value. The average values for each station are as follows: 63.6 ppbv at Xujiahui, 57.1 ppbv at Baoshan, 48.7 ppbv at Pudong, 40.1 ppbv at Xujiahui, and 65.9 ppbv at Jinshan. The top two evident ozone weekend effect in Xujiahui and Jinshan are likely relative to traffic conditions in Shanghai. Xujiahui is a business center in Shanghai, where traffic flow and vehicles tail gas emissions are much less at weekends than those on weekdays. On Monday, ozone average value in Xujiahui is 40.6 ppbv. The minimum of average value presents on Thursdays (37.2 ppbv), whereas the maximum is on Saturdays (43.4 ppbv). Compared to Thursdays, ozone average value on Saturdays is higher by 16.9%. As is shown in Figure 3, NO, NO 2 and CO, as the emissions of ozone precursor gases, also show the same weekly cycle pattern as ozone s. Near-ground ozone photochemical production processes in urban areas are as follows: 1 O + hv O + O( D), O( D) + H2O 2OH, VOCs + OH + O2 RO2 + H2O, RO + NO NO + RO, 2 2 NO + hv NO + O( P), 2 3 O( P) + O + M O + M 2 3 Excluding transportation, the concentration of VOCs and NO x and the photolysis rates of ozone and NO 2 determine whether ozone produces or consumes. When NO is sufficient in the air, reaction O 3 +NO (Titration) will predominate that results in ozone consumption. Analysis results indicate that in Xujiahui, the center of human activities, average values of NO 2 and NO are 31.6 ppbv and 8.7 ppbv at weekends, whereas 34.2 ppbv and 11.9 ppbv on weekdays, which result in 20.4% lower NO/NO 2 ratio at weekends. According to the ozone photochemical theory, obvious change in NO x emission at weekends is the possible chemical cause of ozone weekend effect. Chemical data (O 3, NO, NO 2 and CO) in Xujiahui is divided into two categories. Sunday and Saturday is labeled as WE category and Tuesday to Friday labeled as WD category. Figure 4 illustrates the average concentrations of O 3, NO, NO 2 and CO for every hour in the high ozone day, and concentration-time curve for each compound as well. It has been documented that the diurnal ozone cycle consists of four phases [29] : overnight carryover of ozone precursors (Carry Over), inhibition of ozone formation in the morning due to abundance of new NO emission (O 3 Inhibition), ozone photochemical production (O 3 Accumulation), and ozone consumption during the period from ozone maximum to mid-night (Post Maximum O 3 ). The similar ozone cycle in Xujiahui was repre- 3 Figure 2 Ozone weekend effect in five monitoring sites in Shanghai area TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no

4 Figure 3 The weekly cycles of the concentration of NO, NO 2, CO and TVOC in Xujiahui, Shanghai. The zero lines indicate the corresponding average value, and the bars indicates the difference from daily 1-h ozone maximum to year average value (the average values of NO, NO 2, CO and TVOC are 11.1 ppbv, 33.3 ppbv, ppbv and 48.4 ppbc respectively). Figure 4 The concentration-time curve of O 3, NO, NO 2 and CO in Xujiahui, Shanghai. The dashed lines indicate weekdays (from Monday to Friday) and the real lines indicate weekends (Saturday, Sunday). sented in Figure 4: (1) From mid-night to 5:00 a.m., ozone and NO concentrations remain essentially constant, whereas CO and NO 2 slightly reduce. The corresponding compounds concentrations at weekends are nearly the same as those on weekdays and ozone minimum (10.0 ppbv) maintains during this phase; (2) The TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no

5 5:00-8:00 a.m., numerous fresh NO emissions generated by automobiles in morning traffic rush time on weekdays result in higher NO than those at weekends (on weekdays, NO was 10.0 ppbv; weekdays, 20.0 ppbv). In this ozone inhibition phase, NO, as a reductant, consumes ozone because of weak sun radiation and low NO 2 concentration; (3) Duration of ozone accumulation (08:00-13:00 a.m.) is from the end of the inhibition period to the time when ozone reach its maximum. Ozone increases by more than 20.0 ppbv on account of strengthening in sun radiation, NO 2 increasing and NO 2 photolysis; (4) From 13:00 a.m. to mid-night is ozone reduction phase. On account of vertical mixing, horizontal advection, weak sun radiation and consumption by new NO emission in afternoon traffic rush time, ozone declines to the minimum at 8:00 p.m. Overnight NO emissions in Xujiahui at weekends are the same as those on weekdays. This part of NO consumes ozone by oxidation reaction leading ozone to reach its daily minimum. After 05:00 a.m., especially in the morning rush hour on weekdays, traffic flow increases and abundant new fresh NO complements and then ozone inhibits after sunrise (Figure 4, ozone inhibition). At 07:00 a.m., NO reaches its maximum and is 50% higher on weekdays than that at weekends, which indicates that the degree of ozone inhibition by NO is more significant on weekdays. During the ozone Inhibition phase, CO, VOCs, OH are the main sources of peroxide free radicals (HO 2, RO 2 ). The period of ozone accumulation begins when sufficient NO has been translated into NO 2. On one hand, with vertical mixing in boundary layer, upper overnight ozone effectively transfers downwards, on the other hand, ozone generates by photochemical process. Both of them lead ozone to accumulate rapidly and reach its maximum at 13:00. The point of intersection of NO and ozone in the morning is the marker as the end of ozone inhibition period and the beginning of ozone photochemical production. The period between the point of intersection and the time of ozone maximum is defined as duration of ozone accumulation. The rate of ozone production is the increment of ozone divided by the duration of ozone accumulation. The length of ozone inhibition is determined by the concentrations of NO, NO 2 and NO x. As is shown in Figure 4, duration of ozone inhibition by NO in the weekend morning is half hour shorter than that on weekday as the result of 12.13% NO x (NO+NO 2 ) reduction in the weekend morning (05:00-09:00 a.m.). Therefore, compared with weekdays, longer duration of ozone accumulation and higher average rate of ozone production at weekends are contributed to the increasing of ozone concentration. Ozone, the photochemical production, is a non-linear function related to VOCs and NO x in the air. Figure 5 illustrates the ozone isopleth plot using the Ozone Isopleth Plotting Package Research (OZIPR). It can be found that ozone maximum depends on the given initial concentration of NO x and VOCs and can be identified as the most effective ozone control strategies. Ozone weekend effect is rooted in the complicated photochemical process. The ozone production rate is a function related with the ratio of VOCs and NO x. Either NO x -limited or VOCs-limited region depends by VOCs/NO x ratio. In Figure 5, the left dashed curves are VOC-limited region, and the right NO x -limited region. In VOC-limited, VOCs reduction can depress ozone formation efficiently and achieve the ozone control strategies, while in NO x -limited region, NO x is lower and ozone concentration is only affected by the change of NO x in the air. Average VOCs/NO x ratio in Xujiahui is 4.55 at weekends and 4.73 on weekdays that result in 4.13% higher VOCs/NO x ratio representing at weekends. Figure 5 also shows that Xujiahui locates in VOC-limited region. At weekends, both lower NO x, VOCs and higher VOCs/NO x ratio contributes to ozone increasing from 73 ppbv to 80 ppbv, which is identical Figure 5 the ozone isopleth plot in Xujiahui, Shanghai using the Ozone Isopleth Plotting Package Research (OZIPR) TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no

6 to Xujiahui ozone weekend effect that we discussed above. Figure 6 shows the affect of cloud amount on ozone weekend effect in Xujiahui. Cloud amount data are taken from MICAPS (Meteorological Information Combine Analysis and Process System). Cloud amount data during the period between 08:00 and 17:00 is used, when ozone photochemical process is mainly influence by cloud. The ozone average value is defined as the average of the highest four values during 11:00 to 18:00 p.m. Ozone average values on Saturdays, Sundays and Tuesday-Friday are classified in terms of cloud amount in Xujiahui of Shanghai (see Figure 6). Figure 6 Ozone average value in different MICAPS cloud amount in Xujiahui, Shanghai. For one thing, cloud amounts impact on sun radiation, that is, more cloud amounts, less sun radiation; for another, sun radiation impacts on ozone photochemical process. Effects of cloud amount on photochemical process are significant. First, Figure 6 shows that ozone average value is higher at weekends at any cloud amount, indicating that there is evident ozone weekend effect in Xujiahui, Shanghai; second, Ozone concentration decreases and cloud amount increases at weekends, and so does on weekdays, because of changing ozone photochemical rates along with changing radiation; third, smaller and smaller magnitude of ozone weekend effect in Xujiahui coincides with that the cloud amount gradually switches from lower to higher. The cloud amount varies from 1 to 8, long with the difference of ozone average value between weekends and weekdays decreasing from 15 to 3 ppbv. A negative cloud amount effect on solar radiation and ozone photochemical production suggests that ozone weekend effect nearly disappears because of cloud amount enhance. In conclusion, ozone weekend effect in Xujiahui, Shanghai is rooted in ozone photochemical production. 4 Conclusions Analysis of observed ozone data in 2006 from five monitoring sites (Xujiahui, Chongming, Baoshan, Pudong, Jinshan) in Shanghai, reveals that ozone concentrations in Xujiahui, the urban core, are higher at weekends than those on weekdays, despite the fact that emissions of ozone precursor substances, oxides of nitrogen (NO x ), carbon monoxide (CO) and volatile organic compounds (VOCs) are lower at weekends than those on weekdays. Ozone weekend effect occurs in the downtown (Xujiahui) rather than in the near-urban region (Baoshan and Pudong) and the suburb (Chongming). Ozone average concentration in Xujiahui on Monday is ppbv. The average minimum presents on Thursdays ( ppbv), and the maximum is on Saturdays ( ppbv). Comparing to Thursdays, ozone average concentration is higher by 16.9% on Saturdays. The possible reasons of ozone weekend effect in Xujiahui, Shanghai are concluded as follows: First of all, According to the ozone photochemical production theory, the possible chemical cause of ozone weekend effect is NO 2 /NO ratio increasing by 25.61% at weekends than those on weekdays. NO 2 and NO average value were 31.6 ppbv and 8.7 ppbv respectively at weekends, and 34.2 ppbv and 11.9 ppbv on weekdays. Secondly, because of an average 12.13% reduction in NO x (NO + NO 2 ) in the early morning (05:00-09:00) at weekends than those on weekdays, the ozone inhibition period ends 0.5 h earlier at weekends which result in the longer duration of ozone accumulation and the higher ozone production rates. The rate of ozone production is a function of VOCs and NO x in the air. Either NO x -limited or VOCs-limited region depends on VOCs/NO x ratio. VOCs/NO x ratio in Xujiahui is average 4.55 at weekends and 4.73 on weekdays, indicating that Xujiahui is located in VOC-limited region. Both lower NO x, VOCs and higher VOCs/NO x ratio contribute to ozone increasing from 73 to 80 ppbv, which is identical to Shanghai Xujiahui ozone weekend effect that we discuss. Further research has been conducted using MICAPS cloud amount. Ozone concentrations decrease when cloud amounts increase both at weekends and on weekdays. In addition, smaller and smaller magnitude of ozone weekend effect in Xujiahui, Shanghai accompanies with the cloud amount gradually switches from lower to higher. A negative effect of cloud amount on TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no

7 solar radiation and ozone photochemical production suggests that ozone weekend effect nearly disappears because of cloud amounts enhance. In conclusion, ozone weekend effect in Xujiahui, Shanghai is caused by local ozone photochemical production. The Ozone Isopleth Plotting Package Research (OZIPR) is provided by U.S. Environmental Protection Agency ( 1 Qin Y, Zhao C S. Basis of Atmospheric Chemistry (in Chinese). Beijing: China Meteorology Press, Zhou X J. The Change of Ozone in The Atmosphere of China and Its Influence to the Climatic EnvironmentⅠ(in Chinese). Beijing: China Meteorology Press, Zhou X J. The Change of Ozone in The Atmosphere of China and Its Influence to the Climatic Environment Ⅱ(in Chinese). Beijing: China Meteorology Press, Zhou X J. Research on the interaction between lower atmosphere and ecosystem over Yangtze Delta region. Beijing: China Meteorology Press, Zhao C S, Tie X X, Wang G L, et al. Analysis of air quality in Eastern China and its interaction with other regions of the world. J Atmos Chem, 2006, 55: Tie X X, Brasseur G P, Zhao C S, et al. Chemical characterization of air pollution in Eastern China and the Eastern United States. Atmos Environ, 2006, 40: Wang T, Cheung V T F, Vincent T F, et al. Ozone and related gaseous pollutants in the boundary layer of eastern China: Overview of the recent measurements at a rural site. Geophys Res Lett, 2001, 28(12) : Ma J Z, Liu H L. Summertime tropospheric ozone over China simulated with a regional chemical transport model. 1. Model description and evaluation. J Geophys Res, 2002, 107(D22): Ma J Z, Zhou X J, Hauglustaine D. Summertime tropospheric ozone over China simulated with a regional chemical transport model. 2. Source contributions and budget. J Geophysi Res, 2002, 107(D22): Geng F, Zhao C S, Tang X, et al. Analysis of ozone and VOCs measured in Shanghai: A case study. Atmos Environt, 2007, 41(5): Zhao C S, Peng L, Sun A D, et al. Numerical modeling of tropospheric ozone over Yangtze Delta region. J Environ Sci, 2004, 24(3): Zheng X D, Chen Z Y, Cui H, et al. Characteristics of vertical zone distribution in the lower troposphere in the Yangtze River Delta at Lin an in the spring of Sci China Ser D-Earth Sci, 2005, 48(9): Hu J L, Zhang Y H. Process Analysis of ozone formation in the Yangtze River Delta. Res Environ Sci, 2005, 18(2): Xu X B, Lin W L, Wang T, et al. Long-term trend of tropospheric ozone over the Yangtze Delta Region of China. Adv Climate Change Res, 2006, 2(5): Cleveland W S, Graedel T E, Kleiner B, et al. Sunday and workday variations in photochemical air pollutants in New Jersey and New York. Science, 1974, 186: Lebron F. A comparison of weekend-weekday ozone and hydrocarbon concentrations in the Baltimore-Washington metropolitan area. Atmos Environ, 1975, 9: Vukovich F M. The spatial variation of the weekday/weekend differences in the Baltimore area. J Air Waste Manag Ass, 2000, 50: Elkus B, Wilson K R. Photochemical air pollution: Weekend/weekday differences. Atmos Environ, 1977, 11: Levitt S B, Chock D P. Weekday-weekend pollutant studies of the Los Angeles basin. J Air Poll Control Ass, 1976, 26: Altshuler S, Arcado T D, Lawson D R, et al. Weekday vs. weekend ambient ozone concentrations : Discussion and hypotheses with focus on Northern California. J Air Poll Control Ass, 1995, 45: Diem J E. Comparisons of weekday/weekend ozone: Importance of biogenic volatile organic compound emissions in the semi-arid southwest USA. Atmos Environ, 2000, 34: Pun B K, Seigneur C, White W. Data analysis for a better understanding of the weekday/weekend ozone and PM differences. Atmospheric and Environmental Research, Inc., San Ramon, CA 94583, Pryor S C, Steyn D G. Hebdomadal and diurnal cycles in ozone time series from the Lower Fraser Valley, B.C. Atmos Environ, 1995, 29: Bronninmann S, Neu U. Weekend weekday differences of near-surface ozone concentrations in Switzerland for different meteorological conditions. Atmos Environ, 1997, 31: Pont V, Fontan J. Comparison between Weekend and Weekday Ozone Concentration in Large Cities in France. Atmos Environ, 2001, 35: National Research Council. Rethinking the Ozone Problem in Urban and Regional Air Pollution. Washington, D C: National Academy Press, Bower J S, Broughton G F, Dando M T, et al. Surface ozone concentrations in the U K in Atmos Environ, 1989, 23: Gery M W, Crouse R R. User's Guide for Executing OZIPR. Chemical Processes and Characterization Division, Atmospheric Research and Exposure Assessment Laboratory, US Environment Protection Agency, Fujita, E M, Stockwell W, et al. Weekend/weekday ozone observations in the South Coast Air Basin: Retrospective analysis of ambient and emissions data and refinement of hypotheses. Volume I-Executive Summary. National Renewable Energy Laboratory, Golden, CO 80401, TANG WenYuan et al. Sci China Ser D-Earth Sci Sep vol. 51 no