Science of the Total Environment 550 (2016) 273–284

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Critical role of meteorological conditions in a persistent haze episode in the Guanzhong basin, China Naifang Bei, Bo Xiao, Ning Meng, Tian Feng School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi'an, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The synoptic situations have the dominant role in heavy haze formation in the basin. • Low-level inversion, weak wind, and high relative humidity contribute to the event. • The emissions need to be mitigated by more than 91% to meet the excellent level.

The response of the average PM2.5 level in the Guanzhong basin, China to the anthropogenic emission reduction during the period from December 16 to 26, 2013. Blue line denotes the control simulation; green, red, and brown lines represent simulations with 50%, 75%, and 87.5% reductions of anthropogenic emissions, respectively; Black line is the simulation without anthropogenic emissions. The figure shows that the emissions in the basin need to be mitigated by more than 90% to meet the excellent level of the China National Air Quality Standard under the extremely unfavorable meteorological conditions.

a r t i c l e

i n f o

Article history: Received 26 November 2015 Received in revised form 28 December 2015 Accepted 30 December 2015 Available online xxxx Editor: D. Barcelo Keywords: Guanzhong basin Haze Meteorological conditions Emission reduction

http://dx.doi.org/10.1016/j.scitotenv.2015.12.159 0048-9697/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t In the present study, the critical role of the meteorological condition in a persistent extreme haze episode that occurred in Guanzhong basin of China on December 16 to 25, 2013 has been investigated. Analyses of the large-scale meteorological conditions on 850 hPa during the episode have been performed using the NCEP FNL data set, indicating that synoptic situations generally facilitate the accumulation of pollutants either in horizontal or vertical directions in the basin. The FLEXPART model has been utilized to illustrate the pollutant transport patterns during the episode, further showing the dominant role of synoptic conditions in accumulation of pollutants in the basin. Detailed meteorological conditions, such as temperature inversion, and low-level horizontal wind speed also contribute to the extreme haze episode. In addition, the WRF-CHEM model has been used to evaluate the responses of the surface PM2.5 level to the emission mitigation. Generally, the predicted PM2.5 spatial patterns and temporal variations agree well with the observations at the ambient monitoring sites. Sensitivity studies show that the emissions in the basin need to be mitigated by more than 91% to meet the excellent level of the China National Air Quality Standard under the extremely unfavorable meteorological conditions, demonstrating that it is imperative to implement stringent controls on emissions to improve the air quality. © 2016 Elsevier B.V. All rights reserved.

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1. Introduction Fine particulate matter (or aerosol) with an aerodynamic diameter equal or less than 2.5 μm (PM2.5) becomes a serious environmental issue since it could reduce visibility, negatively affect human health, and further influence weather and climate (e.g., Stocker et al., 2013; Wang et al., 2014c; Wang et al., 2011; Tie et al., 2009; Li et al., 2007). The aerosol concentrations in the atmosphere are mainly determined by several factors, which include pollutant emissions, secondary aerosol formation, atmospheric advection and diffusion, deposition, etc. (He et al., 2001; Yang et al., 2011; Sun et al., 2013). Haze is defined as one kind of weather, in which the horizontal visibility is less than 10 km and the relative humidity is less than 90% (Wu et al., 2007). In haze events, the concentrations of PM2.5 rapidly increase, causing low atmospheric visibility. In addition, the hygroscopic growth of aerosol particles due to increased water vapor in the atmosphere can further enhance their effects on the visibility (Liu et al., 2012, 2013; Quan et al., 2011). In recent 30 years, due to the rapid industrialization and urbanization, China has experienced severe haze pollutions with exceedingly high PM2.5 levels (e.g., Wu et al., 2005; Chan and Yao, 2008; Gao et al., 2011; Kang et al., 2013; Fu et al., 2014; Wang et al., 2014d; Zhang et al., 2015; and Yang et al., 2015). Guanzhong basin is located in the northwest of China (Fig. 1a), nestled between the Qinling Mountains in the south and the Loess Plateau in the north. The particular topography facilitates the accumulation of air pollutants, and along with the rapid increasing industries and city expansions, heavy air pollution events frequently attack the basin, causing the basin to be one of the areas with the most haze days in China. Numerous studies have been performed to investigate the key role of the meteorological conditions in the formation, transformation, diffusion, transport, and removal of the atmospheric pollutants (e.g., Seaman, 2000; Solomon et al., 2000; de Foy et al., 2005, 2006; Banta et al., 2005; Zhang et al., 2007; and Bei et al., 2008, 2010, 2012, 2013, 2014). Most of studies on understanding the role of the meteorological conditions in the haze formation in China have concentrated on the regions of Beijing–Tianjin–Hebei, the Pearl River Delta, and Yangtze River Delta. Guo et al. (2014) have elucidated the coupling between meteorology, local emissions, and aerosol processes during the heavy haze events occurred in Beijing. They have also pointed out that the periodic cycles of severe haze episodes in Beijing are mainly driven by meteorological conditions. Zhang et al. (2015) have showed that the planetary boundary layer (PBL) height and surface wind speeds both decrease

during the transition from the high visibility to low visibility, and the visibility is still very low even with low PM2.5 concentrations when relative humidity is higher than 80%. In addition, Wang et al. (2014a) have demonstrated that the severe haze events that occurred in January, 2013 are mainly attributed to the unfavorable meteorological condition, rather than an abrupt increase in emissions. Wang et al. (2014b) have further shown that a persistent haze episode that occurred in Beijing is affected by both the local stable PBL structure and the pollutants transport from the south in the lower troposphere between 925 and 850 hPa. In the Guanzhong basin, few studies have been performed to investigate the impact of meteorological fields on haze formations. Previous works on the air pollution in the basin mainly focused on the chemical composition, characterization and source apportionment of the aerosols in the atmosphere (e.g., Cao et al., 2009, 2012; Shen et al., 2010, 2011). Therefore, it is imperative to explore the role of the specific meteorological conditions in the haze formation in the basin. Several studies have shown that the haze event in Xi'an exhibits heavy particle pollution and long duration characteristics. Chang et al. (2009) have found that visibilities for all seasons in Xi'an show increasing trends from 1973 to 1995, but decline dramatically from 1995 to 2007. Days with the low visibility (i.e., visual range less than 10 km) increase from 50 days in 1995 to 340 days in 2007. Additionally, the occurrence of “very good” visibility is extremely rare after 1997 (Chang et al., 2009). Cao et al. (2012) have further investigated the impacts of aerosol compositions on visibility impairment in Xi'an, showing that visibility is strongly influenced by anthropogenic air pollution sources and the high secondary inorganic aerosol formation is the main contributor for the visual range less than 5 km. The purpose of the present study is to explore the specific meteorological conditions during a persistent heavy haze episode that occurred from December 16 to 25, 2013 both at synoptic and local scales, and the response of PM2.5 mass concentrations to the emission reduction through the observational analysis and modeling. The models and methodology used in this study are introduced in Section 2. The main results are presented in Section 3. Summary and discussion are given in Section 4. 2. Model and data descriptions The National Centers for Environmental Prediction (NCEP) final operational global gridded analysis (FNL) data is used to analyze the large-scale meteorological conditions influencing the Guanzhong

Fig. 1. (a) WRF and WRF–CHEM model simulation domain with topography and (b) geographic distributions of ambient monitoring stations. In (b), the filled squares with different colors are the ambient monitoring sites for air pollutants in five cities in the Guanzhong basin and the red filled circle is the Jinghe meteorological site. The area surrounded by the black line is the urban region of Xi'an. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

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basin during the persistent haze episode from December 16 to 25, 2013. The geopotential height and wind fields on 850 hPa are utilized to identify the synoptic situations that affect the plume transport patterns in the basin. The vertical high-resolution radiosonde data at Jinghe (Fig. 1b) is used to investigate the vertical local meteorological conditions during the persistent haze episode. Since 2013 January, the China MEP has commenced to release the real-time hourly observations of chemical species at the national ambient monitoring stations, including O3, NO2, CO, SO2, PM2.5, and PM10 (particulate matter with aerodynamic diameter less than 10 μm). Total 33 monitoring sites are distributed in the Guanzhong basin (Fig. 1b). The hourly PM2.5 measurement released by China MEP in 2013 are used to validate the categorized synoptic situations and local meteorological conditions influencing the basin. In order to analyze the corresponding pollutant transport patterns during the episode, the FLEXPART model is employed to calculate the forward Lagrangian particle dispersion (Stohl et al., 1998; Fast and Easter, 2006), driven by the output from the WRF model (Skamarock et al., 2008). The FLEXPART model is set-up with releases of 6000 computational particles within a grid cell of 10 km × 10 km × 0.02 km centered at Xi'an urban area in the morning. Tracer particles are released continuously from 04:00 to 10:00 BJT (Beijing Time) of the day, and traced until 04:00 BJT of next day. For the convenience, all the time used hereafter is BJT. The WRF model adopts one grid with horizontal resolution of 3-km and 35 sigma levels in the vertical direction. The grid cells used for the domain are 201 × 201 (Fig. 1a). The persistent haze episode from December 16 to 25, 2013 is simulated using the WRF model. The WRF model is initialized at 20:00 BJT on December 15 and integrated for 264 h continuously. The NCEP FNL reanalysis data is used to produce the initial and boundary conditions for the WRF model. The physical process parameterization schemes used in simulations included the Grell–Devenyi ensemble scheme for cumulus scheme (Grell and Devenyi, 2002), the WRF Single Moment (WSM) three-class microphysics (Hong et al., 2004), and Mellor–Yamada–Janjic (MYJ) TKE scheme (Janjic, 2002) for the PBL processes. The WRF–CHEM model has been used to further simulate the 10-day severe haze pollution episode and to evaluate the response of the surface PM2.5 level to emissions mitigation in the Guanzhong basin. A specific version of the WRF–CHEM model (Grell et al., 2005) is used in the present study, which has been developed by Li et al. (2010, 2011b, 2012) at the Molina Center for Energy and the Environment, with a new flexible gas phase chemical module and the CMAQ (version 4.6) aerosol module developed by US EPA (Binkowski and Roselle, 2003). The inorganic aerosols are predicted in the WRF–CHEM model using ISORROPIA Version 1.7 (http://nenes.eas.gatech.edu/ISORROPIA/). The SOA formation is simulated using a non-traditional SOA model including the volatility basis-set modeling method in which primary organic components are assumed to be semi-volatile and photochemically reactive and are distributed in logarithmically spaced volatility bins (Li et al., 2011a). Detailed description of the WRF–CHEM model can be found in Li et al. (2010, 2011b, 2012). The meteorological setup in the WRF–CHEM model simulations is same as those in the WRF model, except that the spin-up time of the WRF–CHEM model is one day. The chemical initial and boundary conditions for the WRF–CHEM model simulations are interpolated from the 6-h output of a global chemical transport model for O3 and related chemical tracers (MOZART). The anthropogenic emission inventory (EI) developed by Zhang et al. (2009) is used in the study, including contributions from agriculture, industry, power, residential and transportation sources. The MEGAN model developed by Guenther et al. (2006) is used to calculate on-line biogenic emissions. 3. Results In this section, we first examine the synoptic conditions and the lowlevel meteorological characteristics during this long-lasting severe

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pollution episode. Meanwhile, we also investigate the pollutant transport patterns during the episode by using the FLEXPART model. Finally, we verify the responses of PM2.5 mass concentrations to the emission reductions to meet the China National Air Quality Standard under such unfavorable meteorological conditions in the Guanzhong basin using the WRF–CHEM model. 3.1. Synoptic conditions in the recent 5 years Using the NCEP-FNL reanalysis data, we analyze the average synoptic situations at 08:00 BJT in December influencing the Guanzhong basin in the recent 5 years (2008–2012) in comparison with that in 2013 (Fig. 2). At the middle level of the troposphere (Fig. 2a–b), the Guanzhong basin (the red dot in the figures denotes the location of Xi'an) is located in the southwest of the trough on 500 hPa. The trough in 2013 (Fig. 2b) is apparently weaker than that averaged during the period from 2008 to 2012 (Fig. 3a), which also reflects in wind speeds and geophysical heights, indicating weaker cold dry air on the upper layer over the Guanzhong basin in 2013. On 850 hPa (Fig. 3c–d), the basin is generally located in the northwest of the high with prevailing northwesterly winds. The high in 2013 (Fig. 3d) is more intensified than that averaged during the period from 2008 to 2012 (Fig. 3c), showing the more stable atmosphere over the basin in 2013. Fig. 3e and f highlight that the Guanzhong basin is usually under the control of a high-pressure system at the surface in December of 2013. The intensified surface high pressure system in December, 2013 tends to withhold the pollutants in the basin, due to the stable stratification, calm winds, and the subsidence around the surface high. In summary, the large-scale meteorological conditions in 2013 are more favorable for trapping the pollutants inside the basin than those in the previous 5 years. 3.2. Synoptic overview and plume transport patterns during the haze episode Fig. 3 presents time evolutions of the PM2.5 mass concentrations averaged over 5 cities (Xi'an, Xianyang, Baoji, Weinan, and Tongchuan, see Fig. 1b) in the basin during the severe haze episode from December 16 to 25, 2013. The observed PM2.5 mass concentrations are generally more than 200 μg m− 3, exceeding the standard of severe pollutions (hourly PM2.5 mass concentration exceeding 150 μg m−3) according to China National Air Quality Standard. The basin is most polluted during the period from December 23 to 25, 2013, with the PM2.5 concentration exceeding 400 μg m−3. The daily fluctuation of the PM2.5 concentrations, such as on December 21 and December 26, are caused primarily by the transition between different synoptic situations. In addition, the diurnal cycles of the observed PM2.5 mass concentrations are not clear, demonstrating the obvious regional pollution characteristics in the basin. The PM2.5 level at Xi'an is much higher than the other cities in the basin, caused by the massive local emissions. NCEP-FNL reanalysis data, together with the model output from WRF and FLEXPART are used to investigate the meteorological synoptic situations and the corresponding plume transport patterns during the haze episode. Figs. 4 and 5 show the synoptic conditions on 850 hPa influencing the Guanzhong basin and the corresponding plume transport patterns during the episode. From December 16 to 17 (Fig. 4a, the synoptic situation on December 17 is shown), the basin is located at the north part of the low on 850 hPa (hereafter referred to as “north-low”) with the predominant easterly wind. Due to the low wind speed and the convergence, the pollutants are subject to be trapped inside the basin (Fig. 5a). However, the pollutants might be cleaned up by precipitations because of the favorable dynamical conditions under the “north-low” situation. From December 18 to 19 (Fig. 4b, the synoptic situation on December 19 is shown), the basin is mainly under the control of an inland high (hereafter referred to as “inlandhigh”), in which the prevailing wind is varied, dependent on the exact

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Fig. 2. The geopotential heights and wind vectors at 08:00 BJT in December, 2013 on (a) 500 hPa, (c) 850 hPa, and (e) at the surface, and averaged during the period from 2008 to 2012 on (b) 500 hPa, (d) 850 hPa, and (f) at the surface.

Fig. 3. Hourly PM2.5 concentrations averaged in five cities (Xi'an, Xianyang, Baoji, Weinan, and Tongchuan) in the Guanzhong basin during the period from December 16 to 26, 2013.

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Fig. 4. The geopotential heights and wind vectors on 850 hPa at 08:00 BJT on December (a) 17, (b) 19, (c) 20, (d) 21, (e) 23, and (f) 25, 2013.

location of the high. The weak winds, the subsidence, and the stable stratification facilitate the accumulation of pollutants in the basin (Fig. 5b). On December 20 (Fig. 4c), the synoptic condition is transformed to “southeast-high” (in which, the basin is located in the southeast of the high on 850 hPa), and the northwesterly wind is prevalent, commencing to carry on pollutants to the southeast of the basin and mitigate the PM2.5 mass concentration on December 21. From December 21 to 24, the basin is again controlled by the in-land high (Fig 4d and e, the synoptic situations on December 21 and 23 are shown, respectively), leading to continuous accumulation of pollutants and extremely high PM2.5 mass concentration on December 23 and 24. On December 25, the basin is located in the southwest of the trough (Fig. 4f) and the dominant wind over the basin is northwesterly (hereafter referred to as “southwest-trough”). The cold dry air originated from the northwest is favorable for evacuation of pollutants from the basin (Fig. 5f), and also brings clean air from the north, improving the air quality in the basin dramatically. However, the observed PM2.5 level is still very high in the basin on December 25, and the rapid falloff of PM2.5 mass concentrations occurs until the early morning on December 26, showing the slow evacuation of pollutants caused by gradually

intensified northwesterly winds. During daytime on December 26, the PM2.5 level in the basin approaches the excellent level (hourly PM2.5 concentrations less than 35 μg m−3), maintained by the strong northwesterly wind.

3.3. Low-level meteorological conditions during the episode Using the high resolution radiosonde data (with vertical interval of 100 m) at 07 AM and 19 PM every day at Jinghe site (Fig. 1b), we have further analyzed the temporal and vertical variations of the lowlevel temperature, wind speed, and relative humidity during the haze episode (Fig. 6). The observed temperatures are relatively high during the haze episode because of the warm air mass brought by the inland high (shown in Fig. 4b, d and e). Moreover, except on Decemeber 18 and 20, near surface temperature inversions (below 200 m) occur on each day during daytime. The elevated temperature inversions also exist around the height of 700 m during the period from December 22 to 24. Temperature inversions enhance the atmospheric stability, hindering pollutants dispersions in the vertical direction (Fig. 6a).

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Fig. 5. Plume transport patterns in the Guanzhong basin on December (a) 17, (b) 19, (c) 20, (d) 21, (e) 23, and (f) 25, 2013.

The height-time cross section of the wind speed (Fig. 6b) shows that the horizontal wind speed under the height of 1600 m is generally less than 2 m s−1 from December 18 to 25. Additionally, the wind speed below 300 m is generally less than 1 m s−1. The weak low-level wind directly fosters the accumulation of the pollutants inside the basin. The wind speed starts to increase on December 20 due to the transition of the synoptic situation from “inland-high” to “northeast-high”. Meanwhile, the relative humidity also drops on December 20 caused by the intrusion of cold and dry air from the north. Favorable transport conditions together with the decreasing relative humidity engender the drop of observed PM2.5 concentrations on December 21. Fig. 6c displays the cross-section of the relative humidity during the episode. The relative humidity below 600 m principally exceeds 70%, and is particularly high during daytime from December 18 to 25 when the temperature inversion occurs (except on December 18 and 20). Water vapors play a key role in the heterogeneous reactions for the secondary aerosol formation and lead to rapid escalation of the PM2.5 mass concentration (Huang et al., 2014; Guo et al., 2014; Quan et al., 2011). Temperature inversions, low wind speeds, together with high relative humidity, not only advantage accumulation of pollutants, also accelerate the formation of secondary aerosols, substantially escalating the pollution level in the basin.

After 19:00 BJT on December 25, with the increase of the northerly wind (Fig. 4f), the temperature and relative humidity are both decreased dramatically (Fig. 6c–d), and pollutants in the basin commence to be dispersed efficiently in the vertical and horizontal directions. In addition, the evacuation process from December 25 to 26 is retarded due to the high accumulation of PM2.5 in the basin. 3.4. Response of PM2.5 mass concentrations to the emission mitigation The results of the FLEXPART model presented in Subsection 3.2 only explain the direct impact of the meteorological fields on the plume transport since the chemical processes are not considered in the model. The WRF–CHEM model is therefore used to simulate the actual air quality conditions during the severe haze episode. Based on the reasonable simulation results from the WRF–CHEM model, we have further investigated the responses of PM2.5 mass concentrations to the emission mitigation through numerical experiments in order to estimate the emission reduction necessary to meet the China National Air Quality Standard in the basin under the extremely unfavorable meteorological situations. Fig. 7 shows the simulated and observed temporal profiles of the near surface PM2.5, O3, NO2, SO2, and CO mass concentrations averaged

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279

XN

ðP −Oi Þ2 i¼1 i     2 N P i −O þ Oi −O i¼1

IOA ¼ 1−X

ð3Þ

where Pi and Oi are the simulated and observed variable, respectively. N

Fig. 6. Temporal and height variations of the (a) low-level temperature, (b) wind speed, and (c) relative humidity at Jinghe (close to Xi'an) during the period from December 16 to 26, 2013.

over the 33 monitoring in the basin during the episode. The mean bias (MB), the root mean square error (RMSE), and the index of agreement (IOA) are used to evaluate the WRF–CHEM model simulations of the air pollutants. MB ¼

1 XN ðP −Oi Þ i¼1 i N

RMSE ¼

 X 12 1 N 2 ð P −O Þ i i i¼1 N

ð1Þ

ð2Þ

is the total number of the predictions used for comparisons, and O denotes the average of the prediction and observation, respectively. IOA ranges from 0 to 1, with 1 indicating perfect agreement between model and observation.The WRF–CHEM model generally captures well the observed variations of air pollutants with the IOA more than 0.72, although the model biases still exist, particularly with regard to O3 simulations. The model reproduces well the high PM2.5 level which causes severe haze in the basin, although it tends to somehow underestimate the observation before December 22 and overestimate on and after December 22 (Fig. 8a). Furthermore, the model have successfully captured the drop of the PM2.5 mass concentrations on December 21 and the decrease from December 25 to 26 due to the transition of the large-scale meteorological patterns. On December 21, the synoptic situation is altered from “inland-high” to “southeast-high”, which is favorable for the evacuation of the pollutants in the basin. On December 25, the synoptic meteorological condition becomes “northwest-trough”, which also facilitates the dispersion of the pollutants in the basin. However, the model cannot replicate the diurnal variation of pollutants very well in comparison with the observations on each day, which might be caused by the uncertainties of simulated meteorological fields or emissions inventory (Bei et al., 2012). Fig. 8 presents the spatial distributions of calculated and observed near-surface PM2.5 mass concentrations at 12:00 BJT during the episodes along with the simulated wind fields. The predicted PM2.5 spatial patterns are consistent well with the observations at the ambient monitoring sites in the basin. From December 16 to 17, the basin is controlled by “north-low”; weak easterly winds and blocking of mountains in the west of the basin initiate the buildup of PM2.5, gradually deteriorating the air quality. From December 18 to 19, the calm and varied winds originated by “inland-high” further escalate the PM2.5 level, causing heavy haze in the basin, with the PM2.5 mass concentrations exceeding 400 μg m− 3. On December 20, “southeast-high” dominates the basin and the well organized northwest winds commence to transport the PM2.5 from the west to the east in the basin. However, due to the subsequent transformation of the synoptic situations to “inland-high” on December 21, high PM2.5 mass concentrations are still maintained in the east of the basin. Continuous 4-day “inland-high” situations from December 21 to 24 foster extremely heavy air pollutions in the basin, and the basin average PM2.5 mass concentrations exceed 400 μg m − 3 on December 23 and 24. From December 25 to 26, strong northwest or north winds induced by “southwest-trough” intrude the basin, and transport the PM2.5 outside of the basin, significantly improving the air quality on December 26. Based on the above-mentioned reasonable simulations using the WRF-CHEM model, we have further investigated the responses of the PM2.5 mass concentrations to the emission reduction through conducting numerical experiments. In the first experiment, we have evaluated the background contributions to the PM2.5 level in the basin by turning off the anthropogenic emissions in the simulation. The basin average background PM2.5 mass concentrations are about 20 μg m−3, showing the high PM2.5 contribution from the background. In the rest three sensitivity studies, the anthropogenic emissions are reduced to 50%, 75%, and 87.5%, respectively. The simulated time evolutions of the PM2.5 mass concentrations averaged over the basin are provided in Fig. 9. If excluding the background PM2.5 contribution, the basin average PM2.5 mass concentrations almost linearly decrease with the anthropogenic emission mitigation. When the emission is reduced by 50%, the PM2.5 level is decreased by 53%, and an 87.5%

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Fig. 7. Simulated time evolutions of the (a) PM2.5, (b) O3, (c) NO2, (d) SO2, and (e) CO mass concentrations during the period from December 16 to 26, 2013.

reduction of the emission decreases the PM2.5 level by about 90%, showing the predominant role of synoptic situations in the PM2.5 level over the basin. Considering the high PM2.5 background contribution, only the anthropogenic emission is reduced by more than 91%, the basin average PM2.5 mass concentrations can be decreased to 35 μg m−3 or less and the air quality in the basin can reach the excellent level of the China National Air Quality Standard under such persistent extremely unfavorable meteorological conditions. Furthermore, sensitivity studies have shown that the residential living source plays the most important role in the PM2.5 level in the basin during wintertime, with the contribution

exceeding 40%. Therefore, reducing emissions from residential living is an efficient pathway to alleviate the air pollution in the basin. However, the PM2.5 contribution from the transportation is around 10%, so controlling the traffic during heavy haze episodes does not help remarkably improve the air quality in the basin. 4. Summaries and discussions The critical role of the meteorological conditions in a persistent heavy haze episode occurred in the Guanzhong basin of China during

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December 16 to 25, 2013 is investigated through using NCEP reanalysis data, high-resolution radiosonde data, and simulations from the FLEXPART and WRF–CHEM models. The responses of PM2.5 mass concentrations to emission reductions are further explored using the WRF-CHEM model in order to assess the emission mitigation necessary to meet the China National Air Quality Standard under the extremely unfavorable meteorological conditions in the basin. The analysis of the synoptic situations of December during 2008 to 2013 shows that both 850 hPa and surface high-pressure systems influencing the basin in 2013 is more intensive than those averaged during the period from 2008 to 2012, indicating that the atmosphere over the basin is more stable in December, 2013, and in turn more

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favorable for withholding the pollutants inside the basin. Based on the NCEP reanalysis data and PM2.5 measurements, the large-scale meteorological conditions controlling the Guanzhong basin during the episode are summarized into four types, including “north-low”, “inlandhigh”, “southeast-high”, and “southwest-trough”. The FLEXPART trajectory model has been used to examine the corresponding pollutant transport during the episode, showing the dominant role of the synoptic situation on the pollutants transportation pattern. The synoptic situations from December 16 to 24 are generally unfavorable for the dispersion of pollutants except on December 21 (“southeast-high”), and from December 25 to 26, the synoptic situations (“southwest-trough”) become favorable for the evacuation of pollutants in the basin.

Fig. 8. Pattern comparison of simulated vs. observed near-surface PM2.5 mass 12:00 BJT from December 16 to 26, 2013. Colored squares: PM2.5 observations; color contour: PM2.5 simulations; black arrows: simulated surface winds. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8 (continued).

Analysis of the high resolution radiosonde data shows that the local low-level meteorological conditions are also favorable for the formation of the heavy haze. The observed temperatures are relatively high due to the warm air mass brought by the subtropical high. Frequent occurrence of near-surface and elevated temperature inversions hinders the vertical dispersion of pollutants. The horizontal wind speeds below 1600 and 300 m are generally less than 2 and 1 m s−1, respectively, which is disadvantageous to the horizontal transportation of pollutants in the basin. Furthermore, the relative humidity is high during daytime, considerably enhancing the heterogeneous reactions of secondary aerosols.

The WRF–CHEM model is used to simulate the actual air quality conditions during the episode. The model well reproduces the persistent severe haze episode from December 16 to 25. In addition, the model has successfully captured the drop of the PM2.5 mass concentrations on December 21 and the decrease from December 25 to 26. Sensitivity studies have shown that only the anthropogenic emissions are reduced by more than 91%, the air quality in the basin can meet the excellent level according to the China National Air Quality Standard under such persistent extremely unfavorable meteorological conditions. The results also suggest that it is imperative to implement stringent controls on emissions to improve the air quality in the Guanzhong basin.

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Fig. 9. The response of basin average PM2.5 level to the anthropogenic emission reduction during the period from December 16 to 26, 2013. Blue line: the control simulation; green line: the simulation with a 50% reduction of anthropogenic emissions; red line: the simulation with a 75% reduction of anthropogenic emissions; brown line: the simulation with an 87.5% reduction of anthropogenic emissions; black line: the simulation without anthropogenic emissions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Critical role of meteorological conditions in a persistent haze episode in the Guanzhong basin, China.

In the present study, the critical role of the meteorological condition in a persistent extreme haze episode that occurred in Guanzhong basin of China...
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