Impacts of Himalayas on black carbon over the Tibetan Plateau during summer monsoon

Science of the Total Environment 598 (2017) 307–318

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

Impacts of Himalayas on black carbon over the Tibetan Plateau during summer monsoon Shuyu Zhao a, Xuexi Tie a,b,c,d,⁎, Xin Long a, Junji Cao a,e a

KLACP, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Shanghai Key Laboratory of Meteorology and Health, Shanghai 200030, China d National Center for Atmospheric Research, Boulder, CO 80303, USA e Department of Environmental Science, Xi'an Jiaotong University, Xi'an 710061, China b c

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 trans-Himalayas transport of black carbon from South Asia to the Tibetan Plateau is investigated by the WRFChem model. • The reduction of the Himalayas’ altitude was in favor of more BC over the Himalayas, but not more BC in the TP. • Effects of the Himalayas on BC transport were strongly dependent on the cyclonic activities in the IGP during summer monsoon • In convergent airflows, BC concentrations significantly increased in the southeastern TP, and reached to 0.60.8 μg m-3. • A cyclone located in the eastern IGP, BC transport clearly weakened. While moving to the west, BC transport enhanced.

a r t i c l e

i n f o

Article history: Received 10 February 2017 Received in revised form 13 April 2017 Accepted 13 April 2017 Available online xxxx Editor: D. Barcelo Keywords: Black carbon Monsoon The Himalayas and Tibetan Plateau WRF-Chem model

a b s t r a c t The Tibetan Plateau (TP) plays important roles in global climate and environment. This study combines in-situ BC measurements in the Himalayas and the Indo-Gangetic Plain (IGP) with a regional dynamical and chemical model (WRF-Chem model) to investigate the effect of the trans-Himalayas on black carbon (BC) from the IGP to the TP during Indian summer monsoon. To determine topographic effects of the trans-Himalayas on BC concentrations over the TP, sensitive experiments were conducted by applying the WRF-Chem model. The results showed that the reduction of the altitude of the Himalayas had an important effect on the trans-Himalayas transport of BC. There was an obvious increase in BC concentration over the trans-Himalayas region, but no significant increase over the TP because the TP (a.m.s.l ~4 km) always acted as a wall to prevent BC transport from the IGP to the TP. The trans-Himalayas transport of BC was strongly dependent upon meteorological conditions over the IGP. During summer monsoon, there were three types of cyclones at different locations and one kind of convergent circulation in the IGP. Under the condition of convergent airflows, a strong northeastward wind produced the trans-Himalayas transport of BC. As a result, BC concentrations in the southeastern TP significantly increased to 0.6–0.8 μg m−3. When the cyclone located in the eastern IGP, high BC concentrations over the IGP were

⁎ Corresponding author at: KLACP, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China. E-mail address: [email protected] (X. Tie).

http://dx.doi.org/10.1016/j.scitotenv.2017.04.101 0048-9697/© 2017 Elsevier B.V. All rights reserved.

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transported along the foothill of the Himalayas, resulting in a significant reduction of the trans-Himalayas transport. When the cyclone moved to the west, the dynamical perturbations for the trans-Himalayas transport were weaker than the eastern cyclone, and the trans-Himalayas transport were enhanced in the middle and eastern Himalayas. This study will be helpful to assess the impacts of BC particles emitted from South Asia on regional climate change and ecological environment over the TP in the future. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The Tibetan Plateau (TP) in the north of the Himalayas is away from the influence of the human beings due to an extremely high elevation and severe natural environment. On the contrary, the Indo-Gangetic Plain (IGP) with dense population and massive farming in the south of the Himalayas is increasingly emitting large amount of pollutants, especially black carbon (BC), a light-absorbing particle, into the atmosphere since 1990 (Ramanathan et al., 2005). In recent decades, a few in-situ measurements and pollution events validated BC emitted from South Asia have penetrated into the Himalayas and TP regions. Backward trajectories show that BC particles deposited in the Everest are mainly from South Asia (Ming et al., 2008). An ice core drilled by Xu et al. (2009) in the southeast TP records that BC particles from South Asia are transported into the TP by summer monsoon and the south branch of westerlies since 1990. Aerosol optical depth observations at Nam Co in the central TP record an intense pollution episode during springtime in 2009, and the characteristics of particle size and absorption are similar to the IGP and South Asia, indicating that pollutants come from the IGP (Xia et al., 2011). Therefore, the TP is experiencing a profound impact of BC particles from South Asia. Ramanathan and Carmichael (2008) state that BC in the elevated Himalayas likely plays an important role as carbon dioxide in the melting of snowpacks and glaciers by increasing solar heating. The TP, called as “the third pole”, is an extremely important region where numerous mountain glaciers develop (Menon et al., 2010). However, the region is also too vulnerable to BC particles because the particles decrease ice and snow albedo and increase melting and runoff (Yasunari et al., 2010). It is found that BC particles significantly reduce albedos of visible wavelengths, and 15 μg kg−1 of BC in snow can decrease albedo by 1%– 3% (Warren and Wiscombe, 1980; Light et al., 1998; Flanner et al., 2009). Consequently, increased surface-incident solar radiation induced by BC particles further enhances snow and ice melting, resting in the increase in glacial runoff (Ramanathan et al., 2005; Lau et al., 2010). For example, BC concentration of 26.0–68.2 μg kg−1 in surface snow can increase 70–204 mm water of annual runoff, exerting a profound impact on regional water cycle (Yasunari et al., 2010). The results from numerical simulations show increasing BC emission in India during 2000– 2010 contributes 36% to snow/ice decrease in the Himalayas (Menon et al., 2010). Base on the importance of BC impacts on the cryosphere and consequent climate change over the TP, it is a critical issue to determine how BC from the IGP are transported into the TP while the huge Himalayas mountains lie between them. A previous study suggests that the Himalayas is like a natural barrier that limits exchanges of airflows between the TP and the IGP (Nieuwolt, 1977). Recent studies suggest that the southern slope of the Himalayas is directly exposure to atmospheric brown cloud and that pollutants in the IGP can be lifted to an upper height of 5000 m in the afternoon during pre-monsoon (Bonasoni et al., 2010). Lawrence and Lelieveld (2010) point out that pollutants in South Asia are lifted to mid- and upper troposphere by intense ascending airflows in deep convective clouds, and then transport a long distance in summer. A recent study investigates transport mechanisms of the pollution in such a complex topography of the Himalayas and the TP, founding that westerlies adjustment nearby the Himalayas and the TP interacts with local-scale weather conditions in the IGP, resting in the transport of pollutants from the IGP to the TP (Lüthi et al., 2015). Clearly, available studies

mainly concentrated on how the pollutant from the IGP and South Asia transport over the Himalayas during the pollution episode, but the quantitative calculation by the application of numerical modeling were rare. The present study will use WRF-Chem model to calculate the impacts of the Himalayas region on BC concentrations over the TP during Indian summer monsoon. This study expects to improve the understanding of the Himalayas' impacts on the transport pathways of regional pollution and provide a preliminary support for investigating the impacts of BC emitted from South Asia on the cryosphere in the TP. 2. Data and methods 2.1. WRF-Chem model description Black carbon particles were simulated over South Asia using a stateof-the-art regional dynamical and chemical model (Weather Research and Forecasting Chemical model, WRF-Chem model). The spatial resolution was degraded to 9 × 9 km in the horizontal direction, with 600 grids in longitude and 400 grids in latitude. The domain was concentrated in the IGP and the Himalayas and TP regions, with the center location in the South Tibet (94.44°E, 29.46°N). There were 28 levels in the vertical direction from the surface to 50 hPa. The meteorological fields in WRF-Chem model were driven by NCEP 1° × 1° reanalysis data, with a temporal resolution of 6 h. The lateral BC tracer concentrations were provided by a global chemistry transport model - MOZART4 (Model for OZone And Related chemical Tracers, Version 4), with a 6-h output (Tie et al., 2005; Emmons et al., 2010). The WRF model included online calculation of dynamical inputs (winds, temperature, planetary boundary layer etc.), transport (advection, convection and diffusion), dry deposition (Wesely, 1989) and wet deposition. The physical scheme used Yonsei University (YSU) PBL scheme (Hong et al., 2006), the microphysics scheme (Hong and Lim, 2006), the Noah land-surface model (Chen and Dudhia, 2001), the long-wave radiation parameterization (Mlawer et al., 1997), and the shortwave radiation Table 1 WRF-Chem model configurations. Regions Simulation period Domain size Domain center Horizontal resolution Vertical resolution Microphysics scheme

South Asia and the TP

July 2012 600 × 400 94.4°E, 29.5°N 9 km × 9 km 28 vertical levels from the surface to 50 hPa WSM 5-classes microphysics scheme (Hong and Lim, 2006) Boundary layer scheme YSU PBL scheme (Hong et al., 2006) Surface layer scheme MM5 similarity (Zhang and Anthes, 1982) Land-surface scheme Noah land-surface model (Chen and Dudhia, 2001) Longwave radiation scheme RRTM scheme (Mlawer et al., 1997) Shortwave radiation scheme MM5 shortwave scheme (Dudhia, 1989) Meteorological boundary and NCEP 1° × 1° reanalysis data initial conditions Chemical boundary and MOZART 6-h output (Tie et al., 2005; Emmons et initial conditions al., 2010) Anthropogenic emission Non-residential sources (industry, power, inventory transportation) and residential sources related to fossil fuel and bio-fuel combustions (Zhang et al., 2009; Li et al., 2017)

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Fig. 1. (a) Map of the Himalayas and neighboring regions, the light red is labeled as the Himalayas. (b) The topography distribution of the Himalayas and neighboring regions, the area surrounded by the dark dash line is corresponding to the Himalayas region as Fig. 1a, and the area surrounded by the grey line is the IGP region and the dark solid line is for the crosssection focused by this study. (c) Similar to Fig. 1b, but for BC emission during summer monsoon in the Himalayas and neighboring regions, white circles, diamonds and triangles represent BC measurements at remote, rural and urban sites: Hanle (1), NCO-P (2), Langtang (3), Nainital (4), Mukteshwar (5), Mt. Abu (6), Goa (7), Bangalore (8), Pune (9), New Delhi (10), Ahmedabad (11), Dibrugarh (12), Visakhapatnam (13), Jabalpur (14), Nagpur (15), Delhi (16), Agra (17), Anatapur (18), Varanasi (19). (d) Variations of BC emission and the topographical altitude along the cross section mentioned above, the solid red line is for BC emission and the shallow shading is for the topography, and the deep shading indicates the topographical difference before and after changing the altitude of the Himalayas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

parameterization (Dudhia, 1989). The detailed description is shown by Grell et al. (2005). The black carbon emission inventory with a spatial resolution of 0.25° × 0.25° was downloaded from the Multi-resolution Emission Inventory for China (MEIC) for the year 2010 (http://www. meicmodel.org). The emission source included anthropogenic activities, such as non-residential sources (industry, power, and transportation) and residential sources related to fossil fuel and bio-fuel combustions. The inventory was calculated by the same method as Zhang et al. (2009), but with the updated emission factors that were combined measurements with estimations from Li et al. (2017). The present study ignores natural BC emission from biomass burning because less fire events occur in monsoon seasons, and the fires distribution is archived from http://firms.modaps.eosdis.nasa.gov/firemap/. The WRFChem simulation of BC concentration is conducted for the entire month of July 2012. The detailed model configurations are listed in Table 1. As seen in Fig. 1c, the BC emission had a significant spatial gradient, decreasing from South Asia to the TP, with the highest emissions in the IGP, especially in Eastern India. The Himalayas locates in the buffer between the IGP and the TP, with the lowest emission in the TP. Compared to the spatial distribution of BC emission, the topographic altitude has the opposite gradient, with the lowest altitude in the IGP and the highest altitudes in the Himalayas region. In order to clearly display the trans-transport of Himalayas on BC concentrations over the TP, we choose an oriented southwest-northeast cross-section (shown in black line in Fig. 1b) to illustrate the variations of BC emission and the topographic altitude along the cross section. During monsoon seasons,

southwest winds prevail in South India. However, airflows are forced to divide into two branches in the south foothill of the huge Himalayas. One continues to move towards to the southwest China while the other turns to southeast winds and moves along the foothill of the Himalayas. Therefore, an oriented southwest-northeast cross-section that is intersects with the wind direction is suitable to explain the mountain effects on BC transport. Fig. 1d well illustrates a dramatic change of BC emission along the cross section, e.g., there is a significant decline of BC emission from the IGP to the TP as the terrain elevation increases from the IGP to the Himalayas region. This study used WRF-Chem model to design two sensitive experiments to investigate the topographic effects of the Himalayas region on the transport pathways of BC emitted from South Asia to the TP. One numerical experiment is conducted with the practical NCEP meteorological reanalysis data as the initial and boundary conditions, and the other is exactly same as the first one except that the elevation of the Himalayas is reduced to the half (Fig. 1d). 2.2. BC measurement In order to evaluate the model calculation of BC distributions, surface BC measurements were collected from various studies (detailed information shown in Table 2). These BC measurements included three remote sites in the Himalayas region, three rural sites, and fourteen urban sites over the IGP. The detailed site information is shown in Fig. 1c and Table 2. The information in Table 2 shows that BC measurements during July 2012 were available at three sites. During the summer of

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Table 2 Measured and simulated surface BC concentrations at a single site in the Himalayas and neighboring regions. Region

Site

Remote NCO-P Hanle Langtang Rural Mukteshwar Nainitial Mt. Abu Urban Goa Bangalore Pune New Delhi Ahmedabad Dibrugarh Visakhapatnam Jabalpur Nagpur Delhi Agra Anantapur Varanasi

Lat (°N)

27.9 32.8 28.1 29.4 29.4 24.6 15.5 13.0 18.5 28.4 23.0 27.3 17.7 23.2 21.5 28.9 27.2 15.0 25.5

Lon (°E)

86.8 79.0 85.6 79.6 79.5 72.7 73.8 77.6 73.9 77.1 72.5 94.6 83.3 79.9 79.1 76.8 78.0 78.0 83.6

Alt. (m)

5079 4520 3920 2180 1958 1680 50 960 559 217 55 111 – – 312 – – 331 82

Period

2007.07 2010.07 2000.06–09 2007.06–08 2007.07 2005.07–08 2008.07 2008.06–08 2012.07 2012.07 2008.07 2008.07 2006.06–08 2012.07 2011.07 2011.07 2011.06–08 2013.06 2011.07

Surface BC (μg m−3) Obs.a

Modeled

0.056[1] 0.063[2] 0.095[3] 0.532[4] 0.530[5] 0.200[6] 0.310[7] 1.900[8] 1.600[9] 3.700[10] 2.100[11] 3.400[12] 1.670[13] 5.000[14] 2.000[15] 2.400[16] 5.200[17] 1.200[18] 4.600[19]

0.300 0.122 0.985 1.623 1.635 0.151 0.088 1.580 1.568 2.458 1.514 4.425 2.653 3.394 1.154 3.730 3.254 0.317 4.513

[1]

Marinoni et al. (2010), [2] Babu et al. (2011), [3] Carrico et al. (2003), [4] Hyvärinen et al. (2009), [5] Dumka et al. (2010), [6] Ram et al. (2008), [7] Menon et al. (2014), [8] Satheesh et al. (2011), [9] Safai et al. (2014), [10] Bisht et al. (2015), [11] Ramachandran and Kedia (2010), [12] Pathak et al. (2010), [13] Sreekanth et al. (2007), [14] Panicker et al. (2015), [15] Kompalli et al. (2014), [16] Surendran et al. (2013), [17] Pachauri et al. (2013), [18] Kalluri et al. (2016), [19] Kumar et al. (2016). a In-situ BC measurements at various studies.

2010–2011 and 2013, BC measurements were available at six sites. Measurements at the rest sites covered periods from the year 2000 to 2009. Table 2 also shows that BC concentrations at remote sites were typically less than 0.1 μg m−3, while at rural sites, BC concentrations varied in the range of 0.1 μg m−3 to 1.0 μg m−3. For the urban sites, BC concentrations were higher than 1.0 μg m−3, except the value at the coastal site in western India (Goa). Seen in Fig. 1c, high BC emission mainly concentrated in northern and eastern India, and BC emission in western India is much lower, so BC concentration at Goa is as the same level as rural sites.

Fig. 2. (a) Measured surface BC concentrations at remote (triangles), rural (diamonds) and urban (circles) sites in the Himalayas and surrounding regions, and the shading are simulated mean BC concentration for July 2012. (b) Simulated versus measured monthly mean surface BC concentrations at sites in panel a, the solid line is the linear regression with regression slope (S), correlation coefficient (r) between the measured and simulated BC concentrations and site number with data points (N), and the dash line indicates for 1:1.

3. Model evaluation Fig. 2a shows the comparison of the measured and calculated spatial distributions of the surface BC concentrations. Due to limited in-situ BC measurements during the same period, the surface BC concentrations observed in the region at different years were used to compare and evaluate the model in this study (the comparison results shown in Table 2). The results indicate that model results reproduced the concentration gradient from urban sites, rural sites to remote sites. The model simulation had better representation at the urban sites, but overestimated BC concentrations at the rural and remote sites. One important reason might be due to the bias induced by the interpolation of the emission inventory from the coarse spatial resolution to the fine resolution. The Himalayas region acted as a topographic boundary that divided the highest BC emission in the south and the lowest emission in the north. As a result, such dramatic differences of emission at the topographic boundary can produce some deviations in the interpolation algorithm. However, uncertainty from the emission generally exists in the model results and it is extremely difficult to solve. Another factor for causing the discrepancy was the meteorological conditions. Comparisons between measured and calculated air temperature, relative humidity and wind speed are given in Table 3. There are 19 sites with BC measurements (Table 2), we collected the monitoring of meteorological parameters at five sites of them. They are Agra in the IGP, Ahmedabad in the western India, Bangalore in the southern India,

Dibrugarh in the northern India and Jabalpur in the central India. Overall, the model has a better performance on the temperature and relative humidity, but the modeling of the wind speed is completely failed. The mean bias (MB) shows that the calculated temperature is overestimated

Table 3 Measured and simulated surface meteorological parameters at the sites with BC measurements. City Agra

Ahmedabad

Bangalore

Dibrugarh

Jabalpur

MB RMSE IOA MB RMSE IOA MB RMSE IOA MB RMSE IOA MB RMSE IOA

Temperature

Relative humidity

Wind speed

1.73 3.29 0.71 0.66 2.22 0.81 0.01 1.33 0.94 1.32 2.37 0.76 1.13 2.32 0.85

−12.08 21.10 0.65 −9.61 15.11 0.63 −2.79 9.74 0.87 −17.61 86.95 0.09 −8.28 13.93 0.79

−3.90 65.23 0.03 2.71 2.98 0.28 4.35 4.50 0.31 −4.67 65.30 0.02 −0.55 65.36 0.00

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Fig. 3. (a) Schematic diagram for the temperature gradient between the Indian Ocean and the continent in South Asia during summer monsoon. (b) The calculated distribution of 2-m air temperature (unit: K) over the Himalayas and surrounding regions, and temperature in the continents is about 2 °C higher than that in the Ocean. (c) The calculated distributions of mean surface BC concentrations (unit: μg m−3) and winds (unit: m s−1) in July 2012. The thick white arrow represents the prevailing southwest winds in South Asia, and the thick arrows represent the prevailing southwest airflows divide into two branches at the foothill of the Himalayas. (d) Same as panel c, except that the altitude of the Himalayas is reduced to a half. (e) The difference of BC concentrations (unit: μg m−3) between panel c and panel d, the positive indicates higher BC concentrations when the Himalayas is reduced to a half and the negative indicates lower BC concentrations when the Himalayas is reduced to a half. (f) The percentage change of BC concentrations (unit: %), calculated by ([BC]after − [BC]before)/ [BC]before × 100, the positive indicates BC concentrations increase when the Himalayas is reduced to a half and the negative indicates BC concentrations decrease when the Himalayas is reduced to a half.

at every site, and the mean square error (RMSE) shows that the maximum departure is 3.29 °C. The index of agreement (IOA) varies from 0.71 to 0.94, suggesting a good agreement with the observations. Corresponding to the bias of the temperature, the humidity is underestimated. It is noted that the humidity at Dibrugarh seriously

deviates from the observation. The Dibrugarh locates in a valley (Fig. 1b), so it brings some difficulties for modeling. As for winds, the domain has a quite complicated topography, and it is quite difficult for the model to reproduce the winds. This is agreement with He et al. (2014), and they suggest that complicated mountain meteorology is a

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Fig. 4. The vertical profiles of monthly-mean BC concentration and winds in July 2012 along the cross section in Fig. 1b, (a) with the real altitude of the Himalayas with strong upward motions (the thick dark arrow) and (b) with a half altitude of the Himalayas with weak upward motions (the thin dark arrow), the box represents enhanced horizontal transport in the area of the reduced altitude of the Himalayas.

challenge for the simulations of meteorological conditions in the Himalayas and surrounding regions. In addition, the horizontal resolution of the model is 9 km, too coarse to identify the local winds. Therefore, the departure of winds in the model directly affects the accuracy of BC concentration. The scatter distributions and correlation between calculated and measured BC concentrations are shown in Fig. 2b. The model generally underestimated the measured BC concentrations, with a regression slope of 0.70. Such model performance suggested that the emission inventory used in this study might underestimate BC emission in the IGP. However, the coefficient correlation was 0.83, with a confidence level of p-value b0.001. Therefore, the WRF-Chem model generally well reproduces the distribution of BC concentrations. 4. Results and discussion 4.1. Effect of Himalayas on BC transport during summer monsoon During the Indian summer monsoon, the temperature in Indian Ocean (T_Ocean) is cooler than the continent in South Asia (T_Land) (Fig. 3a and b), generating a horizontal temperature difference (ΔT) that leads to a circulation between the ocean and the continent. Because the horizontal distance (ΔY) between the Bay of Bengal and the Himalayas is narrow, the maximum horizontal temperature gradient (ΔT/ΔY) in this region produces. As a result, there is a strong airflow from the Bay of Bengal towards to the Himalayas, generating a transHimalayas transport pathway for pollutants (Fig. 3a).

Fig. 3c shows the WRF-Chem calculation for the mean surface winds and BC concentrations in July. It was shown that there was a strong south wind from the Bay of Bengal towards to the Himalayas. It also showed that there were two branches of south winds due to the effect of high mountains. One branch was a southeast wind, and the other was a southwest wind. These winds resulted in the trans-Himalayas transport of BC, indicating that there were BC particles crossing the Himalayas to the TP. This trans-Himalayas transport occurred evidently in southeastern Himalayas, producing 0.2–0.6 μg m−3 BC concentrations in the TP. Fig. 3d shows the trans-Himalayas transport of BC concentrations with changing the altitude of the Himalayas. The south wind from the Bay of Bengal to the Himalayas still had two branches. One branch was a southwest wind, the same as Fig. 3c, while the other branch was a southeast wind with a lower speed than Fig. 3c. After removing a half altitude of the trans-Himalayas, the speed of the south wind from the Himalayas to the TP became more continuous and the wind direction became more concurrent. Under this condition, more BC particles were transported to the TP. Fig. 3e shows the difference of calculated BC concentrations, resulting from changing altitude of the Himalayas. Corresponding to Fig. 3e, Fig. 3f calculates the change in percentage of BC concentrations. The result suggested that there was a significant effect by reducing the altitude of the Himalayas, leading to an increase in the trans-Himalayas transport of BC particles. Clearly, an obvious increase was confined in the southern Himalayas, and the largest increase in BC concentration located in the area where the altitude of the Himalayas was reduced, with a maximum increase of 0.6–1.0 μg m−3 (Fig. 3e). In the TP, an increase

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Fig. 5. Four typical surface weather patterns, and the solid dark line is exactly the same cross-section location as Fig. 1b. (a) No cyclone activities but with convergent airflows over the IGP on 17th July 2012. (b) The cyclone is in the east of the cross section (or in the eastern IGP) on 3rd July 2012. (c) The cyclone is in the south of the cross section (or in the middle IGP) on 23th July 2012. (d) The cyclone is in the west of the cross section (or in the western IGP) on 11th July 2012. The blue line is for sea level pressure (hPa). The arrow represents the pathways that the winds move on and thicker arrows indicate higher wind speeds, and vice versa. The data are obtained from http://rda.ucar.edu/datasets/ds083.2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in BC concentration was less than 0.05 μg m−3, but the change in percentage was nearly doubled (Fig. 3f). Fig. 4 calculated the trans-Himalayas transport of BC concentrations along the cross section (the location shown in Fig. 1b). The results showed that there was an obvious enhancement of the trans-Himalayas transport of BC concentrations, with reducing the altitude of the Himalayas. As a result, the BC concentrations on the top of the Himalayas increased from 0.2–0.4 μg m−3 to 0.6–0.8 μg m−3. The result suggested that the high elevation of the Himalayas indeed acted as a wall to prevent the trans-Himalayas transport of BC particles, causing low BC concentrations in the TP. However, the reduction of altitudes of the Himalayas produced complicated changes in meteorological conditions. Comparing the dynamical transport conditions between the two model simulations, there were significant differences in the trans-Himalayas transport of BC particles with the real altitude of the Himalayas (Fig. 4a) and with the reduction of the altitude of the Himalayas (Fig. 4b). Firstly, the horizontal transport of BC particles was enhanced in the area of the reduced altitude of the Himalayas (Fig. 4b). Because of the high altitude of the TP (the mean elevation about 4 km), the reduction of the Himalayas' altitude only plays a limited role in the transport of BC particles in horizontal distance of the TP. Secondly, a lower altitude of the Himalayas produced a weaker upward motion at the edge of the Himalayas (85.4°E, 27.9°N). For example, a relatively high BC concentration (0.4– 0.6 μg m−3) was transported to the height of 7 km with the real altitude of the Himalayas (Fig. 4a). On the contrary, it was only transported to the height of 5 km with a reduced altitude of the Himalayas (Fig. 4b). As a result, the reduction from the altitude of the Himalayas plays a role in a “filling effect” (as shown by the black square in Fig. 4b), and prevents more penetration of BC particles into the TP.

This result suggests that not only a high altitude of the Himalayas played important roles in preventing high BC concentrations being transported from the IGP to the TP, a high altitude of the TP also obstructed BC transport from the IGP to the TP to some extent. 4.2. Effect of meteorological conditions on the trans-Himalayas transport Cyclonic circulations dominate over the IGP region in summer monsoon seasons. These motions produce strong ascending airflows, in favor for lifting near-surface pollutants to mid- even or upper troposphere. Then, the atmospheric advection motions at the mid- and upper troposphere transport pollutants to deposit in the TP, leading to an increase in surface BC concentration. To understand the effects of meteorological conditions on the trans-Himalayas transport of BC concentrations, four different weather conditions were defined based on the location of cyclonic circulations relative to the cross section in this study, i.e., the eastern, western, southern cyclonic circulations, and the convergent airflows. The details of four weather conditions are shown in Fig. 5. The occurrences of the eastern and western cyclones had the similar frequency, contributing 37.9% and 34.5% to the total, respectively. The southern cyclone accounted for 20.7% and the cyclonic circulations with convergent airflows contributed to 6.9%. Fig. 5 showed four types of surface weather patterns that would affect the trans-Himalayas motions of airflows during the modeling period. For the convergent airflows (Fig. 5a, condition-a), there were no cyclone activities. As a result, there was an enhanced trans-Himalayas motion from the Bay of Bengal moving towards to the Himalayas region, resulting in strong northeastward motions from the IGP to the TP. For the eastern cyclone (Fig. 5b, condition-b), there was a cyclone center located nearby the Bay of Bengal. As a result, the airflows from the Bay of Bengal turned

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Fig. 6. Corresponding to weather patterns mentioned in Fig.5, simulated surface BC concentration and horizontal winds distributions on (a) 17th July, (b) 3rd July, (c) 23th July, and (d) 11th July 2012. The white arrows represent the pathways that BC particles move on.

to northwest along the foothill of the Himalayas, resulting in a weak trans-Himalayas transport. For the southern cyclone (Fig. 5c, conditionc), there was a cyclone center located in the middle of the IGP. Consequently, the airflows from the Bay of Bengal had less northwestward component than the condition-b. In this case, the trans-Himalayas transport was stronger, but weaker than the condition-a. For the western cyclone (Fig. 5d, condition-d), there was a cyclone located in the west of the IGP. As a result, the airflows from the Bay of Bengal moving towards to the Himalayas were little influenced by the cyclone. In this case, the trans-Himalayas transport was enhanced again, resulting in significant BC transport from the IGP to the TP. 4.3. The trans-Himalayas transport of BC under different meteorological conditions Fig. 6 shows the distribution of calculated BC concentrations under different meteorological conditions (i.e., condition-a, b, c and d defined in Section 4.2). In the condition-a (Fig. 6a), the dominated surface airflow was from the Bay of Bengal moving towards to the Himalayas, bringing high BC concentrations from the IGP (about 4–6 μg m−3) to the foothill of the Himalayas. Due to strong southwest winds, BC particles were significantly transported across the Himalayas to the TP, resulting in high BC concentrations in southeastern TP (about 0.6– 0.8 μg m−3), which was consistent with the surface measurements (Cao et al., 2010; Zhao et al., 2013). In this condition, a strong transHimalayas transport of high BC concentrations occurred. In the condition-b (Fig. 6b), the trans-Himalayas transport of BC significantly

weakened due to the cyclone center nearby the Bay of Bengal region. As a result, BC particles were transported along the foothill of the Himalayas, resulting in less BC particles to cross over the Himalayas than condition-a. In the condition-c (Fig. 6c), the cyclone center moved towards to the middle of the IGP, and BC particles were still transported along the foothill of the Himalayas. The trans-Himalayas transport of BC occurred in the middle of the Himalayas. In the condition-d (Fig. 6d), the cyclone center moved further west of the IGP. As a result, the surface airflows from the Bay of Bengal moving towards the Himalayas strengthened. It was similar to the condition-a that high BC concentrations were transported from the IGP to the TP, resulting in high BC concentrations in southeastern TP. However, compared with the condition-a, there were some differences. Firstly, the northeastward trans-Himalayas transport of BC was weaker than the condition-a. For example, due to a strong northeastward transport, there was significantly a large amount of BC particles being transported in the TP and reaching to the further north location (95°E, 32°N) in the condition-a. In contrast, BC particles only reached to the location (95°E, 31°N) in the condition-d. Secondly, due to the dynamical perturbation of the western cyclone, there was significant northward transport in western Himalayas, generating a wider area of the trans-Himalayas transport of BC than in the condition-a. In summary, convergent airflows produced strong northeastward winds from the Bay of Bengal to the Himalayas, leading to a transHimalayas transport of BC particles during Indian summer monsoon. As a result, BC concentrations significantly increased in the southeastern TP, reaching to 0.6–0.8 μg m−3. When the cyclone located in eastern IGP,

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Fig. 7. Same as Fig. 6 except that the altitude of the Himalayas is reduced to a half. The closed dash circles represent that BC concentrations over the TP significantly increase after removing a half altitude of the Himalayas.

high BC concentrations over the IGP were transported along the foothill of the Himalayas, and the trans-Himalayas transport weakened. When the cyclone moved to the west, the dynamical perturbations for the trans-Himalayas transport of BC were weaker than the eastern cyclone, and the trans-Himalayas transport were enhanced in the middle and eastern Himalayas region. 4.4. Effect of the Himalayas on BC transport under different meteorological conditions This study aims to investigate the effect of the Himalayas on BC transport from the IGP to the TP. The above section showed that the meteorological conditions, such as the locations of the cyclone over the IGP, played important roles in the trans-Himalayas transport of BC. Fig. 7 shows the distributions of calculated BC concentrations under different meteorological conditions with the reduced altitude of the Himalayas. Fig. 8 shows the differences of BC concentrations before and after removing a half altitude of the Himalayas while Fig. 9 shows the change in percentage of BC concentrations. As shown in Figs 7 and 8, the trans-Himalayas transport was still significantly affected by the meteorological conditions as the altitude of the Himalayas reduced. In the condition-a, the reduction of the high-elevated Himalayas produced a large amount of BC particles penetrated into the TP, and BC concentrations increased by 0.2–0.6 μg m−3 in the southeast TP (Fig. 8a). In this case, the reduction of Himalayas played important roles in the trans-Himalayas transport, and BC concentrations reached to a high level (0.8– 2.0 μg m−3) in the southeast TP (shown in Fig. 7a). The change in

percentages of BC concentrations in Fig. 9a also illustrated BC concentrations over the southeastern TP significantly increased, even doubled after reducing a half altitude of the Himalayas. In the condition-b and c, northward and northeastward winds weakened, the reduction of Himalayas played insignificant roles in the penetration of BC particles into the TP. There were some indications for increases in BC concentrations along the Himalayas region. This was associated with the “a filling effect” shown in Fig. 4. The penetration of BC particles into the hinterland of the TP was blocked by the high-elevated TP itself. However, the change in percentages of BC concentrations was as significant as that in the condition-a, but with different spatial distributions (Figs 9b and c). This suggested that the trans-Himalayas transports of BC particles were quite different under these two meteorological conditions. Fig. 9b showed an obvious increase in the southeastern TP while Fig. 9c showed a little decrease in the southeastern TP after removing a half altitude of the Himalayas. In the condition-d, the northward and northeastward winds were stronger than the conditions b and c, leading to a stronger trans-Himalayas transport to the southeastern TP due to the reduction of Himalayas. As a result, BC concentrations increased by 0.2–1.0 μg m−3 in the middle and southeast of the TP (shown in Fig. 8d), and BC concentrations reached to a high level of 0.8– 2.0 μg m− 3 in these regions (shown in Fig. 7d). The corresponding change in percentage of BC concentrations showed a significantly doubled increase in BC concentration in a larger area, not confined in the southeastern TP (Fig. 9d). This also indicated that the enhanced northward winds were significantly in favor for the trans-Himalayas transport of BC after removing a half altitude of the Himalayas.

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Fig. 8. The difference of BC concentration under four typical surface weather patterns in Fig. 5, (a) on 17th July, (b) on 3rd July, (c) on 23th July, and (d) on 11th July 2012. The positive indicates higher BC concentrations when the Himalayas is reduced to a half altitude and the negative indicates lower BC concentrations when the Himalayas is reduced to a half altitude. The closed dash circles represent that increased BC concentrations over the TP after removing a half altitude of the Himalayas.

5. Conclusions The trans-Himalayas transport of BC particles from the IGP to the TP was studied. In order to determine the effect of Himalayas on BC transport, two sensitive experiments were designed by applying the WRFChem model to investigate topographic effects of the trans-Himalayas region on the transport pathways of BC emitted from South Asia to the TP. One numerical experiment was conducted with the real altitude of the Himalayas, and the other is exactly same as the first one except that the elevation of the Himalayas was reduced to a half of the real altitude. The main results of this study are illustrated as below. (1) During Indian summer monsoon, the highest meridional temperature gradient occurred from the Bay of Bengal towards to the Himalayas. As a result, the trans-Himalayas transport evidently occurred in the southeast of the Himalayas, producing 0.2– 0.6 μg m−3 BC concentrations in the southeastern TP. With the reduction of the altitude of the Himalayas, the trans-Himalayas transport of BC significantly enhanced, and the largest increase occurred on the top of the Himalayas where BC concentrations varied from 0.2–0.4 μg m−3 to 0.6–0.8 μg m−3. This result suggested that the reduction of the high-elevated Himalayas played a filling role in the penetration of BC particles over the Himalayas, and no more BC particles penetrated into the TP. The TP itself with a high elevation (a.m.s.l. 4 km) always acted as a wall to prevent BC transport from the IGP to the TP. (2) Cyclonic circulations in the IGP during Indian summer monsoon had important effects on the trans-Himalayas transport of BC.

In the convergent airflows condition, a strong northeastward wind from the Bay of Bengal to the Himalayas produced the trans-Himalayas transport of BC. As a result, BC concentrations significantly increased in the southeastern TP, and reached to 0.6–0.8 μg m−3. When the cyclone located in eastern IGP, high BC concentrations over the IGP transported along the foothill of the Himalayas, and the trans-Himalayas transport of BC significantly weakened. When the cyclone moved to the west, the dynamical perturbations for the trans-Himalayas transport of BC were weaker than the condition of the eastern cyclone, and the trans-Himalayas transport of BC was enhanced in the middle and eastern Himalayas. This study provides useful information for understanding the effect of the Himalayas on BC transport from South Asia to the TP. Moreover, it is helpful to estimate the contribution of BC emission from South Asia to BC concentrations in the TP and assess the impacts of BC particles from South Asia on regional climate change and ecological environment over the TP in the future. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 41430424 and 41375136. The Atmospheric Air Pollution Grant Funded by the Ministry of Science and Technology of China (2016YFC0203400). The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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Fig. 9. The percentage change of BC concentrations before and after removing a half altitude of the Himalayas under four typical surface weather patterns in Fig. 5, (a) on 17th July, (b) on 3rd July, (c) on 23th July, and (d) on 11th July 2012. The positive indicates BC concentrations increase when the Himalayas is reduced to a half altitude and the negative indicates BC concentrations decrease when the Himalayas is reduced to a half altitude. The closed dash circles represent that BC concentrations increased over the southeastern TP after removing a half altitude of the Himalayas while the closed solid circle represents that BC concentrations decreased over the southeastern TP after removing a half altitude of the Himalayas.

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