Seasonal Transport and Dry Deposition of Black Carbon Aerosol in the Southeastern Tibetan Plateau
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To investigate the regional transport and dry deposition of black carbon (BC) aerosol in the southeastern Tibetan Plateau, continuous equivalent BC (eBC) mass concentrations were measured at a high-altitude remote site of Lulang from July 2008 to July 2009. The bivariate polar plots for eBC mass concentrations showed that large eBC values were often associated with low winds (< 2 m s−1) during the pre-monsoon, post-monsoon, and winter seasons. Moreover, strong winds (> 4 m s−1) from southeast or northeast also contribute to the large eBC loadings during the pre-monsoon, monsoon, and post-monsoon seasons. The concentration-weighted trajectory analysis showed that emissions from the eastern Kingdom of Bhutan, Assam of India, and southern Shannan Prefecture of Tibet had the most important contributions to the eBC pollution at Lulang during the pre-monsoon and monsoon seasons. In contrast, the eBC potential source region shifted to the east and southeast of Lulang during the post-monsoon and to the north India and northwest Nepal during the winter. The estimated eBC deposition rate was the highest for the pre-monsoon (6.3–62.6 μg eBC m−2 day−1), followed by the post-monsoon (4.6–45.9 μg eBC m−2 day−1), winter (4.3–43.1 μg eBC m−2 day−1), and monsoon (2.4–24.5 μg eBC m−2 day−1). Further calculations of eBC concentrations in the snow surface were 33.3–333.2, 61.5–614.7, 27.0–269.9, and 58.8–587.6 μg kg−1 during the pre-monsoon, monsoon, post-monsoon, and winter seasons, respectively, which resulted in the snow albedos being reduced by 2.6–25.3, 4.7–46.6, 2.1–20.5, and 4.5–44.5% accordingly.
KeywordsBlack carbon Regional transport Dry deposition Tibetan Plateau
Black carbon (BC) aerosol is a major particulate light-absorbing substance in the atmosphere (Bond et al. 2013). It is a by-product produced during incomplete combustion processes, including natural and anthropogenic sources (e.g., biomass burning and fossil fuel combustion) (Petzold et al. 2013). BC has substantial effect on the global climate due to its strong absorption of solar radiation, and it has been considered as the second strongest warming forcing agent after carbon dioxide (CO2) (Jacobson 2001; Ramanathan and Carmichael 2008; Bond et al. 2013). BC can further act as cloud condensation nuclei or enhance the evaporation rates in cloud layers, indirectly causing changes in climate (Nenes et al. 2002; Wang 2004). In addition, BC greatly damages air quality (Cao et al. 2012; Wang et al. 2013; Ding et al. 2016), and poses threat to human health (Jansen et al. 2005; Grahame et al. 2014). Since BC has a shorter lifetime compared to other greenhouse gases (e.g., CO2), its reduction can potentially provide a fast solution to mitigate near-term global warming, as well as to improve air pollution, and be beneficial to human health and food security simultaneously (Quinn et al. 2008; Grieshop et al. 2009; Jacobson 2010; Shindell et al. 2012; Bond et al. 2013).
The Tibetan Plateau (TP), covering an area of ~ 2,500,000 km2, is known as the “Third Pole” of the Earth. It holds the largest volume of ice at the low and middle altitudes (Qiu 2008). Serving as the “Water Tower of Asia”, the TP provides critical freshwater source for several major rivers including the Indus, Ganges, Brahmaputra, Yangtze, and Yellow River. These rivers are the lifelines to billions of people living in the downstream (Immerzeel et al. 2010). However, observations and modeling studies show that snowpack and glaciers in the TP have been undergoing rapid melting and shrinking (William et al. 2010; Lee et al. 2013; Loibl et al. 2014; Jacobi et al. 2015; Li et al. 2016; Xu et al. 2016; Ke et al. 2017). Re-analysis of the TP Landsat images and satellite altimetry data show that the glacier area in this region experienced notable shrinkage at a rate of − 0.31 ± 0.04 km2 year−1 during 1976–2013 (Ke et al. 2017). Li et al. (2016) employed multiple remote sensing data to investigate the changes of glaciers in the Dupuchangdake region of northwestern Tibet, and found that the glacier area in this region decreased from 409 to 393 km2 from 1991 to 2013.
BC is an important contributor to the observed rapid glacier retreat (Xu et al. 2009a). Once deposited on snow and ice in the TP, BC can significantly decrease the surface albedo, causing an aggravated surface melting (Ming et al. 2009, 2013; Xu et al. 2009b; Nair et al. 2013; Ménégoz et al. 2014; Qu et al. 2014). Ming et al. (2013) calculated that BC in the winter snowpack reduced the albedo by 11% in the Nam Co region in central Tibet. Qu et al. (2014) reported that the albedo of the Zhadang glacier decreased at a rate of − 0.003 a−1 from 2001 to 2012, and the contribution of BC to albedo reduction was 36% when the glacier was covered by aged snow.
Although anthropogenic activities in the TP are sparse, this region is surrounded by a number of strong BC source areas, including India and China (Wang et al. 2014). Simulated by GEOS-Chem, Kopacz et al. (2011) found that emissions from north India and central China contributed most of BC to the Himalayas, while the TP received a large number of BC from the western and central China, as well as from India, Nepal, the Middle East, and Pakistan. Lu et al. (2012) developed a novel back-trajectory approach to study the origin of BC reaching the Himalayas and the TP, and found that BC received by the Himalayas and the TP increased by 41% from 1996 to 2010. On an annual basis, South Asia was the largest contributor to the Himalayas and TP, accounting for 67%, followed by East Asia (17%), the former USSR region (~ 8%), Middle East (~ 4%), Europe (~ 2%), and Northern Africa (~ 1%). Although the high altitude of Himalayas can act as a “physical wall” that prevents transport of pollutants from South Asia to the TP (Zhao et al. 2017), the valley of Yarlung Tsangpo River created a “leaking wall”, where BC aerosol can be transported up onto the glaciers (Cao et al. 2010). This trans-Himalayas transport of BC is mostly controlled by the meteorological conditions over the Indo-Gangetic Plain. For example, convergent airflows produce a strong northeastward wind from the Bay of Bengal to the Himalayas, resulting in high BC concentrations in the southeastern TP (Zhao et al. 2017).
Currently, studies about BC sources and its impact on snow albedo in the TP based on the seasonal perspective are very limited. In this study, we discussed the impact of regional transport on BC aerosol in the southeastern TP using bivariate polar plot graphical technique (Sect. 3.1) and the concentration-weighted trajectory (CWT) analysis (Sect. 3.2). The BC concentration in the snow surface was calculated based on the assumption of minimal deposition velocity of BC (Sect. 3.3), and its effect on the snow albedo reduction is also assessed in Sect. 3.3.
2 Experimental Methods
2.1 Research Site
2.2 Equivalent BC (eBC) Measurement
Five-minute averaged eBC mass concentrations were measured continuously with an aethalometer (Model AE-16, Magee Scientific Company, Berkeley, CA, USA). The term of eBC is specific to aethalometer measurement (Petzold et al. 2013). The detailed principle of aethalometer has been described previously (Hansen et al. 1984; Virkkula et al. 2007). Briefly, the instrument is based on the optical transmittance at a single wavelength of λ = 880 nm, and it measures the light attenuation (ATN) transmitted through a quartz-fiber filter. Sample air was drawn into the aethalometer at a flow rate of 4 L min−1 using a ~ 2 m length of conductive silicone tubing with a total suspended particle inlet. Before the atmospheric particles entered the instrument, they were dried with a silica gel dyer to avoid any effects of water condensation in the sampling line.
3 Results and Discussion
3.1 Transport Direction
3.2 Identification of eBC Source Regions
3.3 Estimated Impact of eBC Deposition
We further estimated the eBC concentration in surface snow. Since the snow impurities are derived from dry deposition of atmospheric aerosols, the top layer of snow surface contains more impurities than the deeper part of the snow layer (Aoki et al. 2000, 2007). In this study, the concentration of eBC in the top layer of snow (e.g., 2-cm thickness) was estimated based on the assumption of uniform distribution for eBC in the top pure snow. Because eBC concentration is associated with snow water content, and snow density data in Lulang area are not available, the average snow density of 300 kg m−3 at the Parlong No. 4 Glacier was used (Yang et al. 2011). The glacier, located in the upper Yarlung Zangbu River Basin of southeast TP, has an area of ~ 11.7 km2 and a length of nearly 8 km. If the total eBC of 1083.2–10,832.1 μg eBC m−2 during the entire campaign was deposited on 2-cm thickness of pure snow, the eBC concentration in the snow surface would be 180.5–1805.4 μg kg−1. The seasonal variations for eBC concentrations in the snow surface showed that the highest value was obtained during the monsoon (61.5–614.7 μg kg−1), followed by the winter (58.8–587.6 μg kg−1) and pre-monsoon (33.3–333.2 μg kg−1), with the minimum value for post-monsoon (27.0–269.9 μg kg−1). Zhang et al. (2017) collected surface snow and snowpit samples from different glaciers in the southeastern TP in June 2015, and found that the eBC concentrations were 97.3, 318, and 125 μg kg−1 in the snow point samples collected from Dongga, Renlongba, and Demula glaciers, respectively. The amounts of eBC in the fresh snow and snowpits were generally lower than those in the aged snow and surface granular ice. Although direct comparison is not suitable for various studies due to their different analytical methodologies, sampling dates, and snow conditions, the high eBC concentrations in the southeastern glaciers indicate that the polluted air masses transported from South Asia may have a significant impact on these glacier melts.
Surface eBC concentrations were measured with an aethalometer at Lulang, a high-altitude remote site in the southeastern TP, from July 2008 to July 2009. The bivariate polar plots for eBC mass concentrations showed obviously seasonal patterns. For the pre-monsoon season, the large eBC values (> 0.65 µg m−3) were mainly associated with the low wind speed (< 2 m s−1) or with the high wind speed (> 4 m s−1) from southeast, indicating the effects of local accumulation and regional transport. For the monsoon season, high eBC values (> 0.4 µg m−3) occasionally occurred at wind speed > 4 m s−1 from the southeast. For the post-monsoon season, large eBC loadings (> 0.6 µg m−3) were measured at low wind speed < 2 m s−1 or high wind speed > 6 m s−1 from the northeast or southeast. For the winter season, the high eBC values (> 0.6 µg m−3) were concentrated at wind speed < 2 m s−1, indicating that eBC pollution may be mainly caused by local emissions. The CWT analysis together with the distributions of the BC column mass density showed that sources located to the southwest of Lulang were the most important for eBC pollution during the pre-monsoon and monsoon. However, the possible source region for eBC pollution during the post-monsoon shifted to the east and southeast of Lulang, indicating the importance of biofuel burning emissions from the interior of the TP. During the winter, the largest potential source region for eBC pollution was located in north India and northwest Nepal.
The estimated total eBC deposition amount was 1083.2–10,832.1 μg eBC m−2, which corresponds to 3.6–36.1 μg eBC m−2 day−1. The eBC deposition rate showed clear seasonal variations, with a maximum of 6.3–62.6 μg eBC m−2 day−1 for the pre-monsoon, followed by the post-monsoon (4.6–45.9 μg eBC m−2 day−1) and winter (4.3–43.1 μg eBC m−2 day−1), and a minimum during the monsoon (2.4–24.5 μg eBC m−2 day−1). The average eBC concentration on 2-cm thickness of pure snow was estimated to be highest for the monsoon (61.5–614.7 μg kg−1), followed by the winter (58.8–587.6 μg kg−1) and pre-monsoon (33.3–333.2 μg kg−1), with the minimum value for the post-monsoon (27.0–269.9 μg kg−1). Based on these eBC concentrations in the top layer of snow, the albedos reduced by 2.6–25.3, 4.7–46.6, 2.1–20.5, and 4.5–44.5% for the pre-monsoon, monsoon, post-monsoon, and winter, respectively.
This work was supported by the National Natural Science Foundation of China (41230641, 41503118, and 41661144020). The authors are grateful to the Integrated Observation and Research Station for Alpine Environment in South-East Tibet, Chinese Academy of Sciences, for their assistance with field sampling.
Compliance with Ethical Standards
Conflict of interest
On behalf of all the authors, the corresponding author states that there is no conflict of interest.
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