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Desert dust as a significant carrier of atmospheric mercury *
Abstract
The atmospheric circulation plays a critical role in the global transport and deposition of atmospheric pollutants such as mercury (Hg). Desert dust emissions contribute to nearly 60e95% of the global dust budget and thus, desert dust may facilitate atmospheric Hg transport and deposition to the downwind regions worldwide. The role of desert dust in biogeochemical cycling of Hg, however, has not been well recognized by the Hg research community. In this study, we measured the concentration of particulate bound Hg (Hg P) in total suspended particulate (TSP) collected from China's largest desert, Taklimakan Desert, between 2013 and 2017. The results show that Hg P concentrations over the Taklimakan Desert atmosphere are remarkably higher than those observed from background sites in China and are even comparable to those measured in most of the Chinese metropolitan cities. Moreover, Hg P concentrations in the Taklimakan Desert exhibit a distinct seasonal pattern peaking during dust storm outbreak periods in spring and summer (March to August). A preliminary estimation demonstrates that export of total Hg associated with atmospheric dust from the Taklimakan Desert could be 59.7 ± 60.3 (1SD) Mg yr À1. The unexpectedly high Hg P concentrations during duststorms, together with consistent seasonal pattern of Hg revealed from the snow/ice, clearly demonstrate that Asian desert dust could act as a significant carrier of atmospheric Hg to the cryosphere of Western China and even can have further global reach.
Lumbini is a world heritage site located in the southern plains region of Nepal, and is regarded as a potential site for evaluating transboundary air pollution due to its proximity to the border with India. In this study, 82 aerosol samples were collected between April 2013 and July 2014 to investigate the levels of particulate-bound mercury (PBM) and the corresponding seasonality, sources, and influencing factors. The PBM concentration in total suspended particulate (TSP) matter ranged from 6.8 pg m–3 to 351.7 pg m–3 (mean of 99.7 ± 92.6 pg m–3), which exceeded the ranges reported for remote and rural sites worldwide. The Hg content (PBM/TSP) ranged from 68.2 ng g–1 to 1744.8 ng g–1 (mean of 446.9 ± 312.7 ng g–1), indicating anthropogenic enrichment. The PBM levels were higher in the dry season (i.e., winter and the pre-monsoon period) than in the wet season (i.e., the monsoon period). In addition, the δ²⁰²Hg signature indicated that waste/coal burning and traffic were the major sources of Hg in Lumbini during the pre-monsoon period. Meanwhile, precipitation occurring during photochemical processes in the atmosphere may have been responsible for the observed Δ¹⁹⁹Hg values in the aerosol samples obtained during the monsoon period. The PBM concentration was influenced mostly by the resuspension of polluted dust during dry periods and crop residue burning during the post-monsoon period. The estimated PBM deposition flux at Lumbini was 15.7 μg m–2 yr–1. This study provides a reference dataset of atmospheric PBM over a year, which can be useful for understanding the geochemical cycling of Hg in this region of limited data.
The Tibetan Plateau and its surroundings are known as the Third Pole (TP). This region is noted for its high rates of glacier melt and the associated hydrological shifts that affect water supplies in Asia. Atmospheric pollutants contribute to climatic and cryospheric changes through their effects on solar radiation and the albedos of snow and ice surfaces; moreover, the behavior and fates within the cryosphere and environmental impacts of environmental pollutants are topics of increasing concern. In this review, we introduce a coordinated monitoring and research framework and network to link atmospheric pollution and cryospheric changes (APCC) within the TP region. We then provide an up-to-date summary of progress and achievements related to the APCC research framework, including aspects of atmospheric pollution's composition and concentration, spatial and temporal variations, trans-boundary transport pathways and mechanisms, and effects on the warming of atmosphere and changing in Indian monsoon, as well as melting of glacier and snow cover. We highlight that exogenous air pollutants can enter into the TP's environments and cause great impacts on regional climatic and environmental changes. At last, we propose future research priorities and map out an extended program at the global scale. The ongoing monitoring activities and research facilitate comprehensive studies of atmosphere-cryosphere interactions, represent one of China's key research expeditions to the TP and the polar regions and contribute to the global perspective of earth system science.
Total gaseous mercury (TGM) concentrations were continuously measured at Nam
Co Station, a remote high-altitude site (4730 m a.s.l.), on the inland
Tibetan Plateau, China, from January 2012 to October 2014 using a Tekran
2537B instrument. The mean concentration of TGM during the entire monitoring
period was 1.33±0.24 ng m−3 (mean ± standard deviation),
ranking it as the lowest value among all continuous TGM measurements reported
in China; it was also lower than most of sites in the Northern Hemisphere.
This indicated the pristine atmospheric environment on the inland Tibetan
Plateau. Long-term TGM at the Nam Co Station exhibited a slight decrease
especially for summer seasons. The seasonal variation of TGM was
characterized by higher concentrations during warm seasons and lower
concentrations during cold seasons, decreasing in the following order: summer
(1.50±0.20 ng m−3) > spring
(1.28±0.20 ng m−3) > autumn
(1.22±0.17 ng m−3) > winter (1.14±0.18 ng m−3).
Diurnal variations of TGM exhibited uniform patterns in different seasons:
the daily maximum was reached in the morning (around 2–4 h after sunrise),
followed by a decrease until sunset and a subsequent buildup at night,
especially in the summer and the spring. Regional surface reemission and
vertical mixing were two major contributors to the temporal variations of TGM
while long-range transported atmospheric mercury promoted elevated TGM during
warm seasons. Results of multiple linear regression (MLR) revealed that
humidity and temperature were the principal covariates of TGM. Potential
source contribution function (PSCF) and FLEXible PARTicle dispersion model
(WRF-FLEXPART) results indicated that the likely high potential source
regions of TGM to Nam Co were central and eastern areas of the Indo-Gangetic
Plain (IGP) during the measurement period with high biomass burning and
anthropogenic emissions. The seasonality of TGM at Nam Co was in phase with
the Indian monsoon index, implying the Indian summer monsoon as an important
driver for the transboundary transport of air pollution onto the inland
Tibetan Plateau. Our results provided an atmospheric mercury baseline on the
remote inland Tibetan Plateau and serve as new constraint for the assessment
of Asian mercury emission and pollution.
Metal dissolution from atmospheric aerosol deposition to the oceans is important in enhancing and inhibiting phytoplankton growth rates and modifying plankton community structure, thus impacting marine biogeochemistry. Here we review the current state of knowledge on the causes and effects of the leaching of multiple trace metals from natural and anthropogenic aerosols. Aerosol deposition is considered both on short timescales over which phytoplankton respond directly to aerosol metal inputs, as well as longer timescales over which biogeochemical cycles are affected by aerosols.
A severe dust storm event originated from the Gobi Desert in Central and East Asia during 2–7 May 2017. Based on Moderate Resolution Imaging Spectroradiometer (MODIS) satellite products, hourly environmental monitoring measurements from Chinese cities and East Asian meteorological observation stations, and numerical simulations, we analysed the spatial and temporal characteristics of this dust event as well as its associated impact on the Asia-Pacific region. The maximum observed hourly PM10 (particulate matter with an aerodynamic diameter ≤ 10 µm) concentration was above 1000 µg m⁻³ in Beijing, Tianjin, Shijiazhuang, Baoding, and Langfang and above 2000 µg m⁻³ in Erdos, Hohhot, Baotou, and Alxa in northern China. This dust event affected over 8.35 million km², or 87 % of the Chinese mainland, and significantly deteriorated air quality in 316 cities of the 367 cities examined across China. The maximum surface wind speed during the dust storm was 23–24 m s⁻¹ in the Mongolian Gobi Desert and 20–22 m s⁻¹ in central Inner Mongolia, indicating the potential source regions of this dust event. Lidar-derived vertical dust profiles in Beijing, Seoul, and Tokyo indicated dust aerosols were uplifted to an altitude of 1.5–3.5 km, whereas simulations by the Weather Research and Forecasting with Chemistry (WRF-Chem) model indicated 20.4 and 5.3 Tg of aeolian dust being deposited respectively across continental Asia and the North Pacific Ocean. According to forward trajectory analysis by the FLEXible PARTicle dispersion (FLEXPART) model, the East Asian dust plume moved across the North Pacific within a week. Dust concentrations decreased from the East Asian continent across the Pacific Ocean from a magnitude of 10³ to 10⁻⁵ µg m⁻³, while dust deposition intensity ranged from 10⁴ to 10⁻¹ mg m⁻². This dust event was unusual due to its impact on continental China, the Korean Peninsula, Japan, and the North Pacific Ocean. Asian dust storms such as those observed in early May 2017 may lead to wider climate forcing on a global scale.
The Taklimakan Desert (TD) and Gobi Desert (GD) are two of the most important dust sources in East Asia, and have important impact on energy budgets, ecosystems and water cycles at regional and even global scales. To investigate the contribution of the TD and the GD to dust concentrations in East Asia as a whole, dust emissions, transport, and deposition over the TD and the GD in different seasons from 2007 to 2011 were systematically compared, based on the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Dust emissions, uplift, and long-range transport related to these two dust source regions were markedly different due to differences in topography, elevation, thermal conditions, and atmospheric circulation. Specifically, the topography of the GD is relatively flat, and at a high elevation, and the area is under the influence of two jet streams at high altitudes, resulting in high wind speeds in the upper atmosphere. Deep convective mixing enables the descending branch of jet streams to continuously transport momentum downward to the mid-troposphere, leading to enhanced wind speeds in the lower troposphere over the GD which favors the vertical uplift of the GD dust particles. Therefore, the GD dust was very likely to be transported under the effect of strong westerly jets, and thus played the most important role in contributing to dust concentrations in East Asia. Approximately 35% and 31% of dust emitted from the GD transported to remote areas in East Asia in spring and summer, respectively. The TD has the highest dust emission capabilities in East Asia, with emissions of about 70.54 Tg yr⁻¹ in spring, accounting for 42% of the total dust emissions in East Asia. However, the TD is located in the Tarim Basin and surrounded by mountains on three sides. Furthermore, the dominant surface wind direction is eastward and the average wind speed at high altitudes is relatively small over the TD. As a result, the TD dust particles are not easily transported outside the Tarim Basin, such that most of the dust particles are re-deposited after uplift, at a total deposition rate of about 40 g m⁻². It is only when the TD dust particles are uplifted above 4 km, and entrained in westerlies that they begin to undergo a long-range transport. Therefore, the contribution of the TD dust to East Asian dust concentrations was relatively small. Only 25% and 23% of the TD dust was transported to remote areas over East Asia in spring and summer, respectively.
Long-term monitoring of global pollutant such as Mercury (Hg) in the cryosphere is very essential for understanding its bio-geochemical cycling and impacts in the pristine environment with limited emission sources. Therefore, from May 2015 to Oct 2015, surface snow and snow-pits from Xiao Dongkemadi Glacier and glacier melt water were sampled along an elevation transect from 5410 to 5678m a.s.l. in the central Tibetan Plateau (TP). The concentration of Hg in surface snow was observed to be higher than that from other parts of the TP. Unlike the southern parts of the TP, no clear altitudinal variation was observed in the central TP. The peak Total Hg (HgT) concentration over the vertical profile on the snow pits corresponded with a distinct yellowish-brown dust layer supporting the fact that most of the Hg was associated with particulate matter. It was observed that only 34% of Hg in snow was lost when the surface snow was exposed to sunlight indicating that the surface snow is less influenced by the post-depositional process. Significant diurnal variation of HgT concentration was observed in the river water, with highest concentration observed at 7pm when the discharge was highest and lowest concentration during 7-8am when the discharge was lowest. Such results suggest that the rate of discharge was influential in the concentration of HgT in the glacier fed rivers of the TP. The estimated export of HgT from Dongkemadi river basin is 747.43gyr(-1), which is quite high compared to other glaciers in the TP. Therefore, the export of global contaminant Hg might play enhanced role in the Alpine regions as these glaciers are retreating at an alarming rate under global warming which may have adverse impact on the ecosystem and the human health of the region.
Eolian dust transport and deposition are important geophysical processes which influence global biogeochemical cycles. Currently, reliable deposition data are scarce in central and east Asia. Located at the boundary of central and east Asia, Xinjiang Province of northwestern China has long played a strategic role in cultural and economic trade between Asia and Europe. In this paper, we investigated the spatial distribution and temporal variation in dust deposition and ambient PM10 (particulate matter in aerodynamic diameter �10 μm) concentration from 2000 to 2013 in Xinjiang Province. This variation was assessed using environmental monitoring records from 14 stations in the province. Over the 14 years, annual average dust deposition
across stations in the province ranged from 255.7 to 421.4 t km2. Annual dust deposition was greater in southern Xinjiang (663.6 t km2/ than northern (147.8 t km2/ and eastern Xinjiang (194.9 t km2/. Annual average PM10 concentration across stations in the province varied from 100 to 196 μgm3 and was 70, 115 and 239 μgm3 in northern, eastern and southern Xinjiang, respectively. The highest annual dust deposition (1394.1 t km2/ and ambient PM10 concentration (352 μgm3/ were observed in Hotan, which is located in southern Xinjiang and at the southern boundary of the Taklamakan Desert. Dust deposition was more intense
during the spring and summer than other seasons. PM10 was the main air pollutant that significantly influenced regional air quality. Annual average dust deposition increased logarithmically with ambient PM10 concentration (R2 �0.81). While the annual average dust storm frequency remained unchanged from 2000 to 2013, there was a positive relationship between dust storm days and dust deposition and PM10 concentration across stations. This study suggests that sand storms are a major factor affecting the temporal variability
and spatial distribution of dust deposition in northwest China.
This paper provides an up-to-date assessment of global mercury emissions from anthropogenic and natural sources. On an annual basis, natural sources account for 5207 Mg of mercury released to the global atmosphere, including the contribution from re-emission processes, which are emissions of previously deposited mercury originating from anthropogenic and natural sources, and primary emissions from natural reservoirs. Anthropogenic sources, which include a large number of industrial point sources, are estimated to account for 2320 Mg of mercury emitted annually. The major contributions are from fossil-fuel fired power plants (810 Mg yr<sup>−1</sup>), artisanal small scale gold mining (400 Mg yr<sup>−1</sup>), non-ferrous metals manufacturing (310 Mg yr<sup>−1</sup>), cement production (236 Mg yr<sup>−1</sup>), waste disposal (187 Mg yr<sup>−1</sup>) and caustic soda production (163 Mg yr<sup>−1</sup>). Therefore, our current estimate of global mercury emissions suggests that the overall contribution from natural sources (primary emissions + re-emissions) and anthropogenic sources is nearly 7527 Mg per year, the uncertainty associated with these estimates are related to the typology of emission sources and source regions.
Total suspended particulate matter (TSP) and dust fall concentrations in the ambient air of Kashgar, Xinjiang, were measured in a field campaign. The results revealed that the high levels of TSP and dust fall among literature were reported in data. TSP and dust fall showed the similar monthly variations. Monthly TSP level in air ranged between 304.0 and 2030.6 μg/m3, and dust fall loadings varied from 8.6 to 248.1 g/m2 during the field campaign. Wind speed and direction were identified to be the most important meteorological factor affecting the concentrations of TSP. Central Asia and the Tarim Basin were identified to be the two major source regions from where TSP and dust were transported to Kashgar by both westerly and easterly winds. The dominant anion in TSP was SO4
2−, followed by Cl−, NO3
−, and F−. The dominant cation in sampled TSP was Ca2+, followed by Na+, K+, NH4+, and Mg2+. The principle component analysis showed that the first principal component (PC1) of the ion in TSP samples from the Kashgar region accounted for 44.81 % of total data variance, and the second principal component (PC2) accounted for 18.65 %. Mg2+, Na+, K+, Ca2+, NO3
−, SO4
2−, and Cl− have greater loadings in the PC1, and Cl− and NH4
+ have greater loadings in the PC2. This study suggests that desert dust, earth crust, soil, and anthropogenic activities such as fossil fuel combustion were the main sources of TSP in the Kashgar area.
The second Chinese glacier inventory was compiled based on 218 Landsat TM/ETM+ scenes acquired mainly during 2006–10. The widely used band ratio segmentation method was applied as the first step in delineating glacier outlines, and then intensive manual improvements were performed. The Shuttle Radar Topography Mission digital elevation model was used to derive altitudinal attributes of glaciers. The boundaries of some glaciers measured by real-time kinematic differential GPS or digitized from high-resolution images were used as references to validate the accuracy of the methods used to delineate glaciers, which resulted in positioning errors of �10m for manually improved clean-ice outlines and �30m for manually digitized outlines of debris-covered parts. The glacier area error of the compiled inventory, evaluated using these two positioning accuracies, was �3.2%. The compiled parts of the new inventory have a total area of 43 087km2, in which 1723 glaciers were covered by debris, with a total debris-covered area of 1494km2. The area of uncompiled glaciers from the digitized first Chinese glacier inventory is ~8753km2, mainly distributed in the southeastern Tibetan Plateau, where no images of acceptable quality for glacier outline delineation can be found during 2006–10.
This study reports the concentrations and potential sources of speciated atmospheric mercury at the Shangri-La Atmosphere Watch Regional Station (SAWRS), a pristine high-altitude site (3580ma.s.l.) in Tibetan Plateau, China. Total gaseous mercury (TGM, defined as the sum of gaseous elemental mercury, GEM, and gaseous oxidized mercury, GOM), GOM and particulate-bound mercury (PBM) were monitored from November 2009 to November 2010 to investigate the characteristics and potential influence of the Indian summer monsoon (ISM) and the Westerlies on atmospheric transport of mercury. The mean concentrations (+/- standard deviation) of TGM, PBM and GOM were 2.55 +/- 0.73 ngm(-3), 38.82 +/- 31.26 pgm(-3) and 8.22 +/- 7.90 pgm(-3), respectively. A notable seasonal pattern of TGM concentrations was observed with higher concentrations at the beginning and the end of the ISM season. High TGM concentrations (> 2.5 ngm(-3)/were associated with the transport of dry air that carried regional anthropogenic emissions from both Chinese domestic and foreign (e. g., Myanmar, Bay of Bengal, and northern India) sources based on analysis of HYSPLIT4 back trajectories. Somewhat lower PBM and GOM levels during the ISM period were attributed to the enhanced wet scavenging. The high GOM and PBM were likely caused by local photo-chemical transformation under low RH and the domestic biofuel burning in cold seasons.
In global environmental cooperation, legally binding agreements remain a customary way for states to set common goals and standards. This article analyzes the Minamata Convention on Mercury by addressing three questions: First, how did linkages to earlier agreements shape the negotiations? Second, what were the main legal and political issues during the negotiations? Third, what are the major issues moving forward with treaty implementation and mercury abatement? The analysis shows that the decision to start treaty negotiations was influenced by related policy developments on hazardous chemicals as well as differences in national interests. Five sets of issues dominated the negotiations: 1) supply and trade, 2) products and processes, 3) emissions and releases, 4) artisanal and small-scale gold mining, and 5) resources and compliance. The article concludes that future mercury abatement hinges on the parties’ ability to move beyond the initial mandates, as the convention may affect decisions by a wide range of public, private, and civil society actors.
Detailed 3-D structures of Trans-Pacific Asian dust transport occurring during 5–15 May 2007 were in-vestigated using the NASA/CALIOP vertical-resolved mea-surements and a three-dimensional aerosol model (SPRINT-ARS). Both CALIOP and SPRINTARS dust extinctions showed a good agreement along the way of the transport from the dust source regions across North Pacific into North America. A vertically two-layered dust distribution was ob-served over the northeastern Pacific and North America. The lower dust layer originated from a dust storm generated in the Gobi Desert on 5 May. It was transported at an altitude of around 4 km MSL and has mixed with Asian anthropogenic air pollutants during the course of transport. The upper dust layer mainly originated from a dust storm that occurred in the Taklimakan Desert 2–3 days after the Gobi dust storm gen-eration. The upper dust cloud was transported in higher alti-tudes above the major clouds layer during the Trans-Pacific transport. It therefore has remained unmixed with the Asian air pollutants and almost unaffected by wet removal. The decay of its concentration level was small (only one-half after its long-distance transport crossing the Pacific). Our dust budget analysis revealed that the Asian dust flux pass-ing through the longitude plane of 140 • E was 2.1 Tg, and one third of that arrived North America. The cases analyzed in this study revealed that, while the Gobi Desert is an impor-tant source that can contribute to the long-range dust trans-port, the Taklimakan Desert appears to be another important source that can contribute to the dust transport occurring par-ticularly at high altitudes.
The global cycle of desert dust aerosols responds strongly to climate and human perturbations, and, in turn, impacts climate and biogeochemistry. Here we focus on desert dust size distributions, how these are characterized, emitted from the surface, evolve in the atmosphere, and impact climate and biogeochemistry. Observations, theory and global model results are synthesized to highlight the evolution and impact of dust sizes. Individual particles sizes are, to a large extent, set by the soil properties and the mobilization process. The lifetime of different particle sizes controls the evolution of the size distribution as the particles move downwind, as larger particles fall out more quickly. The dust size distribution strongly controls the radiative impact of the aerosols, as well as their interactions with clouds. The size of particles controls how far downwind they travel, and thus their ability to impact biogeochemistry downwind of the source region.
A size-segregated aerosol model that includes most of the major physical processes (generation, transport, and dry and wet deposition) is developed. This model is coupled with a Regional Air Quality Model (RAQM) and is applied to simulate Asian dust storms during the 10-day period of 15–24 March 2002. A nonhydrostatic mesoscale model (MM5) is used to provide meteorological fields. Model results are verified by available observational data including surface weather observations and size-segregated particle concentrations. The validation demonstrates a good capability of this model system in capturing most of the key features of dust evolution and reproducing the particle mass size distribution along the transport pathway of soil dust. An apparent feature has been both observed and reproduced by the model, showing a shift of size range with peak mass concentration from coarse mode to finer mode on the pathway from source regions to distant downwind areas. The maximum dust concentration averaged over 10 days is simulated to be 3000 μg m−3 over the southern China-Mongolia border. Total dry deposition of soil dust for 10 days is up to 30 g m−2 in the Gobi desert along the southern China-Mongolia border. Distribution and magnitude of particle deposition are strongly dependent on both concentration and size-segregated dry deposition velocity and scavenging rate. While dry deposition dominates the removal of dust particles in or in the vicinity of source regions, the influence of wet deposition increases along the transport pathway of soil dust, with high removal efficiency for coarser particles (>2 μm) and very low efficiency for particles in the accumulation mode. Of the total dust emission (43.2 megatons), about 71% is redeposited onto the underlying surface by the dry deposition process, 6% is removed by the wet deposition process, and the remaining 23% is suspended in the atmosphere or subject to long-range transport.
Compilation and analysis of the past 40 years of dust storm reports from China allow estimation of the meteorological conditions of dust storms, dust transport routes, and eolian source regions. Our results indicate that dust storms in China are highly associated with the frontal systems and the Mongolian cyclonic depression. The spatial distribution of dust storms indicates that there are two dominant source regions of eolian dust raised from China and its surrounding regions. The major source is the gobi deserts in Mongolia and northern China. Another source region is the Taklimakan Desert in western China. However, dust entrained from the two sources makes different contributions to downwind deposition regions. In most cases, dust materials entrained from the gobi deserts of Mongolia and China can only be entrained to an elevation of 5000 m and then transported over long distances (~5000 km) by the westerlies. These materials are not the main sources of the dust deposited in the proximal region, such as the Chinese Loess Plateau, but they are important sources of the eolian fraction of pelagic sediment in the remote North Pacific Ocean.
Knowledge of long-range transport and vertical distribution of Asian dust aerosols in the free troposphere is important for estimating their impact on climate. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), surface micropulse lidar (MPL), and standard surface measurements are used to directly observe the long-range transport and vertical distribution of Asian dust aerosols in the free troposphere during the Pacific Dust Experiment (PACDEX). The MPL measurements were made at the Loess Plateau (35.95°N, 104.1°E) near the major dust source regions of the Taklamakan and Gobi deserts. Dust events are more frequent in the Taklamakan, where floating dust dominates, while more intensive, less frequent dust storms are more common in the Gobi region. The vertical distribution of the CALIPSO backscattering/depolarization ratios indicate that nonspherically shaped dust aerosols floated from near the ground to an altitude of approximately 9 km around the source regions. This suggests the possible long-range transport of entrained dust aerosols via upper tropospheric westerly jets. A very distinct large depolarization layer was also identified between 8 and 10 km over eastern China and the western Pacific Ocean corresponding to dust aerosols transported from the Taklamakan and Gobi areas, as confirmed by back trajectory analyses. The combination of these dust sources results in a two-layer or multilayered dust structure over eastern China and the western Pacific Ocean.
Dust aerosol particles produced by wind erosion in arid and semi arid regions affect climate and air quality, but the magnitude of these effects is largely unquantified. The major dust source regions include the Sahara, the Arabian and Asian deserts; global annual dust emissions are currently estimated to range between 1000 and 3000 Mt/yr. Dust aerosol can be transported over long distances of thousands of kilometers, e.g. from source regions in the Saharan desert over the North Atlantic, or from the Asian deserts towards the Pacific Ocean. The atmospheric dust load varies considerably on different timescales. While dust aerosol distribution and dust effects are important on global scales, they strongly depend on dust emissions that are controlled on small spatial and temporal scales.
The Tibetan Plateau (TP), with an average elevation of over 4000 m asl and an area of approximately 2.5 × 106 km2, is the highest and most extensive highland in the world and has been called the 'Third Pole'. The TP exerts a huge influence on regional and global climate through thermal and mechanical forcing mechanisms. Because the TP has the largest cryospheric extent outside the polar region and is the source region of all the large rivers in Asia, it is widely recognized to be the driving force for both regional environmental change and amplification of environmental changes on a global scale. Within China it is recognized as the 'Asian water tower'. In this letter, we summarize the recent changes observed in climate elements and cryospheric indicators on the plateau before discussing current unresolved issues concerning climate change in the TP, including the temporal and spatial components of this change, and the consistency of change as represented by different data sources. Based on meteorological station data, reanalyses and remote sensing, the TP has shown significant warming during the last decades and will continue to warm in the future. While the warming is predominantly caused by increased greenhouse gas emissions, changes in cloud amount, snow-albedo feedback, the Asian brown clouds and land use changes also partly contribute. The cryosphere in the TP is undergoing rapid change, including glacier retreat, inconsistent snow cover change, increasing permafrost temperatures and degradation, and thickening of the active layer. Hydrological processes impacted by glacial retreat have received much attention in recent years. Future attention should be paid to additional perspectives on climate change in the TP, such as the variations of climate extremes, the reliability of reanalyses and more detailed comparisons of reanalyses with surface observations. Spatial issues include the identification of whether an elevational dependency and weekend effect exist, and the identification of spatial contrasts in temperature change, along with their causes. These issues are uncertain because of a lack of reliable data above 5000 m asl.
Mineral dust is usually transported long distances in the lower troposphere. There are examples of Asian dust being transported across the Pacific Ocean1–7, and traces of Asian dust have also been found in ice and snow cores in Greenland8 and the French Alps9. Here, we use measurements from the Cloud–Aerosol Lidar with Orthogonal Polarization10, an air parcel trajectory model and a three-dimensional aerosol transport model to map the transport of dust clouds generated during a storm in China’s Taklimakan Desert during May 2007. We show that the dust-veiled clouds were lofted to the upper troposphere around 8–10km above the Earth’s surface and transported more than one full circuit around the globe in about 13 days. When the dust reached the northwestern Pacific Ocean for the second time, the subsidence of a large-scale high-pressure system caused it to descend into the lower troposphere; some of the dust was then deposited over the ocean. Our analysis also indicates that the dust particles may have acted as ice nuclei in these high-altitude clouds, leading to the formation of cirrus clouds. We suggest that Asian dust can influence the global radiation budget by stimulating cirrus cloud formation and marine ecosystems by supplying nutrients to the open ocean.
An international team of scientists from government agencies and universities in the United States, U.S. Virgin Islands (USVI), Trinidad & Tobago, the Republic of Cape Verde, and the Republic of Mali (West Africa) is working together to elucidate the role Saharan dust may play in the degradation of Caribbean ecosys- tems. The first step has been to identify and quantify the persistent organic pollutants (POPs), trace metals, and viable microorganisms in the atmosphere in dust source areas of West Africa, and in dust episodes at downwind sites in the eastern Atlantic (Cape Verde) and the Caribbean (USVI and Trinidad & Tobago). Preliminary find- ings show that air samples from Mali contain a greater number of pesticides, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) and in higher concentrations than the Caribbean sites. Overall, POP concentrations were similar in USVI and Trinidad samples. Trace metal concentrations were found to be similar to crustal composition with slight enrichment of lead in Mali. To date, hundreds of cultureable micro- organisms have been identified from Mali, Cape Verde, USVI, and Trinidad air samples. The sea fan pathogen, Aspergillus sydowii, has been identified in soil from Mali and in air samples from dust events in the Caribbean. We have shown that air samples from a dust-source region contain orders of magnitude more cultureable micro- organisms per volume than air samples from dust events in the Caribbean, which in turn contain 3-to 4-fold more cultureable microbes than during non-dust conditions. Rev. Biol. Trop. 54 (Suppl. 3): 9-21. Epub 2007 Jan. 15.
1] Using an elemental signature for Asian dust derived from events in April 1998, we probed a long-term set of routine aerosol samples to develop the first empirical assessment of the frequency and intensity of dust transport from Asia to midlatitude North America. Our data reveal a pattern of consistent, frequent transport that contradicts the episodic characterization derived from short-term studies and anecdotal reports. We find that fine (<2.5 mm) Asian dust is a regular component of the troposphere over the eastern Pacific and western North America and is common, at least in spring, across North America. Typical Asian fine dust concentrations (24-hour average) are between 0.2 and 1 mg/m 3 and only very rarely exceed 5 mg/m 3 . Our data also indicate that Asian dust is concentrated in an altitude zone ranging from about 500 to 3000 m MSL, consistent with isentropic transport processes previously observed in the western Pacific.
Dust storms originate in many of the world’s drylands and frequently present hazards to human society, both within the drylands themselves but also outside drylands due to long-range transport of aeolian sediments. Major sources of desert dust include the Sahara, the Middle East, central and eastern Asia, and parts of Australia, but dust-raising occurs all across the global drylands and, on occasion, beyond. Dust storms occur throughout the year and they vary in frequency and intensity over a number of timescales. Long-range transport of desert dust typically takes place along seasonal transport paths. Desert dust hazards are here reviewed according to the three phases of the wind erosion system: where dust is entrained, during the transport phase, and on deposition. This paper presents a synthesis of these hazards. It draws on empirical examples in physical geography, medical geology and geomorphology to discuss case studies from all over the world and in various fields. These include accelerated soil erosion in agricultural zones – where dust storms represent a severe form of accelerated soil erosion – the health effects of air pollution caused by desert aerosols via their physical, chemical and biological properties, transport accidents caused by poor visibility during desert dust events, and impacts on electricity generation and distribution. Given the importance of desert dust as a hazard to human societies, it is surprising to note that there have been relatively few attempts to assess their impact in economic terms. Existing studies in this regard are also reviewed, but the wide range of impacts discussed in this paper indicates that desert dust storms deserve more attention in this respect.
The Kathmandu Valley, located in the Himalayan foothills in Nepal, is heavily polluted. In order to investigate ambient particulate-bound mercury (Hg) in the Kathmandu Valley, a total 64 total suspended particulates (TSP) samples were collected from a sub-urban site in the Kathmandu Valley, the capital region of Nepal during a sampling period of an entire year (April 2013–April 2014). They were analyzed for ambient particulate-bound Hg (PBM) using thermal desorption combined with cold vapor atomic spectroscopy. In our knowledge, it is the first study of ambient PMB in the Kathmandu Valley and the surrounding broader Himalayan foothill region. The average concentration of PBM over the entire sampling period of a year was found to be 850.5 (± 962.8) pg m− 3 in the Kathmandu Valley. This is comparable to those values reported in the polluted cities of China and significantly higher than those observed in most of urban areas in Asia and other regions of world. The daily average Hg contents in TSP (PBM/TSP) ranges from 269.7 to 7613.0 ng g− 1 with an average of 2586.0 (± 2072.1) ng g− 1, indicating the high enrichment of Hg in TSP. The average concentrations of PBM were higher in the winter and pre-monsoon season than in the monsoon and post-monsoon season. The temporal variations in the strength of anthropogenic emission sources combined with other influencing factors, such as ambient temperature and the removal of atmospheric aerosols by wet scavenging are attributable to the seasonal variations of PBM. The considerably high dry deposition flux of PBM estimated by using a theoretical model was 135 μg m− 2 yr− 1 at the Kathmandu Valley. This calls for an immediate attention to addressing ambient particulate Hg in the Kathmandu Valley, including considering it as a key component of future air quality monitoring activities and mitigation measures.
Glaciers, particularly alpine glaciers, have been receding globally at an accelerated rate in recent decades. The glacial melt-induced release of pollutants (e.g., mercury) and its potential impact on the atmosphere and glacier-fed ecosystems has drawn increasing concerns. During 15th–20th August, 2011, an intensive sampling campaign was conducted in Qugaqie Basin (QB), a typical high mountain glacierized catchment in the inland Tibetan Plateau, to investigate the export and transport of mercury from glacier to runoff. The total mercury (THg) level in Zhadang (ZD) glacier ranged from <1 to 20.8 ng L⁻¹, and was slightly higher than levels measured in glacier melt water and the glacier-fed river. Particulate Hg (PHg) was the predominant form of Hg in all sampled environmental matrices. Mercury concentration in Qugaqie River (QR) was characterized by a clear diurnal variation which is linked to glacier melt. The estimated annual Hg exports by ZD glacier, the upper river basin and the entire QB were 8.76, 7.3 and 157.85 g, respectively, with respective yields of 4.61, 0.99 and 2.74 μg m⁻² yr⁻¹. Unique landforms and significant gradients from the glacier terminus to QB estuary might promote weathering and erosion, thereby controlling the transport of total suspended particulates (TSP) and PHg. In comparison with other glacier-fed rivers, QB has a small Hg export yet remarkably high Hg yield, underlining the significant impact of melting alpine glaciers on regional Hg biogeochemical cycles. Such impacts are expected to be enhanced in high altitude regions under the changing climate.
Alpine lake sediments and glacier ice cores retrieved from high mountain regions can provide long-term records of atmospheric deposition of anthropogenic contaminants such as mercury (Hg). In this study, eight lake sediment cores and one glacier ice core were collected from high elevations across the Himalaya-Tibet region to investigate the chronology of atmospheric Hg deposition. Consistent with modeling results, the sediment core records showed higher Hg accumulation rates in the southern slopes of the Himalayas than those in the northern slopes in the recent decades (post-World War II). Despite much lower Hg accumulation rates obtained from the glacier ice core, the temporal trend in the Hg accumulation rates matched very well with that observed from the sediment cores. The combination of the lake sediments and glacier ice core allowed us to reconstruct the longest, high-resolution atmospheric Hg deposition chronology in High Asia. The chronology showed that the Hg deposition rate was low between the 1500s and early 1800, rising at the onset of the Industrial Revolution, followed by a dramatic increase after World War II. The increasing trend continues to the present-day in most of the records, reflecting the continuous increase in anthropogenic Hg emissions from South Asia.
Dust storms, produced by the removal of surface materials from the world's drylands, are a vital component of the environment. This is because of their role in biogeochemical cycling, their potential influence on climate, their role in sediment accumulation and their influence on human affairs. This book, which is exhaustively referenced, explores and summarises recent research on where dust storms originate, why dust storms are generated, where dust is transported and deposited, the nature of dust deposits and the changing frequency of dust storms over a range of time-scales. © Andrew S. Goudie and Nicholas J. Middleton 2006. All rights are reserved.
East Asia is a major dust source in the world. Mineral dusts in the atmosphere and their interactions with clouds and precipitation have great impacts on regional climate in Asia, where there are large arid and semi-arid regions. In this review paper, we summarize the typical transport paths of East Asian dust, which affect regional and global climates, and discuss numerous effects of dust aerosols on clouds and precipitation primarily over East Asian arid and semi-arid regions. We hope to provide a benchmark of our present understanding of these issues. Compared with the aerosols of Saharan dust, those of East Asian dust are more absorptive of solar radiation, and its direct radiative forcing at the top of atmosphere is nearly positive or nil. It means that aerosols of East Asian dust can influence the cloud properties not only by acting as cloud condensation nuclei and ice nuclei (via first indirect effect, second indirect effect and invigoration effect) but also through changing the relative humidity and stability of the atmosphere (via semi-direct effect). Converting visible light to thermal energy, dust aerosols can burn clouds to produce a warming effect on climate, which is opposite to the first and second indirect effects of aerosols. The net dust aerosol radiative effects are still highly unclear. In addition, dust can inhibit or enhance precipitation under certain conditions, thus impacting the hydrological cycle. Over Asian arid and semi-arid regions, the positive feedback loop in the aerosol-cloud-precipitation interaction may aggravate drought in its inner land.
Precipitation samples collected at a remote high elevation site (i.e., Nam Co Station, 4730 m a.s.l.) in the southern Tibetan Plateau were analyzed for total mercury (HgT) between July 2009 and 2011, particulate-bound mercury (HgP) between July 2010 and 2011 and methylmercury (MeHg) from July through August of 2009. The volume-weighted mean (VWM) concentrations and wet deposition fluxes of HgT and MeHg in precipitation were 4.8 ng L−1 and 1.75 μg m−2 yr−1, 0.031 ng L−1 and 0.01 μg m−2 yr−1, respectively. VWM HgT concentration was approximately two times higher during the non-monsoon season than during the monsoon season, while 83% of the HgT wet deposition fluxes occurred during the monsoon season. The HgT and MeHg concentrations are comparable to the reported data for some of the most remote alpine and polar regions worldwide (e.g., Churchill), but the wet deposition fluxes of HgT and MeHg were among the lowest in the world. Analysis of Hg speciation has presented that HgP and MeHg concentrations are high, making up 71.2% and 1.82% of the HgT on average (VWM), respectively. The high HgP%, as well as a significantly positive between HgT and HgP (R2 = 0.91; n = 44; p < 0.001), confirmed that atmospheric deposition of Hg in the Tibetan Plateau was occurring in the form of HgP. A decreasing trend in HgT concentrations with increasing amount of precipitation (R2 = 0.08; N = 101; p < 0.005) was found at Nam Co Station, indicative that scavenging of HgP from the atmosphere was an important mechanism contributing Hg to precipitation. The precipitation amount, rather than HgT concentration, was found to be the governing factor affecting HgT wet deposition flux. Moreover, a comparison between measured wet deposition flux of Hg at Nam Co Station and the estimates from environmental records indicated that both snowpits and lake sediments appear to be reliable archives for estimating historical Hg accumulation rates over the Tibetan Plateau.
The Tibetan Plateau is one of the most extreme cold regions in the world and a source of fresh water to 1.4 billion people. In this study, between 2008 and 2010, surface snow samples were retrieved in replicate from four high-elevation glaciers with an average elevation of 5200 m above sea level from the Tibetan Plateau and were analyzed for total Hg (HgT). The spatial distribution of Hg for glaciers and magnification processes of Hg in snow with increasing altitude over the Tibetan Plateau are investigated. The total Hg concentrations in snow samples ranged from
Urban aerosol samples for PM10 and PM2.5 were collected during summer (August) and winter (December) 2000 in southern Taiwan (Tainan City) to demonstrate the temporal variations of Hg and As in particulate matter (PM). The mean mass concentrations with standard deviations were 80.0±26.8μgm−3 for PM10 and 50.6±16.6μgm−3 for PM2.5. The average PM2.5/PM10 mass ratio for the two periods combined was 63%, indicating that fine particles were a large portion of PM10. Particulate samples of Hg and As were analyzed within 2 days following sampling and weighing, because of the highly volatile nature of PM Hg and As. The average Hg and As values in PM10, PM2.5 and PM2.5–10 in summer were significantly lower than those in winter. PM2.5 Hg constituted 0.34 to 5.8ngm−3 and PM2.5–10 Hg 0.05 to 3.1ngm−3. PM2.5 As constituted 1.09 to 9.51ngm−3 and PM2.5–10 As 0.18 to 4.14ngm−3. In summer and winter PM10, the Hg contents showed regular daily variation, with the higher values at daytime and lower values at nighttime, indicating conversion of gaseous Hg to the particulate phase by reaction with atmospheric oxidants under strong solar radiation during the daytime in both summer and winter. PM As behaved similar to Hg in the summer, but in the winter higher concentrations were observed during the nighttime than during the daytime, implying that the stable temperature inversion during winter nighttime caused the accumulation of PM As near the ground. In summer, SE–WSW winds carried As from an As-emitting fossil power plant to the sample area. In a similar vein, NE–WNW winter winds contributed to aerosol Hg, especially in PM2.5, originating from a waste incinerator located NW of Tainan City.
Measurements of speciated atmospheric mercury were conducted at a remote
mountain-top station (WLG) at the edge of northeastern part of the
Qinghai-Xizang Plateau, western China. Mean concentrations of total
gaseous mercury (TGM), particulate mercury (PHg), and reactive gaseous
mercury (RGM) during the whole sampling campaign were 1.98 ± 0.98
ng m-3, 19.4 ± 18.1 pg m-3, and 7.4
± 4.8 pg m-3, respectively. Levels of speciated Hg at
WLG were slightly higher than those reported from remote areas of North
America and Europe. Both regional emissions and long-rang transport
played a remarkable role in the distribution of TGM and PHg in ambient
air at WLG, whereas RGM showed major links to the regional sources,
likely as well as the in-situ productions by photochemical processes.
Regional sources for speciated Hg were mostly located to the east of
WLG, which is the most developed areas of Qinghai province and accounted
for most of the province's anthropogenic Hg emissions. Potential source
contribution function (PSCF) results showed a strong impact of
long-range transport from eastern Gansu, western Ningxia and Shanxi
Province, with good accordance with locations of urban areas and
industrial centers. Moreover, we found that northern India was also an
important source region of WLG during the sampling campaign, and this is
the first time of direct evidence of long-range transport of atmospheric
Hg from India to northeastern Tibetan Plateau. Seasonal and diurnal
variations of TGM were in contrast with most of the previous studies in
China, with relatively higher levels in warm seasons and night,
respectively. The temporal trend of TGM also highlighted the impact of
long-range transport on the distribution of TGM in ambient air at WLG.
Estimates of atmospheric dust deposition to five Asian/Pacific regions indicate that -800 Tg of Chinese desert dust is injected into the atmosphere annually; about 30% of this is redeposited onto the deserts, 20% is transported over regional scales, primarily within continental China. The remaining 50% of the dust is subject to long-range transport to the Pacific Ocean and beyond. Elemental tracers based on several dust- derived elements (A1, Fe, Mg, and Sc) reveal high-frequency variability in the contributions of the western desert sources versus northern high-dust and low-dust desert sources to eolian deposits from the center of the Loess Plateau. Comparisons of the patterns uncovered with climate signals from the remote North Atlantic region for the last glaciation show that shifts in source areas of Asian dust were synchronous with large-scale variations in atmospheric circulation.
Persistent organic pollutants and mercury are important contaminants due
to their persistence in the environment and potential toxic effects on
ecosystems and humans. Concerns related to these contaminants are
particularly pertinent in Asia where the use of pesticides and mercury
emissions have been increasing dramatically due to changing agricultural
practices and rapidly expanding industrialization. Based on studies in
European and North American mountain regions, evidence is increasing
that alpine regions function as regional convergence zones for selected
organic pollutants due to an effect called orographic cold trapping. It
is hypothesized that such an effect may be particularly pronounced in
the Himalaya because of dramatic elevational temperature and
precipitation gradients relative to contaminant source regions in its
immediate vicinity, and because of the regional monsoon system that has
been shown to deliver particles and inorganic air pollutants to higher
altitudes. We report here evidence for the movement of select
environmentally relevant chlorinated organic pesticides into the Central
Himalaya with strong seasonal differences due to the Indian monsoon.
Atmospheric concentrations of these chemicals are positively correlated
with altitude in summer up to an elevation of 5000m a.s.l and then
decrease at higher elevation. In winter the atmospheric concentrations
are negatively correlated with altitude indicating that during this
season remote sites are above the boundary layer. Soil concentrations
appear to follow the gradient of forestation, with maximum
concentrations at 2600m a.s.l. and then declining above that altitude.
Mercury in three Tibetan snow pits, well above the boundary layer shows
the likelihood of particulate deposition in winter when particulate
concentrations are highest. Strong dust storm activity is the largest
source of mercury deposition on the plateau, though it is unsure if the
mercury is transported on dust long distances or if mercury is
partitioning onto particulates nearby to the deposition site. Both these
deposition mechanisms are likely occurring. The highest elevations of
the Himalaya and Tibetan plateau are protected by the upper level
westerlies above the boundary layer and are likely only affected by
chemicals that have long atmospheric lifetimes such as hexachlorobenzene
and mercury.
An oasis is not only the most concentrated area of human activity in an arid area but also the largest area where artificial disturbances occur at a regional scale. The study of oasis urban expansion and related factors is important to understand the development of cities in arid areas, guide the evolution of rational urban expansion and promote sustainable development of oasis cities. Although there have been several studies on urban expansion in Xinjiang over past decades, a lack of quantitative data and methods impedes further research. In this paper, urban expansion of the studied area in 1990, 2000 and 2007 is evaluated using 39 Landsat satellite images, a geographical information system (GIS) and remote sensing (RS). We also analyse the related factors of urban expansion using quantitative and qualitative methods. We found that economic development and the area of the urban administrative region greatly effect urban expansion. Too rapid growth and outward urbanized expansion paradigms should not be applied blindly in arid areas from the point of view of sustainability. Historic–geographic features of oases and urban planning influence oasis urban morphology. Compact urban morphology is relatively better for protecting precious water and arable resources, and reducing ecology damage to the surrounding oasis. The relationship between urban expansion and population growth was not coordinate before 2000, and effectively only improved after that date. City growth should be moderated, with lower elastic coefficients, slower expansion and higher elasticity coefficients. Oasis cities in Xinjiang are expanding under the combined effects of urbanization, economic development, transportation, environment, resources, policy and planning.
Total particulate mercury (TPM) may constitute, in general, a small percentage by mass of total atmospheric mercury, but under certain conditions it can be a very significant form of atmospheric Hg, and play an important role in the deposition of mercury to terrestrial and aquatic cosystems. To understand the cycling and distribution of mercury in the environment, accurate determination of the total concentration and speciation of TPM are necessary. This article provides a review of scientific and technical literature published in the past 5 yr pertaining to sampling and analytical methodologies for the determination and chemical speciation of TPM in ambient air. Sampling methods discussed include the conventional filtration method and the diffusion denuder-based technique. Analytical techniques, such as gold-amalgamation/cold-vapor spectrometry, and nuclear methods, are summarized.
Ten snowpits were sampled at the Zhadang glacier during 2008 and 2011 to investigate the seasonal variations, speciation, and sources of mercury (Hg) in the southern Tibetan Plateau. In the 2008 snowpit, total Hg (Hg(T)), particulate matter, most of major ions were found in higher concentrations during the non-monsoon season than in the monsoon season. Analysis of Hg speciation indicated that Hg(T) in the 2011 snowpits was dominated by particulate-bound Hg (Hg(P)). Most of particulate matter in the 2008 snowpit was dominated by fine particulates, indicating that the influx of particulate matter and Hg(P) was probably occurring by long-range transportation via general atmospheric circulation. Analysis of dominant ion Ca²⁺ and alkaline pH values has suggested that the long-range transported Hg(P), originating from dust storm activities, may be the most important source for Hg in the Zhadang glacier snowpit during the non-monsoon season. Backward-trajectory analysis indicates the majority of the air masses arriving at the Zhandang glacier originated from the arid regions of northwestern India (e.g., Thar Desert), confirming that arid regions in central and southern Asia are likely the main sources of Hg being deposited in the Zhadang glacier snowpit. This study also suggests that ice core records from the Tibetan Plateau may be useful tools for interpreting long-term historical records of atmospheric Hg deposition, and reconstructing Hg biogeochemical cycling.
Western China is home to the largest aggregate of glaciers outside the polar regions, yet little is known about how the glaciers in this area affect the transport and cycling of mercury (Hg) regionally and globally. From 2005 to 2010, extensive glacier snow sampling campaigns were carried out in 14 snowpits from 9 glaciers over western China, and the vertical distribution profiles of Hg were obtained. The Total Hg (THg) concentrations in the glacier snow ranged from <1 to 43.6 ng L(-1), and exhibited clear seasonal variations with lower values in summer than in winter. Spatially, higher THg concentrations were typically observed in glacier snows from the northern region where atmospheric particulate loading is comparably high. Glacier snowpit Hg was largely dependent on particulate matters and was associated with particulate Hg, which is less prone to postdepositional changes, thus providing a valuable record of atmospheric Hg deposition. Estimated atmospheric Hg depositional fluxes ranged from 0.74 to 7.89 μg m(-2) yr(-1), agreeing very well with the global natural values, but are one to two orders of magnitude lower than that of the neighboring East Asia. Elevated Hg concentrations were observed in refrozen ice layers in several snowpits subjected to intense melt, indicating that Hg can be potentially released to meltwater.
Total particulate mercury (TPM) and reactive gaseous mercury (RGM) concentrations in ambient air on the eastern slope of the Mt. Gongga area, Sichuan Province, Southwestern China were monitored from 25 May, 2005 to 29 April, 2006. Simultaneously, Hg concentrations in rain samples were measured from January to December, 2006. The average TPM and RGM concentrations in the study site were 30.7 and 6.2 pg m−3, which are comparable to values observed in remote areas in Northern America and Europe, but much lower than those reported in some urban areas in China. The mean seasonal RGM concentration was slightly higher in spring (8.0 pg m−3) while the minimum mean concentration was observed in winter (4.0 pg m−3). TPM concentrations ranged across two orders of magnitude from 5.2 to 135.7 pg m−3 and had a clear seasonal variation: winter (74.1 pg m−3), autumn (22.5 pg m−3), spring (15.3 pg m−3) and summer (10.8 pg m−3), listed in decreasing order. The annual wet deposition was 9.1 μg m−2 and wet deposition in the rainy season (May–October) represented over 80% of the annual total. The temporal distribution of TPM and RGM suggested distinguishable dispersion characteristics of these Hg species on a regional scale. Elevated TPM concentration in winter was probably due to regional and local enhanced coal burning and low wet deposition velocity. The RGM distribution pattern is closely related to daily variation in UV radiation observed during the winter sampling period indicating that photo-oxidation processes and diurnal changes in meteorology play an important role in RGM generation.
This paper presents a broad overview and synthesis of current knowledge and understanding pertaining to all major aspects of mercury in the atmosphere. The significant physical, chemical, and toxicological properties of this element and its environmentally relebant species encountered in the atmosphere are examined. Atmospheric pathways and processes considered herein include anthropogenic as well as natural sources of Hg emissions to the atmosphere, aerial transport and dispersion (including spatial and temporal variability), atmospheric transformations (both physical and chemical types), wet and dry removal/deposition processes to Earth's surface. In addition, inter-compartmental (air-water/soil/vegetation) transfer and biogeochemical cycling of mercury are considered and discussed. The section on numerical modelling deals with atmospheric transport models as well as process-oriented models. Important gaps in our current knowledge of mercury in the atmospheric environment are identified, and suggestions for future areas of research are offered.
Contributions of the nine potential dust source regions (North and South Africa, the Arabian Peninsula, Central Asia, eastern and western China, North and South America, and Australia) to the global dust budget are investigated with a global dust transport model. A six-year simulation (1990 to 1995) indicates that the greatest contributor to the global dust budget is found to be North Africa (the Sahara Desert), which accounts for 58% of the total global dust emission and 62% of the total global dust load in the atmosphere. Australian dust dominates the southern hemisphere. The dust emission and atmospheric dust load originating from East Asia (eastern and western China) are estimated to be 214 Tg yr− 1 and 1.1 Tg, respectively, which are 11% and 6% of the total global dust emission and dust load. Dust from East Asia dominates the atmospheric load over China and Mongolia (about 70%), Korea (60%), Japan (50%), and the North Pacific Ocean (40%). The contribution of dust originating from regions other than East Asia to the dust load over these East Asian countries and the North Pacific Ocean cannot be ignored. The simulated total dust deposition flux on Greenland suggests a possible overestimation of the Saharan dust and an underestimation of the East Asian dust in the Arctic region, which may be a common problem with global dust transport models. Possible reasons for the underestimation of the East Asian dust are discussed.
Isotopic (Sr and Nd) and mineralogical analyses of dust extracted from snow deposits from the last decade at the NorthGRIP, Greenland, ice camp (75.1°N, 042.3°W) confirm the eastern Asian source for Greenland dust and extend it to the present day. The 1998 and 1999 samples, which represent a ∼2.4 month resolution of dust deposition, show that there are at least two east Asian sources for the Greenland dust that vary seasonally. The provenance of dust during the major, spring/summer deposition is the Takla Makan desert of northwest China.
This paper discusses the sources, spatial distribution, frequency and trend of dust storms in China. Most dust storms in China originate from one of three geographic areas: the Hexi (River West) Corridor and western Inner Mongolia Plateau, the Taklimakan Desert, and the central Inner Mongolia Plateau. Dust is most likely from deteriorated grasslands, Gobi, alluvial, lacustrine sediments and wadis at the outer edge of deserts. But deserts themselves contribute only slightly to the dust storm directly. Two geographic areas frequently have dust storms: one is in the western Tarim Basin, a ground surface of deteriorated land and wadi, but it only affects its neighboring areas, and the other one is in the western Inner Mongolia Plateau, a ground surface of Gobi, alluvial and lacustrine sediments, but it causes most of the dust storms in north China. Generally speaking, dust storms have reduced in most regions of China from the 1950 to 2000. Dust storms are highly correlated with human activities and climate changes.
The atmosphere is an important transient reservoir of mercury. In addition to its great capacity, the chemical processes transforming mercury between the elemental and divalent states strongly influence the transport characteristics and deposition rate of this toxic metal back to the ground. Modeling efforts to assess global cycling of mercury require an in-depth knowledge of atmospheric mercury chemistry. This review article provides selected physical and chemical properties of atmospheric mercury, and discusses the identified mercury transformation pathways mediated by ozone, S(IV), hydroperoxyl radical, hydroxyl radical, chlorine, nitrate radical and photolysis of Hg(II) complexes. Special attention is paid to the kinetics and mechanisms of the reactions interconverting mercury between elemental and divalent states. The significance and implications of each transformation pathway under atmospheric conditions are addressed. Future research areas that must be pursued to better understand the fate and transformation of mercury in the atmosphere are also projected.
This paper deals with mineral dust emission inventory from surfaces of Northern China. The inventory was calculated with a US EPA formula by inputting the pre-processed Chinese data of pedology and climatology. Mainly, the emission factor (emission rate) of the dust particles whose diameters are less than 0.03 mm increases from east to west of the area by five orders of magnitude and there are two strong emission regions, one is in Takelamagan desert, Xinjiang Province, and the other in Central Gobi-desert, western part of inner-Mongolia plateau. The maximum rate is at center of the Takelamagan desert, i.e., 1.5 ton ha yr−1. Also, the total annual emission amount of the area is equal to some 25 million tons, and spring is the worst dust-emitting season in the area, which takes more than half of the annual emission amount. The results are in good agreement with the previous calculations using a different US EPA formula (Xuan, J., 1999. Dust emission factors for environment of Northern China. Atmospheric Environment 33, 1767–1776).