Atmospheric particulate-bound mercury (PBM10) in a Southeast Asia megacity: Sources and health risk assessment.

Particulate-bound mercury (PBM) is a global environmental concern owing to its large dry deposition velocities and scavenging coefficients, both of which drive Hg into terrestrial and marine ecosystems. PBM observation studies have been widely conducted over East Asia, but comparable studies in Peninsular Southeast Asia (PSEA) remain scarce. This is the first study reporting PBM concentrations for Ho Chi Minh City (HCMC), the biggest metropolitan area in Vietnam. A total of 222 samples were collected in 2018 and contained an average PBM10 (particulate matter - PM with diameter ≤10 µm) concentration and Hg mass fraction (i.e. PBM/PM) of 67.3 ± 45.9 pg m-3 and 1.18 ± 1.12 µg g-1, respectively. Although PBM concentration was lower than those reported in Chinese megacities, the Hg mass fraction was similar to those in China, suggesting strong enrichment from anthropogenic Hg emissions in HCMC. Traffic-induced particulate emission and deposition processes were major factors governing PBM temporal variation at our site. In addition, the prevailing southwest monsoon winds brought air masses that passed through industrial areas and were associated with a higher Hg mass fraction. Statistically significant positive correlations (R2 = 0.11-0.52, p < 0.01) were observed for PBM with PM and the Hg mass fraction, indicating similar PM and Hg sources or oxidized Hg adsorption onto PM via gas-particle partitioning. Moreover, PCA results revealed a higher contribution of primary sources than secondary sources to PBM concentration variability in HCMC. A health risk assessment indicated that the PBM concentrations at HCMC posed minimal non-carcinogenic risks (HI < 1) for children and adults, but dermal contact may act as an important exposure route since lightweight clothing is common among residents. This PBM dataset over PSEA, a region with high atmospheric Hg emissions, provides a valuable resource for the Hg scientific community to improve our understanding of Hg biogeochemical cycle.

Distinct transport mechanisms of East Asian dust and the impact on downwind marine and atmospheric environments.

East Asian dust episodes have a multitude of impacts, including on human health, environment, and climate over near-source and receptor regions. However, the mechanistic understanding of the synoptic conditions of these outbreaks at different altitude layers, and their eventual environmental impacts are less studied. The present study analyzed the synoptic transport patterns of East Asian dust during multiple dust generation episodes that occurred over only a few days apart in northern China, and which eventually delivered high PM10 concentrations to surface level and high-altitude locations in Taiwan. Whether the dust plume was uplifted ahead of or behind the 700 hPa trough over East Asia determined its trajectory and eventual impact on the environment downwind. The total dust (iron) deposition over the ocean surface preceding arrival to Taiwan was 2.4 mg m-2 (0.95 µg m-2) for the episode impacting the surface level and 5.0 mg m-2 (4.6 µg m-2) for the episode impacting high-altitude Taiwan. Dust deposition in marine areas east of China was more intense for the higher altitude transport event that was uplifted behind the 700 hPa trough and resulted in twice higher marine Chl-a concentrations. Furthermore, we estimated a dust-induced direct radiative effect over a high mountainous region in Taiwan of -6.2 to -8.2 W m-2 at the surface, -1.9 to -2.9 W m-2 at the top of the atmosphere and +3.9 to +5.3 W m-2 in the atmosphere. This dust-induced atmospheric warming and surface cooling are non-negligible influences on the atmospheric thermal structure and biogeochemical cycle over the western North Pacific. Overall, this study highlights the significant impacts of dust particles on the marine ecosystem and atmospheric radiation budget over the downwind region, thus lays the foundation for linking these impacts to the initial synoptic conditions in the source area.

Multiscale Temporal Variations of Atmospheric Mercury Distinguished by the Hilbert-Huang Transform Analysis Reveals Multiple El Niño-Southern Oscillation Links.

Atmospheric mercury (Hg) cycling is sensitive to climate-driven changes, but links with various teleconnections remain unestablished. Here, we revealed the El Niño-Southern Oscillation (ENSO) influence on gaseous elemental mercury (GEM) concentrations recorded at a background station in East Asia using the Hilbert-Huang transform (HHT). The timing and magnitude of GEM intrinsic variations were clearly distinguished by ensemble empirical mode decomposition (EEMD), revealing the amplitude of the GEM concentration interannual variability (IAV) is greater than that for diurnal and seasonal variability. We show that changes in the annual cycle of GEM were modulated by significant IAVs at time scales of 2-7 years, highlighted by a robust GEM IAV-ENSO relationship of the associated intrinsic mode functions. With confirmation that ENSO modulates the GEM annual cycle, we then found that weaker GEM annual cycles may have resulted from El Niño-accelerated Hg evasion from the ocean. Furthermore, the relationship between ENSO and GEM is sensitive to extreme events (i.e., 2015-2016 El Niño), resulting in perturbation of the long-term trend and atmospheric Hg cycling. Future climate change will likely increase the number of extreme El Niño events and, hence, could alter atmospheric Hg cycling and influence the effectiveness evaluation of the Minamata Convention on Mercury.

Potential sources, scavenging processes, and source regions of mercury in the wet deposition of South Korea.

In this study, the potential sources, scavenging processes, and emission regions for Hg in wet deposition were investigated in rural (Jeju), suburban (Gwangju), and urban sites (Incheon and Seoul) of South Korea. The annual volume-weighted mean concentrations of Hg in wet deposition were four to five times higher in Incheon (16.6 ng L-1) and Seoul (22.5 ng L-1) than in Jeju (4.0 ng L-1) and Gwangju (4.1 ng L-1). The variations in the Hg concentrations in wet deposition of Jeju and Gwangju were related to Cl-, Na+, Mg2+, and K+ originating from marine and crustal sources, and those in Incheon and Seoul were related to SO42-, NO3-, and NH4+ emitted from anthropogenic sources. The below-cloud scavenging was considered a major inclusion process of Hg in Jeju and Gwangju, while the within-cloud scavenging was suggested in Incheon and Seoul, based on the results of correlation analysis with Hg and major ions in wet deposition, and meteorological data. The cluster analysis of backward trajectories demonstrated that the Hg concentration in wet deposition was highest in the cluster transported from Hebei and Shandong of China in Gwangju, but in Seoul, the Hg concentrations of each cluster were comparable. This suggests that regional transport is the major source of Hg in the wet deposition of Gwangju while local transport provides substantial amount of Hg in the wet deposition of Seoul. This was further supported by the results of concentration-weighted trajectories: the most probable source region was east China for Gwangju, and the mid-west of South Korea and east China for Seoul. It is noted that the peak methylmercury concentrations were found every spring with simultaneous increases in atmospheric Al, Ca, Mg, and Fe concentrations, indicating a concurrence with Asian dust. The formation process of methylmercury in Asian dust should be confirmed in future studies.

Eight-year dry deposition of atmospheric mercury to a tropical high mountain background site downwind of the East Asian continent.

Atmospheric deposition, either dry or wet, has been identified as an important pathway of mercury (Hg) input to terrestrial and aquatic systems. Although East Asia is the major atmospheric Hg emission source region, very few studies have been conducted to quantify atmospheric Hg deposition in its downwind region. In this study, 8-year (2009-2016) atmospheric Hg dry deposition was reported at the Lulin Atmospheric Background Station (LABS), a high mountain forest site in central Taiwan. Dry deposition of speciated Hg was estimated using a bi-directional air-surface flux exchange model for gaseous elemental mercury (GEM) and dry deposition models for gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM), making use of the monitored speciated atmospheric Hg concentrations. Annual total Hg dry deposition ranged from 51.9 to 84.9 µg m-2 yr-1, with a multi-year average of 66.1 µg m-2 yr-1. Among the three forms of atmospheric Hg, GEM was the main contributor to the total dry deposition, contributing about 77.8% to the total, due to the high density of forest canopy as well as the much higher concentration of GEM than GOM and PBM at LABS. Mercury dry deposition is higher in winter and spring than in summer and fall, partly due to the elevated Hg concentrations associated with air masses from East and Southeast Asia where with high atmospheric Hg emissions. The mean annual dry/wet deposition ratio of 2.8 at LABS indicated that Hg deposition to forest landscape was governed by dry rather than wet deposition.

Temporal changes in atmospheric mercury concentrations at a background mountain site downwind of the East Asia continent in 2006-2016.

Atmospheric mercury (Hg) has been monitored at the Lulin Atmospheric Background Station (LABS) in Taiwan since April 2006 and is still continuing. Here we reported the trend in gaseous elemental Hg (GEM) concentrations at LABS between April 2006 and December 2016, before the Minamata Convention on Mercury entered into force in 2017. Previous research indicated nighttime (0-8 am) data collected at LABS are better representative of regional influence. Therefore, only nighttime GEM data were used for trend analysis. A significant decreasing trend in GEM at a rate of -1.5% yr-1 (-0.022 ng m-3 yr-1, p < 0.01) was found, comparable to the decreasing trends observed in Europe, North America, South Africa, and over the North Atlantic Ocean. Five major GEM source regions to the LABS were identified, including northern Indochina Peninsula, China, Northeast Asia, the Pacific Ocean, and South China Sea. Significant decreasing trends in GEM were found for air masses coming from northern Indochina Peninsula (-0.042 ng m-3 yr-1, -2.6% yr-1, p < 0.01), China (-0.041 ng m-3 yr-1, -2.4% yr-1, p < 0.01), Northeast Asia (-0.031 ng m-3 yr-1, -2.0% yr-1, p < 0.05), and the Pacific Ocean (-0.022 ng m-3 yr-1, -1.7% yr-1, p < 0.05). Decreasing GEM trend (-0.020 ng m-3 yr-1, -1.5% yr-1), but insignificant (p > 0.05), was also found for air masses coming from South China Sea. The decreasing trends observed with air from the Pacific Ocean and South China Sea indicated declining background GEM concentrations in Northern Hemisphere. Decrease in GEM concentrations at the LABS was in agreement with the reduction in atmospheric Hg export from the East Asia continent caused by changes in Hg emission quantity and speciation, and temporal and spatial distribution in emission sources that have been suggested by recent research. Additionally, changes in the frequency distribution of air mass origins and transport paths may also contribute to the changes in GEM concentrations at LABS.

A New Monitoring Effort for Asia: The Asia Pacific Mercury Monitoring Network (APMMN).

The Asia Pacific Mercury Monitoring Network (APMMN) cooperatively measures mercury in precipitation in a network of sites operating in Asia and the Western Pacific region. The network addresses significant data gaps in a region where mercury emission estimates are the highest globally, and available measurement data are limited. The reduction of mercury emissions under the Minamata Convention on Mercury also justifies the need for continent-wide and consistent observations that can help determine the magnitude of the problem and assess the efficacy of reductions over time. The APMMN's primary objectives are to monitor wet deposition and atmospheric concentrations of mercury and assist partners in developing their own monitoring capabilities. Network planning began in 2012 with wet deposition sampling starting in 2014. Currently, eight network sites measure mercury in precipitation following standardized procedures adapted from the National Atmospheric Deposition Program. The network also has a common regional analytical laboratory (Taiwan), and quality assurance and data flagging procedures, which ensure the network makes scientifically valid and consistent measurements. Results from our ongoing analytical and field quality assurance measurements show minimal contamination in the network and accurate analytical analyses. We are continuing to monitor a potential concentration and precipitation volume bias under certain conditions. The average mercury concentration in precipitation was 11.3 (+9.6) ng L-1 for 139 network samples in 2018. Concentrations for individual sites vary widely. Low averages compare to the low concentrations observed on the U.S. West Coast; while other sites have average concentrations similar to the high values reported from many urban areas in China. Future APMMN goals are to (1) foster new network partnerships, (2) continue to collect, quality assure, and distribute results on the APMMN website, (3) provide training and share best monitoring practices, and (4) establish a gaseous concentration network for estimating dry deposition.

Investigation of the relationship between atmospheric mercury and concentrations of key greenhouse gases at a mountainous monitoring site.

The concentration of total gaseous mercury (TGM) was monitored, together with some key greenhouse gases (GHGs: carbon dioxide (CO2), methane (CH4), and water (H2O) vapor) at hourly intervals at a mountainous monitoring site close to the highly industrialized city of Seoul, Korea. Correlations between the concentrations of Hg and those of the greenhouse gases were examined to assess their source characteristics and responses to changes in meteorological conditions. The mean Hg levels in this study (3.58 ± 2.13 ng m(-3)) were considerably lower (by, e.g., 24.3%) than those measured previously in other comparable sites during 1999-2006 (4.73 ± 1.34 ng m(-3)). Accordingly, such a reduction in Hg levels suggests the effectiveness of the regulatory measures enforced over the years. The mean Hg level observed in this study is also lower (by approximately 5%) than those in other Asian locations. In contrast, the mean concentrations of the two most important GHGs (CO2 and CH4) were moderately higher than those of other locations across the world (by approximately 4-9%). The results of our analysis indicate that the behavior of Hg is strongly correlated with water vapor and CH4 in terms of their source characteristics, despite notable differences in their diurnal patterns.

Interactions between biomass-burning aerosols and clouds over Southeast Asia: current status, challenges, and perspectives.

The interactions between aerosols, clouds, and precipitation remain among the largest sources of uncertainty in the Earth's energy budget. Biomass-burning aerosols are a key feature of the global aerosol system, with significant annually-repeating fires in several parts of the world, including Southeast Asia (SEA). SEA in particular provides a "natural laboratory" for these studies, as smoke travels from source regions downwind in which it is coupled to persistent stratocumulus decks. However, SEA has been under-exploited for these studies. This review summarizes previous related field campaigns in SEA, with a focus on the ongoing Seven South East Asian Studies (7-SEAS) and results from the most recent BASELInE deployment. Progress from remote sensing and modeling studies, along with the challenges faced for these studies, are also discussed. We suggest that improvements to our knowledge of these aerosol/cloud effects require the synergistic use of field measurements with remote sensing and modeling tools.

PCDD/F measurement at a high-altitude station in Central Taiwan: evaluation of long-range transport of PCDD/Fs during the Southeast Asia biomass burning event.

Recent biomass burning in Southeast Asia has raised global concerns over its adverse effects on visibility, human health, and global climate. The concentrations of total suspended particles (TSPs) and other vapor-phase pollutants (CO and ozone) were monitored at Lulin, an atmospheric background station in central Taiwan in 2008. To evaluate the long-range transport of persistent organic pollutants (POPs) during the Southeast Asia biomass burning event, the atmospheric polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were also measured at Lulin station. The atmospheric PCDD/F and TSP concentrations measured at Lulin station ranged from 0.71-3.41 fg I-TEQ/m(3) and 5.32-55.6 microg/m(3), respectively, during the regular sampling periods. However, significantly higher concentrations of PCDD/Fs, TSPs, CO, and ozone were measured during the spring season. These high concentrations could be the result of long-range transport of the products of Southeast Asia biomass burning. During the Southeast Asia biomass burning event (March 18-24, 2008), an intensive observation program was also carried out at the same station. The results of this observation program indicated that the atmospheric PCDD/F concentration increased dramatically from 2.33 to 390 fg I-TEQ/m(3) (March 19, 2008). The trace gas (CO) of biomass burning also significantly increased to 232 ppb during the same period, while the particle-bound PCDD/Fs in the TSP increased from 28.7 to 109 pg I-TEQ/g-TSP at Lulin station during the burning event. We conclude that there was a significant increase in the PCDD/F concentration in ambient air at a high-altitude background station in central Taiwan during the Southeast Asia biomass burning event.