Monitoring Effect of SO2 Emission Abatement on Recovery of Acidified Soil and Streamwater in Southwest China

Assessing the efficacy of aluminum metal clusters Al13 and Al15 in mitigating NO2 and SO2 pollutants: a DFT investigation

The present investigation delves into the adverse environmental impact of atmospheric pollutant gases, specifically nitrogen dioxide (NO2) and sulfur dioxide (SO2), which necessitates the identification and implementation of effective control measures. The central objective of this study is to explore the eradication of these pollutants through the utilization of aluminum Al13 and Al15 metal clusters, distinguished by their unique properties. The comprehensive evaluation of gas/cluster interactions is undertaken employing density functional theory (DFT). Geometric optimization calculations for all structures are executed using the ωB97XD functional and the Def2-svp basis set. To probe various interaction modalities, gas molecule distribution around the metal clusters is sampled using the bee colony algorithm. Frequency calculations employing identical model chemistry validate the precision of the optimization calculations. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) methodologies are applied for the analysis of intermolecular interactions. This research establishes the robust formation of van der Waals attractions between the investigated gas molecules, affirming aluminum metal clusters as viable candidates for the removal and control of these gases.

A probe into the acid deposition mitigation path in China over the last four decades and beyond

China currently has the highest acid deposition globally, yet research on its status, impacts, causes and controls is lacking. Here, we compiled data and calculated critical loads regarding acid deposition. The results showed that the abatement measures in China have achieved a sharp decline in the emissions of acidifying pollutants and a continuous recovery of precipitation pH, despite the drastic growth in the economy and energy consumption. However, the risk of ecological acidification and eutrophication showed no significant decrease. With similar emission reductions, the decline in areas at risk of acidification in China (7.0%) lags behind those in Europe (20%) or the USA (15%). This was because, unlike Europe and the USA, China's abatement strategies primarily target air quality improvement rather than mitigating ecological impacts. Given that the area with the risk of eutrophication induced by nitrogen deposition remained at 13% of the country even under the scenario of achieving the dual targets of air quality and carbon dioxide mitigation in 2035, we explored an enhanced ammonia abatement pathway. With a further 27% reduction in ammonia by 2035, China could largely eliminate the impacts of acid deposition. This research serves as a valuable reference for China's future acid deposition control and for other nations facing similar challenges.

Delayed recovery of surface water chemistry from acidification in subtropical forest region of China

The three largest acid rain regions of current earth are located in northern and western Europe, eastern North America, and East Asia. Sulfur and nitrate concentrations in headwater streams in Europe and North America decreased as atmospheric sulfur and nitrogen deposition decreased, albeit with a considerable delay. However, how water chemistry responds to the declining sulfur and nitrogen deposition in China is unclear. The regional survey of surface water chemistry during 2010 and 2018 within the Sichuan Basin in southwestern China showed that the recovery of the surface water chemistry was delayed for at least 5 years owing to the release of previously deposited sulfur and nitrogen stored in the soil. After sulfur deposition declined from its peak value, the subregions of purplish soil with low sulfate adsorption capacity still exhibited a net sulfur release in 2010, but this release was no longer evident by 2018. The subregions with the red and yellow soils, which have a high sulfate adsorption capacity, operated as sulfur sinks during 2010 and 2018, indicating a continuous immobilization process through sulfate reduction despite a decrease in sulfur deposition. Additionally, this sulfate reduction countered the release of sulfate caused by sulfur desorption. There was a substantial nitrogen sink within the Sichuan Basin. Nitrogen leaching decreased slowly with the declined nitrogen deposition, except in regions where nitrogen deposition exceeded the critical threshold. Compared to temperate forest regions in Europe, the Sichuan Basin and its surrounding areas have experienced higher decline rates in the leaching of sulfur and nitrogen, highlighting that the subtropical forest region undergoes a faster restoration of surface water chemistry.

Development of science and policy related to acid deposition in East Asia over 30 years

Scientific and public interest in acid deposition and its ecological impacts have increased throughout 1990s in East Asia (Northeast and Southeast Asia). After being established in 2001, the Acid Deposition Monitoring Network in East Asia (EANET) celebrates the 20th anniversary in 2021, and is now being expanded in scope reflecting the shifting social concern from acid deposition to broader air quality and climate change in recent years. This paper reviews the past 30 years of development of scientific research and policy related to acid deposition in East Asia. Since the onset of the twenty-first century, East Asia has had the highest SO₂ and NO_x emissions in the world by continents, with substantial economic developmental inequality among countries. An overview of studies on sulfur and nitrogen deposition, the acidification of inland water and forest soil, and forest decline reveal that although limited acidification of inland water and forest soils have been documented, no decline in the populations of fish and other aquatic biota has been reported in East Asia. After a review of policy-oriented modeling studies on source receptor relationships and the critical loads of sulfur and nitrogen in East Asia, the history of EANET and its success and challenges are discussed. Finally, the importance of epistemic communities as the interface between science and policy in the region is discussed. Regional governance and cooperation are essential for reducing the emission of greenhouse gases, especially short-lived climate pollutants and atmospheric pollutants to realize the co-benefits of global climate change mitigation and improved air quality.

Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H⁺) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha^-1 yr^-1). H⁺ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K⁺, Ca²⁺, Na⁺, Mg²⁺, Al³⁺, NH₄⁺, NO₃^-, SO₄^2-, Cl^-, and H⁺). The result showed that total H⁺ production in the watershed was 5152 mol_c ha^-1 yr^-1. N transformation was the most important H⁺ source (68%), followed by excess plant uptake of cations (25%) and H⁺ deposition (7%). Base cations exchange and weathering of minerals (3842 mol_c ha^-1 yr^-1) dominated H⁺ neutralization, followed by SO₄^2- adsorption (1081 mol_c ha^-1 yr^-1), while H⁺ and Al³⁺ leaching amounted to 431 mol_c ha^-1 yr^-1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H⁺ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230-1080 mol_c ha^-1 yr^-1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.

Fluxes of H2S and SO2 above a subtropical forest under natural and disturbed conditions induced by temporal land-use change

Hydrogen sulfide (H₂S) is one of predominant biogenic sulfur gases, influencing aerosol formation and climate change. There is considerable uncertainty of the global budget of H₂S due to limited field data, especially in subtropical forests. In addition, an interaction between soil-emitted H₂S and ambient sulfur dioxide (SO₂) might exist within forest ecosystems. In this study, the aerodynamic gradient method was applied to consecutively measure H₂S and SO₂ fluxes above a subtropical forest canopy in Southwest China under natural and disturbed conditions induced by temporal land-use changes. The average H₂S concentration and flux under natural conditions were 0.79 ± 0.07 ppbv and 0.04 ± 0.01 g S m^-2 yr^-1, respectively. The emission was larger than that in most croplands and freshwater wetlands. Vegetation emissions might account for about 26% of the total forest H₂S emissions at this site. The deposition of SO₂ was likely balanced by H₂S oxidization under the forest canopy, with the mean concentration and net flux as 1.23 ± 0.11 ppbv and -0.03 ± 0.10 g S m^-2 yr^-1, respectively. Under disturbed conditions with soils excavation and scattering on the forest floor, simultaneously high emissions of H₂S and SO₂ were observed above the canopy, reaching 5.78 ± 0.16 and 1.60 ± 0.87 g S m^-2 yr^-1, respectively. This suggested that land-use change in subtropical forests might lead to release of legacy S in subsoils to the atmosphere in the form of H₂S and SO₂. Regarding the widely documented large S accumulation and expanding deforestation across subtropical forests, potentially high emissions of H₂S and SO₂ from subtropical forests should be carefully considered in regional air quality control and forest management.

Critical loads of headwater streams in China using SSWC model modified by comprehensive F-factor

In order to evaluate the potential risk of surface water acidification in regions with historically-elevated acid deposition and to measure the recovery of such ecosystems after policy changes, critical loads and their exceedances were estimated for 349 headwater streams across China using a modified SSWC model. Such a model considered the acid-neutralizing capacity derived from high base cation deposition and the robust retention of sulfate and nitrate. Results indicated that China's streams had higher critical loads (averaged at 4.7 keq·ha^-1·yr^-1) and were less sensitive to acid deposition as compared to Europe and North America. The proportion of surveyed streams with acid deposition exceeded critical load decreased from 40.4% in 2005 to 29.5% in 2018, indicating a significant decrease in risk of surface water acidification, and thus a benefit from the emission abatement in recent years. Nonetheless, a relatively high risk of acidification still existed in southeast China with lower critical loads and most critical load exceedances. More efforts should be put into implementing emission control policies in the future.

Effect of metal decoration on sulfur-based gas molecules adsorption on phosphorene

Based on first-principles calculation, the adsorption of sulfur-based gas molecules (H2S, SO2, SO3) on various metal-decorated phosphorenes is researched systematically. Eleven metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl) which can avoid the formation of clusters on the phosphorene are considered. Noticeably, all metal decorations can enhance the adsorption strength of phosphorene to sulfur-based gas molecules except for H2S on Tl-decorated phosphorene. Meanwhile, the adsorption energy (Eads) shows the trend of Eads(H2S) < Eads(SO2) < Eads(SO3) for the same metal decoration case. In addition, some metal-decorated phosphorene systems exhibit intriguing magnetic and electrical variation after sulfur-based gas molecule adsorptions, indicating that these systems are promising to be candidates for the detection and removal of sulfur-based gas molecules.

Preparation of a new high-performance calcium-based desulfurizer using a steam jet mill

Dry flue gas desulfurization is an increasingly attractive technique in SO₂ emission control. However, the low efficiency in dry desulphurization is the bottleneck of this technology. To find a high-performance desulfurizer is an urgent task. This research utilized a steam jet mill digestion to prepare a desulfurizer at steam temperature of 220 ℃ and pressure of 0.45 MPa, and compared this product with the conventional digestion desulfurizer. Our results show that the digestion in steam jet mill can transform all the calcium oxide into calcium hydroxide. The calcium hydroxide had good fluidity and with honeycomb morphological characteristics. The experiments of dry flue gas desulfurization demonstrated that under the relative humidity of 15, 30 and 45%, the total dead times were 340, 640 and 720 min, the working time for keeping a 100% desulfurization efficiency were 120, 420 and 580 min, and the total sulfur fixation were 124.05, 274.58 and 332.09 mg. Compared with the desulfurizer by conventional dry digestion, the desulfurizer prepared in this research had a significantly superior performance. This experiment provides a new method for high-performance desulfurizer via quicklime digestion, which is an important step in pushing forward the application of dry flue gas desulfurization.

Subtropical Forests Act as Mercury Sinks but as Net Sources of Gaseous Elemental Mercury in South China

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China in 2014 to quantify Hg (gaseous elemental Hg, Hg⁰, and reactive Hg, Hg^II) input and output fluxes and Hg retention in forests, consequently exploring the roles of subtropical forests in the global Hg cycle. At site Qianyanzhou, representing a background region with an enhanced atmospheric Hg⁰ concentration, the total Hg^II deposition (67.7 μg·m^-2·year^-1, 73% as dry Hg^II deposition) was found to be slightly higher than the Hg⁰ emission above the canopy (58.5 μg·m^-2·year^-1), indicating that the forest is a minor Hg sink but a significant net Hg⁰ source on a yearly basis. In contrast, the forest in the moderately polluted region (site Huitong) acted as a significant Hg sink but a minor net Hg⁰ source with a higher Hg^II deposition (73.7 μg·m^-2·year^-1) and relatively negligible Hg⁰ emission (2.65 μg·m^-2·year^-1). The decreasing atmospheric Hg⁰ concentrations declined the total Hg sink based on the Hg budgets synthesized of this and previous studies and may promote forest Hg⁰ emissions. Consequently, it was expected that the re-emission of historically deposited Hg may be enhanced from subtropical forests by recent decreases in atmospheric Hg⁰ concentrations throughout China.

A Bibliometric Analysis of Research on Acid Rain

With the continuation of industrialization and urbanization, acid rain (AR) has aroused extensive concern because of its potential negative effects on ecosystems. However, analysis of the current status and development trends in AR research area has seldom been systematically studied. Therefore, we motivated to conduct a bibliometric analysis of AR publications (1900–2018) using HistCite and CiteSpace software programs. Compared to traditional reviews by experts, this study offers an alternative method to quantitatively analyze and visualize the development of AR field at a large time scale. The results indicated that the overall concern of AR research studies had increased from 1900 to 2018. The most productive country was the United States, while the institution with the most publications was Chinese Academy of Sciences. “Environmental Sciences” was the most popular subject category, Water, Air, and Soil Pollution was the dominant journal, and C.T. Driscoll was the most prominent author in AR field. There were three hotspots in the field of AR, including analyzing AR status and its control policies in Europe, the United States, and China in the past few decades, investigating the ecological consequences of AR on plant histological, physiological, and biochemical traits, as well as surface water and soil properties, and the model application for quantitatively assessing AR and its effects on terrestrial and aquatic ecosystems at regional scale. Further, “behavior”, “phosphorus”, “fractionation”, “soil acidification”, “corrosion”, “performance”, “recovery”, “rainwater”, “trace element”, and “surface water” have been emerging active topics in recent years. This study can help new researchers to find out the most relevant subject categories, countries, institutions, journals, authors, and articles, and identify research trends and frontiers in the field of AR.

High hydrogen sulfide emissions from subtropical forest soils based on field measurements in south China

The present estimate of global hydrogen sulfide (H₂S) emission from natural sources has large uncertainty mainly due to the lack of valid field data, particularly for subtropical forest soil in China with elevated atmospheric S deposition. In this study, the field observation of H₂S fluxes over subtropical forest soil was conducted for the first time in south China to measure the magnitude of emissions of H₂S, and evaluate its contribution to the large S sink, using the Dynamic Flux Chamber (DFC) method. Daily variations of H₂S fluxes showed an increasing emission with the increasing air temperature in the morning, a peak at the middle of the day, and a decreasing emission thereafter, then approximated to zero at night. The H₂S flux had positive values in all seasons, with the highest in summer, followed by spring, and relatively lower values in fall and winter. The H₂S flux measurements showed relatively large emission with annual average value of 0.028 g S m^-2 yr^-1, possibly due to the elevated sulfate concentration in the soil solution by S deposition, the hot and humid climate, as well as the lower soil pH in subtropical China. Thus, not only tropical, subtropical soils need to be included as significant H₂S sources to accurately portray the global H₂S budget. Although the soil in subtropical forests acted as a strong source for H₂S to atmosphere, H₂S emission from soil had limited contribution (about 0.2%) to the large S sink in this forest catchment.