Taiwan Basin Research Articles

Role of an Ultra-Large Coal-Fired Power Plant in PM2.5 Pollution in Taiwan

Taichung Power Plant (TPP) is Taiwan’s largest coal-fired power plant and is considered a major source of air pollution. During periods of deteriorating air quality, it is often required to reduce the load to reduce emissions. However, frequent power load shedding not only requires cost but also requires safety considerations. Therefore, it is necessary to explore the role that thermal power plant emissions play in air pollution in Taiwan. This study employed the Community Multiscale Air Quality modeling system with the brute-force method to analyze the PM2.5 concentration contributed by TPP. The results showed that among the various air basins in Taiwan, the Yun-Chi-Nan air basin (YCNAB), located to the south of TPP, was most severely affected by TPP’s emissions, with an annual average affected concentration of 1.0 µg m−3 (3.3%). However, when serious PM2.5 pollution events (daily concentration > 70 µg/m3) occurred due to low wind speeds, the Central Taiwan air basin (CTAB), where TPP is located, became the area most severely affected by TPP’s emissions. The low wind speed was caused by the interaction between the easterly wind field around Taiwan and Taiwan’s north–south mountain ranges. When this happens, TPP’s emissions would have a greater impact on the PM2.5 concentration at nearby stations in the CTAB and YCNAB, up to about 11%. Overall, on pollution days caused by low wind speeds, the largest TPP load reduction (40%) still had a certain effect as an emergency measure to improve the high PM2.5 pollution in central and southern Taiwan.

Seismically Imaged Crustal Breakup in the Southwest Taiwan Basin of the Northeastern South China Sea Margin

AbstractThe Southern Depression of the Southwest Taiwan Basin is an ENE‐trending wide rift in the northeastern South China Sea margin. Though known as highly thinned, the maximum degree of crustal extension remains unclear. Here, we present three NW‐trending multi‐channel seismic profiles crossing the depression askew, and another profile traversing it, all of which image the basement and Moho clearly. The basement deepens toward the rift axis while the Moho shoals toward the axis, indicating significant crustal thinning. Faults appear with only small throws and are usually untraceable to depth. As the integral fault offset seems insufficient to match the horizontal crustal extension, the high crustal thinning is mainly accommodated by detachment faults. Only small‐scale post‐rift magmatic bodies and sporadic sills are observed, and syn‐rift magmatism is poor. The depression shows a recumbent V‐shaped valley opening to the northeast with the width increasing from 50 km on the southwestern profile to 96 km on the northeastern profile. Along the valley axis, the crust thins from 8.1 to 3.6 km and more thinner to the northeast, demonstrating enhanced extensional degree toward the northeast. A unique conch‐shaped package (up to ∼2 s two‐way travel time thick) with moderately coherent reflections is imaged atop the axial mantle on the northeastern profile. It was possibly formed by mantle exhumation, syn‐rift sedimentary uplift, or volcanic eruption. All these possibilities indicate that the crust probably broke up there, which represents a special style of crustal thinning different from other magma‐poor margins.

Baseflow Variation in Southern Taiwan Basin

Baseflow is among the most important components of streamflow. It is the main source of streamflow from groundwater systems in the dry season and also plays an important role as a water resource in the ecological environment and for human activities. In recent years, because of climate change, the number of dry season days in Taiwan has increased, and the wet season has been delayed, resulting in fewer rainy days and increased precipitation intensity. In addition, the spatial distribution of rainfall is uneven, and rivers are short and fast-flowing. Taiwan has become a country with abundant rainfall but insufficient water resources; therefore, the assessment of baseflow is important. This study selected eight basins with distinct wet and dry seasons in southern Taiwan as the study area. The baseflow characteristics and their relationships with climate features were assessed using time series analysis, baseflow attribution analysis, and wavelet analysis. The results showed that baseflow has an increasing trend; both precipitation and evapotranspiration have a significant positive correlation with baseflow, and the impact of precipitation is greater than that of evaporation. Sensitivity analysis showed that baseflow increases with increasing evaporation and precipitation; this behavior is related to the concentration of precipitation and evaporation in the wet season. Baseflow attribution analysis showed that the contribution of climate change to baseflow (75.0%) was larger than that of human activities (−2.9%), indicating that climate change was the main factor in the increase in baseflow. Wavelet analysis showed that both the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are correlated with baseflow, where the PDO is more strongly correlated than the ENSO. The main timescales of the ENSO and PDO are a 4–8-year band and an 8-year band, respectively. The ENSO may have a timescale above the 8-year band, and the PDO exhibits periodic correlation changes at a 1-year band.

Marine sediment nitrogen isotopes and their implications for the nitrogen cycle in the sulfate-methane transition zone

IntroductionRecent work has proposed that the nitrogen isotopes in marine sediments can be impacted by anaerobic oxidation of methane (AOM), since nitrogen uptake by anaerobic methanotrophic archaea (ANME) modifies the nitrogen isotope compositions of bulk sediment. Thus, unraveling the AOM-driven nitrogen cycle in the sulfate-methane transition zone (SMTZ) becomes significant. Additional study of the nitrogen cycle between sediment and interstitial water in SMTZ is needed.MethodsTo better understand the nitrogen cycle in the SMTZ, we analyzed NH4+ concentrations of interstitial water and nitrogen isotopes of sediment in the core GC10 from the southwestern Taiwan Basin in the South China Sea.ResultsThe defined SMTZ is located at 560–830 cmbsf, based on methane and sulfate concentrations, as well as TS/TOC ratios, δ13CTIC and δ34S values. In the SMTZ, the NH4+ concentration decreases, the δ15NTN shows a negative excursion, δ15Ndecarb displays a positive excursion.DiscussionsNH4+ concentration decrease is interpreted by sulfate-reducing ammonium oxidation (SRAO). The δ15NTN shows negative excursion, which is most likely interpreted to N2 (δ15N=0‰) released from SRAO that was fixed into marine sediment via ANME nitrogen fixation. The δ15Ndecarb shows a negative correlation with NH4+ concentrations, indicating that it was controlled by organic matter decomposition. In the SMTZ, the methane competes with organic matter for becoming the substrate of sulfate reduction bacteria, which possibly decreases the organic matter degradation rate and causes δ15Ndecarb relative positive excursion. Although δ15Ndecarb is controlled by organic matter degradation, δ15NTN still reveals a negative excursion in the SMTZ. This likely indicates that nitrogen uptake by ANME/AOM microbial consortiums mainly modifies the nitrogen isotope of soluble nitrogen in the SMTZ.ConclusionsThis study indicates unique geochemistry processes in SMTZ will modify nitrogen characteristics in sediment/interstitial water, and the latter can serve as a proxy for AOM.

Prediction of soil erosion and sediment transport in a mountainous basin of Taiwan

Soil erosion substantially implicates global nutrient and carbon cycling of the land surface. Its monitoring is crucial for assessing and managing global land productivity and socio-economy. The Zhuoshui River Basin, the largest catchment, in Taiwan is highly susceptible to soil erosion by water due to extremely high rainfall, rugged terrain, easily eroded soil, and intensively agricultural cultivation over the steep land. Hence, this study examines the annual soil erosion rate for 2005, 2011, and 2019 and the average long-term soil erosion and sediment yield (SY) during 2005–2019. Coupling of the Revised Universal Soil Loss Equation (RUSLE) and sediment delivery ratio (SDR) models is implemented using remote sensing and GIS techniques. The soil erosion rate is classified into five classes, namely mild (0–10 t ha−1 year−1), moderate (10–50 t ha−1 year−1), moderately severe (50–100 t ha−1 year−1), severe (100–150 t ha−1 year−1), and very severe (> 150 t ha−1 year−1). Over one half of the total area is categorized as moderate and moderately severe classes, and one-third of the whole basin as severe and very severe classes. Recently, mild and moderate classes increase, while moderately severe, severe, and very severe decrease. During 2005–2019, the annual soil loss rate ranges from 0.00 to 6,881.88 t ha−1 year−1 with an average rate of 122.94 t ha−1 year−1. Among the SDR models, the RUSLE combined with the SDR model with the length and slope gradient of mainstream shows satisfactory sediment yield estimation. Predictably, the downstream receives a massive sediment delivery from all upper streams (246.06 × 106 t year−1), and the percent bias values for all sub-basins are below ± 39.0%. The study provides a rapid approach to investigate soil erosion and sediment yield, and it can be applied to the other basins in Taiwan. More importantly, information about spatial patterns of soil erosion and SY is critical to establish suitable measures to achieve effective watershed planning and optimize the regional productivity and socio-economy. The proposed approach is potentially to identify risk areas, conduct scenario estimation for management, and perform spatiotemporal comparison of soil erosion, while adjustment in the empirical formulas of the proposed approach may be needed when it is applied to the other regions, especially outside Taiwan.

Spatial distribution and origin of the high-velocity lower crust in the northeastern South China Sea

The origin of seismically-defined high-velocity lower crust (HVLC) is an important scientific issue that has attracted much debate. This study compiles 12 published wide-angle seismic velocity models from the northeastern South China Sea (SCS) to define the spatial distribution and origin of the HVLC. We have compiled the P-wave seismic velocities and interface depths of sediment, crust (including the HVLC), and mantle from each seismic profile. A pseudo-3D seismic velocity model of the northeastern SCS is then built using Kriging interpolation. Sediment thickness is 4–12 km in the Southwest Taiwan Basin and is 8–12 km at the Hengchun Ridge due to an abundant source of sediments. On a regional scale, the crust gradually thins from continental shelf and Taiwan Island toward ocean basin. Moreover, a very high crustal stretching factor (5–7) within the northeastern Southwest Taiwan Basin shows an extremely extended crust that is interpreted as a failed rift. The HVLC is spatially uneven in distribution, and seems to be divided into two parts along the paleo-Pacific subduction zone. The northwestern HVLC, located in front of the paleo-Pacific subduction zone, is distributed in a continuous stripe shape and overlapped almost with high magnetic anomalies; whereas the southeastern HVLC is distributed in the form of discrete blocks. The former one is formed by residual magmatic materials in response to the paleo-Pacific subduction, and the latter one is inferred to originate from post-rift magmatic modification.

Cenozoic evolution of the Yangtze River: Constraints from detrital zircon U[sbnd]Pb ages

The development of modern drainage patterns in eastern Tibet is closely related to uplift of the Tibetan Plateau. The Yangtze River is the longest river in Asia. When and how this river formed is still vigorously debated. Here we compile published detrital zircon UPb ages from the Chuxiong, Sichuan, Jianghan, Jianchuan, Simao, Vientiane, Song Hong-Yinggehai basins, marginal sea basins in East China and Taiwan to reconstruct the evolution of the Yangtze River. Reassessment of published detrital zircon data from the broader region sheds new light on the development of the Yangtze. Detrital zircon data from the Jianghan Basin, marginal sea basins in East China and Taiwan suggest that the pre-Miocene Yangtze River did not include the present upper reaches. The detrital zircon ages from the Song Hong-Yinggehai Basin suggest that the upper Yangtze River did not flow southward into the Red River since the Oligocene. Instead, similarities in detrital zircon UPb ages between the Sichuan, Chuxiong and Simao basins imply that the paleo-upper Yangtze flowed southwestward into the Vientiane Basin via the Chuxiong and Simao basins before the Miocene. The appearance of Cenozoic zircons in the Miocene sediment in the Jianghan Basin suggests that the southwestward-flowing paleo-upper Yangtze River was captured by the mid-lower Yangtze between the late Oligocene and the early Miocene.

Response of Terrigenous Sediment Input to Sea Level Change and East Asian Monsoon Evolution Since 30kyr in the Southwestern Taiwan Basin

Response of Terrigenous Sediment Input to Sea Level Change and East Asian Monsoon Evolution Since 30kyr in the Southwestern Taiwan Basin

Spatiotemporal Variation of the Meteorological and Groundwater Droughts in Central Taiwan

In recent years, Taiwan has been facing severe water shortages due to extreme drought. In addition, changes in rainfall patterns have resulted in an increasingly notable drought phenomenon, which affects the management and utilization of water resources. Therefore, this work examines basins in Central Taiwan. Long-term records from 13 rainfall and 17 groundwater stations were selected. The Standardized Precipitation Index (SPI) and Standardized Groundwater Level Index (SGI) were used to analyze the drought characteristics of this region. The rainfall and groundwater level data from basins in Central Taiwan were analyzed in this study. The results show that the year 2015 experienced extreme drought conditions due to a correlation with SPI and SGI signals. In addition, with regard to groundwater drought, more drought events occurred in the Da'an River basin; however, the duration and intensity of these events were relatively low, in contrast to those of the Wu River basin. Finally, the correlation between SPI and SGI was observed to vary in different basins, but a certain degree of correlation was observed in all basins. The results show that drought intensity increases with longer drought durations. Moreover, severe droughts caused by rainfall tend to occur at a greater frequency than those caused by groundwater.

Characteristics of a laccolith along the LRTPB fault zone between Pearl River Mouth Basin and Southwest Taiwan Basin

Characteristics of a laccolith along the LRTPB fault zone between Pearl River Mouth Basin and Southwest Taiwan Basin

Attribution of Streamflow Variations in Southern Taiwan

Climate change and anthropogenic activity are the main factors impacting the hydrological environment. For sustainable water utilization, identifying the impact contribution of these two factors on the streamflow variations is an important topic in recent research. In this study, seven river basins in southern Taiwan were selected as the study area to evaluate the annual streamflow from 1980 to 2017. The decomposition and elasticity methods based on the Budyko hypothesis were applied to quantify the contribution of climate and anthropogenic factors to the streamflow variations. In addition, the normalized difference vegetation index (NDVI) was used to represent the actual situation of land cover and verify the parameters in the Budyko equation. The two quantitative methods consistently demonstrated that the streamflow variations from pre- to post-period occurred due to the climate factor. The elasticity coefficient of variables demonstrated that the streamflow change is more sensitive to precipitation and this influence reduces from pre- to post-period as the streamflow increase. In the NDVI variations, except for the Yanshui and the Linbain rivers, the Budyko equation parameters changed consistently with NDVI. The present study provides effective results on the contribution of streamflow variations in southern Taiwan to serve as a reference for future water management.

Climate Change Impacts on Soil Erosion and Sediment Yield in a Watershed

This study analyzed the influence of climate change on sediment yield variation, sediment transport and erosion deposition distribution at the watershed scale. The study was based on Gaoping River basin, which is among the largest basins in southern Taiwan. To carry out this analysis, the Physiographic Soil Erosion Deposition (PSED) model was utilized. Model results showed a general increase in soil erosion and deposition volume under the A1B-S climate change scenario. The situation is even worsened with increasing return periods. Total erosion volume and total sediment yield in the watershed were increased by 4–25% and 8–65%, respectively, and deposition volumes increased by 2–23%. The study showed how climate change variability would influence the watershed through increased sediment yields, which might even worsen the impacts of natural disasters. It has further illustrated the importance of incorporating climate change into river management projects.

Stochastic Model for Drought Forecasting in the Southern Taiwan Basin

The global rainfall pattern has changed because of climate change, leading to numerous natural hazards, such as drought. Because drought events have led to many disasters globally, it is necessary to create an early warning system. Drought forecasting is an important step toward developing such a system. In this study, we utilized the stochastic, autoregressive integrated moving average (ARIMA) model to predict drought conditions based on the standardized precipitation index (SPI) in southern Taiwan. We employed data from 1967 to 2006 to train the model and data from 2007 to 2017 for model validation. The results showed that the coefficients of determination (R2) were over 0.80 at each station, and the root-mean-square error and mean absolute error were sufficiently low, indicating that the ARIMA model is effective and adequate for our stations. Finally, we employed the ARIMA model to forecast future drought conditions from 2019 to 2022. The results yielded relatively low SPI values in southern Taiwan in future summers. In summary, we successfully constructed an ARIMA model to forecast drought. The information in this study can act as a reference for water resource management.

Using the Markov Chain to Analyze Precipitation and Groundwater Drought Characteristics and Linkage with Atmospheric Circulation

In recent years, Taiwan has been facing water shortages due to the impact of climate change, which has resulted in many serious drought events, especially in southern Taiwan. Long-term records from 25 rainfall stations and 17 groundwater stations in the southern Taiwan basin were used in this study. We used the Standardized Precipitation Index (SPI) and the Standardized Groundwater Level Index (SGI) and employed the first-order Markov chain model and wavelet transform to determine the drought characteristics and propagation, including the steady-state probabilities of drought events and the mean duration for each station. The Drought Index (DI) was also used to investigate the effects of rainfall on groundwater drought. The results show that the steady-state probability of the meteorological drought in the Yanshui River basin in southern Taiwan is higher than that in other basins. The area with the longer mean duration is located in the Yanshui River basin and the Erren River basin, and overall, the mean duration ranges from 3 to 7 months. In addition, the results from the drought proneness analysis indicated that when rainfall causes a longer drought duration, there will be a higher degree of proneness to groundwater drought in the future. Finally, the results show that the mean duration of groundwater droughts are longer than those of meteorological droughts. The results of the wavelet analysis revealed a positive correlation at long-term scales, which may be related to large-scale atmospheric circulation. The information from this research could be used as a reference for water resource management in the future.

Evaluation of basin storage–discharge sensitivity in Taiwan using low‐flow recession analysis

AbstractSensitivity analysis of the hydrological behaviour of basins has mainly focused on the correlation between streamflow and climate, ignoring the uncertainty of future climate and not utilizing complex hydrological models. However, groundwater storage is affected by climatic change and human activities. The streamflow of many basins is primarily sourced from the natural discharge of aquifers in upstream regions. The correlation between streamflow and groundwater storage has not been thoroughly discussed. In this study, the storage–discharge sensitivity of 22 basins in Taiwan was investigated by means of daily streamflow and rainfall data obtained over more than 30 years. The relationship between storage and discharge variance was evaluated using low‐flow recession analysis and a water balance equation that ignores the influence of rainfall and evapotranspiration. Based on the obtained storage–discharge sensitivity, this study explored whether the water storage and discharge behaviour of the studied basins is susceptible to climate change or human activities and discusses the regional differences in storage–discharge sensitivity. The results showed that the average storage–discharge sensitivities were 0.056 and 0.162 mm−1 in the northern and southern regions of Taiwan, respectively. In the central and eastern regions, the values were both 0.020 mm−1. The storage–discharge sensitivity was very high in the southern region. The regional differences in storage–discharge sensitivity with similar climate conditions are primarily due to differences in aquifer properties. Based on the recession curve, other factors responsible for these differences include land utilization, land coverage, and rainfall patterns during dry and wet seasons. These factors lead to differences in groundwater recharge and thus to regional differences in storage–discharge sensitivity.

Carbon and oxygen isotope composition of carbonate in bulk sediment in the southwest Taiwan Basin, South China Sea: Methane hydrate decomposition history and its link to mud volcano eruption

Abstract Anaerobic oxidation of methane (AOM) coupled with sulfate reduction (SR) generates authigenic carbonate which has the unique carbon and oxygen isotope composition in sulfate methane transition zone (SMTZ). The carbon and oxygen isotope composition of carbonate in sediments, which can be used to revel the variable methane flux, is affected by the percentage of AOM-driven carbonate in sediments. To investigate the methane hydrate decomposition history in the southwest Taiwan Basin, we analyzed a 13 m-long sediment core for grain size, carbonate and elemental carbon contents and stable carbon and oxygen isotopes composition of carbonate (δ13Ccarb and δ18Ocarb). The AMS 14C dating results and carbonate content reveal that a turbidity current occurred at around 14 ka B.P., which reversed sediment accumulation between 455 and 885 cm in depth. Combining the chronological framework and δ13Ccarb values, we identified that the methane hydrate decomposition scale began to decrease at 14 ka B.P. Moreover, we discovered that three transiently enhanced methane hydrate decomposition events occurred from then on. Although the δ13Ccarb and δ18Ocarb variability are as expected, the low δ18Ocarb values are inconsistent with the previous theory which favors 18O-enriched in AOM-driven authigenic carbonate. Thus, we suggest that a large amount of 18O-depleted water which originally migrated from southwest Taiwan island was injected into surface sediment along with the eruption of mud volcanos. As the terminal electron accepter of AOM-SR reaction, the exogenous 18O-depleted water leads its special oxygen isotope to be inherited by the authigenic AOM-driven carbonate. The proposed theory provides new insights on the authigenic carbonate in methane hydrate area and the association between methane hydrates and mud volcanoes.

Carbon Geochemistry of Gas Hydrate‐associated Sediments in the Southwestern Taiwan Basin

AbstractMarine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate‐bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973‐3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon (DIC) and high total organic carbon (TOC) (mean = 0.46%) originated from terrigenous organic matter (mean δ13CTOC value of −23.6±) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon (IC), and the depleted δ13CIC, coupled with the increase of TOC and the decrease of δ13CTOC values owing to the methanotroph ‘Methanosarcinales/ANME’ in 430–840 cm. Environmental factors and archaeal communities in core 973‐3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate‐associated sediments.

Intensity of methane seepage reflected by relative enrichment of heavy magnesium isotopes in authigenic carbonates: A case study from the South China Sea

Changing conditions including steep redox gradients result in different Ca-Mg-carbonates forming at methane seeps as a consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Mg2+ is one of the main cations in Ca-Mg-carbonates, Mg isotope composition may reflect the mode of mineral authigenesis at seeps. Seep carbonates were collected from the Shenhu area and the Southwest (SW) Taiwan basin in the South China Sea. The carbonate phases in Shenhu samples are dolomite accompanied by accessory Mg-calcite, while SW Taiwan samples consist of Mg-calcite and dolomite. Correspondingly, Shenhu carbonates show systematically higher Mg/Ca ratios. Low δ13C values of seep carbonates confirm their derivation from the oxidation of methane, δ18O values reflect formation in equilibrium with coeval seawater. The δ26Mg value of a reference sample of biodetrital carbonate (−4.28‰) is lower than those of seep carbonates (−3.25 to −2.95‰). Since only little variability of δ26Mg values of pore waters is expected based on previous work, differences in the δ26Mg values of seep carbonates were apparently caused by changing degrees of isotopic fractionation during precipitation. Trends between δ26Mg values and Mg/Ca ratios and between δ26Mg and δ13C values suggest that Mg isotope fractionation was controlled by a kinetic mechanism affecting the incorporation of Mg2+ ions into the carbonate lattice in the course of SD-AOM. By consuming sulfate and by producing sulfide, SD-AOM reduces the energy differences for the dehydration of ions of light and heavy Mg isotopes, lowering isotope fractionation. Additionally, the two trends have been found to be steeper for Shenhu samples, suggesting more pronounced SD-AOM in the Shenhu area. Our study indicates that Mg isotope composition of methane-derived carbonates is affected by the process that drives carbonate precipitation – SD-AOM. Future work is required to confirm the utility of Mg isotopes as a new proxy for this major biogeochemical process.

Origin of the Dongsha Event in the South China Sea

Post-rift tectonic activities have been widely observed in the northern continental margin of the South China Sea, especially during the late Miocene. Large numbers of faults became active. Unconformities, uplift of faulted blocks, sequence tilting, erosion along the Dongsha massif and canyon incision were also discriminated at this stage in the Pearl River Mouth basin (PRMB) and the area to the east. This tectonism has been named Dongsha Event. A number of hypotheses have been put forward to explain the mechanism of the Dongsha Event, such as high-velocity lower crustal flow, magmatic underplating, and arc–continent collision. To investigate the tectonic dynamics, sequence contact relationships, fault activities, and magmatism were analyzed along large numbers of seismic profiles that cover the eastern PRMB and Southwest Taiwan Basin. The timing, affected regions, and differences in the intensity of tectonic deformation were assessed, upon which the plate bending model was favored. In order to check the reasonableness of plate bending model, effective elastic thickness and other geodynamic parameters were calculated constrained by uplift area width and regarding the trench as sediment filling. A maximum Te value of ~27 km and a minimum value of ~4 km were obtained. Integrating with the former stress field calculation, we conclude that the Dongsha Event was mainly affected by subduction and collision of the South China Sea toward the Philippine Sea plate. This event commenced at about 10 Ma and peaked at around 3.6 Ma. Although the high effective elastic thickness required is a problem to be addressed, this research provides by far the most comprehensive evidences to the mechanism of the Dongsha Event.

Modeling approach for emissions reduction of primary PM2.5 and secondary PM2.5 precursors to achieve the air quality target

The study proposes an approach to quantify necessary emission reductions of various related pollutants to meet national PM2.5 air quality standard (AQS). Many scenarios of emission reductions are simulated to establish the relationships between the reduction ratios of air pollutants and improvements of atmospheric PM2.5 concentration for specific air basins in Taiwan. The air pollutants include primary PM2.5 and secondary PM2.5 precursor species, such as nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic compounds (VOCs), and ammonia (NH3). These relationships were used to estimate the required amounts of emission reduction for various pollutants to achieve the national PM2.5 AQS of annual average with 15μgm−3. Three air basins of KPAB, YCNAB and CTAB with severe PM2.5 pollution in Taiwan were selected as case studies. The results follow: (1) The primary PM2.5 emission reductions and the PM2.5 concentration improvements exhibit linear relationships, and its effects are relatively significant. (2) Among the secondary PM2.5 precursor species, emission reductions of NOx and NH3 will have greater effectiveness, but higher proportions (>50%) of emission reductions are needed. (3) In terms of mitigating PM2.5 pollution, the priorities for pollutant emission control should be in the order of primary PM2.5, NOx, and SOx, while NMHC and NH3 should not currently be a focus. (4) If three air basins of KPAB, YCNAB and CTAB with high PM2.5 levels are required to meet the AQS, it is necessary to reduce primary PM2.5, NOx and SOx emissions simultaneously in case that LRT effect remains constant. Under the premise of meeting the AQS for the three air basins, the reduction of primary PM2.5, NOx and SOx emission in Taiwan should be up to 76%, 93% and 20% respectively. Fortunately, many efforts have been and will be done to improve the PM2.5 concentration in China. Those efforts will mitigate significantly the loading of emission reduction and AQS should be more possible to achieve for all air basins in Taiwan.