A Perspective on the Controversy over Global Emission Fluxes of Microplastics from Ocean into the Atmosphere.

Since the transfer of microplastic across the sea-air interface was first reported in 2020, numerous studies have been conducted on its emission flux estimation. However, these studies have shown significant discrepancies in the estimated contribution of oceanic sources to global atmospheric microplastics, with evaluations ranging from predominant to negligible, varying by 4 orders of magnitude from 7.7 × 10-4 to 8.6 megatons per year, thereby creating considerable confusion in the research on the microplastic cycle. Here, we provide a perspective by applying the well-established theory of particulate transfer through the sea-air interface. The upper limit of global sea-air emission flux microplastics was calculated, aiming to constrain the controversy in the previously reported fluxes. Specifically, the flux of sub-100 µm microplastic cannot exceed 0.01 megatons per year, and for sub-0.1 µm nanoplastics, it would not exceed 3 × 10-7 megatons per year. Bridging this knowledge gap is crucial for a comprehensive understanding of the sea-air limb in the "plastic cycle", and facilitates the management of future microplastic pollution.

Rivers of plastic: A socio-economic and topographic approach to modeling plastic transport from catchment to sea.

Marine litter caused by discharge of mismanaged plastic waste is considered to be one of the major environmental challenges by the international society. With the annual increase of plastic production, a growing number of plastic products are being used in people's daily lives. A large number of these plastics end up as waste emitted into rivers and subsequently into oceans through the effects of downpours or wind, posing a threat to the marine ecosystem. In this study, we developed a riverine plastic transport model based on catchment topography and social-economic factors. By applying reasonable compromise on the complexity of the model, this compromised simplified process-based model has the innovative capability to estimate plastic emissions effectively under the current conditions of limited data availability for model inputs. Compared to existing models, this novel model can also resolve challenges related to the contributions of various land use types and transport stages to plastic emissions into the oceans. To further explore the applicability of our results on a global scale, certain input parameter such as the proportion of mismanaged waste is crucial for users to acquire. Here, taking the S river catchment as our study area, the tourism-driven seasonal variation of land-based plastic emissions was quantified. According to our estimation, the emission flux in S river catchment in 2020 was 68 to 280 tons. 62.4% of riverine plastics reached the ocean. Although urban areas are the predominant contributors to the total emission flux, the contributions from other land use types such as forests and cultivated areas are also unignorable. For instance, forests and cultivated areas contribute 25.7% and 6.3%, respectively, even surpassing the contributions from high tourist activity (5.8%). Stricter waste collection legislations are imperatively needed particularly in these regions.

VOCs emission from a final landfill cover system induced by ground surface air temperature and barometric pressure fluctuation.

Volatile organic compounds (VOCs) emission flux and their concentration profiles were measured at a final municipal solid waste (MSW) landfill cover in Hangzhou, China. The influencing parameters, especially ground surface air temperature and pressure were monitored concomitantly. Furthermore, a numerical model incorporating coupled thermo-hydro-chemical interaction to assess VOCs emission from this final landfill cover (LFC) system was developed and validated with the field test results. The tested total VOC emission flux from the final cover is 0.0124 µg/m2/s, which indicates that the total amount of VOCs emitted into the atmosphere is 391 mg/m2 annually. Among these, dichloromethane (DCM) dominated VOCs emission flux during May, comprising 51.8% of the total emission flux. The numerical simulation results indicated that the diffusive emission flux of VOCs varied consistently with the fluctuation of atmospheric temperature. Whereas, the advective flux varied inversely with the fluctuation of barometric pressure. The highest difference in diffusive emission flux induced by temperature variation is 183 µg/m2/day and occurred in spring. Moreover, the results demonstrated that the impact of atmospheric temperature and pressure fluctuation on the emission of VOC from final covers is non-negligible when reasonably assessing the risks of landfill and landfill gas emission budget.

Multi-dimension analysis of volatile sulfur compound emissions from an urban wastewater treatment plant.

Long-term monitoring of volatile sulfur compounds (VSCs) released at the water-air interface from different treatment units of an anaerobic/oxic (A/O) wastewater treatment plant (WWTP) was carried out to assess the temporal and spatial emission characteristics of VSCs, to explore relationships between wastewater quality and VSC release. The VSC from non-aerated and aerated units were collected using dynamic and static chambers, respectively, and determined using gas chromatography. The VSC emission fluxes diminished in the order of primary sedimentation tank (PST) > anaerobic areas (ANA) > oxic section 1 (OX1). VSCs were not detected in the oxic section 2 (OX2), the oxic areas section 3 (OX3), and the final setting basin (FSB). Release capacities of VSCs descended in the order of summer > fall > spring > winter, with July, August, and September being the months with the highest VSC release capacities. VSC emission fluxes correlated well with wastewater temperatures, sulfate concentrations, and COD. VSC emission flux empirical equations based on wastewater temperature, sulfate concentrations, and COD were established. Based on the established VSC emission empirical equation, a control strategy to reduce the operating costs of deodorization facilities was proposed. This strategy is economically efficient and reduces the consumption of electrical energy.

Quantifying emission fluxes of atmospheric pollutants from mobile differential optical absorption spectroscopic (DOAS) observations.

This study demonstrates a mobile passive differential optical absorption spectroscopy (DOAS) based remote sensing method for quantifying the emission fluxes of soot pollutants. First, the mobile DOAS system scans the plume emitted from urban sources. Then, the DOAS method retrieves the total columns of pollutant gases along the measurement path. Combining the longitude, latitude, and mobile speed recorded by vehicle GPS, the net emission fluxes of NO2 and SO2 in the measurement area are calculated by coupling with the wind field data. The NO2 flux in the region is combined with the NO to NO2 concentration ratio in the Copernicus Atmospheric Monitoring Service (CAMS) model to calculate NOx net emission flux in the measurement period. We conducted the mobile DOAS measurements in the coal production area and obtained the distribution of pollutant gases along the measurement path. Meanwhile, the NO2 concentration distribution of the city and surrounding areas were reconstructed by using TROPOMI satellite data. During the mobile measurement, the net NO2 emission flux measured by mobile DOAS are in good agreement with satellite observations (R2 = 0.66). This study verified that the flux calculation method based on mobile DOAS can be used to detect urban soot pollutant gas emissions.

Dynamic chamber as a more reliable technique for measuring methane emissions from aquatic ecosystems.

Aquatic ecosystems are the largest natural source of atmospheric methane ("CH4") worldwide. However, the current estimation of CH4 emissions from aquatic ecosystems still has extensive uncertainty due to large spatiotemporal variations in CH4 emissions as well as significant uncertainty in measurement methods. In this study, we initially investigated CH4 fluxes from a simulated eutrophic water body by using static chamber method ("SC") during an incubation period of 36 days. Approximately 23 % of the total flux measurements were unsuccessful because they lacked a linear correlation between the accumulation of CH4 concentrations and enclosure time. CH4 fluxes could be achieved for most measurements. However, 5 min after enclosing, the initial CH4 concentrations measured in the chambers were too high (up to 507.4 ppm) to greatly suppress CH4 emissions from the diffusion process. Therefore, a dynamic chamber method ("DC") was developed to overcome the shortcomings of the SC. To achieve the DC, air samples must be continuously collected at the inlet and outlet of the dynamic chamber at fixed flow rates. In contrast to the SC, effective CH4 flux data could be obtained by the DC for each measurement at different frequencies. The DC measured the diel and daily variations in CH4 fluxes and the displayed CH4 emissions from the simulated water were highly irregular. The displayed emissions had variations up to more than two orders of magnitude. These results implied that the SC measured few intermittent fluxes that were difficult to represent the actual CH4 emissions from eutrophic water. The DC developed in this study considers the temporal variations in CH4 emissions from aquatic ecosystems. Thus, the DC is expected to be applicable in the field flux measurements of CH4 as well as other greenhouse gases to reduce emissions uncertainties.

Mitigation of methane emissions from three Danish landfills using different biocover systems.

The establishment of biocover systems is an emerging methodology in reducing methane (CH4) emissions from landfills. This study investigated the performance of three biocover systems with different designs (biowindow and passively and actively loaded biofilters) in mitigating CH4 emissions from three landfills in Denmark. A series of field tests were carried out to evaluate the functionality of each system, and total CH4 emissions from relevant landfill sections or the entire landfill were measured before and after biocover implementation. Surface CH4 concentration screening and local CH4 fluxes showed generally low emissions from the biowindow/biofilters (mostly < 5 g CH4 m-2 d-1), although some hotspots were identified on two actively loaded biofilters. One passively loaded biofilter exhibited high CH4 emissions, mainly due to gas overloading into the system. Gas concentration profiles measured at different locations suggested uneven gas distribution in the biofilters, and significant CH4 oxidation occurred in both the gas distribution layer (when oxygen was fed into the system) and the CH4 oxidation layer. High CH4 oxidation efficiencies of above 95% were found in all systems except for one biofilter (55%). Whole-site emission measurements showed CH4 reduction efficiencies between 29 and 72% after implementing biocover systems at the three landfills, suggesting that they were efficient in reducing CH4 emissions. The most challenging task for the passively loaded biocover systems was to control gas flow and secure homogenous gas distribution, while for actively loaded biocovers, it might be more important to eliminate emission hotspots for better functionality.

Study on carbon dioxide emission from reservoirs with different regulation types and its empirical prediction model.

The construction of artificial reservoirs with various regulation types on river is currently an important form of comprehensive utilization of water energy and water resources in river basins. The type of regulation is important in controlling the residence time, which in turn affects the photosynthesis-respiration balance in the water. This process has a significant impact on carbon dioxide (CO2) emissions from reservoirs. In this study, seasonal observations were carried out from September 2020 to July 2021 at five artificial reservoirs in the Qiantang River Basin, eastern China, to reveal the characteristics of CO2 emission from the water-air interface of reservoirs with different regulating types. The results showed that the annual average CO2 emission flux of the studied reservoirs varied significantly, ranging from 4.2 to 155.3 mmol m-2 day-1 with an average of 48.4 mmol m-2 day-1, which also had a significant negative correlation with the hydraulic retention time. While downstream of the dam, the annual average CO2 emission flux was quite high with a range of 105.8 to 543.0 mmol m-2 day-1, averaging 381.6 mmol m-2 day-1. This is mainly due to the release of water with high-concentration CO2 from the bottom of the reservoir. Additionally, using related data of reservoirs around the world, a CO2 emission model with hydraulic retention time, air temperature, and reservoir age as the primary parameters was developed, which was conducive to evaluate reservoir CO2 emissions on a larger scale and provided theoretical support for effective reservoir management.

[Estimation of Nitrous Oxide Emission from River System Based on Water Discharge and Dissolved Nitrous Oxide Concentration].

Due to increasing active nitrogen pollution loads, river systems have become an important source of nitrous oxide (N2O) in many areas. Due to the lack of monitoring data in many studies as well as the difficulty in estimating intermediate parameters and expressing temporal-spatial variability in current methods, a high level of uncertainty remains in the estimates of riverine N2O emission quantity. Based on the monthly monitoring efforts conducted for 10 sampling sites across the Yonganxi River system in Zhejiang Province from June 2016 to July 2019, the temporal and spatial dynamics of riverine N2O dissolved concentrations ρ(N2O), N2O fluxes, and their influencing factors were addressed. A multiple regression model was then developed for predicating riverine N2O emission flux to estimate annual N2O emission quantity for the entire river system. The results indicated that observed riverine ρ(N2O) (0.03-2.14 µg·L-1) and the N2O fluxes[1.32-82.79 µg·(m2·h)-1] varied by 1-2 orders of magnitude of temporal-spatial variability. The temporal and spatial variability of ρ(N2O) were mainly influenced by the concentrations of nitrate, ammonia, and dissolved organic carbon, whereas the N2O emission fluxes were mainly affected by river water discharges and ρ(N2O). A multiple regression model that incorporates variables of river water discharge and ρ(N2O) could explain 90% of the variability in riverine N2O emission fluxes and has high accuracy. The model estimated N2O emission quantity from the entire Yonganxi River system of 3.67 t·a-1, with 29% from the main stream and 71% from the tributaries. The IPCC default emission factor method might greatly overestimate and underestimate N2O emission quantities for rivers impacted by low and high pressures of human activities, respectively. This study advances our quantitative understanding of N2O emission for the entire river system and provides a reference method for estimating riverine N2O emission with more accuracy.

Significance of soil moisture on temperature dependence of Hg emission.

Soil moisture is a key factor for mercury (Hg) emission from soil. Despite its significance for Hg emissions, the effect of soil moisture on Hg flux and fractions has not been thoroughly investigated. The objective of this study was to elucidate the influences of soil moisture and temperature on Hg fluxes from soils and Hg fractions. A kinetic study was performed to measure Hg emission fluxes of six soil samples under different temperature (T) (15 °C, 20 °C, 25 °C, 30 °C, and 35 °C) and moisture conditions (0%, 10%, and 20% added water). The results showed that the Hg fluxes increased with increases in T and soil moisture. A linear correlation was found between ln (Hg emission flux) and 1/T for the six soil samples at different moisture contents (R2 = 0.73-0.99). The range of activation energy (Ea) values was 25.31-57.86 kJ/mol. The Hg fractions in soils of different moisture content were determined by a sequential extraction method. The results demonstrated that soil moisture affected the Hg fractions in soils. The Ea values had different relationships with soil moisture in different soils. There were correlations between Ea and the elemental and mercuric sulfide fractions for air-dried soils. However, for moist soils, Ea was negatively correlated with the water-soluble and acid-soluble fractions. Collectively, the combination of the Hg emission kinetics and Hg fraction measurement of different moist soils indicated that Hg emission was affected by both total Hg concentration and Hg fractions.

Modeling pesticides in global surface soils: Exploring relationships between continuous and discrete emission patterns.

Continuous pesticide emission at constant rate does not occur in reality, but can be a useful and simple concept in modeling studies. To explore the relationship between continuous and discrete emission patterns, we introduced a simple equivalent approach based on a comparison of simulated surface soil pesticide concentrations. The simulated results indicate that, at high soil pesticide dissipation rates and low emission frequencies, the average concentrations under the continuous and discrete emission scenarios were very similar. We demonstrated that the continuous emission model that used the simple average method to calculate the emission rate always overestimated the simulated pesticide concentrations in the surface soil compared to the discrete emission model when using a one-year period based on agricultural practices. In addition, we incorporated the equivalent approach into the USEtox model (a screening-level tool), which can approximate the average pesticide concentrations in surface soil using the time-integrated fate factors at different emission frequencies. The results indicate that the continuous-emission simulations agree with the discrete emission for at least 90% of the selected pesticides based on annual or semi-annual emission patterns. Further studies into other topics, such as random emission patterns and simulation periods, are required to improve the model. Nevertheless, the equivalent approach presented in this study can aid in transforming discrete emission patterns into continuous-emission-based models and improve surface soil pesticide management.

Characterizing ammonia emissions from water bodies using dynamic floating chambers.

Ammonia (NH3) is the most important alkaline gas in the atmosphere and plays a central role in atmospheric pollution and the global N cycle. Water bodies receive increasing nitrogen inputs from effluents and atmospheric deposition due to anthropogenic activities and are regarded as the major natural NH3 and NH4+ sinks. In this work, floating dynamic flux chambers were deployed at four types of freshwater (rivers, large reservoirs, medium-sized reservoirs and ponds) systems and a coastal seawater system to estimate the water-air NH3 emission fluxes. The NH3 emission fluxes of rivers (26.4 µg NH3 m-2 h-1) were significantly higher than those of other types of freshwater systems, and the NH3 flux of offshore water was unexpectedly high (3.9 µg NH3 m-2 h-1). The ammonium content and water temperature were the most important factors driving NH3 emissions from water bodies. The global NH3 emissions from water bodies reached 8.88 TgN a-1, and this value will increase persistently with global warming and water quality deterioration. Water bodies that are relatively eutrophic and directly affected by anthropogenic activities should be considered reservoirs of inputted N instead of permanent sinks.

Nitrous oxide (N2O) emissions from the high dam reservoir in longitudinal range-gorge regions on the Lancang-Mekong River, southwest China.

Nitrous oxide (N2O) is a greenhouse gas that should not be overlooked, and its emissions from plain reservoirs as well as small- and medium-sized reservoirs have been extensively studied; however, N2O emission patterns from high-dam reservoirs in longitudinal range-gorge regions remain unclear. In this study, the N2O concentration and emission flux from the high-dam Xiaowan Reservoir were investigated using static headspace gas chromatography and a boundary layer approach in the Lancang River. The factors influencing N2O production and emissions, especially the influence of damming, were explored. Our results demonstrated that the Xiaowan Reservoir, a source of N2O emissions, had an N2O emission flux of 15.48 ± 2.87 µmol m2·d-1 in 2019; the N2O concentration and emission flux exhibited an increasing trend along the flow direction within the Xiaowan Reservoir but decreased downstream of the dam. During the two water seasons, water temperature, the concentration of DO, NO3- and NH4+are all influencing factors of the N2O concentration in the XWR. the N2O in the XWR during the wet season was produced by nitrification, during the dry season the production mechanism of N2O was relatively complicated, but mainly produced by nitrification. This study advances our knowledge of N2O emissions from high-dam reservoirs in longitudinal range-gorge regions.

Mitigation of methane and trace gas emissions through a large-scale active biofilter system at Glatved landfill, Denmark.

Biocover systems are a cost-effective technology utilised to mitigate methane (CH4) and trace gas emissions from landfills. A full-scale biofilter system was constructed at Glatved landfill, Denmark, consisting of three biofilters with a total area of 3950 m2. Landfill gas collected mainly from shredder waste cells was mixed with ambient air and fed actively into the biofilter, resulting in an average load of 60-75 g m-2 d-1 for CH4 and 0.15-0.21 g m-2 d-1 for trace gases (e.g., aromatics, chlorofluorocarbons (CFCs), aliphatic hydrocarbons). The initial CH4 surface screening showed uneven gas distribution into the system, and elevated surface concentrations were observed close to the gas inlet. Both positive and negative CH4 fluxes, ranging from -0.36 to 4.25 g m-2 d-1, were measured across the surface of the biofilter. Total trace gas emissions were between -0.005 and 0.042 g m-2 d-1, and the emission flux of individual compounds were generally small (10-8 to 10-3 g m-2 d-1). Vertical gas concentration profiles showed that the oxidation of CH4 and easily degradable trace compounds such as aromatics and aliphatic hydrocarbons happened in the aerobic zones, while CFCs were degraded in the anaerobic zone inside the compost layer. In addition, oxidation/degradation of CH4 and trace gases also occurred in the gas distribution layer, which contributed significantly to the overall mitigation efficiency of the biofilter system. Overall, the biofilter system showed mitigation efficiencies of nearly 100% for both CH4 and trace gases, and it might have the potential to work under higher loads.

A highly agricultural river network in Jurong Reservoir watershed as significant CO2 and CH4 sources.

Freshwaters are receiving growing concerns on atmospheric carbon dioxide (CO2) and methane (CH4) budget; however, little is known about the anthropogenic sources of CO2 and CH4 from river network in agricultural-dominated watersheds. Here, we chose such a typical watershed and measured surface dissolved CO2 and CH4 concentrations over 2 years (2015-2017) in Jurong Reservoir watershed for different freshwater types (river network, ponds, reservoir, and ditches), which located in Eastern China and were impacted by agriculture with high fertilizer N application. Results showed that significantly higher gas concentrations occurred in river network (CO2: 112 ± 36 µmol L-1; CH4: 509 ± 341 nmol L-1) with high nutrient concentrations. Dissolved CO2 and CH4 concentrations were supersaturated in all of the freshwater types with peak saturation ratios generally occurring in river network. Temporal variations in the gas saturations were positively correlated with water temperature. The saturations of CO2 and CH4 were positively correlated with each other in river network, and both of these saturations were also positively correlated with nutrient loadings, and negatively correlated with dissolved oxygen concentration. The highly agricultural river network acted as significant CO2 and CH4 sources with estimated emission fluxes of 409 ± 369 mmol m-2 d-1 for CO2 and 1.6 ± 1.2 mmol m-2 d-1 for CH4, and made a disproportionately large, relative to the area, contribution to the total aquatic carbon emission of the watershed. Our results suggested the aquatic carbon emissions accounted for 6% of the watershed carbon budget, and fertilizer N and watersheds land use played a large role in the aquatic carbon emission.

Deriving emission fluxes of volatile organic compounds from tower observation in the Pearl River Delta, China.

Accurate estimation of speciated emissions of volatile organic compounds (VOCs) is challenging due to the complexity of both species and sources. Evaluation of the bottom-up emission inventory (EI) by atmospheric observation is needed to better understand the VOC emissions and then to control air pollutions caused by VOCs. This study conducts vertical measurements of VOCs between November 3 and 11, 2018 at the Canton Tower in the urban core of Pearl River Delta (PRD), China. A mixed layer gradient (MLG) technique is applied to the tower observation data to derive emission fluxes for individual VOC. The results show that the measured VOCs concentrations at ground level were always higher than those at the heights of 118 m and 488 m. Obvious vertical gradients of concentrations were found for VOC species, such as benzene, toluene and isoprene. The emission flux was estimated to be largest for propane (3.29 mg m-2 h-1), followed by toluene (2.55 mg m-2 h-1), isoprene (2.24 mg m-2 h-1), n-butane (2.10 mg m-2 h-1) and iso-pentane (1.73 mg m-2 h-1). The total VOC emission fluxes were around 3 times larger than those in the EI, suggesting 1.5-2 times underestimations of ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) by current EI. Substantial underestimations (3-20 times) were found for C2-C5 alkanes by current EI. Due to unmeasured input parameters, limited sample size and short sampling period, there are still large uncertainties (40%-117%) in the estimated emission fluxes for individual species. Whereas, this study shows that the tower observation and emission estimation using MLG method could provide useful information for better understanding vertical distributions and emission fluxes of VOCs, and pioneer in assessing the existing emission inventories at species-level and hour-level.

Comparison of Ammonia Emission Estimation between Passive Sampler and Chamber System in Paddy Soil after Fertilizer Application.

Ammonia (NH3) is an important precursor for particulate secondary aerosol formation. This study was conducted to evaluate the applicability of a passive sampler (PAS) for estimating the NH3 emission from chemical fertilizer application (85 kg-N·ha-1) at field scale and to compare the results with a chamber system for the calculation of NH3 emission flux at lab scale. The application of chemical fertilizer increased the ambient NH3 concentration from 7.11 to 16.87 µg·m-3. Also, the ambient NH3 concentration measured by the PAS was found to be highly influenced by not only the chemical fertilizer application but also the weather (temperature and rainfall). Wind rose diagram data can be useful for understanding the distribution of ambient NH3 concentration. In the case of a chamber with few environmental variables, NH3 was emitted very quickly in the early stages and gradually decreased, whereas it was delayed at intervals of about one week at the site. It was found that daily temperature range, atmospheric disturbance by wind and rainfall, changes in soil moisture, and the presence of a flooded water table were the main influencing factors. The PAS data and the chamber system data were observed to have significant differences in spatial-temporal scale. In order to reduce the gap, it seems to be necessary to further develop a chamber system, in order to improve the precision of field analysis and to strengthen the connection between experimental results.

[Relationship Between CO2 and CH4 Emissions in Urban Rivers and Sewage Discharging from a Municipal Drainage Network].

The increasing carbon emission of polluted rivers in urban areas is an environmental problem commonly faced by many cities in China, especially the megacities with vast populations. In this study, two typical rivers located in the megacity of Shanghai, including the suburban river network R1 and urban river R2 (in the central city), were investigated for their emission characteristics of CO2 and CH4 in dry and wet weather. We also analyzed the relationship between the state and type of river pollution and CO2 and CH4 emissions, and further explained the mechanisms of CO2 and CH4 emissions in urban rivers impacted by sewage discharged from the municipal drainage network. The results show that:① In dry weather, the average fluxes of CO2 and CH4 emitted from the river in the central city (R2) were (2.48±1.02) mmol·(m2·h)-1 and (1.21×10-2±0.71×10-2) mmol·(m2·h)-1, respectively. The average fluxes of CO2 and CH4 from the suburban river (R1) network were (1.53±0.39) mmol·(m2·h)-1 and (9.26×10-3±9.18×10-3) mmol·(m2·h)-1, respectively. In wet weather, affected by sewage from the municipal drainage network, CH4 flux emitted from the surface water of the R2 river downstream of the pump station P increased by up to 119 times that in dry weather. ② Global carbon emission statistics, involving the data from our study and from other rivers around the world, seemed to imply a relationship between the carbon emission flux and the pollution state of an urban river, i.e., the rivers with high pollution showed significant carbon emission intensity. ③ According to the results of PCA, organic matter can be an essential factor in driving the variation of carbon emissions, and this trend is evident in all the rivers in urban and suburban areas. The relationship between carbon emissions and nitrogen pollution in a river varies with different types of underlying riparian surface. In the less polluted urban rivers, the aquatic physical factor can also be an essential factor. ④ In the short term, with massive quantities of sewage discharged into urban rivers, a large amount of CH4 flux can be emitted. In contrast, in the long run, the carbon cycle can be strengthened when the carbon storage is increased, and thus the emission potential of CO2 and CH4 is improved.

Emission of CO2 and CH4 from a multi-ditches system in rice cultivation region: Flux, temporal-spatial variation and effect factors.

Man-made multi-level ditches system is designed to irrigate, drain and collect runoff from surrounding fields. It is not only the conduit of water and field carbon, but also the linear-like wetland with complex carbon cycling. However, the contribution of ditches system to CO2 and CH4 emission has rarely been assessed. To understand the emission pattern of CO2 and CH4 from ditches, this study investigated the emission fluxes of CO2 and CH4 in a three-level ditches system in Chengdu Plain, China. The results showed that the emission of CO2 and CH4 ranged from 70.38 to 950.40 mg C m-2 h-1 and 6.51-74.99 mg C m-2 h-1, respectively, and was higher in spring and summer than other seasons in all ditches (P < 0.05). On the other hand, the emission of CO2 and CH4 increased along with the decreasing ditches size. Besides, it is found that the precipitation, water table depth and water DO concentration might contribute to the emission of CO2, while CH4 was possibly influenced by precipitation, water table depth, temperature, water DO and DOC concentration. Moreover, it is suggested that terrestrial external input and in-situ metabolism might be the main sources of C emission, and in-situ source might largely contribute to CH4 emission. To reduce the C emission, it is necessary to improve fertilization and irrigation methods, limit soil pollutants transferring into ditches, and frequently dredge sediments in future.

Seasonal and diurnal variations of greenhouse gas emissions from a saline mangrove constructed wetland by using an in situ continuous GHG monitoring system.

Wetland systems play important roles in the issues of global warming. That is because wetlands can not only intake carbon and nitrogen in the plants and sediments, but could also release carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere during the microbial decomposition processes in the water. In the past, greenhouse gases (GHGs) were measured mostly by using the manual sampling technique; however, it is difficult to measure the diurnal variation of greenhouse gas emissions. In this study, a floating chamber used to collect GHGs, which were then in-site monitored with a continuous GHG analyzer, through a Teflon tube connected to the top of the chamber was designed. The data for seasonal and diurnal variations of GHGs, emission fluxes of GHGs, and equivalent carbon dioxide emissions (CO2-e) were measured to explore the extent of a saline mangrove constructed wetland contributing to global warming. In addition, the correlation and regression analysis among greenhouse gas emissions, water quality, and other environmental factors were analyzed statistically. The results of continuous monitoring of GHGs showed that the concentrations of CO2 emitted from a saline mangrove constructed wetland ranged from 383.5 ± 25.9 to 476.8 ± 24.2 ppm. The diurnal variation of GHG concentrations was significant, which showed that the GHG concentrations in the daytime were generally lower than those at nighttime. The emissions of methane (CH4) from the wetland were monitored in a range between 3.7 ± 1.4 and 28.6 ± 7.6 ppm, while the concentrations of nitrous oxide (N2O) emitted from the wetlands ranged from 3.1 ± 2.6 to 0.9 ± 0.6 ppm. Hence, the diurnal variations of CH4 and N2O concentrations showed higher values in the daytime than those at nighttime. The correlation analytical results among GHG emissions, water quality, and other environmental factors presented that the emission fluxes of CO2 and BOD5 were positively correlated, but a negative correlation was shown for global solar radiation (GSR). Moreover, the concentrations of CH4 and N2O had moderately positive correlation with both air temperature and water temperature.