Covariation Research Articles

Emission and in-cylinder combustion characteristics of a spark ignition engine operated on binary mixtures of gas and liquid fuels

In this study, three different gas fuels such as hydrogen, propane, methane and six different liquid fuels such as, benzene, gasoline, toluene, hexane, methanol and ethanol have been combined and combustion characteristics have been simulated for a spark ignition engine (SIE). Variation of in-cylinder temperature, in-cylinder pressure, CO, CO2 and NO has been investigated. The gas fuel ratios in the mixtures were changed between 0 and 100% by mass. It was investigated that how the addition of gaseous fuels to liquid fuels at different ratios affects the combustion characteristics and emission formation. The gas fuel ratios considerably affected emission and combustion specifications of the engine. Increasing liquefied gas fuel ratio in the mixture leads to decrement in the temperature of charge. The maximum pressure first increases with increasing gas fuel ratio and then start to minimize after maximum pressure. The maximum peak pressure values are observed at 50% gasoline and 50% methane, 75% gasoline and 25% propane, 50% gasoline-50% hydrogen as 48.1 bar, 46 bar and 62.4 bar for different gas fuel mixtures. The minimum peak pressure value is displayed at 100% methanol as 44.06 bar. The maximum temperature value is observed at 100% benzene as 2582 K and its minimum value seems at 100% hydrogen as 2059 K. The maximum mass fractions of CO, CO2, NO are observed at 100% benzene as 0.0271, 0.161, 0.0076, the minimum mass fraction of CO2 is observed at 100% methane as 0.095. The minimum mass fraction of CO is observed at 100% methanol as 0.01243. The minimum mass fraction of NO is observed at 100% hydrogen as 0.00085. The fuels are ranked using a multi-criteria decision-making algorithm called AHP after evaluating the values of all performance criteria and their respective sub-criteria, including in-cylinder pressure, in-cylinder temperature, NO, CO2 and CO to get preference rank between fuels.

Changes in CO2 concentration and degassing of eutrophic urban lakes associated with algal growth and decline

Urban lakes are numerous in the world, but their role in carbon storage and emission is not well understood. This study aimed to answer the critical questions: How does algal growing season influence carbon dioxide concentration (cCO2) and exchange flux (FCO2) in eutrophic urban lakes? We investigated trophic state, seasonality of algal productivity, and their association with CO2 dynamics in four urban lakes in Central China. We found that these lightly-to moderately-eutrophic urban lakes showed a shifting pattern of CO2 source-sink dynamics. In the non-algal bloom phase, the moderately-eutrophic lakes outgassed on average of 12.18 ± 24.37 mmol m−2 d−1 CO2; but, during the algal bloom phase, the lakes sequestered an average 1.07 ± 6.22 mmol m−2 d−1 CO2. The lightly-eutrophic lakes exhibited lower CO2 emission in the algal bloom (0.60 ± 10.24 mmol m−2 d−1) compared to the non-algal bloom (3.84 ± 12.38 mmol m−2 d−1). Biological factors such as Chl-a (chlorophyll a) and AOU (apparent oxygen utilization), were found to be important factors to potentially affect the shifting pattern of lake CO2 source-sink dynamics in moderately-eutrophic lakes, explaining 48% and 34% of the CO2 variation in the non-algal and algal bloom phases, respectively. Moreover, CO2 showed positive correlations with AOU, and negative correlations with Chl-a in both phases. In the lightly-eutrophic lakes, biological factors explained a higher proportion of CO2 variations (29%) in the non-algal bloom phase, with AOU accounting for 19%. Our results indicate that algal growth and decline phases largely affect dissolved CO2 level and exchange flux by regulating in-lake respiration and photosynthesis. Based on the findings, we conclude that shallow urban lakes can act as both sources and sinks of CO2, with algal growth seasonality and trophic state playing pivotal roles in controlling their carbon dynamics.

Comparing the variation and influencing factors of CO2 emission from subsidence waterbodies under different restoration modes in coal mining area

Subsidence waterbodies play an important role in carbon cycle in coal mining area. However, little effort has been made to explore the carbon dioxide (CO2) release characteristics and influencing factors in subsidence waterbodies, especially under different restoration modes. Here, we measured CO2 release fluxes (F(CO2)) across Anguo wetland (AW), louts pond (LP), fishpond (FP), fishery-floating photovoltaic wetland (FFPV), floating photovoltaic wetland (FPV) in coal mining subsidence area, with unrestored subsidence waterbodies (SW) and unaffected normal Dasha river (DR) as the control area. We sampled each waterbody and tested which physical, chemical, and biological characteristics of water and sediment related to variability in CO2. The results indicated that F(CO2) exhibited the following patterns: FFPV > FPV > FP > SW > DR > LP > AW. Trophic lake index (TLI) and microbial biomass carbon content (MBC) in sediment had a positive impact on F(CO2). The dominant archaea Euryarchaeota and Thaumarchaeota, and dominant bacteria Proteobacteria promoted F(CO2). This study can help more accurately quantify CO2 emissions and guide CO2 future emission reduction and subsidence waterbodies estoration.

On the Large Variation in Atmospheric CO2 Concentration at Shangdianzi GAW Station during Two Dust Storm Events in March 2021

Dust storms have large impacts on air quality and meteorological elements; however, their relationships with atmospheric greenhouse gases (e.g., CO2) and radiation components remain uncertain. In this study, the co-variation of dust and CO2 concentrations and its possible influencing mechanism are examined using observations at the Shangdianzi (SDZ) regional Global Atmosphere Watch (GAW) station along with simulations of the Vegetation Photosynthesis and Respiration Model coupled with the Weather Research and Forecasting model (WRF-VPRM), during two dust storm events on 15 and 28 March 2021. During these events, hourly CO2 concentrations decreased by 40–50 ppm at SDZ while dust concentrations increased to 1240.6 and 712.4 µg m−3. The elevated dust increased diffusive shortwave irradiance by 50–60% and decreased direct shortwave irradiance by ~60% along with clouds. The dust events were attributed to the passages of two cold front systems over northern China. At SDZ, during the frontal passages, wind speed increased by 3–6 m s−1, and relative humidity decreased by 50–60%. The CO2 variations associated with the frontal systems were captured by the WRF-VPRM despite the overestimated surface CO2 level at SDZ. Biogenic CO2 flux plays an indistinctive role in the large CO2 variation at SDZ, as it is weak during the non-growing season. The cold fronts pushed polluted air southeastward over the North China Plain and replaced it with low-CO2 air from Northwest China, leading to the decline in CO2. These findings demonstrate that mesoscale synoptic conditions significantly affect the regional transport and dispersion of CO2, which can influence the prediction of terrestrial carbon balance on a regional scale.

Effects of paleogeographic changes and CO2 variability on northern mid-latitudinal temperature gradients in the Cretaceous

The Cretaceous ‘greenhouse’ period (~145 to ~66 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs.

Gross primary productivity and the predictability of CO2: more uncertainty in what we predict than how well we predict it

Abstract. The prediction of atmospheric CO2 concentrations is limited by the high interannual variability (IAV) in terrestrial gross primary productivity (GPP). However, there are large uncertainties in the drivers of GPP IAV among Earth system models (ESMs). Here, we evaluate the impact of these uncertainties on the predictability of atmospheric CO2 in six ESMs. We use regression analysis to determine the role of environmental drivers in (i) the patterns of GPP IAV and (ii) the predictability of GPP. There are large uncertainties in the spatial distribution of GPP IAV. Although all ESMs agree on the high IAV in the tropics, several ESMs have unique hotspots of GPP IAV. The main driver of GPP IAV is temperature in the ESMs using the Community Land Model, whereas it is soil moisture in the ESM developed by the Institute Pierre Simon Laplace (IPSL-CM6A-LR) and in the low-resolution configuration of the Max Planck Earth System Model (MPI-ESM-LR), revealing underlying differences in the source of GPP IAV among ESMs. Between 13 % and 24 % of the GPP IAV is predictable 1 year ahead, with four out of six ESMs showing values of between 19 % and 24 %. Up to 32 % of the GPP IAV induced by soil moisture is predictable, whereas only 7 % to 13 % of the GPP IAV induced by radiation is predictable. The results show that, while ESMs are fairly similar in their ability to predict their own carbon flux variability, these predicted contributions to the atmospheric CO2 variability originate from different regions and are caused by different drivers. A higher coherence in atmospheric CO2 predictability could be achieved by reducing uncertainties in the GPP sensitivity to soil moisture and by accurate observational products for GPP IAV.

Evolution of Atmospheric Carbon Dioxide and Methane Mole Fractions in the Yangtze River Delta, China

As the most economically developed region in China, the Yangtze River Delta (YRD) region contributed to ~17% of the total anthropogenic CO2 emissions from China. However, the studies of atmospheric CO2 and CH4 in this area are relatively sparse and unsystematic. Here, we analyze the changing characters of those gases in different development periods of China, based on the 11-year atmospheric CO2 and CH4 records (from 2010 to 2020) at one of the four Chinese sites participating in the World Meteorological Organization/Global Atmospheric Watch (WMO/GAW) program (Lin’an regional background station), located in the center of YRD region, China. The annual average atmospheric CO2 and CH4 mole fractions at LAN have been increasing continuously, with growth rates of 2.57 ± 0.14 ppm yr−1 and 10.3 ± 1.3 ppb yr−1, respectively. Due to the complex influence of regional sources and sinks, the characteristics of atmospheric CO2 and CH4 varied in different periods: (i) The diurnal and seasonal variations of both CO2 and CH4 in different periods were overall similar, but the amplitudes were different. (ii) The elevated mole fractions in all wind sectors tended to be uniform. (iii) The potential source regions of both gases expanded over time. (iv) The growth rate in recent years (2016–2020) changed significantly less than that in the earlier period (2010–2015). Our results indicated that the CO2 and CH4 mole fractions were mainly correlated to the regional economic development, despite the influence of special events such as the G20 Summit and COVID-19 lockdown.

High-northern-latitude forcing modulated fluvial‒aeolian interplay during the Last Glacial Period in the Mu Us Desert, northern central China

Substantial stratigraphic evidence from the Late Quaternary at orbital- and millennial‒scales indicates that the formation and evolution of aeolian‒sand landscapes results from the interaction between regional aeolian and fluvial‒lacustrine activity. In many deserts (sandy lands) across the planet, fluvial‒aeolian interplay at the glacial‒interglacial scale since the Late Quaternary has been responsive to monsoonal precipitation, and linked to atmospheric CO2 and periodic variations in insolation. However, the patterns of fluvial‒aeolian interplay and their relationship with climate change are not always consistent over a given timescale during the Late Pleistocene. The paucity of studies on fluvial‒aeolian interplay in the Mu Us Desert (MUD) during the Last Glacial Period (LGP) hinders our understanding of the relationships between the dynamics of aeolian and fluvial‒lacustrine landscapes and their drivers. Here, based on a comprehensive field survey and stratigraphic dating of a typical fluvial‒aeolian sequence in the MUD, we synthesized the chronology of fluvial‒lacustrine and aeolian strata during the LGP. Our results show that extensive development of a fluvial‒lacustrine environment in the MUD during the LG mainly occurred during the late Last Glacial Maximum (LGMa, 20.1–18.4 ka BP) and Bølling/Allerød interstadial (14.8–12.6 ka BP), while thicker aeolian sands extensively accumulated during the Heinrich Stadial 2, Heinrich Stadial 1, and Younger Dryas periods. However, the development of fluvial‒lacustrine environments showed considerable spatial heterogeneity, mainly occurring on the southern and eastern margins of the desert, while they were extremely rare in the central and western parts of the desert owing to strong wind erosion. Comparisons with other high-resolution geological records from the Asian monsoon region reveal that the fluvial‒aeolian interplay in the MUD during the LGP was highly consistent with the evolution pattern of the East Asian summer monsoon (EASM) on the millennial‒centennial scale, and indicate that the development of fluvial‒lacustrine environments was mainly controlled by an increase in monsoonal precipitation during strengthening of the EASM. Consistency with other broad changes at high latitudes in the Northern Hemisphere suggests that fluvial‒aeolian interplay during the LGP in northern Chinese deserts, as represented by the MUD, was mainly controlled by changes in high-northern-latitude ice volume and the Atlantic Meridional Overturning Circulation.

Impacts of reclamation marsh restoration on greenhouse gas emission in the Sanjiang Plain, China.

To understand the variations in greenhouse gas fluxes during the process of returning cropland to wetland in the Sanjiang Plain, we selected naturally restored wetlands of 4, 7, 11, 16 and 20 years as research objects to compare with a cultivated site (soybean plantation for 13 years) and an uncultivated marsh dominated by Deyeuxia purpurea and Carex schmidtii. We measured carbon dioxide (CO2) and methane (CH4) fluxes using a static chamber-gas chromatography and explored the main influencing factors. The results showed that there were seasonal variations in growing-season CO2 and CH4 fluxes of the restored wetlands, with the seasonal trends in greenhouse gases becoming gradually similar to that of natural marsh with increasing restoration time. The mean growing-season CO2 fluxes increased during the early stage of restoration, but then decreased during the late stage, which decreased from 893.4 mg·m-2·h-1 to 494.0 mg·m-2·h-1 in the 4-year and 20-year sites, respectively. Mean CH4 fluxes increased with restoration time, ranging from a weak CH4 sink (soybean fields, -0.6 mg·m-2·h-1) to a CH4 source of 87.8 mg·m-2·h-1(20-year restored site). The CH4 fluxes of experimental plots were consistently lower than that of natural marsh (96.4 mg·m-2·h-1). Increases in water level and soil conductivity resulting from restoration were the main driving factors for the decrease in CO2 fluxes. The increases in water level and soil dissolved organic carbon resulting from restoration were the primary drivers for the increase of CH4 fluxes in the restored wetlands. The global warming potentials increased with restoration time, ranging from 27.8 t·CO2-eq·hm-2(soybean fields) to 130.8 t·CO2-eq·hm-2(plot of 20-year restoration), which gradually approached that of natural marsh (156.3 t·CO2-eq·hm-2). The emission of GHGs from restored wetlands in the Sanjiang Plain gradually approached those of natural marsh. Further monitoring is required to identify the maturity of restored wetlands for achieving greenhouse gas emissions equivalent to that of natural marshland.

Outgassing of CO2 dominates in the coastal upwelling off the northwest African coast

The Eastern Boundary Upwelling System off northwest Africa is among the most productive regions of the ocean. In 2019, two merchant ships equipped with an underway CO2 system sampled the region following exactly the same track from 10°N to 36°N. We determine the sources and sinks of CO2 and the seasonal cycle along the track. A weak permanent upwelling (WPU), a permanent upwelling (PU) and an open ocean regions are identified. The WPU (26°N to 33°N) is a source of CO2 in summer and autumn, and a sink of CO2 in winter and spring. Thermodynamic warming and cooling processes mainly drive the CO2 variations in this region. The PU (20°N to 26°N) is a sink of CO2 in spring and a source of CO2 in other seasons. This region is the most productive and exhibits the largest variability of the CO2 flux. The supply of CO2 from subsurface waters dominate over the carbon uptake by biology, which leads to a strong outgassing, especially in winter and autumn. A sink of CO2 occurs in spring only in the PU. Near Cape Blanc (20°N) in July 2019, a source of CO2 is observed around 20°N within ± 1° of latitude and becomes a sink of CO2 a few days later when the ship samples back the same area. South of 18°N, out of the influence of the coastal upwelling, the region is a sink of CO2 in winter only and the region is mainly controlled by physical processes. Using voyages from 2010 to 2022, seawater fCO2 significantly increases at a rate ranging from 1.82 μatm yr−1 to 2.10 μatm yr−1 close to the atmospheric increase. This is associated with a pH decrease between 0.0016 and 0.0022 yr−1. Nevertheless, there is no clear trend of the CO2 flux in any region.

Coupled weather-biosphere model (WRF-VPRM) response to extreme land cover scenarios over Ganga basin: a cross-component analysis

Deforestation can have both biophysical and biochemical effects. In our study we evaluate the impacts of extreme idealized land cover scenarios over Ganga basin, India using an online coupled weather-biosphere model. We present an analysis of the carbon stored, based on introduced afforestation (AFF) and deforestation (DEF) in the Ganga basin of India. WRF-VPRM model simulations were carried out at horizontal resolution of 20 km using optimized downscaling configuration. For DEF case, we found two-fold increase in surface temperatures whereas, AFF scenario exhibited cooling effect. The average carbon sequestration capability for AFF was 0.3 g C m−2 day−1 more than control run (CTL). The CTL simulations exhibited carbon sequestration capability of −0.15 g C m−2 day−1 which for a year accounts for around 59.3 Mt C yr−1. AFF scenario showed relative increase in net sequestration compared to DEF scenario. Most importantly, the model simulations showed that the croplandscan increase net carbon sequestration rather than just deciduous forest type. Our study highlights the possible effects of land use management practices on atmospheric CO2 variability.

Spatiotemporal variability of near-surface CO2 and its affecting factors over Mongolia

We investigate the spatiotemporal variability of near-surface CO2 concentrations in Mongolia from 2010 to 2019 and the factors affecting it over four climate zones of Mongolia based on the Köppen-Geiger climate classification system, including arid desert climate (BWh), arid steppe climate (BSk), dry climate (Dw), and polar frost climate (ET). Initially, we validate the near-surface CO2 datasets obtained from the Greenhouse Gases Observing Satellite (GOSAT) using ground-based CO2 observations obtained from the World Data Center for Greenhouse Gases (WDCGG) and found good agreement. The results showed that CO2 concentrations over Mongolia steadily increased from 389.48 ppmv in 2010 to 409.72 ppmv in 2019, with an annual growth rate of 2.24 ppmv/year. Spatially, the southeastern Gobi desert region has the highest annual average CO2 concentration, while the northwestern Alpine and Meadow steppe region exhibits the most significant growth rate. Additionally, significant monthly and seasonal variations were observed in each climate zone, with CO2 levels decreasing to a minimum in summer and reaching a maximum in spring. Furthermore, our findings revealed a negative correlation between CO2 concentrations and vegetation parameters (NDVI, GPP, and LAI) during summer when photosynthesis is at its peak, while a positive correlation was observed during spring and autumn when the capacity for carbon sequestration is lower. Understanding CO2 concentrations in different climate zones and the uptake capacity of vegetation may help improve estimates of carbon sequestration in ecosystems such as deserts, steppes and forests.

Experimental study on cooling heat transfer performance of supercritical CO2 in zigzag printed circuit heat exchanger

The zigzag printed circuit heat exchanger (PCHE) is promising as the precooler in the application of supercritical CO2 (S-CO2) Brayton cycle. In this study, the experimental investigation is conducted to explore the heat transfer performance of supercritical CO2 in the mini channels of zigzag PCHE. The experimental range of pressure, temperature and mass flux is 7.5–9.0MPa, 50–90°C and 300–600kg/(m2·s), respectively. The structural parameters of diameter, pitch, and bend angle are D=2.0mm, Lp = 7.24mm and θ = 40°, respectively. The effects of system pressure, inlet temperature, and mass flux on the heat transfer performance of supercritical CO2 are investigated by measuring multiple temperature points along the zigzag channel. The results show that the heat transfer coefficient exhibits a higher level near the pseudocritical region due to the dramatic thermophysical properties variation of supercritical CO2. Meanwhile, both the change of mass flux and pressure have a significant influence on the S-CO2 heat transfer coefficient, while the effect of inlet temperature is relatively small. Furthermore, the comparison is conducted between the existing heat transfer correlations and the experimental results. Four new cooling heat transfer correlations are proposed and compared by considering the effect of velocity change and cross-sectional thermophysical property variation on heat transfer performance. It is found that adding variates to the correlation can improve the prediction accuracy of the results while iterative calculation becomes more difficult. In order to improve the robustness and prediction accuracy, it is determined to choose the correlation without the density form, which predict 91.5% of the experimental data within ±25% error band.

Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes.

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO2 sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO2 and CO2 degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO2 in contrast to the atmospheric equilibrium (~400 μatm). The pCO2 values in the lake water during the dry season were relatively low in comparison, and the pCO2 levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetlands. The estimated water-to-air CO2 degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×107g CO2 year-1, 13.8×107g CO2 year-1, and 9.17×107g CO2 year-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO2, indicating that when the FDOM and DO level is decreased, the amount of pCO2 values increases.

Asymmetries in climate change and livestock productivity: non-linear evidence from autoregressive distribution lag mode

IntroductionThe livestock sector is extremely important to Socioeconomic growth in Pakistan, yet it is also quite vulnerable to weather changes. Climate change reduces livestock production by changing ecosystem services such as water availability, feed quality and quantity, disease outbreaks, animal heat stress, and a decline in livestock variety and breeds. Climate change has a direct impact on ecological and animal health. As a consequence of climate change, animal diseases, and infections are becoming more widespread. With the non-linearities of climate change in the livestock industry in mind, the present study investigated the asymmetric influence of climatic and non-climatic variables on livestock productivity across Pakistan. The empirical analysis was conducted utilizing secondary time series data from 1980 to 2021.MethodThe non-linear autoregressive distributive lag (NARDL) model is used to examine the asymmetric behavior of climatic variability in the livestock sector. We included CO2 emissions, mean temperature (MT), and precipitation (PERC) as climatic variables in the current study, along with additional control factors.Results and discussionOur research discovered that CO2, MT, and PREC had asymmetries in their impacts on livestock. Variations in CO2, MT, and PREC have contradictory effects on livestock productivity in the long and short term. A percent increase in LCO2 leads to a fall in livestock production insignificantly by 1.0062% for Model I and significantly by 5.7613% and 5.3929% for Models II and III, respectively. A percent decrease in LCO2 significantly lowers livestock production by 4.1739% for Model I and improves livestock production by 8.5928% and 6.7901%, respectively, for Model II and Model III. A unit increase in MT significantly improves livestock productivity by 1.5520% and 0.8149% for Models II and III, respectively, while a unit decrease in MT insignificantly improves livestock production by 0.1316% and 0.2122% for Models II and III, respectively. A unit increase and decrease in PREC significantly lowers and insignificantly improve livestock productivity respectively by 0.002% in both cases for Model III. To protect the livestock industry from the negative effects of climate change, this study suggests that livestock producers use new environmentally friendly technologies and ecological agricultural systems.

Greenhouse gas assemblages (CO2, CH4 and N2O) in the continental shelf of the Gulf of Cadiz (SW Iberian Peninsula)

This study examines the simultaneous water-atmosphere exchange of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) on the continental shelf of the Gulf of Cadiz, as well as the effect it has in terms of the radiative balance in the atmosphere, between 2014 and 2016.The experimental database consists of new measurements of the spatial and seasonal distribution of CO2 partial pressure (pCO2) and N2O concentration in 2016. pCO2 shows a wide range of variation influenced mainly by seasonal thermal variations (8.0 μatm 0C-1), as well as with the relative intensity of biological activity. There is experimental evidence of a progressive increase of pCO2 over the last 2 decades, with an estimated gradient of 4.2 ± 0.7 μatm y−1. During 2016, the Gulf of Cadiz acted as a slight source of CO2 to the atmosphere, with a mean flux of 0.4 ± 2.2 mmol m−2 d−1. The analysis of concentration variations in the water column shows that nitrification is the main N2O production process in the study area, although in the more coastal zone there are signs of inputs related to continental and sediment contributions, most probably induced by denitrification processes. In 2016, the Gulf of Cadiz acted as a weak sink of atmospheric N2O, with a mean flux of −0.1 ± 0.9 μmol m−2 d−1.From previous studies, performed with a similar methodology, an interannual database (2014–2016) of water-atmosphere fluxes of CO2, CH4 and N2O, normalized to the mean wind speed in the area, has been generated. Considering their respective Global Warming Potential (GWP) a joint greenhouse gasses (GHG) flux, expressed in CO2 equivalents of 0.6 ± 2.0 mmol m−2 d−1, has been estimated, which extended to the area of study indicates an approximate emission of 67.9 Gg CO2 y−1. However, although there is a high uncertainty associated with the spatial, temporal and interannual variations of CO2, CH4 and N2O fluxes in the Gulf of Cadiz, the exchange of greenhouse gasses could be influencing a radiative forcing increase in the atmosphere. When considering the available information on local and global estimates, the uncertainty about the effect of the joint exchange of GHGs to the atmosphere from the coastal seas increases significantly.

Variable ventilation ages in the equatorial Indian Ocean thermocline during the LGM

Variations of atmospheric CO2 during the Pleistocene ice-ages have been associated with changes in the drawdown of carbon into the deep-sea. Modelling studies suggest that about one third of the glacial carbon drawdown may not be associated to the deep ocean, but to the thermocline or intermediate ocean. However, the carbon storage capacity of thermocline waters is still poorly constrained. Here we present paired 230Th/U and 14C measurements on scleractinian cold-water corals retrieved from ~ 450 m water depth off the Maldives in the Indian Ocean. Based on these measurements we calculate ∆14C, ∆∆14C and Benthic-Atmosphere (Batm) ages in order to understand the ventilation dynamics of the equatorial Indian Ocean thermocline during the Last Glacial Maximum (LGM). Our results demonstrate a radiocarbon depleted thermocline as low as -250 to -345‰ (∆∆14C), corresponding to ~ 500–2100 years (Batm) old waters at the LGM compared to ~ 380 years today. More broadly, we show that thermocline ventilation ages are one order of magnitude more variable than previously thought. Such a radiocarbon depleted thermocline can at least partly be explained by variable abyssal upwelling of deep-water masses with elevated respired carbon concentrations. Our results therefore have implications for radiocarbon-only based age models and imply that upper thermocline waters as shallow as 400 m depth can also contribute to some of the glacial carbon drawdown.

Measurement strategies to account for the temporal variation in soil-respired CO2 and its δ13C value in an irrigated oasis agroecosystem

Soil respiration is an important ecosystem process and fluctuates daily and seasonally with abiotic and biotic factors, inevitably leading to uncertainty in the estimation of daily, monthly or seasonal means from short-term measurements. The measurement strategies are poorly understood and are commonly analyzed using noncontinuous measurements. The objectives of this study were to evaluate the optimum measurement time and frequency of soil-respired CO2 (Rs) and its δ13C (δs) value, as well as soil temperature (ST) and soil water content (SWC) during the growing season. To provide more refined recommendations, we performed long-term continuous measurements of Rs and δs value with a multichannel and dual-cycle observation system under non-steady-state conditions in an irrigated oasis agroecosystem in Northwest China. The daily optimum measurement time of Rs was 9:00–10:00, which accounted for 89.0% of the total measurement days and was within the previously recommended time scale of 9:00–12:00. δs showed little diurnal variation, and each hourly mean approximately represented the daily mean. The Rs and δs measured at 9:00–11:00 did not significantly differ from the daily mean, yet the soil temperature (ST) and soil water content (SWC) measured at 9:00–11:00 were lower than the daily mean values. The number of days needed to obtain values ±10% for Rs, δs, ST and SWC was 101, 23, 13, and 25, respectively (95% confidence interval (CI)), which sharply decreased to 11, 3, 1, and 3 days, respectively (95% CI), when the desired accuracy expanded to ±30% for Rs, δs, ST and SWC in the growing season. That means the measurement interval of Rs and δs were about is 14 days and 53 days during the growing season in agroecosystems (95% CI, ±30% mean). Overall, this study analyzed the optimum measurement times for Rs and δs at daily, monthly and seasonal time scales and provided guidelines for improving measurement frequencies under noncontinuous measurements.

Metabolism and carbonate buffering drive seasonal dynamics of CO2 emissions from two German reservoirs

Biological metabolism drives much of the variation in CO2 in terrestrial ecosystems but does not explain CO2 oversaturation and emission in net autotrophic lakes and reservoirs. The unexplained CO2 could be attributed to the equilibria between CO2 and the carbonate buffering system, which is seldom integrated into CO2 budgets, let alone its interplay with metabolism on CO2 emissions. Here, we perform a process-based mass balance modeling analysis based on an 8-year dataset from two adjacent reservoirs with similar catchment sizes but contrasting trophic states and alkalinity. We find that in addition to the well-acknowledged driver of net metabolic CO2 production, carbonate buffering also determines the total amount and seasonal dynamics of CO2 emissions from the reservoirs. Carbonate buffering can contribute up to nearly 50% of whole-reservoir CO2 emissions, by converting the ionic forms of carbonate to CO2. This results in similar seasonal CO2 emissions from reservoirs with differing trophic state, even in low alkalinity system. We therefore suggest that catchment alkalinity, instead of trophic state, may be more relevant in predicting CO2 emissions from reservoirs. Our model approach highlights the important role of carbonate buffering and metabolism that generate and remove CO2 throughout the reservoirs on a seasonal scale. The inclusion of carbonate buffering could diminish a major uncertainty in the estimation of reservoir CO2 emissions and increase the robustness of aquatic CO2 emission estimates.

Variation of AIRS-Retrieved CO2 over Uttarakhand, India, During 2010–2017

Variation of AIRS-Retrieved CO2 over Uttarakhand, India, During 2010–2017