| 31. |
- Baltar, Federico, et al.
(författare)
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Significance of non-sinking particulate organic carbon and dark CO2 fixation to heterotrophic carbon demand in the mesopelagic Atlantic
- 2010
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L09602-
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Tidskriftsartikel (refereegranskat)abstract
- It is generally assumed that sinking particulate organic carbon (POC) constitutes the main source of organic carbon supply to the deep ocean's food webs. However, a major discrepancy between the rates of sinking POC supply (collected with sediment traps) and the prokaryotic organic carbon demand (the total amount of carbon required to sustain the heterotrophic metabolism of the prokaryotes; i.e., production plus respiration, PCD) of deep-water communities has been consistently reported for the dark realm of the global ocean. While the amount of sinking POC flux declines exponentially with depth, the concentration of suspended, buoyant non-sinking POC (nsPOC; obtained with oceanographic bottles) exhibits only small variations with depth in the (sub) tropical Northeast Atlantic. Based on available data for the North Atlantic we show here that the sinking POC flux would contribute only 4-12% of the PCD in the mesopelagic realm (depending on the primary production rate in surface waters). The amount of nsPOC potentially available to heterotrophic prokaryotes in the mesopelagic realm can be partly replenished by dark dissolved inorganic carbon fixation contributing between 12% to 72% to the PCD daily. Taken together, there is evidence that the mesopelagic microheterotrophic biota is more dependent on the nsPOC pool than on the sinking POC supply. Hence, the enigmatic major mismatch between the organic carbon demand of the deep-water heterotrophic microbiota and the POC supply rates might be substantially smaller by including the potentially available nsPOC and its autochthonous production in oceanic carbon cycling models. Citation: Baltar, F., J. Aristegui, E. Sintes, J. M. Gasol, T. Reinthaler, and G. J. Herndl (2010), Significance of non-sinking particulate organic carbon and dark CO2 fixation to heterotrophic carbon demand in the mesopelagic northeast Atlantic.
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| 32. |
- Bannan, T. J., et al.
(författare)
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A Large Source of Atomic Chlorine From ClNO2 Photolysis at a UK Landfill Site
- 2019
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 46:14, s. 8508-8516
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Tidskriftsartikel (refereegranskat)abstract
- Nitryl chloride (ClNO2) acts as a source of highly reactive chlorine atoms as well as an important NOx reservoir. Measurements of ClNO2 at an operational U.K. landfill site are reported here for the first time. A peak concentration of 4 ppb of ClNO2 was found with a peak mean nighttime maximum of 0.9 ppb. Using models based upon the photolysis of observed ClNO2 and atmospheric chlorine chemistry, chlorine atom concentrations reaching in excess of 1.20 x 10(5) molecules/cm(3) in the early morning following sunrise are calculated. These concentrations are approximately 10 times higher than previously reported in the United Kingdom, suggesting a significant impact on the oxidizing capacity around such sites. Given the ubiquity of landfill sites regionally and globally, and the large abundances of Cl atoms from the photolysis of ClNO2, chlorine chemistry has a significant impact on ozone formation and volatile organic compounds oxidation as shown by WRF-Chem modeling. Plain Language Summary Landfill sites are a known source of traces gases into the atmosphere, but measurements often focus predominately on methane and carbon dioxide. A small subsection of trace gas measurements at landfill sites have shown, however, that these sites may be important halogen sources that could have subsequent impacts on air quality and climate. Spatially limited field measurements have previously been reported of a halogen species, ClNO2, showing that this species is consistently formed during nighttime hours, but no such measurements before now have been made at any landfill site. ClNO2 undergoes photolysis upon sunrise, releasing the extremely reactive Cl as well as NO2 into the atmosphere and therefore plays an important part in the total budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species, all of which are important to air quality. Here we present mass spectrometry measurements of ClNO2 taken at an undisclosed landfill, which show high concentrations in comparison to any other global study of this type. We use predictive modeling techniques to show the importance of this halogen species to air quality, using indicators such as ozone formation. Based on these results we recommend that landfill sources of Cl should be included in future air quality studies.
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| 33. |
- Bao, Fangling, et al.
(författare)
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Advancing Cloud Classification Over the Tibetan Plateau: A New Algorithm Reveals Seasonal and Diurnal Variations
- 2024
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Ingår i: GEOPHYSICAL RESEARCH LETTERS. - 0094-8276 .- 1944-8007. ; 51:13
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Tidskriftsartikel (refereegranskat)abstract
- The cloud classification algorithm widely used in the International Satellite Cloud Climatology Project (ISCCP) tends to underestimate low clouds over the Tibetan Plateau (TP), often mistaking water clouds for high-level clouds. To address this issue, we propose a new algorithm based on cloud-top temperature and optical thickness, which we apply to TP using Advanced Himawari Imager (AHI) geostationary satellite data. Compared with Clouds and the Earth's Radiant Energy System cloud-type products and ISCCP results obtained from AHI data, this new algorithm markedly improved low-cloud detection accuracy and better aligned with cloud phase results. Validation with lidar cloud-type products further confirmed the superiority of this new algorithm. Diurnal cloud variations over the TP show morning dominance shifting to afternoon high clouds and evening mid-level clouds. Winter is dominated by high clouds, summer by mid-level clouds, spring by daytime low clouds and nighttime high clouds, and autumn by low and mid-level clouds. The accurate identification of low clouds over the Tibetan Plateau (TP) is crucial for climate regulation, ecosystems, aviation safety, research, and modeling. However, satellite-based methods often miss these clouds, misclassifying them as high-level clouds. To remedy this, we developed a new algorithm using cloud-top temperature and optical thickness, applied to Advanced Himawari Imager data. This significantly improves low-cloud detection, better aligning with actual cloud phases. Simultaneously, we analyzed diurnal cloud variations over the TP with the new algorithm. Cloud types at different altitudes in the TP exhibit strong seasonality. The dominant cloud types in winter and summer are high and mid-level, respectively. In spring, low clouds dominate during the day (2:00-10:00 UTC), transitioning to high clouds at night (10:00-18:00 UTC), with mid-level clouds prevailing at other times. In autumn, low clouds dominate during the day, transitioning to mid-level clouds at other times, with fewer occurrences of high clouds. Employing cloud-top temperature instead of pressure resolves classification-phase inconsistencies for clouds in the Tibetan Plateau (TP) Lidar validation shows new algorithm's low cloud detection outperforms the conventional International Satellite Cloud Climatology Project algorithm for both TP and plains The study reveals significant diurnal and seasonal variations in low clouds over the TP
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| 34. |
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| 35. |
- Barchyn, Thomas E., et al.
(författare)
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Subglacial bed form morphology controlled by ice speed and sediment thickness
- 2016
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 43:14, s. 7572-7580
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Tidskriftsartikel (refereegranskat)abstract
- Subglacial bed forms (drumlins, ribbed moraines, and megascale glacial lineations) are enigmatic repetitive flow-parallel and flow-transverse landforms common in glaciated landscapes. Their evolution and morphology are a potentially powerful constraint for ice sheet modeling, but there is little consensus on bed form dynamics or formative mechanisms. Here we explore shallow sediment bed form dynamics via a simple model that iterates (i) down-flow till flux, (ii) pressure gradient-driven till flux, and (iii) entrainment and deposition of sediment. Under various boundary conditions, replicas of subglacial bed forms readily emerge. Bed form dynamics mirror those in subaqueous and aeolian domains. Transitions between ribbed moraines and elongate flow-parallel bed forms are associated with increasing ice speeds and declining sediment thickness. These simulations provide quantitative flux estimates and suggest that widely observed transitions in shallow sediment subglacial bed forms (e.g., ribbed moraines to drumlinoids to megascale glacial lineations) are manifestations of subtle variations in ice velocity and sediment thickness.
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| 36. |
- Bassiouni, Maoya
(författare)
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Plant Water Uptake Thresholds Inferred From Satellite Soil Moisture
- 2020
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Ingår i: Geophysical Research Letters. - 0094-8276. ; 47
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Tidskriftsartikel (refereegranskat)abstract
- Empirical functions are widely used in hydrological, agricultural, and Earth system models to parameterize plant water uptake. We infer soil water potentials at which uptake is downregulated from its well-watered rate and at which uptake ceases, in biomes with <60% woody vegetation at 36-km grid resolution. We estimate thresholds through Bayesian inference using a stochastic soil water balance framework to construct theoretical soil moisture probability distributions consistent with empirical distributions derived from satellite soil moisture observations. The global median Nash-Sutcliffe efficiency between empirical soil moisture distributions and theoretical distributions using reference constants, inferred median parameters per biome, and spatially variable inferred parameters are 0.38, 0.59, and 0.8, respectively. Spatially variable thresholds capture location-specific vegetation and climate characteristics and can be connected to biome-level water uptake strategies. Results demonstrate that satellite soil moisture probability distributions encode information, valuable to understanding biome-level ecohydrological adaptation and resistance to climate variability.Plain Language Summary Vegetation regulates a large fraction of the terrestrial water and carbon cycles as it adapts and responds to changing environmental conditions such as soil moisture availability, yet our ability to characterize diversity in vegetation soil water-use behavior at large scales is limited. In this study, we analyze satellite observations to estimate thresholds that are commonly used to approximate when vegetation extracts water from the soil. We show that the newly found values are more consistent with global patterns of soil moisture compared to constants found in the literature. Spatially variable plant water uptake thresholds reflect land cover and climate characteristics and can be connected to water-use strategies in biomes not dominated by trees.
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| 37. |
- Basu, N. B., et al.
(författare)
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Nutrient loads exported from managed catchments reveal emergent biogeochemical stationarity
- 2010
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37:L23404
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Tidskriftsartikel (refereegranskat)abstract
- Complexity of heterogeneous catchments poses challenges in predicting biogeochemical responses to human alterations and stochastic hydro‐climatic drivers. Human interferences and climate change may have contributed to the demise of hydrologic stationarity, but our synthesis of a large body of observational data suggests that anthropogenic impacts have also resulted in the emergence of effective biogeochemical stationarity in managed catchments. Long‐term monitoring data from the Mississippi‐Atchafalaya River Basin (MARB) and the Baltic Sea Drainage Basin (BSDB) reveal that inter‐annual variations in loads (LT) for total‐N (TN) and total‐P (TP), exported from a catchment are dominantly controlled by discharge (QT) leading inevitably to temporal invariance of the annual, flow‐weighted concentration, = (LT/QT). Emergence of this consistent pattern across diverse managed catchments is attributed to the anthropogenic legacy of accumulated nutrient sources generating memory, similar to ubiquitously present sources for geogenic constituents that also exhibit a linear LT‐QT relationship. These responses are characteristic of transport‐limited systems. In contrast, in the absence of legacy sources in less‐managed catchments, values were highly variable and supply limited. We offer a theoretical explanation for the observed patterns at the event scale, and extend it to consider the stochastic nature of rainfall/flow patterns at annual scales. Our analysis suggests that: (1) expected inter‐annual variations in LT can be robustly predicted given discharge variations arising from hydro‐climatic or anthropogenic forcing, and (2) water‐quality problems in receiving inland and coastal waters would persist until the accumulated storages of nutrients have been substantially depleted. The finding has notable implications on catchment management to mitigate adverse water‐quality impacts, and on acceleration of global biogeochemical cycles.
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| 38. |
- Becerra Garcia, Marley, et al.
(författare)
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On the velocity of positive connecting leaders associated with negative downward lightning leaders
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2
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Tidskriftsartikel (refereegranskat)abstract
- A self-consistent leader propagation model is used to estimate the velocity of upward connecting positive leaders initiated from a tall tower under the influence of downward negative lightning leaders. The propagation of upward connecting leaders has been found to be influenced not only by the average velocity of the downward leader but also by the prospective return stroke current, the lateral position of the downward leader channel as well as by the ambient electric field. This result show that the velocity and propagation time of upward connecting positive leaders change from flash to flash due to the variations in these parameters.
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| 39. |
- Beck, Lisa J., et al.
(författare)
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Differing Mechanisms of New Particle Formation at Two Arctic Sites
- 2021
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 48:4
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Tidskriftsartikel (refereegranskat)abstract
- New particle formation in the Arctic atmosphere is an important source of aerosol particles. Understanding the processes of Arctic secondary aerosol formation is crucial due to their significant impact on cloud properties and therefore Arctic amplification. We observed the molecular formation of new particles from low-volatility vapors at two Arctic sites with differing surroundings. In Svalbard, sulfuric acid (SA) and methane sulfonic acid (MSA) contribute to the formation of secondary aerosol and to some extent to cloud condensation nuclei (CCN). This occurs via ion-induced nucleation of SA and NH3 and subsequent growth by mainly SA and MSA condensation during springtime and highly oxygenated organic molecules during summertime. By contrast, in an ice-covered region around Villum, we observed new particle formation driven by iodic acid but its concentration was insufficient to grow nucleated particles to CCN sizes. Our results provide new insight about sources and precursors of Arctic secondary aerosol particles.
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| 40. |
- Behar, Etienne, et al.
(författare)
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Mass loading at 67P/Churyumov-Gerasimenko : A case study
- 2016
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 43:4, s. 1411-1418
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of the interaction between the solar wind ions and a partially ionized atmosphere around a comet, at a distance of 2.88 AU from the Sun during a period of low nucleus activity. Comparing particle data and magnetic field data for a case study, we highlight the prime role of the solar wind electric field in the cometary ion dynamics. Cometary ion and solar wind proton flow directions evolve in a correlated manner, as expected from the theory of mass loading. We find that the main component of the accelerated cometary ion flow direction is along the antisunward direction and not along the convective electric field direction. This is interpreted as the effect of an antisunward polarization electric field adding up to the solar wind convective electric field.
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