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1. Assessing the influence of soil freeze–thaw cycles on catchment water storage–flux–age interactions using a tracer-aided ecohydrological model

   https://doi.org/10.5194/hess-23-3319-2019  

   Smith, Aaron ; Tetzlaff, Doerthe ; Laudon, Hjalmar ; Maneta, Marco ; Soulsby, Chris ( January 2019 , Hydrology and Earth System Sciences)

   Abstract. Ecohydrological models are powerful tools to quantify the effects that independent fluxes may have on catchment storage dynamics. Here, we adapted the tracer-aided ecohydrological model, EcH₂O-iso, for cold regions with the explicit conceptualization of dynamic soil freeze–thaw processes. We tested the model at the data-rich Krycklan site in northern Sweden with multi-criterion calibration using discharge, stream isotopes and soil moisture in three nested catchments. We utilized the model's incorporation of ecohydrological partitioning to evaluate the effect of soil frost on evaporation and transpiration water ages, and thereby the age of source waters. The simulation of stream discharge, isotopes, and soil moisture variability captured the seasonal dynamics at all three stream sites and both soil sites, with notable reductions in discharge and soil moisture during the winter months due to the development of the frost front. Stream isotope simulations reproduced the response to the isotopically depleted pulse of spring snowmelt. The soil frost dynamics adequately captured the spatial differences in the freezing front throughout the winter period, despite no direct calibration of soil frost to measured soil temperature. The simulated soil frost indicated a maximum freeze depth of 0.25 m below forest vegetation. Water ages of evaporation and transpiration reflect themore »influence of snowmelt inputs, with a high proclivity of old water (pre-winter storage) at the beginning of the growing season and a mix of snowmelt and precipitation (young water) toward the end of the summer. Soil frost had an early season influence of the transpiration water ages, with water pre-dating the snowpack mainly sustaining vegetation at the start of the growing season. Given the long-term expected change in the energy balance of northern climates, the approach presented provides a framework for quantifying the interactions of ecohydrological fluxes and waters stored in the soil and understanding how these may be impacted in future.

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2. Airborne and ground-based observations of ammonium-nitrate-dominated aerosols in a shallow boundary layer during intense winter pollution episodes in northern Utah

   https://doi.org/10.5194/acp-18-17259-2018  

   Franchin, Alessandro ; Fibiger, Dorothy L. ; Goldberger, Lexie ; McDuffie, Erin E. ; Moravek, Alexander ; Womack, Caroline C. ; Crosman, Erik T. ; Docherty, Kenneth S. ; Dube, William P. ; Hoch, Sebastian W. ; et al ( January 2018 , Atmospheric Chemistry and Physics)

   Abstract. Airborne and ground-based measurements of aerosol concentrations, chemicalcomposition, and gas-phase precursors were obtained in three valleys innorthern Utah (USA). The measurements were part of the Utah Winter FineParticulate Study (UWFPS) that took place in January–February 2017. Totalaerosol mass concentrations of PM₁ were measured from a Twin Otteraircraft, with an aerosol mass spectrometer (AMS). PM₁ concentrationsranged from less than 2µgm⁻³ during clean periods to over100µgm⁻³ during the most polluted episodes, consistent withPM_2.5 total mass concentrations measured concurrently at groundsites. Across the entire region, increases in total aerosol mass above∼2µgm⁻³ were associated with increases in theammonium nitrate mass fraction, clearly indicating that the highest aerosolmass loadings in the region were predominantly attributable to an increase inammonium nitrate. The chemical composition was regionally homogenous fortotal aerosol mass concentrations above 17.5µgm⁻³, with 74±5% (average±standard deviation) ammonium nitrate, 18±3%organic material, 6±3% ammonium sulfate, and 2±2%ammonium chloride. Vertical profiles of aerosol mass and volume in the regionshowed variable concentrations with height in the polluted boundary layer.Higher average mass concentrations were observed within themore »first few hundredmeters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid (HNO₃) and ammonia (NH₃) duringthe pollution episodes revealed that in the Cache and Utah valleys, partitioningof inorganic semi-volatiles to the aerosol phase was usually limited by theamount of gas-phase nitric acid, with NH₃ being in excess. The inorganicspecies were compared with the ISORROPIA thermodynamic model. Total inorganicaerosol mass concentrations were calculated for various decreases in totalnitrate and total ammonium. For pollution episodes, our simulations of a50% decrease in total nitrate lead to a 46±3% decrease in totalPM₁ mass. A simulated 50% decrease in total ammonium leads to a36±17%µgm⁻³ decrease in total PM₁ mass, over the entirearea of the study. Despite some differences among locations, ourresults showed a higher sensitivity to decreasing nitric acid concentrationsand the importance of ammonia at the lowest total nitrate conditions. In theSalt Lake Valley, both HNO₃ and NH₃ concentrations controlledaerosol formation.

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3. Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India

   https://doi.org/10.5194/cp-14-1869-2018  

   Kathayat, Gayatri ; Cheng, Hai ; Sinha, Ashish ; Berkelhammer, Max ; Zhang, Haiwei ; Duan, Pengzhen ; Li, Hanying ; Li, Xianglei ; Ning, Youfeng ; Edwards, R. Lawrence ( January 2018 , Climate of the Past)

   Abstract. A large array of proxy recordssuggests that the “4.2ka event” marks an approximately300-year long period (∼3.9 to 4.2ka) ofmajor climate change across the globe. However, the climatic manifestation ofthis event, including its onset, duration, and termination, remains lessclear in the Indian summer monsoon (ISM) domain. Here, we present new oxygenisotope (δ¹⁸O) data from a pair of speleothems (ML.1 and ML.2)from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record ofISM variability during a period (∼3.78 and 4.44ka) that fullyencompasses the 4.2ka event. The sub-annually to annually resolved ML.1δ¹⁸O record is constrained by 18 ²³⁰Th dates with anaverage dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability withunprecedented detail. The inferred pattern of ISM variability during theperiod contemporaneous with the 4.2ka event shares broad similarities andkey differences with the previous reconstructions of ISM from the MawmluhCave and other proxy records from the region. Our data suggest that the ISMintensity, in the context of the length of our record, abruptly decreased at∼4.0ka ($\sim \pm \mathrm{13}$more »xlink:href="cp-14-1869-2018-ie00001.svg" width="32pt" height="10pt" src="cp-14-1869-2018-ie00001.png"/> years), marking the onset of a multi-centennialperiod of relatively reduced ISM, which was punctuated by at least twomulti-decadal droughts between ∼3.9 and 4.0ka. The latter stands outin contrast with some previous proxy reconstructions of the ISM, in which the4.2ka event has been depicted as a singular multi-centennial drought.

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4. Contributions of biomass-burning, urban, and biogenic emissions to the concentrations and light-absorbing properties of particulate matter in central Amazonia during the dry season

   https://doi.org/10.5194/acp-19-7973-2019  

   de Sá, Suzane S. ; Rizzo, Luciana V. ; Palm, Brett B. ; Campuzano-Jost, Pedro ; Day, Douglas A. ; Yee, Lindsay D. ; Wernis, Rebecca ; Isaacman-VanWertz, Gabriel ; Brito, Joel ; Carbone, Samara ; et al ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Urbanization and deforestation have important impacts on atmosphericparticulate matter (PM) over Amazonia. This study presents observations andanalysis of PM₁ concentration, composition, and opticalproperties in central Amazonia during the dry season, focusing on theanthropogenic impacts. The primary study site was located 70 km downwind ofManaus, a city of over 2 million people in Brazil, as part of theGoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol massspectrometer (AMS) provided data on PM₁ composition, and aethalometermeasurements were used to derive the absorption coefficient b_abs,BrC ofbrown carbon (BrC) at 370 nm. Non-refractory PM₁ mass concentrationsaveraged 12.2 µg m⁻³ at the primary study site, dominated byorganics (83 %), followed by sulfate (11 %). A decrease inb_abs,BrC was observed as the mass concentration of nitrogen-containingorganic compounds decreased and the organic PM₁ O:C ratio increased,suggesting atmospheric bleaching of the BrC components. The organic PM₁was separated into six different classes by positive-matrix factorization(PMF), and the mass absorption efficiency E_abs associated with eachfactor was estimated through multivariate linear regression ofb_abs,BrC on the factor loadings. The largest E_abs values wereassociated with urban (2.04±0.14 m² g⁻¹) and biomass-burning(0.82±0.04 to 1.50±0.07 m² g⁻¹) sources. Together, these sources contributed at least 80 % ofb_abs,BrCmore »while accounting for 30 % to 40 % of the organic PM₁ massconcentration. In addition, a comparison of organic PM₁ compositionbetween wet and dry seasons revealed that only part of the 9-foldincrease in mass concentration between the seasons can be attributed tobiomass burning. Biomass-burning factor loadings increased by 30-fold,elevating its relative contribution to organic PM₁ from about 10 % inthe wet season to 30 % in the dry season. However, most of the PM₁mass (>60 %) in both seasons was accounted for by biogenicsecondary organic sources, which in turn showed an 8-fold seasonalincrease in factor loadings. A combination of decreased wet deposition andincreased emissions and oxidant concentrations, as well as a positivefeedback on larger mass concentrations are thought to play a role in theobserved increases. Furthermore, fuzzy c-means clustering identified threeclusters, namely “baseline”, “event”, and “urban” to representdifferent pollution influences during the dry season. The baseline cluster,representing the dry season background, was associated with a mean massconcentration of 9±3 µg m⁻³. This concentration increasedon average by 3 µg m⁻³ for both the urban and the event clusters.The event cluster, representing an increased influence of biomass burningand long-range transport of African volcanic emissions, was characterized byremarkably high sulfate concentrations. The urban cluster, representing theinfluence of Manaus emissions on top of the baseline, was characterized byan organic PM₁ composition that differed from the other two clusters.The differences discussed suggest a shift in oxidation pathways as well asan accelerated oxidation cycle due to urban emissions, in agreement withfindings for the wet season.

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5. Decadal-scale hotspot methane ebullition within lakes following abrupt permafrost thaw

   https://doi.org/10.1088/1748-9326/abc848  

   Walter Anthony, K. M. ; Lindgren, P. ; Hanke, P. ; Engram, M. ; Anthony, P. ; Daanen, R. P. ; Bondurant, A. ; Liljedahl, A. K. ; Lenz, J. ; Grosse, G. ; et al ( February 2021 , Environmental Research Letters)

   Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO₂) and methane (CH₄) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH₄ebullition with (a) an assessment of lake area changes delineated from high-resolution (1–2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH₄ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release¹⁴C-depleted CH₄from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH₄hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older¹⁴C ages of CH₄released from hotspot seepsmore »in older, expanding thermokarst lakes (¹⁴C_CH420 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger¹⁴C_CH4in new lakes (¹⁴C_CH48526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger¹⁴C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH₄-hotspot seep emissions following formation of thermokarst lakes as climate warms.

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