52 research outputs found
Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers,
[1] In recent decades, the diatom Didymosphenia geminata has emerged as nuisance species in river systems around the world. This periphytic alga forms large "blooms" in temperate streams, presenting a counterintuitive result: the blooms occur primarily in oligotrophic streams and rivers, where phosphorus (P) availability typically limits primary production. The goal of this study is to examine how high algal biomass is formed under low P conditions. We reveal a biogeochemical process by which D. geminata mats concentrate P from flowing waters. First, the mucopolysaccaride stalks of D. geminata adsorb both iron (Fe) and P. Second, enzymatic and bacterial processes interact with Fe to increase the biological availability of P. We propose that a positive feedback between total stalk biomass and high growth rate is created, which results in abundant P for cell division. The affinity of stalks for Fe in association with ironphosphorus biogeochemistry suggest a resolution to the paradox of algal blooms in oliogotrophic streams and rivers
Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers,
[1] In recent decades, the diatom Didymosphenia geminata has emerged as nuisance species in river systems around the world. This periphytic alga forms large "blooms" in temperate streams, presenting a counterintuitive result: the blooms occur primarily in oligotrophic streams and rivers, where phosphorus (P) availability typically limits primary production. The goal of this study is to examine how high algal biomass is formed under low P conditions. We reveal a biogeochemical process by which D. geminata mats concentrate P from flowing waters. First, the mucopolysaccaride stalks of D. geminata adsorb both iron (Fe) and P. Second, enzymatic and bacterial processes interact with Fe to increase the biological availability of P. We propose that a positive feedback between total stalk biomass and high growth rate is created, which results in abundant P for cell division. The affinity of stalks for Fe in association with ironphosphorus biogeochemistry suggest a resolution to the paradox of algal blooms in oliogotrophic streams and rivers
The convective transport of active species in the tropics (Contrast) experiment
The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was conducted from Guam (13.5degN, 144.8degE) during January-February 2014. Using the NSF/NCAR Gulfstream V research aircraft, the experiment investigated the photochemical environment over the tropical western Pacific (TWP) warm pool, a region of massive deep convection and the major pathway for air to enter the stratosphere during Northern Hemisphere (NH) winter. The new observations provide a wealth of information for quantifying the influence of convection on the vertical distributions of active species. The airborne in situ measurements up to 15-km altitude fill a significant gap by characterizing the abundance and altitude variation of a wide suite of trace gases. These measurements, together with observations of dynamical and microphysical parameters, provide significant new data for constraining and evaluating global chemistry climate models. Measurements include precursor and product gas species of reactive halogen compounds that impact ozone in the upper troposphere/lower stratosphere. High-accuracy, in situ measurements of ozone obtained during CONTRAST quantify ozone concentration profiles in the upper troposphere, where previous observations from balloon-borne ozonesondes were often near or below the limit of detection. CONTRAST was one of the three coordinated experiments to observe the TWP during January-February 2014. Together, CONTRAST, Airborne Tropical Tropopause Experiment (ATTREX), and Coordinated Airborne Studies in the Tropics (CAST), using complementary capabilities of the three aircraft platforms as well as ground-based instrumentation, provide a comprehensive quantification of the regional distribution and vertical structure of natural and pollutant trace gases in the TWP during NH winter, from the oceanic boundary to the lower stratosphere
Global Atmospheric Budget of Acetone: Air-Sea Exchange and the Contribution to Hydroxyl Radicals
Acetone is one of the most abundant oxygenated volatile organic compounds (VOCs) in the atmosphere. The oceans impose a strong control on atmospheric acetone, yet the oceanic fluxes of acetone remain poorly constrained. In this work, the global budget of acetone is evaluated using two global models: CAMâchem and GEOSâChem. CAMâchem uses an online airâsea exchange framework to calculate the bidirectional oceanic acetone fluxes, which is coupled to a dataâoriented machineâlearning approach. The machineâlearning algorithm is trained using a global suite of seawater acetone measurements. GEOSâChem uses a fixed surface seawater concentration of acetone to calculate the oceanic fluxes. Both model simulations are compared to airborne observations from a recent globalâscale, multiseasonal campaign, the NASA Atmospheric Tomography Mission (ATom). We find that both CAMâchem and GEOSâChem capture the measured acetone vertical distributions in the remote atmosphere reasonably well. The combined observational and modeling analysis suggests that (i) the ocean strongly regulates the atmospheric budget of acetone. The tropical and subtropical oceans are mostly a net source of acetone, while the highâlatitude oceans are a net sink. (ii) CMIP6 anthropogenic emission inventory may underestimate acetone and/or its precursors in the Northern Hemisphere. (iii) The MEGAN biogenic emissions model may overestimate acetone and/or its precursors, and/or the biogenic oxidation mechanisms may overestimate the acetone yields. (iv) The models consistently overestimate acetone in the upper troposphereâlower stratosphere over the Southern Ocean in austral winter. (v) Acetone contributes up to 30â40% of hydroxyl radical production in the tropical upper troposphere/lower stratosphere
Global Atmospheric Budget of Acetone: AirâSea Exchange and the Contribution to Hydroxyl Radicals
Acetone is one of the most abundant oxygenated volatile organic compounds (VOCs) in the atmosphere. The oceans impose a strong control on atmospheric acetone, yet the oceanic fluxes of acetone remain poorly constrained. In this work, the global budget of acetone is evaluated using two global models: CAMâchem and GEOSâChem. CAMâchem uses an online airâsea exchange framework to calculate the bidirectional oceanic acetone fluxes, which is coupled to a dataâoriented machineâlearning approach. The machineâlearning algorithm is trained using a global suite of seawater acetone measurements. GEOSâChem uses a fixed surface seawater concentration of acetone to calculate the oceanic fluxes. Both model simulations are compared to airborne observations from a recent globalâscale, multiseasonal campaign, the NASA Atmospheric Tomography Mission (ATom). We find that both CAMâchem and GEOSâChem capture the measured acetone vertical distributions in the remote atmosphere reasonably well. The combined observational and modeling analysis suggests that (i) the ocean strongly regulates the atmospheric budget of acetone. The tropical and subtropical oceans are mostly a net source of acetone, while the highâlatitude oceans are a net sink. (ii) CMIP6 anthropogenic emission inventory may underestimate acetone and/or its precursors in the Northern Hemisphere. (iii) The MEGAN biogenic emissions model may overestimate acetone and/or its precursors, and/or the biogenic oxidation mechanisms may overestimate the acetone yields. (iv) The models consistently overestimate acetone in the upper troposphereâlower stratosphere over the Southern Ocean in austral winter. (v) Acetone contributes up to 30â40% of hydroxyl radical production in the tropical upper troposphere/lower stratosphere
An Observationally Constrained Evaluation of the Oxidative Capacity in the Tropical Western Pacific Troposphere
Hydroxyl radical (OH) is the main daytime oxidant in the troposphere and determines the atmospheric lifetimes of many compounds. We use aircraft measurements of O3, H2O, NO, and other species from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, which occurred in the tropical western Pacific (TWP) during JanuaryâFebruary 2014, to constrain a photochemical box model and estimate concentrations of OH throughout the troposphere. We find that tropospheric column OH (OHCOL) inferred from CONTRAST observations is 12 to 40% higher than found in chemical transport models (CTMs), including CAM-chem-SD run with 2014 meteorology as well as eight models that participated in POLMIP (2008 meteorology). Part of this discrepancy is due to a clear-sky sampling bias that affects CONTRAST observations; accounting for this bias and also for a small difference in chemical mechanism results in our empirically based value of OHCOL being 0 to 20% larger than found within global models. While these global models simulate observed O3 reasonably well, they underestimate NOx (NOâ+âNO2) by a factor of two, resulting in OHCOL ~30% lower than box model simulations constrained by observed NO. Underestimations by CTMs of observed CH3CHO throughout the troposphere and of HCHO in the upper troposphere further contribute to differences between our constrained estimates of OH and those calculated by CTMs. Finally, our calculations do not support the prior suggestion of the existence of a tropospheric OH minimum in the TWP, because during JanuaryâFebruary 2014 observed levels of O3 and NO were considerably larger than previously reported values in the TWP
The Defensive Role of Volatile Emission and Extrafloral Nectar Secretion for Lima Bean in Nature
Lima bean (Phaseolus lunatus) features two indirect anti-herbivore defensesâemission of volatile organic compounds (VOCs) and secretion of extrafloral nectar (EFN)âwhich are both inducible upon herbivore damage. In a previous field study, Lima bean benefited from the simultaneous induction of the two defenses, yet it remained unclear whether both had contributed to plant protection. Our experimental approach aimed at studying the defensive role of both indirect defenses simultaneously. Tendrils were sprayed with jasmonic acid (JA) to induce both defenses, and performance was compared to that of others that were treated with a synthetic blend of either EFN or VOCs. Confirming earlier results, JA treatment and application of the VOC mixture induced EFN secretion in treated tendrils in quantitatively similar amounts. The composition of the applied synthetic blend of EFN was adjusted to match the concentration of EFN secreted from JA- and VOC-treated tendrils. Repeated application of either enhanced the performance of several fitness-relevant plant parameters such as growth rate and flower production. Tendrils treated with JA showed a similar trend, yet some fitness-related parameters responded less to this treatment. This suggests a minor importance of any putative JA-dependent direct defense traits or higher costs of JA-elicited responses as compared to VOCS and EFN, as otherwise JA-treated tendrils should have outperformed VOC- and EFN-treated tendrils. Moreover, the beneficial effect of applying synthetic EFN alone equaled or exceeded that of VOCs and JA. Ants were by far the dominant group among the arthropods that was attracted to JA-, VOC-, or EFN-treated tendrils. The results suggest that EFN plays a more important role as an indirect defense of lima bean than VOCs or any other JA-responsive trait
A Lagrangian Model Diagnosis of Stratospheric Contributions to Tropical Midtropospheric Air
Airborne in situ observations during the Convective Transport of Active Species in the Tropics campaign in JanuaryâFebruary 2014 revealed a large region over the tropical western Pacific where the midtroposphere had a layered structure with a distinct chemical signature of high ozone and low water vapor (HOLW). The observed anticorrelation between ozone and water vapor is a strong indication of transport from the midlatitude upper troposphere and lower stratosphere. This work presents a diagnosis of stratospheric air in the tropical western Pacific midtroposphere through isentropic transport and mixing. Using the Chemical Lagrangian Model of the Stratosphere, we characterize and quantify the contribution of transported stratospheric air to the observed HOLW layers. The result indicates that the isentropic transport is an effective process for stratospheric air to mix into the tropical midtroposphere. Using the modeled stratospheric tracer and 3âD back trajectories, we identified that 60% of the observed HOLW air masses contain significant stratospheric influence. We have also examined possible contribution to the HOLW layer from ozone production related to biomass burning emissions. Clear chemical signature of this process is found in âŒ8% of the HOLW air masses, identified by positive correlations among O3, HCN, and CO. This analysis provides the first quantitative diagnosis of the contribution from the stratosphereâtoâtroposphere transport, highlights the importance of mixing in chemical transport, and demonstrates the limitations of pure Lagrangian trajectory calculations in quantifying transport
On the origin of pronounced O3 gradients in the thunderstorm outflow region during DC3
Unique in situ measurements of CO, O3, SO2, CH4, NO, NOx, NOy, VOC, CN, and rBC were carried out with the German Deutsches Zentrum fĂŒr Luft- und Raumfahrt (DLR)-Falcon aircraft in the central U.S. thunderstorms during the Deep Convective Clouds and Chemistry experiment in summer 2012. Fresh and aged anvil outflow (9â12âkm) from supercells, mesoscale convective systems, mesoscale convective complexes, and squall lines were probed over Oklahoma, Texas, Colorado, and Kansas. For three case studies (30 May and 8 and 12 June) a combination of trace species, radar, lightning, and satellite information, as well as model results, were used to analyze and design schematics of major trace gas transport pathways within and in the vicinity of the probed thunderstorms
Use of Airborne In Situ VOC Measurements to Estimate Transit Time Spectrum: An ObservationâBased Diagnostic of Convective Transport
Convective transport from the marine boundary layer to the upper troposphere (UT) is investigated using airborne in situ measurements of chemical species over the tropical western Pacific. Using 42 volatile organic compounds with photochemical lifetimes ranging from shorter than a day to multiple decades, we derive a transit time spectrum G(t) and the associated modal and mean transit times for the UT air mass over the convectively dominant tropical western Pacific region. G(t) describes relative contributions of air masses transported from the marine boundary layer to the UT via all transport paths with different transit times. We further demonstrate that the volatile organic compoundâderived transit time scale is broadly comparable to that estimated from convective mass flux. The observationâbased transit time spectrum not only provides insights into convective transport pathways, but also has the potential to serve as an effective diagnostic for evaluating the representation of convective transport in global models.
Plain Language Summary
Tropical deep convection is an important mechanism whereby air mass and chemical species are transported from near the surface to the upper troposphere and lower stratosphere. This transport process is difficult to quantify by observations or represent in models because of the small spatial scales and short temporal scales involved. In this study, we present a method to characterize convective transport using the framework of transit time spectrum, which describes the relative contributions of different transport paths with different transit times. We demonstrate that convective transport transit time spectrum can be derived using airborne in situ measurements of chemical species with a wide range of lifetimes, and we illustrate the wealth of information they provide for quantifying transport processes. This analysis method has the potential to serve as a unique diagnostic for evaluating the representation of convective transport in global models.
Key Points
Airborne in situ measurements of VOCs with a wide range of lifetimes are used to quantify a convectiveâtransport transit time spectrum
The transit time scale derived from VOC measurements is broadly comparable to that estimated from convective mass flux
The estimated transit time spectrum has the potential to serve as an effective diagnostic for evaluating convective transport in CCM
- âŠ