ABSTRACT The transitional millisecond pulsar (MSP) binary system PSR J1023+0038 re-entered an accreting state in 2013 June in which it bears many similarities to low-mass X-ray binaries (LMXBs) in ...quiescence or near-quiescence. At a distance of just 1.37 kpc, PSR J1023+0038 offers an unsurpassed ability to study low-level accretion onto a highly magnetized compact object. We have monitored PSR J1023+0038 intensively using radio imaging with the Karl G. Jansky Very Large Array, the European VLBI Network and the Low Frequency Array, seeing rapidly variable, flat spectrum emission that persists over a period of six months. The flat spectrum and variability are indicative of synchrotron emission originating in an outflow from the system, most likely in the form of a compact, partially self-absorbed jet, as is seen in LMXBs at higher accretion rates. The radio brightness, however, greatly exceeds extrapolations made from observations of more vigorously accreting neutron star LMXB systems. We postulate that PSR J1023+0038 is undergoing radiatively inefficient "propeller-mode" accretion, with the jet carrying away a dominant fraction of the liberated accretion luminosity. We confirm that the enhanced γ-ray emission seen in PSR J1023+0038 since it re-entered an accreting state has been maintained; the increased γ-ray emission in this state can also potentially be associated with propeller-mode accretion. Similar accretion modes can be invoked to explain the radio and X-ray properties of the other two known transitional MSP systems XSS J12270-4859 and PSR J1824-2452I (M28I), suggesting that radiatively inefficient accretion may be a ubiquitous phenomenon among (at least one class of) neutron star binaries at low accretion rates.
We study the evolution of tropospheric ozone over the period 1979–2010 using a chemistry‐climate model employing a stratosphere‐troposphere chemistry scheme. By running with specified dynamics, the ...key feedback of composition on meteorology is suppressed, isolating the chemical response. By using historical forcings and emissions, interactions between processes are realistically represented. We use the model to assess how the ozone responds over time and to investigate model responses and trends. We find that the chlorofluorocarbon (CFC)‐driven decrease in stratospheric ozone plays a significant role in the tropospheric ozone burden. Over the period 1979–1994, the decline in transport of ozone from the stratosphere, partially offsets an emissions‐driven increase in tropospheric ozone production. From 1994–2010, despite a leveling off in emissions, increased stratosphere‐to‐troposphere transport of ozone drives a small increase in the tropospheric ozone burden. These results have implications for the impact of future stratospheric ozone recovery on air quality and radiative forcing.
Plain Language Summary
We use a modeling approach to study the effect of stratospheric ozone depletion on the composition of the troposphere. We focus on the period 1979–2010 and use a chemistry‐climate model employing historical emissions, climate forcing, and meteorology. Our model has a good description of both stratospheric and tropospheric ozone chemistry and allows us to calculate the effect of exchange between stratosphere and troposphere. We show that stratospheric ozone depletion over the period 1979–2010 has a critical effect on tropospheric composition – with less ozone in the lower stratosphere, there is less transport to the troposphere, and this offsets an emissions‐driven increase in ozone production in the troposphere. Such combined studies are important to quantify the future effects of stratospheric ozone recovery on the evolution of tropospheric composition.
Key Points
We show that stratospheric ozone recovery has an affect on tropospheric ozone levels
Stratospheric ozone loss over the period 1979–1994 offset an emissions‐driven increase in tropospheric ozone
Future stratospheric ozone recovery and changes to the rate of stratosphere‐to‐troposphere transport will be important in the future
Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or ...cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Isoprene is a~precursor to tropospheric ozone, a key pollutant and greenhouse gas. Anthropogenic activity over the coming century is likely to cause large changes in atmospheric CO2 levels, climate ...and land use, all of which will alter the global vegetation distribution leading to changes in isoprene emissions. Previous studies have used global chemistry-climate models to assess how possible changes in climate and land use could affect isoprene emissions and hence tropospheric ozone. The chemistry of isoprene oxidation, which can alter the concentration of ozone, is highly complex, therefore it must be parameterised in these models. In this work, we compare the effect of four different reduced isoprene chemical mechanisms, all currently used in Earth system models, on tropospheric ozone. Using a box model we compare ozone in these reduced schemes to that in a more explicit scheme (the Master Chemical Mechanism) over a range of NOx and isoprene emissions, through the use of O3 isopleths. We find that there is some variability, especially at high isoprene emissions, caused by differences in isoprene-derived NOx reservoir species. A global model is then used to examine how the different reduced schemes respond to potential future changes in climate, isoprene emissions, anthropogenic emissions and land use change. We find that, particularly in isoprene-rich regions, the response of the schemes varies considerably. The wide-ranging response is due to differences in the model descriptions of the peroxy radical chemistry, particularly their relative rates of reaction towards NO, leading to ozone formation, or HO2, leading to termination. Also important is the yield of isoprene nitrates and peroxyacyl nitrate precursors from isoprene oxidation. Those schemes that produce less of these NOx reservoir species, tend to produce more ozone locally and less away from the source region. We also note changes in other key oxidants such as NO3 and OH (due to the inclusion of additional isoprene-derived HOx recycling pathways). These have implications for secondary organic aerosol formation, as does the inclusion of an epoxide formation pathway in one of the mechanisms. By combining the emissions and O3 data from all of the global model integrations, we are able to construct isopleth plots comparable to those from the box model analysis. We find that the global and box model isopleths show good qualitative agreement, suggesting that comparing chemical mechanisms with a box model in this framework is a useful tool for assessing mechanistic performance in complex global models. We conclude that as the choice of reduced isoprene mechanism may alter both the magnitude and sign of the ozone response, how isoprene chemistry is parameterised in perturbation experiments such as these is a crucially important consideration. More measurements and laboratory studies are needed to validate these reduced mechanisms especially under high-volatile-organic-compound, low-NOx conditions.
Gravitationally bound three-body systems have been studied for hundreds of years and are common in our Galaxy. They show complex orbital interactions, which can constrain the compositions, masses and ...interior structures of the bodies and test theories of gravity, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7, 8; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar MSymbol: see text(1.4378(13), where MSymbol: see textis the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15)MSymbol: see text and 0.4101(3))MSymbol: see text, as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Field campaigns have been carried out with the FAGE (fluorescence assay by gas expansion) technique in remote biogenic environments in the last decade to quantify the in situ concentrations of OH, ...the main oxidant in the atmosphere. These data have revealed concentrations of OH radicals up to a factor of 10 higher than predicted by models, whereby the disagreement increases with decreasing NO concentration. This was interpreted as a major lack in our understanding of the chemistry of biogenic VOCs (volatile organic compounds), particularly isoprene, which are dominant in remote pristine conditions. But interferences in these measurements of unknown origin have also been discovered for some FAGE instruments: using a pre-injector, all ambient OH is removed by fast reaction before entering the FAGE cell, and any remaining OH signal can be attributed to an interference. This technique is now systematically used for FAGE measurements, allowing the reliable quantification of ambient OH concentrations along with the signal due to interference OH. However, the disagreement between modelled and measured high OH concentrations of earlier field campaigns as well as the origin of the now-quantifiable background OH is still not understood. We present in this paper the compelling idea that this interference, and thus the disagreement between model and measurement in earlier field campaigns, might be at least partially due to the unexpected decomposition of a new class of molecule, ROOOH, within the FAGE instruments. This idea is based on experiments, obtained with the FAGE set-up of the University of Lille, and supported by a modelling study. Even though the occurrence of this interference will be highly dependent on the design and measurement conditions of different FAGE instruments, including ROOOH in atmospheric chemistry models might reflect a missing piece of the puzzle in our understanding of OH in clean atmospheres.
The year when total column ozone (TCO) returns to 1980 levels is commonly used to quantify recovery from disturbance caused by ozone‐depleting substances. This date is reasonable but somewhat ...arbitrary. Here we borrow an indicator from climate change research, the signal‐to‐noise (S/N) metric to investigate how TCO might return to statistically similar levels to those of the pre‐ozone hole era (1960–1970). We calculate S/N for ozone projections from the Chemistry‐Climate Model Initiative. In most regions, a return to 1980 levels doesn't represent TCO recovery to pre‐disturbance conditions because it does not account for internal variability or reflect when statistically significant TCO losses occurred. Future projections show that, in most regions, TCO “de‐emerges” (i.e., S/N becomes less than one standard deviation from the pre‐disturbance mean) before it returns to its 1980 value. We conclude that S/N is an appropriate, perhaps complementary metric for determining when TCO returns to a pre‐disturbance state.
Plain Language Summary
The stratospheric ozone layer is expected to recover from the damaging effects of chlorofluorocarbons (CFCs) and other halogen‐containing source gases this century. Ozone recovery is traditionally but somewhat arbitrarily defined as the year when total column ozone (TCO) abundances return to 1980 levels. Here, we use a metric commonly used in the climate change emergence literature—signal‐to‐noise (S/N)—to quantify statistically significant changes in TCO in models and observations relative to a period before CFC‐induced ozone depletion began. The S/N framing ties recovery to unperturbed environmental conditions, rather than to the environment in a specific year. We argue that tying recovery to unperturbed local environmental conditions is at least as reasonable as tying it to the same global date. Our calculations, which account for natural variability, show that 1980 does not reflect the onset of statistically significant ozone loss. Using a return of TCO to 1980 levels underestimates the time of recovery in Antarctica, where losses have been most significant, and overestimates the time to recovery in other regions. We conclude that signal‐to‐noise is a useful metric to assess the return of TCO to its baseline state and evaluate the effectiveness of the Montreal Protocol.
Key Points
We present signal‐to‐noise calculations of stratospheric ozone depletion in chemistry‐climate models relative to 1960–1970
Statistically significant ozone depletion does not emerge in 1980, which is traditionally used as a benchmark for ozone recovery
In most regions de‐emergence occurs before ozone returns to 1980 levels except in Antarctica where de‐emergence occurs 25 years later
Tropospheric ozone in CMIP6 simulations Griffiths, Paul T; Murray, Lee T; Zeng, Guang ...
Atmospheric chemistry and physics,
03/2021, Letnik:
21, Številka:
5
Journal Article
Recenzirano
Odprti dostop
The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled ...atmosphere–ocean chemistry–climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 ± 36 Tg in 1850 to a mean value of 356 ± 31 Tg for the period 2005–2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 ± 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 ± 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3-LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere–troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850–2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden.
A stratosphere-resolving configuration of the Met Office's Unified Model (UM) with the United Kingdom Chemistry and Aerosols (UKCA) scheme is used to investigate the atmospheric response to changes ...in (a) greenhouse gases and climate, (b) ozone-depleting substances (ODSs) and (c) non-methane ozone precursor emissions. A suite of time-slice experiments show the separate, as well as pairwise, impacts of these perturbations between the years 2000 and 2100. Sensitivity to uncertainties in future greenhouse gases and aerosols is explored through the use of the Representative Concentration Pathway (RCP) 4.5 and 8.5 scenarios. The results highlight an important role for the stratosphere in determining the annual mean tropospheric ozone response, primarily through stratosphere–troposphere exchange (STE) of ozone. Under both climate change and reductions in ODSs, increases in STE offset decreases in net chemical production and act to increase the tropospheric ozone burden. This opposes the effects of projected decreases in ozone precursors through measures to improve air quality, which act to reduce the ozone burden. The global tropospheric lifetime of ozone (τO3) does not change significantly under climate change at RCP4.5, but it decreases at RCP8.5. This opposes the increases in τO3 simulated under reductions in ODSs and ozone precursor emissions. The additivity of the changes in ozone is examined by comparing the sum of the responses in the single-forcing experiments to those from equivalent combined-forcing experiments. Whilst the ozone responses to most forcing combinations are found to be approximately additive, non-additive changes are found in both the stratosphere and troposphere when a large climate forcing (RCP8.5) is combined with the effects of ODSs.
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