2,685 research outputs found
ROOOH: A missing piece of the puzzle for OH measurements in low-NO environments?
Abstract. 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.
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X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Dossier on Govan Young: Exploring children’s historical consciousness through film and archaeology
Govan Young (2017) is a 30-minute documentary in which schoolchildren from
Glasgow learn of the area’s important but largely unknown medieval history. This
dossier brings together four essays that reflect on the film from various academic
perspectives – film studies, archaeology and education – to explore how
schoolchildren might learn about the past, and develop a historical consciousness,
by participating in film-making projects. The dossier also reflects on how educators
can learn from those whom they are supposedly teaching, thereby highlighting that
experimental pedagogical projects often bring unexpected learning outcomes
into being. Consequently, educators must resist the pressures to predict the
outcomes of projects, and must strive to keep the future open-ended
No detectable radio emission from the magnetar-like pulsar in Kes 75
The rotation-powered pulsar PSR J1846-0258 in the supernova remnant Kes 75
was recently shown to have exhibited magnetar-like X-ray bursts in mid-2006.
Radio emission has not yet been observed from this source, but other
magnetar-like sources have exhibited transient radio emission following X-ray
bursts. We report on a deep 1.9 GHz radio observation of PSR J1846-0258 with
the 100-m Green Bank Telescope in late 2007 designed to search for radio
pulsations or bursts from this target. We have also analyzed three shorter
serendipitous 1.4 GHz radio observations of the source taken with the 64-m
Parkes telescope during the 2006 bursting period. We detected no radio emission
from PSR J1846-0258 in either the Green Bank or Parkes datasets. We place an
upper limit of 4.9 \mu Jy on coherent pulsed emission from PSR J1846-0258 based
on the 2007 November 2 observation, and an upper limit of 27 \mu Jy around the
time of the X-ray bursts. Serendipitously, we observed radio pulses from the
nearby RRAT J1846-02, and place a 3\sigma confidence level upper limit on its
period derivative of 1.7 * 10^{-13}, implying its surface dipole magnetic field
is less than 2.6 * 10^{13} G.Comment: 15 pages, 2 figures, submitted to Ap
Binary evolution with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
Extension, development, and evaluation of the representation of the OH-initiated dimethyl sulfide (DMS) oxidation mechanism in the Master Chemical Mechanism (MCM) v3.3.1 framework
Understanding dimethyl sulfide (DMS) oxidation can help us constrain its contribution to Earth's radiative balance. Following the discovery of hydroperoxymethyl thioformate (HPMTF) as a DMS oxidation product, a range of new experimental chamber studies have since improved our knowledge of the oxidation mechanism of DMS and delivered detailed chemical mechanisms. However, these mechanisms have not undergone formal intercomparisons to evaluate their performance.
This study aimed to synthesise the recent experimental studies and develop a new, near-explicit, DMS mechanism, through a thorough literature review. A simple box model was then used with the mechanism to simulate a series of chamber experiments and evaluated through comparison with four published mechanisms. Our modelling shows that the mechanism developed in this work outperformed the other mechanisms on average when compared to the experimental chamber data, having the lowest fractional gross error for 8 out of the 14 DMS oxidation products studied. A box model of a marine boundary layer was also run, demonstrating that the deviations in the mechanisms seen when comparing them against chamber data are also prominent under more atmospherically relevant conditions.
Although this work demonstrates the need for further experimental work, the mechanism developed in this work has been evaluated against a range of experiments, which validate the mechanism and reduce the bias from individual experiments. Our mechanism provides a good basis for a near-explicit DMS oxidation mechanism that would include other initiation reactions (e.g. halogens) and can be used to compare the performance of reduced mechanisms used in global models.</p
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