174 research outputs found
The effect of instrumental stray light on Brewer and Dobson total ozone measurements
Dobson and Brewer spectrophotometers are the primary, standard
instruments for ground-based ozone measurements under the World
Meteorological Organization's (WMO) Global Atmosphere Watch program. The
accuracy of the data retrieval for both instruments depends on a knowledge of
the ozone absorption coefficients and some assumptions underlying the data
analysis. Instrumental stray light causes nonlinearity in the response of
both the Brewer and Dobson to ozone at large ozone slant paths. In addition,
it affects the effective ozone absorption coefficients and extraterrestrial
constants that are both instrument-dependent. This effect has not been taken
into account in the calculation of ozone absorption coefficients that are
currently recommended by WMO for the Dobson network. The ozone absorption
coefficients are calculated for each Brewer instrument individually, but in
the current procedure the effect of stray light is not considered. This
study documents the error caused by the effect of stray light in the Brewer
and Dobson total ozone measurements using a physical model for each
instrument. For the first time, new ozone absorption coefficients are
calculated for the Brewer and Dobson instruments, taking into account the
stray light effect. The analyses show that the differences detected between
the total ozone amounts deduced from Dobson AD and CD pair wavelengths are
related to the level of stray light within the instrument. The discrepancy
introduced by the assumption of a fixed height for the ozone layer for ozone
measurements at high latitude sites is also evaluated. The ozone data
collected by two Dobson instruments during the period of December 2008 to
December 2014 are compared with ozone data from a collocated double
monochromator Brewer spectrophotometer (Mark III). The results illustrate the
dependence of Dobson AD and CD pair measurements on stray light.</p
A calibration procedure which accounts for non-linearity in single-monochromator Brewer ozone spectrophotometer measurements
It is now known that single-monochromator Brewer spectrophotometer
ozone and sulfur dioxide measurements suffer from non-linearity at large
ozone slant column amounts due to the presence of instrumental stray light
caused by scattering within the optics of the instrument. Because of the
large gradient in the ozone absorption spectrum in the near-ultraviolet, the
atmospheric spectra measured by the instrument possess a very large gradient
in intensity in the 300 to 325 nm wavelength region. This results in a
significant sensitivity to stray light when there is more than 1000 Dobson
units (DU) of ozone in the light path. As the light path (air mass) through
ozone increases, the stray-light effect on the measurements also increases.
The measurements can be of the order of 10 %, low for an ozone column of
600 DU and an air mass factor of 3 (1800 DU slant column amount), which is an
example of conditions that produce large slant column amounts.
Primary calibrations for the Brewer instrument are carried out at Mauna Loa
Observatory in Hawaii and Izana Observatory in Tenerife. They are done
using the Langley plot method to extrapolate a set of measurements made
under a constant ozone vertical column to an extraterrestrial calibration
constant. Since the effects of a small non-linearity at moderate ozone
paths may still be important, a better calibration procedure should account
for the non-linearity of the instrument response. Studies involving the
scanning of a laser source have been used to characterize the stray-light
response of the Brewer (Fioletov et al., 2000), but until recently
these data have not been used to elucidate the relationship between the
stray-light response and the ozone measurement non-linearity.
In a study done by Karppinen et al. (2015), a method for correcting stray
light has been presented that uses an additive correction, which is
determined via instrument slit characterization and a radiative transfer
model simulation and is then applied to the single Brewer data
(Karppinen et al., 2015).
The European Brewer Network is also applying stray-light corrections, which
includes an iterative process that results in correcting the single Brewer
data to agree with double Brewer data (Rimmer et
al., 2018; Redondas et al., 2018). The
first model requires measurements of the slit function and the latter method
relies on a calibrated instrument, such as a double Brewer, to characterize
the instrument and to determine a correction for stray light.
This paper presents a simple and practical method to correct for the
effects of stray light, which includes a mathematical model of the instrument
response and a non-linear retrieval approach that calculates the best values
for the model parameters. The model can then be used in reverse to provide
more accurate ozone values up to a defined maximum ozone slant path. The
parameterization used was validated using an instrument physical model
simulation. This model can be applied independently to any Brewer instrument
and correct for the effects of stray light.</p
Upper Tropospheric Water Vapour Variability at High Latitudes- Part 1: Influence of the Annular Modes
Seasonal and monthly zonal medians of water vapour in the upper troposphere and lower stratosphere (UTLS) are calculated for both Atmospheric Chemistry Experiment (ACE) instruments for the northern and southern high-latitude regions (60-90° N and 60-90°S). Chosen for the purpose of observing high-latitude processes, the ACE orbit provides sampling of both regions in 8 of 12 months of the year, with coverage in all seasons. The ACE water vapour sensors, namely MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) and the Fourier Transform Spectrometer (ACE-FTS) are currently the only satellite instruments that can probe from the lower stratosphere down to the mid-troposphere to study the vertical profile of the response of UTLS water vapour to the annular modes. The Arctic oscillation (AO), also known as the northern annular mode (NAM), explains 64 % (r = -0.80) of the monthly variability in water vapour at northern high latitudes observed by ACE-MAESTRO between 5 and 7 km using only winter months (January to March, 2004-2013). Using a seasonal time step and all seasons, 45% of the variability is explained by the AO at 6.5 ± -0.5 km, similar to the 46 % value obtained for southern high latitudes at 7.5 ± 0.5 km explained by the Antarctic oscillation or southern annular mode (SAM). A large negative AO event in March 2013 produced the largest relative water vapour anomaly at 5.5-km (+70 %) over the ACE record. A similarly large event in the 2010 boreal winter, which was the largest negative AO event in the record (1950-2015), led to \u3e 50 % increases in water vapour observed by MAESTRO and ACE-FTS at 7.5 km
A Global Ozone Climatology from Ozone Soundings via Trajectory Mapping: A Stratospheric Perspective
This study explores a domain-filling trajectory approach to generate a global ozone climatology from sparse ozonesonde data. Global ozone soundings of 51,898 profiles at 116 stations over 44 years (1965-2008) are used, from which forward and backward trajectories are performed for 4 days, driven by a set of meteorological reanalysis data. Ozone mixing ratios of each sounding from the surface to 26 km altitude are assigned to the entire path along the trajectory. The resulting global ozone climatology is archived monthly for five decades from the 1960s to the 2000s with grids of 5 degree 5 degree 1 km (latitude, longitude, and altitude). It is also archived yearly from 1965 to 2008. This climatology is validated at 20 ozonesonde stations by comparing the actual ozone sounding profile with that found through the trajectories, using the ozone soundings at all the stations except one being tested. The two sets of profiles are in good agreement, both individually with correlation coefficients between 0.975 and 0.998 and root mean square (RMS) differences of 87 to 482 ppbv, and overall with a correlation coefficient of 0.991 and an RMS of 224 ppbv. The ozone climatology is also compared with two sets of satellite data, from the Satellite Aerosol and Gas Experiment (SAGE) and the Optical Spectrography and InfraRed Imager System (OSIRIS). Overall, the ozone climatology compares well with SAGE and OSIRIS data by both seasonal and zonal means. The mean difference is generally under 20 above 15 km. The comparison is better in the northern hemisphere, where there are more ozonesonde stations, than in the southern hemisphere; it is also better in the middle and high latitudes than in the tropics, where assimilated winds are imperfect in some regions. This ozone climatology can capture known features in the stratosphere, as well as seasonal and decadal variations of these features. Furthermore, it provides a wealth of detail about longitudinal variations in the stratosphere such as the spring ozone maximum over the Canadian Arctic. It also covers higher latitudes than current satellite data. The climatology shows clearly the depletion of ozone from the 1970s to the mid 1990s and ozone recovery in the 2000s. When this climatology is used as the upper boundary condition in an Environment Canada operational chemical forecast model, the forecast is improved in the vicinity of the upper tropospherelower stratosphere region. As this ozone climatology is neither dependent on a priori data or photochemical modeling, it provides independent information and insight that can supplement satellite data and model simulations and enhance our understanding of stratospheric ozone
Upper tropospheric water vapour variability at high latitudes – Part 1: Influence of the annular modes
Seasonal and monthly zonal medians of water vapour in the upper troposphere
and lower stratosphere (UTLS) are calculated for both Atmospheric Chemistry
Experiment (ACE) instruments for the northern and southern high-latitude
regions (60–90° N and 60–90° S). Chosen for the purpose
of observing high-latitude processes, the ACE orbit provides sampling of both
regions in 8 of 12 months of the year, with coverage in all seasons. The ACE
water vapour sensors, namely MAESTRO (Measurements of Aerosol Extinction in
the Stratosphere and Troposphere Retrieved by Occultation) and the Fourier
Transform Spectrometer (ACE-FTS) are currently the only satellite instruments
that can probe from the lower stratosphere down to the mid-troposphere to
study the vertical profile of the response of UTLS water vapour to the
annular modes.
The Arctic oscillation (AO), also known as the northern annular mode (NAM),
explains 64 % (r = −0.80) of the monthly variability in water vapour at
northern high latitudes observed by ACE-MAESTRO between 5 and 7 km using
only winter months (January to March, 2004–2013). Using a seasonal time step
and all seasons, 45 % of the variability is explained by the AO at
6.5 ± 0.5 km, similar to the 46 % value obtained for southern high
latitudes at 7.5 ± 0.5 km explained by the Antarctic oscillation or
southern annular mode (SAM). A large negative AO event in March 2013 produced
the largest relative water vapour anomaly at 5.5 km (+70 %) over the
ACE record. A similarly large event in the 2010 boreal winter, which was the
largest negative AO event in the record (1950–2015), led to
> 50 % increases in water vapour observed by MAESTRO and
ACE-FTS at 7.5 km
Verification of passive cooling techniques in the Super-FRS beam collimators
The Super FRagment Separator (Super-FRS) at the FAIR facility will be the largest in-flight separator of heavy ions in the world. One of the essential steps in the separation procedure is to stop the unwanted ions with beam collimators. In one of the most common situations, the heavy ions are produced by a fission reaction of a primary 238U-beam (1.5 GeV/u) hitting a 12C target (2.5 g/cm^2). In this situation, some of the produced ions are highly charged states of 238U. These ions can reach the collimators with energies of up to 1.3 GeV/u and a power of up to 500 W. Under these conditions, a cooling system is required to prevent damage to the collimators and to the corresponding electronics. Due to the highly radioactive environment, both the collimators and the cooling system must be suitable for robot handling. Therefore, an active cooling system is undesirable because of the increased possibility of malfunctioning and other complications. By using thermal simulations (performed with NX9 of Siemens PLM), the possibility of passive cooling is explored. The validity of these simulations is tested by independent comparison with other simulation programs and by experimental verification. The experimental verification is still under analysis, but preliminary results indicate that the explored passive cooling option provides sufficient temperature reduction
Observation of a charged charmoniumlike structure in at GeV
We study the process at a
center-of-mass energy of 4.26GeV using a 827pb data sample obtained with
the BESIII detector at the Beijing Electron Positron Collider. Based on a
partial reconstruction technique, the Born cross section is measured to be
pb. We observe a structure near the
threshold in the recoil mass spectrum, which we denote as the
. The measured mass and width of the structure are
MeV/c and MeV, respectively. Its
production ratio is determined to be . The first uncertainties
are statistical and the second are systematic.Comment: 7 pages, 4 figures, 1 table; version accepted to be published in PR
Search for C-parity violation in and
Using events recorded in
collisions at 3.686 GeV with the BESIII at the BEPCII collider, we
present searches for C-parity violation in and decays via . No significant
signals are observed in either channel. Upper limits on the branching fractions
are set to be and
at the 90\%
confidence level. The former is one order of magnitude more stringent than the
previous upper limit, and the latter represents the first limit on this decay
channel.Comment: 7 pages, 2 figure
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