487 research outputs found
Optimizing the Performance of Aerosol Photoacoustic Cells using a Finite Element Model. Part 1: Method Validation and Application to Single-Resonator Multipass Cells
This is the final version. Available on open access from Taylor & Francis via the DOI in this recordData access statement:
For data related to this paper, please contact Michael I. Cotterell ([email protected]) or Justin M. Langridge ([email protected]).Photoacoustic spectroscopy is the technique-of-choice for non-contact and in situ measurements of light absorption coefficients for aerosols. For most aerosol photoacoustic (PA) detectors, a key process is the amplification of the acoustic pressure wave generated from light absorption through excitation of a pressure eigenmode of a PA cell. To our knowledge, no modelling of the acoustics, sensitivity or signal-to-background ratio (SBR) has been performed for the PA cells applied commonly to aerosol absorption measurements. In this Part 1 manuscript, we develop a finite element method (FEM) framework to simulate the acoustic response and SBR of photoacoustic cells. Furthermore, we validate this modelling framework by comparing FEM predictions of single-resonator PA cells with measurements using a prototype single-resonator cell, the geometry of which can be readily adjusted. Indeed, single-resonator cells are applied commonly to aerosol absorption measurements. We show that our model predicts accurately the trends in acoustic properties with changes to cell geometry. We investigate how common geometric features, used to supress detection of background and noise processes, impact on the SBR of single-resonator PA cells. Such features include using multiple acoustic buffer volumes and tuneable air columns. The FEM model and measurements described in this paper provide the foundation of a companion paper that reports the acoustic properties and optimization of a two-resonator PA cell used commonly in aerosol research.Defence Science and Technology Laboratory (DSTL)Royal Society of ChemistryAnalytical Chemistry Trust FundNatural Environment Research Council (NERC
Weird inflects but OK : Making sense of morphological generation errors
We conduct a manual error analysis of the CoNLL-SIGMORPHON 2017 Shared Task on Morphological Reinflection. In this task, systems are given a word in citation form (e.g., hug) and asked to produce the corresponding inflected form (e.g., the simple past hugged). This design lets us analyze errors much like we might analyze children's production errors. We propose an error taxonomy and use it to annotate errors made by the top two systems across twelve languages. Many of the observed errors are related to inflectional patterns sensitive to inherent linguistic properties such as animacy or affect; many others are failures to predict truly unpredictable inflectional behaviors. We also find nearly one quarter of the residual "errors" reflect errors in the gold data. © 2019 Association for Computational Linguistics.Peer reviewe
Non-invasive respiratory support in the management of acute COVID-19 pneumonia: considerations for clinical practice and priorities for research
Non-invasive respiratory support (NIRS) has increasingly been used in the management of COVID-19-associated acute respiratory failure, but questions remain about the utility, safety, and outcome benefit of NIRS strategies. We identified two randomised controlled trials and 83 observational studies, compromising 13 931 patients, that examined the effects of NIRS modalities-high-flow nasal oxygen, continuous positive airway pressure, and bilevel positive airway pressure-on patients with COVID-19. Of 5120 patients who were candidates for full treatment escalation, 1880 (37%) progressed to invasive mechanical ventilation and 3658 of 4669 (78%) survived to study end. Survival was 30% among the 1050 patients for whom NIRS was the stated ceiling of treatment. The two randomised controlled trials indicate superiority of non-invasive ventilation over high-flow nasal oxygen in reducing the need for intubation. Reported complication rates were low. Overall, the studies indicate that NIRS in patients with COVID-19 is safe, improves resource utilisation, and might be associated with better outcomes. To guide clinical decision making, prospective, randomised studies are needed to address timing of intervention, optimal use of NIRS modalities-alone or in combination-and validation of tools such as oxygenation indices, response to a trial of NIRS, and inflammatory markers as predictors of treatment success
The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band
Photoacoustic
spectroscopy is a sensitive in situ technique for measuring the absorption
coefficient for gas and aerosol samples. Photoacoustic spectrometer (PAS)
instruments require accurate calibration by comparing the measured
photoacoustic response with a known level of absorption for a calibrant.
Ozone is a common calibrant of PAS instruments, yet recent work by Bluvshtein
et al. (2017) has cast uncertainty on the validity of ozone as a calibrant at
a wavelength of 405 nm. Moreover,
Fischer and Smith (2018) demonstrate
that a low O2 mass fraction in the bath gas can bias the measured PAS
calibration coefficient to lower values for wavelengths in the range
532–780 nm. In this contribution, we present PAS sensitivity measurements
at wavelengths of 405, 514 and 658 nm using ozone-based calibrations with
variation in the relative concentrations of O2 and N2 bath
gases. We find excellent agreement with the results of Fischer and Smith at
the 658 nm wavelength. However, the PAS sensitivity decreases significantly
as the bath gas composition tends to pure oxygen for wavelengths of 405 and
514 nm, which cannot be rationalised using arguments presented in previous
studies. To address this, we develop a model to describe the variation in PAS
sensitivity with both wavelength and bath gas composition that considers
Chappuis band photodynamics and recognises that the photoexcitation of
O3 leads rapidly to the photodissociation products O(3P)
and O2(X, v > 0). We show that the rates of two
processes are required to model the PAS sensitivity correctly. The first
process involves the formation of vibrationally excited
O3(X̃) through the reaction of the nascent
O(3P) with bath gas O2. The second process involves the
quenching of vibrational energy from the nascent O2(X,
v > 0) to translational modes of the bath gas. Both of these
processes proceed at different rates in collisions with N2 or
O2 bath gas species. Importantly, we show that the PAS sensitivity is
optimised for our PAS instruments when the ozone-based calibration is
performed in a bath gas with a similar composition to ambient air and
conclude that our methods for measuring aerosol absorption using an
ozone-calibrated PAS are accurate and without detectable bias. We emphasise
that the dependence of PAS sensitivity on bath gas composition is
wavelength-dependent, and we recommend strongly that researchers characterise
the optimal bath gas composition for their particular instrument.</p
Tuning photochemistry:substituent effects on πσ* state mediated bond fission in thioanisoles
The electronic branching in the thiophenoxyl radicals formed by UV photolysis of thioanisole can be tuned by placing electron withdrawing/donating substituents at the 4-position.</p
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