Accurate observations of near-infrared solar spectral irradiance and water vapour continuum

This thesis contains analyses of the solar spectral irradiance (SSI) and near-infrared water vapour continuum from high-resolution observations by a ground-based, sun-pointing Fourier transform spectrometer in the wavenumber region 2000-10000 cm-1 (1-5 μm). This was performed primarily using the Langley method on observations during 18 September 2008. Particular focus was placed on a detailed assessment of the uncertainty budget for each of these analyses. The solar spectral irradiance was found to be ~8% lower than the commonly-used satellite-based ATLAS3 SSI in the region 4000-7000 cm-1 (where ATLAS3 is most uncertain). This disagreement with ATLAS3 is in line with several other modern analyses. There is good agreement with ATLAS3 and other spectra in the 7000-10000 cm-1 region (where these spectra are considered more accurate). This thesis contains the first published results of water vapour continuum absorption in the atmosphere in the 1.6 and 2.1 μm atmospheric windows (in which laboratory measurements show some significant disagreement) with robust uncertainties. The derived water vapour continuum in these windows is stronger than the widely-used MT_CKD model (v3.2) by a factor of ~100 and ~5 respectively. These results also show that MT_CKD is a reasonably accurate representation of the continuum in the 4 μm window. These results are broadly consistent with laboratory measurements of the foreign continuum, but are inconsistent with the highest such measurements of the self-continuum. The effect of the self and foreign continuum in atmospheric conditions is assessed, with comparisons to the Langley-derived spectra from this work. These results show that the difference between MT_CKD and this work in the 1.6 and 2.1 μm windows may come primarily from the observed differences in the foreign continuum, with a smaller contribution from the self-continuum. These results show inconsistency with several sets of laboratory room temperature spectra within the experimental uncertainties

Atmospheric observations of the water vapour continuum in the near-infrared windows between 2500 and 6600 cm-1

Water vapour continuum absorption is potentially important for both closure of the Earth's energy budget and remote sensing applications. Currently, there are significant uncertainties in its characteristics in the near-infrared atmospheric windows at 2.1 and 1.6 µm. There have been several attempts to measure the continuum in the laboratory; not only are there significant differences amongst these measurements, but there are also difficulties in extrapolating the laboratory data taken at room temperature and above to temperatures more widely relevant to the atmosphere. Validation is therefore required using field observations of the real atmosphere. There are currently no published observations in atmospheric conditions with enough water vapour to detect a continuum signal within these windows or where the self-continuum component is significant. We present observations of the near-infrared water vapour continuum from Camborne, UK, at sea level using a Sun-pointing, radiometrically calibrated Fourier transform spectrometer in the window regions between 2000 and 10 000 cm−1. Analysis of these data is challenging, particularly because of the need to remove aerosol extinction and the large uncertainties associated with such field measurements. Nevertheless, we present data that are consistent with recent laboratory datasets in the 4 and 2.1 µm windows (when extrapolated to atmospheric temperatures). These results indicate that the most recent revision (3.2) of the MT_CKD foreign continuum, versions of which are widely used in atmospheric radiation models, requires strengthening by a factor of ∼5 in the centre of the 2.1 µm window. In the higher-wavenumber window at 1.6 µm, our estimated self- and foreign-continua are significantly stronger than MT_CKD. The possible contribution of the self- and foreign-continua to our derived total continuum optical depth is estimated by using laboratory or MT_CKD values of one, to estimate the other. The obtained self-continuum shows some consistency with temperature-extrapolated laboratory data in the centres of the 4 and 2.1 µm windows. The 1.6 µm region is more sensitive to atmospheric aerosol and continuum retrievals and therefore more uncertain than the more robust results at 2.1 and 4 µm. We highlight the difficulties in observing the atmospheric continuum and make the case for additional measurements in both the laboratory and field and discuss the requirements for any future field campaign

Roles and mechanisms of action of the L-cysteine cystathionine-gamma-lyase hydrogen sulphide pathway in the heart

Hydrogen sulphide (H2S) is a naturally occurring gas and originally the primary focus of research was to investigate its toxicity. In 1989 a physiological role of H2S was proposed after endogenous levels were detected in the rat brain and normal human post-mortem tissue. This discovery has led to an explosion of interest in H2S as a biological mediator. Identification of H2S synthesising enzymes in the cardiovascular system has led to a number of studies examining specific regulatory actions of H2S. The hypothesis underlying the studies in this thesis was that H2S synthesising enzymes exist in the myocardium and the resulting H2S provides cardioprotection against ischaemia-reperfusion injury. This was investigated using a broad range of experimental techniques including Langendorff isolated perfused rat heart models, biochemical H2S stimulation and detection assays, PCR, and Western blotting. The principal findings can be summarised as follows: 1. Rat myocardium has the potential to express both CSE and CBS H2S synthesising enzymes, due to the confirmed detection of mRNA. 2. Furthermore it was possible to exogenously stimulate the CSE enzyme, with its substrate L-cysteine, to produce H2S gas which limited infarct size during regional ischaemia-reperfusion. 3. Endogenous H2S levels were up-regulated during ischaemia-reperfusion, consistent with an endogenous protective role within the myocardium. 4. Simple and complex H2S/thiol containing compounds produced cardioprotection during regional ischaemia-reperfusion, with a mechanism that involves PI3k and Akt activation, implicating recruitment of downstream kinases within the RISK pathway. The studies presented have provided a significant advancement in understanding the involvement of H 2S in cardioprotection during ischaemia-reperfusion. It has also raised questions such as the exact mechanism of action of H2S donor/thiol containing compounds and highlighted the need for more robust H2S donors. The scope for H2S as an endogenous mediator also stems beyond that of cardioprotection, as the range of body systems and cell types are continually expanding

Can measurements of the near-infrared solar spectral irradiance be reconciled? A new ground-based assessment between 4000-10000 cm-1

The near-infrared solar spectral irradiance (SSI) is of vital importance for understanding the Earth’s radiation budget, and in Earth observation applications. Differences between previously published solar spectra (including the commonly-used ATLAS3 spectrum) reach up to 10% at the low-wavenumber end of the 4000-10000 cm-1 (2.5 – 1 μm) spectral region. The implications for the atmospheric sciences are significant, since this spectral region contains 25% of the incoming total solar irradiance. This work details an updated analysis of the CAVIAR SSI, featuring additional analysis techniques and an updated uncertainty budget using a Monte Carlo method. We report good consistency with ATLAS3 in the 7000-10000 cm-1 region where there is confidence in these results due to agreement with other spectra, but ~7% lower in the 4000-7000 cm-1 region, in general agreement with several other analyses

Water vapour self-continuum in near-visible IR absorption bands: Measurements and semiempirical model of water dimer absorption

The nature of the water vapour continuum has been of great scientific interest for more than 60 years. Here, water vapour self-continuum absorption spectra are retrieved at temperatures of 398 K and 431 K and at vapour pressures from 1000 to 4155 mbar in the 8800 and 10,600 cm−1 absorption bands using high-resolution FTS measurements. For the observed conditions, the MT_CKD-3.2 model underestimates the observed continuum on average by 1.5–2 times. We use the hypothesis that water dimers contribute to the continuum absorption to simulate the experimentally-retrieved self-continuum absorption spectra, and to explain their characteristic temperature dependence and spectral behaviour. The values of the effective equilibrium constant are derived for the observed temperatures. We find that the dimer-based model fits well to the measured self-continuum from this and previous studies, but requires a higher effective equilibrium constant compared to the modern estimates within the temperature range (268–431 K) and spectral region studied. It is shown that water dimers are likely responsible for up to 50% of the observed continuum within these bands. Possible causes of the incomplete explanation of the continuum are discussed. Extrapolating these measurements to atmospheric temperatures using the dimer-based model, we find that the newly-derived self-continuum reduces calculated surface irradiances by 0.016 W m−2 more than the MT_CKD-3.2 self-continuum in the 8800 cm−1 band for overhead-Sun mid-latitude summer conditions, corresponding to a 12.5% enhancement of the self-continuum radiative effect. The change integrated across the 10,600 cm−1 band is about 1%, but with significan

Codd et al_suppl. data

raw and processed EMG data file

Codd et al_gator EMG analysis

raw and processed EMG data file

Cross-sections for heavy atmospheres:H₂O continuum

Most of the exoplanets detected up to now transit in front of their host stars, allowing for the generation of transmission spectra; the study of exoplanet atmospheres relies heavily upon accurate analysis of these spectra. Recent discoveries mean that the study of atmospheric signals from low-mass, temperate worlds are becoming increasingly common. The observed transit depth in these planets is small and more difficult to analyze. Analysis of simulated transmission spectra for two small, temperate planets (GJ 1214 b and K2-18 b) is presented, giving evidence for significant differences in simulated transit depth when the water vapor continuum is accounted for when compared to models omitting it. These models use cross-sections from the CAVIAR lab experiment for the water self-continuum up to 10,000 cm$^{-1}$; these cross-sections exhibit an inverse relationship with temperature, hence lower-temperature atmospheres are the most significantly impacted. Including the water continuum strongly affects transit depths, increasing values by up to 80 ppm, with the differences for both planets being detectable with the future space missions Ariel and JWST. It is imperative that models of exoplanet spectra move toward adaptive cross-sections, increasingly optimized for H$_2$O-rich atmospheres. This necessitates including absorption contribution from the water vapor continuum into atmospheric simulations

Cross-sections for heavy atmospheres: H2O continuum

Most of the exoplanets detected up to now transit in front of their host stars, allowing for the generation of transmission spectra; the study of exoplanet atmospheres relies heavily upon accurate analysis of these spectra. Recent discoveries mean that the study of atmospheric signals from low-mass, temperate worlds are becoming increasingly common. The observed transit depth in these planets is small and more difficult to analyze. Analysis of simulated transmission spectra for two small, temperate planets (GJ 1214 b and K2-18 b) is presented, giving evidence for significant differences in simulated transit depth when the water vapor continuum is accounted for when compared to models omitting it. These models use cross-sections from the CAVIAR lab experiment for the water self-continuum up to 10,000 cm ; these cross-sections exhibit an inverse relationship with temperature, hence lower-temperature atmospheres are the most significantly impacted. Including the water continuum strongly affects transit depths, increasing values by up to 60 ppm, with the differences for both planets being detectable with the future space missions Ariel and JWST. It is imperative that models of exoplanet spectra move toward adaptive cross-sections, increasingly optimized for H O-rich atmospheres. This necessitates including absorption contribution from the water vapor continuum into atmospheric simulations