The impact of volcanic eruptions in the period 2000-2013 on global mean temperature trends evaluated in the HadGEM2-ES climate model

This is the final version of the article. Available from Wiley via the DOI in this record.The slow-down in global warming over the last decade has lead to significant debate about whether the causes are of natural or anthropogenic origin. Using an ensemble of HadGEM2-ES coupled climate model simulations we investigate the impact of overlooked modest volcanic eruptions. We deduce a global mean cooling of around -0.02 to -0.03K over the period 2008-2012. Thus while these eruptions do cause a cooling of the Earth and may therefore contribute to the slow-down in global warming, they do not appear to be the sole or primary cause. © 2014 Royal Meteorological Society.JMH, AJ, and GSJ were supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output

Can reducing black carbon and methane below RCP2.6 levels keep global warming below 1.5C?

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordMethane and black carbon aerosols have been identified as exerting the two strongest positive radiative forcings after carbon dioxide and therefore drastic reductions in these atmospheric constituents could potentially offer strong leverage in reducing global warming. Using the HadGEM2-ES model we reduce concentrations of methane and black carbon while holding all other emissions at representative concentration pathway RCP2.6 levels to examine whether we can achieve the target of keeping global-mean temperature rise below 1.5 oC relative to the pre-industrial level during the remainder of the 21st century. We find that even total cessation of black carbon aerosol emissions is ineffective in attaining this goal. Reducing methane concentrations at four times the rate assumed in RCP2.6 is able to return warming levels to below 1.5 oC by the 2070s but overshoots the target level prior to that. As RCP2.6 represents an optimistic scenario relative to the Intended Nationally Determined Contributions our results highlight the importance of deep and rapid reductions in both CO2 and methane emissions if humanity is serious about attaining the 1.5 oC target.This work was supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). CDJ was also supported by the European Union's Horizon 2020 research and innovation programme under grant agreement No 64181

The impact of bath gas composition on the calibration of photoacoustic spectrometers with ozone at discrete visible wavelengths spanning the Chappuis band

This is the author accepted manuscript. The final version is available from EGU via the DOI in this record.For data related to this paper, please contact Michael I. Cotterell (or Justin M. LangridgePhotoacoustic 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(X˜) 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.Analytical Chemistry Trust FundResearch Council on Norwa

On the accuracy of aerosol photoacoustic spectrometer calibrations using absorption by ozone

This is the final version of the article. Available from EGU via the DOI in this record.In recent years, photoacoustic spectroscopy has emerged as an invaluable tool for the accurate measurement of light absorption by atmospheric aerosol. Photoacoustic instruments require calibration, which can be achieved by measuring the photoacoustic signal generated by known quantities of gaseous ozone. Recent work has questioned the validity of this approach at short visible wavelengths (404 nm), indicating systematic calibration errors of the order of a factor of 2. We revisit this result and test the validity of the ozone calibration method using a suite of multipass photoacoustic cells operating at wavelengths 405, 514 and 658 nm. Using aerosolised nigrosin with mobility-selected diameters in the range 250-425 nm, we demonstrate excellent agreement between measured and modelled ensemble absorption cross sections at all wavelengths, thus demonstrating the validity of the ozone-based calibration method for aerosol photoacoustic spectroscopy at visible wavelengths.This work was funded by the Met Office. In addition, Nicholas W. Davies was supported by a NERC/Met Office Industrial Case studentship (ref 640052003). Michael I. Cotterell was supported by a Tom West Analytical Chemistry Trust Fund Fellowship. Michael I. Cotterell and Jim M. Haywood were supported by the CLARIFY-2017 Natural Environment Research Council funded proposal (NE/L013797/1)

Sensitivity of volcanic aerosol dispersion to meteorological conditions: A Pinatubo case study

This is the final version of the article. Available from American Geophysical Union via the DOI in this record.Using a global climate model (Hadley Centre Global Environment Model version 2-Carbon Cycle Stratosphere ) with a well-resolved stratosphere, we test the sensitivity of volcanic aerosol plume dispersion to meteorological conditions by simulating 1 day Mount Pinatubo-like eruptions on 10 consecutive days. The dispersion of the volcanic aerosol is found to be highly sensitive to the ambient meteorology for low-altitude eruptions (16–18 km), with this variability related to anomalous anticyclonic activity along the subtropical jet, which affects the permeability of the tropical pipe and controls the amount of aerosol that is retained by the tropical reservoir. Conversely, a high-altitude eruption scenario (19–29 km) exhibits low meteorological variability. Overcoming day-to-day meteorological variability by spreading the emission over 10 days is shown to produce insufficient radiative heating to loft the aerosol into the stratospheric tropical aerosol reservoir for the low eruption scenario. This results in limited penetration of aerosol into the southern hemisphere (SH) in contrast to the SH transport observed after the Pinatubo eruption. Our results have direct implications for the accurate simulation of past/future volcanic eruptions and volcanically forced climate changes, such as Intertropical Convergence Zone displacement.A.C.J. was funded by a NERC/CASE PhD studentship (ref. 580 009 138, with CASE partner being the Met Office); J.M.H. and A.J. were supported by the Joint UK DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). The authors would like to thank Larry Thomason for supplying the SAGE II data. Data are freely available by contacting A.C.J

Optimizing the Performance of Aerosol Photoacoustic Cells using a Finite Element Model. Part 2: Application to a Two-Resonator Cell

This is the author accepted manuscript. The final version is available from Taylor & Francis via the DOI in this recordData availability: For data related to this paper, please contact Michael I. Cotterell ([email protected]).Photoacoustic spectroscopy (PAS) measures aerosol absorption in a non-contact manner, providing accurate absorption measurements that are needed to improve aerosol optical property representations in climate models. Central to PAS is resonant amplification of the acoustic pressure wave generated from laser-heated aerosol transferring heat to surrounding gas by a photoacoustic cell. Although this cell amplifies pressure sources from aerosol absorption (signal), it also amplifies noise and background sources. It is important to maximise the cell signal-to-background ratio (SBR) for sensitive absorption measurements. Many researchers have adopted the two-resonator cell design described by Lack et al. (2006). We show that the uncertainty in PAS measurements of aerosol absorption using this two-resonator cell is significantly degraded by its large sensitivity to background contributions from laser scattering and absorption at the cell windows. In Part 1, we described the use of a finite element method (FEM) to predict cell acoustic properties, validated this framework by comparing model predictions to measurements, and used FEM to test various strategies applied commonly to single-resonator cell optimisation. In this second part, we apply FEM to understand the excitation of resonant modes of the two-resonator cell, with comparison measurements demonstrating accurate predictions of acoustic response. We perform geometry optimisation studies to maximise the SBR and demonstrate that the laser-window interaction background is reduced to undetectable levels for an optimal cell. This optimised two-resonator cell will improve the sensitivity and accuracy of future aerosol absorption measurements.Defence Science and Technology Laboratory (DSTL)Royal Society of ChemistryAnalytical Chemistry Trust FundNatural Environment Research Council (NERC

Spatial distribution of dust's optical properties over the Sahara and Asia inferred from Moderate Resolution Imaging Spectroradiometer

There is great uncertainty regarding the role of mineral dust aerosols in Earth’s climate system. One reason for this uncertainty is that the optical properties of mineral dust, such as its single scattering albedo (the ratio of scattering to total extinction), are poorly constrained because ground observations are limited to a few locations and satellite standard products are not available due to the excessively bright surface of the desert in the visible wavelength, which makes robust retrievals difficult. Here, we develop a method to estimate the spatial distributions of the aerosol single scattering albedo (ω0) and optical depth (τa), with daily 1◦ × 1 ◦ spatial resolution using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) as well as model simulations of radiative transfer. This approach is based on the “critical surface reflectance” method developed in the literature, which estimates ω0 from the top of the atmospheric radiance. We estimate the uncertainties in ω0 over the Sahara (Asia) to be approximately 0.020 and 0.010 (0.023 and 0.017) for bands 9 and 1, respectively, while the uncertainty in τa is approximately 0.235 and 0.228 (0.464 and 0.370) for bands 9 and 1, respectively. The 5–95 % range of the spatial distribution of ω0 over the Sahara (Asia) is approximately 0.90–0.94 and 0.96–0.99 (0.87–0.94 and 0.89–0.97) for bands 9 and 1, respectively, and that of τa over the Sahara (Asia) is approximately 0.8–1.4 and 0.8–1.7 (0.7–2.0 and 0.7–1.9) for bands 9 and 1, respectively. The results for the Sahara indicate a good correlation between ω0 and the surface reflectance, and between ω0 and τa. However, the relationships between ω0, τa, and surface reflectance are less clear in Asia than in the Sahara, and the ω0 values are smaller than those in the Sahara. The regions with small ω0 values are consistent with the regions where coal-burning smoke and carbonaceous aerosols are reported to be transported in previous studies. Because the coal-burning and carbonaceous aerosols are known to be more absorptive and have smaller ω0 values than dust aerosols, our results indicate that the dust aerosols in Asia are contaminated by these anthropogenic aerosols. The spatial distribution of dust optical properties obtained in our work could be useful in understanding the role of dust aerosols in Earth’s climate system, most likely through future collaboration with regional and global modelling studies.The authors are grateful to the Open CLASTER project for allowing us to use the RSTAR package for this research. We would like to thank the AERONET project and its staff for establishing and maintaining the Tamanrasset, Agoufou, Banizoumbou and Saada sites considered in this investigation. We would also like to thank the SKYNET project and its staff for establishing and maintaining the Dunhuang site. Finally, we appreciate the valuable discussions and support provided by Ben Johnson, Satoru Fukuda, Yosuke Sato, Eiji Oikawa, Makiko Hashimoto, Yasushi Mitomi and Matthew Collins. One of the authors was supported by projects by JAXA/EarthCARE and GCOM/C, MEXT/VL for Climate System Diagnostics, MOE/Global Environment Research Fund A-1101, NIES/GOSAT, and MEXT/RECCA/SALSA

Sensitivity and accuracy of refractive index retrievals from measured extinction and absorption cross sections for mobility-selected internally mixed light absorbing aerosols

This is the final version. Available on open access from Taylor & Francis via the DOI in this record. Data availability: For data related to this article, please contact Michael I. Cotterell ([email protected]).Aerosol refractive index (RI) is related to particle composition and density, is used in optical spectroscopy studies to probe aerosol physiochemical properties during chemical reactions and gas-particle partitioning, and is important in atmospheric physics. Indeed, aerosol radiative forcing calculations require accurate descriptions of the real (n) and imaginary (k) RI components and their dependence on wavelength, humidity and particle mixing state. Using cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS) to measure the extinction and absorption cross sections for mobility-selected aerosols is recognized as a good approach to retrieving n and k accurately. However, little work has assessed rigorously the sensitivity and accuracy of the retrieved values from this approach. This work investigates RI retrievals from CRDS- and PAS-measured optical properties for mobility-selected aerosols composed of ammonium sulfate (non-absorbing), nigrosin (strongly light absorbing) or a mixture of these two species. Importantly, we assess the sensitivity in our RI retrievals and then apply a Monte Carlo error propagation analysis to quantify the retrieval accuracy. Our Monte Carlo analysis is the first to account for the full range of uncertainties involved in RI retrievals from optical measurements on mobility-selected aerosol. We also report the first experimental validation of predictive RI mixing rules for non-aqueous internally mixed light absorbing aerosols by comparing mixing rule predictions with measurements for aerosol composed of internal mixtures of ammonium sulfate and nigrosin. The commonplace volume fraction mixing rule fails to predict refractive indices accurately and mixing rules with a physical basis must be used.Natural Environment Research Council (NERC

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

Antipyretic medication for a feverish planet

This is the final version. Available on open access from Springer Nature via the DOI in this record. University of Genev