Atmospheric isoprene measurements reveal larger-than-expected Southern Ocean emissions.

Isoprene is a key trace component of the atmosphere emitted by vegetation and other organisms. It is highly reactive and can impact atmospheric composition and climate by affecting the greenhouse gases ozone and methane and secondary organic aerosol formation. Marine fluxes are poorly constrained due to the paucity of long-term measurements; this in turn limits our understanding of isoprene cycling in the ocean. Here we present the analysis of isoprene concentrations in the atmosphere measured across the Southern Ocean over 4 months in the summertime. Some of the highest concentrations ( >500 ppt) originated from the marginal ice zone in the Ross and Amundsen seas, indicating the marginal ice zone is a significant source of isoprene at high latitudes. Using the United Kingdom Earth System Model we show that current estimates of sea-to-air isoprene fluxes underestimate observed isoprene by a factor >20. A daytime source of isoprene is required to reconcile models with observations. The model presented here suggests such an increase in isoprene emissions would lead to >8% decrease in the hydroxyl radical in regions of the Southern Ocean, with implications for our understanding of atmospheric oxidation and composition in remote environments, often used as proxies for the pre-industrial atmosphere.

Characterization of Fourier Transform Infrared, Cavity Ring-Down Spectroscopy, and Optical Feedback Cavity-Enhanced Absorption Spectroscopy Instruments for the Analysis of Ammonia in Biogas and Biomethane.

Novel traceable analytical methods and reference gas standards were developed for the detection of trace-level ammonia in biogas and biomethane. This work focused on an ammonia amount fraction at an upper limit level of 10 mg m-3 (corresponding to approximately 14 µmol mol-1) specified in EN 16723-1:2016. The application of spectroscopic analytical methods, such as Fourier transform infrared spectroscopy, cavity ring-down spectroscopy, and optical feedback cavity-enhanced absorption spectroscopy, was investigated. These techniques all exhibited the necessary ammonia sensitivity at the required 14 µmol mol-1 amount fraction. A 29-month stability study of reference gas mixtures of 10 µmol mol-1 ammonia in methane and synthetic biogas is also reported.

Understanding Sources and Drivers of Size-Resolved Aerosol in the High Arctic Islands of Svalbard Using a Receptor Model Coupled with Machine Learning.

Atmospheric aerosols are important drivers of Arctic climate change through aerosol-cloud-climate interactions. However, large uncertainties remain on the sources and processes controlling particle numbers in both fine and coarse modes. Here, we applied a receptor model and an explainable machine learning technique to understand the sources and drivers of particle numbers from 10 nm to 20 µm in Svalbard. Nucleation, biogenic, secondary, anthropogenic, mineral dust, sea salt and blowing snow aerosols and their major environmental drivers were identified. Our results show that the monthly variations in particles are highly size/source dependent and regulated by meteorology. Secondary and nucleation aerosols are the largest contributors to potential cloud condensation nuclei (CCN, particle number with a diameter larger than 40 nm as a proxy) in the Arctic. Nonlinear responses to temperature were found for biogenic, local dust particles and potential CCN, highlighting the importance of melting sea ice and snow. These results indicate that the aerosol factors will respond to rapid Arctic warming differently and in a nonlinear fashion.

Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland.

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state-of-the-art FORest Canopy-Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave-drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress-induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model-measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long-term ecosystem-scale observations to enable improved model representation of atmosphere-biosphere interactions in a changing global climate.

An investigation of the matrix sensitivity of refinery gas analysis using gas chromatography with flame ionisation detection.

The response of a flame ionisation detector (FID) on a gas chromatograph to methane, ethane, propane, i-butane and n-butane in a series of multi-component refinery gas standards was investigated to assess the matrix sensitivity of the instrument. High-accuracy synthetic gas standards, traceable to the International System of Units, were used to minimise uncertainties. The instrument response exhibited a small dependence on the component amount fraction: this behaviour, consistent with that of another FID, was thoroughly characterised over a wide range of component amount fractions and was shown to introduce a negligible bias in the analysis of refinery gas samples, provided a suitable reference standard is employed. No significant effects of the molar volume, density and viscosity of the gas mixtures on the instrument response were observed, indicating that the FID is suitable for the analysis of refinery gas mixtures over a wide range of component amount fractions provided that appropriate drift-correction procedures are employed.

Kinetic studies of the BrO + ClO cross-reaction over the range T = 246-314 K.

The kinetics of the atmospherically important gas phase radical reaction between BrO and ClO have been studied over the temperature range T = 246-314 K by means of laser flash photolysis coupled with UV absorption spectroscopy. Charge-coupled-device (CCD) detection allowed simultaneous monitoring of both free radicals and the OClO product using 'differential' spectroscopy, which minimised interference from underlying UV absorbing species. In this way, the total rate coefficient for BrO + ClO → products (1) was measured, along with that for the OClO producing channel of this process BrO + ClO → OClO + Br (1c). These reaction rate coefficients are described by the Arrhenius expressions: k1/cm(3) molecule(-1) s(-1) = (2.5 ± 2.2) × 10(-12) exp[(630 ± 240)/T] and k(1c)/cm(3) molecule(-1) s(-1) = (4.6 ± 3.0) × 10(-12) exp[(280 ± 180)/T], where errors are 2σ, statistical only. An extensive sensitivity analysis was performed to quantify the potential additional systematic uncertainties in this work arising from uncertainties in secondary chemistry, absorption cross-sections and precursor concentrations. This analysis identified the reactions of initial and secondarily generated bromine atoms (specifically Br + O3 and Br + Cl2O) as particularly important, along with the reversible combination of ClO with OClO forming Cl2O3. Potential uncertainty in this latter process was used to define the lowest temperature of the present study. Results from this work indicate larger absolute values for k1 and k(1c) than those reported in previous studies, but a weaker negative temperature dependence for k(1c) than previously observed, resulting in a branching ratio for with a positive temperature dependence, in disagreement with previous studies. is the principal source of OClO in the polar stratosphere and is commonly used in atmospheric models as an indicator of stratospheric bromine chemistry. Thus these measurements might lead to a reinterpretation of modelled stratospheric OClO, which has also been suggested by previous comparisons of observations with atmospheric model studies.

Kinetic studies on the temperature dependence of the BrO + BrO reaction using laser flash photolysis.

The BrO self-reaction, BrO + BrO → products (1), has been studied using laser flash photolysis coupled with UV absorption spectroscopy over the temperature range T = 266.5-321.6 K, under atmospheric pressure. BrO radicals were generated via laser photolysis of Br(2) in the presence of excess ozone. Both BrO and O(3) were monitored via UV absorption spectroscopy using charge-coupled device (CCD) detection. Simultaneous fitting to both temporal concentration traces allowed determination of the rate constant of the two channels of , BrO + BrO → 2Br + O(2) (1a); BrO + BrO → Br(2) + O(2) (1b), hence the calculation of the overall rate of and the branching ratio, α: k(1a)/cm(3) molecule(-1) s(-1) = (1.92 ± 1.54) × 10(-12) exp[(126 ± 214)/T], k(1b)/cm(3) molecule(-1) s(-1) = (3.4 ± 0.8) × 10(-13) exp[(181 ± 70)/T], k(1)/cm(3) molecule(-1) s(-1) = (2.3 ± 1.5) × 10(-12) exp(134 ± 185 /T) and α = k(1a)/k(1) = (0.84 ± 0.09) exp[(-7 ± 32)/T]. Errors are 1σ, statistical only. Results from this work show a weaker temperature dependence of the branching ratio for channel (1a) than that found in previous work, leading to values of α at temperatures typical of the Polar Boundary Layer higher than those reported by previous studies. This implies a shift of the partitioning between the two channels of the BrO self-reaction towards the bromine atom and hence directly ozone-depleting channel (1a).

Kinetic and thermochemical studies of the ClO + ClO + M <=> Cl(2)O(2) + M reaction.

Recent work by von Hobe et al. [Atmos. Chem. Phys., 2007, 7, 3055] has highlighted significant inconsistencies between laboratory results, theoretical calculations and field observations concerning the ClO dimer ozone destruction cycle. This work investigates the temperature dependence of the equilibrium constant of one of the key reactions in this cycle, ClO + ClO + M <=> Cl(2)O(2) + M (1, -1), by means of laser flash photolysis and time-resolved UV absorption spectroscopy. ClO radicals were generated via laser flash photolysis of Cl(2)/Cl(2)O mixtures in synthetic air. Radicals were monitored via UV absorption spectroscopy: the use of a charge coupled device (CCD) detector allowed time resolution over a broad range of wavelengths giving unequivocal concentrations of radicals. The equilibrium constant K(eq) was determined as the ratio of the rate constants of the forward and reverse over the temperature range T = 256.55-312.65 K. Second Law and Third Law thermodynamic methods were employed to determine the standard enthalpy and entropy changes of , Δ(r)H° and Δ(r)S°, from the measured equilibrium constants. The values obtained from Second Law analysis were Δ(r)H° = - 80.7 ± 2.2 kJ mol(-1) and Δ(r)S° = -168.1 ± 7.8 J K(-1) mol(-1). Third Law analysis gave Δ(r)H° = -74.65 ± 0.4 kJ mol(-1) and Δ(r)S° = -148.0 ± 0.4 J K(-1) mol(-1). These values are in good agreement with previous work by Nickolaisen et al. [J. Phys. Chem., 1994, 98, 155] but greater in (negative) magnitude than current JPL-NASA recommendations [Sander et al., Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, JPL Publication 06-2, NASA Jet Propulsion Laboratory, Pasadena, 2006 (interim update to this reference, 2009)]. The discrepancy between the Second and Third Law analyses also agrees with Nickolaisen et al., possibly indicating an aspect of the ClO recombination reaction not yet fully elucidated. The atmospheric implications of the results and their impact on the current understanding on polar ozone depletion are briefly discussed.