Abstract. Residential wood burning (RWB) has become an increasingly significant source of carbonaceous aerosols (CAs) in the UK and worldwide. Black Carbon (BC) and Organic Aerosol (OA) fractions of CAs are of particular concern due to their impacts on climate and human health. Measuring the mixing state of CA is important as the mixing state can influence key aerosol properties including light absorption, hygroscopicity, cloud scavenging, atmospheric lifetime, and toxicity. While emissions and mixing states from traditional stoves have been previously reported and characterized, key uncertainties remain in the influence of user behaviour on emissions and the emission characteristics from the modern ‘Ecodesign’ appliances. Recent emissions tests imply that these can emit more pollutants such as polycyclic aromatic hydrocarbons (PAHs) and BC under certain circumstances and here we utilise online instrumentation to probe the mechanisms behind this and whether user behaviour has a role in impacting emissions. Hardwood logs were burned in a controlled test system using a UK Ecodesign-compliant woodstove under five operating protocols: standard, overload, underload, hot-reload and open-door protocols. Instantaneous particle emissions were quantified using a Single Particle Soot Photometer (SP2), a Differential Mobility Sizer (DMS500) and an Aerosols Mass Spectrometer (AMS). Modified combustion efficiency (MCE) was derived from CO and CO2 concentrations measured by a Fourier-Transform Infrared (FTIR) spectrometer. Three typical combustion phases were observed: a pre-ignition phase, a flaming phase (with rich and non-rich flaming distinguished by an MCE of 0.95), and a smouldering phase. Stove operation can affect emissions by altering particle size, leading to ultrafine particle (UFP) formation (e.g., open-door, underload and overload conditions) and also prolong the rich flaming phase (e.g., hot-reload and overload), increasing PAH emissions. The findings here demonstrate how user behaviour may increase emissions of certain pollutants from modern stoves, partially offsetting the benefits compared to older designs.

Open-to-air triethylborane (Et3B) is a well-known room-temperature initiator of free radical reactions. Current consensus is that the initiating species in this system are ethyl radicals formed in the triethylborane autoxidation cycle, although some early studies suggested that reactions of the oxidation products can also contribute to the initiation. We report a quantitative mechanistic study which demonstrates that most ethyl radicals in the triethylborane initiator system are not formed in the autoxidation cycle but instead are generated by a Molecule-Induced Radical Formation (MIRF) reaction between triethylborane and its main oxidation product, ethylperoxyborane Et2BOOEt (secondary initiation). A simplified kinetic model allowed us to predict optimal conditions for the initiation, and propose the MIRF reaction as an alternative, homogeneous initiator system. The high efficiency of this initiator has been demonstrated in several model radical reactions.

Nineteen structurally different alkene ozonolysis reactions studied herein proceed via a 1,3-cycloaddition step to produce a short-lived primary ozonide, which then breaks down to form a Criegee intermediate (CI) and an aldehyde/ketone co-product. Both steps of each ozonolysis reaction are examined here using a high-level computational chemistry approach (DF-HF/DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ), and a rate constant and product branching ratio are produced for each reaction. The reactions are then categorized into broadly defined taxonomic groups on the basis of how the ozonolysis chemistry is affected by functional groups, steric bulk and the spatial arrangement of the substituent groups. The five alkene taxonomic groups used for classification are monosubstituted alkenes, trisubstituted alkenes, E-2-alkenes, Z-2-alkenes and haloalkenes. The general cycloaddition reactivity trend identified for these alkene groups is k(THEO) (haloalkenes) < k(THEO) (monosubstituted alkenes) < k(THEO) (E-2-alkenes) similar to k(THEO) (Z-2-alkenes) < k(THEO) (trisubstituted alkenes). Within these categories, one secondary trend was that if one or more substituents was small and rich in hyperconjugative alpha-H atoms, such as a methyl group, a higher alkene rate and a higher CI yield would be induced, compared to a bulky and alpha-H-poor substituent, such as a tert-butyl (Bu-t) group. Furthermore, bulky or electronegative substituents were also shown to prompt a reduction in syn-CI yields. Also highlighted in the study is the theoretical mechanism of how the ozonolysis of haloalkenes generates significant yields of tropospheric CF3CHO, a species which can undergo photolysis to produce the strong greenhouse gas fluoroform (CHF3).

Including practical atmospheric chemistry in the undergraduate syllabus presents numerous challenges: most research grade equipment is too complex and expensive, and can require extensive safety training for students to use independently and efficiently. Herein we report an undergraduate analytical project that can be effectively performed in teaching laboratories equipped with standard UV-vis spectrometers, centred around an important application of photochemical kinetics in atmospheric research. Carbonyl containing compounds in dilute cyclohexane solutions may be considered as in a "quasi-gas-phase" environment. Measurements of absorption spectra in such solutions can therefore provide useful information into the atmospheric photochemistry of carbonyl compounds - a critically understudied aspect of atmospheric kinetics. We have delivered this course for two years and found that a project centred around collecting, processing, and understanding the atmospheric implications of these spectra is accessible to a range of student abilities and is an excellent tool for introducing the wider concepts and impacts of atmospheric chemistry.

Domestic biomass burning is a significant source of organic aerosol (OA) to the atmosphere; however, the understanding of OA composition under different burning conditions and after oxidation is largely unknown. Compositional analysis of OA is often limited by the lack of analytical standards available for quantification; however, semi-quantitative non-target analysis (NTA) can overcome these limitations by enabling the detection of thousands of compounds and quantification via surrogate standards. A series of controlled-burn experiments were conducted at the Manchester Aerosol Chamber to investigate domestic biomass-burning OA (BBOA) under different burning conditions and the impact of atmospheric ageing. Insights into the chemical composition of fresh and aged OA from flaming-dominated and smouldering-dominated combustion were obtained via a newly developed semi-quantitative NTA approach using ultra-high-performance liquid chromatography high-resolution mass spectrometry. Aerosol from smouldering-dominated burns contained significant organic carbon content, whereas under flaming-dominated conditions it was primarily black carbon. The detectable OA mass from both conditions was dominated by oxygenated compounds (CHO) (≈ 90 %) with smaller contributions from organonitrogen species. Primary OA (POA) had a high concentration of C8–C17CHO compounds, with both burns exhibiting a peak between C8–C11. However, flaming-dominated POA exhibited a greater contribution of C13–C17 CHO species. More than 50 % of the CHO mass in POA was determined as aromatic by the aromaticity index, largely in the form of functionalised monoaromatic compounds. After ageing, the aromatic contribution to the total CHO mass decreased with a greater loss for smouldering (−53 %) than flaming (−16 %) due to the increased reduction of polyaromatic compounds under smouldering conditions. The O:C ratios of the aged OA from flaming and smouldering were consistent with those from the oxidation of aromatic compounds (0.57–1.00), suggesting that compositional changes upon ageing were driven by the oxidation of aromatic compounds and the loss of aromaticity. However, there was a greater probability of O:C ratios ≥ 0.8 in aged smouldering OA, indicating the presence of more oxidised species. This study presents the first reported quantitative non-target compositional analysis of domestic BBOA using retention window scaling and demonstrates that compositional changes between burn phase and after ageing may have important consequences for exposure to such emissions in residential settings.

Isoprene is the dominant nonmethane volatile organic compound (VOC) emitted into the atmosphere globally, with important atmospheric chemistry impacts on air quality and climate. One crucial intermediate in its gas-phase oxidation is isoprene epoxydiol (IEPOX), which contributes significantly to the formation of secondary organic aerosols (SOA). It is generally accepted that IEPOX is efficiently formed in remote forested regions with a sufficiently low NO/HO2 ratio. Here, we show that the oxidation of isoprene hydroxynitrates (IHN) can be an alternative, efficient, NO-driven pathway leading to the formation of IEPOX in urban areas where moderate to high NO concentrations exist. Field measurements from the megacity of Beijing show that this pathway contributes to more than 50% of IEPOX production during the morning and early afternoon. The results improve our understanding of the NOx dependence of SOA formation in polluted areas, where anthropogenic emissions can significantly enhance biogenic SOA formation.

Short-lived radical intermediates, such as reactive oxygen species (ROS), play a crucial role in oxidative stress, aging, and carcinogenesis. However, understanding of these processes is impeded by limitations of existing radical characterisation techniques. Previous investigations demonstrated that the recently-developed SH2′ radical trapping technique could be used to probe biochemical oxidation mechanisms. In this study, SH2′ radical trapping and mass spectrometry (MS) techniques were used to investigate ●OH-initiated degradation of thymine, relevant to oxidative damage of DNA. Numerous substrate-derived radical intermediates were trapped and characterised, aiding elucidation of, and offering validation to, proposed mechanisms. Isotope exchange and chromatographic separations helped to distinguish between isomeric structures. Observation of thymine dimer radicals and products suggests that this methodology can be useful for studying radical cross-linking of nucleobases. Overall, SH¬2′ radical trapping was demonstrated to be a powerful technique for investigating oxidation pathways in biological systems.

Two applications of a radical trap based on a homolytic substitution reaction (SH2') are presented for the trapping of short-lived radical intermediates in organic reactions. The first example is a photochemical cyanomethylation catalyzed by a Ru complex. Two intermediate radicals in the radical chain propagation have been trapped and detected using mass spectrometry (MS), along with the starting materials, products and catalyst degradation fragments. Although qualitative, these results helped to elucidate the reaction mechanism. In the second example, the trapping method was applied to study the radical initiation catalyzed by a triethylboronoxygen mixture. In this case, the concentration of trapped radicals was sufficiently high to enable their detection by nuclear magnetic resonance (NMR). Quantitative measurements made it possible to characterize the radical flux in the system under different reaction conditions (including variations of solvent, temperature and concentration) where modelling was complicated by chain reactions and heterogeneous mass transfer.

Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policymakers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation and the creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system that have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other, resulting in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g., low-NO scenarios, wildfires, mega- and gigacities) and that require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity and aerosol chemistry impacting climate, human and ecosystem health.

Nontarget analysis (NTA) by liquid chromatography coupled to high-resolution mass spectrometry improves the capacity to comprehend the molecular composition of complex mixtures compared to targeted analysis techniques. However, the detection of unknown compounds means that quantification in NTA is challenging. This study proposes a new semi-quantitative methodology for use in the NTA of organic aerosol. Quantification of unknowns is achieved using the average ionization efficiency of multiple quantification standards which elute within the same retention time window as the unknown analytes. In total, 110 authentic standards constructed 25 retention time windows for the quantification of oxygenated (CHO) and organonitrogen (CHON) species. The method was validated on extracts of biomass burning organic aerosol (BBOA) and compared to quantification with authentic standards and had an average prediction error of 1.52 times. Furthermore, 70% of concentrations were estimated within a factor of 2 (prediction errors between 0.5 and 2 times) from the authentic standard quantification. The semi-quantification method also showed good agreement for the quantification of CHO compounds compared to predictive ionization efficiency-based methods, whereas for CHON species, the prediction error of the semi-quantification method (1.63) was significantly lower than the predictive ionization efficiency approach (14.94). Application to BBOA for the derivation of relative abundances of CHO and CHON species showed that using peak area underestimated the relative abundance of CHO by 19% and overestimated that of CHON by 11% compared to the semi-quantification method. These differences could lead to significant misinterpretations of source apportionment in complex samples, highlighting the need to account for ionization differences in NTA approaches.

In recent years, clean air policies have led to reductions in air pollution across China. Alongside this, emerging carbon neutrality (CN) policies that aim to address the impacts of climate change may also deliver air quality (AQ) co-benefits or climate penalties. Different CN policies will lead to different changes in volatile organic compound (VOC), NOx and particulate matter (PM) emissions, which will in turn impact the photochemical production of secondary pollutants such as ozone (O3). It is currently unclear how different combinations of AQ and CN policies may impact in situ O3 production across China in the future. A detailed chemical box model incorporating the Master Chemical Mechanism was developed to investigate the impact of combined AQ and CN policies on O3 formation in Beijing. The Multi-resolution Emission Inventory model for Climate and air pollution research (MEIC) and the Dynamic Projection model for Emissions in China (DPEC) were used to estimate future pollutant mixing ratios, relative to ambient observations of 35 VOCs, NOx, CO and aerosol surface area (ASA) during the APHH-Beijing 2017 summer campaign. The most ambitious policy scenario, “Ambitious Pollution 1.5D Goals”, led to the largest reduction in O3 production by 2060 but was not the most impactful scenario for reducing O3 production between 2030–2045. Larger reductions were observed under the “Ambitious Pollution Neutral Goals” policy, which focuses on achieving net zero by 2060. O3 production was found to be most sensitive to changes in the OLE2 group of VOCs (alkenes where kOH>7×104 ppm−1 min−1; a 5 % increase in OLE2 increased simulated O3 production by 1.12 %). However, reducing less reactive but higher concentration species in Beijing (such as short-chain alkanes) led to larger reductions in O3 production under all scenarios. O3 production was not sensitive to changes in ASA, with a 69 % decrease in ASA leading to a change of <1 % in O3. However, doubling biogenic VOCs in the model further increased O3 production in 2060 under all future scenarios by up to 18 %, indicating that the influence of future climate-induced changes in biogenic emissions may have a significant impact on in situ O3 formation in Beijing. This study highlights that the emission trajectories of certain specific VOCs are highly influential in determining possible future O3 air quality effects that may arise from increasing ambient temperatures and decarbonisation in Beijing.

A radical trapping method based on an S(H)2' homolytic substitution reaction was applied to study the mechanism of a photochemical spirocyclisation of indole-ynones in the presence of thiols. Starting material, products and a range of trapped radical intermediates were simultaneously detected in reaction mixtures by mass spectrometry (MS). The trapped intermediates included both initiating and main chain propagating radicals. These data made it possible to propose a self-initiation mechanism consistent with the originally postulated photoexcitation of an intramolecular electron donor-acceptor complex of the substrate. The effect of thiol structure on the MS peak intensity of the reaction components was rationalised in terms of the relative stability of the radical intermediates. The results were compared to a simpler related reaction, a photochemical thiol-ene addition where reagents, products and trapped intermediate radicals were also detected by MS. Relative MS peak intensities were again explained by a combination of electronic and steric effects on the stability of intermediate radicals. Overall, S(H)2' radical trapping was demonstrated to be a powerful experimental technique for providing mechanistic evidence on photochemical and other organic radical reactions.

AbstractIn this chapter, we focus on aspects of analysis of typical simulation chamber experiments and recommend best practices in term of data analysis of simulation chamber results relevant for both gas phase and particulate phase atmospheric chemistry. The first two sections look at common gas-phase measurements of relative rates and product yields. The simple yield expressions are extended to account for product removal. In the next two sections, we examine aspects of particulate phase chemistry looking firstly at secondary organic aerosol (SOA) yields including correction for wall losses, and secondly at new particle formation using a variety of methods. Simulations of VOC oxidation processes are important components of chamber work and one wants to present methods that lead to fundamental chemistry and not to specific aspects of the chamber that the experiment was carried out in. We investigate how one can analyse the results of a simulation experiment on a well-characterized chemical system (ethene oxidation) to determine the chamber-specific corrections. Finally, we look at methods of analysing photocatalysis experiments, some with a particular focus on NOx reduction by TiO2-doped surfaces. In such systems, overall reactivity is controlled by both chemical processes and transport. Chambers can provide useful practical information, but care needs to be taken in extrapolating results to other conditions. The wider impact of surfaces on photosmog formation is also considered.

AbstractIn order to perform experiments in the chamber, characterization of physical properties is essential for the evaluation and interpretation of experiments. In this chapter, recommendations are given how to measure physical parameters such as temperature and pressure. For photochemistry experiments, knowledge of the radiation either provided by the sun or lamps is key to calculate photolysis frequencies. Standard protocols are described how to validate the calculation of the radiation inside the chamber using actinometry experiments. In addition, the characterization of loss processes for gas-phase species as well as for aerosol is discussed. Reference experiments can be used to test the state of the chamber. Different types of reference experiments focusing on gas-phase photo-oxidation experiments are recommended and described in detail in this chapter.

Delhi, India, suffers from periods of very poor air quality, but little is known about the chemical production of secondary pollutants in this highly polluted environment. During the postmonsoon period in 2018, extremely high nighttime concentrations of NOx (NO and NO2) and volatile organic compounds (VOCs) were observed, with median NOx mixing ratios of ∼200 ppbV (maximum of ∼700 ppbV). A detailed chemical box model constrained to a comprehensive suite of speciated VOC and NOx measurements revealed very low nighttime concentrations of oxidants, NO3, O3, and OH, driven by high nighttime NO concentrations. This results in an atypical NO3 diel profile, not previously reported in other highly polluted urban environments, significantly perturbing nighttime radical oxidation chemistry. Low concentrations of oxidants and high nocturnal primary emissions coupled with a shallow boundary layer led to enhanced early morning photo-oxidation chemistry. This results in a temporal shift in peak O3 concentrations when compared to the premonsoon period (12:00 and 15:00 local time, respectively). This shift will likely have important implications on local air quality, and effective urban air quality management should consider the impacts of nighttime emission sources during the postmonsoon period.

Isoprene and monoterpene emissions to the atmosphere are generally dominated by biogenic sources. The oxidation of these compounds can lead to the production of secondary organic aerosol; however the impact of this chemistry in polluted urban settings has been poorly studied. Isoprene and monoterpenes can form secondary organic aerosol (SOA) heterogeneously via anthropogenic–biogenic interactions, resulting in the formation of organosulfate (OS) and nitrooxy-organosulfate (NOS) species. Delhi, India, is one of the most polluted cities in the world, but little is known about the emissions of biogenic volatile organic compounds (VOCs) or the sources of SOA. As part of the DELHI-FLUX project, gas-phase mixing ratios of isoprene and speciated monoterpenes were measured during pre- and post-monsoon measurement campaigns in central Delhi. Nocturnal mixing ratios of the VOCs were substantially higher during the post-monsoon (isoprene: (0.65±0.43) ppbv; limonene: (0.59±0.11) ppbv; α-pinene: (0.13±0.12) ppbv) than the pre-monsoon (isoprene: (0.13±0.18) ppbv; limonene: 0.011±0.025 (ppbv); α-pinene: 0.033±0.009) period. At night, isoprene and monoterpene concentrations correlated strongly with CO during the post-monsoon period. Filter samples of particulate matter less than 2.5 µm in diameter (PM2.5) were collected and the OS and NOS content analysed using ultra-high-performance liquid chromatography tandem mass spectrometry (UHPLC-MS2). Inorganic sulfate was shown to facilitate the formation of isoprene OS species across both campaigns. Sulfate contained within OS and NOS species was shown to contribute significantly to the sulfate signal measured via AMS. Strong nocturnal enhancements of NOS species were observed across both campaigns. The total concentration of OS and NOS species contributed an average of (2.0±0.9) % and (1.8±1.4) % to the total oxidized organic aerosol and up to a maximum of 4.2 % and 6.6 % across the pre- and post-monsoon periods, respectively. Overall, this study provides the first molecular-level measurements of SOA derived from isoprene and monoterpene in Delhi and demonstrates that both biogenic and anthropogenic sources of these compounds can be important in urban areas.

Liquid chromatography coupled to electrospray ionisation high resolution mass spectrometry is an extremely powerful technique for both targeted and non-targeted analysis of organic aerosol.

Abstract In order to perform experiments in the chamber, characterization of physical properties is essential for the evaluation and interpretation of experiments. In this chapter, recommendations are given how to measure physical parameters such as temperature and pressure. For photochemistry experiments, knowledge of the radiation either provided by the sun or lamps is key to calculate photolysis frequencies. Standard protocols are described how to validate the calculation of the radiation inside the chamber using actinometry experiments. In addition, the characterization of loss processes for gas-phase species as well as for aerosol is discussed. Reference experiments can be used to test the state of the chamber. Different types of reference experiments focusing on gas-phase photo-oxidation experiments are recommended and described in detail in this chapter.

Abstract. Lab-based experimental and computational methods were used to study the atmospheric degradation of two promising “green” solvents: pinacolone, (CH3)3CC(O)CH3, and methyl pivalate, (CH3)3CC(O)OCH3. Pulsed laser photolysis coupled to pulsed laser-induced fluorescence was used to determine absolute rate coefficients (in 10−12 cm3 molec.−1 s−1) of k1(297 K) = (1.2 ± 0.2) for OH + (CH3)3CC(O)CH3 (Reaction R1) and k2(297 K) = (1.3 ± 0.2) for OH + (CH3)3CC(O)OCH3 (Reaction R2), in good agreement with one previous experimental study. Rate coefficients for both reactions were found to increase at elevated temperature, with k1(T) adequately described by k1(297–485 K) = 2.1 × 10−12 exp(-200/T) cm3 molec.−1 s−1. k2(T) exhibited more complex behaviour, with a local minimum at around 300 K. In the course of this work, k3(295–450 K) was obtained for the well-characterised reaction OH + C2H5OH (ethanol; Reaction R3), in satisfactory agreement with the evaluated literature. UV–Vis spectroscopy experiments and computational calculations were used to explore cross-sections for (CH3)3CC(O)CH3 photolysis (Reaction R4), while (CH3)3CC(O)OCH3 showed no sign of absorption over the wavelengths of interest. Absorption cross-sections for (CH3)3CC(O)CH3, σ4(λ), in the actinic region were larger, and the maximum was red-shifted compared to estimates (methyl ethyl ketone (MEK) values) used in current state-of-science models. As a consequence, we note that photolysis (Reaction R4) is likely the dominant pathway for removal of (CH3)3CC(O)CH3 from the troposphere. Nonetheless, large uncertainties remain as quantum yields φ4(λ) remain unmeasured. Lifetime estimates based upon Reactions (R1) and (R4) span the range 2–9 d and are consequently associated with a poorly constrained estimated photochemical ozone creation potential (POCPE). In accord with previous studies, (CH3)3CC(O)OCH3 did not absorb in the actinic region, allowing for straightforward calculation of an atmospheric lifetime of ≈ 9 d and a small POCPE ≈ 11.

We report a new general method for trapping short-lived radicals, based on a homolytic substitution reaction SH2'. This departure from conventional radical trapping by addition or radical-radical cross-coupling results in high sensitivity, detailed structural information, and general applicability of the new approach. The radical traps in this method are terminal alkenes possessing a nitroxide leaving group (e.g., allyl-TEMPO derivatives). The trapping process thus yields stable products which can be stored and subsequently analyzed by mass spectrometry (MS) supported by well-established techniques such as isotope exchange, tandem MS, and high-performance liquid chromatography-MS. The new method was applied to a range of model radical reactions in both liquid and gas phases including a photoredox-catalyzed thiol-ene reaction and alkene ozonolysis. An unprecedented range of radical intermediates was observed in complex reaction mixtures, offering new mechanistic insights. Gas-phase radicals can be detected at concentrations relevant to atmospheric chemistry.