A Two-Decade Anthropogenic and Biogenic Isoprene Emissions Study in a London Urban Background and a London Urban Traffic Site

Nonstatistical Unimolecular Decay of the CH2OO Criegee Intermediate in the Tunneling Regime

Unimolecular decay of the formaldehyde oxide (CH2OO) Criegee intermediate proceeds via a 1,3 ring-closure pathway to dioxirane and subsequent rearrangement and/or dissociation to many products including hydroxyl (OH) radicals that are detected. Vibrational activation of jet-cooled CH2OO with two quanta of CH stretch (17-18 kcal mol-1) leads to unimolecular decay at an energy significantly below the transition state barrier of 19.46 ± 0.25 kcal mol-1, refined utilizing a high-level electronic structure method HEAT-345(Q)Λ. The observed unimolecular decay rate of 1.6 ± 0.4 × 106 s-1 is 2 orders of magnitude slower than that predicted by statistical unimolecular reaction theory using several different models for quantum mechanical tunneling. The nonstatistical behavior originates from excitation of a CH stretch vibration that is orthogonal to the heavy atom motions along the reaction coordinate and slow intramolecular vibrational energy redistribution due to the sparse density of states.

Investigation of Ozone Formation Chemistry during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA)

Salt Lake City (SLC), UT, is an urban area where ozone (O3) concentrations frequently exceed health standards. This study uses an observationally constrained photochemical box model to investigate the drivers of O3 production during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA), which took place from August to September 2022 in SLC. During SAMOZA, a suite of volatile organic compounds (VOCs), oxides of nitrogen (NOx), and other parameters were measured at the Utah Technical Center, a high-NOx site in the urban core. We examined four high-O3 cases: 4 August and 3, 11, and 12 September, which were classified as a nonsmoky weekday, a weekend day with minimal smoke influence, a smoky weekend day, and a smoky weekday, respectively. The modeled O3 production on 4 August and 3 September was highly sensitive to VOCs and insensitive to NOx reductions of ≤50%. Box model results suggest that the directly emitted formaldehyde contributed to the rapid increase in morning O3 concentrations on 3 September. Model sensitivity tests for September 11-12 indicated that smoke-emitted VOCs, especially aldehydes, had a much larger impact on O3 production than NOx and/or anthropogenic VOCs. On 11 and 12 September, smoke-emitted VOCs enhanced model-predicted maximum daily 8 h average O3 concentrations by 21 and 13 parts per billion (ppb), respectively. Overall, our results suggest that regionwide VOC reductions of at least 30-50% or NOx reductions of at least 60% are needed to bring SLC into compliance with the national O3 standard of 70 ppb.

Master Equation Studies of the Unimolecular Decay of Thermalized Methacrolein Oxide: The Impact of Atmospheric Conditions

Master equation simulations of the unimolecular reaction dynamics of the Criegee intermediate methacrolein oxide (MACR oxide) have been performed under a variety of temperature and pressure conditions. These simulations provide insight into how the unimolecular kinetics vary across temperatures spanning the range 288-320 K. This work has incorporated a new potential energy surface and includes the anti-to-syn and cis-to-trans conformational dynamics of MACR oxide, as well as the unimolecular reactions to form dioxirane and dioxole species. The competition between the unimolecular reactivity of MACR oxide and previously documented bimolecular reactivity of MACR oxide with water vapor is explored, focusing on how this competition is affected by changes in atmospheric conditions. The impact on the role of MACR oxide as an atmospheric oxidant of SO₂ is noted.

Analysis of Essential Isoprene Metabolic Pathway Proteins in Variovorax sp. Strain WS11

Isoprene monooxygenase (IsoMO, encoded by isoABCDEF) initiates the oxidation of the climate-active gas isoprene, with the genes isoGHIJ and aldH nearly always found adjacent to isoABCDEF in extant and metagenome-derived isoprene degraders. The roles of isoGHIJ and aldH are uncertain, although each is essential to isoprene degradation. We report here the characterization of these proteins from two model isoprene degraders, Rhodococcus sp. strain AD45 and Variovorax sp. strain WS11. The genes isoHIJ and aldH from Variovorax and aldH from Rhodococcus were expressed individually in Escherichia coli as maltose binding protein fusions to overcome issues of insolubility. The activity of two glutathione S-transferases from Variovorax, IsoI and IsoJ was assessed with model substrates, and the conversion of epoxyisoprene to the intermediate 1-hydroxy-2-glutathionyl-2-methyl-3-butene (HGMB) was demonstrated. The next step of the isoprene metabolic pathway of Variovorax is catalyzed by the dehydrogenase IsoH, resulting in the conversion of HGMB to 2-glutathionyl-2-methyl-3-butenoic acid (GMBA). The aldehyde dehydrogenases (AldH) from Variovorax and Rhodococcus were examined with a variety of aldehydes, with both exhibiting maximum activity with butanal. AldH significantly increased the rate of production of NADH when added to the IsoH-catalyzed conversion of HGMB to GMBA (via GMB), suggesting a synergistic role for AldH in the isoprene metabolic pathway. An in silico analysis of IsoG revealed that this protein, which is essential for isoprene metabolism in Variovorax, is an enzyme of the formyl CoA-transferase family and is predicted to catalyze the formation of a GMBA-CoA thioester as an intermediate in the isoprene oxidation pathway. IMPORTANCE Isoprene is a climate-active gas, largely produced by trees, which is released from the biosphere in amounts equivalent to those of methane and all other volatile organic compounds combined. Bacteria found in many environments, including soils and on the surface of leaves of isoprene-producing trees, can grow on isoprene and thus may represent a significant biological sink for this globally significant volatile compound and remove isoprene before it escapes to the atmosphere, thus reducing its potency as a climate-active gas. The initial oxidation of isoprene by bacteria is mediated by isoprene monooxygenase encoded by the genes isoABCDEF. In isoprene-degrading bacteria, a second gene cluster, isoGHIJ, is also present, although the exact role in isoprene degradation by the proteins encoded by these genes is uncertain. This investigation sheds new light on the roles of these proteins in the isoprene oxidation pathway in two model isoprene-degrading bacteria of the genera Rhodococcus and Variovorax.

'Omics-guided prediction of the pathway for metabolism of isoprene by Variovorax sp. WS11

Bacteria that inhabit soils and the leaves of trees partially mitigate the release of the abundant volatile organic compound, isoprene (2-methyl-1,3-butadiene). While the initial steps of isoprene metabolism were identified in Rhodococcus sp. AD45 two decades ago, the isoprene metabolic pathway still remains largely undefined. Limited understanding of the functions of isoG, isoJ and aldH and uncertainty in the route of isoprene-derived carbon into central metabolism have hindered our understanding of isoprene metabolism. These previously uncharacterised iso genes are essential in Variovorax sp. WS11, determined by targeted mutagenesis. Using combined 'omics-based approaches, we propose the complete isoprene metabolic pathway. Isoprene is converted to propionyl-CoA, which is assimilated by the chromosomally encoded methylmalonyl-CoA pathway, requiring biotin and vitamin B12, with the plasmid-encoded methylcitrate pathway potentially providing robustness against limitations in these vitamins. Key components of this pathway were induced by both isoprene and its initial oxidation product, epoxyisoprene, the principal inducer of isoprene metabolism in both Variovorax sp. WS11 and Rhodococcus sp. AD45. Analysis of the genomes of distinct isoprene-degrading bacteria indicated that all of the genetic components of the methylcitrate and methylmalonyl-CoA pathways are not always present in isoprene degraders, although incorporation of isoprene-derived carbon via propionyl-CoA and acetyl-CoA is universally indicated.

Investigating the Variation of Benzene and 1,3-Butadiene in the UK during 2000-2020

The concentrations of benzene and 1,3-butadiene in urban, suburban, and rural sites of the U.K. were investigated across 20 years (2000-2020) to assess the impacts of pollution control strategies. Given the known toxicity of these pollutants, it is necessary to investigate national long-term trends across a range of site types. We conclude that whilst legislative intervention has been successful in reducing benzene and 1,3-butadiene pollution from vehicular sources, previously overlooked sources must now be considered as they begin to dominate in contribution to ambient pollution. Benzene concentrations in urban areas were found to be ~5-fold greater than those in rural areas, whilst 1,3-butadiene concentrations were up to ~10-fold greater. The seasonal variation of pollutant concentration exhibited a maximum in the winter and a minimum in the summer with summer: winter ratios of 1:2.5 and 1:1.6 for benzene and 1,3-butadiene, respectively. Across the period investigated (2000-2020), the concentrations of benzene decreased by 85% and 1,3-butadiene concentrations by 91%. A notable difference could be seen between the two decades studied (2000-2010, 2010-2020) with a significantly greater drop evident in the first decade than in the second, proving, whilst previously successful, legislative interventions are no longer sufficiently limiting ambient concentrations of these pollutants. The health impacts of these pollutants are discussed, and cancer impact indices were utilized allowing estimation of cancer impacts across the past 20 years for different site types. Those particularly vulnerable to the adverse health effects of benzene and 1,3-butadiene pollution are discussed.

Isoprene-Chlorine Oxidation in the Presence of NOx and Implications for Urban Atmospheric Chemistry

Fine particulate matter (PM_2.5) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM_2.5 concentration. Discrepancies between modeling and field measurements of SOA indicate missing sources and formation mechanisms. Recent studies report elevated concentrations of reactive chlorine species in inland and urban regions, which increase the oxidative capacity of the atmosphere and serve as sources for SOA and particulate chlorides. Chlorine-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon, is known to produce SOA under pristine conditions, but the effects of anthropogenic influences in the form of nitrogen oxides (NO_x) remain unexplored. Here, we investigate chlorine-isoprene reactions under low- and high-NO_x conditions inside an environmental chamber. Organic chlorides including C₅H₁₁ClO₃, C₅H₉ClO₃, and C₅H₉ClO₄ are observed as major gas- and particle-phase products. Modeling and experimental results show that the secondary OH-isoprene chemistry is significantly enhanced under high-NO_x conditions, accounting for up to 40% of all isoprene oxidized and leading to the suppression of organic chloride formation. Chlorine-initiated oxidation of isoprene could serve as a source for multifunctional (chlorinated) organic oxidation products and SOA in both pristine and anthropogenically influenced environments.

Kinetics of Unimolecular Decay of Methyl Vinyl Ketone Oxide, an Isoprene-Derived Criegee Intermediate, under Atmospherically Relevant Conditions

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere. Ozonolysis of isoprene produces methyl vinyl ketone oxide (MVKO), which may react with atmospheric SO₂, formic acid, and other important species at substantial levels. In this study, we utilized ultraviolet absorption to monitor the unimolecular decay kinetics of syn-MVKO in real time at 278-319 K and 100-503 Torr. After removing the contributions of radical reactions and wall loss, the unimolecular decay rate coefficient of syn-MVKO was measured to be k_uni = 70 ± 15 s^-1 (1σ uncertainty) at 298 K with negligible pressure dependence. In addition, k_uni increases from ca. 30 s^-1 at 278 K to ca. 175 s^-1 at 319 K with an effective Arrhenius activation energy of 8.3 ± 2.5 kcal mol^-1, k_uni(T) = (9.3 × 10⁷)exp(-4200/T) s^-1. Our results indicate that unimolecular decay is the major sink of MVKO in the troposphere. The data would improve the estimation for the steady-state concentrations of MVKO and thus its oxidizing ability.

Microbial metabolism of isoprene: a much-neglected climate-active gas

The climate-active gas isoprene is the major volatile produced by a variety of trees and is released into the atmosphere in enormous quantities, on a par with global emissions of methane. While isoprene production in plants and its effect on atmospheric chemistry have received considerable attention, research into the biological isoprene sink has been neglected until recently. Here, we review current knowledge on the sources and sinks of isoprene and outline its environmental effects. Focusing on degradation by microbes, many of which are able to use isoprene as the sole source of carbon and energy, we review recent studies characterizing novel isoprene degraders isolated from soils, marine sediments and in association with plants. We describe the development and use of molecular methods to identify, quantify and genetically characterize isoprene-degrading strains in environmental samples. Finally, this review identifies research imperatives for the further study of the environmental impact, ecology, regulation and biochemistry of this interesting group of microbes.

An Assessment of the Suitability of Active Green Walls for NO2 Reduction in Green Buildings Using a Closed-Loop Flow Reactor

Nitrogen dioxide (NO2) is a common urban air pollutant that is associated with several adverse human health effects from both short and long term exposure. Additionally, NO2 is highly reactive and can influence the mixing ratios of nitrogen oxide (NO) and ozone (O3). Active green walls can filter numerous air pollutants whilst using little energy, and are thus a candidate for inclusion in green buildings, however, the remediation of NO2 by active green walls remains untested. This work assessed the capacity of replicate active green walls to filter NO2 at both ambient and elevated concentrations within a closed-loop flow reactor, while the concentrations of NO and O3 were simultaneously monitored. Comparisons of each pollutant’s decay rate were made for green walls containing two plant species (Spathiphyllum wallisii and Syngonium podophyllum) and two lighting conditions (indoor and ultraviolet). Biofilter treatments for both plant species exhibited exponential decay for the biofiltration of all three pollutants at ambient concentrations. Furthermore, both treatments removed elevated concentrations of NO and NO2, (average NO2 clean air delivery rate of 661.32 and 550.8 m3∙h−1∙m−3 of biofilter substrate for the respective plant species), although plant species and lighting conditions influenced the degree of NOx removal. Elevated concentrations of NOx compromised the removal efficiency of O3. Whilst the current work provided evidence that effective filtration of NOx is possible with green wall technology, long-term experiments under in situ conditions are needed to establish practical removal rates and plant health effects from prolonged exposure to air pollution.

One-Year Characterization and Reactivity of Isoprene and Its Impact on Surface Ozone Formation at A Suburban Site in Guangzhou, China

Isoprene has a potentially large effect on ozone (O3) formation in the subtropical, highly polluted city of Guangzhou. Online measurements of isoprene in Guangzhou city are scarce; thus, isoprene levels were monitored for one year at the Guangzhou Panyu Atmospheric Composition Station (GPACS), a suburban site in Guangzhou, using an online gas chromatography-flame ionization detector (GC–FID) system to investigate the characterization and reactivity of isoprene and its effect on the O3 peak profile in different seasons. The results showed that the daily average mixing ratios of isoprene at GPACS were 0.40, 2.20, 1.40, and 0.13 mixing ratio by volume (ppbv) in spring, summer, autumn, and winter, respectively. These values were considerably higher than the mixing ratios of isoprene in the numerous other subtropical and temperate cities around the world. Furthermore, isoprene ranked first with regard to O3 formation potential (OFP) and propylene-equivalent mixing ratio among 56 measured non–methane hydrocarbons (NMHCs). The ratios of isoprene to cis-2-butene, an exhaust tracer, were determined to estimate the fractions of biogenic and anthropogenic emissions. The results revealed a much greater contribution from biogenic than anthropogenic factors during the daytime in all four seasons. In addition, night-time isoprene emissions were mostly associated with vehicles in winter, and the residual isoprene that remained after photochemical loss during the daytime also persisted into the night. The high levels of isoprene in summer and autumn may cause the strong and broad peaks of the O3 profile because of its association with the most favorable meteorological conditions (e.g., high temperature and intense solar radiation) and the highest OH mixing ratio, which could affect human health by exposing people to a high O3 mixing ratio for prolonged periods. The lower mixing ratios of isoprene resulted in a weak and sharp peak in the O3 profile in both spring and winter. The high level of isoprene in the subtropical zone could accentuate its large impact on atmospheric oxidant capacity and air quality in Guangzhou city.