The carbon dioxide radical anion, CO2●−, is a highly reactive radical species involved in the reduction of the CO2 greenhouse gas, organic synthesis, atmospheric aerosol chemistry, and treatment of halogenated compounds. In recent years, CO2●− has emerged as a strong reductant, or single electron donor. Here we present techniques used to generate CO2●− and we discuss applications to degrading pollutants such as halogenated alkanes. The potential occurrence of such reductions in water and aqueous aerosols is discussed, notably for the degradation of perfluoroalkyl substances. In the laboratory, CO2●− is directly generated by either direct electrochemical reduction of CO2 or hydrogen atom transfer of either formate salts with and without catalysts or ferrioxalate through photochemical or radiolytic processes. The CO2●− has an ultraviolet spectrum, and CO2●− vibration modes are characterized by fast kinetics using infrared and Raman spectroscopy. The second-order rate constants of the reactions of CO2●− with halogenated alkanes, of -1.84 ± 0.22 V, are generally slower than that of the hydrated electron, of -2.87 V, and give a negative linear relationship with energy of lower unoccupied molecular orbital, suggesting single electron transfer mechanism in reducing the halogenated compounds.

Nitrogen dioxide (NO2) is a critical atmospheric pollutant and ozone precursor, yet biogenic soil sources remain poorly constrained. Current models assume soil NO2 flux is exclusively depositional. Here we demonstrate that soils can produce NO2 through microbial superoxide (O2ˉ) production. Using controlled factorial slurry experiments, native microbial communities produced approximately 10 times more NO2 than sterile controls following nitric oxide (NO) exposure. Stimulating superoxide production with NADH increased NO2 formation 15- to 26-fold, while inhibiting NADH oxidase reduced production toward baseline levels. Superoxide dismutase decreased NO2 production by 46-71%, and O2ˉ concentration explained 60% of variation in NO2 production rates. Addition of peroxynitrite to soil increased headspace NO2, confirming this intermediate as the mechanistic link. These findings reveal a pathway linking carbon and nitrogen cycling where heterotrophic decomposers facilitate biogenic NO-to-NO2 conversion via superoxide chemistry, potentially explaining discrepancies between satellite observations and modelled soil NOx emissions. Soil microbes produce superoxide, which reacts with nitric oxide to form nitrogen dioxide directly in soil. This overlooked pathway means soils can emit NO₂ as well as NO, with implications for how nitrogen emissions are modelled.

The emergence of periodical cicadas from soil every 13 or 17 years is a unique ecological phenomenon with the potential to affect soil biogeochemistry in forests, with increased emissions of climate-relevant gases as a consequence. While it's well-known that cicada carcasses create resource pulses of carbon and nitrogen (N) in soil when they die in mass, the processes underlying these effects, as well as the consequences of these effects for N losses, are poorly known. We investigated how the emergence of Brood X cicadas (Magicicada spp.) in 2021 affected soil microbial communities - particularly N cycling taxa - in forests of the United States. We found that decaying carcasses led to emissions of nitrous oxide (N2O) and ammonia (NH3) gas at around 0.53 mg-N m- 2 h- 1, estimated to be a - 35-fold increase over -21 days from the annual average emissions from US forest soils (0.015 mg-N m- 2 h- 1), with the greatest effects occurring at the interface between carcasses and soil surface. Using amplicon sequencing and qPCR, we determined the potential microbial mechanisms behind N2O and NH3 production, including correlations between taxa capable of carrying out less well studied processes DNRA and nitrifier denitrification, and increased emissions of N2O and NH3. Although distinguishing the relative contributions of DNRA, denitrification, and nitrifier denitrification requires direct rate measurements, our results suggest these processes working together contribute to previously unrecognised greenhouse gas emissions following insect emergence events. Collectively, our results indicate that cicadas significantly affect nutrient cycling in forests with the potential to alter soil microbial communities in ways that may enhance ecosystem N emissions.

In the lower atmosphere, reactive nitrogen oxides (NO y ) are temporarily sequestered as organic nitrate esters (RONO2), which partition into aerosols and can contribute to elevated NO y levels and ozone (O3) formation in remote regions. However, knowledge of the reaction mechanisms of RONO2 to NO y in aerosols is scarce. In this study, we investigated the multiphase chemistry releasing NO y via the photochemical and dark reactions of RONO2 with iron on surfaces that mimic mineral dust and iron-containing aerosols. We found that irradiation of Fe3+-citrate (Fe3+Cit) with UV-visible light produces Fe2+ that directly reduces RONO2, releasing reactive nitrogen including nitrogen dioxide (NO2), nitrous acid (HONO) and nitric oxide (NO) into the atmosphere. We believe subsequent reactions involving Fe2+ convert NO2 to HONO, which is further reduced to NO. Evidence for the involvement of Fe2+ comes from experiments done under anoxic conditions, which favor higher yields of Fe2+ and thus NO y products, and direct spectroscopic evidence showing the formation of an Fe2+ - NO intermediate. Our investigation reveals for the first time that iron-promoted reduction is an important sink for RONO2 in aerosols, mineral dust, and cloudwater, in addition to the more traditional OH-driven oxidation chemistry. Reduction mechanisms involving photochemistry of Fe (hydr)oxides, Fe-organic complexes, and other Fe2+ sources should be considered when evaluating the ability of particle phase RONO2 to recycle NO y in the atmosphere.

Photolysis of nitrate (NO3-) in the presence of photosensitizers is thought to promote the release of atmospherically important reactive nitrogen species (NOy), such as HONO, via mechanisms that are poorly understood. To address this knowledge gap, we conducted photochemistry studies on mixtures of NO3-, a model photosensitizer [4-benzoylbenzoic acid (4-BBA)], and various aliphatic organic matter proxies. We found that aliphatic organic matter enhances the production of NOy and superoxide (O2-) from bulk aqueous nitrate photolysis, while the addition of 4-BBA decreases NOy and O2- yields in most cases. This effect was not observed in heterogeneous systems, where 4-BBA enhanced photochemical NOy production when coadsorbed with NO3- on silica surfaces. This demonstrates that photosensitizers act as both oxidants and reductants, regulating the yields of NOy and reactive oxygen species from nitrate photochemistry. Given sufficient residence time in the bulk aqueous phase, nitrate photoproducts can be scavenged via secondary reactions with photosensitizers. In heterogeneous and aerosol systems with high surface area-to-volume ratios, however, volatilization of NOy photoproducts is favored, and NOy production is less affected by chromophoric organic matter sinks in the bulk phase. This work demonstrates the intricate role of aliphatic and chromophoric moieties in natural organic matter in the photochemical conversion of NO3- to NOy (i.e., renoxification) in the atmosphere.

The emergence of billions of periodical cicadas affects plant and animal communities profoundly, yet little is known about cicada impacts on soil carbon fluxes. We investigated the effects of Brood X cicadas (Magicicada septendecim, M. cassinii and M. septendeculain) on soil CO2 fluxes (RS ) in three Indiana forests. We hypothesized RS would be sensitive to emergence hole density, with the greatest effects occurring in soils with the lowest ambient fluxes. In support of our hypothesis, RS increased with increasing hole density and greater effects were observed near AM-associating trees (which expressed lower ambient fluxes) than near EcM-associating trees. Additionally, RS from emergence holes increased the temperature sensitivity (Q10 ) of RS by 13%, elevating the Q10 of ecosystem respiration. Brood X cicadas increased annual RS by ca. 2.5%, translating to an additional 717 Gg of CO2 across forested areas. As such, periodical cicadas can have substantial effects on soil processes and biogeochemistry.

Heterotrophic bacteria and fungi are responsible for the decomposition of organic matter in soil. During this process, reactive oxygen species (ROS) can be produced directly and indirectly as extracellular byproducts of respiration. It is well known that nitrogen (N) cycle processes lead to the formation of volatile reactive nitrogen oxides (NOy), a group of climate-active gases that contribute to atmospheric chemistry and negatively impact human health. Primary microbial sources of NOy include ammonia-oxidising bacteria and denitrifying bacteria and fungi. Despite soil being a significant source of global NOy emissions, studies to date have primarily focused on N emissions from agricultural soils and there remains a large scope for investigating mechanisms of soil NOy production in a wider context in order to better constrain terrestrial and climate process models. Here, we propose a potential microbial mechanism involving ROS that could influence the production of NOy from soil. We utilised metagenomics and metatranscriptomics alongside continuous gas flux measurements and analysis of soil properties to evaluate the connection between microbial ROS and soil-sourced NOy. Our findings suggest that more NOy, particularly nitric oxide (NO), is produced in the presence of increased abundance of ammonia-oxidising and denitrifying taxa. NO can be lost to the environment or reacts with superoxide, an ROS produced via the enzymatic activity of soil organic matter (SOM) decomposers. This reaction produces peroxynitrite (ONOO-), which we have demonstrated to enhance nitrogen dioxide (NO2) emissions from soil. We have shown that the extent of NOy production through this pathway may be dependent on SOM composition, and the associated variability in carbon and nitrogen content. ### Competing Interest Statement The authors have declared no competing interest.

Nitrogen oxides (NO y ) such as NO, NO2, and HONO control the oxidative capacity of the lower atmosphere. Studies have shown that photolysis of nitrate on atmospheric surfaces is an efficient source of nitrogen oxides through a process termed "renoxification;" however, the mechanisms responsible for this process remain poorly understood, leading to difficulties in modeling atmospheric composition. This work aims to elucidate the mechanism of NO y formation from nitrate photolysis on model boundary layer surfaces comprised of mixtures of organic matter (citrate and Suwanee River fulvic acid) and environmentally relevant metals (e.g., Al3+ and Fe3+). Results show that in the presence of organic matter, photochemical yields of NO y were enhanced by a factor of between 5 and 15 compared with photolysis of pure nitrate controls. Known nitrate photochemistry mechanisms are unable to explain this enhancement, suggesting that a fraction of nitrate is directly converted to NO y by strong reductants produced photochemically from organic matter. The addition of Fe (hydr)oxides catalyzed both the reduction of NO2 to HONO and further reduction of HONO to NO via Fe2+, which is formed through photoreduction of Fe-organic matter coordination complexes. In addition, this study assesses the contribution of surface acidity and visible light attenuation on the product yields. The results support a growing body of evidence that strong reductants generated photochemically via organic matter are an important and unrecognized pathway for renoxification on both soil and airborne surfaces (e.g., mineral dust and aerosols).

Halogen atoms are important atmospheric oxidants that have unidentified daytime sources from photochemical halide oxidation in sea salt aerosols. Here, we show that the photolysis of nitrate in aqueous chloride solutions generates nitryl chloride (ClNO2) in addition to Cl2 and HOCl. Experimental and modeling evidence suggests that O(3P) formed in the minor photolysis channel from nitrate oxidizes chloride to Cl2 and HOCl, which reacts with nitrite to form ClNO2. This chemistry is different than currently accepted mechanisms involving chloride oxidation by OH and could shift our understanding of daytime halogen cycling in the lower atmosphere.

Organicnitrate ester compounds (RONO2) are an importantsink for nitrogen oxides (NO x & EQUIV;NO + NO2) that impact ozone and organic aerosol formationin the lower atmosphere. While RONO2 formation from gasphase oxidation chemistry and heterogeneous reactions involving reactiveuptake of NO3 into organic aerosols is well documented, in situ bulk-aqueous-phase production of RONO2 within organic aerosols has not been explored as extensively. Here,we provide evidence that RONO2 species are produced duringaqueous phase reactions of nitronium (NO2 (+))with humic and fulvic acids, which are used as surrogates for aerosolorganic matter. X-ray photoelectron spectroscopy (XPS) and high-resolutionmass spectrometry were used to characterize nitrogen functional groupsand changes in bulk chemical composition during nitration reactions.Correlations between the organic N abundance in reacted humic andfulvic acids and functional group abundance measured by solid-state C-13 NMR indicate that NO2 (+) targets thelipid fraction of the organic matter. Fourier transform infrared spectroscopy(FTIR) analysis of model lipid compounds shows that the reaction ofNO(2) (+) with terminal alcohols (ROH) and alkenesis a source of stable RONO2 compounds. Further, the low-pHenvironment of most nitrate aerosols is expected to enhance the totalterminal ROH pool through acid-catalyzed ester hydrolysis within thelipid-like fraction of natural organic matter. Our findings demonstratethat multiphase oxidation of organic matter by NO2 (+), along with air-particle partitioning of products from gas-phasechemistry, may be an important source of particle-phase organic nitrates.

A selective catalytic converter has been developed to quantify nitrous acid (HONO), a photochemical precursor to NO and OH radicals that drives the formation of ozone and other pollutants in the troposphere. The converter is made from a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) that was found to convert HONO to NO with unity yield under specific conditions. When coupled to a commercially available NOx (=NO + NO2) chemiluminescence (CL) analyzer, the system measures HONO with a limit of detection as low as 64 parts-per-trillion (ppt) (1 min average) in addition to NOx. The converter is selective for HONO when tested against other common gas-phase reactive nitrogen species, although loss of O3 on Nafion is a potential interference. The sensitivity and selectivity of this method allow for accurate measurement of atmospherically relevant concentrations of HONO. This was demonstrated by good agreement between HONO measurements made with the Nafion-CL method and those made with chemical ionization mass spectrometry in a simulation chamber and in indoor air. The observed reactivity of HONO on Nafion also has significant implications for the accuracy of CL NOx analyzers that use Nafion to remove water from sampling lines.

Unique surface properties of aluminosilicate clay minerals arise from anisotropic distribution of surface charge across their layered structures. Yet, a molecular-level understanding of clay mineral surfaces has been hampered by the lack of analytical techniques capable of measuring surface charges at the nanoscale. This is important for understanding the reactivity, colloidal stability, and ion-exchange capacity properties of clay minerals, which constitute a major fraction of global soils. In this work, scanning ion conductance microscopy (SICM) is used for the first time to visualize the surface charge and topography of dickite, a well-ordered member of the kaolin subgroup of clay minerals. Dickite displayed a pH-independent negative charge on basal surfaces whereas the positive charge on edges increased from pH 6 to 3. Surface charges responded to malonate addition, which promoted dissolution/precipitation reactions. Results from SICM were used to interpret heterogeneous reactivity studies showing that gas-phase nitrous acid (HONO) is released from the protonation of nitrite at Al-OH2+ groups on dickite edges at pH well above the aqueous pKa of HONO. This study provides nanoscale insights into mineral surface processes that affect environmental processes on the local and global scale.

Volatile nitrogen oxides (N2O, NO, NO2, HONO, …) can negatively impact climate, air quality, and human health. Using soils collected from temperate forests across the eastern United States, we show microbial communities involved in nitrogen (N) cycling are structured, in large part, by the composition of overstory trees, leading to predictable N‐cycling syndromes, with consequences for emissions of volatile nitrogen oxides to air. Trees associating with arbuscular mycorrhizal (AM) fungi promote soil microbial communities with higher N‐cycle potential and activity, relative to microbial communities in soils dominated by trees associating with ectomycorrhizal (ECM) fungi. Metagenomic analysis and gene expression studies reveal a 5 and 3.5 times greater estimated N‐cycle gene and transcript copy numbers, respectively, in AM relative to ECM soil. Furthermore, we observe a 60% linear decrease in volatile reactive nitrogen gas flux (NOy ≡ NO, NO2, HONO) as ECM tree abundance increases. Compared to oxic conditions, gas flux potential of N2O and NO increase significantly under anoxic conditions for AM soil (30‐ and 120‐fold increase), but not ECM soil—likely owing to small concentrations of available substrate ( NO3‐ ) in ECM soil. Linear mixed effects modeling shows that ECM tree abundance, microbial process rates, and geographic location are primarily responsible for variation in peak potential NOy flux. Given that nearly all tree species associate with either AM or ECM fungi, our results indicate that the consequences of tree species shifts associated with global change may have predictable consequences for soil N cycling.

Nitrate anion (NO3) is ubiquitous in the environment, and its photochemistry produces nitrous acid (HONO), a major source of tropospheric hydroxyl radical (OH). Enhanced HONO (g) emissions have been observed from NO3(aq)- photolysis in field studies, although the underlying reasons for this enhancement are debated. Here, we show that the enhancement is in part caused by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Increased yields of superoxide radical (O-2(-)) and HONO were observed when NO3- solutions (pH 6) were photolyzed in the presence of DAOM surrogates of varying solubility. In an additional experiment, nitrate titrated with additional DAOM showed a further simultaneous increase in the levels of O-2((aq))- and HONO(g) with decreased yields of gaseous nitric oxide (NO) and nitrogen dioxide (NO2). To the best of our knowledge, this is the first time that superoxide was directly observed as an intermediate in nitrate photolysis experiments, produced through DOAM oxidation by OH(aq). Herein, we suggest that enhanced HONO(g) emissions from NO3 (aq) photolysis result from the reaction of O-2((aq))- with NO(aq) and NO2(aq) to form perox-ynitrate (OONO2-) and peroxynitrite (OONO-), respectively, which are precursors to nitrite (NO2-). Overall, this points to an important role of O-2((aq)) in aqueous aerosol chemistry, which is currently underappreciated.

Oxidation flow reactors (OFRs) are an emerging technique for studying the formation and oxidative aging of organic aerosols and other applications. In these flow reactors, hydroxyl radicals (OH), hydroperoxyl radicals (HO2), and nitric oxide (NO) are typically produced in the following ways: photolysis of ozone (O3) at λ=254 nm, photolysis of H2O at λ=185 nm, and via reactions of O(1D) with H2O and nitrous oxide (N2O); O(1D) is formed via photolysis of O3 at λ=254 nm and/or N2O at λ=185 nm. Here, we adapt a complementary method that uses alkyl nitrite photolysis as a source of OH via its production of HO2 and NO followed by the reaction NO + HO2 → NO2 + OH. We present experimental and model characterization of the OH exposure and NOx levels generated via photolysis of C3 alkyl nitrites (isopropyl nitrite, perdeuterated isopropyl nitrite, 1,3-propyl dinitrite) in the Potential Aerosol Mass (PAM) OFR as a function of photolysis wavelength (λ=254 to 369 nm) and organic nitrite concentration (0.5 to 20 ppm). We also apply this technique in conjunction with chemical ionization mass spectrometer measurements of multifunctional oxidation products generated following the exposure of α-Pinene to HOx and NOx obtained using both isopropyl nitrite and O3 + H2O + N2O as the radical precursors.

Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above-canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above-canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state-of-science atmospheric model (GEOS-Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below-canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.

Reactive nitrogen oxides (NOy; NOy = NO + NO2 + HONO) decrease air quality and impact radiative forcing, yet the factors responsible for their emission from nonpoint sources (i.e., soils) remain poorly understood. We investigated the factors that control the production of aerobic NOy in forest soils using molecular techniques, process-based assays, and inhibitor experiments. We subsequently used these data to identify hotspots for gas emissions across forests of the eastern United States. Here, we show that nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil chemical characteristics (pH and C:N), and nitrogen (N) transformation rates (net nitrification). We find that, while nitrification rates are controlled primarily by chemoautotrophic ammonia-oxidizing archaea (AOA), the production of NOy is mediated in large part by chemoautotrophic ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in forest communities, as stands dominated by arbuscular mycorrhizal (AM) trees had greater N transformation rates and NOy fluxes than stands dominated by ectomycorrhizal (ECM) trees. Given mapped distributions of AM and ECM trees from 78,000 forest inventory plots, we estimate that broadleaf forests of the Midwest and the eastern United States as well as the Mississippi River corridor may be considered hotspots of biogenic NOy emissions. Together, our results greatly improve our understanding of NOy fluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.