An important property of the compounds comprising organic aerosol is volatility, typically described in terms of saturation vapor pressure or saturation concentration (measured in µg/m3). The volatility of aerosol constituents can be estimated based on their molecular formula using different parametrizations or measured experimentally by using a chemical ionization mass spectrometer (CIMS) coupled to a filter inlet for gases and aerosols (FIGAERO). In this technique, aerosol sample is collected semi-online and evaporated via gradually heated nitrogen flow desorbing organic constituents to be measured by CIMS. From the temperature at which detected chemical species reach their maximum signal, it is possible to determine the respective compositions’ volatility.In 2024, the FIGAERO-CIMS was deployed at the CHANEL (household chemicals amplifying urban aerosol pollution) measurement campaign at the SAPHIR chamber at Jülich Research Centre, Germany. During the campaign, complex reactive mixtures representing urban air scenarios were injected into the chamber, and exposed to both day- and night-time oxidation via opening or closing the roof to natural sunlight. We developed a multi-peak fitting algorithm to fully fit each composition’s thermogram (signal vs. desorption temperature), resulting in multiple nominal saturation concentrations per detected composition. We interpret these as combinations of simple volatility-driven desorption and decomposition (typically at higher temperatures) of larger compounds, such as accretion products.We tracked the chemical composition and volatility of secondary organic aerosol throughout its formation and subsequent aging in the chamber over several hours. The chemical composition measured by FIGAERO-CIMS was compared with other co-located online mass spectrometric techniques, e.g., CIMS following online aerosol evaporation by a heated sheath flow (WALL-E). Our initial results show how aerosol volatilities typically decreased with age, as more oxygen was incorporated. Further, night-time conditions resulted in both increased organonitrate formation and lower product volatility relative to day-time conditions.

Effective management of air quality and climate change requires recognition of their fundamental coupling through atmospheric oxidation capacity (AOC), which governs the atmosphere’s self-cleansing capacity. However, policies often overlook the nonlinear chemical feedbacks inherent to AOC, leading to fragmented strategies that risk unintended consequences. Here, we demonstrate that uncoordinated strategies, such as reducing fossil fuel-related NOX without concurrent methane controls, can suppress OH radicals, inadvertently prolonging methane’s lifetime. This “chemical lockdown paradox,” observed during COVID-19 lockdowns, reveals critical trade-offs, where short-term air-quality gains may increase methane accumulation, offsetting the climate benefits of CO2 abatement. Given AOC’s spatial heterogeneity, such effects can extend beyond local scales. We thus propose a regulatory framework integrating AOC dynamics through coordinated multi-pollutant controls, advanced multi-scale AOC monitoring, and improved Earth system models with fully coupled chemical feedbacks. This framework paves a science-based pathway for synergistically managing air quality and climate mitigation throughout the decarbonization transition.

Chemical ionization mass spectrometry (CIMS) offers high time-resolution measurements for diverse compounds, but atmospheric quantification remains challenging. Here, we combine a recently published method for determining the collision limit using a single reagent ion with a voltage scanning approach for assessing the relative sensitivities of diverse adduct ions. We used voltage scanning in a Multi-Reagent Chemical Ionization Mass Spectrometer (MR-CIMS) to assess ion-molecule adduct strength. The sensitivities to most detectable compounds were calculated based on this relationship using a collision-limit sensitivity of 13.87 +/- 0.69 ncps pptv-1 determined for alpha-pinene using the benzene channel. Following previously published work, the collision limit sensitivity of the other reagent ions used was assumed to be equal to that of the benzene channel and was further examined using the binding energy and measured sensitivity of nitrophenol in the bromide channel. Calibration of 13 molecules, including nitric acid, formic acid, and oxygenated VOCs, was performed to obtain a universal relationship between the sensitivities and the voltage at which the adduct signal halves (dV50). Quantification uncertainties stayed below 20 % for compounds with sensitivities above 5.69 and 5.30 ncps pptv-1 in bromide and iodide channels, respectively. High-level quantum chemical calculations indicated that the detected compounds predominantly form hydrogen-bonded clusters with bromide and iodide. In a large photochemical chamber, estimated sensitivities of more than 260 compounds were determined. Based on this, we achieved their quantification with multiple negative reagent ions. The quantification was validated by comparing nitrous acid concentrations measured using MR-CIMS with those obtained from a calibrated iterative cavity-enhanced differential optical absorption spectroscopy (ICAD) (R2 = 0.891, slope = 1.24). Six of the organic compounds were taken as examples to show the results of this method for a daytime oxidation chamber experiment. The measurement uncertainties for these pptv-level compounds ranged from 8.9 % to 39.0 %, depending on their sensitivities and concentrations. Further theoretical and experimental investigations showed that halogen compounds can form intermolecular halogen bonds with strength comparable to their intramolecular bonds, preventing this approach from being applied to determine their sensitivities. This work highlights that voltage scanning is a useful approach to the determination ion-molecule adduct sensitivity in CIMS.

The hydroxyl radical (OH) is the most important daytime oxidant in the troposphere, initiating chemical degradation of volatile organic compounds (VOCs) and hence contributing to the formation of secondary pollutants such as ozone (O3). In the oxidation process of VOCs, peroxy radicals (RO2) and hydroperoxy radicals (HO2) are formed. In polluted areas, characterised by the presence of nitric oxide (NO), the OH radical is regenerated by the reaction of HO2 with NO, enhancing atmospheric oxidation. Ozone is mainly produced from the photolysis of nitrogen dioxide (NO2) which is formed in the reaction of HO2 and RO2 with NO, where the latter reaction also leads to the formation of an alkoxy radical (RO). Depending on the fate of the RO radical, additional O3 may be produced. Large discrepancies between measured and modelled HO2 and RO2 radical concentrations have been observed during daytime for several urban environments, both for low (< 1 ppbv) and high (> 3 ppbv) NO. As measured and modelled radical concentrations are commonly used to determine the instantaneous ozone production rate (P(Ox)), a large model-measurement discrepancy was also found for P(Ox) at high NO. A systematic study of the photo-oxidation of different anthropogenic VOCs (propane, propene, iso-pentane, n-hexane, trans-2-hexene), associated with traffic emissions and involving different alkoxy chemistry, was conducted at the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany, for NO mixing ratios below 1 ppbv and between 3 and 5 ppbv. Measured radicals as well as precursors and oxidation products are compared with results from a zero-dimensional box model using the Master Chemical Mechanism (MCM v3.3.1) which is complemented by structure-activity relationships (SAR). When including SAR, an improved model-measurement agreement of HO2 and RO2 radical concentrations was specifically found for n-hexane and trans-2-hexene. In addition, the Ox (= NO2 + O3) formation per oxidised VOC (P(Ox)VOC) could be derived from modelled radical concentrations and measured Ox concentrations. Overall, a good agreement between the different P(Ox)VOC was found.

The seasonal variations of aerosol sources and their atmospheric evolution are investigated using observations from the year-long JULIAC (Jülich Atmospheric Chemistry Project) campaign (January–November 2019) in Jülich, Germany. Non-refractory submicron aerosol components were continuously measured alongside oxidants (OH, O3, NO3), trace gases, and meteorological conditions. Organic aerosols (OA) dominated the aerosol composition throughout the year (39 %–58 %), with secondary formation being the major source. OA, including organic nitrate and organosulfate, peaked during a summer heatwave event due to enhanced daytime and nighttime secondary OA formation driven by elevated concentrations of atmospheric oxidants. Changes in the OA composition during the heatwave suggest a shift in the formation pathways, where isoprene may play an important role. Biomass-burning, mainly wildfires and anthropogenic activities (e.g., heating, industry), is the dominant primary OA source (45 %–83 %), which may grow in influence due to climate change and the expected energy transition. Air masses containing OA from regional transport from marine and wildfire sources are identified through source apportionment. Analysis and modeling prove this method to be more reliable than traditional tracer-based methods. Regional transport to this study site typically shows a cleansing effect on the aerosol concentration, except in winter. Furthermore, seasonal variations in the effects of regional transport are seen, where identical transport pathways led to different influences on aerosol properties, driven by seasonal differences in biogenic and anthropogenic emissions. This study enhances understanding of seasonal variation in submicron aerosol properties in response to their sources, atmospheric evolution, and transport.

Ozone (O3) is a criteria air pollutant in the troposphere and powerful oxidant that damages cellular tissue along our respiratory tract, causing distress, and crops and other plants, decreasing primary productivity in the environment. O3 is also a greenhouse gas that is responsible for ~12% of the anthropogenic global warming since 1750. Unlike other criteria air pollutants that have major primary emission sources, O3 is entirely a secondary pollutant and has a complicated non-linear dependence on its precursors nitrogen oxides (NOx) and volatile organic compounds (VOCs). For instance, lowering some O3 precursors can actually increase local O3 concentrations under certain conditions. Ozone concentrations observed at monitoring sites not only depend on the local chemistry but also on the transport of air masses containing O3 from other locations, and on dry deposition. It is therefore essential to clearly understand how these physicochemical processes impact ozone budgets to design efficient mitigation measures at targeted sites. Simultaneous measurements of ozone production rates, P(O3), and ozone concentrations can provide a detailed picture of the ozone budget at a monitoring site, including a critical assessment of the O3-driving processes mentioned above. In this study, we will present how a chemical amplifier can be used to infer P(O3) from peroxy radical measurements and we will discuss the reliability of this methodology. We will present results from ozone production experiments that were performed in the SAPHIR atmospheric chamber during the ROxCOMP (ROx Comparison) campaign. We will show how P(O3) values inferred from the chemical amplifier compare to values derived from observed Ox (O3+NO2) changes in the chamber for various experiments using contrasting conditions of VOCs and NOx. Acknowledgments. This work is supported by the French national research agency (ANR) under LABEX-CaPPA (ANR-11-LABX-005-01), the CPER-CLIMIBIO program, the French national program LEFE/CHAT INSU and the Hauts-de-France region of France. This project has received funding from the European Union’s Horizon 2020 research and innovation programme through the ATMO-ACCESS Integrating Activity under grant agreement No 101008004.

Aromatic hydrocarbons play a critical role in the formation of urban ozone and secondary organic aerosols, impacting air pollution, climate change, and human health. However, their oxidation mechanisms remain uncertain, particularly under low nitric oxide (NO) conditions (<5 ppb). Using the SAPHIR chamber, this study investigated the yields of key oxidation products and their NO dependence under initial NO concentrations varying from 150 ppt to 20 ppb. For toluene (m-xylene) oxidation, the low-NO experimental yields of benzaldehyde, phenolic products, and products from bicyclic ring-opening, bicyclic ring-retaining, and H-shift reactions of organic peroxy radicals (RO2) were 8% (5%), 21% (9%), 45% (53%), 6% (8%), and 6% (7%), respectively. The yields from the bicyclic pathway showed the strongest NO sensitivity. A comparison between the MCMv3.3.1 mechanism-simulated yields and experimental results revealed notable discrepancies, with model─measurement differences of +9% and +25% for the bicyclic ring-opening and ring-retaining products in toluene oxidation and +7%, -16%, and +14% for the phenolic, bicyclic ring-opening, and ring-retaining products in m-xylene oxidation. Notably, products from H-shift RO2 reactions, not included in the model, showed yields comparable to bicyclic closed-ring products, contributing 5-8% to carbon closure in toluene and 6-8% in m-xylene oxidation.

The photooxidation of five anthropogenic volatile organic compounds (propane, propene, isopentane, n-hexane, trans-2-hexene) at different levels of nitric oxide (NO) was investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich. Measured time series of trace gases and radical concentrations are compared to zero-dimensional box model calculations, based on the Master Chemical Mechanism (agreement within 30%) and complemented by state-of-the-art structure–activity relationships (SAR). Including RO2 isomerization reactions from SAR, validated with theoretical calculations, improves particularly the model–measurement agreement by ∼20% for n-hexane. The photooxidation of the chosen compounds generates different types of peroxy radicals (RO2) which produce HO2 after one or multiple RO2+NO reaction steps, depending on the formed alkoxy radical (RO). Measurements show that the HO2/RO2 ratio is up to ∼40% lower and the number of odd oxygen (Ox = O3+NO2) formed per OH+VOC reaction (P(Ox)VOC) is up to ∼30% higher if RO regenerates RO2 instead of forming HO2 directly. Though, the formation of organic nitrates nearly completely compensates for the ozone production from the second NO reaction step for nitrate yields higher than 20%. Measured and modelled HO2/RO2 ratios agree well as does P(Ox)VOC, derived from measured/modelled radical concentrations and calculated from measured Ox.

Oxidized Organic Aerosol (OOA), a major component of fine atmospheric particles, impacts climate and human health. Previous experiments and atmospheric models emphasize the importance of nocturnal OOA formation from NO3· oxidation of biogenic VOCs. This seasonal study extends the understanding by showing that nocturnal oxidation of biomass-burning emissions can account for up to half of total OOA production in fall and winter. It is the first to distinguish nocturnal OOA characteristics from daytime OOA across all seasons using bulk aerosol measurements. Summer observations of nocturnal OOA align well with regional chemistry transport model predictions, but discrepancies in other seasons reveal a common model deficiency in representing biomass-burning emissions and their nocturnal oxidation. This study underscores the significance of near-ground nocturnal OOA production, proposes a method to differentiate it using bulk aerosol measurements, and suggests model optimization strategies. These findings enhance the understanding and prediction of nighttime OOA formation.

Tropospheric ozone pollution is a critical air-quality concern in China. However, the most effective mitigation approach remains unclear, with prioritizing the reduction of volatile organic compounds or nitrogen oxides (NOX) currently still under debate. Here we analyse observational measurements of ozone in August, as well as its precursors, from urban Beijing between 2006 and 2020. We show that, despite a continuous increase in the primary atmospheric oxidant (hydroxyl radical, OH), ozone increased until 2014 and then decreased. This ozone trend can be explained by changes in OH turnover rate, primarily determined by the reactivity ratio between volatile organic compounds and NOX. Overall, reactive abatement of volatile organic compounds should be a near-future priority for ozone-pollution control in China, followed by further NOX controls. Summer ozone in urban Beijing increased until 2014 and then decreased, according to 15 years of measurements in August from 2006 to 2020.

The formation of peroxynitrates (RO2NO2) from the reaction of peroxy radicals (RO2) and nitrogen dioxide (NO2) and their subsequent redissociation are typically not included in chemical mechanisms. This is often done to save computational time as the assumption is that the equilibrium is strongly towards the RO2 + NO2 reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO2 radicals due to the long lifetime of peroxyacyl nitrates RO2NO2 (PANs). In this study, the nighttime oxidation of cis-2-butene and trans-2-hexene in the presence of NO2 is investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum J & uuml;lich, Germany, at atmospherically-relevant conditions at different temperatures (approximate to 276 K, approximate to 293 K, approximate to 305 K). Measured concentrations of peroxy and hydroperoxy radicals as well as other trace gases (ozone, NO2, volatile organic compounds) are compared to state-of-the-art zero-dimensional box model calculations. Good model-measurement agreement can only be achieved when reversible RO2 + NO2 reactions are included for all RO2 species using literature values available from the latest SAR by [Jenkin et al., Atmos. Chem. Phys., 2019, 19, 7691]. The good agreement observed gives confidence that the SAR, derived originally for aliphatic RO2, can be applied to a large range of substituted RO2 radicals, simplifying generalised implementation in chemical models. RO2NO2 concentrations from non-acyl RO2 radicals of up to 2 x 10 cm(-3) are predicted at 276 K, impacting effectively the kinetics of RO2 radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO2NO2. Based on this study, 60% of RO2 radicals would be stored as RO2NO2 at a temperature of 10 degrees C and in the presence of a few ppbv of NO2. The fraction increases further at colder temperatures and/or higher NO2 mixing ratios. This does not only affect the expected concentrations of RO2 radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO2 radicals as well as increase the production of NOx (= NO + NO2) in different locations depending on their lifetime. Omitting this chemistry from the kinetic model can lead to misinterpreted product formation and may prevent reconciling observations and model predictions.

Correction for 'Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry' by Michelle Farber et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d3cp04163h.

How much do specific climbing plants contribute to the cleansing or absorption of harmful greenhouse and pollutant gases; often regarded as the main environmental threat in cities due to their adverse effects on human health? One of the main hurdles in the quantification of such ecosystem services is associated with the difficulty to obtain and design systems that provide detailed information on the interaction between various gases and the plant in question. To tackle these questions, two highly precise and accurate instruments, namely a mid-infrared laser absorption spectrometer (TDL) and a cavity-ring-down spectrometer (CRDS) were used to monitor the fate of gases when exposed to façade climbing plants like ivy. In a laboratory setting, a relaxation type of experiment was used consisting of a reaction chamber equipped with plant species and continuously flushed by synthetic air. This setup was used to determine the timescales of decay after short injections of the above-mentioned gases. After these injections, all gases followed simple exponential decay curves. N2O, a non-reactive (inert) tropospheric gas, was used as a reference to which all other gases were compared and thereby quantified. This paper focuses on the detailed description of methods and processes to analyse the gas-absorptive behaviour of plants when exposed to gaseous pollutants. For demonstration purposes, quantified absorption features of nitrogen oxide (NO2) are presented for ivy of the variety Hedera helix “Plattensee”. Results of this method of quantification showed that - as compared to N2O (control), - NO2 had a reduced residence time (time scale) of 100 s, while N2O resulted in a 600 s residence time (indicating no interference with the plant). This is equivalent to a 0.3 cm/s deposition velocity/ absorption rate of NO2 under light conditions.

Correction for ‘Impact of temperature-dependent non-PAN peroxynitrate formation, RO 2 NO 2 , on nighttime atmospheric chemistry’ by Michelle Färber et al. , Phys. Chem. Chem. Phys. , 2024, https://doi.org/10.1039/d3cp04163h.

Correction for 'Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry' by Michelle Farber et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d3cp04163h.

In this work, we used a Zeppelin NT equipped with six sensor setups, each composed of four different low-cost electrochemical sensors (ECSs) to measure nitrogen oxides (NO and NO2), carbon monoxide, and Ox (NO2+O3) in Germany. Additionally, a MIRO MGA laser absorption spectrometer was installed as a reference device for in-flight evaluation of the ECSs. We report not only the influence of temperature on the NO and NO2 sensor outputs but also find a shorter timescale (1 s) dependence of the sensors on the relative humidity gradient. To account for these dependencies, we developed a correction method that is independent of the reference instrument. After applying this correction to all individual sensors, we compare the sensor setups with each other and to the reference device. For the intercomparison of all six setups, we find good agreements with R2≥0.8 but different precisions for each sensor in the range from 1.45 to 6.32 ppb (parts per billion). The comparison to the reference device results in an R2 of 0.88 and a slope of 0.92 for NOx (NO+NO2). Furthermore, the average noise (1σ) of the NO and NO2 sensors reduces significantly from 6.25 and 7.1 to 1.95 and 3.32 ppb, respectively. Finally, we highlight the potential use of ECSs in airborne applications by identifying different pollution sources related to industrial and traffic emissions during multiple commercial and targeted Zeppelin flights in spring 2020. These results are a first milestone towards the quality-assured use of low-cost sensors in airborne settings without a reference device, e.g., on unmanned aerial vehicles (UAVs).

The gas-phase reaction of isoprene with the nitrate radical (NO3) was investigated in experiments in the outdoor SAPHIR chamber under atmospherically relevant conditions specifically with respect to the chemical lifetime and fate of nitrato-organic peroxy radicals (RO2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) the Master Chemical Mechanism, which simplifies the NO3 isoprene chemistry by only considering one RO2 isomer; (2) the chemical mechanism derived from experiments in the Caltech chamber, which considers different RO2 isomers; and (3) the FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the Caltech mechanism includes equilibrium reactions of RO2 isomers, unimolecular reactions of nitrate RO2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO3 oxidation of isoprene. Hydroperoxy aldehyde (HPALD) species, which are specific to unimolecular reactions of nitrate RO2, were detected even in the presence of an OH scavenger, excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, ion signals at masses that can be attributed to epoxy compounds, which are specific to the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the Caltech mechanism cannot compete with the ring-closure reactions predicted by quantum chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by a good agreement between measured and simulated hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to product species such as nitrate hydroperoxides, nitrate carbonyl and nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is small on the timescale of one night (12 h) for typical oxidant concentrations. However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss.