In spite of the importance of organic peroxy radicals in the oxidizing capacity of the atmosphere few techniques exist to monitor them individually, leaving large uncertainties in the understanding of the atmospheric oxidation cycles. This presentation will give an overview of the on-going ERC-advanced project EPHEMERAL focusing on the detection of these radicals using proton transfer mass spectrometry and aiming at monitoring them in ambient air. Current advances in their detection and in that of related intermediates will be presented. The main advantages and limits of the technique will be discussed, in particular the challenges and various strategies for determining the radical absolute concentration (calibrations). While the technique is still being improved, the performances already obtained allow to study new reactions, such as the interactions of the gas-phase radicals with surfaces, which will be also rapidly presented.
How to cite: Noziere, B., Durif, O., Piel, F., and Wisthaler, A.: The Speciated Detection of Gas-phase Organic Peroxy Radicals and Related Intermediates by Proton Transfer Mass Spectrometry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1237, https://doi.org/10.5194/egusphere-egu25-1237, 2025.
Atmospheric radicals are central to the oxidation capacity and self-cleansing ability of the troposphere, driving the formation of secondary air pollution and the removal of short-lived climate forcers such as methane (CH4) and hydrofluorocarbons (HFCs). Understanding radical chemistry is thus critical for air quality improvement and climate change mitigation. While tropospheric radical chemistry has been extensively studied since the 1990s, significant gaps remain, particularly in the underestimation of hydroxyl (OH) and hydroperoxyl (HO2) radicals under low- and high-NO concentration regimes, respectively. These gaps hinder the development of effective pollution control and climate mitigation strategies.
In this study, we introduce the Ensembled eXperiment of Atmospheric oxidation Capacity in the Troposphere (EXACT) campaign conducted in China. This comprehensive initiative employs state-of-the-art instrumentation to measure key radicals (OH, HO2, RO2, NO3) and their precursors, covering diverse chemical and environmental conditions across urban, regional, and background settings in the North China Plain. Seasonal campaigns conducted in autumn, winter, spring, and summer aim to unravel the molecular-level sources and transformation mechanisms of atmospheric radicals. The research further seeks to elucidate the evolution patterns and driving mechanisms of atmospheric oxidation in critical regions of China.
This presentation will provide an overview of the EXACT campaign design and methodology, alongside preliminary results and discussions from the completed autumn and winter campaigns. These findings offer new insights into diurnal radical sources, transformation pathways, and the broader implications for atmospheric oxidation dynamics in China.
How to cite: Ma, X., Tan, Z., Lu, K., Hu, R., and Lou, S.: Elucidating Tropospheric Radical Chemistry and Atmospheric Oxidation Capacity: Insights from the EXACT Campaign in China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17264, https://doi.org/10.5194/egusphere-egu25-17264, 2025.
Despite well over a half century of research, gaps remain in our understanding of ozone formation chemistry. Net ozone formation results from the oxidation of NO to NO2 by peroxy radicals (HO2 and RO2) followed by photolysis of NO2. Measurements of peroxy radicals made by several analytical methods over the past decade in numerous locations across the world have revealed discrepancies under high NOx conditions ([NO] > 1 ppb), with zero-dimensional models apparently underestimating peroxy radical concentrations and ozone production rates (P(O3)) by up to a factor of eight. These findings suggest that models may misidentify when ozone formation is NOx-limited vs. NOx-saturated (VOC-limited) and that our knowledge of the relevant reactions is incomplete. To investigate these anomalously high P(O3) values at high NOx, we used the Drexel University Ethane Chemical AMPlifier (ECHAMP) instrument to measure total peroxy radicals at a roof-top site in Manhattan (NYC) as part of the NOAA AEROMMA/NYC-METS project. A wide assortment of other measurements were made by spectroscopic and mass spectrometric methods. We will present results from this field project with special emphasis on our measurements during a few “high-NOx” periods (roughly defined as daytime periods with NO mixing ratios greater than 1 ppb).
How to cite: Wood, E., Joy, K., Lindsay, A., Feinman, L., Roscioli, R., Daube, C., Claflin, M., Canagaratna, M., Lerner, B., Blomdahl, D., and Gentner, D.: Ozone Formation under high NOx conditions: Insights from the AEROMMA NYC-METS field campaign, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12780, https://doi.org/10.5194/egusphere-egu25-12780, 2025.
Volatile organic compounds (VOCs) play crucial roles in regulating the photochemical formation of ozone. However, limited knowledge on the interactions between vertical VOCs change and ozone formation in the planetary boundary layer (PBL) has hindered effective ozone control strategies, particularly in large cities. To address concern, we investigated the vertical changes in concentrations, compositions, and key drivers of a large suite of VOCs using online gradient measurements taken from a 325 m tall tower in urban Beijing, China. The impacts of these vertical VOC variations on ozone formation were also analyzed using box model simulations. We find that VOCs exhibited differentiated vertical gradients due to their differences in both sources and chemical reactivities, along with the diurnal PBL evolution. In daytime, reactive VOCs (e.g., hydrocarbons) are rapidly oxidized and their concentrations generally decreased with height, accompanied by the formation and accumulation of oxygenated VOCs (OVOCs) in the middle and upper layers. We also find that the formation of ozone responds positively to changes in both NOx and VOCs. As a result, the production rate of ozone declines with height due to the simultaneous decreases in concentrations of reactive VOCs and NOx, but remains high in the middle and upper layers due to the presence of high OVOCs concentrations. Therefore, careful consideration should be given to the vertical variations in both ozone production rates and formation regimes in the whole PBL when developing regional ozone control strategies.
How to cite: Yuan, B., Li, X., Song, X., and Huangfu, Y.: Vertical changes in volatile organic compounds (VOCs) and impacts on ozone formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14586, https://doi.org/10.5194/egusphere-egu25-14586, 2025.
The hydroxyl radical (OH) sets the oxidative capacity of the atmosphere and determines the lifetime of reactive greenhouse gases such as methane (CH4). The response of OH to climate warming is influenced by uncertain and compensating processes involving weather-sensitive chemistry and emissions. In this study, we extend the idealized aquaplanet configuration of the Community Earth System Model (CESM) Community Atmosphere Model version 6 (CAM6) to include atmospheric chemistry (“AquaChem”). Beyond the aquaplanet’s zonally symmetric sea surface temperatures (SSTs) and lack of seasonality, we further simplify the spatial variability of trace gas and aerosol emissions. We show that the AquaChem configuration generates a robust OH chemical budget, including both production and loss pathways, with relatively short simulations. Thus, the AquaChem model serves as an effective tool for rapidly assessing the sensitivity of OH chemistry, including both production and loss pathways. Rapid convergence allows us to assess the sensitivity of OH chemistry to surface warming. The strongest direct response in OH to increased surface temperatures is an increase in “primary” OH production due to higher water vapor concentrations. We then test the sensitivity of OH sources and sinks to different assumptions regarding the response of emissions to rising temperatures. AquaChem simulations indicate that biogenic emissions are a dominant factor influencing the OH response to climate warming. In tropical regions, climate warming enhances biogenic emissions and increases the OH loss rate, outweighing the increase in OH production resulting from rising water vapor and resulting in a decrease in OH abundance.
How to cite: Zhu, Q., Neumann, N., Fiore, A., Pincus, R., Guan, J., Milly, G., Singer, C., Medeiros, B., and Giani, P.: Biogenic Emissions Modulate the Tropospheric Hydroxyl Radical (OH) Response to Climate Warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7156, https://doi.org/10.5194/egusphere-egu25-7156, 2025.
RO2 autoxidation is the most important class of chemical reactions for modelling instantaneous formation of low-volatility organics in the atmosphere. However, systematic inclusion of these reactions in atmospheric chemistry models is tricky for reasons both fundamental (huge environmental variability in the importance of individual reactions) and technical (reaction rate data relies on experiments on complex radicals with complex reaction branching). This being the case, automatic mechanism generation based on structure-activity relationships (SAR) are crucial for the development of autoxidation-including atmospheric chemistry models. We thus aim to update the mechanism generator GECKO-A (Aumont et al, ACP, 2005) with an autoxidation module based on up-to-date SARs for RO2 H-shift (Vereecken & Nozière, ACP, 2020), ring-closure (Vereecken et al, PCCP, 2021), as well as linear RC(O)O2 H-shift reactions (Seal et al, PCCP, 2023).
At the meeting we will briefly present our strategy for adapting autoxidation mechanisms for different environments and discuss the impact of H-scrambling isomerisations on RO2 chemistry, as these are the largest challenges in developing the autoxidation module. In addition, we present our computational efforts to expand the above SARs in order to more accurately represent the most rapid reactions. Our calcualtions include H-shifts from aldehyde groups in RC(O)O2, ring-closures and allylic H-shifts in unsaturated RC(O)O2, and H-shifts from enol groups in RO2, which have all been found to be exceptionally rapid in previous studies (Rissanen et al, JPCA, 2015; Ojala et al, In Preparation; Peeters & Nguyen, JPCA, 2012). Out of these, we especially highlight the RC(O)O2 ring-closures, as this appears to be the main fate of unsaturated RC(O)O2 in most environments.
In summary, we are aiming to develop the most complete and chemically explicit autoxidation mechanism generator that can be achieved with our current knowledge, and we hope that the modelling community will make great use of it when more specialized truncated models are developed.
How to cite: Franzon, L., Valorso, R., Aumont, B., Camredon, M., Lee-Taylor, J., Orlando, J., Savolainen, A., Iyer, S., Rissanen, M., and Kurtén, T.: Towards automated inclusion of representative autoxidation chemistry in explicit models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15619, https://doi.org/10.5194/egusphere-egu25-15619, 2025.
Atmospheric Oxidation Capacity (AOC) quantifies the ability of atmosphere to oxidize primary species. It plays a crucial role in initiating atmospheric chemical processes and impacts the formation of secondary pollutants, such as ozone (O₃) and secondary aerosols. AOC is fundamentally determined by the concentrations and reactivity of atmospheric oxidants, including O₃, hydroxyl radicals (OH), and nitrate radicals (NO₃). Due to the inherent challenges in direct measurement, AOC is typically inferred through numerical modeling. However, the chemical mechanisms implemented in commonly used 3-D chemical transport models (CTMs) often simplify organic species, leading to underestimations of radical concentrations and AOC.
The Mechanism for Air pollution compleX version 1.0 (MAX1) describing detailed tropospheric chemical processes has been therefore developed to improve the simulation of radicals. MAX1 contains 940 reactions including photolysis, gaseous reactions and heterogeneous reactions of 300 species, which is adequate for both box model and CTM applications. Detailed chemical processes of chlorine chemistry, chemistry of Criegee radicals and heterogeneous uptake of HO2 and N2O5 have been implemented and updated. With this level of explicitness, MAX1 can support investigations on the quantification of secondary pollutant productions and the chemical behavior of the crucial intermedia such as organic peroxy radicals. MAX1 has been validated in box model and regional models. Simulations of MAX1 well captured the variation of O₃ in all cases tested. Meanwhile, significant improvement was made on predictions of radicals compared to other mechanisms, especially under the low NOx environment.
How to cite: Liu, Y., Geng, C., Bai, W., Zhang, N., Yang, W., Zhou, H., Wei, W., Chen, X., Zhou, M., Ma, X., Yang, X., Song, H., Chen, X., Wang, H., Tan, Z., Wang, Z., Zhang, Y., and Lu, K.: Improved Simulation of Atmospheric Oxidation Capacity Using the MAX1 Chemical Mechanism in North China Plain, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19714, https://doi.org/10.5194/egusphere-egu25-19714, 2025.
Both sulfuric acid (H2SO4) (SA) and hydroxyl radical (OH) play critical roles in the atmospheric chemistry processes. In most environments, from the pristine Tibet plateau to highly populated megacities, SA is the decisive nucleation precursor. OH, on the other hand, dominates the atmospheric oxidation capacity under most circumstances. Therefore, accurate measurements of SA and OH are important for atmospheric chemistry studies. This study developed an instrument based on chemical ionization mass spectrometry (CIMS) to measure SA and OH in the air simultaneously. The working principle was based on the nitrate (NO3-) CIMS. SA was ionized by NO3- directly to form bisulfate anion (HSO4-), which was then detected with a high-resolution time-of-flight mass spectrometer (HR-ToF-MS). OH was first converted into SA by excess sulfur dioxide (SO2) and then detected as SA total, which is the sum of ambient SA, OH-converted SA, and background signals due to the high concentration of SO2 (~1ppmv). In order to minimize wall losses, a 3-cm ID, 50-cm long sample inlet was used, and a blower was employed to suck in ambient air at ~100 L min-1. Two 1/16 in gas injectors were installed at the front end of the inlet and 30 cm downstream of the front injector. The instrument was operated in three sequential modes, i.e., the ambient SA mode, OH mode, and background (BG) mode. No reagent gases were injected in the SA mode, and ambient SA was detected directly. During the OH mode, SO2 was injected into the sample flow through the first injector. For the BG mode, both SO2 and pure propane (C3H8) were injected into the sample flow through the first injector. Since C3H8 concentration was about a few hundred ppmv, nearly two orders of magnitude higher than SO2, OH was completely scavenged during the BG mode, and only ambient SA and SO2 BG were detected. A constant stream of propane was injected into the inlet through the second injector to prevent further free radical cycling. Therefore, the difference between the OH mode and the BG mode was the ambient OH signal. Each mode was operated for about 3-min, and the 9-min detection limits of SA and OH were ~2×105 molecules cm-3 and 4×105 molecules cm-3 (3σ), respectively. The instrument was calibrated by known concentrations of SA standards generated by a low-pressure Hg-lamp, the intensity of which was determined by the N2O-actinometry. The instrument was field tested in a mountain site located in Longquan Mountain, Chengdu, China. Both SA and OH showed clear diurnal patterns following the solar radiation, ranging from less than the detection limit to a few 106 molecules cm-3. Further intercomparison between the measurements and model simulations was also conducted to verify the measurement results.
How to cite: Zheng, J. and Ma, Y.: A versatile instrument for simultaneous detection of atmospheric H2SO4 and OH radicals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10634, https://doi.org/10.5194/egusphere-egu25-10634, 2025.
Improving air quality is one of the main challenges in achieving a sustainable future. The mitigation strategies for air quality and climate change require accurate knowledge of both the amount of trace gases present in the atmosphere and chemical processes involving these trace gases. The primary removal mechanism of trace species such as methane (CH4), volatile organic compounds (VOCs), and NOx (NOx = NO + NO2) in the atmosphere is the reaction with hydroxyl radical (OH), which leads to the formation of secondary pollutants such as ozone (O3) and secondary organic aerosol (SOA). Thus, understanding the behavior of OH in the atmosphere is critical to understanding the lifetimes of many trace species and the reaction pathways leading to the production of secondary pollutants. Although it is not possible to quantify all species in the atmosphere that react with OH, it is possible to quantify their impacts on air quality and climate through measurements of OH reactivity (kOH) (Kovacs and Brune, 2001). OH reactivity is the total pseudo-first-order coefficient describing the loss of OH, which is the inverse of the OH chemical lifetime, and defined as 
, where 
is the rate coefficient for reaction of OH with species Xi.
Measurements of kOH have been made successfully in the field using several techniques (Sadanaga et al., 2004; Sinha et al., 2008; Stone et al., 2016), but long-term continuous measurements have proved challenging, particularly in high NOx environments (Fuchs et al., 2017). In this work we describe the development of a novel instrument based on laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy to make long-term measurements in a wide range of environments. In the field configuration, the instrument has a limit of detection (LOD) of kOH around 1.5 s-1 and LOD of [OH] around 5 × 1010 molecules cm-3. We will present details about the instrument development, characterisation and the field intercomparison with a laser-induced fluorescence (LIF) instrument.
How to cite: George, M., Luke, T., Babu, A., Whalley, L., Heard, D., Blitz, M., and Stone, D.: Development of a novel instrument for long-term measurements of OH reactivity , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18818, https://doi.org/10.5194/egusphere-egu25-18818, 2025.
The total OH reactivity (kOH′), which is equal to the reciprocal of the lifetime (τOH) of the hydroxyl (OH) radical in the atmosphere, is an important parameter for quantitatively assessing the atmospheric oxidation capacity. Although kOH′ was first measured in the laboratory more than 20 years ago, the required instrumentation is costly and complex, and only a few research groups can perform such measurements. Long-term observation of kOH′ remains challenging and difficult to achieve. In this presentation, we report the development of a portable laser-flash photolysis Faraday rotation spectroscopy (LP-FRS) instrument for real-time and in-situ measurement of kOH′. OH decay is directly measured using a time-resolved FRS spectrometer at 2.8 μm. Since FRS relies on the detection of the rotation of the polarization state of the probe light induced by paramagnetic molecules in a longitudinal magnetic field, the laser noise and molecule interferences are significantly reduced, which enables the FRS system to directly and highly sensitive monitor OH concentration without chemical interferences. The LP-FRS instrument has a kOH′ detection precision of 1.0 s-1 with an averaging time of 300 s. The instrument’s optical box measures 130 cm × 40 cm × 35 cm, making its convenient for field applications.
How to cite: Fang, B., Zhao, W., Wei, N., Zhang, W., and Chen, W.: Portable laser-flash photolysis Faraday rotation spectrometer for real-time in-situ measurement of total OH reactivity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16408, https://doi.org/10.5194/egusphere-egu25-16408, 2025.
Gaseous nitrous acid (HONO), a critical precursor of hydroxyl radicals (OH), plays a key role in the atmosphere’s oxidizing capacity, driving the production of secondary pollutants. However, large uncertainties in its formation and removal mechanisms impede accurate simulation of HONO levels using chemical transport models (CTMs). In this study, a deep neural network (DNN) model was established based on routine air quality data (NO2, CO, O3, PM2.5) and meteorological parameters (temperature, relative humidity, solar zenith angle and season) collected from four typical megacity clusters in China. The DNN model exhibited robust performance on both train sets (slope = 1.0, r2 = 0.94, RMSE = 0.29 ppbv) and two independent test sets (slope = 1.0, r2 = 0.79, RMSE = 0.39 ppbv). It demonstrated excellent capability in reproducing the spatial temporal variations of HONO and outperformed an observation-constrained box model incorporated with newly proposed HONO formation mechanisms. Nitrogen dioxide (NO2) was identified as the most impactful features for HONO prediction using the SHapely Additive exPlanation (SHAP) approach, highlighting the contribution of NO2 conversion in HONO formation. The DNN model was applied to predict future change of HONO levels under different NOx mitigation scenarios, which is expected to decrease 27-44% under 30-50% NOx reduction, consistent with the box model outputs. These results suggest a dual effect brought by NOx abatement, leading to not only reduction of O3 and nitrate precursors but also decrease in HONO levels and hence primary radical production rates. The model was further employed to construct an hourly-resolved nationwide HONO dataset for China spanning 2015-2023, offering a valuable tool for constraining ozone production in the CTMs.
The construction and application of the DNN model has been published on Environ. Sci. & Tech. (Environ. Sci. Technol. 2024, 58, 29, 13035–13046).
How to cite: Li, X., Ye, C., Lu, K., Wang, C., and Zhang, Y.: Accurately Predicting Spatiotemporal Variations of Near-Surface Nitrous Acid (HONO) Based on a Deep Learning Approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3673, https://doi.org/10.5194/egusphere-egu25-3673, 2025.
The hydroxyl radical (OH), the main atmospheric oxidant, removes most pollutants including potent greenhouse gas methane. However, its short lifetime precludes direct observational quantification, so its temporal variability is poorly understood. Here, we used a three-dimensional chemical transport model GEOS-Chem, to investigate global tropospheric OH concentrations changes (ΔOH) from 1999 to 2019, driven by nitrogen oxides and carbon monoxide. We showed that ΔOH relative to 1999 increased rapidly from 2000 to 2008, dropped until 2010 and then continued to grow from 2011 to 2019, culminating in a 4.7% increase by 2019. The increase in ΔOH is primarily attributed to emissions from land (63%), followed by emissions from aircraft (24%) and shipping (13%). Notably, the tropics, particularly East Asia, Southeast Asia, South Asia, and Central America, contributed 74% of the global ΔOH burden in 2019. We also performed fractional simulations to separate the influence of land emissions influence from changes in the spatial distribution (LandS) and magnitude of emissions (LandM). We found that as land NOx emissions shifted equatorward from middle and high latitudes to low latitudes, the influence of landS increased persistently, exceeding LandM by 2014, and the relative contribution of LandS to ΔOH due to land emissions reached 58% in 2019. Looking forward, with the continued global southward shift in anthropogenic emissions, the role of LandS in global OH levels should not be overlooked. These insights underscore the need to consider anthropogenic emission patterns in projecting future OH concentrations and developing climate mitigation strategies.
How to cite: Zhu, Y., Shen, L., Liu, G., and Peng, S.: Global southward shift in anthropogenic emisisons enhance tropospheric hydroxyl radicals during 1999-2019, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5603, https://doi.org/10.5194/egusphere-egu25-5603, 2025.
The troposphere is considered an oxidizing atmospheric environment with radical chemical reactions. During the day, hydroxyl radicals (OH) are the primary oxidants, while at night, nitrate radicals (NO3) take on this role. NO3 radicals can react with volatile organic compounds (VOCs), especially alkenes, to form organic aerosols. In addition, NO3 radicals can also generate nitrate aerosols through the heterogeneous reaction of N2O5, leading to severe air pollution issues. Nighttime atmospheric oxidizing capacity refers to the ability of oxidants to convert primary pollutants into secondary pollutants. Since NO3 radicals are the main oxidants at night, the nitrate radical production rate (PNO3) is often used as an indicator of nighttime atmospheric oxidizing capacity. However, unlike the well mixed during the day, nighttime air exhibits strong vertical stratification due to the cooling of the ground. As a result, there are significant differences in the concentration of pollutants and chemical reaction processes at different heights, leading to substantial variations in atmospheric oxidizing capacity with altitude. Therefore, ground-based observations cannot fully represent the entire nighttime boundary layer. To accurately describe the oxidative characteristics of the nighttime atmosphere, we combined vertical tower observation data to analyze the distribution characteristics of PNO3 with altitude and classified the distribution patterns of PNO3 under different environmental conditions.
How to cite: Qin, Y., Wang, H., Fan, S., Brown, S., and Lu, K.: Enhanced nocturnal oxidation chemistry in the upper mixing layer of megacities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3493, https://doi.org/10.5194/egusphere-egu25-3493, 2025.
Carbonyls are an important class of volatile organic compounds (VOC) in the atmosphere, being both directly emitted and produced by the oxidation of other VOCs. UV-B carbonyl photolysis produces radicals and so is an important driver of atmospheric radical cycles, that are a key route for the breakdown of primary pollutants and formation of secondary pollutants such as ozone and organic aerosol.
However, photochemical data concerning atmospheric photolysis of carbonyl compounds (i.e. photolysis cross-sections and quantum yields) are limited, and air quality research tools such as the Master Chemical Mechanism (MCM; mcm.york.ac.uk) must rely on using parameters from a small number of surrogate compounds to estimate photolysis rates for a larger suite of photo-labile VOCs.
To address this, we have developed a new laboratory flow reactor that utilises UV-LED technology to study photolysis quantum yields. This uses nitric oxide as a tracer for the peroxy radical photoproducts, assisted by a zero-dimensional chemical box model of the reactor system. Preliminary results for acetaldehyde and butanone show reasonable agreement with literature values, and the technique is fast and relatively easy to apply to a range of previously understudied compounds such as longer chain and branched ketones.
A wider understanding of the structure-reactivity trends of carbonyl quantum yields will improve atmospheric modelling capabilities, including better predictions of atmospheric impacts on air quality and human health.
How to cite: Winkless, R., Rickard, A., and Dillon, T.: Photolysis of atmospherically important carbonyls: Quantum yield measurements using an NO radical tracer method, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3713, https://doi.org/10.5194/egusphere-egu25-3713, 2025.
Nocturnal oxidation driven by nitrate radicals is an important process in atmospheric chemistry, regulating the fate of volatile organic compounds and nitrogen oxide, and affecting the particulate pollution levels. While detecting NO3 is challenging due to its extremely low concentration. Currently, techniques such as Cavity Ring-Down Spectroscopy (CRDS) and Cavity-enhanced absorption Spectroscopy (CEAS) are widely used in NO3 measurement but suffer from the sampling loss due to its high reactivity. Here, we try to develop an open CEAS system to detect the ambient NO3, which eliminates the sampling loss. However, this method has its technical challenges, including the interferences of water absorptions during the NO3 absorption window near 662 nm and the effects of particle extinction and relative humidity and temperature during the field observation. We applied a small cavity cage (~40 cm high reflectivity mirror distance) during the hardware design, which features great stability. In addition, we calculated the real-time water vapor cross-section by measuring the ambient temperature and relative humidity to retrieve the water vapor concentration with high accuracy. And we proposed an I0-database method to eliminate the effects of particle extinction and variations in environmental meteorological conditions. Finally, we will present the instrumental performance in the laboratory tests and field applications.
How to cite: Wang, Y., Wang, H., Chen, X., and Fan, S.: The development of Open path cavity-enhanced absorption spectroscopy for detecting ambient nitrate radicals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5762, https://doi.org/10.5194/egusphere-egu25-5762, 2025.
Nitryl chloride (ClNO₂), an important precursor of chlorine radicals (Cl•), significantly enhances atmospheric oxidative capacity (AOC) during early morning hours. Previous studies have shown that ClNO₂ exhibits distinct vertical characteristics, with concentrations typically higher in coastal areas, raising concerns about chlorine-induced pollution. This study investigates the vertical formation of ClNO₂ in the Pearl River Delta (PRD), focusing on the contribution of sea spray aerosols (SSA). Using field observations and WRF-CMAQ model simulations, we assess the impact of SSA on nocturnal heterogeneous reactions driving ClNO₂ formation. The observations show that ClNO₂ mixing ratios are significantly higher in the upper boundary layer (200 m) compared to surface measurements, with peak mixing ratios occurring in the early morning. Air mass trajectory analysis shows that the marine air masses are primarily responsible for elevated ClNO₂ levels aloft. The maximum contribution of SSA to ClNO₂ yield is found to be more than 97% of the total yield. Process analysis identifies the upper boundary layer as the critical region for ClNO₂ formation, with SSA playing a dominant role. Moreover, SSA not only enhances ClNO₂ production but also increases the mixing ratios of chlorine and hydroxyl radicals at higher altitudes the following day (400 m), significantly boosting AOC, with an increase in AOC of up to 10%. These findings highlight the pivotal role of SSA in modulating vertical ClNO₂ formation and its broader impacts on regional air quality, particularly in coastal areas.
How to cite: Zhu, Y. and Liu, Y.: Significant contributions of sea spray aerosol to vertical ClNO2 formation over coastal South China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7933, https://doi.org/10.5194/egusphere-egu25-7933, 2025.
A full suite of radical measurements (OH, HO2, RO2, and kOH) was established in Yangze River Delta (YRD) region to accurately elucidate the limitations of oxidation processes in the chemical-complex atmosphere. The diurnal peaks of radicals exhibited considerable variations in 3.6 to 27.1×106 cm-3 for OH, 2.1 to 33.2×108 cm-3 for HO2, and 4.9 to 30.5×108 cm-3 for RO2. The simulated results provided by the RACM2-LIM1 mechanism failed to adequately match the observed data both in radical concentration and experimental budget at a heavy ozone pollution episode. Sensitivity tests utilizing a comprehensive set of radical measurements revealed that the reactive aldehyde chemistry effectively complements the regeneration of OH radicals with 4.4% - 6.0% compared to the base scenario, while the concentrations of HO2 and RO2 radicals have shown increments of about 7.4% and 12.5%, respectively. The incorporation of larger alkoxy radicals stemming from monoterpenes has refined the consistency between measurements and modeling in the context of ozone production under elevated NO levels, diminishing the disparity from 4.17 to 2.33. Moving forward, by implementing a comprehensive radical detection approach, further investigations should concentrate on a broader range of OVOCs to rectify the imbalance associated with RO2 radicals, thereby providing a more precise understanding of oxidation processes during severe ozone pollution episodes.
How to cite: Zhang, G., Hu, R., Xie, P., Hu, C., and Liu, W.: Accurate Elucidation of Oxidation Under Heavy Ozone Pollution: A Full Suite of Radical Measurement In the Chemical-complex Atmosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10283, https://doi.org/10.5194/egusphere-egu25-10283, 2025.
Air quality is an important issue to human health and the climate. Poor air quality has been proven to be a risk factor in a wide range of cardiovascular and respiratory diseases, and it contributes to 6.9 million premature deaths worldwide [1]. Air quality also has complex links to climate change, with primary and secondary pollutants affecting the radiative forcing on Earth [2]. Therefore, accurate measurements of air pollutants, alongside understanding of their emissions and atmospheric sinks, is crucial information for informing policy on air quality.
Volatile organic compounds (VOCs), emitted from both anthropogenic and biogenic sources, impact air quality, as they are involved in processes that produce secondary pollutants like ozone and secondary organic aerosol. There is an estimated >100,000 VOCs found in ambient air [3], and they cannot all be measured using traditional direct techniques such as gas chromatography and mass spectrometry. As an alternative to measuring the total quantities of each individual VOC, techniques used to measure OH reactivity have been developed over the last 25 years. OH reactivity (kOH), the inverse of the chemical lifetime of OH, provides a quantitative measure of the total reactive pollutant loading in an air mass. This information can be used to determine the extent to which measured OH sinks contribute to the OH loss rate and can be used to determine the total impact of VOCs. The measured OH reactivity can be compared to modelled OH reactivity to assess the completeness of models used to assess and predict air quality. Current instruments designed for measurements of OH reactivity are precise and accurate but are often technically challenging and expensive, which limits current OH reactivity measurements to short intensive field campaigns.
We have recently developed an instrument designed to make continuous long-term measurements of OH reactivity based on laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy. We will present measurements of OH reactivity made in Leeds, UK, using the UV absorption instrument, as well as results obtained using an instrument based on laser-induced fluorescence (LIF) spectroscopy [4] during an intercomparison exercise. The instruments both sampled ambient air from an urban site at the University of Leeds during February and March 2024. Results from the intercomparison will be presented, which indicated that OH reactivity varied between 0.5 and 45.5 s-1, with a mean reactivity of 9.7 s-1. It was found that the agreement between instruments was good and that the new instrument can successfully measure OH reactivity over a range of environmental conditions.
The instrument has been successfully deployed in the Birmingham Air Quality Supersite at the University of Birmingham, UK since November 2024. Initial measurements from this long-term campaign will be presented, alongside detailed chemical modelling using the Master Chemical Mechanism.
References:
[1] World Health Organisation, https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health [Accessed 20/07/2024] (2022).
[2] Intergovernmental Panel on Climate Change, Camb. Uni. Press, 923–1054 (2023).
[3] A. H. Goldstein & I. E. Galbally, Env. Sci. & Tech. 41(5), 1514-1521 (2007)
[4] D. Stone et al., Atmos. Meas. Tech, 9 2827–2844 (2016).
How to cite: Luke, T., George, M., Vallipparambil Babu, A., Hou, S., Wynn, T., Bloss, W., Whalley, L., Heard, D., and Stone, D.: Long-term field measurements of OH reactivity using laser flash photolysis coupled with time-resolved broadband UV absorption spectroscopy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10586, https://doi.org/10.5194/egusphere-egu25-10586, 2025.
Inlet-pre-injectors (IPIs) are now increasingly used by the HOx measurement community (e.g. Mao et al., 2012; Novelli et al., 2014; Woodward-Massey et al., 2020; Cho et al., 2021) when measuring ambient OH by laser-induced fluorescence (LIF) to provide an interference-free measurement.
Traditionally, OH is detected by LIF in a low-pressure detection cell by tuning the laser wavelength on and off an OH transition to distinguish the OH fluorescence signal from the instrument background signal which is comprised of laser and solar scatter and any detector dark counts. However, this method of OH detection, which is often known as OHWAVE, does not allow the signal contribution from any OH generated within the detection cell to be differentiated from the ambient OH signal. The injection of propane (C3H8) or perfluoropropene (C3F6) scavenger via an IPI before the OH sampling nozzle leads to rapid removal of ambient OH and provides a measure of the instrument background signal whilst tuned to the OH transition. By this method, which is often known as OHCHEM, any signal from OH generated internally within the detection cell contributes to the background signal and so is distinguished from the ambient OH signal.
The concentration of OH scavenger injected via the IPI before sampling by the LIF instrument must be high enough to rapidly remove ambient OH, but not too high such that there is also removal of any internally-generated OH. A point-source of OH, generated by a Hg lamp in a humidified zero air flow, can be used to optimise the concentration of scavenger required. Here we will show, however, that higher concentrations of scavenger are often required to fully remove ambient OH as the scavenger must out-compete the reactions occurring under ambient conditions (e.g. HO2+NO) that are continually producing OH within the IPI.
Taking examples from previous OH measurement campaigns in two contrasting environments (forested and urban), the efficiency of an IPI to remove ambient OH is investigated using a detailed chemistry box-model based on the Master Chemical Mechanism (MCMv3.3.1) and constrained to measurements made during the campaigns. Taking typical residence times between scavenger injection and sampling by the low-pressure LIF detection cell, and varying the concentration of scavenger added, we show that under certain scenarios, >25 % of ambient OH could remain and may be erroneously considered as an OH interference.
Cho et al., Atmospheric Measurement Techniques, 14, 1851 – 1877, 2021
Mao et al., Atmospheric Chemistry and Physics, 12, 8009 – 8020, 2012
Novelli et al., Atmospheric Measurement Techniques, 7, 3413 – 3430, 2014
Woodward-Massey et al., Atmospheric Measurement Techniques, 13, 3119 – 3146, 2020
How to cite: Whalley, L. and Heard, D.: A modelling study investigating the efficiency of inlet-pre-injectors in removing ambient OH under different atmospheric conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10751, https://doi.org/10.5194/egusphere-egu25-10751, 2025.
Nitrate radicals (NO3) in the nocturnal boundary layer are key oxidants that influence nighttime atmospheric oxidation capacity and the nitrogen cycle. However, their low concentrations, short lifetimes, and complex nighttime chemistry pose challenges for large-scale spatiotemporal observations. In this study, we couple a machine learning model with the CMAQ model and historical observation data to predict NO3 over the long term. This approach combines an exact physical-chemical framework with observational data support to better capture the spatial and temporal characteristics of NO3. Our results show that: (1) CMAQ accurately simulates NO2 and O3 concentrations and also performs well for N2O5, indicating that the nighttime NO3 reaction framework in CMAQ is correct. (2) Combined with the CMAQ result, the stacking model improves the R of NO3 predictions by an average of 0.17 compared with single models, and its SHAP results align with current atmospheric chemistry. (3) After predicting NO3 levels and comparing summer and winter conditions in Shanghai and Beijing, our results reveal a notable decrease in NO3 in Shanghai during summer, likely due to declining nighttime O3. However, reduced heterogeneous hydrolysis during Shanghai’s winter nights may lead to a slight rise in NO3 concentrations. Additionally, NO3 in Beijing do not show a strong decrease and even increases slightly.
How to cite: Fu, W., Qin, M., and Hu, J.: Prediction of NO3 Radical Concentrations from 2015 to 2020 in Beijing and Shanghai Based on Air Quality Models and Machine Learning Methods, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12750, https://doi.org/10.5194/egusphere-egu25-12750, 2025.
Chlorine species play a crucial role as precursors to Cl radicals, which can significantly impact the atmospheric oxidation capacity and influence the levels of trace gases related to climate and air quality. We developed The Anthropogenic Chlorine Emission Inventory for China (ACEIC), which was the first chlorine emission inventory for China based on local data, and explored the impact of chlorine species emissions on secondary pollutants in 2019 in China using the CMAQ model. Considering chlorine emissions, the concentration of chlorine radicals (Cl·) in China increased by about 1000 molecules/cm3 on average, with a maximum increase of more than 6000 molecules/cm3 in major cities. Cl2 and HOCl emissions were the most important contributors to the increase of Cl·, with both Cl2 and HOCl emissions originating mainly from the residential sector. Regarding monthly variation, the increase in Cl· was most significant in summer due to intensified human activities. Regarding daily variation, the increase in Cl· peaked around 9 am and decreased to zero at night. Process analyses showed that the main reactions affecting the change in Cl· were the photolysis reactions of Cl2 and HOCl and the consumption reactions of Cl· and VOCs. As an important precursor of Cl·, the concentration of Nitryl chloride (ClNO2) in China increased by about 50 ppt on average, with a maximum increase of more than 150 ppt in major cities. HCl and fine particulate Cl- emissions were the most important contributors to the increase of ClNO2. The increase in ClNO2 was most significant in winter, peaked around 6 am and decreased to zero at daytime. Process analysis identified the upper boundary layer as the critical region for ClNO2 formation. Chlorine emissions caused some increase in O3 concentration. Maximum Daily 8-hour Average O3 (MDA8 O3) concentration increased by about 1 ppb, with O3 increasing much higher in winter than in summer. In the daily variation, the ozone increase was most significant at 12 am, with a maximum increase of more than 1.5 ppb. These findings highlight the significant contribution of chlorine emissions to secondary pollutants and can aid in the formulation of emission control strategies to mitigate secondary pollution in China.
How to cite: Shen, A., Liu, Y., and Fan, Q.: Contributions of Anthropogenic Chlorine Emissions to secondary pollutions in China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14854, https://doi.org/10.5194/egusphere-egu25-14854, 2025.
Isoprene, the most abundantly volatile organic compound (VOC), plays a significant role in atmospheric chemistry, particularly in the upper troposphere. In the tropics, it is emitted in large quantities by rainforests. Driven by prevailing high solar radiation, temperature, and humidity, Isoprene rich air is transported from the boundary layer to the upper troposphere by deep convective systems. Without nighttime photo-oxidation, isoprene can accumulate in this region, where it reacts with hydroxyl radicals during the day, contributing to aerosol formation (Shen et al, 2024, Curtius et al., 2024). This study explores the oxidation processes of isoprene at low temperatures (-50°C), typical of the upper troposphere, with a focus on the effects of varying NOx concentrations (low and high NOx) on these mechanisms. Experiments were conducted in the CLOUD chamber at CERN, simulating these atmospheric processes under controlled conditions.
While previous research has largely focused on isoprene oxidation at relatively high near-surface temperatures, the chemistry at low temperatures, particularly radical recycling, has not been sufficiently studied. Our study's cold-temperature measurements are particularly relevant for understanding upper tropospheric processes. We aim to elucidate the oxidation mechanisms of isoprene by analyzing radical production, concentration, and recycling under various chemical conditions at low temperatures. The findings will enhance our understanding of atmospheric chemistry in the upper troposphere and improve the accuracy of climate and air quality models.
References:
Shen, J., et al. New particle formation from isoprene under upper-tropospheric conditions. Nature 636, 115–123 (2024).
Curtius, J., et al. Isoprene nitrates drive new particle formation in Amazon’s upper troposphere. Nature 636, 124–130 (2024).
How to cite: Kunkler, F., Holzbeck, P., Russell, D., Shen, J., Mentler, B., Hansel, A., Kirkby, J., He, X.-C., Curtius, J., Lelieveld, J., and Harder, H.: Investigating Isoprene Oxidation in the Upper Troposphere: Insights from Cold-Temperature Chamber Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17088, https://doi.org/10.5194/egusphere-egu25-17088, 2025.
Understanding the OH and peroxy radical chemistry in different environments is essential to predict atmospheric lifetimes and chemical transformations of compounds emitted to the atmosphere of both biogenic and anthropogenic origin. The extent to which insight into radical chemistry can be gained by comparing simulated and measured radical concentrations has been found to depend on the environment. In particular, significant discrepancies between modeled and measured OH and peroxy radical concentrations have been observed in forested regions characterized by relatively high VOCs and low NO concentrations. The objective of this study was to assess the importance of different radical production and loss processes as well as the role of OH in the production of sulfuric acid, a major precursor of newly formed atmospheric particles, in a sub-urban temperate forest.
Measurements were performed as part of the ACROSS project (Atmospheric ChemistRy Of the Suburban foreSt) during June-July of 2022 at a forested site in Rambouillet located along the path of pollution plumes from Paris. OH radicals were measured in a forest clearing at ground-level (about 6 m). Co-located measurements of HO2+RO2 (i.e. the sum of hydroperoxy and organic peroxy radicals) and gas-phase H2SO4 were also made on top of a 40 m tower (~20 m above the forest canopy). A budget analysis was performed using steady-state calculations for OH and H2SO4 using other available measurements on the ground and on the tower (photolysis rates, NOx, O3, VOCs, OH reactivity, aerosol particle size distribution, etc.). A detailed budget analysis for OH and peroxy radicals was performed with a box-model using a MCM derived mechanism.
Calculated daytime and nighttime OH concentrations on the ground, using measured OH reactivity, showed good correlation with the measurements and reproduced the observed daytime maximum and nighttime levels of about 4×106 molecule cm-3 and (2-6)×105 molecule cm-3, respectively. The production of OH radicals in the clearing and above the canopy during the day was found to be dominated by its regeneration in reactions of HO2 and RO2 with nitric oxide. During the night, the ozonolysis of monoterpenes was a significant OH production pathway with its contribution depending on the nighttime NO concentrations. The box-model resulted in significant underestimation, up to a factor of two in daytime OH and an overestimation of OH reactivity. At the same time, the model sum of peroxy radicals was larger than measurements, especially during the night with lowest observed NO concentration. However, most of the time the model reproduced the observed peroxy radical temporal behavior on the ground and above the canopy, as well as their slightly lower concentrations over the canopy during the day.
The formation of H2SO4 was observed every day during the measurement period, with the median maximum H2SO4 concentration of 2.5×106 molecule cm-3 similar to that observed at some other forested sites. The calculated and measured daytime H2SO4 concentrations were highly correlated with formation of sulfuric acid via SO2+OH reaction accounting for (90±20)% of the observed H2SO4.
How to cite: Jiao, Y. and Kukui, A. and the ACROSS Rambouillet Measurements Team: Chemical budgets of OH, HO2+RO2 and H2SO4 in a sub-urban temperate forest near Paris, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17736, https://doi.org/10.5194/egusphere-egu25-17736, 2025.
Urban ozone (O3) pollution correlates with temperature, and higher O3 often occurs during heat waves, threatening public health. However, limited data on how anthropogenic volatile organic compound (AVOC) precursor emissions vary with temperature hinders understanding their impact on O3. Here, we show that the increase in non-combustion AVOC emissions (e.g., from volatile chemical products) during a heat wave in Shanghai contributes significantly to increased O3, based on ambient measurements, emission testing, and air quality modelling. AVOC concentrations increase ~2 when the temperature increases from 25 °C to 35 °C due to air stagnation and increased emissions. During the heat wave, higher concentrations result in an 82% increase in VOC OH reactivity. Air quality simulations reveal that temperature-driven AVOC emission increases account for 8% (1.6 s-1) of this reactivity increase and enhance O3 by 4.6 ppb. Moreover, we predict a more profound (2 ) increase in OH reactivity of oxygenated VOCs, facilitating radical production and O3 formation. Enhanced AVOC emissions trigger O3 enhancements in large cities in East China during the heat wave, and similar effects may also happen in other AVOC-sensitive megacities globally. Reducing AVOC emissions, particularly non-combustion sources, which are currently less understood and regulated, could mitigate potential O3 pollution in urban environments during heat waves.
How to cite: Qin, M.: Increased Urban Ozone in Heat Waves due to Temperature-Induced Emissions of Anthropogenic Volatile Organic Compounds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18072, https://doi.org/10.5194/egusphere-egu25-18072, 2025.
Nitrous acid (HONO) is a crucial precursor to hydroxyl radical (OH) in the atmosphere and significantly influences atmospheric photochemical processes. Despite extensive field observations, the quantitative understanding of HONO formation processes in varying environments remains elusive, particularly due to the divergent parameters used in different studies that have led to contradictory conclusions. This study measured HONO at four sites characterized by contrasting environments: an urban site in the Yellow River Delta, a mountain site in the North China Plain, and two coastal sites at varying distances from the sea in Qingdao. The HONO concentration at the urban site is the highest, with peaks occurring at night, consistent with previous observations in other urban areas, yet markedly different from the daytime HONO peaks observed at other three clean sites. Using an observation-based model incorporated with unified parametrizations, we identify the dominant HONO formation pathways in these sites. Results show that the model effectively reproduces HONO concentrations and diurnal variations at both urban and mountainous sites. In urban areas, HONO formation is primarily driven by the heterogeneous conversion of NO2 and the photolysis of particulate nitrate, while in the polluted mountainous region, the photolysis of nitrate plays a more significant role, with vertical transport potentially contributing to morning HONO increases. However, the model could not replicate daytime HONO peaks at the two coastal sites, suggesting the presence of an unidentified or underestimated marine HONO source. At all four sites, HONO contributes significantly to atmospheric OH radicals and ozone production, and the missing marine source may affect evaluations of its influence in coastal regions. In summary, this study provides a comprehensive analysis of HONO sources in contrasting environments, underscoring the need for further observations, especially in clean marine/coastal atmospheres.
How to cite: zhong, X., shen, H., and xue, L.: Atmospheric Nitrous Acid (HONO) in Contrasting Environments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11702, https://doi.org/10.5194/egusphere-egu25-11702, 2025.
The concentrations of HO2, a critical radical in the atmosphere, are often overestimated in atmospheric models (1). These discrepancies have sometimes been attributed to the heterogeneous uptake to atmospheric aerosols.
However, the correct treatment of heterogeneous chemistry in models is a significant source of uncertainty, partly due to the complexity of atmospheric aerosols and the need for laboratory experiments to formulate a robust parameterisation of HO2 uptake. There is a significant lack of experimental data for the uptake coefficient γ(HO2) of HO2 onto secondary organic aerosols (SOAs), even though they represent a high proportion of atmospheric aerosols and a significant fraction of particulate matter below 2.5 μm (PM2.5).
We report the first γ(HO2) measurements onto SOAs over a range of relative humidities (30 - 85 %). Atmospherically relevant SOA has been produced in a Potential Aerosol Mass Chamber (PAM) from the oxidation, by OH and ozone, of the volatile organic compounds α-pinene, ∆-limonene, 1,3,5 – trimethyl benzene (TMB) and toluene. An aerosol flow tube coupled to a Fluorescence Assay Gas Expansion (FAGE) detection cell, which utilises laser-induced fluorescence (LIF) spectroscopy, is used to measure the uptake of gas-phase HO2 onto the aerosols, with a limit of detection of γ(HO2) = 0.003.
Results show that the aerosol liquid water (ALW) content plays an important role in heterogeneous reactions by enhancing HO2 uptake onto aerosols. The measured γ(HO2) was low (γ ≤ 0.004) for TMB and toluene SOA and undetectable for both α-pinene and ∆-limonene, and no correlation was observed between RH and γ(HO2). The aerosol size distribution of the SOA remained constant over the range of relative humidities, suggesting the RH had little effect on the ALW of purely organic aerosols. Whereas, when ammonium sulfate seed aerosols are added to enhance SOA formation, the measured γ(HO2) for toluene-derived SOA increases from 0.006 to 0.03 with an increase in RH from 38 – 84 %. The increase in the geometric mean of the toluene-derived SOA at higher RH suggests that the ALW increases. Thus, the presence of seed particles results in a more significant γ(HO2), which increases with RH and potentially impacts the atmospheric abundance of HOx.
1. Dyson, Joanna E., et al. "Impact of HO2 aerosol uptake on radical levels and O3 production during summertime in Beijing." Atmospheric Chemistry and Physics Discussions(2022): 1-43.
How to cite: McConnell, A., Stone, D., and Heard, D.: Investigating HO2 uptake onto the surface of secondary organic , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12714, https://doi.org/10.5194/egusphere-egu25-12714, 2025.
Volcanic gas emissions influence the composition of the atmosphere and therefore also our climate. For some gas species, volcanoes represent even the most important natural source. In atmospheric research, however, volcanoes are often neglected as an important source of many gas species and are still little studied, partly also because of the challenges in terms of the necessary technology and logistics.
In volcanic gas mixtures near the source, very high OH mixing ratios (ppb-ppm) are often assumed, usually based on thermodynamic equilibrium calculations (e.g. Gerlach, 2004). However, no OH measurements have been successfully performed in such environments.
Here we report on CO/CO2 measurements in the downwind volcanic gas plume of Mount Etna, which were obtained by taking air core samples with an UAV and analysed immediately afterwards with Cavity Ring-Down Spectroscopy in July 2024 (T. Schuck et al., 2025). We relate these results to previous near-source CO/CO2 emission measurements and to calculated CO/CO2 emission ratios from petrological studies. The observed change of more than two orders of magnitude in the CO/CO2 ratio can only be explained by the oxidation of CO and therefore allows us to estimate the amount of OH necessary to explain the high proportion of CO oxidation. Our estimate based on kinetic chemistry, happening after the first seconds of the gas release, leads indeed to results of unusual high amounts of OH in the source region.
Gerlach, T. M. (2004). Volcanic sources of tropospheric ozone‐depleting trace gases. Geochemistry, Geophysics, Geosystems, 5(9).
Schuck, T., Degen J., Bobrowski, N., Metais-Bossard M., Boucher L., Chen, H., Geil, B.H., Giuffrida, G.B. van Heuven, S., Hoffmann, T., Ortenzi G., and Engel A. (2025). First deployment of a drone-borne active AirCore in a volcanic plume at Mount Etna, submitted to EGU
How to cite: Bobrowski, N., Ortenzi, G., Boucher, L., Degen, J., Engel, A., Geil, B., Giuffrida, G., Metais-Bossard, M., Schuck, T., and Hoffmann, T.: Volcanic vents – OH mixing ratios as in a Bunsen burner flame?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16213, https://doi.org/10.5194/egusphere-egu25-16213, 2025.
Inverse modeling can provide valuable insights into the sources of atmospheric tracers based on observations and a set of boundary conditions. However, biases in these boundary conditions can lead to biases in the optimized emissions. In this study, we present a series of global inversion experiments of carbon monoxide (CO) emissions using the TM5-4dvar inverse modeling suit constrained by satellite data from the TROPOspheric Monitoring Instrument (TROPOMI) and surface flask measurements from NOAA. These experiments are designed to systematically assess the impact of different boundary conditions, with a particular focus on hydroxyl radical (OH) distributions, a key determinant of both the sources and sinks of CO.
Methyl chloroform (MCF) measurements are commonly used to constrain global atmospheric OH climatological fields. We find that our OH fields that are modeled with global atmospheric chemistry calculations are biased high. Despite this bias, these OH fields often provide more realistic lateral distributions than climatological OH fields, particularly in the tropical boundary layer. Another critical boundary condition for inverse modeling of CO is its secondary production from Volatile Organic Compounds (VOCs) and methane. Due to the challenges of directly measuring secondary CO production, model-based estimates are used instead.
Our results show that combining modeled secondary CO production estimates with modeled OH fields leads to a closed budget, reducing aliasing across emission categories and enhancing confidence in the optimized anthropogenic and biomass burning emissions. Although the individual budget terms of both the secondary production and the chemical loss of CO may be overestimated, their combined effect yields realistic steady-state CO mixing ratios, as validated by TROPOMI CO observations. This study emphasizes the critical need for improved OH fields to accurately estimate CO emissions, and advances the understanding of potential biases in future inversions.
How to cite: Nüß, J. R., Daskalakis, N., Gkouvousis, A., Kanakidou, M., Krol, M. C., and Vrekoussis, M.: Exploring the influence of OH fields and secondary CO production on CO emission estimates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9998, https://doi.org/10.5194/egusphere-egu25-9998, 2025.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot 5
EGU25-21194 | Posters virtual | VPS3
Photooxidation of Oxygenated Volatile Organic Compounds as a Major Source of Hydroperoxyl Radicals Driving Ozone Formation in the Yangtze River Delta RegionWed, 30 Apr, 14:00–15:45 (CEST) | vP5.42
Oxygenated volatile organic compounds (OVOCs) significantly contribute to the radical formation in the troposphere, enhancing atmospheric oxidation capacity and driving secondary pollutant production. However, uncertainties in OVOC emissions hinder accurate assessments of their regional impacts. This study updates OVOC emission profiles for the Yangtze River Delta (YRD) region and integrates them into the Community Multiscale Air Quality (CMAQ) model to refine OVOC estimations. The updated model effectively captures the diurnal variations of most OVOCs, significantly reducing biases compared to simulations based on previous inventories. OVOCs, particularly formaldehyde (HCHO), are key precursors of hydroperoxyl radicals (HO2), which play a dominant role in ozone production across the YRD. Anthropogenic emissions, primarily from industrial activities and vehicular sources, account for 40−60% of total OVOCs. Sensitivity simulations reveal that reducing emissions of reactive OVOCs, such as HCHO and glyoxal, effectively lowers regional ozone levels. These findings underscore the pivotal role of OVOCs in radical chemistry and ozone formation, providing insights for mitigating ozone pollution in rapidly urbanizing regions like the YRD.
How to cite: Li, J.: Photooxidation of Oxygenated Volatile Organic Compounds as a Major Source of Hydroperoxyl Radicals Driving Ozone Formation in the Yangtze River Delta Region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21194, https://doi.org/10.5194/egusphere-egu25-21194, 2025.
EGU25-14804 | ECS | Posters virtual | VPS3
Investigation of the Cyclohexene Oxidation Mechanism Through the Direct Measurement of Organic Peroxy RadicalWed, 30 Apr, 14:00–15:45 (CEST) | vP5.43
Monoterpenes, the second most abundant biogenic volatile organic compounds globally, are crucial in forming secondary organic aerosols, making their oxidation mechanisms vital for addressing climate change and air pollution. This study utilized cyclohexene as a surrogate to explore first-generation products from its ozonolysis through laboratory experiments and mechanistic modeling. We employed proton transfer reaction mass spectrometry with NH4+ ion sources (NH4+-CIMS) and a custom-built OH calibration source to quantify organic peroxy radicals (RO2) and closed-shell species. Under near-real atmospheric conditions in a Potential Aerosol Mass-Oxidation Flow Reactor, we identified 30 ozonolysis products, expanding previous data sets of low-oxygen compounds. Combined with simulations based on the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere and relevant literature, our results revealed that OH dominates over ozone in cyclohexene oxidation at typical atmospheric oxidant levels with H-abstraction contributing 30% of initial RO2 radicals. Highly oxidized molecules primarily arise from RO2 autoxidation initiated by ozone, and at least 15% of ozone oxidation products follow the overlooked nonvinyl hydroperoxides pathway. Gaps remain especially in understanding RO2 cross-reactions, and the structural complexity of monoterpenes further complicates research. As emissions decrease and afforestation increases, understanding these mechanisms becomes increasingly critical.
How to cite: Li, Y., Ma, X., Lu, K., and Zhang, Y.: Investigation of the Cyclohexene Oxidation Mechanism Through the Direct Measurement of Organic Peroxy Radical, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14804, https://doi.org/10.5194/egusphere-egu25-14804, 2025.
