To investigate the effect of oxidation on the formation of secondary pollution in different environmental conditions, a comprehensive field campaign (STORM) was carried out from October to November 2018 at Peking University Shenzhen Graduate School (DXC, 22.60 °N, 113.97 °E) in Shenzhen, Guangdong Province. Precursor concentrations introduced typical urban characteristics on the modeled OH reactivity (k_OH) levels with a broad range between 15.0 and 30.0 s^-1. The daily maxima of the observed OH and HO₂ radical concentrations were (2.3–12.8) × 10⁶ cm^-3 and (1.3–9.1) × 10⁸ cm^-3, respectively. In the polluted period, abundant photolysis sources (e.g., HONO, HCHO, and O₃) intensified photochemistry. The OH radical concentration peaked 6.0 × 10⁶ cm^-3 at noon (11:00-13:00). The low k_OH (12.0 s^-1) in the clean atmosphere suggested that the reduction in termination efficiency prolonged the OH lifetime, so that the period maintained a sustained OH concentration comparable to the polluted period. In terms of meteorology, the dominating air mass was isolated from the east and northeast directions, which promoted the transition of ozone from mild pollution to severe pollution. The abundant precursor emissions at urban sites first compensated for the negative effect of declined solar radiation, and then they amplified radical propagation. Simultaneously amplified radical propagation promoted oxidation capacity, and increased the chain length (ChL) from 3.6 to 4.1, and P(O₃) has lifted effectively by approximately 13.3 %. The stable wind direction and velocity reduced physical dilution losses and thus led to the rapid rise in O_x around 16:00. Further detailed investigations are required on the environmental causes of ozone pollution to address the influence of the oxidation processes on secondary pollution formation under other environmental conditions.

How to cite: Zhang, G., Guo, J., Hu, R., Lin, C., and Xie, P.: Local radical chemistry driven ozone pollution in a megacity: A case study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14073, https://doi.org/10.5194/egusphere-egu24-14073, 2024.

Correspondence: Min Qin (mqin@aiofm.ac.cn)

Nitrous acid (HONO) and nitrogen oxides (NO_x) play a crucial role in tropospheric photochemistry by contributing to the hydroxyl radical (OH) and influencing atmospheric oxidization capacity. Recent research has found that soil HONO emissions are considered to be the main source of atmospheric HONO. Here, an aerodynamic gradient (AG) method combined with a BroadBand Cavity Enhanced Absorption Spectrometer (BBCEAS) system was developed to measure HONO and NOx emission flux from agricultural fields in the Huaihe River Basin. Measurements were taken at two different heights and included various agricultural management activities such as rotary tillage, flood irrigation, fertilization, transplanting rice seedlings, and top-dressing. For HONO and NO, upward fluxes were observed (0.07 ± 0.22 and 0.19 ± 0.53 nmol /(m^2·s)), while NO₂ was deposited to the ground (-0.37 ± 0.47 nmol /(m^2·s)). The maximum emission fluxes of HONO and NO occurred at around 24°C, which is close to the optimal temperature (25°C) for soil microbial nitrification processes. This indicates that surface microbial processes may contribute to gas emissions. Specifically, during rotary tillage, continuous peaks in HONO flux and NO flux were observed. P_OH(HONO)_net and P_OH(O₃)_net were 1.42 ppb/h and 1.35 ppb/h, respectively, with HONO and O₃ photolysis accounted for 51% and 49% of the total yield. The peak of P_OH(HONO)_net coincides with the peak of HONO emissions from agricultural fields, revealing the significant contribution of agricultural HONO emissions to atmospheric oxidizing capacity. After irrigation in agricultural fields, the increase in soil moisture content (~80% water filled pore space) restricts oxygen availability, thereby suppressing the HONO emission. Overall, this study provides valuable insights into the dynamics of soil HONO and NO_x emissions in agricultural fields, shedding light on their environmental implications and the role of agricultural activities in atmospheric chemistry.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (U21A2028) and the National key Reearch and Development Program of China (2022YFC3701100).

How to cite: Han, B., Qin, M., Meng, F., Fang, W., Xie, J., Shao, D., Liao, Z., and Xie, P.: Surface exchange flux measurement of HONO and NOx in agricultural fields of the Huaihe River Basin, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15664, https://doi.org/10.5194/egusphere-egu24-15664, 2024.

The oxidation pathways of dimethyl sulfide (DMS), the largest natural source of sulfur in theatmosphere (Bates et al., 1992), and its impact on Earth’s radiative balance are not fully understood yet. One of the major open questions in the DMS oxidation scheme is the chemical kinetics of hydroperoxymethyl thioformate (HPMTF) formation and breakdown, which also leads to substantial HPMTF abundances in the global atmosphere as detetcted by Veres et al. in 2020 by means of aircraft observations. HPMTF is formed from methylthiomethyl peroxy radical (MTMP), one of the major oxidation products of DMS, and was first suggested by Wu et al. (2015) based on ab initio calculations. Meanwhile, several atmospheric simulation chamber studies have taken place to better understand DMS oxidation (e.g., Jernigan et al. (2022), Ye et al. (2022), Shen et al. (2022) and, von Hobe et al. (2023)).

In autumn 2023 a DMS oxidation campaign took place at the SAPHIR* chamber at the Forschungszentrum Jülich to address these issues. SAPHIR* (SAPHIR-STAR; Simulation of Atmospheric PHotochemistry In a large Reaction Chamber – STirred Atmospheric tank Reactor) is a continuously stirred tank reactor made of glass with a total volume of 2000 l. All experiments in SAPHIR* are carried out under steady state conditions. The chamber is inside a thermostat, features two independent UV light systems at 254nm (UV-C) and 365nm (UV-A), a state of the art control and gas supply system. Measurements are performed under extremely controlled conditions regarding temperature, relative humidity (RH), flows and composition/constituents. SAPHIR* features a set of standard measurements (O₃, NO/NO_x, CO, CO₂, CH₄). During the campaign three Chemical Ionisation Mass Spectrometers (CIMS) were used during the campaign: a CIMS-API-LTOF with a nitrate and bromide ion source, which was also used for frequent voltage scanning during the steady states, a CIMS-HTOF with iodide ion source and, FunMass-C, a rebuilt aircraft CTOF instrument employing CF₃O^- ion chemistry. Every of the four reagent ions is capable to detect HPMTF. Moreover, a proton-transfer-reaction mass spectrometer (PTR-MS) and a gas chromatography mass spectrometer (GC-MS) measured DMS. For FunMass-C, this was the first time measuring under non-laboratory conditions over weeks.

As precursor of OH, H₂O₂ photolysis was used. This allowed for HPMTF production even at mostly dry conditions (RH < 1%) as well as the investigation of the influence of additional water vapour (RH = 20, 40 and 60 %) under atmospherically relevant OH levels (~1*10⁷ molecules*cm^-3). In addition, the dependence of several NO levels (5, 10, 20 and 30 ppbv NO) on HPMTF formation under dry conditions was investigated. Lastly, the HPMTF formation caused by nitrate radicals in the dark (100 ppb O₃ and 25 ppb NO) was investigated, as well as the influence of water vapour on this reaction pathway (RH = 20 %). For all experiments the initial DMS mixing ratio in the chamber was 10 or 20 ppb. The presentation gives an overview of these experiments and discusses preliminary results.

How to cite: Alber, S., Steffes, W., Stroh, F., Wang, H., Wu, R., Zanders, A., and Zorn, S. R.: DMS oxidation experiments in the SAPHIR* simulation chamber: Constraining chemical kinetics of the HPMTF channel, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16059, https://doi.org/10.5194/egusphere-egu24-16059, 2024.

Accurate knowledge of the rate coefficient of the reaction of volatile organic compounds (VOCs) with the hydroxyl radical (OH) is fundamental for understanding atmospheric chemical processes and for predictions of air quality forecasting models. Nevertheless, the temperature dependence of reaction rates of many organic compounds is not well investigated as measurements are challenging.

In this work, we present absolute measurements of reactions rates with the OH radical for a series of VOCs for different temperatures. An OH reactivity instrument previously used for field and chamber measurements was further developed to enable these accurate measurements of reaction rates. The OH reactivity (kOH) is the inverse lifetime of the OH radical expressed by the product of the OH reactant concentrations and its reaction coefficient with OH. In this instrument, OH reactivity is measured by the direct observation of the decay rate of OH that is produced in a flow tube by ozone laser flash photolysis at 266 nm. The decay of OH radicals is measured at a high time resolution of 1 ms. If a known concentration of an organic compound is added to the bath gas (humidified synthetic air) the decay time directly gives the rate coefficient. As the air temperature can be controlled, the Arrhenius expression of the rate coefficient can be determined.

VOC mixtures in synthetic air were prepared in canisters. The concentration was determined by the total organic carbon method, in which all carbon atoms are combusted to CO₂ on heated catalysts and the CO₂ concentration is measured by a commercial cavity ring-down instrument.

The method was validated with several alkanes for which reaction rates are well-known. Excellent agreement within a few percent with recommendations by IUPAC and NASA-JPL demonstrates the high accuracy of the new method. The Arrhenius expression of OH reaction rates was determined for several monoterpenes and aromatic hydrocarbons.

How to cite: Novelli, A., Berg, F., Dubus, R., Wahner, A., and Fuchs, H.: Determination of temperature dependent rate coefficients of the reaction of hydroxyl radicals with volatile organic compounds using a new OH reactivity instrument , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16085, https://doi.org/10.5194/egusphere-egu24-16085, 2024.

Biomass burning (BB), including wildfires, agricultural burning and domestic heating results in the emission of complex mixture of particle and gas phase species, among which volatile organic compounds and organic aerosols (primary organic aerosol – POA) are prevailing, in the atmosphere. These emissions and their chemical aging products, including both gas-phase pollutants and secondary organic aerosols (SOA), under different atmospheric and combustion conditions, remain poorly understood and have significant but still uncertain impacts on air quality, human health and climate.

The experimental campaign in the EUPHORE outdoor photoreactors focused on studying the gas and particle phase emissions and aging processes from the combustion of different types of wood (orange tree, vineyard and beech), as well as a diesel engine car exhaust, under diurnal and nocturnal operating regimes. It consisted of an ambitious campaign that involved a wide range of analytical instrumentation, highlighting state-of-the-art mass spectrometers (PTR-ToF-MS and API-ToF-CIMS coupled to a Figaero inlet) for both gas and particle phase analysis and a newly released 9λ Aethalometer model AE36s (Aerosol Magee Scientific) for improved characterization of aerosol optical properties. This work focuses on the analysis of the particulate phase emissions and their aged SOA products using the instrument API-ToF-Iodide-CIMS+Figaero inlet.

From the analysis of the chemical composition of the particulate phase in this extensive set of experiments, significant information, similarities and differences are derived, both between different types of wood fuels and between distinct combustion regimes (smoldering vs. flaming) and aging conditions (daytime vs. night-time). In the mass spectra (MS) analysis, a general emission of higher POA m/Q compounds is observed in the smoldering combustion compared to the flaming one. After several hours of aging, evolution and differentiation from the initial fume mass spectra is observed, with a gradual transition to MS with a more significant presence of higher molecular weight compounds. Diverse compound types have been identified, including phenolic, furanic, organonitrate products, non-oxygenated and oxygenated PAHs and other BB tracers, such as levoglucosan.

Combined dark and light aging experiments reveal changing trends in the formation and evolution of numerous compounds due to light conditions shift. Aerosol night-time aging periods lead to the formation and accumulation of organonitrate BB derived products, such as nitroanisole, nitroguaiacol and nitrocatechol, leading to its degradation after the chamber opening and the exposure to natural light. This is an ongoing work currently under investigation with great potential to provide with new valuable information that can enhance our understanding of the complex aging pathways of biomass-burning pollutants and to improve model results.

This work is part of a project supported by the European Commission under the Horizon 2020 – Research and Innovation Framework Programme through the ATMO-ACCESS Integrating Activity (grant agreement N. 101008004) and by the R+D project ATMOBE (PID2022-142366OB-I00), funded by MCIN/AEI/10.13039/501100011033/, the "ERDF A way of making Europe”, the Valencian Regional Government (GVA) and the EVER project CIPROM/20200/37.

How to cite: Soler, R., Ródenas, M., Vera, T., Borrás, E., Gómez, T., Chen, M., Rigler, M., Gregorič, A., Alföldy, B., Yubero, E., Crespo, J., and Muñoz, A.: Analysis of biomass burning derived aerosol under different combustion and oxidation regimes at the EUPHORE chambers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18998, https://doi.org/10.5194/egusphere-egu24-18998, 2024.

Despite the global phase-out of leaded gasoline, lead (Pb) pollution remains a persistent and serious issue, particularly in developing nations. To portray the temporal dynamics of Pb emissions and their responses to evolving policy interventions in Asia, we reconstructed a 60-year record of Pb concentration (Pb/Ca) and isotopic composition (i.e., ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb) in a coral from Dongsha Atoll, the South China Sea. Our study reveals a fourfold increase in coral Pb/Ca levels from 1953 to mid-2000s, followed by a ~ 60% decline in the subsequent decade. In the early era from 1953 to 1960, Pb isotopic compositions in the Dongsha coral align closely with the values in natural marine sediments. When anthropogenic Pb sources became to prevail, we observe an increase in Pb/Ca values and altered isotopic compositions, attributable mostly to automotive Pb emissions but also with significant contribution from industrial Pb emissions especially after 1990s. With Pb isotopic analysis on the coral, we identify that, after 1990s, the Chinese Pb was the primary endmember that impacts the regional Pb. The influence of Chinese Pb is also registered in other East and Southeast Asian coral records, probably also in those from the Indian Ocean. The increasing trend of Pb in these records underscores the necessity to reduce Pb emissions in the post-leaded gasoline era, particularly in developing countries in the region. On the other hand, the decline in Pb/Ca in our record after the mid-2000s provides encouraging evidence of the positive impact of environmental policies in safeguarding the environment and public health.

How to cite: Lin, K., Chen, M., Chen, Y.-G., and Wang, X.: Rise and fall of lead (Pb) pollution in East Asia:  lessons learnt from a 60-yr long modern coral, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19488, https://doi.org/10.5194/egusphere-egu24-19488, 2024.

We present the measurement of the temperature dependent rate coefficient of the OH reaction with methacrolein between 280 and 340 K using an OH reactivity instrument that allows to accurately determine the loss rate of OH reactants. Experiments were performed in synthetic air (presence of oxygen). In the case of methacrolein, OH radicals are regenerated by the 1,4 H-shift reaction of the aldehyde group of the MACR-1-OH-O₂ radical which is the main peroxy radicals formed its OH reaction. Therefore, the observed OH decay deviates from a single-exponential decay expected from a pseudo-first order loss reaction. This allows to determine the rate coefficient of the peroxy radical isomerization reaction using model calculations, in which reaction rates are optimized to best describe the observed OH radical decay. This method is one of few direct measurements of atmospherically relevant peroxy radical isomerization reaction coefficients that have been reported in literature so far. Experimentally derived values are compared to quantum-chemical calculations of the 1,4 H-shift reaction rate and to rate coefficients reported in literature for the isomerization reaction and the OH reactions of methacrolein.

How to cite: Fuchs, H., Berg, F., Dubus, R., Vereecken, L., Wahner, A., and Novelli, A.: Temperature dependent rate coefficients of the OH reaction with methacrolein and of the 1,4 H-shift reaction of the organic peroxy radical from methacrolein, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1747, https://doi.org/10.5194/egusphere-egu24-1747, 2024.

A profound understanding of the correlation between ozone and particulate matter is crucial for the collaborative prevention and control of these pollutants. However, existing research predominantly focuses on near-surface analysis, with limited exploration into the vertical variations of the ozone-particulate matter correlation. Leveraging two years of observation data from the Canton Tower Atmospheric Pollution Vertical Gradient Observation Platform, this study investigates the correlation between ozone and particulate matter at different heights in the near-surface layer of Guangzhou urban area. The results indicate that: ①The correlation between ozone and particulate matter is more pronounced in summer and autumn at different heights but diminishes in winter; ②At lower levels, the correlation between ozone and particulate matter is weaker, ascending with altitude, and peaks at higher levels; ③Regardless of the height, the correlation between ozone and particulate matter is positively influenced by smaller particle sizes. In summary, within the near-surface layer below 500 meters in the urban area of Guangzhou, especially under meteorological conditions conducive to ozone generation, a significant positive correlation between ozone and particulate matter is evident. As altitude increases and particle size decreases, the correlation coefficient significantly rises, particularly in high-level ultrafine-sized particles, suggesting shared chemical mechanisms in the formation processes of ozone and particulate matter. This study contributes novel observational evidence supporting the cooperative control principle of ozone and particulate matter.

How to cite: Zhang, J. and Wang, X.: Correlation between Ozone and Particulate Matter in the Near-surface Layer of Guangzhou Urban Area, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12263, https://doi.org/10.5194/egusphere-egu24-12263, 2024.

Chengdu Plain Urban Agglomeration (CPUA) is one of the most serious areas of ozone pollution in China. A comprehensive field experiment focused on the ozone episode characteristics, and temporal and spatial variations of ozone production rate was conducted at CPUA in the summer of 2019. Six sampling sites were set and two ozone pollution episodes were recognized. The daily maximum 8-h average (MDA8) O₃ concentration reached 137.9 ppbv in the urban sites during the ozone episode. The high concentration of O₃ was closely related to intense solar radiation, high temperatures, and precursor emissions. Based on the calculation of OBM, the OH-HO₂-RO₂ radical chemistry and ozone production rate (P(O₃)) was analyzed. The OH daily maximum is in the range of 3-13×10⁶ molecules cm⁻³, and HO₂ and RO₂ are in the range of 2–14×10⁸ molecules cm⁻³ during ozone episodes, varying by the location of sites. During ozone episode, the average maximum of P(O₃) in suburb sites (about 30 ppb h^-1.) were compared with urban sites, while the maximum of P(O₃) was 18 ppb h^-1 in rural sites. The relative incremental reactivity (RIR) and empirical kinetic modeling approach (EKMA) results demonstrate that centered on the urban area of Chengdu, where it was a VOC-limited regime, the northern and southern suburban area was transition region. In the remote rural area of the southern CPUA, it was highly NOx-limited. Local ozone production driven by the photochemical process is important for CPUA. The geographically differentiated recognition of the ozone regime found by this study can help to tailor control strategies for local conditions and avoid the negative effects of a one-size-fits-all approach.

How to cite: Zhou, M., Liu, Y., and Lu, K.: Ozone production sensitivity analysis for the Chengdu Plain Urban Agglomeration based on a muti-site and two-episode observation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2984, https://doi.org/10.5194/egusphere-egu24-2984, 2024.

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultra-violet (UV) absorption cross-section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. We performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometer and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultra-pure standard HONO source were utilized for laboratory intercomparisons. Results shows a consistent ~22% negative bias in measurements of the IBBCEAS compared with SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation in the absolute UV absorption cross-sections through careful analysis of possible uncertainties. We quantified the absorption cross-section of gaseous HONO (360-390nm) utilizing a custom-built IBBCEAS instrument, and the results was found to be 22%-34% lower than the previously published absorption cross-sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross-sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross-section of HONO and the re-evaluation of the previously reported HONO budgets.

How to cite: Li, X., Dong, H., Lu, K., and Zhang, Y.: Revisiting the Ultra-Violet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3007, https://doi.org/10.5194/egusphere-egu24-3007, 2024.

Measurement of OH and HO₂ radicals was conducted at a coastal site in the Pearl River Delta in October 2019. The mixing of air masses of continental and marine origins can lead to more variability in radical concentrations. In the ocean-atmosphere period (OCM), the observed OH and HO₂ radicals could be reflected by the RACM2-LIM1 chemical mechanism. However, the heterogeneous uptake process has a certain effect on the HOx radical chemistry, but the influence of the halogen mechanism is limited due to the NOx level. Land mass (LAM) was associated with a higher net ozone generation rate (5.52 ppb/h), and the daily maximum OH and HO₂ concentrations were 7.1 × 10⁶ cm⁻³ and 5.2 × 10⁸ cm⁻³, respectively. Rapid oxidation process was accompanied by a higher diurnal nitrous acid (HONO) concentration (> 400 ppt). The particularity of HONO chemistry increases the ozone generation rate of the coastal atmosphere by ~40%. Without HONO constraint, simulated ozone concentration drops from ~75 ppb to a global background (~35 ppb). Therefore, the promotion of oxidation by elevated precursors deserves a lot of attention when aiding pollution mitigation policies.

How to cite: Hu, R., Zhang, G., Xie, P., and Liu, W.: Intensive photochemical oxidation in the marine atmosphere: Evidence from direct radical measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5080, https://doi.org/10.5194/egusphere-egu24-5080, 2024.

OH radicals play a key role for the self-cleaning and oxidation capacity of the atmosphere. Therefore, the detection of OH radicals in water is of great interest for atmospheric research. Terephthalate is a commonly used reagent to detect OH radicals in the aqueous system, forming the fluorescent product 2-hydroxytherephthalate. This reaction is reported to be a selective and sensitive method that can detect OH radicals in the nanomolar range. By irradiating a solution of disodium terephthalate in ultrapure water with different wavelengths between 250 nm and 310 nm the influence of photochemical excitation on the formation of 2-hydroxyterephthalate was studied using fluorescence spectroscopy. First results indicate that in the aqueous system the fluorescent 2-hydroxyterephthalate is not only formed by OH radicals but also through a photochemical reaction upon direct excitation of terephthalate. This photochemically driven hydroxylation may lead to an overestimation of the actual OH radical concentration, which could have a significant effect on the measurement outcome, particularly when measuring at the edge of the detection limit. Therefore, it can be concluded that the excitation wavelength should be carefully considered when using terephthalate as a detection reagent for OH radicals in aqueous solutions.

How to cite: Werner, L., Naumann, R., and Theis, A.: Detection of OH radicals in aqueous solution using terephthalate: Overestimation of the OH radical concentration due to photochemical hydroxylation of terephthalate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13055, https://doi.org/10.5194/egusphere-egu24-13055, 2024.

The heterogeneous uptake reaction of dinitrogen pentoxide (N₂O₅) has a great effect on generation of soluble nitrate, nocturnal atmospheric chemistry and regulating NO_x. Organic matter is one of the important components of atmospheric particles, and has been proved that can significantly affect the N₂O₅ heterogeneous uptake coefficients (γ(N₂O₅)). In this study, an aerosol flow tube system was used for γ(N₂O₅) measurements on ammonium sulphate aerosols ((NH₄)₂SO₄, AS) and three kinds of core-shell structured secondary organic aerosols (SOA), and explored the effects of relative humidity (RH), phase state and organic coating thickness on γ(N₂O₅). Three kinds of SOA with core-shell structures were generated successfully by seed particle AS, organic gas (α-pinene, isoprene and toluene) and O₃, and characterized by a scanning mobility particle sizer (SMPS) and TEM. The SOA yields, i.e., organic coating thicknesses, of the three organics were ranked, given the same organic gas concentration was α-pinene (17.1±2.2, 4.8 ppm) ＞ isoprene (4.7±0.04, 4.8 ppm) ＞ toluene (1.3±0.7, 4.8 ppm). The experimental results showed that the γ(N₂O₅) on AS were in the range of 0.002~0.017 (RH=1~50%), whereas that of SOA were between 3.7×10^-5~2.6×10^-3 (RH=30~50%), and the order of γ(N₂O₅) at the same RH was isoprene SOA＞ toluene SOA＞ α-pinene SOA. γ(N₂O₅) of the SOA was reduced by 1-3 orders of magnitude when compared with that of AS, which suggests that organic coating significantly inhibits the diffusion of N₂O₅ on the surface and in the bulk phase of particles. However, the γ(N₂O₅) of these three SOA were not completely correlated with the organic coating thickness. The organic coating thickness of two BVOCs showed a nonlinear negative correlation with γ(N₂O₅) (R²＞0.9), while toluene SOA with the thinnest coating thickness greatly inhibited γ(N₂O₅) due to its greater influence on particle hygroscopicity. Therefore, due to the large number of organic species, it is not possible to generalize the degree of inhibition of γ(N₂O₅) only by coating thickness. It is necessary to further consider the influence of properties and structures of organics in the future, and widely applicable parameters for evaluating the effect of organic effect need to be proposed, so as to improve the accuracy of γ(N₂O₅) estimation.

How to cite: Li, J., Xie, S., Xu, H., and Lu, K.: N2O5 heterogeneous uptake on secondary organic aerosol: the effect of organic coating thickness, relative humidity and phase state, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14056, https://doi.org/10.5194/egusphere-egu24-14056, 2024.

In the tropics, intense solar radiation drives photochemistry and strong convection, transporting air from the boundary layer to the upper troposphere. Conditions in the tropics are characterized by high humidity and UV intensity enhancing hydroxyl (OH) radical production. In addition, OH radicals and ozone (O₃) are formed through reactions of HO_x (OH + HO₂) with nitrogen oxides (NO_x), the latter being produced by lightning in abundant convective systems. The convection also transports volatile organic compounds (VOCs), notably from emissions by the tropical rainforest. The VOCs are oxidized by radicals and O₃, resulting in secondary species contributing to new particle formation. To understand and characterize the atmospheric chemistry in these conditions, the Chemistry of the Atmosphere Field Experiment (CAFE) Brazil was conducted from December 2022 to January 2023 with the High Altitude and Long Range Research Aircraft (HALO) in the Amazon region.

In this study, we present preliminary results measured with the Hydroxyl Radical measurement Unit based on fluorescence Spectroscopy (HORUS), focusing on vertical HO_x profiles measured during different times of the day over both the continent and the ocean, including the outflow of both electrified and non-electrified convective systems. In contrast to the conditions over the continents where lightning-generated NO_x aids in the efficient recycling of radicals, over the ocean, the limited availability of NO hinders recycling and results in radical termination. The conditions over the continent are compared to those measured over the Atlantic Ocean during the CAFE Africa expedition in summer 2018 based in the Cape-Verde islands. This unique dataset provides valuable insights into the atmospheric chemistry and oxidation capacity in these tropical regions.

How to cite: Holzbeck, P., Rohloff, R., Sreekumar, S., Monteiro, C., Tsokankunku, A., Marno, D., Martinez, M., Nussbaumer, C., Dienhart, D., Tripathi, N., Wang, N., Edtbauer, A., Bohn, B., Obersteiner, F., Williams, J., Fischer, H., Curtius, J., Pöhlker, M., Lelieveld, J., and Harder, H.: Hydroxyl Radicals and Oxidation Capacity in the Tropical Troposphere: Measurements from CAFE Field Campaigns using HORUS, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14641, https://doi.org/10.5194/egusphere-egu24-14641, 2024.

Hydroxyl radical (OH) is recognized to be one of the most reactive oxidants in the troposphere and plays a critical role in driving the photochemistry of the troposphere. The reactions with OH initiate the oxidation of most trace gases in the troposphere, e.g. NO_x and volatile organic compounds (VOCs), which leads to the formation of secondary oxidation products such as ozone (O₃) and particular matter (e.g. PM_2.5), thus changing the atmospheric composition and affecting the climate. In this study, an observation-based model (OBM) for evaluating the OH concentrations in Guangdong province, China has been developed. We employed the OBM to derive the OH radicals in seven representative districts/cities in Guangdong province from 2015 to 2020. The average OH concentrations in the seven cities gradually increased from an average of (3.3±0.8)×10⁶ cm^–3 at 08:00 to (6.7±0.7)×10⁶ cm^–3 at 13:00 local time. The OH concentrations derived by the OBM were found in reasonable agreement with previous observations as well as modeling studies in Guangdong. In addition, the average OH concentrations derived by the OBM method in the seven cities in Guangdong Province were about five times higher than those in clean background regions such as the Amazonas, Brazil, and pre-industrial Guangdong province. This is a highly significant point in terms of the impacts of the changing OH on the atmospheric capacity and chemical composition, which can have detrimental effects on human health, ecosystem and the climate.

How to cite: Wei, L.: An observation-based analysis of hydroxyl radicals in Guangdong, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5208, https://doi.org/10.5194/egusphere-egu24-5208, 2024.

Organic peroxy radicals (RO2), derived from volatile organic compounds (VOCs) oxidation by hydroxyl radical (OH), ozone (O3), and nitrate radical (NO3) etc., play an important role in atmospheric chemistry. Direct measurements of speciated RO2 are challenging but necessary for understanding their atmospheric fate and impact. In this study, we introduce a newly developed Proton Transfer Reaction measurement method (PTR3-TOF) using NH4+ as the ion source to measure the multifunctional complex RO2 and related reactive oxidation products based on the principle of chemical ionization mass spectrometry (CIMS). We establish a calibration system that combines a home-built OH calibration source with various standard VOC gases, enabling to quantify both RO2 radicals and closed-shell species with a detection limit of about 1.6×107 molecule cm-3 (2σ, 60 s) and an uncertainty of 10%. This technique is first applied to study the gas-phase ozonolysis of cyclohexene in our laboratory, with experiments carried out using the O3 OFR mode in a PAM-OFR system under near-real atmosphere conditions. As a consequence, a total of 30 cyclohexene ozonolysis reaction products are detected in the first step of cyclohexene ozonolysis, including 9 types of RO2 radicals and 21 types of closed-shell species, with oxygen atoms ranging from 1 to 8. We also establish a mechanistic model for the first-generation products of ozonolysis of cyclohexene, and the simulation and measurement results agree within a factor of 2-3 for the respective formulas except for some differences in a few species.

How to cite: Li, Y., Ma, X., Lu, K., and Zhang, Y.: Detection of RO2 radicals and other products from oxidation of VOCs in PAM chamber with NH4+ ionization mass spectrometry , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4899, https://doi.org/10.5194/egusphere-egu24-4899, 2024.