This study presents results from an Intensive Measurement Period (IMP2022) conducted during the European heatwave of July 2022, focusing on ozone, volatile organic compounds (VOCs), and carbonaceous aerosols at 31 sites across Europe. The episode featured persistent high-pressure systems, record-breaking temperatures, widespread ozone exceedances and concurrent atmospheric new particle formation and growth events. Coordinated measurements and chemistry transport modelling were used to examine the spatial variability of ozone, VOC composition, and secondary organic aerosol (SOA) formation under extreme meteorological conditions. Oxygenated VOCs (O-VOCs) constituted the largest fraction of total measured VOC mixing ratios, followed by non-methane hydrocarbons (NMHCs) and aromatics, with contributions from both anthropogenic and biogenic sources. Sensitivity simulations indicate that ozone formation was predominantly NOx-limited across most regions during IMP2022. However, the highest ozone peaks occurred under conditions of elevated NOx in combination with enhanced BVOC emissions. In contrast, SOA formation was slightly enhanced under low-NOx conditions and reduced in elevated NOx. Isoprene, aliphatic NMHCs, and O-VOCs dominated the ozone formation potential, while aromatics and monoterpenes were major contributors to SOA potential. Model simulations indicated that higher NOx concentrations can reduce SOA formation by about 10 %. The campaign also highlighted observational gaps underscoring the need for broader and higher-resolution VOC monitoring across Europe. Overall, further reductions in NOx emissions, alongside targeted control of key anthropogenic VOCs, would benefit air quality under future climate extremes.

Port environments are characterized by complex and highly variable emissions from shipping, industrial and traffic activities, which results in distinct spatial and temporal heterogeneity in air pollutant concentrations. Fuel cells used in port activities, whether stationary or mobile, can be adversely affected by elevated pollutant concentrations. Therefore, we investigate the concentrations of volatile organic compounds (VOCs), trace gases, and ultrafine particles in the Port of Rotterdam as part of the KaLiBer joint project. MOBILAB serves as the mobile measuring platform equipped with state-of-the-art instrumentation for particle and gas-phase analysis, enabling both mobile transects and stationary monitoring. VOCs were measured using proton-transfer-reaction mass spectrometry (PTR-MS), capturing a wide range of oxygenated and non-oxygenated species, alongside simultaneous observations of CO, CO₂, NOₓ, SO₂, NH₃, and ultrafine particle (UFP) number concentrations. The mobile measurements were conducted over 10 days along selected routes in the port area and subsequent 30 days of stationary monitoring during summer. PTR-MS measurements covered approximately 130 masses, allowing a detailed characterization of both oxygenated and non-oxygenated VOCs.The mobile measurements capture distinct spatial gradients and short-lived concentration spikes linked to local emission plumes. In contrast, the stationary data set reveals long-term variability and background conditions with additional plumes from ships passing by. Average concentrations and prominent diurnal patterns suggest distinct emission sources from shipping, heavy-duty traffic, and industrial activities. Spatial analysis reveals elevated VOC concentrations accompanied by high UFP and CO2 levels, along port roadways. This suggests a dominant contribution from traffic-related sources. In the urban background, mixing ratios decreased due to chemical transformation and atmospheric dilution. The aromatic-to-oxygenated VOC (OVOC) ratios reveal distinct differences between fresh emissions along traffic-affected routes and more chemically aged air masses at the stationary site. This emphasizes the dynamic interplay between primary emissions and atmospheric processing. Although toluene/benzene ratios were comparable during mobile and stationary periods, they reflect mixed contributions from traffic, shipping, and industrial sources.The combined mobile–stationary dataset demonstrates the significance of high-time-resolution measurements for capturing emission variability, chemical processing, and source contributions in complex port environments. Such insights are vital for quantifying transport-sector contributions to urban air pollution. They are essential for assessing potential health impacts, and effective emission mitigation and air quality management. Beyond atmospheric characterization, the observed concentration ranges, variability, and occurrence of short-lived pollutant peaks are important for designing and optimizing air filtration systems that protect fuel cell technology. In particular, understanding the temporal occurrence of VOCs and acidic gases is essential for minimizing catalyst poisoning, membrane degradation, and performance losses. This, in turn, improves fuel cell durability and operational lifetime under real-world port conditions.This work is funded by the Federal Ministry for Economic Affairs and Energy based on a resolution of the German Bundestag under funding code 03EN5043D.

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

Oxidation of volatile organic compounds (VOCs) involving hydroxyl radicals (OH center dot) and nitrogen oxides (NOx), or nitrate radicals (NO3 center dot) forms organic nitrates that undergo gas-particle partitioning, changing the lifetime of nitrogen and their deposition on ecosystems. In urban areas, VOC composition is complex, with contributions from traffic, cooking, volatile chemical products (VCPs), and biogenic emissions. Secondary organic aerosol (SOA) formation from urban VOC mixtures was investigated using chamber experiments during the SAPHIR-CHANEL campaign under realistic VOC-NOx and oxidation conditions. The yield of total organic nitrates is higher for precursor mixtures with a higher percentage of unsaturated VOCs, such as those from traffic and cooking sources (11 %-21 %), compared to VCPs and complex urban emission replicas (2 %-7 %). Enhanced particle-phase partitioning is observed under nighttime oxidation (by NO3 center dot) versus daytime oxidation (by OH center dot). Particulate organic nitrates have a higher average molecular weight under nighttime conditions (330 +/- 80 gmol(-1)) than under daytime conditions (250 +/- 30 gmol(-1)) mainly due to a higher dimer fraction. Similarly, the mass fraction of the total organic aerosol that is organic nitrate is 2.6-4.5 times higher under nighttime than daytime conditions, likely due to higher molecular weight and lower temperatures. Although gas-phase organic nitrate composition varies substantially between precursor mixtures, bulk organic nitrate partitioning is generally similar to that of modeled oxidized monoterpene nitrates (10-4-10-2 m(3)mu g(-1) at 18-40 degrees C). These findings improve understanding of bulk organic nitrate sources and properties in complex urban environments, allowing better simulations of air quality and nitrate deposition.

Europe is one of the most studied areas related to biogenic volatile organic compound (BVOC) emissions. However, our knowledge of these atmospheric reactive compounds is still quite limited even there. Total hydroxyl radical (OH) reactivity studies indicate that half of the atmospheric reactive compounds are still unknown especially in the forested areas (Yang et al. 2016) and OH and ozone reactivity studies of our group have shown high fractions of reactivity from biogenic emissions (Praplan et al. 2020 and Thomas et al. 2023).Globally, isoprene is the primary emitted BVOC. While boreal forests in Northern Europe are mainly considered as monoterpene emitters, Central Europe is expected to be dominated by isoprene (e.g. Messina et al. 2016). However, our results from a campaign at 17 stations over Europe in summer 2022 indicated that BVOC mixing ratios are highly variable and some areas also in Central Europe may be dominated by monoterpenes.Sesquiterpenes and diterpenes have very high potential for secondary organic aerosol formation, but much less is known on their emissions and atmospheric concentrations. Our studies show that birches and spruces may be strong sesquiterpene emitters. We have also found that some urban trees in Montreal and wetlands in Lapland known as isoprene emitters may also release significant amounts of sesquiterpenes. Additionally, forest floor represents a potential source of sesquiterpenes.Compared to terrestrial sources very little is known on the marine emissions of BVOCs. There are studies on dimethyl sulphide, but our recent results on an island in Baltic Sea suggest that other sulphuric compounds, like methanethiol, may be important too and could have strong impacts on SO2 production and therefore also on new particle and cloud formation. Furthermore, our recent campaign at the coast of Baltic Sea indicates that phytoplankton and macrophytes could be a source of isoprene and monoterpenes (Thakur et al., 2024 publication under prep).Compounds classified as BVOCs (e.g. monoterpenes) can also be emitted from anthropogenic sources, such as construction sites (e.g. from wooden material), as well as cleaning and personal care products. Our studies in a street canyon in Helsinki in 2022 indicates that they strongly impact local atmospheric chemistry even in wintertime. Messina, P., Lathière, J., Sindelarova, K., Vuichard, N., Granier, C., Ghattas, J., Cozic, A., and Hauglustaine, D. A.: Global biogenic volatile organic compound emissions in the ORCHIDEE and MEGAN models and sensitivity to key parameters, Atmos. Chem. Phys., 16, 14169–14202, https://doi.org/10.5194/acp-16-14169-2016, 2016Praplan, A. P., Tykkä, T., Schallhart, S., Tarvainen, V., Bäck, J., and Hellén, H.: OH reactivity from the emissions of different tree species: investigating the missing reactivity in a boreal forest, Biogeosciences, 17, 4681–4705, https://doi.org/10.5194/bg-17-4681-2020, 2020.Thomas, S. J., Tykkä, T., Hellén, H., Bianchi, F., and Praplan, A. P.: Undetected biogenic volatile organic compounds from Norway spruce drive total ozone reactivity measurements, Atmos. Chem. Phys., 23, 14627–14642, https://doi.org/10.5194/acp-23-14627-2023, 2023.Yang, Y., Shao, M., Wang, X., Nölscher, A. C., Kessel, S., Guenther, A., and Williams, J.: Towards a quantitative understanding of total OH reactivity: A review, Atmos. Environ., 134, 147–161, https://doi.org/10.1016/j.atmosenv.2016.03.010, 2016.

On 8 and 9 July 2018 extensive observations were conducted under fair-weather conditions in the German city of Stuttgart and its surroundings. This intensive observation period, part of the four weeks Urban Climate Under Change (UC)2 campaign, intended to provide a comprehensive data set to understand the complex interactions of thermally induced wind systems, vertical turbulent mixing and air pollutant concentration distribution in the atmospheric boundary layer of the city. Stuttgart has a very special and complex topography with a city center located in a basin surrounded by hills with heights of 250 to 300 m influencing the wind and flow system, reducing the wind speed, causing inhibited dispersion of air pollutants. Cold air flows from the surrounding plains can penetrate into the urban areas and influence the urban climate including the air quality. For investigating these effects with a focus on urban climate, combinations of different measurement platforms and techniques were used, such as in situ stationary and mobile measurements with cars, vertical profiling by means of tethered balloons, radiosondes, a drone, and aircraft observations, remote sensing devices and satellite-based instruments. Numerous atmospheric processes in an urban area regarding boundary layer evolution, inversion, local wind systems, urban heat island, etc. were observed. Some important findings are: Temperature observations provide local information about the warmest areas in the city and about the city and its surroundings. The urban heat island effect was evident from the results of stationary and mobile air temperature measurements as higher air temperature was measured in Stuttgart basin compared to its surroundings. Considerable spatio-temporal differences concerning the wind (speed and direction), turbulence and the convective boundary depth are evident. Lower wind speeds were observed during the nighttime and the main wind direction in the Stuttgart valley was measured to be southwest, which carried cold air from the hillsides into the city and pollutants to the windward side of the city into the Neckar valley. The low wind speed favored the accumulation of pollutants in a shallow nocturnal boundary layer close to the surface. During the day, the overall pollutant concentration was reduced by vertical convective mixing. The vertical profile measurements have shown that the applied techniques provided a good overview to understand the vertical characteristics of meteorological parameters and pollutants as well as the stability of the atmosphere and extent of the urban boundary layer. It also showed that the extent of atmospheric mixing determines the dispersion, dilution and mixing of emitted pollutants.Finally, the additional comprehensive air-chemical observations (surface and satellite based) allow understanding the diurnal cycle of air pollutants in the atmospheric boundary layer of the city of Stuttgart. Satellite-based observations from Sentinel-5P/TROPOMI have shown their potential for mapping urban pollution islands and urban pollution plumes even in cities with a complex terrain like Stuttgart. These observations assisted to obtain a comprehensive data set intended for the validation of a novel urban climate model, PALM-4U.

The oxidation of limonene by the hydroxyl (OH) radical and ozone (O3) was investigated in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) in experiments performed at different nitric oxide (NO) mixing ratios from nearly 0 up to 10 ppbv. For the experiments dominated by OH oxidation, the formaldehyde (HCHO) yield was experimentally determined and found to be (12 ± 3), (13 ± 3), and (32 ± 5) % for experiments with low (∼ 0.1 ppbv), medium (∼ 0.3 ppbv), and high NO (5 to 10 ppbv), respectively. The yield in an ozonolysis-only experiment was (10 ± 1) %, which agrees with previous laboratory studies. The experimental yield of the first-generation organic nitrates from limonene–OH oxidation is calculated as (34 ± 5) %, about 11 % higher than the value in the Master Chemical Mechanism (MCM), which is derived from structure–activity relationships (SARs). Time series of measured radicals, trace-gas concentrations, and OH reactivity are compared to results from zero-dimensional chemical box model calculations applying MCM v3.3.1. Modeled OH reactivity is 5 to 10 s−1 (25 % to 33 % of the OH reactivity at the start of the experiment) higher than measured values at the end of the experiments under all chemical conditions investigated, suggesting either that there are unaccounted loss processes of limonene oxidation products or that products are less reactive toward OH. In addition, model calculations underestimate measured hydroperoxyl radical (HO2) concentrations by 20 % to 90 % and overestimate organic peroxyl radical (RO2) concentrations by 50 % to 300 %. The largest deviations are found in low-NO experiments and in the ozonolysis experiment. An OH radical budget analysis, which uses only measured quantities, shows that the budget is closed in most of the experiments. A similar budget analysis for RO2 radicals suggests that an additional RO2 loss rate constant of about (1–6) × 10−2 s−1 for first-generation RO2 is required to match the measured RO2 concentrations in all experiments. Sensitivity model runs indicate that additional reactions converting RO2 to HO2 at a rate constant of about (1.7–3.0) × 10−2 s−1 would improve the model–measurement agreement of NOx, HO2, and RO2 concentrations and OH reactivity. Reaction pathways that could lead to the production of additional OH and HO2 are discussed, which include isomerization reactions of RO2 from the oxidation of limonene, different branching ratios for the reaction of RO2 with HO2, and a faster rate constant for RO2 recombination reactions. As the exact chemical mechanisms of the additional HO2 and OH sources could not be identified, further work needs to focus on quantifying organic product species and organic peroxy radicals from limonene oxidation.

The last 2 decades have seen substantial technological advances in the development of low-cost air pollution instruments using small sensors. While their use continues to spread across the field of atmospheric chemistry, the air quality monitoring community, and for commercial and private use, challenges remain in ensuring data quality and comparability of calibration methods. This study introduces a seven-step methodology for the field calibration of low-cost sensor systems using reference instrumentation with user-friendly guidelines, open-access code, and a discussion of common barriers to such an approach. The methodology has been developed and is applicable for gas-phase pollutants, such as for the measurement of nitrogen dioxide (NO2) or ozone (O-3). A full example of the application of this methodology to a case study in an urban environment using both multiple linear regression (MLR) and the random forest (RF) machine-learning technique is presented with relevant R code provided, including error estimation. In this case, we have applied it to the calibration of metal oxide gas-phase sensors (MOSs). Results reiterate previous findings that MLR and RF are similarly accurate, though with differing limitations. The methodology presented here goes a step further than most studies by including explicit transparent steps for addressing model selection, validation, and tuning, as well as addressing the common issues of autocorrelation and multicollinearity. We also highlight the need for standardized reporting of methods for data cleaning and flagging, model selection and tuning, and model metrics. In the absence of a standardized methodology for the calibration of low-cost sensor systems, we suggest a number of best practices for future studies using low-cost sensor systems to ensure greater comparability of research.

The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s−1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.

Kurzfassungen der Meteorologentagung DACH DACH2022-213, 2022 https://doi.org/10.5194/dach2022-213 DACH2022 © Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License.

<p>Langzeitmessungen der atmosphärischen Zusammensetzung sind von zentraler Bedeutung, um die Atmosphärenchemie und den Klimawandel zu verstehen. ACTRIS (Aerosol, Cloud and Trace Gases Research Infrastructure) hat sich zum Ziel gesetzt, ein europaweites Netzwerk von Beobachtungsstationen aufzubauen, die qualitativ hochwertige Daten und Informationen zu kurzlebigen atmosphärischen Bestandteilen liefern und für Nutzer auf der ganzen Welt offen zugänglich machen. Stickstoffmonoxid (NO) und Stickstoffdioxid (NO<sub>2</sub>), die sogenannten Stickoxide (NO<sub>x</sub>), spielen eine Schlüsselrolle in der Atmosphärenchemie, da sie zur Bildung von troposphärischem Ozon, Smog und saurem Regen beitragen. Darüber hinaus ist die kurz- und langfristige Exposition mit NO<sub>2</sub> mit negativen Auswirkungen auf das menschliche Atmungssystem in Verbindung gebracht worden. Die Hauptquellen von NO<sub>x</sub> in bewohnten Gebieten sind Verbrennungsprozesse, z.B. von Fahrzeugen und bei industriellen Aktivitäten. NO<sub>x</sub>-Messungen werden derzeit meist indirekt über Chemilumineszenz-Instrumente durchgeführt, die Korrekturen für Feuchte und Ozon erfordern. Jüngste technologische Fortschritte (z. B. Cavity Attenuated Phase Shift, CAPS, oder Tunable Diode Laser Systeme) erlauben die direkte Detektion von NO<sub>x</sub>-Komponenten, was Interferenzen vermeidet, die durch die Umwandlung von NO<sub>2</sub> in NO hervorgerufen werden. Messvergleiche zeigen aber, dass auch hier neben bekannten Problemen wie Reaktionen in den Einlassleitungen auch unerwartete Artefakte beobachtet werden können. Messvergleiche aber zeigen auch hier, dass neben bekannten Problemen wie Reaktionen in den Einlassleitungen auch unerwartete auftreten können. Um genaue und präzise NO<sub>x</sub> Messungen mit einer Vielzahl von NO<sub>x</sub>-Messsystemen in verschiedenen Stationen sicherzustellen, müssen neben der Standardisierung von Messprotokollen und Kalibrierungsverfahren auch an zentraler Stelle durch Messvergleiche und Auditierungen Unterschiede der verschiedenen Messverfahren dokumentiert werden.</p> <p>ACTRIS setzt sich aus central facilities (CFs) und national facilities (NFs) zusammen. Die NFs bilden den explorativen und beobachtenden Teil der Forschungsinfrastruktur. Die CFs sind von grundlegender Bedeutung für die Bereitstellung von harmonisierten und hochpräzisen Daten und stellen eine Vielzahl von Dienstleistungen zur Verfügung. Eines der CFs ist das Reactive Trace Gases In Situ Measurements (CiGas), das für die Überwachung der Datenqualität reaktiver Spurengase verantwortlich ist. Für die Qualitätssicherung (QA) und Qualitätskontrolle (QC) der Stickoxidmessungen an den NFs innerhalb von CiGas ist das Forschungszentrum Jülich (FZJ) zuständig, das auch das World Calibration Center (WCC) für Stickoxide im Global Atmosphere Watch (GAW) Netzwerk beheimatet. Seine Aufgaben umfassen i) die Verbindung von Spurengasmessungen von ACTRIS mit denen anderer Netzwerke, ii) die Beratung und Organisation von Schulungen, iii) die Bereitstellung von Mess- und Auswerteverfahren, iv) das Labelling und die Auditierung von NFs, v) die Implementierung neuer wissenschaftlicher und technologischer Entwicklungen.</p> <p>Es ist vorgesehen, bis 2025 ein zertifiziertes und funktionsfähiges Netzwerk von ACTRIS-Stationen aufzubauen. Es soll der wissenschaftlichen Gemeinschaft qualitativ hochwertige Daten liefern, die die Grundlage für fundierte Entscheidungen der politischen Entscheidungsträger bilden können.</p>

Limonene is the fourth-most abundant monoterpene in the atmosphere, which upon oxidation leads to the formation of secondary organic aerosol (SOA) and thereby influences climate and air quality.In this study, the oxidation of limonene by OH at different atmospherically relevant NO and HO2 levels (NO: 0.1 – 10 ppb; HO2: 20 ppt) was investigated in simulation experiments in the SAPHIR chamber at Forschungszentrum Jülich. The analysis focuses on comparing measured radical concentrations (RO2, HO2, OH) and OH reactivity (kOH) with modeled values calculated using the Master Chemical Mechanism (MCM) version 3.3.1.At high and medium NO concentrations, RO2 is expected to quickly react with NO. An HO2 radical is produced during the process that can be converted back to an OH radical by another reaction with NO. Consistently, for experiments conducted at medium NO levels (~0.5 ppb, RO2 lifetime ~10 s), simulated RO2, HO2, and OH agree with observations within the measurement uncertainties, if the OH reactivity of oxidation products is correctly described.At lower NO concentrations, the regeneration of HO2 in the RO2 + NO reaction is slow and the reaction of RO2 with HO2 gains importance in forming peroxides. However, simulation results show a large discrepancy between calculated radical concentrations and measurements at low NO levels (<0.1 ppb, RO2 lifetime ~ 100 s). Simulated RO2 concentrations are found to be overestimated by a factor of three; simulated HO2 concentrations are underestimated by 50 %; simulated OH concentrations are underestimated by about 35%, even if kOH is correctly described. This suggests that there could be additional RO2 reaction pathways that regenerate HO2 and OH radicals become important, but they are not taken into account in the MCM model.

<p>Langzeitmessungen der atmosph&#228;rischen Zusammensetzung sind von zentraler Bedeutung, um die Atmosph&#228;renchemie und den Klimawandel zu verstehen. ACTRIS (Aerosol, Cloud and Trace Gases Research Infrastructure) hat sich zum Ziel gesetzt, ein europaweites Netzwerk von Beobachtungsstationen aufzubauen, die qualitativ hochwertige Daten und Informationen zu kurzlebigen atmosph&#228;rischen Bestandteilen liefern und f&#252;r Nutzer auf der ganzen Welt offen zug&#228;nglich machen. Stickstoffmonoxid (NO) und Stickstoffdioxid (NO₂), die sogenannten Stickoxide (NO_x), spielen eine Schl&#252;sselrolle in der Atmosph&#228;renchemie, da sie zur Bildung von troposph&#228;rischem Ozon, Smog und saurem Regen beitragen. Dar&#252;ber hinaus ist die kurz- und langfristige Exposition mit NO₂ mit negativen Auswirkungen auf das menschliche Atmungssystem in Verbindung gebracht worden. Die Hauptquellen von NO_x in bewohnten Gebieten sind Verbrennungsprozesse, z.B. von Fahrzeugen und bei industriellen Aktivit&#228;ten. NO_x-Messungen werden derzeit meist indirekt &#252;ber Chemilumineszenz-Instrumente durchgef&#252;hrt, die Korrekturen f&#252;r Feuchte und Ozon erfordern. J&#252;ngste technologische Fortschritte (z. B. Cavity Attenuated Phase Shift, CAPS, oder Tunable Diode Laser Systeme) erlauben die direkte Detektion von NO_x-Komponenten, was Interferenzen vermeidet, die durch die Umwandlung von NO₂ in NO hervorgerufen werden. Messvergleiche zeigen aber, dass auch hier neben bekannten Problemen wie Reaktionen in den Einlassleitungen auch unerwartete Artefakte beobachtet werden k&#246;nnen. Messvergleiche aber zeigen auch hier, dass neben bekannten Problemen wie Reaktionen in den Einlassleitungen auch unerwartete auftreten k&#246;nnen. Um genaue und pr&#228;zise NO_x Messungen mit einer Vielzahl von NO_x-Messsystemen in verschiedenen Stationen sicherzustellen, m&#252;ssen neben der Standardisierung von Messprotokollen und Kalibrierungsverfahren auch an zentraler Stelle durch Messvergleiche und Auditierungen Unterschiede der verschiedenen Messverfahren dokumentiert werden.</p> <p>ACTRIS setzt sich aus central facilities (CFs) und national facilities (NFs) zusammen. Die NFs bilden den explorativen und beobachtenden Teil der Forschungsinfrastruktur. Die CFs sind von grundlegender Bedeutung f&#252;r die Bereitstellung von harmonisierten und hochpr&#228;zisen Daten und stellen eine Vielzahl von Dienstleistungen zur Verf&#252;gung. Eines der CFs ist das Reactive Trace Gases In Situ Measurements (CiGas), das f&#252;r die &#220;berwachung der Datenqualit&#228;t reaktiver Spurengase verantwortlich ist. F&#252;r die Qualit&#228;tssicherung (QA) und Qualit&#228;tskontrolle (QC) der Stickoxidmessungen an den NFs innerhalb von CiGas ist das Forschungszentrum J&#252;lich (FZJ) zust&#228;ndig, das auch das World Calibration Center (WCC) f&#252;r Stickoxide im Global Atmosphere Watch (GAW) Netzwerk beheimatet. Seine Aufgaben umfassen i) die Verbindung von Spurengasmessungen von ACTRIS mit denen anderer Netzwerke, ii) die Beratung und Organisation von Schulungen, iii) die Bereitstellung von Mess- und Auswerteverfahren, iv) das Labelling und die Auditierung von NFs, v) die Implementierung neuer wissenschaftlicher und technologischer Entwicklungen.</p> <p>Es ist vorgesehen, bis 2025 ein zertifiziertes und funktionsf&#228;higes Netzwerk von ACTRIS-Stationen aufzubauen. Es soll der wissenschaftlichen Gemeinschaft qualitativ hochwertige Daten liefern, die die Grundlage f&#252;r fundierte Entscheidungen der politischen Entscheidungstr&#228;ger bilden k&#246;nnen.</p>

This repository contains data for the manuscript: "Unraveling a black box: An open-source methodology for the field calibration of small air quality sensors." This includes: Raw data from the low-cost prototype EarthSense Zephyrs, as well as raw data from reference instrumentation. SC stands for "Summer Campaign" and WC stands for "Winter Campaign", denoting the two different campaigns assessed in this study. Abstract The last two decades have seen substantial technological advances in the development of low-cost air pollution instruments using small sensors. While their use continues to spread across the field of atmospheric chemistry, challenges remain in ensuring data quality and comparability of calibration methods. This study introduces a seven-step methodology for the field calibration of low-cost sensors using reference instrumentation with user-friendly guidelines, open access code, and a discussion of common barriers to such an approach. The methodology has been developed and is applicable for gas-phase pollutants, such as for the measurement of nitrogen dioxide (NO2) or ozone (O3). A full example of the application of this methodology to a case study in an urban environment using both Multiple Linear Regression (MLR) and the Random Forest (RF) machine-learning technique is presented with relevant R code provided, including error estimation. In this case, we have applied it to the calibration of metal oxide gas-phase sensors (MOS). Results reiterate previous findings that MLR and RF are similarly accurate, though with differing limitations. The methodology presented here goes a step further than most studies by including explicit, transparent steps for addressing model selection, validation, and tuning, as well as addressing the common issues of autocorrelation and multicollinearity. We also highlight the need for standardized reporting of methods for data cleaning and flagging, model selection and tuning, and model metrics. In the absence of a standardized methodology for the calibration of low-cost sensors, we suggest a number of best practices for future studies using low-cost sensors to ensure greater comparability of research.