Abstract As large wildfires become more frequent and severe in North America due to heightened fuel aridity and fire weather conditions driven by anthropogenic climate change, wildfire smoke has an increasingly important influence on air quality. Transported smoke can impact urban boundary layer ozone (O 3 ) directly, via the entrainment of O 3 formed in smoke, or indirectly, through the influence of pyrogenic volatile organic compounds (VOCs) and aerosols on local O 3 production. Widespread smoke impacts from the record‐breaking 2023 Canadian wildfire season coincided with the July–August 2023 Airborne Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) aircraft‐based field campaign, which provided extensive in situ observations of urban chemistry in North American megacities. Vertically resolved trace gas and aerosol measurements from the August 2 AEROMMA research flight in Chicago indicated the entrainment of diffuse smoke from an aged, lofted wildfire plume into the urban boundary layer and were used to constrain a zero‐dimensional photochemical model and a radiative transfer model. We perturbed the models across a range of smoke densities and urban nitrogen oxide emissions and found net pyrogenic O 3 enhancements of 3–17 ppbv, with the largest contribution from transported smoke O 3 (65%–84%), followed by locally produced photochemical O 3 from smoke VOCs (18%–46%), and finally smoke aerosol shading (−12%–0%). Although these results are specific to the observed August 2 smoke plume and Chicago urban chemistry, the modeling framework presented could be applied to other smoke‐impacted locations to disentangle the various effects of wildfire smoke on urban O 3 .

In recent years, the use of automatic lidars and ceilometers (ALC) for atmospheric research has increased. Originally, these instruments were developed to measure cloud base height automatically, utilising the LIDAR principle. However, multiple studies have shown their usability for aerosol remote sensing and planetary boundary layer height detection. It is not only possible to calibrate a ceilometer and derive the attenuated backscatter signal, but also to retrieve aerosol extinction coefficients and aerosol mass concentrations by means of estimated extinction to mass coefficients (EMC). The ACTRIS (Aerosol, Clouds and Trace Gases Research Infrastructure) national facility JOYCE (Julich Observatory for Cloud Evolution) offers a multiyear dataset of cloud remote sensing measurements and ceilometer observations. So far, a method for measuring aerosol properties has been missing to use this dataset to quantify aerosol-cloud interaction. The goal of this study is to evaluate the applicability of a ceilometer aerosol retrieval to prove the value of this dataset for aerosol remote sensing. We present the workflow, starting with the raw ceilometer data, followed by a calibration of the backscatter coefficient profiles and a retrieval of aerosol properties. To evaluate the result of this workflow for the JOYCE ceilometer (Vaisala CT25k), in situ aerosol measurements at the Julich meteorological tower were performed, where an optical particle sizer (OPS) was installed at 120 m above ground. Aerosol extinction coefficients sigma a were retrieved from the ceilometer attenuated backscatter signal with sigma a correlated with the in situ total aerosol mass concentration with R = 0.73. For our measurement set-up, aerosol mass concentrations can be derived from the retrieved sigma a with a mean absolute percentage error of 39 %. However, the extinction to mass conversion factor EMC = (2.2 +/- 0.9) m(2) g(-1) derived from the measurements for a wavelength of 906 nm was found to be greater by a factor of about 1.8 compared to literature and to EMC calculated from Mie simulations based on the in situ aerosol size distributions. The mismatch is tentatively attributed to the limited aerosol size range of the OPS and a temperature bias inside this instrument.

The photolysis of nitrous acid (HONO) produces hydroxyl radicals (OH), the most important cleaning agent of the troposphere. For decades, HONO has been measured in concentrations which exceed the photo-stationary concentration arising from its gas phase formation via the reaction NO + OH and destruction by photolysis. Several photochemical and heterogeneous formation mechanisms, including the photolysis of nitrate have been proposed which may explain this excess HONO. This study reports on airborne remote sensing measurements of the mini-DOAS instrument over continental Europe, Southeast Asia, and the tropical Atlantic. The observations form a C-shaped profile in the troposphere with maximum volume mixing ratios of approximately 150 ppt in the planetary boundary layer, about 10 ppt in the free troposphere and up to 100 ppt in the tropical upper troposphere. These measurements of HONO throughout the troposphere exceed model predictions by up to an order of magnitude. Together with a host of other measured species and parameters, various formation mechanisms are explored to investigate in situ HONO sources. Although a precise formation mechanism in the polluted boundary layer remains elusive, the photolysis of particulate nitrate may explain excess HONO in the marine boundary layer. The excess HONO observed in the upper troposphere requires a gas phase source with a formation rate of up to 300 ppt h(-1). The possible role of peroxynitrous acid (HOONO), formed by the reactions NO + HO2 + M and NO2 + OH + M, and further oxidation by reactions with NO or O-3, is explored.

Declining nitrogen oxide (NOx = NO + NO2) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO2), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O3). Under lower-NO conditions, alternative RO2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO2 fate across urban environments. We infer RO2 bimolecular lifetimes (τbi) as a proxy for isomerization potential, finding longer τbi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τbi is widespread across urban North America, suggesting that declining NOx is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.

Airborne measurements provide valuable information about the vertical distribution of pollutants enabling the complex transport and dispersion pathways within and above the boundary layer (BL) to be investigated. In this study, the transport of pollution within the Po Plain, a major atmospheric pollution hotspot in Europe, was explored by exploiting airborne measurements made within the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) project in combination with in situ and ground-based remote-sensing measurements over the whole Po basin. The analysis considers three areas where pollution, emission and mixing are dominated by different processes: the Gulf of Venice to the east, the central part of the Po Plain, and the Gulf of Genoa to the west. Wind fields and backward trajectories during the days of the flights indicate the impact of the sea and mountain breezes on the BL distribution of pollutants, and of synoptic scale transport above it. Overall, the extensive data set of primary and secondary trace gases and aerosols at different altitudes provides insight into the effect of vertical and horizontal dynamical mixing on the chemical processing of pollutants within the BL. In this context, mixing of pollution plumes during stagnant conditions within the BL, stratification, venting and export of pollutants towards the Adriatic coast were observed. In addition, desert dust layers of Saharan origin at different altitudes confirm the mixing of naturally occurring dust and its impact on air quality of the lowermost atmosphere over the Po Plain.

Abstract. Numerical simulation of environmental chamber experiments is essential for improving models of atmospheric composition. This task often relies on box models, which are not integral to three-dimensional atmospheric chemical transport models. Here, we present an application of the Modular Earth Submodel System (MESSy) DWARF model for chamber simulation experiments. It was developed as a zero-dimensional chemical box model of atmospheric oxidation. Chamber-specific features—including the injection of trace gases, variable radiation conditions, gas- and aqueous-phase chemistry, wall emissions, and dilution—are represented using existing and adapted MESSy submodels. We describe the multi-phase kinetic framework, the estimation of aerosol liquid water content and a simplified treatment of wall losses. We tested the MESSy chamber DWARF setup with experiments from three environmental chambers: The SAPHIR and SAPHIR-STAR chambers at Forschungszentrum Jülich, and the BATCH chamber at the University of Bayreuth. These chambers cover diverse designs and experimental conditions. The model reproduces observed time series of radicals, nitrogen oxides and ozone during an experiment in SAPHIR in which 2-methyl-3-butene-2-ol was oxidised. In another test the model was used to check the consistency of the experimental boundary conditions in the BATCH chamber. Further chamber DWARF tests also show that the framework can predict organic aerosol mass concentration via kinetic partitioning between the gas phase and deliquescent particles in SAPHIR-STAR. Modeled organic mass concentration during this experiment where α-pinene was oxidised agrees well with observations, supporting the framework’s ability to simulate the chemical evolution in both the gaseous and aqueous phase. The MESSy chamber DWARF provides a useful tool for developing and evaluating kinetic models. Within this framework, we can test new laboratory findings directly for their atmospheric relevance. Thanks to the MESSy framework, we can use the same kinetic model for global simulations.

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

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

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 HOx (OH + HO2) with nitrogen oxides (NOx), 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 HOx 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 NOx 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.

Despite ongoing reductions in emissions of ozone (O3) precursors, nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs), the three largest urban areas in the United States ─ New York City (NYC), Chicago, and Los Angeles (LA) ─ continue to exceed national air quality standards for O3. Airborne measurements during the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign investigated nonlinear O3 photochemistry in these cities. We report mean ozone production efficiency (OPE), the enhancement ratio of Ox (= O3 + NO2) to NOx oxidation products, of 9 ± 4 (1σ), 6 ± 3, and 6 ± 3 ppbv ppbv-1 in NYC, Chicago, and LA, respectively. Compared to historical values, OPE has increased in NYC but remains constant in LA. We find that OPE during AEROMMA has a nonlinear, inverse relationship with total reactive nitrogen (NOy, a proxy for initial NOx) and a positive correlation with the nonmethane VOC to NOy enhancement ratio. A zero-dimensional photochemical model supports these observed OPE dependences on NOx and VOCs and shows that OPE is a distinct metric from total O3 production that is informative to the development of O3 pollution control strategies. We find that OPE values have higher variability, and a larger increase with NOx emissions reductions, in areas that experience NOx-sensitive rather than NOx-saturated O3 photochemistry; nonetheless, NOx reductions under NOx-sensitive conditions still reduce total O3 production despite the corresponding increase in OPE.

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

Atmospheric simulation chambers (ASCs) are one of the most advanced tools for the experimental investigation of the oxidation of volatile organic compounds (VOCs) and the subsequent secondary organic aerosol (SOA) formation. Toluene is one of the most prevalent anthropogenic VOCs. Its photo-oxidation yields a wide range of products in the gas phase and a significant amount of SOA. Some of the remaining uncertainties about toluene atmospheric chemistry are possibly linked with chamber artifacts. In this study, several atmospheric simulation chambers, characterized by a great diversity (size, shape, material of walls, light source, instrumentation, measurement techniques, etc.), performed several toluene photo-oxidation experiments under different pre-set conditions (levels of toluene, NOx, and relative humidity, presence, or lack of seeds). A model based on the Master Chemical Mechanism (MCM) and a SOA production module were used to facilitate the synthesis of the results. The results of the multiple-chamber toluene experiments suggest that a combination of facilities can provide a better picture of the overall behavior and that significant gaps remain in our understanding of the system, especially in the later oxidation stages. For cresol, a first-generation product, the observed gas-phase yields, ranging from 3

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

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

New particle formation (NPF) in the tropical upper troposphere is a globally important source of atmospheric aerosols1, 2, 3-4. It is known to occur over the Amazon basin, but the nucleation mechanism and chemical precursors have yet to be identified2. Here we present comprehensive in situ aircraft measurements showing that extremely low-volatile oxidation products of isoprene, particularly certain organonitrates, drive NPF in the Amazonian upper troposphere. The organonitrates originate from OH-initiated oxidation of isoprene from forest emissions in the presence of nitrogen oxides from lightning. Nucleation bursts start about 2 h after sunrise in the outflow of nocturnal deep convection, producing high aerosol concentrations of more than 50,000 particles cm-3. We report measurements of characteristic diurnal cycles of precursor gases and particles. Our observations show that the interplay between biogenic isoprene, deep tropical convection with associated lightning, oxidation photochemistry and the low ambient temperature uniquely promotes NPF. The particles grow over time, undergo long-range transport and descend through subsidence to the lower troposphere, in which they can serve as cloud condensation nuclei (CCN) that influence the Earth's hydrological cycle, radiation budget and climate1,4, 5, 6, 7-8.

Hydroxyl radicals are the most predominant daytime initiator for atmospheric oxidation processes. Due to the COVID -19 pandemic, there was a considerable reduction in emissions from industry and all means of transportation during spring 2020. The main objective of the BLUESKY campaign is to understand the effect of these reduced emissions on atmospheric composition such as trace gases, aerosols, and cloud properties. OH and HO2 were measured with HORUS (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy), during eight research flights from the boundary layer up to 14 km. Here we present the impact of reduced aircraft emissions and the meteorological situation on the HOx Chemistry in the upper troposphere over Europe during the COVID-19 lockdown. We contrast the findings during BLUESKY with results from previous campaigns and analyze the occurrence of HOx during daytime in the outflow of electrified and non-electrified convective systems.