Atmospheric CH4 mole fractions have strongly increased since 1750 due to human activity and continue to rise. Reducing CH4 emissions is often easily feasible and also economically interesting, especially from fossil fuel sources (e. g. leakages). For the development of effective reduction strategies and to prioritize actions, CH4 emissions, their spatial distribution and their variability must be well constrained. This study presents airborne top-down emission estimates from Gabonese offshore oil installations as well as emissions from the Libreville urban area. A correlation with installation age and oil production is discussed, and a comparison with reported data and other top-down studies is presented. Further, co-emitted species such as C2H6, CO2, CO and NOy are shown for both offshore fossil fuel sources and the mixture of different urban sources, which include contributions from fossil fuel and biogenic origins (e.g. landfills).

Abstract. Asia accounts for approximately half of global anthropogenic emissions of major pollutants, yet emission inventories remain uncertain and ground-based monitoring is sparse across much of the region. To address these challenges, this study applies a multi-species satellite data assimilation framework to estimate emissions and concentrations of key chemical species during the NASA's ASIA-AQ campaign. The assimilation improves agreement with airborne observations for O3, NOx, CO, and CH2O, with the largest gains for CO (correlation increasing from 0.63–0.64 to 0.77) and CH2O (biases reduced by 41–70 %). Domain-wide, the optimized emissions show increases of 15 % for NOx and 9 % for CO, and a 52 % reduction in isoprene. Comparisons with multiple emission inventories reveal large discrepancies, with normalized standard deviation ranging from 11 % for NOx in mainland China to 68 % for CO in Taiwan. Over Thailand, the assimilation increases fire emissions from 3.6 Tg (GFASv1.2) to 8.3 Tg, while FINNv2.7 produces estimates roughly twice as high, highlighting persistent divergence among fire emission estimates. Source-receptor analysis reveals strong meteorological control on transboundary pollutant: long-range transport contributes up to 62 % of surface O3 in Manila during strong monsoon conditions, whereas Seoul exhibits local NOx-saturated chemistry. During a strong transport episode, Indian emissions account for 72–78 % of the free tropospheric O3 response over Taipei and Chiang Mai, and 24 % over Seoul, highlighting an overlooked transport pathway affecting Asian air quality. These results highlight the value of satellite data assimilation and the need for improved inventories and coordinated action on local and transboundary pollution.

Abstract. Quantifying greenhouse gas (GHG) emissions from urban areas is critical for assessing progress toward climate goals. Aircraft-based measurements provide valuable information on urban emission fluxes; however, their interpretation is often limited by uncertainties in atmospheric background concentrations. Here, we present an emission estimate of Berlin city emissions that explicitly includes background concentrations from a Eulerian background run as state vector elements in a Bayesian inversion framework. Using data from the 20th of July flight of the 2018 3DO aircraft campaign of the [UC]2 (“Urban Climates Under Change”) project over Berlin and high-resolution GHG and meteorological simulations using Weather Research and Forecasting (WRF) model, we derive a CO2 emission estimate for the city of Berlin. We find an optimized Berlin emission rate of 13.4 ± 4.6 MtCO2a−1. The emissions rate is about 48 % higher than the prior total annual emissions of the TNO anthropogenic bottom-up inventory (time-scaled emissions, 10.8 MtCO2a−1) and VPRM biogenic (time-varying, -1.7 MtCO2a−1) and about 30 % of the estimate from the traditional mass balance approach (44 ± 24 MtCO2a−1), while reducing the uncertainty by over a factor of five. We verify our posterior background concentration with independent concentration measurements above the boundary layer. This study provides an improved independent estimate of urban emissions and demonstrates that explicit treatment of background concentrations in inversions improves aircraft-based urban flux estimates.

Abstract. Nitrous oxide (N2O) is the most dominant precursor of ozone-depleting substances and the third most important anthropogenic greenhouse gas, with agriculture contributing the largest share of emissions (56 %). Over the past two decades, airborne in-situ measurements have become increasingly important for studying emission and transport of N2O in the atmosphere, also driven by the development of more precise and simultaneously faster Quantum Cascade Laser (QCL)-based spectrometers. However, many QCL-based spectrometers exhibit sensitivities to environmental and flight-related parameters that can vary rapidly (≤ 1 s), such as static or cabin pressure and aircraft roll and pitch angles. The impact of changing ambient water vapor is particularly critical due to both dilution and quantum-mechanical effects. Because the variability of N2O in the lower troposphere is often very small (<1 ppb) relative to its high background (∼ 338 ppb), even minor variations in these parameters can significantly affect data quality and must be corrected. Although many instruments can resolve such small concentration changes, their typical temporal resolution of 1 Hz may limit the application of advanced measurement techniques such as eddy covariance on fast-moving aircraft. Here, we present and evaluate a new instrument setup for precise (< 0.2 ppb) and high-frequency (10 Hz) airborne in-situ N2O measurements in altitudes up to 4500 m based on a MIRO MGA3 QCL spectrometer (MIRO Analytical AG). The instrument was successfully deployed during two airborne science missions, namely onboard the unpressurized DLR Cessna during the Greenhouse Gas Monitoring (GHGMon) campaign in 2023 in the Netherlands, as well as onboard the NASA DC8 during the Satellite Investigation of the Asian Air Quality (ASIA-AQ) campaign in 2024. Specifically, we evaluate and compare different water vapor correction approaches using ASIA-AQ data sampled within the tropical boundary layer over South Korea, the Philippines, Thailand and Taiwan, which partly were characterized by specifically high ambient humidity (up to 30000 ppm H2O). The water vapor correction methods include an empirical approach that relies on both native and corrected MIRO in-flight water vapor measurements, as well as an approach by an updated version of the MIRO specific fitting software. Comparison with N2O measurements from a well-established instrument onboard the NASA DC8 shows agreement within combined measurement uncertainties for all tested water vapor correction approaches, albeit special caution is needed for humidities larger than 15000 ppm. The new water vapor corrected data shows a 42 % better precision than with original default settings of the instrument. We further show that the instrument setup is insensitive to flight parameter changes such as roll and pitch angle of the aircraft, and allows for stable measurements even under challenging conditions such as in the turbulent boundary layer. This instrument setup enables improved characterization of N2O emissions and sources from the agricultural sector which is, in particular, relevant for tropical regions with strong agricultural activity and high humidity, where observational data remain scarce.

Hydrogen-powered aircraft are considered a key technology for reducing the climate impact of aviation, particularly for short- and medium-range applications. However, the assessment of their environmental effects critically depends on a robust characterization of emissions under realistic operating conditions. To date, in-flight emission data for hydrogen combustion engines at cruise altitude are essentially nonexistent, leading to large uncertainties in climate impact assessments and model representations.Here, we present results from the Blue Condor campaign, a collaboration of DLR, Airbus, The Perlan Project, and AV Experts, which aimed to characterize in-flight emissions and contrails from a hydrogen-fueled turbojet engine and to directly compare them with emissions from a kerosene-fueled engine. Both engines were mounted on Arcus J gliders and operated under the same atmospheric and operational conditions.An autonomous and modular emission measurement system was successfully developed and integrated onto the high-altitude research aircraft Grob EGRETT. The system included instrumentation for non-volatile and total particle number (tPM) concentrations and particle size distributions, optimized for formation-flight sampling in aircraft exhaust plumes.The measurement system was deployed during formation flights behind the hydrogen-powered turbojet engine, enabling the first in-flight measurements of a hydrogen combustion engine at cruise altitude (FL 180-320). Particle emissions were sampled at distances of several tens to hundreds of meters downstream of the engine exhaust, ensuring representative plume conditions. For reference, analogous measurements were conducted behind a kerosene-fueled engine at similar flight levels and ambient conditions.The tPM emissions were found to be three orders of magnitude lower for the hydrogen engine. However, the tPM emission indices of the hydrogen engine range from 1011 to 1015 particles of kg fuel burned, depending on engine conditions.The hydrogen engine particle emissions exhibit a distinctparticle size distribution, in contrast to the particle size distributions typically observed in kerosene combustion, both in terms of modal structure and emission indices.the observed particle emissions most likely originate from non-combustion-related sources, such as lubrication oil. This interpretation is supported by complementary laboratory measurements.These results provide the first observational constraints on in-flight particle emissions from hydrogen-fueled aircraft engines which are precursors for contrail formation and represent an important step toward assessing their atmospheric and climate impacts.

This study reports on the first successful deployment of a new airborne eddy covariance (EC) setup to better characterize and quantify non-CO 2 greenhouse gas emissions from agriculture. The system was deployed aboard the DLR research aircraft Cessna Caravan to quantify growing-season emissions of methane (CH 4 ) and nitrous oxide (N 2 O) in Friesland, an agricultural region in the Netherlands, in early summer 2023. The EC system consists of a commercial quantum cascade laser spectrometer, specifically adapted for airborne observations and providing 10 Hz data of N 2 O and CH 4 , and the meteorological measurement suite METPOD, delivering data of the vertical wind, horizontal winds, water vapor and temperature. Our measurements are a novelty for N 2 O, since they are the first implementation of quantifying agricultural emissions with airborne EC, combining the advantages of regional-scale coverage, while maintaining high spatial resolution and hence are well suited to capture the spatial complexity of this dominant emission sector. The system provides fluxes with minimal low- and high-frequency distortions, low detection limits, and total uncertainties (30 %–100 %) comparable to other airborne methods, despite the complexity of agricultural emissions. During measurements in Friesland, we identified clear N 2 O emission hotspots and hot-moments, with peak fluxes of 0.34 µg m −2 s −1 on the regional-scale after intensive precipitation following a relatively dry period. Single small-scale hotspot emissions were as high as 1 µg m −2 s −1 In contrast, CH 4 fluxes showed less temporal variations around a mean flux of 1.62 µg m −2 s −1 throughout the three-week campaign. N 2 O emissions were relatively high compared to other agricultural regions worldwide, and preliminary comparisons with EDGAR v8.0 and the Dutch emission inventory Emissieregistratie suggest substantial underestimation of growing-season N 2 O emissions in current inventories and the lack of an appropriate annual cycle. Our results further document the urgent need for independent verification of reported N 2 O and CH 4 emissions from agriculture, which is the most dominant anthropogenic sector of non-CO 2 greenhouse gas emissions and is expected to become even more dominant in the future, with an increasing world population and food demand.

In October 2023, Boeing and NASA conducted a joint flight and ground experiment based out of Paine Field in Everett, Washington. This experiment measured the emissions and contrail properties of a Boeing 737-10 equipped with CFM LEAP-1B engines, utilizing the NASA DC-8 airborne laboratory as a chase aircraft. The experiment measured and evaluated particulate emissions and contrail properties from three fuel types over 11 flights and two ground tests. In-situ flight measurements on the DC-8 were typically conducted at distances ranging from 1 to 20 nautical miles behind and within a vertical range of +/- 100 feet of the altitude of the 737-10 for both contrail and plume sampling. Atmospheric conditions ranged from ice supersaturation for persistent contrails to subsaturated temporary contrails to non-contrail conditions. The fuels tested included low sulfur Jet A, 100% paraffinic sustainable aviation fuel, and local Jet A. Significant efforts were made to minimize fuel mixing to avoid contamination with sulfur and/or aromatics across fuel types.Here we present preliminary data analyses from both the ground test and in-flight measurements, focusing on the measurements of total particles, non-volatile particles measured after passing through a 350C thermal denuder, and ice particle concentration measured using a Cloud Aerosol Spectrometer (CAS) and contrasting the results for different fuel types. Differences in particulate matter with fuel type were also captured by the NASA high-spectral-resolution lidar (HSRL) High Altitude Lidar Observatory (HALO) Water Vapor differential absorption lidar (DIAL) instrument, which sampled approximately 1,000-5,000 ft above the contrails made by the Boeing 737-10. Challenges of using instruments originally developed for particle measurements in clear air to the more complicated environment of an ice cloud such as a contrail will be discussed.This presentation will also highlight challenges related to logistics, fuel handling, and sampling in contrails, which highlights the complex interactions between atmospheric chemistry and microphysical processes in cloud formation and ice nucleation. Next steps will encompass future campaign needs, outstanding research questions, and measurement techniques.

Abstract. Aviation forces the climate via multiple pathways: The emission of carbon dioxide (CO2) from conventional propulsion technologies has a warming effect, but also non-CO2 effects like contrail cirrus play a crucial role. For new hydrogen (H2) based propulsion concepts there are no direct CO2 and soot emissions, but higher water vapor emissions, which can affect the formation, micro-physical properties and the climate impact of contrails. In-flight measurements of relevant contrail and emission parameters of fuel cell propulsion systems are non-existent. In order to study contrail formation in the wake of fuel cell powered aircraft, a fuel cell exhaust emulator (FCEE) was designed by Airbus to produce the low temperature and high humidity exhaust conditions expected from these systems. Due to their unique exhaust composition and thermodynamic emission characteristic, processes such as plume mixing with ambient air are crucial to understand contrail formation. Further, to compare engine and fuel-dependent contrail properties, in situ methods derive a dilution corrected apparent ice emission index (AEI) using conservative tracers, like CO2 or nitrogen oxides (NOX). As the FCEE emits no such inert trace gas, a newly developed artificial dilution tracer system was used. The design and characterization of this system, as well as the significance and application for exhaust dilution and emission index assessment of fuel cell-powered aircraft in flight, are described. As part of the Nephele flight test campaign, in December 2023 in Minnesota, USA, the prototype FCEE and artificial dilution tracer release system were installed on board a Convair C131. The sampling system was installed on a Piper Cheyenne 400LS chaser aircraft, together with instrumentation for emission and contrail measurement, both aircraft operated by AV Experts LLC. A proof-of-concept of the artificial tracer system was demonstrated in contrail and emission flights. Contrails formed on aerosol particles emitted by the FCEE near the homogeneous freezing threshold (at ambient temperatures around 230 K) were sampled at 400 hPa in distances between 50 and 1500 m behind the aircraft. An increase of AEI from 1.4·1013 to 4·1015 kg-1H2 with increasing temperature difference of 3 to 5 K below the homogeneous freezing temperature was observed. As contrails from fuel cell systems already form at higher ambient temperatures compared to conventional propulsion systems, these are the first contrail measurements in this temperature range. Complementary model simulations suggest that droplet freezing is responsible for the evolution of ice crystal numbers in this temperature range.

In 2018, the CoMet series of airborne missions was launched under the auspices of the German Aerospace Center (DLR). From this point onward, large-scale field campaigns are organized regularly every few years, in collaboration with partner institutions, to understand the anthropogenic and natural fluxes of the greenhouse gases CO2 and CH4, using the German research aircraft HALO. In addition to scientific objectives, the CoMet field campaigns aim to promote technological developments necessary for new and future Earth observation satellites, validate current greenhouse gas satellite measurements, and prepare new satellite missions, in particular the upcoming German-French methane lidar mission MERLIN. One of the core instruments of CoMet is the IPDA lidar CHARM-F, which was developed at DLR as an airborne demonstrator, testbed, and validation tool for MERLIN.

Contrail cirrus clouds are a main contributor to the climate forcing from aviation1. Yet, the number of contrail ice crystals forming behind aircraft with modern lean-burn engines is unknown. Theory spans a four orders of magnitude range in ice crystal numbers2,3-rendering related climate effects unpredictable. Here we show that lean-burn combustion reduces soot particle number emissions by three orders of magnitude compared with conventional rich-quench-lean engines4,5-but does not significantly decrease volatile particles or contrail ice crystal numbers-both can exceed 1015 particles per kg of burned fuel. Our findings arise from in-flight observations behind an A321neo aircraft with lean-burn engines, thus providing real-world confirmation of some laboratory work6 and narrowing the range of theoretical expectations. Our results indicate that the tested lean-burn engine configurations alone are unlikely to reduce the warming effect of contrails, suggesting that modifications of fuel composition and lubrication oil venting architecture may be required. We show that contrail ice particle numbers in the low-soot regime can be reduced by using low-sulfur fuels and that organic fuel constituents and lubrication oil vapours can increase contrail ice particle numbers. Future research should explore how reductions in volatile particles, apart from soot, affect contrail ice formation.

This study reports on the first successful deployment of a new airborne eddy covariance (EC) setup to better characterize and quantify non-CO2 greenhouse gas emissions from agriculture. The system was deployed aboard the DLR research aircraft Cessna Caravan to quantify growing-season emissions of methane (CH4) and nitrous oxide (N2O) in Friesland, an agricultural region in the Netherlands, in early summer 2023. The EC system consists of a commercial quantum cascade laser spectrometer, specifically adapted for airborne observations and providing 10 Hz data of N2O and CH4, and the meteorological measurement suite METPOD, delivering data of the vertical wind, horizontal winds, water vapor and temperature. Our measurements are a novelty for N2O, since they are the first implementation of quantifying agricultural emissions with airborne EC, combining the advantages of regional-scale coverage, while maintaining high spatial resolution and hence are well suited to capture the spatial complexity of this dominant emission sector. The system provides fluxes with minimal low- and high-frequency distortions, low detection limits, and total uncertainties (30 %-100 %) comparable to other airborne methods, despite the complexity of agricultural emissions. During measurements in Friesland, we identified clear N2O emission hotspots and hot-moments, with peak fluxes of 0.34 mu g m-2 s-1 on the regional-scale after intensive precipitation following a relatively dry period. Single small-scale hotspot emissions were as high as 1 mu g m-2 s-1 In contrast, CH4 fluxes showed less temporal variations around a mean flux of 1.62 mu g m-2 s-1 throughout the three-week campaign. N2O emissions were relatively high compared to other agricultural regions worldwide, and preliminary comparisons with EDGAR v8.0 and the Dutch emission inventory Emissieregistratie suggest substantial underestimation of growing-season N2O emissions in current inventories and the lack of an appropriate annual cycle. Our results further document the urgent need for independent verification of reported N2O and CH4 emissions from agriculture, which is the most dominant anthropogenic sector of non-CO2 greenhouse gas emissions and is expected to become even more dominant in the future, with an increasing world population and food demand.

Global warming is proceeding rapidly and quick actions are required to suspend the increasing temperatures globally. Here we give an overview of a series of measurement studies, supported and funded by UNEP´s International Methane Emissions Observatory (IMEO) in 2022-2023. Methane (CH4) is the primary focus of all these studies, since it is one of the most potent greenhouse gases, which at the same time has a relative short lifetime. Due to these specific characteristics, CH4 is presently the prime target for mitigating emissions from industrial activities.Our approach focused on a variety of CH4 emissions from the coal, oil and gas (O&G), and waste industry in Poland and in the Middle East within the framework of METHANE-To-Go-Poland and METHANE-To-Go-Oman. A unique helicopter-towed probe, HELiPOD, was equipped with in situ CH4 instrumentation complemented by mobile ground-based CH4 measurements. The well-known mass balance approach was applied to quantify the CH4 emissions from the targeted sources. Final comparisons of our top-down estimates with bottom-up industry or inventory data have been carried out to assist the involved companies and related governments in prioritizing their CH4 emission mitigation actions and policies for future endeavours. Several non-captured CH4 source strengths, compared to the available bottom-up data, were discovered in the course of these top-down studies. For a number of reasons evaluated during our operations, the novel HELiPOD set-up is proposed to be a suitable platform for upcoming satellite evaluation studies focusing on CH4. In particular, the HELiPOD measurements (CH4 mixing ratio plus 3D wind) can capture the whole vertical and horizontal extension of targeted CH4 plumes, which is necessary for the CH4 mass flux quantification, a number that can be directly comparable to available satellite-based flux rates in UNEP´s Methane Alert and Response System (MARS).

The amount of methane released to the atmosphere from the Nord Stream subsea pipeline leaks remains uncertain, as reflected in a wide range of estimates1–18. A lack of information regarding the temporal variation in atmospheric emissions has made it challenging to reconcile pipeline volumetric (bottom-up) estimates1–8 with measurement-based (top-down) estimates8–18. Here we simulate pipeline rupture emission rates and integrate these with methane dissolution and sea-surface outgassing estimates9,10 to model the evolution of atmospheric emissions from the leaks. We verify our modelled atmospheric emissions by comparing them with top-down point-in-time emission-rate estimates and cumulative emission estimates derived from airborne11, satellite8,12–14 and tall tower data. We obtain consistency between our modelled atmospheric emissions and top-down estimates and find that 465 ± 20 thousand metric tons of methane were emitted to the atmosphere. Although, to our knowledge, this represents the largest recorded amount of methane released from a single transient event, it is equivalent to 0.1% of anthropogenic methane emissions for 2022. The impact of the leaks on the global atmospheric methane budget brings into focus the numerous other anthropogenic methane sources that require mitigation globally. Our analysis demonstrates that diverse, complementary measurement approaches are needed to quantify methane emissions in support of the Global Methane Pledge19. Modelling of the evolution of atmospheric methane emissions from the 2022 Nord Stream subsea pipeline leaks shows that the event emitted the largest recorded amount of methane from a single transient event.

This study introduces a helicopter-borne mass balance approach, utilizing the HELiPOD platform, to accurately quantify methane (CH4) emissions from coal mining activities. Compared to conventional research aircraft, the use of an external sling load configuration eliminates the need for aeronautical certifications, facilitates easier modifications and enables local helicopter companies to conduct flights. Furthermore, it allows for plume probing as close as several hundred meters downwind of an emission source and offers comprehensive vertical coverage from 50 m to 3 km altitude, making the HELiPOD an ideal tool to distinguish, capture, and quantify emissions from single sources in complex emission landscapes worldwide. Our approach serves as an independent emission verification tool, bridging the gap between ground-based, drone, near-field and far-field airborne measurements and supports identification of CH4 emission mitigation opportunities. Nineteen mission flights were conducted in the Upper Silesian Coal Basin of Southern Poland in June and October 2022 that targeted CH4 emissions from multiple coal mine ventilation shafts and several drainage stations. The comparison of top-down HELiPOD mass flux estimates against those calculated from bottom-up in-mine CH4 safety sensor and air flow measurements revealed very good agreement with relative deviations of 0 % to 25 %. This indicates, notwithstanding associated uncertainties, that the two independent approaches are capable of estimating CH4 emissions from coal mine ventilation shafts accurately. However, the accuracy and representativeness of derived in-mine data is application-specific and should be evaluated by independent measurements.With measured CH4 emission rates up to 3000 kg h-1 from individual coal mine ventilation shafts we confirm prior research, while revealing that emission strengths from drainage stations can be of comparable magnitude and should be investigated further. The possibility to detect emissions at rates as low as 20 kg h-1 with the HELiPOD was demonstrated through a controlled release experiment. This emphasises the wide range of potential applications in quantifying sources within a wide range of CH4 emission rates, i.e. from relatively small sources, e.g. biodigesters, landfills, cattle feedlots and manure pits to larger industrial sources including those from the coal, oil and gas sectors.

Atmospheric methane (CH4) concentrations have more than doubled since the beginning of the industrial age, making CH4 the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). The oil and gas (O&G) sectors are one of the major anthropogenic CH4 sources accounting for 22% of global anthropogenic CH4 emissions. The METHANE-To-Go Africa (MTGA) scientific aircraft campaign in September 2022 was conducted as part of UNEP’s International Methane Emissions Observatory (IMEO). During the campaign, we conducted the first large scale methane measurements of the O&G sector in West Africa. The study provides an initial empirical understanding of the magnitude and location of emissions in this important but previously unobserved source region. The emissions of O&G facilities were determined using an aircraft-based mass balance method. The entire emissions of the Angolan offshore O&G sector and the liquid natural gas (LNG) plant were observed to be in the range of emissions reported by the Angolan operators. This is much less than the estimates from scientific emission inventories like EDGAR and CAMS-GLOB-ANT. For the regional scale emission estimates, the Angolan O&G facilities are aggregated in blocks, a local operator-wise separation of assets. Most blocks have low emissions of methane. We observed medium emissions at one block and high emissions at two blocks. These three blocks are close to the coast, in shallow water, and the facilities are generally older than further out at sea. Often the emissions of individual facilities or groups of facilities could be discerned from the mass balance flights. We deduced emission estimates for 31 individual facilities and 10 groups of facilities. The emission estimates on different days are consistent for all facilities, showing little temporal variation. The generally older shallow-water facilities show higher emissions than the deep and ultra-deep water facilities, which have a higher oil production. The additional trace gases CO2, SO2, NOy and aerosol particles were also observed from the aircraft. This data is used to further investigate the source of CH4 emissions: flaring, fugitives, venting, or burning of fuel gas. The CH4-CO2 ratio indicates that most CH4 emissions result from fugitives and venting, not flaring. Ten different flare exhaust plumes were sampled at close distance. The flaring observations will be further analyzed including information on gas composition from the operators. Overall, this study gathered a unique dataset in its coverage providing extraordinarily comprehensive measurements of the CH4 emissions from the O&G industry off the coast of West Africa.

Alternative aviation fuels represent a promising approach to reduce contrails climate effect. In the frame of VOLCAN (“VOL avec Carburants Alternatifs Nouveaux”) project (DGAC funding, collaboration with AIRBUS, Safran Aircraft Engines and DLR, financed by Neofuels), the influence of Sustainable Alternative Fuels (SAF) composition on exhaust plumes emission, and therefore on contrails, is investigated using the 1D detailed microphysical code MoMiE (Modèle Microphysique pour Effluents) developed at ONERA1,2. The VOLCAN measurement campaigns have been able to provide estimations of ice particle number emission indexes within contrails formed by different fuel types (classical kerosene Jet A-1 and biofuel HEFA) and different combustion modes (“rich” and “lean” burn). These are complementing the observations obtained for Sustainable Alternative Fuels with Emission and “CLimate Impact of alternative Fuels” (ECLIF) campaigns3,4, recently compared to the results of the “Aerosol and Contrail Microphysics” (ACM) model developed at the University of Albany5.In its most recent version ONERA’s code MoMiE has been adapted to Sustainable Alternative Fuels (SAF)2. It includes heterogenous freezing with soot activation by sulfur and organic species, as well as homogeneous freezing of liquid droplets of hydrated sulfates and organics, accounting for the competition between both nucleation modes. Chemiionization, brownian coagulation of particles, ice sublimation and condensation are also represented. The code computes the different aerosols distributions (size and number) of sulfates, organics, dry soot, activated soot, and ice particles, homogeneously (no solid nucleus) and heterogeneously (soot solid nucleus) formed.The work proposed here aims first at presenting and analyzing the results obtained with the model in comparison to some of the VOLCAN measurements. The sensitivity of contrail formation to the different fuel types, combustion modes and emission characteristics, as ion emission index, which is known to play a significant role in the coagulation process, are studied. The model is also confronted to the ECLIF measurements3,4 and the microphysics model results from University of Albany5. Advancement and results of this study will be presented and discussed during the conference.1Vancassel X. et al., Numerical simulation of aerosols in an aircraft wake using a 3D LES solver and a detailed microphysical model, International Journal of Sustainable Aviation, 20142Rojo C. et al., Impact of alternative jet fuels on aircraft-induced aerosols, Fuel, 20143Voigt C. et al., Cleaner burning aviation fuels can reduce contrail cloudiness, communications earth & environment, 20214Märkl R. S. et al., Powering aircraft with 100% sustainable aviation fuel reduces ice crystals in contrails, Atmospheric Chemistry and Physics, 20245Yu F. et al., Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles, Environmental Science & Technology, 2024

Within the framework of the Oil and Gas Methane Partnership 2.0 (OGMP 2.0), initiated by the United Nations Environment Programme (UNEP), companies in the Oil and Gas (O&G) sector have committed to monitor and to reduce their methane (CH4) emissions. Presently, more than 120 companies have joined OGMP 2.0 covering operations in 70 countries around the world, one of which is Oman. Methane is one of the most potent greenhouse gases after carbon dioxide and the focus of worldwide initiatives to combat global warming. This includes UNEP’s International Methane Emissions Observatory (IMEO), which focuses on improved data collection and delivery not only from O&G, but also from other emission sectors including waste. According to Oman’s latest Biennial Update Report, the solid waste sector is the second largest CH4 emitter behind the O&G sector and represents 15% of Oman’s CH4 emissions. However, until now, no sector-specific measurement-based studies on such emissions exist for Oman.Here, we present a novel measurement study, supported and funded by UNEP´s IMEO. The approach involves measuring CH4 emissions from both O&G installations and landfills using the unique helicopter-towed probe HELiPOD equipped with in situ CH4 instrumentation complemented by mobile ground-based CH4 measurements. Quantifications of CH4 mass fluxes from individual sources or clusters can be provided from these measurements. The methodology was deployed during the METHANE-To-Go-Oman field experiment lasting from November to December 2023 in collaboration with partners from the O&G and waste industry in Oman. Within four weeks, more than 70 flight hours were successfully flown with a helicopter in the northern and southern parts of Oman, which required a complex setup. For each of the 26 flights, different flight strategies were implemented depending on the wind situation at the probed sites, which was characterized by a continuously running wind lidar. The HELiPOD probe (weight 325 kg, length 5 m) was equipped with a sensor system measuring the 3D wind vector and in situ instrumentation (Picarro G2401-m and Licor-7700) to measure CH4 with a high precision (1 ppb) and temporal resolution (up to 40 Hz), which is necessary for a precise calculation of the CH4 mass flux. An initial overview of the measurements is presented focusing on a showcase from a landfill.By comparing our collected data (top-down approach) with methane mass flux estimates provided by the industry (bottom-up approach), we aim to assist the involved companies and related governments in prioritizing their methane emission mitigation actions and policies for future endeavours.

In recent efforts to reduce the radiative forcing of aviation, fuel design has gained increased attention. Sustainable Aviation Fuels are seeing wider adoption, and their positive impact on carbon dioxide and non-volatile soot particle emissions is well-established. However, the effects of the reduction in fuel sulfur content on volatile particle emissions and contrails are unknown. This study presents observations from in-flight measurements of emissions and contrails of an Airbus A350-900 burning fuels with different sulfur contents. We find a reduction in volatile particles and contrail ice crystals for low-sulfur fuels. For higher fuel sulfur contents, our findings demonstrate an additional contrail ice particle source through activation of sulfate aerosols. Our data-driven results need to be consolidated by in-flight observations with different fuels and engines. Eventually, climate impact estimates as well as regulations should account for the modulating effect of the fuel sulfur content on contrail ice particle numbers.