Understanding the composition of carbonaceous aerosols, black carbon (BC) and organic aerosols (OA), remains a major challenge in atmospheric science. Using data from two aircraft campaigns with identical instrumentation over Europe and East Asia, we analyze statistical relationships between concentrations of five trace gases (CO, NO2, HCHO, O3, and SO2) with BC and OA in order to estimate carbonaceous aerosol in urban pollution plumes. We show that across both campaigns, CO is the best proxy for BC (R2 approximate to 0.6). In plumes, OA shows statistical links with NO2, O3, and CO, reflecting the combined influence of emissions, and secondary organic aerosol formation. Linear regressions based on trace gases remain limited, especially for OA, whereas the use of nonlinear machine-learning regression improves the quantification of BC and OA (R2 approximate to 0.9 for BC, R2 approximate to 0.7 for OA). However, the number of flights is limited, the results should not be interpreted as applicable to flights in other regions and seasons. Our findings indicate that co-emitted and co-produced trace gases contain information for quantifying carbonaceous aerosol in urban pollution plumes. This potential is more robust for BC, whereas OA remains more complex to estimate because it depends on multiple predictors.

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.

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.

Accurate estimates of short-lived climate forcer emissions are essential for effective climate change mitigation, yet large uncertainties remain across Asia. This study integrates the Weather Research and Forecasting/Community Multiscale Air Quality version 5.0.2 (WRF/CMAQv5.0.2) simulations with Effect of Megacities on the Transport and Transformation of Pollutants at Regional and Global Scales (EMeRGe) airborne observations from spring 2018 to refine combustion-related emissions of black carbon (BC) and CO over East Asia. Aircraft observations, which are less affected by near-surface processes, confirmed the accuracy of the Global Fire Emissions Database (GFEDv4.1s) inventory near Thailand. In contrast, anthropogenic BC and CO from the Regional Emission Inventory for Asia (REASv2.1) in the Philippines showed negative biases. For Chinese air masses, the Hemispheric Transport of Air Pollution with corrections (HTAPv2.2z) resulted in overestimation for BC and underestimation for CO, agreeing with our prior ground-based findings on Fukue Island. Scaling Chinese BC emissions using an observation/model ratio (0.48 +/- 0.13) gave our best estimate of 0.65 +/- 0.25 TgBCyr-1. Chinese emissions were further estimated at 166 +/- 65 TgCOyr-1 and 12.4 +/- 4.8 PgCO2yr-1 using the BC/CO and CO/CO2 ratios from observations. These results suggest HTAPv2.2z requires revisions of 50 % downward for BC and 20 % upward for CO, particularly over central East China during spring. Other inventories including Community Emissions Data System (CEDSv2021_02_05) remain notably biased, while Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants (ECLIPSEv6b) and Tropospheric Chemistry Reanalysis (TCR2) estimates fall within our identified uncertainty ranges.

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.

The Nord Stream pipeline leaks on 26 September 2022 released 465 ± 20 kt of methane into the atmosphere, which is the largest recorded transient anthropogenic methane emission event. While most of the gas escaped directly to the atmosphere, a fraction dissolved in the water. So far, studies on the fate of this dissolved methane rely on pipeline volumetric estimates or spatially sparse concentration measurements and ocean models. Here, we use atmospheric measurements with broad spatial coverage obtained from an airborne platform to estimate outgassing of 19-48 t h−1 on 5 October 2022. Our results broadly agree with ocean models but reveal uncertainties such as inaccuracies in their spatial emission distribution. Thus, we provide a data-driven constraint on the fate of the methane from the Nord Stream pipelines in the Baltic Sea. These results demonstrate the benefit of a fast-response airborne mission to track a dynamic methane emission event. Following the Nord Stream pipeline explosions in 2022, an uncertain amount of methane dissolved in the Baltic Sea. Here, Reum et al. estimate subsequent outgassing from a fast-response airborne survey, providing a constraint for dissolution models.

Abstract. The concentration of carbonaceous aerosols, black carbon (BC) and organic aerosol (OA), in the atmosphere is related to co-emitted or co-produced trace gases. In this study, we investigate the most relevant proportional relationships between both BC and OA with the following trace gases: carbon monoxide (CO), formaldehyde (HCHO), nitrogen dioxide (NO2), ozone (O3), and sulfur dioxide (SO2). One motivation for selecting these trace gases is that they can be observed using remote sensing measurements from satellite instrumentation, and could therefore be used to predict spatial changes in the amounts of BC and OA. Airborne measurements are optimal for the analysis of both the composition of aerosols and trace gases in different environments ranging from unpolluted oceanic air masses to those in heavily polluted city plumes. The two aircraft campaigns of the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) project have created a unique database, with flight plans dedicated to studying city plumes in two regions, Europe (2017) and East Asia (2018), along with identical instrumental payload. Using linear regression analysis, three relevant relationships between carbonaceous aerosol and trace gases are identified: - The BC/OA ratio observed in the Asian campaign is three times higher (≈ 0.3) than in the European campaign (≈ 0.1), whereas the Pearson correlation coefficient (R) between BC and OA is much higher in Europe (R ≈ 0.8) than in Asia (R ≈ 0.6). - The CO/BC ratio is also observed higher in the Asian campaign (≈ 240) than in the European campaign (≈ 170), whereas the R-value between CO and BC is similar for both campaigns (R ≈ 0.7). - The HCHO/OA ratio is similar in both campaigns (≈0.32), but the observed R-values between HCHO and OA is higher in Europe than in the Asia (R ≈ 0.7 compared to ≈ 0.3). By focusing on heavily polluted air masses sampled downwind in the city plumes, the ratios between the observed carbonaceous aerosols and the five trace gases change, and the R-values increase with O3 for both BC and OA (R ≈ 0.5). To assess the performance of atmospheric models with respect to the most relevant observed relationships, an air quality model ensemble is used to represent the current state of atmospheric modeling, consisting of two global and two regional simulations. The evaluation shows that these proportional relationships are not satisfactorily reproduced by the model ensemble. The relationships between BC and OA or between CO and BC are modeled with stronger correlations than the observed ones, and their higher ratios observed in Asia compared to Europe are not reproduced. Furthermore, the modeled HCHO/OA ratio is underestimated in the Asian campaign and overestimated in the European campaign. This analysis of the proportional relationships between carbonaceous aerosols and trace gases implies that the observed relationships can be used to constrain models and improve anthropogenic emission inventories. In addition, it implies that information about the lower tropospheric concentration of carbonaceous aerosols can potentially be inferred from satellite retrievals of trace gases, particularly in the plumes from megacities.

Abstract. An air quality model ensemble is used to represent the current state-of-the-art in atmospheric modeling, composed of two global forecasts and two regional simulations. The model ensemble assessment focuses on both carbonaceous aerosols, i.e. black carbon (BC) and organic aerosol (OA), and five trace gases during two aircraft campaigns of the EMeRGe (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global Scales) project. These campaigns, designed with similar flight plans for Europe and Asia, along with identical instrumentation, provide a unique opportunity to evaluate air quality models with a specific focus on city plumes. The observed concentration ranges for all pollutants are reproduced by the ensemble in the various environments sampled during the EMeRGe campaigns. The evaluation of the air quality model ensemble reveals differences between the two campaigns, with carbon monoxide (CO) better reproduced in East Asia, while other studied pollutants exhibit a better agreement in Europe. These differences may be associated to the modeling of biomass burning pollution during the EMeRGe Asian campaign. However, the modeled CO generally demonstrates good agreement with observations with a correlation coefficient (R) of ≈ 0.8. For formaldehyde (HCHO), nitrogen dioxide (NO2), ozone (O3) and BC the agreement is moderate (with R ranging from 0.5 to 0.7), while for OA and SO2 the agreement is weak (with R ranging from 0.2 to 0.3). The modeled wind speed shows very good agreement (R ≈ 0.9). This supports the use of modeled pollutant transport to identify flight legs associated with pollution originating from major population centers targeted among different flight plans. City plumes are identified using a methodology based on numerical tracer experiments, where tracers are emitted from city centers. This approach robustly localizes the different city plumes in both time and space, even after traveling several hundred kilometers. Focusing on city plumes, the fractions of high concentration are overestimated for BC, OA, HCHO, and SO2, which degrades the performance of the ensemble. This assessment of air quality models with collocated airborne measurements provides a clear insight into the existing limitations in modeling the composition of carbonaceous aerosols and trace gases, especially in city plumes.

The quickly developing drone technology can be used efficiently in the field of pipeline leak detection. The aim of this article is to provide drone mission concepts for detecting releases from pipelines. It provides an overview of the current applications of natural gas pipeline surveys, it considers environmental conditions by plume modelling, it discusses suitable commercially available sensors, and develops concepts for routine monitoring of pipelines and short term missions for localising and identifying a known leakage. Suitable platforms depend on the particular mission and requirements concerning sensors and legislation. As an illustration, a feasibility study during a release experiment is introduced. The main challenge of this study was the variability of wind direction on a time scale of minutes, which produces considerable differences to the plume simulations. Nevertheless, the leakage rates derived from the observations are in the same order of magnitude as the emission rates. Finally the results from the modeling, the release experiment and possible drone scenarios are combined and requirements for future application derived.

Abstract. Accurate estimates of short-lived climate forcers (SLCFs) emissions are required to allow efficient strategies that mitigate climate change to be developed. However, there remain large uncertainties about emissions of SLCFs from Asia. We identified and improved the constraints of combustion-related emissions of black carbon (BC) and CO using the WRF/CMAQ model (v5.0.2) and the EMeRGe airborne observation data for East Asia in spring 2018. We performed case studies of air masses containing emissions from fires near Thailand and emissions from urban areas in the Philippines and China. Chinese emissions were analysed in depth. Unlike observations at ground-based stations, the observations from aircraft used here would not have been strongly influenced by local emissions and near-surface processes. We confirmed that the GFEDv4.1s inventory provided accurate data for emissions from fires near Thailand. However, anthropogenic BC and CO emissions from the Philippines (REASv2.1) were negatively biased. Marked positive and negative differences were found for BC (+1.62 µg m−3) and CO (−400 ppbv) from the HTAPv2.2z emission inventory for Chinese air masses, consistent with the results of previous ground-based studies. The Chinese BC/CO emission ratio, 3.5±0.1 ng m−3 ppb−1, calculated using data from airborne observations in the altitude range ~0.3–1 km also agreed with the ground-based results. Linearly scaling BC emissions using an observation/model ratio (E(BC) = 0.48±0.13) gave our best estimate of 0.65±0.25 (Tg BC) yr-1. The calculated BC/CO and CO/CO2 ratios led us to estimate that emissions from China are 166±65 (Tg CO) yr-1 and 12.4±4.8 (Pg CO2) yr-1. The results suggested that downward and upward revisions of Chinese emissions of BC (−50 %) and CO (+20 %), respectively, are required in HTAPv2.2z emission inventory.

Recent atmospheric methane concentrations show an accelerated increase, but the contributions of the underlying emitters are poorly understood. Recording the stable carbon isotope ratio of methane (δ13C(CH4)) is a powerful tool for CH4 source attribution and the understanding of the global methane budget. The airborne measurement of δ13C(CH4) provides the advantages of reaching remote areas and covering large-scale regions, but is challenging regarding sufficient precision while maintaining high spatial measurement density. The state of the art technique is to collect airborne gas samples for subsequent laboratory analysis by isotope ratio mass spectrometry, with high δ13C(CH4) precision of 0.05 ‰. Here we present an innovative in situ airborne system for the measurement of δ13C(CH4), called MIRACLE. MIRACLE consists of a conventional Picarro cavity ring down greenhouse gas analyzer (G2210-i) for the measurement of CH4 and δ13C(CH4), and a sampler unit. The sampler enables the collection of six gas samples in 2 l stainless steel tanks, in a short time (20 s each) via a metal bellows pump, which allows for the specific sampling of small-scale features, such as point source emissions. The sampling is followed by an extended period of subsequent analysis (up to 10 min). Using this setup, we achieve sufficient δ13C(CH4) precision (1σ uncertainty of 0.34 ‰) and an average of five samples per flight hour, allowing for a large number of samples for long flights. Due to the resulting dense coverage with sufficient precision, this novel approach allows for airborne δ13C(CH4) characterization of small-scale methane emitters and large-scale gradients. We employed MIRACLE aboard the research aircraft HALO during the CoMet 2.0 Arctic campaign in summer 2022, which focused on characterizing natural and anthropogenic methane sources in Canada. In this presentation, a proof of concept for the instrument is elaborated, including the investigation of sample purity and measurement comparisons with other instruments. Additionally, we show δ13C(CH4) signatures revealed by the method of Keeling analysis of measurements obtained during CoMet 2.0 and compare them to previous studies. The airborne operation of the MIRACLE instrument combines the advantages of increased precision δ13C(CH4) measurements, typically only possible under stable laboratory conditions, with the in situ, near real time data analysis and the large-scale sampling of secluded areas. MIRACLE will be deployed during the DLR GHGMon campaign (June 2023) to investigate the δ13C(CH4) ratio of agricultural sources of methane in the Netherlands.

The Upper Silesian Coal Basin in southern Poland belongs to one of the strongest emitting regions of anthropogenic methane (CH4) in Europe. A major part of these CH4 emissions is related to the coal mining industry, which are in focus of the METHANE-To-Go-Poland project presented here. For the first time, a unique helicopter towed probe (HELiPOD) was used to capture CH4 plumes from selected coal mine ventilation shafts. The HELiPOD probe (weight 325 kg, length 5 m) was equipped with a 3D wind anemometer and trace gas 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. In June and October 2022, repeated upwind and downwind probing of the plumes from selected shafts (4 shafts, 16 flights) were performed at different horizontal distances from the source (~500 m - 5 km) and altitudes (~20 m – 2 km). This way, both the inflow amount of CH4 and the horizontal/vertical dispersion of the CH4 plumes from the shafts were captured. Depending on wind speed, wind direction and stability, suitable flight patterns were developed for every flight. In addition, two controlled CH4 releases were successfully carried out to prove the novel measurement concept. Mobile ground-based CH4 measurements complemented the airborne probing.In this presentation, mass flux calculations based on measurements from the two airborne CH4 instruments (with different measurement techniques) will be compared and uncertainties determined. Furthermore, CH4 mass flux calculations resulting from coinciding satellite measurements (GHGSat: swath width <15 km, spatial resolution <27 m) over the same ventilation shafts combined with high-resolved GEOS-FP wind data are presented. Finally, the uncertainties of the two different top-down approaches (air- and satellite-borne) are compared, in addition to different flight strategies. Comparisons with production data from the Polish coal mine industry are foreseen in near future (bottom-up approach). Subsequently, the same kind of airborne concept is envisaged for the METHANE-To-Go-Oman field experiment in autumn 2023, which will focus on CH4 emissions from the on-shore oil and gas exploration and production in Oman. Our collected data, funded by the International Methane Emissions Observatory (IMEO), will help coal, oil and gas companies as well as governments, to prioritize their CH4 emission mitigation strategies, actions and policies.

The sabotage of the Nord Stream pipelines on 26 September 2022 led to the largest event of methane venting to the atmosphere on record. The pipelines contained about 300 000 tonnes of methane, and an estimate based on Europe's ICOS network quantified the emissions to the atmosphere at 90 000-300 000 tonnes of methane in the first days of the event (Ramonet et al., 2022). The vast majority of methane that vented from the pipelines into the water likely escaped to the atmosphere near-instantly via bubble transport, which had largely ceased by 1 October 2022. However, a fraction dissolved into the water. To investigate the possibility of a "long tail" of release of this dissolved methane to the atmosphere, we conducted airborne surveys of the leak area on 5 October 2022. Methane data were recorded with a Picarro G2401m onboard the HELiPOD platform, a drag probe attached to a helicopter with a rope, down to 30 m above sea level. We observed methane enhancements of up to 300 ppb above atmospheric background, in an area about 30 km both up- and downwind of the leak locations. Using an inverse model of atmospheric transport, we show that the atmospheric data can be explained by an area source and estimate a source strength on the order of tens of tonnes of methane per hour on the day of observations. To better understand the spatial distribution of the emissions, especially emissions upwind of the leak locations, we further run a model of oceanic transport for tracers released at the leak locations and couple it to a Wanninkhof-model of diffusive emissions. The areal emission distribution we find with this model has some similarities to the emission pattern retrieved using the airborne measurements. We conclude that a significant amount of methane was dissolved in the Baltic Sea during the outgassing event following the Nord Stream explosions. Methane that was initially dissolved in the surface layer still escaped to the atmosphere days after the initial outgassing event.

The number of large urban agglomerations is steadily increasing worldwide. At a local scale, their emissions lead to air pollution, directly affecting people's health. On a global scale, their emissions lead to an increase of greenhouse gases, affecting climate. In this context, in 2017 and 2018, the airborne campaign EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) investigated emissions of European and Asian major population centres (MPCs) to improve the understanding and predictability of pollution outflows. Here, we present two methods to identify and characterise pollution outflows probed during EMeRGe. First, we use a set of volatile organic compounds (VOCs) as chemical tracers to characterise air masses by specific source signals, i.e. benzene from anthropogenic pollution of targeted regions, acetonitrile from biomass burning (BB, primarily during EMeRGe-Asia), and isoprene from fresh biogenic signals (primarily during EMeRGe-Europe. Second, we attribute probed air masses to source regions and estimate their individual contribution by constructing and applying a simple emission uptake scheme for the boundary layer which combines FLEXTRA back trajectories and EDGAR carbon monoxide (CO) emission rates (acronyms are provided in the Appendix). During EMeRGe-Europe, we identified anthropogenic pollution outflows from northern Italy, southern Great Britain, the Belgium–Netherlands–Ruhr (BNR) area and the Iberian Peninsula. Additionally, our uptake scheme indicates significant long-range transport of pollution from the USA and Canada. During EMeRGe-Asia, the pollution outflow is dominated by sources in China and Taiwan, but BB signals from Southeast Asia and India contribute as well. Outflows of pre-selected MPC targets are identified in less than 20 % of the sampling time, due to restrictions in flight planning and constraints of the measurement platform itself. Still, EMeRGe combines in a unique way near- and far-field measurements, which show signatures of local and distant sources, transport and conversion fingerprints, and complex air mass compositions. Our approach provides a valuable classification and characterisation of the EMeRGe dataset, e.g. for BB and anthropogenic influence of potential source regions and paves the way for a more comprehensive analysis and various model studies.

During winter 2015/2016, the Arctic stratosphere was characterized by extraordinarily low temperatures in connection with a very strong polar vortex and with the occurrence of extensive polar stratospheric clouds. From mid-December 2015 until mid-March 2016, the German research aircraft HALO (High Altitude and Long-Range Research Aircraft) was deployed to probe the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar Stratosphere in a Changing Climate) mission. More than 20 flights have been conducted out of Kiruna, Sweden, and Oberpfaffenhofen, Germany, covering the whole winter period. Besides total reactive nitrogen (NOy), observations of nitrous oxide, nitric acid, ozone, and water were used for this study. Total reactive nitrogen and its partitioning between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. The vertical redistribution of total reactive nitrogen was evaluated by using tracer–tracer correlations (NOy–N2O and NOy–O3). The trace gases are well correlated as long as the NOy distribution is controlled by its gas-phase production from N2O. Deviations of the observed NOy from this correlation indicate the influence of heterogeneous processes. In early winter no such deviations have been observed. In January, however, air masses with extensive nitrification were encountered at altitudes between 12 and 15 km. The excess NOy amounted to about 6 ppb. During several flights, along with gas-phase nitrification, indications for extensive occurrence of nitric acid containing particles at flight altitude were found. These observations support the assumption of sedimentation and subsequent evaporation of nitric acid-containing particles, leading to redistribution of total reactive nitrogen at lower altitudes. Remnants of nitrified air masses have been observed until mid-March. Between the end of February and mid-March, denitrified air masses have also been observed in connection with high potential temperatures. This indicates the downward transport of air masses that have been denitrified during the earlier winter phase. Using tracer–tracer correlations, missing total reactive nitrogen was estimated to amount to 6 ppb. Further, indications of transport and mixing of these processed air masses outside the vortex have been found, contributing to the chemical budget of the winter lowermost stratosphere. Observations within POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from the overlying Arctic winter stratosphere. The comparison of the observations with CLaMS model simulations confirm and complete the picture arising from the present measurements. The simulations confirm that the ensemble of all observations is representative of the vortex-wide vertical NOy redistribution.

Table S1.Equipment information of observations used

Abstract. Sulfate is among the major components of atmospheric aerosols or fine particulate matters. Aerosols loaded with sulfate result in low air quality, damage to ecosystems, and influences on climate change. Sulfate aerosols could originate from that directly emitted to the atmosphere and that produced by atmospheric physicochemical processes. The latter is generated from sulfur dioxide (SO2) via oxidation either in the gas phase reactions or in the aqueous phase. Several mechanisms of SO2 oxidation have been proposed, but the differentiation of the various mechanisms and identification of the sources remain challenging. To meet this need, a new method to estimate the contribution of the gas-phase reaction between hydroxyl radical (OH) and SO2 to the sulfate aerosol is proposed and investigated. Briefly, we consider the OH-reaction rates of the respective trace gases that compete for OH radicals with SO2 in the troposphere, and estimate the fraction of SO2-OH reaction in the total OH reactivity. Then the relationship between sulfate concentration and the SO2-OH reaction is analyzed statistically to investigate the sources of sulfate in aerosols. We test this method using the data from ground-based observations and aircraft measurements made during the Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales in Asia (EMeRGe-Asia) over the western Taiwan and West Pacific regions. Our results show that the estimated SO2-OH reactivity fraction is well-correlated with sulfate concentration. The sulfate production from SO2-OH reaction accounts for approximately 30 % of the total sulfate in aerosols collected at the surface and near-surface (altitude < 600 m) in our study area, comparable to the estimates from other model simulations. Within its assumptions and limitations, this new method provides a valuable approach to investigate the significance of SO2-OH reaction regionally and globally.

We report on airborne measurements of tropospheric mixing ratios and vertical profiles of formaldehyde (CH2O), glyoxal (C2H2O2), methylglyoxal and higher carbonyls (C3H4O2*) (see below), and carbon monoxide (CO) over the Amazon Basin during the ACRIDICON-CHUVA campaign from the German High Altitude and Long-range research aircraft (HALO) in autumn 2014. The joint observation of in situ CO and remotely measured CH2O, C2H2O2, and C3H4O2*, together with visible imagery and air mass back-trajectory modelling using NOAA HYSPLIT (National Oceanic Atmospheric Administration, HYbrid Single-Particle Lagrangian Integrated Trajectory), allows us to discriminate between the probing of background tropical air, in which the concentration of the measured species results from the oxidation of biogenically emitted volatile organic compounds (VOCs, mostly isoprene), and measurements of moderately to strongly polluted air masses affected by biomass burning emissions or the city plume of Manaus. For 12 near-surface measurements of fresh biomass burning plumes, normalized excess mixing ratios of C2H2O2 and C3H4O2* with respect to CH2O are inferred and compared to recent studies. The mean glyoxal-to-formaldehyde ratio RGF=0.07 (range 0.02–0.11) is in good agreement with recent reports which suggest RGF to be significantly lower than previously assumed in global chemical transport models (CTMs). The mean methylglyoxal-to-formaldehyde ratio RMF=0.98 (range 0.09–1.50) varies significantly during the different observational settings but overall appears to be much larger (up to a factor of 5) than previous reports suggest even when applying a correction factor of 2.0±0.5 to account for the additional dicarbonyls included in the C3H4O2* measurements. Using recently reported emission factors of CH2O for tropical forests, our observations suggest emission factors of EFG=0.25 (range 0.11 to 0.52) g kg−1 for C2H2O2 and EFM = 4.7 (range 0.5 to 8.64) g kg−1 for C3H4O2*. While EFG agrees well with recent reports, EFM is (like RMF) slightly larger than reported in other studies, presumably due to the different plume ages or fuels studied. Our observations of these critical carbonyls and intermediate oxidation products may support future photochemical modelling of air pollution over tropical vegetation, as well as validate past and present space-borne observations of the respective species.

Column-averaged dry-air mole fractions of carbon dioxide (XCO2) and methane (XCH4) measured by a solar viewing portable Fourier transform spectrometer (FTS, EM27/SUN) have been characterized and validated by comparison using in situ profile measurements made during the transfer flights of two aircraft campaigns: Korea-United States Air Quality Study (KORUS-AQ) and Effect of Megacities on the Transport and Transformation of Pollutants at Regional and Global Scales (EMeRGe). The aircraft flew over two Total Carbon Column Observing Network (TCCON) sites: Rikubetsu, Japan (43.46∘ N, 143.77∘ E), for the KORUS-AQ campaign and Burgos, Philippines (18.53∘ N, 120.65∘ E), for the EMeRGe campaign. The EM27/SUN was deployed at the corresponding TCCON sites during the overflights. The mole fraction profiles obtained by the aircraft over Rikubetsu differed between the ascending and the descending flights above approximately 8 km for both CO2 and CH4. Because the spatial pattern of tropopause heights based on potential vorticity values from the ERA5 reanalysis shows that the tropopause height over the Rikubetsu site was consistent with the descending profile, we used only the descending profile to compare with the EM27/SUN data. Both the XCO2 and XCH4 derived from the descending profiles over Burgos were lower than those from the ascending profiles. Output from the Weather Research and Forecasting Model indicates that higher CO2 for the ascending profile originated in central Luzon, an industrialized and densely populated region about 400 km south of the Burgos TCCON site. Air masses observed with the EM27/SUN overlap better with those from the descending aircraft profiles than those from the ascending aircraft profiles with respect to their properties such as origin and atmospheric residence times. Consequently, the descending aircraft profiles were used for the comparison with the EM27/SUN data. The EM27/SUN XCO2 and XCH4 data were derived by using the GGG2014 software without applying air-mass-independent correction factors (AICFs). The comparison of the EM27/SUN observations with the aircraft data revealed that, on average, the EM27/SUN XCO2 data were biased low by 1.22 % and the EM27/SUN XCH4 data were biased low by 1.71 %. The resulting AICFs of 0.9878 for XCO2 and 0.9829 for XCH4 were obtained for the EM27/SUN. Applying AICFs being utilized for the TCCON data (0.9898 for XCO2 and 0.9765 for XCH4) to the EM27/SUN data induces an underestimate for XCO2 and an overestimate for XCH4.