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Evaluating the global chemistry models in the upper troposphere – lower stratosphere (UTLS) is an important step toward a further understanding of its chemical composition. The latter is regularly sampled through in situ measurements based on passenger aircraft, in the framework of the In-service Aircraft for a Global Observing System (IAGOS) research infrastructure. This study focuses on the comparison of the IAGOS measurements in ozone, carbon monoxide (CO), nitrogen reactive species (NOy) and water vapour, with a 25-year simulation output from the LMDZ-OR-INCA chemistry-climate model. For this purpose, we present and apply an extension of the Interpol-IAGOS software that projects the IAGOS data onto any model grid, in order to derive a gridded IAGOS product and a masked model product that are directly comparable to one another. Climatologies are calculated in the upper troposphere (UT) and in the lower stratosphere (LS) separately, but also in the UTLS as a whole, as a demonstration for the models that do not sort out the physical variables necessary to distinguish between the UT and the LS. In the northern extratropics, the comparison in the UTLS layer suggests that the geographical distribution in the tropopause height is well reproduced by the model. In the separated layers, the model simulates well the water vapour climatologies in the UT, and the ozone climatologies in the LS. The opposite biases in CO in both UT and LS suggest that the cross-tropopause transport is overestimated. The NOy observations highlight the difficulty of the model in parameterizing the lightning emissions. In the tropics, the upper-tropospheric climatologies are remarkably well simulated for water vapour, as the observed CO peaks due to biomass burning in the most convective systems, and the ozone latitudinal variations. Ozone is more sensitive to lightning emissions than to biomass burning emissions, whereas the CO sensitivity to biomass burning emissions strongly depends on the location and on the season. Through this evaluation, the present study demonstrates that the Interpol-IAGOS software is a tool facilitating the assessment of the global model simulations in the UTLS, potentially useful for any modelling experiment involving chemistry-climate and chemistry-transport models.

Tropospheric ozone is an important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Previous studies, using models, aircraft and remote observation datasets, have shown that the tropospheric ozone has increased significantly in the tropical regions. Sensitivities studies have also showed that the ozone precursor emissions in these tropical regions have been increasing for the past three decades. For this paper, we will work with worldwide scientists to investigate how the emission evolves in the tropical regions from 1995 to 2019, and how these changes have contributed to the global and other receptor regions tropospheric ozone burden increases, by using ensemble state-of-the-art global and regional chemical transport models.

As part of the Ozone and Precursors in the Tropics (OPT) working group of the Tropospheric Ozone Assessment Report Phase 2 (TOAR-II), we present the first results on the distribution of tropospheric ozone (O3) and its precursors (carbon monoxide, CO; formaldehyde, HCHO; nitrogen dioxide, NO2) in the tropics over the past 20-25 years. The goal is to give an overview of the seasonal, geographical and vertical variabilities of tropical tropospheric O3 and its precursors. To do so, we use in situ measurements of O3 and its precursors from surface sites, sounding balloons (SHADOZ) and instrumented aircraft (IAGOS and ATOM), as well as ground-based (FTIR) and spatial (IASI, OMI, GOME-2) remote-sensed observations. The observations are averaged monthly over the longest available time-period as well as over the first five years of the time period. The results for these two time periods give the context to interpret distributions and variabilities of O3 and its precursors over the most recent five years as we call it “Present-day”. Special emphasis is given to the differences of O3 and its precursors’ distributions between remote and polluted regions and to the relationships between the gaseous species. Model output will be included to fill gaps in space and time when necessary to help the interpretation of the analysis based on observations.From IAGOS measurements, the highest O3 and CO maxima occur in the lower troposphere of Northern Hemisphere Africa, which remains the most influenced by biomass burning. CO maxima are attributed using SOFT-IO model to anthropogenic emissions by 60%. Second maxima are observed in the lower troposphere of Asia, mostly due to anthropogenic emissions. The highest amount of transported CO in the tropics originates from Africa.

As part of the Tropospheric Ozone Assessment Report Phase 2 (TOAR-II), the Ozone and Precursors in Tropics (OPT) working group counts three deliverables dedicated to quantifying 1) the distribution, 2) the trends of tropospheric ozone (O3) and its precursors (carbon monoxide, CO; formaldehyde, HCHO; nitrogen dioxide, NO2) in the tropics over the past 20-25 years, and 3) their impacts on a global scale. This presentation will focus on the trends estimate. We answer the following scientific questions: How have tropical tropospheric ozone and its precursors changed with time? What sources (e.g. anthropogenic emissions, biomass burning, lightning) drive these trends and to what extent?To accomplish this, we use both observations and model output. The observations include in situ measurements of O3 and its precursors from surface sites, sounding balloons (SHADOZ) and instrumented aircraft (IAGOS), as well as ground-based (FTIR) and spatial (IASI, OMI, GOME-2) remote-sensed observations. Global model output come from ECHAM6-HAMMOZ, LMDZ-OR-INCA, MIROC-CHASER, CAM4-Chem, GISS-E2 and the CESM2-WACCM6 ensemble.The trends estimates are based on monthly anomalies and are calculated after considering climate variabilities such as El Niño- Southern oscillation (ENSO) and quasi-biennial oscillation (QBO).From IAGOS and SHADOZ ozone profiles, we estimate positive ozone trends between 1994 and 2019 throughout the troposphere above the Americas, Africa, India, Southeast Asia and Malaysia/Indonesia. Trends may reach up to 6 ± 1.6 ppb/decade in the free troposphere and up to 12.5 ± 2 ppb/decade in the boundary layer. There is also considerable regional variability. For example, trends are +0 to 4 ppb/decade in the free troposphere above the remote Pacific and Atlantic SHADOZ stations (1998-2019). According to OMI satellite retrievals, tropospheric ozone burden increases between 2004 and 2021 across the tropical latitude band (20˚N-20˚S) and the trends range between 0.09 Tg/yr (OMI CCD retrieval in the southern hemisphere) and 0.3 Tg/year (OMI/MLS retrieval in the northern hemisphere).The presentation will also include trends estimates of observed ozone’s precursors such as CO, NO2 and formaldehyde as well as trends estimate of ozone and its precursors from model output.

The data consists of output from the GEOS-Chem model. Two sets of simulations were performed:Full Chemistry and Tagged ozone simulation. We used ozone and stratospheric ozone variables for the days 4June 2007, 25 May 2009, 11 October 2013 and 9 December 2016.

The characteristics and seasonal variability of the tropical tropospheric distributions of ozone (O3) and carbon monoxide (CO) were analysed based on in situ measurements provided by the In-service Aircraft for a Global Observing System (IAGOS) program since 1994 and 2002 respectively, combined with observations from the Infrared Atmospheric Sounding (IASI) instrument on board the Met-op A satellite since 2008. The SOFT-IO model, which couples back trajectories with CO emissions inventories, was used to explore the origins and sources of the tropical CO observed by IAGOS. The highest tropical O3 and CO maxima occur over Northern Hemisphere (NH) Africa in the low troposphere (LT) (80 ppb and 850 ppb respectively at 2.5 km over Lagos) during the dry season (January). Despite the active local fires, local anthropogenic (AN) emissions (60 %) are dominant for the CO, and consequently the O3 maxima. The importance of the local AN emissions are highlighted over Central Africa, as they cause a persistent polluted surface layer during the transition seasons (40 % in October and 86 % in April). The second highest O3 and CO maxima are observed over Asia. Local or regional Asian AN emissions cause the CO maximum in the LT (0.5 km) in January, and the O3 maximum in the free troposphere (at 6 km) in the post-monsoon season (April). South China is the only Asian site where O3 peaks in the LT (75 ppb at 2.5 km), due to local fires (30 %) in addition to the local (52 %) and regional (15 %) AN emissions. The highest amount of transported CO in the tropics originates from Africa. The main transport pathway is from the dry-season African regions towards the wet-season ones. Contributions from NH Africa are found over Arabia and Eastern Africa (up to 70 %), and India (40 % in the mid (MT) and 60 % in the upper (UT) troposphere) during the dry season. Transport towards NH South America is found all year long, with significant contributions in the MT and UT (30–40 % over Caracas on an annual basis). In contrast, the impact of the Asian emissions in the LT and MT is limited on a local or regional scale. Export of polluted Asian air masses is important in the UT during the Asian summer monsoon and post-monsoon seasons, when convection is active. The AN Asian contributions are mostly found over Arabia and Eastern Africa (up to 80 %) during the Asian summer monsoon. During the post-monsoon, CO impacted by the Indonesian fires (resp. SouthEast Asian AN emissions) are transported towards Eastern Africa (64 % and 16 %) due to the Tropical Easterly Jet. The lowest O3 and CO levels are observed over South America, due to less strong local emissions in comparison to Asia and Africa. The only important CO and O3 enhancement is observed in the MT during the local fires (October), when O3 and precursors impacted by the local AN and fire emissions are trapped in an anticyclone and transported towards South Africa (5–10 ppb from SH and NH South America respectively).

Observational records of meteorological and chemical variables are imprinted by an unknown combination of anthropogenic activity, natural forcings, and internal variability. With a 15-member initial-condition ensemble generated from the CESM2-WACCM6 chemistry-climate model for 1950-2014, we extract signals of anthropogenic (‘forced’) change from the noise of internally arising climate variability on observed tropospheric ozone trends. Positive trends in free tropospheric ozone measured at long-term surface observatories, by commercial aircraft, and retrieved from satellite instruments generally fall within the ensemble range. CESM2-WACCM6 tropospheric ozone trends are also bracketed by those in a larger ensemble constructed from five additional chemistry-climate models. Comparison of the multi-model ensemble with observed tropospheric column ozone trends in the northern tropics implies an underestimate in regional precursor emission growth over recent decades. Positive tropospheric ozone trends clearly emerge from 1950 to 2014, exceeding 0.2 DU yr-1 at 20-40N in all CESM2-WACCM6 ensemble members. Tropospheric ozone observations are often only available for recent decades, and we show that even a two-decade record length is insufficient to eliminate the role of internal variability, which can produce regional tropospheric ozone trends oppositely signed from ensemble mean (forced) changes. By identifying regions and seasons with strong anthropogenic change signals relative to internal variability, initial-condition ensembles can guide future observing systems seeking to detect anthropogenic change. For example, analysis of the CESM2-WACCM6 ensemble reveals year-round upper tropospheric ozone increases from 1995-2014, largest at 30S-40N during boreal summer. Lower tropospheric ozone increases most strongly in the winter hemisphere, but internal variability leads to trends of opposite sign (ensemble overlaps zero) north of 40N during boreal summer. This decoupling of ozone trends in the upper and lower troposphere suggests a growing prominence for tropospheric ozone as a greenhouse gas despite regional efforts to abate warm season ground-level ozone.

Quantification and attribution of long-term tropospheric ozone trends are critical for understanding the impact of human activity and climate change on atmospheric chemistry but are also challenged by the limited coverage of long-term ozone observations in the free troposphere where ozone has higher production efficiency and radiative potential compared to that at the surface. In this study, we examine observed tropospheric ozone trends, their attributions, and radiative impacts from 1995–2017 using aircraft observations from the In-service Aircraft for a Global Observing System database (IAGOS), ozonesondes, and a multi-decadal GEOS-Chem chemical model simulation. IAGOS observations above 11 regions in the Northern Hemisphere and 19 of 27 global ozonesonde sites have measured increases in tropospheric ozone (950–250 hPa) by 2.7 ± 1.7 and 1.9 ± 1.7 ppbv per decade on average, respectively, with particularly large increases in the lower troposphere (950–800 hPa) above East Asia, the Persian Gulf, India, northern South America, the Gulf of Guinea, and Malaysia/Indonesia by 2.8 to 10.6 ppbv per decade. The GEOS-Chem simulation driven by reanalysis meteorological fields and the most up-to-date year-specific anthropogenic emission inventory reproduces the overall pattern of observed tropospheric ozone trends, including the large ozone increases over the tropics of 2.1–2.9 ppbv per decade and above East Asia of 0.5–1.8 ppbv per decade and the weak tropospheric ozone trends above North America, Europe, and high latitudes in both hemispheres, but trends are underestimated compared to observations. GEOS-Chem estimates an increasing trend of 0.4 Tg yr−1 of the tropospheric ozone burden in 1995–2017. We suggest that uncertainties in the anthropogenic emission inventory in the early years of the simulation (e.g., 1995–1999) over developing regions may contribute to GEOS-Chem's underestimation of tropospheric ozone trends. GEOS-Chem sensitivity simulations show that changes in global anthropogenic emission patterns, including the equatorward redistribution of surface emissions and the rapid increases in aircraft emissions, are the dominant factors contributing to tropospheric ozone trends by 0.5 Tg yr−1. In particular, we highlight the disproportionately large, but previously underappreciated, contribution of aircraft emissions to tropospheric ozone trends by 0.3 Tg yr−1, mainly due to aircraft emitting NOx in the mid-troposphere and upper troposphere where ozone production efficiency is high. Decreases in lower-stratospheric ozone and the stratosphere–troposphere flux in 1995–2017 contribute to an ozone decrease at mid-latitudes and high latitudes. We estimate the change in tropospheric ozone radiative impacts from 1995–1999 to 2013–2017 is +18.5 mW m−2, with 43.5 mW m−2 contributed by anthropogenic emission changes (20.5 mW m−2 alone by aircraft emissions), highlighting that the equatorward redistribution of emissions to areas with strong convection and the increase in aircraft emissions are effective for increasing tropospheric ozone's greenhouse effect.

SOFT-IO-LI is a new tool merging space and ground based lightning observations and aircraft NOx measurements to provide a database of new parameters characterizing lightning-NOx air masses observed at the global scale, to the scientific community.The tool takes lightning-NOx air masses measured by IAGOS (In-service Aircraft for a Global Observing System) a European Research Infrastructure for global observations of atmospheric composition using commercial aircraft. FLEXPART the Lagrangian transport and dispersion model is used, together with ERA5 reanalysis weather data from ECMWF, to perform a backward trajectory of these air masses, resulting in hourly mass residence times for lightning-NOx particles for the days prior to the flight measurements.We will present, for IAGOS transatlantic flights, the comparison of these mass residence times with ground based NLDN (National Lightning Detection Network) and space based GLM (Global Lightning Mapper) lightning observations, as well as with ABI (Advanced Baseline Imager) cloud data. We will show that SOFT-IO-LI is capable of relating lightning events and chemical species observed in situ, in order to determine characteristics of the different lightning-related air masses.

IAGOS (www.iagos.org) is a European research infrastructure using commercial aircraft to measure the atmospheric composition. In particular, IAGOS provides regular carbon monoxide (CO) data since December 2001. In this study we use eighteen years of available data (from 2002 to 2019) to investigate CO anomalies throughout the entire flight i.e. vertical profiles over airports and upper troposphere/lower stratosphere (UTLS) at cruise altitude.IAGOS flight track is divided into four distinctive vertical groups: boundary layer, middle troposphere, upper troposphere and lower stratosphere. The entire IAGOS data set has been split in 18 regions according to the geographical variability (e.g. continents over northern mid-latitudes, tropics, etc ...) and the different seasonal cycles of CO. CO anomalies are defined as air masses with CO mixing ratios above the 95th/99th percentile of the regional/seasonal/vertical distribution. This unique data set allows us to look at the variety of CO anomalies between regions and seasons.Soft-io module which couples emission inventories and Lagrangian modelling along IAGOS flight track is used to quantify in which proportion those anomalies are linked to biomass burning and anthropogenic emissions.The origin of those events presents high seasonal discrepancies (drought season and cold season) but also inter-annual variabilities. Anomalies coming from anthropogenic sources hit the most heavily on the lower part of the atmosphere of densely populated areas. However, none of the region, whatever the altitude range, are spared by anthropogenic pollution. Anomalies coming from biomass burning present large regional variability caused by weather conditions and biomass differences. We quantified these local and temporal variabilities to better understand processes affecting CO anomalies in the troposphere and UTLS.

Cirrus clouds and their potential formation regions, so-called ice supersaturated regions (ISSRs), with values of relative humidity with respect to ice exceeding 100 %, occur frequently in the tropopause region. It is assumed that ISSRs and cirrus clouds can change the tropopause structure by diabatic processes, driven by latent heating due to phase transition and interaction with radiation. For many research questions, a three-dimensional picture including a sufficient temporal resolution of the water vapour fields in the tropopause region is required. This requirement is fulfilled nowadays by reanalysis products such as the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis. However, for a meaningful investigation of water vapour in the tropopause region, a comparison of the reanalysis data with measurement is advisable, since it is difficult to measure water vapour and to assimilate meaningful measurements into reanalysis products. Here, we present an intercomparison of high-resolution in situ measurements aboard passenger aircraft within the European Research Infrastructure IAGOS (In-service Aircraft for a Global Observing System; http://www.iagos.org, last access: 15 January 2020) with ERA-Interim. Temperature and humidity data over the North Atlantic from 2000 to 2009 are compared relative to the dynamical tropopause. The comparison of the temperature shows good agreement between the measurement and ERA-Interim. While ERA-Interim also shows the main features of the water vapour measurements of IAGOS, the variability of the data is clearly smaller in the reanalysis data set. The combination of temperature and water vapour leads to the relative humidity with respect to ice (RHi). Here, ERA-Interim deviates from the measurements concerning values larger than RHi=100 %, both in number and strength of supersaturation. Also, pathlengths of ISSRs along flight tracks are investigated, representing macrophysical properties as linked to atmospheric flows. The comparison of ISSR pathlengths shows distinct differences, which can be traced back to the spatial resolution of both data sets. Also, the seasonal cycle and height dependence of pathlengths changes for the different data sets due to their spatial resolution. IAGOS shows a significantly greater amount of smaller ISSRs compared to ERA-Interim. Good agreement begins only at pathlengths on the order of the ERA-Interim spatial resolution and larger.

The European Research Infrastructure IAGOS (In-service Aircraft for a Global Observing System) equips commercial aircraft with a system for measuring atmospheric composition. A range of essential climate variables and air quality parameters are measured throughout the flight, from take-off to landing, giving high resolution information in the vertical in the vicinity of international airports, and in the upper-troposphere/lower-stratosphere during the cruise phase of the flight. Six airlines are currently involved in the programme, achieving a quasi-global coverage under normal circumstances. During the COVID-19 crisis, many airlines were forced to ground their fleets due to a fall in passenger numbers and imposed travel restrictions. Deutsche Lufthansa, a partner in IAGOS since 1994 was able to operate a IAGOS-equipped aircraft during the COVID-19 lockdown, providing regular measurements of ozone and carbon monoxide at Frankfurt airport. The data form a snapshot of an unprecedented time in the 27 year time-series. In May 2020, we see a 39 % increase in ozone near the surface with respect to the 26 year climatology, a magnitude similar to that of the 2003 heatwave. The anomaly in May is driven by an increase in ozone at nighttime which might be linked to the reduction of NO during the COVID-19 lockdowns. The anomaly diminishes with altitude becoming a slightly negative anomaly in the free troposphere. The ozone precursor carbon monoxide shows an 11 % reduction in MAM near the surface. There is only a small reduction of CO in the free troposphere due to the impact of long-range transport on the CO from emissions in regions outside Europe. This is confirmed by IASI-SOFRID CO retrievals which display a clear drop of CO at 800 hPa over Europe in March but otherwise show little change to the abundance of CO in the free troposphere.

Eight years of upper tropospheric (UT) ice crystal measurements with the Backscatter Cloud Probe (BCP), operated as part of the In-Service Aircraft for a Global Observing System (IAGOS), have been analyzed to assess the frequency and characteristics of extreme ice crystal events (EIE), defined in this study as encounters with clouds that have number concentrations exceeding 5000 L−1. A total of 6490 events, in clouds of horizontal extent ≥ 2.5 km, were identified during the period from December 2011 to March 2020 in the global band between 30° S and 30° N. Evaluation of in situ measurements of carbon monoxide in these UT clouds, combined with back-trajectories and carbon monoxide emission inventories, allow the identification of regional anthropogenic sources. An evaluation of low- and upper-level kinematic variables from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), combined with spatial distribution of aerosol optical depth and regions of biomass burning, reveal the physical mechanism by which the particles are lofted to flight levels in regions of deep convection. The maps of lightning frequency derived from the World Wide Lightning Location Network (WWLLN) contribute additional evidence of the role of deep convection lofting ice forming aerosols to the UT. The MERRA-2 analysis shows clear spatial correlations that link dust, black carbon (BC), organic carbon (OC) and sulfate particles with regions of EIE. Given the composition of the source aerosols and the nature of their transport to the UT, the ice clouds are likely of liquid origin whereby droplets formed on these particles are lofted and freeze heterogeneously as have been reported in previous studies. The results from this study have important ramifications related to climate change, satellite measurement validation, weather forecasting and aircraft operations.

Reactive nitrogen in the upper troposphere (~8-12 km) impacts global climate, air quality and the oxidizing capacity of the whole troposphere. Here we use aircraft observations from instruments onboard the NASA DC8 aircraft for campaigns from 1997 (SONEX) to the recent ATom campaign (2016-2018) and the MOZAIC commercial aircraft campaign (2003-2005) to address uncertainties in the dynamics of reactive nitrogen (NOy = NOx + NOx reservoir compounds) in the global upper troposphere (UT). Our initial analysis of the DC8 aircraft observations is consistent with previous work in that PAN is the dominant NOy component (average: 43%; range: 40-60%), followed by NOx (on average, 21%), with smaller contributions (on average, 3.5-12.5%) from pernitric acid (HNO4), organonitrate (RONO2) and nitric acid (HNO3). We go on to compare multiyear mean NOy from MOZAIC to the combination of all NASA DC8 campaigns to determine whether we can build a near-global climatology of NOy and its components to compare to GEOS-Chem to assess our understanding of these very important atmospheric components.

Between December 2005 and 2013, the In-service Aircraft for a Global Observing System (IAGOS) program produced almost daily in situ measurements of CO and O3 between Europe and southern Africa. IAGOS data combined with measurements from the IASI instrument onboard the Metop-A satellite (2008–2013) are used to characterize meridional distributions and seasonality of CO and O3 in the African upper troposphere (UT). The FLEXPART particle dispersion model and the SOFT-IO model which combines the FLEXPART model with CO emission inventories are used to explore the sources and origins of the observed transects of CO and O3. We focus our analysis on two main seasons: December to March (DJFM) and June to October (JJASO). These seasons have been defined according to the position of Intertropical Convergence Zone (ITCZ), determined using in situ measurements from IAGOS. During both seasons, the UT CO meridional transects are characterized by maximum mixing ratios located 10° from the position of the ITCZ above the dry regions inside the hemisphere of the strongest Hadley cell (132 to 165 ppb at 0–5° N in DJFM and 128 to 149 ppb at 3–7° S in JJASO), and decreasing values south- and north-ward. The O3 meridional transects are characterized by mixing ratio minima of ~ 42–54 ppb at the ITCZ (10–16° S in DJFM and 5–8° N in JJASO) framed by local maxima (~ 53–71 ppb) coincident with the wind shear zones North and South of the ITCZ. O3 gradients are strongest in the hemisphere of the strongest Hadley cell. IASI UT O3 distributions in DJFM have revealed that the maxima are a part of a crescent-shaped O3 plume above the Atlantic Ocean around the Gulf of Guinea. CO emitted at the surface is transported towards the ITCZ by the trade winds and then convectively uplifted. Once in the upper troposphere, CO enriched air masses are transported away from the ITCZ by the upper branches of the Hadley cells and accumulate within the zonal wind shear zones where the maximum CO mixing ratios are found. Anthropogenic and fires both contribute, by the same order of magnitude, to the CO budget of the African upper troposphere. Local fires have the highest contribution, drive the location of the observed UT CO maxima, and are related to the following transport pathway: CO emitted at the surface is transported towards the ITCZ by the trade winds and further convectively uplifted. Then UT CO enriched air masses are transported away from the ITCZ by the upper branches of the Hadley cells and accumulate within the zonal wind shear zones where the maxima are located. Anthropogenic CO contribution is mostly from Africa during the entire year, with a low seasonal variability, and is related to similar transport circulation than fire air masses. There is also a large contribution from Asia in JJASO related to the fast convective uplift of polluted air masses in the Asian monsoon region which are further westward transported by the tropical easterly jet (TEJ) and the Asian monsoon anticyclone (AMA). O3 minima correspond to air masses that were recently uplifted from the surface where mixing ratios are low at the ITCZ. The O3 maxima correspond to old high altitude air masses uplifted from either local or long distance area of high O3 precursor emissions (Africa and South America during all the year, South Asia mainly in JJASO), and must be created during transport by photochemistry. This analysis of meridional transects contribute to a better understanding of distributions of CO and O3 in the intertropical African upper troposphere and the processes which drive these distributions. Therefore, it provides a solid basis for comparison and improvement of models and satellite products in order to get the good O3 for the good reasons.

The In-service Aircraft for a Global Observing System (IAGOS) is a European research infrastructure that equips the Airbus A340/330 with a system for monitoring atmospheric composition. The IAGOS instruments have three different configurations: IAGOS-Core, IAGOS – Measurement of Ozone and Water Vapor by Airbus In Service Aircraft (IAGOS-MOZAIC) and IAGOS – Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (IAGOS-CARIBIC). Since 1994, there have been a total of 17 aircraft equipped. In this study, we perform an intercomparison of about 8000 landing and takeoff profiles to compare the O3 and CO measurements performed from these different configurations. The collocated profiles used in the study met various selection criteria. The first was a maximal 1 h time difference between an ascent or descent by two different aircraft at the same airport and the second was a selection based on the similarity of air masses based on the meteorological data acquired by the aircraft. We provide here an evaluation of the internal consistency of the O3 and CO measurements since 1994. For both O3 and CO, we find no drift in the bias amongst the different instrument units (six O3 and six CO IAGOS-MOZAIC instruments, nine IAGOS-Core Package1 and the two instruments used in the IAGOS-CARIBIC aircraft). This result gives us confidence that the entire IAGOS database can be treated as one continuous program and is therefore appropriate for studies of long-term trends. IAGOS gratefully acknowledges financial support during its preparation, implementation and operation phase from the European Commission in FP6 and FP7 programs, the IAGOS for the GMES Atmospheric Service national (IGAS), research programs in Germany (BMBF), France (INSU-CNRS, MESR, CNES) and the UK (NERC), in addition to institutional resources in Germany (Helmholtz Association, Max Planck Society, Leibniz Association), France (Université de Toulouse, Météo-France) and the UK (University of Manchester, University of Cambridge), and the continuing support of participating airlines (Lufthansa, Air France and Iberia in Europe; China Airlines and Cathay Pacific in Asia). IAGOS wishes to emphasize the excellence of the industrial partners involved in the technical development: Sabena Technics Bordeaux for aircraft systems definition and certification, LGM Ingénierie for the instruments realization and aeronautic qualification and Enviscope GmbH Frankfurt for operating the maintenance center of IAGOS. Peer Reviewed "Article signat per14 autors/es: Romain Blot, Philippe Nedelec, Damien Boulanger, Pawel Wolff, Bastien Sauvage, Jean-Marc Cousin, Gilles Athier, Andreas Zahn, Florian Obersteiner, Dieter Scharffe, Hervé Petetin, Yasmine Bennouna, Hannah Clark, and Valérie Thouret"

As part of the ChArMEx-ADRIMED campaign (summer 2013), ground-based in-situ observations were conducted at the Ersa site (northern tip of Corsica; 533 m asl) to characterize the physical, optical and chemical properties of aerosols. During the observation period, three different aerosol regimes have been identified, including a dust outbreak (Dust) originating from Algeria/Tunisia, a primary marine aerosols (PMA) event from both the Gulf of Lion and emissions near the sample site, and a pollution period from Eastern Europe, which includes anthropogenic and biomass burning sources (BBP). The chemical, physical and optical properties of the observed aerosols as well as their local shortwave (SW) direct radiative forcing (DRF) in clear-sky conditions are compared for these three periods in order to assess the direct radiative impact of PMA above the Western Mediterranean Basin. The PMA period is characterized by a mean sea salt mass concentration up to 6.5 μg m−3, representing 40 % of the total PM10 mass concentration, and a relatively low ratio of chloride to sodium (average of 0.57) indicating a generally "aged" sea salt aerosol at Ersa. In this work, an original dataset, obtained from on-line real-time instruments (ATOFMS, PILS-IC) have been used to characterize the ageing of PMA. The majority of PMA had surprisingly undergone chemical reactions and were mostly advected from long-range transport. During PMA period, the mixing between fresh and aged PMA originated from both local and regional (Gulf of Lion) emissions. The aerosol optical properties, obtained for the whole atmospheric column and at the surface, indicate a single scattering albedo (SSA) near unity (at 440 nm), indicating almost purely scattering particles, associated to a relatively low aerosol optical depth (AOD) close to 0.1 (at 500 nm), and an aerosol angstrom extinction exponent (AE) equal to 1.3 ± 0.4 (between 440 and 870 nm), suggesting a possible mixing of the PMA with fine particles (probably of continental origin). AERONET retrievals indicate a relatively low local SW DRF during the PMA period with mean values of −11 ± 4 W m−2 at the surface and −8 ± 3 W m−2 at the top of the atmosphere (TOA). In comparison, our results indicate that the dust outbreak observed at our site during the campaign, although of moderate intensity (AOD of 0.3–0.4 at 440 nm and column-integrated SSA of 0.90–0.95), induced a local instantaneous SW DRF nearly three times the forcing calculated during the PMA period, with maximum values up to −40 W m−2 at the surface. On average, the SW DRF was about −21 ± 11 and −14 ± 6 W m−2, at the surface and at TOA, respectively, during this dust outbreak. Finally, the BBP period was characterized by a significant increase of the aerosol PM1 mass concentration (from 3.7 μg m−3 to 7.2 μg m−3) due to the influence of biomass-burning and anthropogenic aerosols transported from Eastern Europe. The influence of polluted/smoke particles led to a significant decrease in SSA (0.90 at 440 nm), showing the important absorbing characteristics of such particles. For this period, the SW DRF at the surface and TOA also exhibit higher mean values compared to the PMA period (with values of −23 ± 6 W m−2 and −15 ± 4 W m−2, respectively) and similar range of values as the Dust period.

The StratoClim airborne campaign took place in Nepal from 27 July to 10 August 2017 to document the physical and chemical properties of the South Asian Upper Troposphere Lower Stratosphere (UTLS) during the Asian Summer Monsoon (ASM). In the present paper, simulations with the Meso-NH cloud-chemistry model at a horizontal resolution of 15 km are performed over the Asian region to characterize the impact of monsoon deep convection on the composition of Asian Monsoon Anticyclone (AMA) and on the formation of the Asian Tropopause Aerosol Layer (ATAL) during the StratoClim campaign. StratoClim took place during a break phase of the monsoon with an intense convective activity over south China and Sichuan. Comparisons between Brightness Temperature (BT) at 10.8 microns observed by satellite sensors and simulated by Meso-NH highlight the ability of the model to correctly reproduce the life cycle of deep convective clouds. Comparison between CO and O3 concentrations from Meso-NH and airborne observations (StratoClim and IAGOS) demonstrates that the model captures most of the observed variabilities. Nevertheless, for both gases, the model tends to overestimate the concentrations and misses some thin CO plumes related to local convective events probably because of a too coarse resolution, but the convective uplift of pollution is very well captured by the model. We have therefore focused on the impact of Sichuan convection on the AMA composition. A dedicated sensitivity simulation showed that the 7 August convective event brought large amounts of CO deep into the AMA and even across the 380 K isentropic level located at 17.8 km. This Sichuan contribution enhanced the CO concentration by ~ 15 % to reach more than 180 ppbv over a large area around 15 km height. Noteworthy, Meso-NH captures the impact of the diluted Sichuan plume on the CO concentration during a StratoClim flight south of Kathmandu highlighting its ability to reproduce the transport pathway of Sichuan pollution. According to the model, primary organic aerosol and black carbon particles originating from Sichuan are transported following the same pathway as CO. The large particles are heavily scavenged within the precipitating part of the convective clouds but remain the most important contributor to the particle mass in the AMA. Over the whole AMA region, the 7 August convective event resulted in a 0.5 % increase of CO over the 10–20 km range that lasted about 2 days. The impact of pollution uplift from three regions (India, China and Sichuan) averaged over the first 10 days of August has also been evaluated with sensitivity simulations. Even during this monsoon break phase, the results confirm the predominant role of India relative to China with respective contributions of 11 and 7 % to CO in the 10–15 km layer. Moreover, during this period a large part (35 %) of the Chinese contribution comes from the Sichuan basin alone.; The StratoClim airborne campaign took place in Nepal from 27 July to 10 August 2017 to document the physical and chemical properties of the South Asian Upper Troposphere Lower Stratosphere (UTLS) during the Asian Summer Monsoon (ASM). In the present paper, simulations with the Meso-NH cloud-chemistry model at a horizontal resolution of 15 km are performed over the Asian region to characterize the impact of monsoon deep convection on the composition of 20 Asian Monsoon Anticyclone (AMA) and on the formation of the Asian Tropopause Aerosol Layer (ATAL) during the StratoClim campaign. StratoClim took place during a break phase of the monsoon with an intense convective activity over south China and Sichuan. Comparisons between Brightness Temperature (BT) at 10.8 microns observed by satellite sensors and simulated by Meso-NH highlight the ability of the model to correctly reproduce the life cycle of deep convective clouds. Comparison between CO and O 3 concentrations from Meso-NH and airborne observations (StratoClim and IAGOS) 25 demonstrates that the model captures most of the observed variabilities. Nevertheless, for both gases, the model tends to overestimate the concentrations and misses some thin CO plumes related to local convective events probably because of a too coarse resolution, but the convective uplift of pollution is very well captured by the model. We have therefore focused on the impact of Sichuan convection on the AMA composition. A dedicated sensitivity simulation showed that the 7 August convective event brought large amounts of CO deep into the AMA and even across the 380 K isentropic level located at 17.8 30 km. This Sichuan contribution enhanced the CO concentration by ~15 % to reach more than 180 ppbv over a large area around 15 km height. Noteworthy, Meso-NH captures the impact of the diluted Sichuan plume on the CO concentration during a StratoClim flight south of Kathmandu highlighting its ability to reproduce the transport pathway of Sichuan pollution. According to the model, primary organic aerosol and black carbon particles originating from Sichuan are 2 transported following the same pathway as CO. The large particles are heavily scavenged within the precipitating part of the 35 convective clouds but remain the most important contributor to the particle mass in the AMA. Over the whole AMA region, the 7 August convective event resulted in a 0.5% increase of CO over the 10−20 km range that lasted about 2 days. The impact of pollution uplift from three regions (India, China and Sichuan) averaged over the first 10 days of August has also been evaluated with sensitivity simulations. Even during this monsoon break phase, the results confirm the predominant role of India relative to China with respective contributions of 11 and 7 % to CO in the 10-15 km layer. Moreover, during this 40 period a large part (35 %) of the Chinese contribution comes from the Sichuan basin alone.

In this paper, the fate of biomass burning emissions of carbon monoxide is studied with the global chemistry–transport model MOCAGE (MOdélisation de Chimie Atmosphérique à Grande Échelle) and IAGOS (In-Service Aircraft for a Global Observing System) airborne measurements for the year 2013. The objectives are firstly to improve their representation within the model and secondly to analyse their contribution to carbon monoxide concentrations in the upper troposphere. At first, a new implementation of biomass burning injection is developed for MOCAGE, using the latest products available in Global Fire Assimilation System (GFAS) biomass burning inventory on plume altitude and injection height. This method is validated against IAGOS observations of CO made in fire plumes, identified thanks to the SOFT-IO source attribution data. The use of these GFAS products leads to improved MOCAGE skill to simulate fire plumes originating from boreal forest wildfires. It is also shown that this new biomass burning injection method modifies the distribution of carbon monoxide in the free and upper troposphere, mostly at northern boreal latitudes. Then, MOCAGE performance is evaluated in general in the upper troposphere and lower stratosphere in comparison to the IAGOS observations and is shown to be very good, with very low bias and good correlations between the model and the observations. Finally, we analyse the contribution of biomass burning to upper tropospheric carbon monoxide concentrations. This is done by comparing simulations where biomass are toggled on and off in different source regions of the world to assess their individual influence. The two regions contributing the most to upper tropospheric CO are found to be the boreal forests and equatorial Africa, in accordance with the quantities of CO they emit each year and the fact that they undergo fast vertical transport: deep convection in the tropics and pyroconvection at high latitudes. It is also found that biomass burning contributes more than 11 % on average to the CO concentrations in the upper troposphere and up to 50 % at high latitudes during the wildfire season.

In the framework of the MOZAIC–IAGOS programme, vertical profiles of ozone (O3) and carbon monoxide (CO) have been available since 1994 and 2002, respectively. This study investigates the variability and trend of both species in three tropospheric layers above the German airports Frankfurt and Munich. About 21 300 flights have taken place over the period 1994–2012, which represents the worldwide densest vertical in situ data set of O3 and CO (with ∼ 96 flights per month on average). The mean vertical profile of ozone shows a strong gradient in the first kilometre during the whole year and in the tropopause region in spring and summer. The mean vertical profile of CO is characterised by high mixing ratios at the ground, a strong decrease in the first kilometre, in particular in winter and autumn, and a moderate one in the free troposphere. O3 minimises in November–December and shows a broad spring/summer maximum in the lower and mid-troposphere and a sharp maximum in summer in the upper troposphere. The seasonal variation of CO shows a broad minimum in July–October close to the surface and in September–October it occurs higher in the troposphere, while the maximum occurs in February–April in the whole troposphere. Over the period 1994–2012, O3 has changed insignificantly (at a 95 % confidence level), except in winter where a slightly significant increase (from +0.83 [+0.13;+1.67] % yr−1 in the LT to +0.62 [+0.02;+1.22] % yr−1 in the UT, relative to the reference year 2000) is found. The O3 5th percentile shows similar upward trends at the annual scale in all three tropospheric layers. All trends remain insignificant for the O3 95th percentile. In contrast, for CO the mean as well as its 5th and 95th percentiles decrease both at the annual scale and at the seasonal scale in winter, spring and summer (although not always in all three tropospheric layers) with trends ranging between −1.22 [−2.27;−0.47] and −2.63 [−4.54;−1.42] % yr−1, relative to the reference year 2004. However, all CO trends remain insignificant in autumn. The phase of the seasonal variation of O3 was found to change in the troposphere. The O3 maxima moves forward in time at a rate of −17.8 ± 11.5 days decade−1 in the lower troposphere, in general agreement with previous studies. Interestingly, this seasonal shift is shown to persist in the mid-troposphere (−7.8 ± 4.2 days decade−1) but turns insignificant in the upper troposphere.

The research infrastructure IAGOS (In-Service Aircraft for a Global Observing System) equips commercial aircraft with instruments to monitor the composition of the atmosphere during flights around the world. In this article, we use data from two China Airlines aircraft based in Taipei (Taiwan) which provided daily measurements of ozone, carbon monoxide and water vapour throughout the summer of 2016. We present time series, from the surface to the upper troposphere, of ozone, carbon monoxide and relative humidity near Taipei, focusing on periods influenced by the passage of typhoons. We examine landing and take-off profiles in the vicinity of tropical cyclones using ERA-5 reanalyses to elucidate the origin of the anomalies in the vertical distribution of these chemical species. Results indicate a high ozone content in the upper- to middle-troposphere track of the storms. The high ozone mixing ratios are generally correlated with potential vorticity and anti-correlated with relative humidity, suggesting stratospheric origin. These results suggest that tropical cyclones participate in transporting air from the stratosphere to troposphere and that such transport could be a regular feature of typhoons. After the typhoons passed Taiwan, the tropospheric column was filled with substantially lower ozone mixing ratios due to the rapid uplift of marine boundary layer air. At the same time, the relative humidity increased, and carbon monoxide mixing ratios fell. Locally, therefore, the passage of typhoons has a positive effect on air quality at the surface, cleansing the atmosphere and reducing the mixing ratios of pollutants such as CO and O3.

Cirrus clouds and their potential formation regions, so-called ice-supersaturated regions (ISSRs) occur frequently in the tropopause region. It is assumed that ISSRs and cirrus clouds can change the tropopause structure by diabatic processes, driven by latent heating due to phase transition and interaction with radiation. For many research questions a three-dimensional picture including a sufficient temporal resolution of the water vapour fields in the tropopause region is required. This requirement is fulfilled nowadays by reanalysis products such as the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis. However, for a meaningful investigation of water vapour in the tropopause region a comparison of the reanalysis data with measurement is advisable, since it is difficult to measure water vapour and to assimilate meaningful measurements into reanalysis products. Here, we present an intercomparison of high-resolution in-situ measurements aboard passenger aircraft within the European Research Infrastructure IAGOS (In-service Aircraft for a Global Observing System; http://www.iagos.org) with ERA-Interim. Temperature and humidity data over the North Atlantic from 2000 to 2010 are compared relative to the dynamical tropopause. The comparison of the temperature shows a good agreement between measurement and ERA-Interim. While ERA-Interim can reproduce the main features of the water vapour measurements of IAGOS, the variability of the data is underestimated by the reanalysis data. The combination of temperature and water vapour leads to the relative humidity with respect to ice (RHi). Here ERA-Interim deviates from the measurements concerning values of larger than RHi=100 %, both in number and strength of supersaturation. The comparison of ISSR pathlengths shows distinct differences, which can be traced back to the spatial resolution of both data sets. IAGOS shows significantly more smaller ISSRs compared to ERA-Interim. A good agreement begins only at pathlengths in the order of the ERA-Interim spatial resolution and larger.

International audience; Climatology and long-term evolution of ozone and carbon monoxide in the upper troposphere-lower stratosphere (UTLS) at northern midlatitudes, as seen by IAGOS from Abstract. In situ measurements in the upper troposphere-lower stratosphere (UTLS) have been performed in the framework of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System) for ozone since 1994 and for carbon monoxide (CO) since 2002. The flight tracks cover a wide range of longitudes in the northern extratropics, extending from the North American western coast (125 • W) to the eastern Asian coast (135 • E) and more recently over the northern Pacific Ocean. Several tropical regions are also sampled frequently, such as the Brazilian coast, central and southern Africa, southeastern Asia, and the western half of the Maritime Continent. As a result, a new set of climatologies for O 3 (in the upper troposphere (UT), tropopause layer, and lower stratosphere (LS) are made available, including gridded horizontal distributions on a semi-global scale and seasonal cycles over eight well-sampled regions of interest in the northern extratropics. The seasonal cycles generally show a summertime maximum in O 3 and a springtime maximum in CO in the UT, in contrast to the systematic springtime maximum in O 3 and the quasi-absence of a seasonal cycle of CO in the LS. This study highlights some regional variabilities in the UT, notably (i) a west-east difference of O 3 in boreal summer with up to 15 ppb more O 3 over central Russia compared with northeast America, (ii) a systematic west-east gradient of CO from 60 to 140 • E, especially noticeable in spring and summer with about 5 ppb by 10 degrees longitude, (iii) a broad spring/summer maximum of CO over northeast Asia, and (iv) a spring maximum of O 3 over western North Amer-ica. Thanks to almost 20 years of O 3 and 12 years of CO measurements, the IAGOS database is a unique data set to derive trends in the UTLS at northern midlatitudes. Trends in O 3 in the UT are positive and statistically significant in most regions, ranging from +0.25 to +0.45 ppb yr −1 , characterized by the significant increase in the lowest values of the distribution. No significant trends of O 3 are detected in the LS. Trends of CO in the UT, tropopause, and LS are almost all negative and statistically significant. The estimated slopes range from −1.37 to −0.59 ppb yr −1 , with a nearly homogeneous decrease in the lowest values of the monthly distribution (5th percentile) contrasting with the high interregional variability in the decrease in the highest values (95th percentile).

Aerospace electrical systems are continuing to increase their voltage levels to meet the on-board power demands of more-electric aircraft (MEA) wherehydraulic and pneumatic systems are replaced with electrical equivalents. This trend will only continue as hybrid and allelectric aircraft are developed. These higher power demands require the use of higher voltages and as such it is essential to explore the behaviour of the insulation system in the aerospaceenvironment. This insulation must operate in an environment where the operating temperatures range from 250°C to -65°C, the air pressure is around one tenth of that at ground level and where the levels of humidity and ozone vary rapidly. Understanding the impact of these variables on aircraft highvoltage insulation systems is crucial in predicting their behaviour and lifetime. Our work with the IAGOS atmospheric dataset presents worst-case and typical flight environments for higher than expected cruising altitudes, and uses thefindings to compare the relative rates of degradation of insulating materials at ground and cruising altitudes.

For the first time, a plume-in-grid approach is implemented in a chemical transport model (CTM) to parameterize the effects of the nonlinear reactions occurring within high concentrated NOx plumes from lightning NOx emissions (LNOx) in the upper troposphere. It is characterized by a set of parameters including the plume lifetime, the effective reaction rate constant related to NOx–O3 chemical interactions, and the fractions of NOx conversion into HNO3 within the plume. Parameter estimates were made using the Dynamical Simple Model of Atmospheric Chemical Complexity (DSMACC) box model, simple plume dispersion simulations, and the 3-D Meso-NH (non-hydrostatic mesoscale atmospheric model). In order to assess the impact of the LNOx plume approach on the NOx and O3 distributions on a large scale, simulations for the year 2006 were performed using the GEOS-Chem global model with a horizontal resolution of 2° × 2.5°. The implementation of the LNOx parameterization implies an NOx and O3 decrease on a large scale over the region characterized by a strong lightning activity (up to 25 and 8 %, respectively, over central Africa in July) and a relative increase downwind of LNOx emissions (up to 18 and 2 % for NOx and O3, respectively, in July). The calculated variability in NOx and O3 mixing ratios around the mean value according to the known uncertainties in the parameter estimates is at a maximum over continental tropical regions with ΔNOx [−33.1, +29.7] ppt and ΔO3 [−1.56, +2.16] ppb, in January, and ΔNOx [−14.3, +21] ppt and ΔO3 [−1.18, +1.93] ppb, in July, mainly depending on the determination of the diffusion properties of the atmosphere and the initial NO mixing ratio injected by lightning. This approach allows us (i) to reproduce a more realistic lightning NOx chemistry leading to better NOx and O3 distributions on the large scale and (ii) to focus on other improvements to reduce remaining uncertainties from processes related to NOx chemistry in CTM.

The Indian subcontinent (IS) is a region of increasing economic growth, urbanization, and consequently, anthropogenic emissions, altering tropospheric ozone (O3) over the region with impacts on the lives and health of 1.3 billion people. We have developed a comprehensive data set of the tropospheric O3 for 16 years (2000–2015) for the region between 50-115°E and 0–45°N, focusing on the IS. The data set included available balloon-borne, aircraft, and satellite-based measurements. We used a global three-dimensional chemical transport model, GEOS-Chem, at a 2° × 2.5° resolution to calculate daily tropospheric O3 over the region. The simulated O3 abundances in the boundary layer and lower, mid, and upper troposphere were compared with ozonesonde, aircraft, and satellite observations. The statistical analyses indicate that the model simulated boundary layer and lower, mid, and upper tropospheric O3 column abundances reasonably well with a mean bias ~1–3 DU in comparison to observations, but within the uncertainties of the observations. The model reproduced the vertical profiles of O3 and CO with a bias of less than 20% over different regions in the IS. The simulated tropospheric column NO2 was higher by a factor of ~1.5 compared to satellite observations. The model reproduced the regional difference in seasonal variations of tropospheric column O3 as observed by the Ozone Monitoring Instrument. We conclude that the CO emissions from the IS are underestimated while those of NOx are overestimated, both by around 20–30%.

This paper investigates in an innovative way the climatological vertical stratification of relative humidity (RH) and ozone (O3) and carbon monoxide (CO) mixing ratios within the planetary boundary layer (PBL) and at the interface with the free troposphere (FT). The climatology includes all vertical profiles available at northern mid-latitudes over the period 1994–2016 in both IAGOS (In-service Aircraft for a Global Observing System) and WOUDC (World Ozone and Ultraviolet Radiation Data Centre) databases, which represents more than 90,000 vertical profiles. For all individual profiles, apart from the specific case of surface-based temperature inversions (SBIs), the PBL height is estimated following the elevated temperature inversion (EI) method. Several features of both SBIs and EIs are analysed, including their diurnal and seasonal variations. Based on these PBL height estimates (denoted h), the original approach introduced in this paper consists in building a so-called PBL-referenced vertical distribution of O3, CO and RH by averaging all individual profiles beforehand expressed as a function of z/h rather than z (with z the altitude). Using this vertical coordinate system allows to highlight the features existing at the PBL-FT interface that would have been smoothed otherwise. Results demonstrate that the frequently assumed well-mixed PBL remains an exception for both chemical species. Within the PBL, CO profiles are characterized by a mean vertical stratification (here defined as the standard deviation of the CO profile between the surface and the PBL top, normalized by the mean) of 11 %, with moderate seasonal and diurnal variations. A higher vertical stratification is observed for O3 mixing ratios (18 %), with stronger seasonal and diurnal variability (from ~ 10 % in spring/summer midday/afternoon to ~ 25 % in winter/fall night). This vertical stratification is distributed heterogeneously in the PBL with stronger vertical gradients observed at both the surface (due to dry deposition and titration by NO for O3; and due to surface emissions for CO) and the PBL-FT interface. These gradients vary with the season from lowest values in summer to highest ones in winter. Contrary to CO, the O3 vertical stratification was found to vary with the surface potential temperature following an interesting bell shape with weakest stratification for both lowest (typically negative) and highest temperatures, which could be due to a much lower O3 dry deposition under the presence of snow. Therefore, results demonstrate that EIs act as a geophysical interface separating air masses of distinct chemical composition and/or chemical regime. This is further supported by the analysis of the correlation of O3 and CO mixing ratios between the different altitude levels in the PBL and FT (the so-called vertical autocorrelation). Results indeed highlight lower correlations apart from the PBL-FT interface and higher correlations within each of the two atmospheric compartments (PBL and FT).

International audience; This study investigates the role of biomass burning and long-range transport in the anomalies of carbon monoxide (CO) regularly observed along the tropospheric vertical profiles measured in the framework of the In-service Aircraft for a Global Observing System (IAGOS). Considering the high interannual variability of biomass burning emissions and the episodic nature of long-range pollution transport, one strength of this study is the amount of data taken into account, namely 30 000 vertical profiles at nine clusters of airports in Europe, North America, Asia, India and southern Africa over the period 2002-2017. As a preliminary, a brief overview of the spatiotemporal variability, latitudinal distribution, interannual variability and trends of biomass burning CO emissions from 14 regions is provided. The distribution of CO mixing ratios at different levels of the troposphere is also provided based on the entire IAGOS database (125 million CO observations). This study focuses on the free troposphere (altitudes above 2 km) where the long-range transport of pollution is favoured. Anomalies at a given airport cluster are here defined as departures from the local seasonally averaged cli-matological vertical profile. The intensity of these anomalies varies significantly depending on the airport, with maximum (minimum) CO anomalies of 110-150 (48) ppbv in Asia (Eu-rope). Looking at the seasonal variation of the frequency of occurrence, the 25 % strongest CO anomalies appear reasonably well distributed throughout the year, in contrast to the 5 % or 1 % strongest anomalies that exhibit a strong season-ality with, for instance, more frequent anomalies during summertime in the northern United States, during winter/spring in Japan, during spring in southeast China, during the non-monsoon seasons in southeast Asia and south India, and during summer/fall in Windhoek, Namibia. Depending on the location, these strong anomalies are observed in different parts of the free troposphere. In order to investigate the role of biomass burning emissions in these anomalies, we used the SOFT-IO (SOft at-tribution using FlexparT and carbon monoxide emission inventories for In-situ Observation database) v1.0 IAGOS added-value products that consist of FLEXible PARTicle dispersion model (FLEXPART) 20-day backward simulations along all IAGOS aircraft trajectories, coupled with anthro-pogenic Monitoring Atmospheric Composition and Climate (MACC)/CityZEN EU projects (MACCity) and biomass burning Global Fire Assimilation System (GFAS) CO emission inventories and vertical injections. SOFT-IO estimates the contribution (in ppbv) of the recent (less than 20 days) primary worldwide CO emissions, tagged per source region. Biomass burning emissions are found to play an important role in the strongest CO anomalies observed at most airport clusters. The regional tags indicate a large contribution from boreal regions at airport clusters in Europe and North America during the summer season. In both Japan and south India, the anthropogenic emissions dominate all throughout the year, except for the strongest summertime anomalies observed in Japan that are due to Siberian fires. The strongest CO anomalies at airport clusters located in southeast Asia are induced by fires burning during spring in southeast Asia Published by Copernicus Publications on behalf of the European Geosciences Union. 17278 H. Petetin et al.: The role of biomass burning and during fall in equatorial Asia. In southern Africa, the Windhoek airport was mainly impacted by fires in Southern Hemisphere Africa and South America. To our knowledge, no other studies have used such a large dataset of in situ vertical profiles for deriving a climatol-ogy of the impact of biomass burning versus anthropogenic emissions on the strongest CO anomalies observed in the troposphere, in combination with information on the source regions. This study therefore provides both qualitative and quantitative information for interpreting the highly variable CO vertical distribution in several regions of interest.

Nigeria has a high population density and large fossil fuel resources but very poorly managed energy infrastructure. Satellite observations of formaldehyde (HCHO) and glyoxal (CHOCHO) reveal very large sources of anthropogenic nonmethane volatile organic compounds (NMVOCs) from the Lagos megacity and oil/gas operations in the Niger Delta. This is supported by aircraft observations over Lagos and satellite observations of methane in the Niger Delta. Satellite observations of carbon monoxide (CO) and nitrogen dioxide (NO 2 ) show large seasonal emissions from open fires in December–February (DJF). Ventilation of central Nigeria is severely restricted at that time of year, leading to very poor ozone air quality as observed from aircraft (MOZAIC) and satellite (TES). Simulations with the GEOS-Chem chemical transport model (CTM) suggest that maximum daily 8-h average (MDA8) ozone exceeds 70 ppbv over the region on a seasonal mean basis, with significant contributions from both open fires (15–20 ppbv) and fuel/industrial emissions (7–9 ppbv). The already severe ozone pollution in Nigeria could worsen in the future as a result of demographic and economic growth, although this would be offset by a decrease in open fires.

In situ measurements in the upper troposphere – lower stratosphere (UTLS) are performed in the framework of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System) for ozone since 1994 and for carbon monoxide since 2002. The flight tracks cover a wide range of longitudes in the northern extratropics, extending from the North American western coast (125° W) to the eastern Asian coast (135° E), and more recently over the northern Pacific ocean. Different tropical regions are also sampled frequently, such as the Brazilian coast, central and southern Africa, southeastern Asia and the western Maritime Continent. As a result, a new set of climatologies for O3 (Aug. 1994–Dec. 2013) and CO (Dec. 2001–Dec. 2013) in the upper troposphere (UT), tropopause layer and lower stratosphere (LS) are made available, including quasi-global gridded horizontal distributions, and seasonal cycles over eight well sampled regions of interest in the northern extratropics. The seasonal cycles generally show a summertime maximum in O3 and a springtime maximum in CO in the UT, in contrast with the systematic springtime maximum in O3 and the quasi-absence of seasonal cycle of CO in the LS. This study highlights some regional variabilities in the UT notably (i) a west-east difference of O3 in boreal summer with up to 15 ppb more O3 over central Russia compared with northeast America, (ii) a systematic west-east gradient of CO from 60° E to 140° E (especially noticeable in spring and summer with about 5 ppb by 10 degrees longitude), (iii) a broad spring/summer maximum of CO over North East Asia, and (iv) a spring maximum of O3 over Western North America. Thanks to almost 20 years of O3 and 12 years of CO measurements, the IAGOS database is a unique data set to derive trends in the UTLS. Trends in O3 in the UT are positive and statistically significant in most regions, ranging from +0.25 to +0.45 ppb yr−1, characterized by the significant increase of the lowest values of the distribution. No significant trends of O3 are detected in the LS. Trends of CO in the UT, tropopause and LS are all negative and statistically significant. The estimated slopes range from −1.37 to −0.59 ppb yr−1 , with a nearly homogeneous decrease of the lowest values of the monthly distribution (fifth percentile) contrasting with the high inter-regional variability of the highest values (95th percentile).

As part of the ChArMEx-ADRIMED campaign (summer 2013), ground-based in situ observations were conducted at the Ersa site (northern tip of Corsica; 533 m a.s.l.) to characterise the optical, physical and chemical properties of aerosols. During the observation period, a major influence of primary marine aerosols was detected (22–26 June), with a mass concentration reaching up to 6.5 µg m−3 and representing more than 40 % of the total PM10 mass concentration. Its relatively low ratio of chloride to sodium (average of 0.57) indicates a fairly aged sea salt aerosol at Ersa. In this work, an original data set, obtained from online real-time instruments (ATOFMS, PILS-IC) has been used to characterise the ageing of primary marine aerosols (PMAs). During this PMA period, the mixing of fresh and aged PMAs was found to originate from both local and regional (Gulf of Lion) emissions, according to local wind measurements and FLEXPART back trajectories. Two different aerosol regimes have been identified: a dust outbreak (dust) originating from Algeria/Tunisia, and a pollution period with aerosols originating from eastern Europe, which includes anthropogenic and biomass burning sources (BBP). The optical, physical and chemical properties of the observed aerosols, as well as their local shortwave (SW) direct radiative effect (DRE) in clear-sky conditions, are compared for these three periods in order to assess the importance of the direct radiative impact of PMAs compared to other sources above the western Mediterranean Basin. As expected, AERONET retrievals indicate a relatively low local SW DRF during the PMA period with mean values of −11 ± 4 at the surface and −8 ± 3 W m−2 at the top of the atmosphere (TOA). In comparison, our results indicate that the dust outbreak observed at our site during the campaign, although of moderate intensity (AOD of 0.3–0.4 at 440 nm and column-integrated SSA of 0.90–0.95), induced a local instantaneous SW DRF that is nearly 3 times the effect calculated during the PMA period, with maximum values up to −40 W m−2 at the surface. A similar range of values were found for the BBP period to those during the dust period (SW DRF at the surface and TOA of −23 ± 6 and −15 ± 4 W m−2 respectively). The multiple sources of measurements at Ersa allowed the detection of a PMA-dominant period and their characterisation in terms of ageing, origin, transport, optical and physical properties and direct climatic impact.

Since 1994, the In-service Aircraft for a Global Observing System (IAGOS) program has produced in-situ measurements of the atmospheric composition during more than 51000 commercial flights. In order to help analyzing these observations and understanding the processes driving the observed concentration distribution and variability, we developed the SOFT-IO tool to quantify source/receptor links for all measured data. Based on the FLEXPART particle dispersion model (Stohl et al., 2005), SOFT-IO simulates the contributions of anthropogenic and biomass burning emissions from the ECCAD emission inventory database for all locations and times corresponding to the measured carbon monoxide mixing ratios along each IAGOS flight. Contributions are simulated from emissions occurring during the last 20 days before an observation, separating individual contributions from the different source regions. The main goal is to supply added-value products to the IAGOS database by evincing the geographical origin and emission sources driving the CO enhancements observed in the troposphere and lower stratosphere. This requires a good match between observed and modeled CO enhancements. Indeed, SOFT-IO detects more than 95 % of the observed CO anomalies over most of the regions sampled by IAGOS in the troposphere. In the majority of cases, SOFT-IO simulates CO pollution plumes with biases lower than 10–15 ppbv. Differences between the model and observations are larger for very low or very high observed CO values. The added-value products will help in the understanding of the trace-gas distribution and seasonal variability. They are available in the IAGOS data base via http://www.iagos.org. The SOFT-IO tool could also be applied to similar data sets of CO observations (e.g. ground-based measurements, satellite observations). SOFT-IO could also be used for statistical validation as well as for inter-comparisons of emission inventories using large amounts of data.

The ozone (O3) variability over south Asia during the 2008 post-monsoon season has been assessed using measurements from the MetOP-A/IASI instrument and O3 profiles retrieved with the SOftware for a Fast Retrieval of IASI Data (SOFRID). The information content study and error analyses carried out in this paper show that IASI Level 1 data can be used to retrieve tropospheric O3 columns (TOC, surface-225 hPa) and UTLS columns (225–70 hPa) with errors smaller than 20%. Validation with global radiosonde O3 profiles obtained during a period of 6 months show the excellent agreement between IASI and radiosonde for the UTLS with correlation coefficient R > 0.91 and good agreement in the troposphere with correlation coefficient R > 0.74. For both the UTLS and the troposphere Relative Standard Deviations (RSD) are lower than 23%. Comparison with in-situ measurements from the MOZAIC program around Hyderabad demonstrates that IASI is able to capture the TOC inter and intra-seasonal variability in central India. Nevertheless, the agreement is mitigated by the fact that the smoothing of the true O3 profiles by the retrieval results in a reduction of the TOC variability detected by IASI relative to the variability observed by in situ instruments. The post-monsoon temporal variability of the vertical profile of O3 around Hyderabad has been investigated with MOZAIC observations. These observations from airborne instruments show that tropospheric O3 is steadily elevated during most of the studied period with the exception of two sharp drops following the crossing of tropical storms over India. Lagrangian simulations with the FLEXPART model indicate that elevated O3 concentrations in the middle troposphere near Hyderabad are associated with the transport of UTLS air-masses that have followed the Subtropical Westerly Jet (SWJ) and subsided over northern India together with boundary layer polluted air-masses transported from the Indo-gangetic plain by the north-easterly trades. Low O3 concentrations result from the uplift and westward transport of pristine air-masses from the marine boundary layer of the Bay of Bengal by tropical storms. In order to extend the analysis of tropospheric O3 variability to the whole of south Asia, we have used IASI-SOFRID O3 data. We show that IASI O3 data around Hyderabad were able to capture the fast variability revealed by MOZAIC. Furthermore, their spatio-temporal coverage demonstrates that the behaviour of tropospheric O3 observed near Hyderabad extended over most of central and south India and part of the Bay of Bengal. This result highlights the ability of the IASI sensor to capture fast changes in chemical composition related to dramatic tropical weather conditions.

Within the African Monsoon Multidisciplinary Analysis (AMMA), we investigate the impact of nitrogen oxides produced by lightning (LiNOx) and convective transport during the West African Monsoon (WAM) upon the composition of the upper troposphere (UT) in the tropics. For this purpose, we have performed simulations with 4 state-of-the-art chemistry transport models involved within AMMA, namely MOCAGE, TM4, LMDz-INCA and p-TOMCAT. The model intercomparison is complemented with an evaluation of the simulations based on both spaceborne and airborne observations. The baseline simulations show important differences between the UT CO and O3 distributions simulated by each of the 4 models when compared to measurements from the MOZAIC program and fom the Aura/MLS spaceborne sensor. We show that such model discrepancies can be explained by differences in the convective transport parameterizations and, more particularly, the altitude reached by convective updrafts (ranging between ~200–125 hPa). Concerning UT O3, the models exhibit a good agreement with the main observed features. Nevertheless the majority of models simulate low O3 concentrations compared to both MOZAIC and Aura/MLS observations south of the equator, and rather high concentrations in the Northern Hemisphere. Sensitivity studies are performed to quantify the effect of deep convective transport and the influence of LiNOx production on the UT composition. These clearly indicate that the CO maxima and the elevated O3 concentrations south of the equator are due to convective uplift of air masses impacted by Southern African biomass burning, in agreement with previous studies. Moreover, during the WAM, LiNOx from Africa are responsible for the highest UT O3 enhancements (10–20 ppbv) over the tropical Atlantic between 10° S–20° N. Differences between models are primarily due to the performance of the parameterizations used to simulate lightning activity which are evaluated using spaceborne observations of flash frequency. Combined with comparisons of in-situ NO measurements we show that the models producing the highest amounts of LiNOx over Africa during the WAM (INCA and p-TOMCAT) capture observed NO profiles with the best accuracy, although they both overestimate lightning activity over the Sahel.

The transport pathways of carbon monoxide (CO) in the African Upper Troposphere (UT) during the West African Monsoon (WAM) is investigated through the assimilation of CO observations by the Aura Microwave Limb Sounder (MLS) in the MOCAGE Chemistry Transport Model (CTM). The assimilation setup, based on a 3-D First Guess at Assimilation Time (3-D-FGAT) variational method is described. Comparisons between the assimilated CO fields and in situ airborne observations from the MOZAIC program between Europe and both Southern Africa and Southeast Asia show an overall good agreement around the lowermost pressure level sampled by MLS (~215 hPa). The 4-D assimilated fields averaged over the month of July 2006 have been used to determine the main dynamical processes responsible for the transport of CO in the African UT. The studied period corresponds to the second AMMA (African Monsoon Multidisciplinary Analyses) aircraft campaign. At 220 hPa, the CO distribution is characterized by a latitudinal maximum around 5° N mostly driven by convective uplift of air masses impacted by biomass burning from Southern Africa, uplifted within the WAM region and vented predominantly southward by the upper branch of the winter hemisphere Hadley cell. Above 150 hPa, the African CO distribution is characterized by a broad maximum over northern Africa. This maximum is mostly controlled by the large scale UT circulation driven by the Asian Summer Monsoon (ASM) and characterized by the Asian Monsoon Anticyclone (AMA) centered at 30° N and the Tropical Easterly Jet (TEJ) on the southern flank of the anticyclone. Asian pollution uplifted to the UT over large region of Southeast Asia is trapped within the AMA and transported by the anticyclonic circulation over Northeast Africa. South of the AMA, the TEJ is responsible for the tranport of CO-enriched air masses from India and Southeast Asia over Africa. Using the high time resolution provided by the 4-D assimilated fields, we give evidence that the variability of the African CO distribution above 150 hPa and north of the WAM region is mainly driven by the synoptic dynamical variability of both the AMA and the TEJ.

A meso-scale model was used to understand and describe the dynamical processes driving high ozone concentrations observed during both dry and monsoon season in monthly climatologies profiles over Lagos (Nigeria, 6.6° N, 3.3° E), obtained with the MOZAIC airborne measurements (ozone and carbon monoxide). This study focuses on ozone enhancements observed in the upper-part of the lower troposphere, around 3000 m. Two individual cases have been selected in the MOZAIC dataset as being representative of the climatological ozone enhancements, to be simulated and analyzed with on-line Lagrangian backtracking of air masses. This study points out the role of baroclinic low-level circulations present in the Inter Tropical Front (ITF) area. Two low-level thermal cells around a zonal axis and below 2000 m, in mirror symmetry to each other with respect to equator, form near 20° E and around 5° N and 5° S during the (northern hemisphere) dry and wet seasons respectively. They are caused by surface gradients – the warm dry surface being located poleward of the ITF and the cooler wet surface equatorward of the ITF. A convergence line exists between the poleward low-level branch of each thermal cell and the equatorward low-level branch of the Hadley cell. Our main conclusion is to point out this line as a preferred location for fire products – among them ozone precursors – to be uplifted and injected into the lower free troposphere. The free tropospheric transport that occurs then depends on the hemisphere and season. In the NH dry season, the AEJ allows transport of ozone and precursors westward to Lagos. In the NH monsoon (wet) season, fire products are transported from the southern hemisphere to Lagos by the southeasterly trade that surmounts the monsoon layer. Additionally ozone precursors uplifted by wet convection in the ITCZ can also mix to the ones uplifted by the baroclinic cell and be advected up to Lagos by the trade flow.

The purpose of this study is to perform a global search of the upper troposphere (z≥10 km) for enhancements of nitrogen dioxide and determine their sources. This is the first application of satellite-based limb scattering to study upper tropospheric NO2. We have searched two years (May 2003–May 2005) of OSIRIS (Optical Spectrograph and Infrared Imager System) operational NO2 concentrations (version 2.3/2.4) to find large enhancements in the observations by comparing with photochemical box model calculations and by identifying local maxima in NO2 volume mixing ratio. We find that lightning is the main production mechanism responsible for the large enhancements in OSIRIS NO2 observations as expected. Similar patterns in the abundances and spatial distribution of the NO2 enhancements are obtained by perturbing the lightning within the GEOS-Chem 3-dimensional chemical transport model. In most cases, the presence of lightning is confirmed with coincident imagery from LIS (Lightning Imaging Sensor) and the spatial extent of the NO2 enhancement is mapped using nadir observations of tropospheric NO2 at high spatial resolution from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) and OMI (Ozone Monitoring Instrument). The combination of the lightning and chemical sensors allows us to investigate globally the role of lightning to the abundance of NO2 in the upper troposphere (UT). Lightning contributes 60% of the tropical upper tropospheric NO2 in GEOS-Chem simulations. The spatial and temporal distribution of NO2 enhancements from lightning (May 2003–May 2005) is investigated. The enhancements generally occur at 12 to 13 km more frequently than at 10 to 11 km. This is consistent with the notion that most of the NO2 is forming and persisting near the cloud top altitude in the tropical upper troposphere. The latitudinal distribution is mostly as expected. In general, the thunderstorms exhibiting weaker vertical development (e.g. 11≤z≤13 km) extend latitudinally as far poleward as 45° but the thunderstorms with stronger vertical development (z≥14 km) tend to be located within 33° of the equator. There is also the expected hemispheric asymmetry in the frequency of the NO2 enhancements, as most were observed in the northern hemisphere for the period analyzed.

In the framework of the MOZAIC-IAGOS program, ozone and carbon monoxide vertical profiles are available since 1994 and 2002, respectively. This study investigates the variability and trends of both species at several tropospheric layers above the Frankfurt and Munich airports where about 21 600 flights have been performed over the 1994–2012 period, which represents the densest dataset in the world (about 75 flights per month on average). Over that period, most mean ozone trends are positive but insignificant at a 95 % confidence level, except in winter where significant upward trends (around +0.38 ± 0.24 ppb yr−1) are found. Conversely, a significant increase of annual background ozone is highlighted, mostly during winter and autumn. Mean annual ozone trends increase with altitude, the largest increase being found in summer due to a noticeable decrease of highest ozone concentrations observed in the lower troposphere during the second half period. Over the 2002–2012 period, most mean CO trends are significantly negative, the decrease being higher in the lower troposphere compared to the mid- and upper troposphere with again, major differences in summer. Trends in the ozone seasonal cycle are also investigated, with a focus on the phase. Ozone maxima occur earlier and earlier with a shift around −10.6 ± 2.9 days decade−1 in the lower troposphere, in agreement with previous studies. The analysis of other ozone datasets in Europe (including surface stations and ozone soundings) confirms this trend, but highlights strong heterogeneities in the phase change. Interestingly, this shift is shown to decrease with altitude, with trends of −4.3 ± 2.4 and −2.0 ± 1.7 days decade−1 in the mid- and upper troposphere, respectively. The geographic origin of the air masses sampled by aircraft is analysed with FLEXPART backward simulations and suggests, together with trends and phase changes results, that an increase of the Asian contribution to ozone in the upper troposphere may compensate during summer the decrease of European and North American contributions associated to emission control over these two regions.