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  • Open Access English
    Authors: 
    Appel, Oliver; Köllner, Franziska; Dragoneas, Antonis; Hünig, Andreas; Molleker, Sergej; Schlager, Hans; Mahnke, Christoph; Weigel, Ralf; Port, Max; Schulz, Christiane; +4 more
    Project: EC | STRATOCLIM (603557), EC | EXCATRO (321040)

    Aircraft-borne in situ measurements of the chemical aerosol composition were conducted in the Asian tropopause aerosol layer (ATAL) over the Indian subcontinent in the summer of 2017, covering particle sizes below ∼3 µm. We have implemented a recently developed aerosol mass spectrometer, which adopts the laser desorption technique as well as the thermal desorption method for quantitative bulk information (i.e., a modified Aerodyne AMS), aboard the high-altitude research aircraft M-55 Geophysica. The instrument was deployed in July and August 2017 during the StratoClim EU campaign (Stratospheric and upper tropospheric processes for better Climate predictions) over Nepal, India, Bangladesh, and the Bay of Bengal, covering altitudes up to 20 kma.s.l. For particles with diameters between 10 nm and ∼3 µm, the vertical profiles of aerosol number densities from the eight research flights show significant enhancements in the altitude range of the ATAL. We observed enhancements in the mass concentrations of particulate nitrate, ammonium, and organics in a similar altitude range between approximately 13 and 18 km (corresponding to 360 and 410 K potential temperature). By means of the two aerosol mass spectrometry techniques, we show that the particles in the ATAL mainly consist of ammonium nitrate (AN) and organics. The single-particle analysis from laser desorption and ionization mass spectrometry revealed that a significant particle fraction (up to 70 % of all analyzed particles by number) within the ATAL results from the conversion of inorganic and organic gas-phase precursors, rather than from the uplift of primary particles from below. This can be inferred from the fact that the majority of the particles encountered in the ATAL consisted solely of secondary substances, namely an internal mixture of nitrate, ammonium, sulfate, and organic matter. These particles are externally mixed with particles containing primary components as well. The single-particle analysis suggests that the organic matter within the ATAL and in the lower stratosphere (even above 420 K) can partly be identified as organosulfates (OS), in particular glycolic acid sulfate, which are known as components indicative for secondary organic aerosol (SOA) formation. Additionally, the secondary particles are smaller in size compared to those containing primary components (mainly potassium, metals, and elemental carbon). The analysis of particulate organics with the thermal desorption method shows that the degree of oxidation for particles observed in the ATAL is consistent with expectations about secondary organics that were subject to photochemical processing and aging. We found that organic aerosol was less oxidized in lower regions of the ATAL (<380 K) compared to higher altitudes (here 390–420 K). These results suggest that particles formed in the lower ATAL are uplifted by prevailing diabatic heating processes and thereby subject to extensive oxidative aging. Thus, our observations are consistent with the concept of precursor gases being emitted from regional ground sources, subjected to rapid convective uplift, and followed by secondary particle formation and growth in the upper troposphere within the confinement of the Asian monsoon anticyclone (AMA). As a consequence, the chemical composition of these particles largely differs from the aerosol in the lower stratospheric background and the Junge layer.

  • Open Access English
    Authors: 
    Appel, Oliver; Köllner, Franziska; Dragoneas, Antonis; Hünig, Andreas; Molleker, Sergej; Schlager, Hans; Mahnke, Christoph; Weigel, Ralf; Port, Max; Schulz, Christiane; +4 more
    Project: EC | STRATOCLIM (603557), EC | EXCATRO (321040)

    Aircraft borne in-situ measurements of the chemical aerosol composition were conducted in the Asian Tropopause Aerosol layer (ATAL) over the Indian subcontinent in summer 2017 covering particle sizes below 3 µm. We have implemented a recently developed aerosol mass spectrometer, which adopts the laser desorption technique as well as the thermal desorption method for quantitative bulk information (i.e. a modified Aerodyne AMS), aboard the high altitude research aircraft M-55 Geophysica. The instrument was deployed in July and August 2017 during the StratoClim EU campaign (Stratospheric and upper tropospheric processes for better climate predictions) over Nepal, India, Bangladesh, and the Bay of Bengal, covering altitudes up to 20 km a.s.l. For particles with diameters between 10 nm and ~3 µm the vertical profiles of aerosol number densities from the eight research flights show significant enhancements in the altitude range of the ATAL. We observed enhancements in the mass concentrations of particulate nitrate, ammonium, and organics in a similar altitude range between approximately 13 km and 18 km (corresponding to 360 K and 410 K potential temperature). By means of the two aerosol mass spectrometry techniques, we show that the particles in the ATAL mainly consist of ammonium nitrate and organics. The single particle analysis from laser desorption and ionizaton mass spectrometry revealed that a significant particle fraction (up to 70 % of all analyzed particles by number) within the ATAL results from the conversion of inorganic and organic gas-phase precursors, rather than from the uplift of primary particles from below. This can be inferred from the fact that the majority of the particles encountered in the ATAL consisted solely of secondary substances, namely an internal mixture of nitrate, ammonium, sulfate, and organic matter. These particles are externally mixed with particles containing primary components as well. The single particle analyses suggest that the organic matter within the ATAL and in the lower stratosphere (even above 420 K) can partly be identified as organosulfates, in particular glycolic acid sulfate, which are known as components indicative for secondary organic aerosol formation. Also, the secondary particles are smaller in size compared to those containing primary components (mainly potassium, metals, and elemental carbon). The analysis of particulate organics with the thermal desorption method shows that the degree of oxidation for particles observed in the ATAL is consistent with expectations about secondary organics that were subject to photochemical processing and ageing. We found that organic aerosol was less oxidized in lower regions of the ATAL (< 380 K) compared to higher altitudes (here 390–420 K). These results suggest that particles formed in the lower ATAL are uplifted by diabatic heating processes and thereby subject to extensive oxidative ageing. Thus, our observations are consistent with the concept of precursor gases being emitted from regional ground sources, subjected to rapid convective uplift, and followed by secondary particle formation and growth in the upper troposphere within the confinement of the Asian monsoon anticyclone. As a consequence the chemical composition of these particles largely differs from the aerosol in the lower stratospheric background and the Junge layer.

  • Open Access English
    Authors: 
    Clyne, Margot; Lamarque, Jean-Francois; Mills, Michael J.; Khodri, Myriam; Ball, William; Bekki, Slimane; Dhomse, Sandip S.; Lebas, Nicolas; Mann, Graham; Marshall, Lauren; +13 more
    Project: UKRI | The North Atlantic Climat... (NE/N018001/1), SNSF | SPARC International Proje... (138017), NSF | Decadal Prediction Follow... (1430051), EC | STRATOCLIM (603557), UKRI | Reconciling Volcanic Forc... (NE/S000887/1)

    As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt. Tambora eruption (VolMIP-Tambora ISA ensemble). The pre-study provided the ancillary ability to assess intermodel diversity in the radiative forcing for a large stratospheric-injecting equatorial eruption when the volcanic aerosol cloud is simulated interactively. An initial analysis of the VolMIP-Tambora ISA ensemble showed large disparities between models in the stratospheric global mean aerosol optical depth (AOD). In this study, we now show that stratospheric global mean AOD differences among the participating models are primarily due to differences in aerosol size, which we track here by effective radius. We identify specific physical and chemical processes that are missing in some models and/or parameterized differently between models, which are together causing the differences in effective radius. In particular, our analysis indicates that interactively tracking hydroxyl radical (OH) chemistry following a large volcanic injection of sulfur dioxide (SO2) is an important factor in allowing for the timescale for sulfate formation to be properly simulated. In addition, depending on the timescale of sulfate formation, there can be a large difference in effective radius and subsequently AOD that results from whether the SO2 is injected in a single model grid cell near the location of the volcanic eruption, or whether it is injected as a longitudinally averaged band around the Earth.

  • Open Access English
    Authors: 
    Radenz, Martin; Seifert, Patric; Baars, Holger; Floutsi, Athena Augusta; Yin, Zhenping; Bühl, Johannes;
    Project: EC | BACCHUS (603445), EC | ACTRIS-2 (654109)

    Height-resolved air mass source attribution is crucial for the evaluation of profiling ground-based remote sensing observations, especially when using lidar (light detection and ranging) to investigate different aerosol types throughout the atmosphere. Lidar networks, such as EARLINET (European Aerosol Research Lidar Network) in the frame of ACTRIS (Aerosol, Clouds and Trace Gases), observe profiles of optical aerosol properties almost continuously, but usually, additional information is needed to support the characterization of the observed particles. This work presents an approach explaining how backward trajectories or particle positions from a dispersion model can be combined with geographical information (a land cover classification and manually defined areas) to obtain a continuous and vertically resolved estimate of an air mass source above a certain location. Ideally, such an estimate depends on as few as possible a priori information and auxiliary data. An automated framework for the computation of such an air mass source is presented, and two applications are described. First, the air mass source information is used for the interpretation of air mass sources for three case studies with lidar observations from Limassol (Cyprus), Punta Arenas (Chile) and ship-borne off Cabo Verde. Second, air mass source statistics are calculated for two multi-week campaigns to assess potential observation biases of lidar-based aerosol statistics. Such an automated approach is a valuable tool for the analysis of short-term campaigns but also for long-term data sets, for example, acquired by EARLINET.

  • Open Access English
    Authors: 
    Zouzoua, Maurin; Lohou, Fabienne; Assamoi, Paul; Lothon, Marie; Yoboue, Véronique; Dione, Cheikh; Kalthoff, Norbert; Adler, Bianca; Babić, Karmen; Pedruzo-Bagazgoitia, Xabier; +1 more
    Project: EC | DACCIWA (603502)

    Within the framework of the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project and based on a field experiment conducted in June and July 2016, we analyze the daytime breakup of continental low-level stratiform clouds in southern West Africa. We use the observational data gathered during 22 precipitation-free occurrences at Savè, Benin. Our analysis, which starts from the stratiform cloud formation usually at night, focuses on the role played by the coupling between cloud and surface in the transition towards shallow convective clouds during daytime. It is based on several diagnostics, including the Richardson number and various cloud macrophysical properties. The distance between the cloud base height and lifting condensation level is used as a criterion of coupling. We also make an attempt to estimate the most predominant terms of the liquid water path budget in the early morning. When the nocturnal low-level stratiform cloud forms, it is decoupled from the surface except in one case. In the early morning, the cloud is found coupled with the surface in 9 cases and remains decoupled in the 13 other cases. The coupling, which occurs within the 4 h after cloud formation, is accompanied by cloud base lowering and near-neutral thermal stability in the subcloud layer. Further, at the initial stage of the transition, the stratiform cloud base is slightly cooler, wetter and more homogeneous in coupled cases. The moisture jump at the cloud top is usually found to be lower than 2 g kg−1 and the temperature jump within 1–5 K, which is significantly smaller than typical marine stratocumulus and explained by the monsoon flow environment in which the stratiform cloud develops over West Africa. No significant difference in liquid water path budget terms was found between coupled and decoupled cases. In agreement with previous numerical studies, we found that the stratiform cloud maintenance before sunrise results from the interplay between the predominant radiative cooling, entrainment and large-scale subsidence at its top. Three transition scenarios were observed depending on the state of coupling at the initial stage. In coupled cases, the low-level stratiform cloud remains coupled until its breakup. In five of the decoupled cases, the cloud couples with the surface as the lifting condensation level rises. In the eight remaining cases, the stratiform cloud remains hypothetically decoupled from the surface throughout its life cycle since the height of its base remains separated from the condensation level. In cases of coupling during the transition, the stratiform cloud base lifts with the growing convective boundary layer roughly between 06:30 and 08:00 UTC. The cloud deck breakup, occurring at 11:00 UTC or later, leads to the formation of shallow convective clouds. When the decoupling subsists, shallow cumulus clouds form below the stratiform cloud deck between 06:30 and 09:00 UTC. The breakup time in this scenario has a stronger variability and occurs before 11:00 UTC in most cases. Thus, we argue that the coupling with the surface during daytime hours has a crucial role in the low-level stratiform cloud maintenance and its transition towards shallow convective clouds.

  • Open Access English
    Authors: 
    Schneider, Johannes; Weigel, Ralf; Klimach, Thomas; Dragoneas, Antonis; Appel, Oliver; Hünig, Andreas; Molleker, Sergej; Köllner, Franziska; Clemen, Hans-Christian; Eppers, Oliver; +17 more
    Project: EC | ACTRIS-2 (654109), EC | STRATOCLIM (603557), EC | EXCATRO (321040), NSERC

    We analyse aerosol particle composition measurements from five research missions between 2014 and 2018 to assess the meridional extent of particles containing meteoric material in the upper troposphere and lower stratosphere (UTLS). Measurements from the Jungfraujoch mountaintop site and a low-altitude aircraft mission show that meteoric material is also present within middle- and lower-tropospheric aerosol but within only a very small proportion of particles. For both the UTLS campaigns and the lower- and mid-troposphere observations, the measurements were conducted with single-particle laser ablation mass spectrometers with bipolar-ion detection, which enabled us to measure the chemical composition of particles in a diameter range of approximately 150 nm to 3 µm. The five UTLS aircraft missions cover a latitude range from 15 to 68∘ N, altitudes up to 21 km, and a potential temperature range from 280 to 480 K. In total, 338 363 single particles were analysed, of which 147 338 were measured in the stratosphere. Of these total particles, 50 688 were characterized by high abundances of magnesium and iron, together with sulfuric ions, the vast majority (48 610) in the stratosphere, and are interpreted as meteoric material immersed or dissolved within sulfuric acid. It must be noted that the relative abundance of such meteoric particles may be overestimated by about 10 % to 30 % due to the presence of pure sulfuric acid particles in the stratosphere which are not detected by the instruments used here. Below the tropopause, the observed fraction of the meteoric particle type decreased sharply with 0.2 %–1 % abundance at Jungfraujoch, and smaller abundances (0.025 %–0.05 %) were observed during the lower-altitude Canadian Arctic aircraft measurements. The size distribution of the meteoric sulfuric particles measured in the UTLS campaigns is consistent with earlier aircraft-based mass-spectrometric measurements, with only 5 %–10 % fractions in the smallest particles detected (200–300 nm diameter) but with substantial (> 40 %) abundance fractions for particles from 300–350 up to 900 nm in diameter, suggesting sedimentation is the primary loss mechanism. In the tropical lower stratosphere, only a small fraction (< 10 %) of the analysed particles contained meteoric material. In contrast, in the extratropics the observed fraction of meteoric particles reached 20 %–40 % directly above the tropopause. At potential temperature levels of more than 40 K above the thermal tropopause, particles containing meteoric material were observed in much higher relative abundances than near the tropopause, and, at these altitudes, they occurred at a similar abundance fraction across all latitudes and seasons measured. Above 440 K, the observed fraction of meteoric particles is above 60 % at latitudes between 20 and 42∘ N. Meteoric smoke particles are transported from the mesosphere into the stratosphere within the winter polar vortex and are subsequently distributed towards low latitudes by isentropic mixing, typically below a potential temperature of 440 K. By contrast, the findings from the UTLS measurements show that meteoric material is found in stratospheric aerosol particles at all latitudes and seasons, which suggests that either isentropic mixing is effective also above 440 K or that meteoric fragments may be the source of a substantial proportion of the observed meteoric material.

  • Open Access English
    Authors: 
    Schrod, Jann; Thomson, Erik S.; Weber, Daniel; Kossmann, Jens; Pöhlker, Christopher; Saturno, Jorge; Ditas, Florian; Artaxo, Paulo; Clouard, Valérie; Saurel, Jean-Marie; +3 more
    Project: EC | BACCHUS (603445)

    Ice particle activation and evolution have important atmospheric implications for cloud formation, initiation of precipitation and radiative interactions. The initial formation of atmospheric ice by heterogeneous ice nucleation requires the presence of a nucleating seed, an ice-nucleating particle (INP), to facilitate its first emergence. Unfortunately, only a few long-term measurements of INPs exist, and as a result, knowledge about geographic and seasonal variations of INP concentrations is sparse. Here we present data from nearly 2 years of INP measurements from four stations in different regions of the world: the Amazon (Brazil), the Caribbean (Martinique), central Europe (Germany) and the Arctic (Svalbard). The sites feature diverse geographical climates and ecosystems that are associated with dissimilar transport patterns, aerosol characteristics and levels of anthropogenic impact (ranging from near pristine to mostly rural). Interestingly, observed INP concentrations, which represent measurements in the deposition and condensation freezing modes, do not differ greatly from site to site but usually fall well within the same order of magnitude. Moreover, short-term variability overwhelms all long-term trends and/or seasonality in the INP concentration at all locations. An analysis of the frequency distributions of INP concentrations suggests that INPs tend to be well mixed and reflective of large-scale air mass movements. No universal physical or chemical parameter could be identified to be a causal link driving INP climatology, highlighting the complex nature of the ice nucleation process. Amazonian INP concentrations were mostly unaffected by the biomass burning season, even though aerosol concentrations increase by a factor of 10 from the wet to dry season. Caribbean INPs were positively correlated to parameters related to transported mineral dust, which is known to increase during the Northern Hemisphere summer. A wind sector analysis revealed the absence of an anthropogenic impact on average INP concentrations at the site in central Europe. Likewise, no Arctic haze influence was observed on INPs at the Arctic site, where low concentrations were generally measured. We consider the collected data to be a unique resource for the community that illustrates some of the challenges and knowledge gaps of the field in general, while specifically highlighting the need for more long-term observations of INPs worldwide.

  • Open Access English
    Authors: 
    Rowlinson, Matthew J.; Rap, Alexandru; Hamilton, Douglas S.; Pope, Richard J.; Hantson, Stijn; Arnold, Steve R.; Kaplan, Jed O.; Arneth, Almut; Chipperfield, Martyn P.; Forster, Piers M.; +1 more
    Project: EC | BACCHUS (603445), EC | CONSTRAIN (820829), UKRI | NERC Science @ Leeds and ... (NE/L002574/1), EC | LUC4C (603542)

    Tropospheric ozone concentrations are sensitive to natural emissions of precursor compounds. In contrast to existing assumptions, recent evidence indicates that terrestrial vegetation emissions in the pre-industrial era were larger than in the present day. We use a chemical transport model and a radiative transfer model to show that revised inventories of pre-industrial fire and biogenic emissions lead to an increase in simulated pre-industrial ozone concentrations, decreasing the estimated pre-industrial to present-day tropospheric ozone radiative forcing by up to 34 % (0.38 to 0.25 W m−2). We find that this change is sensitive to employing biomass burning and biogenic emissions inventories based on matching vegetation patterns, as the co-location of emission sources enhances the effect on ozone formation. Our forcing estimates are at the lower end of existing uncertainty range estimates (0.2–0.6 W m−2), without accounting for other sources of uncertainty. Thus, future work should focus on reassessing the uncertainty range of tropospheric ozone radiative forcing.

  • Open Access English
    Authors: 
    Pullinen, Iida; Schmitt, Sebastian; Kang, Sungah; Sarrafzadeh, Mehrnaz; Schlag, Patrick; Andres, Stefanie; Kleist, Einhard; Mentel, Thomas F.; Rohrer, Franz; Springer, Monika; +6 more
    Project: EC | PIMMS (287382)

    The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for α-pinene and β-pinene photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NOx, and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing NOx concentration (a) the portion of HOM organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers. Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from the termination reactions of HOM peroxy radicals with NOx, and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles (γeff>0.5 on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in γeff for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of NOx on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups. According to their γeff all HOM-ONs with more than six O atoms will contribute to organic bound nitrate (OrgNO3) in the particulate phase. However, the fraction of OrgNO3 stored in condensable HOMs with molecular masses > 230 Da appeared to be substantially higher than the fraction of particulate OrgNO3 observed by aerosol mass spectrometry. This result suggests losses of OrgNO3 for organic nitrates in particles, probably due to hydrolysis of OrgNO3 that releases HNO3 into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO3 alone could not explain the observed suppressing effect of NOx on particle mass formation from α-pinene and β-pinene. Instead we can attribute most of the reduction in SOA mass yields with increasing NOx to the significant suppression of gas phase HOM-ACCs, which have high molecular mass and are potentially important for SOA mass formation at low-NOx conditions.

  • Open Access English
    Authors: 
    Schneider, Johannes; Weigel, Ralf; Klimach, Thomas; Dragoneas, Antonis; Appel, Oliver; Hünig, Andreas; Molleker, Sergej; Köllner, Franziska; Clemen, Hans-Christian; Eppers, Oliver; +17 more
    Project: EC | STRATOCLIM (603557), EC | EXCATRO (321040)

    In this paper we analyze aerosol particle composition measurements from five research missions conducted between 2014 and 2018 sampling the upper troposphere and lower stratosphere (UTLS), to assess the meridional extent of particles containing meteoric material. Additional data sets from a ground based study and from a low altitude aircraft mission are used to confirm the existence of meteoric material in lower tropospheric particles. Single particle laser ablation techniques with bipolar ion detection were used to measure the chemical composition of particles in a size range of approximately 150 nm to 3 μm. The five UTLS aircraft missions cover a latitude range from 15 to 68° N, altitudes up to 21 km, and a potential temperature range from 280 to 480 K. In total, 338 363 single particles were analyzed, of which 147 338 particles were measured in the stratosphere. Of these particles, 50 688 were characterized by high abundances of magnesium, iron, and rare iron oxide compounds, together with sulfuric acid. This particle type was found almost exclusively in the stratosphere (48 610 particles) and is interpreted as meteoric material immersed or dissolved within stratospheric sulfuric acid particles. Below the tropopause, the observed fraction of this particle type decreases sharply. However, small fractional abundances were observed below 3000 m a.s.l. in the Canadian Arctic and also at the Jungfraujoch high altitude station (3600 m a.s.l.). Thus, the removal pathway by sedimentation and/or mixing into the troposphere is confirmed. In the tropical lower stratosphere, only a small fraction (

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