Displays

Abstract

The dust emission simulated within the up-to-date global and regional models differs by almost an order of magnitude. The models are tuned to reproduce the observed aerosol optical depth (AOD) that, with some caveats, reflects the dust mass retained in the atmosphere. However, the amount of dust suspended in the atmosphere is controlled independently by the dust emission and deposition; therefore, only AOD observations are insufficient to constrain both these processes. To calculate the dust emission over the Middle East (ME), in this study, we employ dust deposition observations, AERONET AOD, micro-pulse lidar, and satellite observations to constrain the WRF-Chem simulations. The dust deposition is measured on a monthly bases for 2015-2019 using passive samplers over six sites over land and the sea. We compare the WRF-Chem simulations, conducted with 10-km grid spacing, with the recent MERRA-2 and CAMS reanalysis. WRF-Chem is configured with the GOCART dust scheme. We calculate the meteorological and aerosol initial and boundary conditions using the MERRA-2 reanalysis. 

We evaluated the dust regional mass balance controlled by emission, deposition, and cross-boundary transport. The smallest dust particles are transported at vast distances while the heaviest ones deposit inside of the domain. Since the model accounts for dust particles with radii<10 um, we process the deposition samples to extract the weight of particles smaller than 10 um. WRF-Chem was tuned to reproduce the observed AOD and monthly deposition of dust particles with radii < 10 um. We found that the ME dust emission comprises about 30% of the global annual dust emission. MERRA-2 underestimates the ME dust emission by about 15%.

How to cite: Mostamandi, M. S., Stenchikov, G., Ukhov, A., Shevchenko, I., Engelbrecht, J., Alshehri, Y., and Anisimov, A.: Test of Dust Emission Over the Middle East, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1893, https://doi.org/10.5194/egusphere-egu2020-1893, 2020.

A significant fraction (>50%) of cloud condensation nuclei (CCN) in the atmosphere arises from new particle formation (Dunne et al., 2016). While particle nucleation has been observed almost everywhere in the atmosphere, the mechanisms governing this process are still poorly understood and subject of on-going research. For example, it is still largely unknown which components participate in new-particle formation. Laboratory experiments and quantum chemical calculations have identified potential candidates that may play a role, including sulphuric acid, ions, ammonia, amines and highly oxygenated organic molecules (Kirkby et al., 2011; Almeida et al., 2013; Bianchi et al., 2016; Bianchi et al., 2019).

Here we present observations of the formation and growth of newly formed particles measured during intense volcano activities.

The measurements were conducted at Chacaltaya mountain station (5240 m a.s.l.) in Bolivia. The station is located on top of the Cordillera Real. It has air masses coming from the Amazon forest, La Paz and the Bolivian altiplano.

With the Chemical Ionization Atmospheric Pressure interface Time-Of-Light mass spectrometers (CI-APi-TOF) we measured H₂SO₄, the APi-TOF retrieved the chemical composition of positive and negative ions. Ion and particle size distributions were measured with the NAIS (Neutral cluster and Air Ion Spectrometer) and the SMPS (Scanning Mobility Particle Sizer), respectively. The PSM (Particle Sizer Magnifier) measured particles with a cut off that varied from 1-4 nm. Finally, with the ACSM (Aerosol Chemical Speciation Monitor) and the FIGAERO (Filter Inlet for Gases and AEROsols) we retrieved the aerosol chemical composition. Besides this set of instruments, other parameters were measured at the Chacaltaya GAW station.

During the intensive measurement campaign, air masses coming directly from volcano eruptions were detected by all our instruments. We were therefore able to determine the gas and particle chemical composition of the air mass. In addition to that, we observed several NPF events triggered by air masses coming from volcanic emissions. With this set of instruments, we were able to retrieve the chemical composition of the vapours leading to the formation of new particles. It was found that all the nucleation event observed during the volcano activity were triggered by sulphuric acid and ammonia. In the presentation we will show more details on the chemical and physical mechanism behind this process.

Almeida, J., et al., (2013) Nature 502, 359-363.

Bianchi, F., et al., (2016) Science 6289, 1109-1112.

Bianchi, F., et al., (2019) Chemical Review 119, 3472−3509

Dunne et al., (2016) Science 354, 1119-1124.

Kirkby, J., et al., (2011) Nature 476, (7361), 429-433.

How to cite: Bianchi, F., Aliaga, D., Zha, Q., Heikkinen, L., Andrade, M., Kulmala, M., and Mohr, C.: The influence of volcano activity on aerosol formation over the Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4763, https://doi.org/10.5194/egusphere-egu2020-4763, 2020.

Mineral dust is an important component of the climate system, interacting with radiation, clouds, and biogeochemical systems and impacting atmospheric circulation, air quality, aviation, and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) >2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d=0.1 to >100 µm) at different stages during transport with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties using airborne observations over the Sahara (from the Fennec field campaign) and in the Saharan Air Layer (SAL) over the tropical eastern Atlantic (from the AER-D field campaign).

Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara compared to 4 µm in the SAL. Excluding giant particles over the Sahara results in significant underestimation of mass concentration (40 %), as well as underestimates of both shortwave and longwave extinction (18 % and 26 %, respectively, from scattering calculations), while the effects in the SAL are smaller but non-negligible. The larger impact on longwave extinction compared to shortwave implies a bias towards a radiative cooling effect in dust models, which typically exclude giant particles and underestimate coarse-mode concentrations.

A compilation of the new and published effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 d, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 d, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear and warrant further investigation in order to improve dust transport schemes and the associated radiative effects of coarse and giant particles in models.

This work has been recently published in ACP (Ryder, C. L., Highwood, E. J., Walser, A., Seibert, P., Philipp, A., and Weinzierl, B.: Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara, Atmos. Chem. Phys., 19, 15353–15376, https://doi.org/10.5194/acp-19-15353-2019, 2019).

How to cite: Ryder, C., Highwood, E., Walser, A., Walser, P., Philipp, A., and Weinzierl, B.: Coarse and giant particles are ubiquitous in Saharan dust export regions and are radiatively significant over the Sahara, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5722, https://doi.org/10.5194/egusphere-egu2020-5722, 2020.

The emission of sea spray aerosol (SSA) and dimethylsulfide (DMS) from the ocean results in marine boundary layer aerosol particles that can impact Earth’s radiation balance by directly scattering solar radiation and by acting as cloud condensation nuclei (CCN), thereby altering cloud properties. The surface ocean is projected to warm by 1.3 to 2.8°C globally over the 21^st century. Impacts of this warming on plankton blooms, ocean ecosystems, and ocean-to-atmosphere fluxes of aerosols and their precursor gases are highly uncertain. A fundamental understanding of linkages between surface ocean ecosystems and ocean-derived aerosols is required to address this uncertainty. One approach for improved understandings of these linkages is simultaneous measurements of relevant surface ocean and aerosol properties in an ocean region with seasonally varying plankton blooms and a minimally polluted overlying atmosphere. The western North Atlantic hosts the largest annual phytoplankton bloom in the global ocean with a large spatial and seasonal variability in plankton biomass and composition. Periods of low aerosol number concentrations associated with unpolluted air masses allow for the detection of linkages between ocean ecosystems and ocean-derived aerosol.

Five experiments were conducted in the western North Atlantic between 2014 and 2018 with the objective of finding links between the bloom and marine aerosols. These experiments include the second Western Atlantic Climate Study (WACS-2) and four North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) cruises. This series of cruises was the first time the western North Atlantic bloom was systematically sampled during every season with extensive ocean and atmosphere measurements able to assess how changes in the state of the bloom might impact ocean-derived aerosol properties. Measurements of unheated and heated number size distributions, cloud condensation nuclei (CCN) concentrations, and aerosol composition were used to identify primary and secondary aerosol components that could be related to the state of the bloom. Only periods of clean marine air, as defined by radon, particle number concentration, aerosol light absorption coefficient, and back trajectories, were included in the analysis.

CCN concentrations at 0.1% supersaturation were best correlated (r² = 0.73) with accumulation mode nss SO₄⁼. Sea spray aerosol (SSA) was only correlated with CCN during November when bloom accumulation had not yet occurred and dimethylsulfide (DMS) concentrations were at a minimum. The fraction of CCN attributable to SSA was less than 20% during March, May/June, and September, indicating the limited contribution of SSA to the CCN population of the western North Atlantic atmosphere. The strongest link between the plankton bloom and aerosol and cloud properties appears to be due to biogenic non-seasalt SO₄⁼.

How to cite: Quinn, P., Bates, T., Saltzman, E., Bell, T., and Behrenfeld, M.: Finding linkages between ocean ecosystems and natural marine aerosols in the minimally polluted North Atlantic atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5796, https://doi.org/10.5194/egusphere-egu2020-5796, 2020.

With low concentrations of tropospheric aerosol, the Southern Ocean offers a "natural laboratory" for studies of aerosol–cloud interactions. Aerosols over the Southern Ocean are produced from biogenic activity in the ocean, which generates sulfate aerosol via dimethylsulfide (DMS) oxidation, and from strong winds and waves that lead to bubble bursting and sea spray emission. Here, we evaluate the representation of Southern Ocean aerosols in the Hadley Centre Global Environmental Model version 3, Global Atmosphere 7.1 (HadGEM3-GA7.1) chemistry–climate model. Compared with aerosol optical depth (AOD) observations from two satellite instruments (the Moderate Resolution Imaging Spectroradiometer, MODIS-Aqua c6.1, and the Multi-angle Imaging Spectroradiometer, MISR), the model simulates too-high AOD during winter and too-low AOD during summer. By switching off DMS emission in the model, we show that sea spray aerosol is the dominant contributor to AOD during winter. In turn, the simulated sea spray aerosol flux depends on near-surface wind speed. By examining MODIS AOD as a function of wind speed from the ERA-Interim reanalysis and comparing it with the model, we show that the sea spray aerosol source function in HadGEM3-GA7.1 overestimates the wind speed dependency. We test a recently developed sea spray aerosol source function derived from measurements made on a Southern Ocean research voyage in 2018. In this source function, the wind speed dependency of the sea spray aerosol flux is less than in the formulation currently implemented in HadGEM3-GA7.1. The new source function leads to good agreement between simulated and observed wintertime AODs over the Southern Ocean; however, it reveals partially compensating errors in DMS-derived AOD. While previous work has tested assumptions regarding the seawater climatology or sea–air flux of DMS, we test the sensitivity of simulated AOD, cloud condensation nuclei and cloud droplet number concentration to three atmospheric sulfate chemistry schemes. The first scheme adds DMS oxidation by halogens and the other two test a recently developed sulfate chemistry scheme for the marine troposphere; one tests gas-phase chemistry only, while the second adds extra aqueous-phase sulfate reactions. We show how simulated sulfur dioxide and sulfuric acid profiles over the Southern Ocean change as a result and how the number concentration and particle size of the soluble Aitken, accumulation and coarse aerosol modes are affected. The new DMS chemistry scheme leads to a 20% increase in the number concentration of cloud condensation nuclei and cloud droplets, which improves agreement with observations. Our results highlight the importance of atmospheric chemistry for simulating aerosols and clouds accurately over the Southern Ocean.

How to cite: Revell, L., Kremser, S., Hartery, S., Harvey, M., Mulcahy, J., Williams, J., Morgenstern, O., McDonald, A., Varma, V., Bird, L., and Schuddeboom, A.: The sensitivity of Southern Ocean aerosol concentrations to sea spray and DMS emissions in the HadGEM3-GA7.1 chemistry–climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5976, https://doi.org/10.5194/egusphere-egu2020-5976, 2020.

Mineral dust aerosols participate in the climate system and biogeochemistry processes due to its interactions with key components of Earth Systems: radiation, clouds, soil and chemical components. A central element to improve our understanding of mineral dust is through its modeling with Earth Systems Models where all these interactions are included. However, current simulations of dust variability exhibit important uncertainties and biases, which are model-dependent, whose cause is our imperfect knowledge about how to best represent the dust life cycle. For these reasons a continuous evaluation of the performance and properties of the different models compared against measurements is a crucial step to improve our knowledge of the dust cycle and its role in the climate system and biogeochemical cycles. Here we present an exhaustive evaluation of mineral dust aerosols in CRESCEND-ESMs over global, regional and local scales. We compare models against three networks of instruments for total dust deposition flux, yearly surface concentrations, and optical depths. Global and regional dust optical depths are compared with MODIS and MISR derived products. Specific analyses are done over the Sahel region where improved and compressive dust observational datasets are available. The results indicate that all the models capture the general properties of the global dust cycle, although the role of larger particles remains challenging. Differences are partially due to surface winds as nudged simulations improve the inter-model comparison and the performance in optical depth compared to MODIS. At the regional scale, there is an optical depth reasonable agreement over main source areas, but a joint inter-comparison including fluxes and concentration indicates larger differences. At the local scale, the uncertainties increase and current models are not able to reproduce together several observables at the same time.

How to cite: Checa-Garcia, R., Balkanski, Y., Bergman, T., Carslaw, K., Dalvi, M., Marticorena, B., Michou, M., Nabat, P., Nieradzik, L., Noije, T. V., O’Donnell, D., Olivie, D., O'Connor, F., Schulz, M., and Scott, C.: Properties and challenges of mineral dust aerosol modelling in the latest Earth System Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9236, https://doi.org/10.5194/egusphere-egu2020-9236, 2020.

We, together with 50 of our colleagues present a review on the interaction between tropical and boreal forests and the atmosphere, especially addressing their influence in the climate system. With its emissions of VOCs, aerosols and trace gases, with strong atmosphere interactions, forests are a key component of the climate system. These emissions and atmospheric processing regulates atmospheric chemistry and are the major source of cloud condensation nuclei (CCN) affecting cloud formation and development, and thus temperature and precipitation. Emissions from forests are thus closely connected to the hydrological and the carbon cycles, being  an essential integrated part of the climate system.

In terms of meteorology, tropical and boreal forests are very different. Temperature, solar radiation, precipitation, evapotranspiration, albedo, cloud structure and cover, convection etc., are all very different. However, the aerosols in the two systems show similarities as Primary Biological Aerosol Particles are the major component (70%) of coarse mode particles in Amazonia while Secondary Organic Aerosol in the tropics are mainly isoprene driven giving a slightly more hygroscopic SOA than the boreal monoterpene driven SOA. The organics constitutes 70 to 85% of PM1 mass for both boreal and tropical forests. In Amazonia, sulfates, nitrates and BC shows very low concentrations, while the boreal sites shows 2-3 times higher concentrations. The Siberian continental site and Amazonian site show remarkable similarities in the lack of new particle formation (NPF) which will be  discussed.

In the tropics dry season and boreal spring and early summer, increasing biomass burning emissions in both forest types dominates the aerosol composition, with high OC and BC concentrations while anthropogenic pollution influences boreal forest atmospheric composition during wintertime. The changes in diffuse to direct radiation due to scattering aerosols has important effects in tropical forests but minor in boreal, enhancing the net ecosystem exchange by 30% and 10% respectively. Thus the natural forest emissions affects the direct as well as the indirect forcing.

An Amazonia high altitude NPF process chain was recently observed at the top of the troposphere, and is an interesting interaction between forest emissions, cloud transport and processing and particle formation and aging at high altitudes that are brought back to the boundary layer, populating the CCN. For boreal forests, the complex relationship between GPP, BVOC, SOA, CCN, clouds, radiation, temperature and CO₂ show multiple pathways and feedbacks, and some of them can be quantified. All showing the complexity of the interaction between forests, atmosphere and climate.

How to cite: Hansson, H.-C., Artaxo, P., Andreae, M., and Kulmala, M.: Composition and Properties of the Natural Aerosol over the Boreal and Tropical Forests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11662, https://doi.org/10.5194/egusphere-egu2020-11662, 2020.

Absorption of shortwave radiation by dust depends on its iron oxide content. Iron oxides amount to just a few percents of dust mineralogy. In the Sahel, the amount of iron oxide in soils is significantly greater than over the rest of North Africa. Recent measurements from the AER-D campaign have evidenced the presence of large dust particles over Northern African sources, which measurements showed absorb higher shortwave radiation than smaller ones.

I present two 100-years simulations of the earth system model IPSLCM6, one with a detailed description of dust and one without dust. Over the summer months (JJAS), dust absorption amounts to 25 W.m^-2 over the region. The changes caused by this absorption to the water budget are analyzed. Dust absorption causes an increase of 16% of summer Western Sahel precipitation, whereas in the Eastern Sahel, summer precipitation is increased by 7%. The analysis is extended to evaporation, surface relative humidity, low-level clouds and total cloud liquid water path, all of which show a significant increase caused by absorbing dust.

The water budget over the Sahel is computed over an airshed that covers the region, 16W:36E and 10N:20N from the surface to 200mb, contrasting the water flux with and without aerosol absorption. Dust absorption causes a change in the mean circulation between 1000 and 800mb that induces an increased inflow of moist air at these levels at the western and southern Sahel boundaries during the summer monsoon.  Hence, it is important to understand the influence of aerosol absorption when studying the causes of variations in Sahel precipitation.

How to cite: Balkanski, Y.: The effect of dust absorption on Sahel precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13495, https://doi.org/10.5194/egusphere-egu2020-13495, 2020.

The effects of atmospheric aerosol particles on the Earth’s radiative balance are a major source of uncertainty in global climate models. A distinction and quantification between natural and anthropogenic atmospheric aerosol concentration and their sources has to be made to reduce this uncertainty. Therefore, the natural pre-industrial aerosol concentration of the atmosphere must be determined. Ice cores are climate archives that enable the reconstruction of past atmospheric composition changes.

For such a reconstruction, an ice core from the Swiss Alps, which covers the years from 1682-1985, was examined for secondary organic aerosol (SOA) compounds. A non-target analysis (NTA) was used to determine the chemical composition of small organic molecules in the ice. The analytical method of the melted ice samples is based on solid-phase extraction, liquid chromatography and high-resolution mass spectrometry. The result of the NTA showed more than 630 features statistically different from the blank. A hierarchical cluster analysis was performed, in which compounds with a similar trend over time were grouped (clustered) together. The cluster analysis separated the considered features into two main groups. The first cluster showed a good correlation with the dissolved organic carbon concentration (DOC) of non-fossil origin (R = 0.75) while the second main group correlated excellently with the fossil DOC (R = 0.95), attributed based on the radiocarbon content. This leads to the presumption that compounds represented in the first cluster originated from biogenic sources while compounds in the second cluster are anthropogenic emissions or SOA formed by anthropogenically emitted precursors. This hypothesis is supported by the temporal trend of the two groups. The potential biogenic compounds show a relative stable behavior throughout time.  At the beginning of the 20th century a decrease of biogenic SOA is recorded. No compounds from the anthropogenic cluster were detected in pre-industrial times, they increase slowly from 1800 and more and more from 1900. Based on the division into the two main clusters, a detailed graphical evaluation of their chemical composition was performed. We show that the suspected biogenic cluster consists mainly of oxidation products of volatile organic compounds (VOC). The presumed anthropogenic cluster consists mainly of organosulfates, nitrooxy-organosulfate, aromatic compounds and mono- and dinitroaromatics.

How to cite: Bachmeier, F., Vogel, A. L., Lauer, A., Fang, L., Arturi, K., Baltensperger, U., Bjelić, S., El Haddad, I., and Schwikowski, M.: Cluster analysis of organic molecules in an alpine ice core: the transition from the pre-industrial to the industrial era, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17620, https://doi.org/10.5194/egusphere-egu2020-17620, 2020.

Increasing the current understanding of how the Earth will respond to a warming climate requires a more accurate representation of aerosols by Earth system models (ESMs). Reducing current uncertainties associated with model estimates of climate change sensitivity to greenhouse gas emissions is hampered by our understanding of the impact aerosol particles have on the radiative budget via their interactions with clouds. The complexity of such interactions leads to their imperfect representation in models.

Emissions of marine organic aerosols play a relevant role on cloud formation in regions where there is a high concentration of phytoplankton, for example in the Southern Ocean (SO). Comparisons between GCMs and satellite observations over the SO show that the models simulate too little reflection of shortwave radiation and this is strongly linked with modelled cloud properties. A potential cause of this issue is a source of missing aerosols in the ESM.

In this study we evaluate the ability of a state-of-the-art ESM, UKESM1, in reproducing aerosol particles originating from organic marine sources that reach a measurement station in pristine air through long-range transport. UKESM1 is developed by the Met. Office and our simulation is nudged by reanalysis datasets for a fair comparison with observations. This ESM is unique in using its ocean biogeochemistry module to interactively simulate the emission of marine organic aerosols. To this end, a novel Lagrangian trajectory framework for evaluating GCMs has been developed. This method makes use of satellite measurements of chlorophyll concentration (a proxy of phytoplankton abundance in the sea surface) at the sea surface, together with the cloud condensation nuclei (CCN) concentration at 0.5% of supersaturation measured at Cape Grim (Southern Ocean, Tasmania) station. Chlorophyll and wind speed data are collocated along air mass trajectories, which are calculated through the HYSPLIT model. A source-receptor analysis is then performed to look for potential spatial correlation between the collocated chlorophyll concentration experienced by air parcel paths coming from a defined clean air sector of the boundary layer (to avoid anthropogenic influences) and CCN measurements. Additionally, a temporal correlation analysis is performed in this framework. This method is applied to both UKESM1 output data and observations to evaluate aerosol processes in climate models.

Preliminary results show a positive correlation in model data between marine organic activity and CCN production that is found also in the observations. Despite the model well representing the seasonal variability of CCN at the station, the model struggles to reproduce the positive relationship obtained from the observations between wind speed and CCN concentration during the winter season. This can be attributed to a potential missing source in the model.

How to cite: Tovazzi, E., Partridge, D., Haywood, J., Sellar, A., Mohit, D., and Kim, P.: Lagrangian evaluation of marine aerosols sources in an Earth System Model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19556, https://doi.org/10.5194/egusphere-egu2020-19556, 2020.

North Africa is very likely to warm over the coming century, but there is fundamental disagreement among climate model projections over the predicted response of rainfall to that warming. Geological records of wind-blown dust accumulating in marine sediment cores in the North Atlantic Ocean provide a way to assess the response of rainfall climate in the region to past intervals of global warmth.

Dust is transported to the North Atlantic Ocean from North Africa via two routes, a summer (northern) route and a winter (southern) route. Virtually everything we have learnt so far from marine sediment cores about North African hydroclimate has come from drill sites located beneath the summer (northern) dust plume. Here we report (i) geochemical records (radiogenic isotope (⁸⁷Sr/⁸⁶Sr and e_Nd) and XRF core scanning) from Ocean Drilling Project (ODP) Site 662 in the eastern equatorial Atlantic spanning the last 200,000 years and (ii) new ⁸⁷Sr/⁸⁶Sr and e_Nd data from North African dust sources. We redefine existing dust Preferential Source Areas (PSAs) into three geochemically distinct (Western, Central and Eastern) source regions. We show that ODP Site 662 is well-situated to study the palaeo-history of the previously under-studied African winter (southern) dust plume. We find that the primary source of terrigenous material to Site 662 throughout the past 200,000 years is palaeolake Megachad in the Central source region. This palaeolake basin is often described as the largest single dust source on Earth but comparatively little is known on geological timescales about its history. We show that its dust contribution to ODP Site 662 varies on orbital timescales, and that it reaches a minimum during insolation maxima, such as the last African Humid Period, coincident with lake high-stands. Large excursions in radiogenic isotope data reveal extreme variability in the relative strength of aeolian dust and distal riverine sources of terrigenous material, associated with hydroclimate change over the last 200 thousand years.

How to cite: Jewell, A., Burton, W., Kunkelova, T., Crocker, A., Röhl, U., Cooper, M., James, R., Xuan, C., Pike, A., Bakker, N., Bristow, C., Drake, N., and Wilson, P.: Provenance of the Saharan winter dust plume and its response to climatic variability over the last 200 kyr, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20321, https://doi.org/10.5194/egusphere-egu2020-20321, 2020.

Low-level clouds and fogs play a key role in the radiative balance over the Arctic pack ice by regulating surface energy fluxes. The radiative features of clouds are dependent on the amount of airborne aerosol particles and their properties, since the particles can act as CCN (cloud condensation nuclei) and INP (ice nucleating particles). As the Arctic climate is currently warming, the local emissions and formation mechanisms of aerosols are expected to change, possibly leading to altered cloud properties.

We measured aerosol chemical composition using FIGAERO-CIMS (Chemical Ionization Mass Spectrometer coupled to a Filter Inlet for Gases and AEROsols) analysis of samples collected during the MOCCHA campaign (Microbiology-Ocean-Cloud-Coupling in the High Arctic) close to the North Pole in 2018. The goal of the campaign was to investigate natural aerosol emissions from the ocean to the atmosphere during summertime in terms of local sources and potential contribution to cloud formation. The sampling period was therefore around the seasonal sea ice minimum in September. With our CIMS setup, the sample molecules are ionised by iodide ions (I^-). The negatively charged adducts are then separated by mass, allowing for characterisation on a molecular level. This is the first time aerosol chemical composition of High Arctic aerosols has been measured using this technique. As the current knowledge about the atmospheric composition in this region is low, our results suggest a potential for using this method for further aerosol chemical characterisation in the pristine Arctic environment.

Our analysis shows that sulphur-containing compounds were most abundant in the aerosol samples, including sulphuric acid, sulphur trioxide, methanesulphonic acid (MSA) and dimethyl sulphoxide (DMSO). MSA and DMSO are oxidation products of dimethyl sulphide (DMS), which is released by marine phytoplankton to the atmosphere under ice-free conditions. Non-sea-salt sulphate (nss-SO₄^2-) aerosols are known to be efficient CCN. The results will be compared to aerosol samples from the NASCENT campaign (Ny-Ålesund Aerosol and Cloud Experiment), analysed using the same CIMS technique. The campaign runs for a year during 2019-2020 at the Zeppelin station in Svalbard. Our findings are expected to contribute to better understanding of the connection between aerosols and cloud formation in the polar regions and the effects on the ocean and pack ice.

How to cite: Siegel, K., Zieger, P., Salter, M., Riipinen, I., Ekman, A. M. L., and Mohr, C.: Chemical composition of summertime High Arctic aerosols using chemical ionization mass spectrometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21012, https://doi.org/10.5194/egusphere-egu2020-21012, 2020.

The main observable quantity used on a global scale to map aerosols is aerosol optical depth (AOD), from ground-based and satellite remote sensing. It is at the same time an optical property and a vertically integrated quantity, and it is commonly used as the main metric towards which to pull aerosol models, through data assimilation, verification, and tuning. Here we introduce a few reflections on how to better constrain our knowledge of the Saharan Air Layer and its associated mineral dust, following results from the AER-D campaign.

AER-D was a small field experiment in the Eastern Atlantic during August 2015, based on the opportunity given by the simultaneous ICE-D experiment. The purpose of AER-D was to investigate the physical properties of the Saharan Air Layer, and to assess and validate remote sensing and modelling products. The FAAM atmospheric research aircraft was used as a flying laboratory, and it carried a full set of instruments aimed at both in-situ sampling and remote sensing.

A broad distribution of particle sizes was consistently observed, with a significant giant mode up to 80 µm, generally larger than what was observed in previous experiments: we ascribe this to the set of instruments used, able to capture the full spectrum. We will discuss the representation of the particle size in operational models, and we will show that despite predicting an extinction coefficient of the correct order of magnitude, the particle size is generally underestimated. We will also discuss the implication of the giant particles for the ground-based remote sensing of columnar size-distributions from the SKYNET and AERONET networks (Sunphotometer Airborne Validation Experiment, which was a component of AER-D).

We will present the vertical structure of the Saharan Air Layer, and in particular one episode when the structure was very different than the one generally accepted in the conceptual model. Moreover, the comparison with the operational models showed that they can predict a correct aerosol optical depth (AOD, a vertically integrated quantity) despite missing the vertical distribution.

These findings lead to a series of reflections on how to better constrain our knowledge of the Saharan Air Layer and its representation in operational models. Size-resolved properties and the vertical distribution are essential companions of the global AOD observations commonly used operationally. We will also discuss objectives and ideas for future field experiments.

How to cite: Marenco, F., Ryder, C., Estelles, V., and O'Sullivan, D.: Is aerosol optical depth a good metric to map dust properties? Lessons learned from AER-D, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22415, https://doi.org/10.5194/egusphere-egu2020-22415, 2020.