ALBERT

Description: Southern West Africa (SWA) is an African pollution hotspot but a relatively poorly sampled region of the world. We present an overview of in situ aerosol optical measurements collected over SWA in June and July 2016 as part as of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) airborne campaign. The aircraft sampled a wide range of air masses, including anthropogenic pollution plumes emitted from the coastal cities, long-range transported biomass burning plumes from central and southern Africa and dust plumes from the Sahara and Sahel region, as well as mixtures of these plumes. The specific objective of this work is to characterize the regional variability of the vertical distribution of aerosol particles and their spectral optical properties (single scattering albedo: SSA, asymmetry parameter, extinction mass efficiency, scattering Ångström exponent and absorption Ångström exponent: AAE). The first findings indicate that aerosol optical properties in the planetary boundary layer were dominated by a widespread and persistent biomass burning loading from the Southern Hemisphere. Despite a strong increase in aerosol number concentration in air masses downwind of urban conglomerations, spectral SSA were comparable to the background and showed signatures of the absorption characteristics of biomass burning aerosols. In the free troposphere, moderately to strongly absorbing aerosol layers, dominated by either dust or biomass burning particles, occurred occasionally. In aerosol layers dominated by mineral dust particles, SSA varied from 0.81 to 0.92 at 550 nm depending on the variable proportion of anthropogenic pollution particles externally mixed with the dust. For the layers dominated by biomass burning particles, aerosol particles were significantly more light absorbing than those previously measured in other areas (e.g. Amazonia, North America), with SSA ranging from 0.71 to 0.77 at 550 nm. The variability of SSA was mainly controlled by variations in aerosol composition rather than in aerosol size distribution. Correspondingly, values of AAE ranged from 0.9 to 1.1, suggesting that lens-coated black carbon particles were the dominant absorber in the visible range for these biomass burning aerosols. Comparison with the literature shows a consistent picture of increasing absorption enhancement of biomass burning aerosol from emission to remote location and underscores that the evolution of SSA occurred a long time after emission. The results presented here build a fundamental basis of knowledge about the aerosol optical properties observed over SWA during the monsoon season and can be used in climate modelling studies and satellite retrievals. In particular and regarding the very high absorbing properties of biomass burning aerosols over SWA, our findings suggest that considering the effect of internal mixing on absorption properties of black carbon particles in climate models should help better assess the direct and semi-direct radiative effects of biomass burning particles.

Description: Vast stretches of agricultural land in southern and central Africa are burnt between June and September each year, which releases large quantities of aerosol into the atmosphere. The resulting smoke plumes are carried west over the Atlantic Ocean at altitudes between 2 and 4 km. As only limited observational data in West Africa have existed until now, whether this pollution has an impact at lower altitudes has remained unclear. The Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) aircraft campaign took place in southern West Africa during June and July 2016, with the aim of observing gas and aerosol properties in the region in order to assess anthropogenic and other influences on the atmosphere. Results presented here show that a significant mass of aged accumulation mode aerosol was present in the southern West African monsoon layer, over both the ocean and the continent. A median dry aerosol concentration of 6.2 µg m−3 (standard temperature and pressure, STP) was observed over the Atlantic Ocean upwind of the major cities, with an interquartile range from 5.3 to 8.0 µg m−3. This concentration increased to a median of 11.1 µg m−3 (8.6 to 15.7 µg m−3) in the immediate outflow from cities. In the continental air mass away from the cities, the median aerosol loading was 7.5 µg m−3 (5.9 to 10.5 µg m−3). The accumulation mode aerosol population over land displayed similar chemical properties to the upstream population, which implies that upstream aerosol is a significant source of aerosol pollution over the continent. The upstream aerosol is found to have most likely originated from central and southern African biomass burning. This demonstrates that biomass burning plumes are being advected northwards, after being entrained into the monsoon layer over the eastern tropical Atlantic Ocean. It is shown observationally for the first time that they contribute up to 80 % to the regional aerosol loading in the monsoon layer over southern West Africa. Results from the COSMO-ART (Consortium for Small-scale Modeling – Aerosol and Reactive Trace gases) and GEOS-Chem models support this conclusion, showing that observed aerosol concentrations over the northern Atlantic Ocean can only be reproduced when the contribution of transported biomass burning aerosol is taken into account. As a result, the large and growing emissions from the coastal cities are overlaid on an already substantial aerosol background. Simulations using COSMO-ART show that cloud droplet number concentrations can increase by up to 27 % as a result of transported biomass burning aerosol. On a regional scale this renders cloud properties and precipitation less sensitive to future increases in anthropogenic emissions. In addition, such high background loadings will lead to greater pollution exposure for the large and growing population in southern West Africa. These results emphasise the importance of including aerosol from across country borders in the development of air pollution policies and interventions in regions such as West Africa.

Description: During the monsoon season, pollutants emitted by large coastal cities and biomass burning plumes originating from central Africa have complex transport pathways over southern West Africa (SWA). The Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) field campaign has provided numerous dynamical and chemical measurements in and around the super-site of Savè in Benin (≈185 km away from the coast), which allows quantification of the relative contribution of advected pollutants. Through the combination of in situ ground measurements with aircraft, radio-sounding, satellite, and high-resolution chemistry-transport modeling with the CHIMERE model, the source attribution and transport pathways of pollutants inland (here, NOx and CO) are carefully analyzed for the 1–7 July 2016 period. The relative contributions of different sources (i.e., emissions from several large coastal cities) to the air quality in Savè are characterized. It is shown that a systematic diurnal cycle exists with high surface concentrations of pollutants from 18:00 to 22:00 UTC. This evening peak is attributed to pollution transport from the coastal city of Cotonou (Benin). Numerical model experiments indicate that the anthropogenic pollutants are accumulated during the day close to the coast and transported northward as soon as the daytime convection in the atmospheric boundary layer ceases after 16:00 UTC, reaching 8∘ N at 21:00 UTC. When significant biomass burning pollutants are transported into continental SWA, they are mixed with anthropogenic pollutants along the coast during the day, and this mixture is then transported northward. At night, most of the coastal anthropogenic plumes are transported within the planetary boundary layer (below about 500 m above ground level), whereas the biomass burning pollutants are mostly transported above it, thus generally not impacting ground level air quality.

Description: In this study, vertical profiles and column-integrated aerosol properties retrieved by the GRASP (Generalized Retrieval of Atmosphere and Surface Properties) algorithm are evaluated with in situ airborne measurements made during the ChArMEx-ADRIMED field campaign in summer 2013. In the framework of this campaign, two different flights took place over Granada (Spain) during a desert dust episode on 16 and 17 June. The GRASP algorithm, which combines lidar and sun–sky photometer data measured at Granada, was used to retrieve aerosol properties. Two sun-photometer datasets are used: one co-located with the lidar system and the other in the Cerro Poyos station, approximately 1200 m higher than the lidar system but at a short horizontal distance. Column-integrated aerosol microphysical properties retrieved by GRASP are compared with AERONET products showing a good agreement. Differences between GRASP retrievals and airborne extinction profiles are in the range of 15 to 30 %, depending on the instrument on board the aircraft used as reference. On 16 June, a case where the dust layer was coupled to the aerosol layer close to surface, the total volume concentration differences between in situ data and GRASP retrieval are 15 and 36 % for Granada and Cerro Poyos retrievals, respectively. In contrast, on 17 June the dust layer was decoupled from the aerosol layer close to the surface, and the differences are around 17 % for both retrievals. In general, all the discrepancies found are within the uncertainly limits, showing the robustness and reliability of the GRASP algorithm. However, the better agreement found for the Cerro Poyos retrieval with the aircraft data and the vertical homogeneity of certain properties retrieved with GRASP, such as the scattering Ångström exponent, for cases with aerosol layers characterized by different aerosol types, shows that uncertainties in the vertical distribution of the aerosol properties have to be considered. The comparison presented here between GRASP and other algorithms (i.e. AERONET and LIRIC) and with airborne in situ measurements shows the potential to retrieve the optical and microphysical profiles of the atmospheric aerosol properties. Also, the advantage of GRASP versus LIRIC is that GRASP does not assume the results of the AERONET inversion as a starting point.

Description: Comprehensive field campaigns dedicated to fog life cycle observation were conducted during the winters of 2010–2013 at the Instrumented Site for Atmospheric Remote Sensing Research (SIRTA) observatory in a suburb of Paris. In order to document their properties, in situ microphysical measurements collected during 23 fog events induced by both radiative cooling and stratus lowering are examined here. They reveal large variability in number, concentration and size of both aerosol background before the fog onset and fog droplets according to the different cases. The objective of this paper is to evaluate the impact of aerosol particles on the fog microphysics. To derive an accurate estimation of the actual activated fog droplet number concentration Nact, we determine the hygroscopicity parameter κ, the dry and the wet critical diameter and the critical supersaturation for each case by using an iterative procedure based on the κ-Köhler theory that combines cloud condensation nuclei (CCN), dry particle and droplet size distribution measurements. Our study reveals low values of the derived critical supersaturation occurring in fog with a median of 0.043 %. Consequently, the median dry and wet activation diameters are 0.39 and 3.79 µm, respectively, leading to a minor fraction of the aerosol population activated into droplets. The corresponding Nact values are low, with median concentrations of 53.5 and 111 cm−3 within the 75th percentile. The activated fraction of aerosols exhibits remarkably low correlation with κ values, which reflects the chemical composition of the aerosols. On the contrary, the activated fraction exhibits a strong correlation with the inferred critical diameter throughout the field campaigns. This suggests that the variability in the activated fraction is mostly driven by particle size, while variations in aerosol composition are of secondary importance. Moreover, our analysis suggests that the supersaturation reached in fog could be lowered by the aerosol number concentration, which could contribute to the sink term of water vapor during the radiative cooling. Although radiative fogs are usually associated with higher aerosol loading than stratus-lowering events, our analysis also reveals that the activated fraction at the beginning of the event is similar for both types of fog. However, the evolution of the droplet concentration during the fog life cycle shows significant differences between both types of fog. This work demonstrates that an accurate calculation of supersaturation is required to provide a realistic representation of fog microphysical properties in numerical models.

Description: As part of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project, an airborne campaign was designed to measure a large range of atmospheric constituents, focusing on the effect of anthropogenic emissions on regional climate. The presented study details results of the French ATR42 research aircraft, which aimed to characterize gas-phase, aerosol and cloud properties in the region during the field campaign carried out in June/July 2016 in combination with the German Falcon 20 and the British Twin Otter aircraft. The aircraft flight paths covered large areas of Benin, Togo, Ghana and Côte d'Ivoire, focusing on emissions from large urban conurbations such as Abidjan, Accra and Lomé, as well as remote continental areas and the Gulf of Guinea. This paper focuses on aerosol particle measurements within the boundary layer ( 15 nm) of 735 cm−3 stp. Regarding submicron aerosol composition (considering non-refractory species and black carbon, BC), organic aerosol (OA) is the most abundant species contributing 53 %, followed by SO4 (27 %), NH4 (11 %), BC (6 %), NO3 (2 %) and minor contribution of Cl (

Description: The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. This was the only flight conducted over the ocean during which a downward-looking lidar was operational. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou, and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations associated with the land–sea surface temperature contrast and orography over Ghana and Togo, as typically observed on hot, cloud-free summer days such as 2 July 2016. Furthermore, we show that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean, in the area of aircraft operation. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Given the general direction of the monsoon flow, the tracer experiments indicate no contribution from Lagos emissions to the atmospheric composition of the area west of Cotonou, where our airborne observations were gathered. The tracer plume does not extend very far south over the ocean (i.e. less than 100 km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain, and land–sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations can distribute atmospheric pollutants over the densely populated SWA region.

Description: Aerosol radiative properties are investigated in southeastern Spain during a dust event on 16–17 June 2013 in the framework of the ChArMEx/ADRIMED (Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) campaign. Particle optical and microphysical properties from ground-based sun/sky photometer and lidar measurements, as well as in situ measurements on board the SAFIRE ATR 42 French research aircraft, are used to create a set of different levels of input parameterizations, which feed the 1-D radiative transfer model (RTM) GAME (Global Atmospheric ModEl). We consider three datasets: (1) a first parameterization based on the retrievals by an advanced aerosol inversion code (GRASP; Generalized Retrieval of Aerosol and Surface Properties) applied to combined photometer and lidar data, (2) a parameterization based on the photometer columnar optical properties and vertically resolved lidar retrievals with the two-component Klett–Fernald algorithm, and (3) a parameterization based on vertically resolved optical and microphysical aerosol properties measured in situ by the aircraft instrumentation. Once retrieved, the outputs of the RTM in terms of both shortwave and longwave radiative fluxes are compared against ground and in situ airborne measurements. In addition, the outputs of the model in terms of the aerosol direct radiative effect are discussed with respect to the different input parameterizations. Results show that calculated atmospheric radiative fluxes differ no more than 7 % from the measured ones. The three parameterization datasets produce a cooling effect due to mineral dust both at the surface and the top of the atmosphere. Aerosol radiative effects with differences of up to 10 W m−2 in the shortwave spectral range (mostly due to differences in the aerosol optical depth) and 2 W m−2 for the longwave spectral range (mainly due to differences in the aerosol optical depth but also to the coarse mode radius used to calculate the radiative properties) are obtained when comparing the three parameterizations. The study reveals the complexity of parameterizing 1-D RTMs as sizing and characterizing the optical properties of mineral dust is challenging. The use of advanced remote sensing data and processing, in combination with closure studies on the optical and microphysical properties from in situ aircraft measurements when available, is recommended.

Description: Mineral dust from arid areas is a major component of global aerosol and has strong interactions with climate and biogeochemistry. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) to investigate atmospheric chemistry and its impacts in the Mediterranean region, an intensive field campaign was performed from mid-June to early August 2013 in the western basin including in situ balloon-borne aerosol measurements with the light optical aerosol counter (LOAC). LOAC is a counter/sizer that provides the aerosol concentrations in 19 size classes between 0.2 and 100 µm, and an indication of the nature of the particles based on dual-angle scattering measurements. A total of 27 LOAC flights were conducted mainly from Minorca Island (Balearic Islands, Spain) but also from Ile du Levant off Hyères city (SE France) under 17 light dilatable balloons (meteorological sounding balloons) and 10 boundary layer pressurised balloons (quasi-Lagrangian balloons). The purpose was to document the vertical extent of the plume and the time evolution of the concentrations at constant altitude (air density) by in situ observations. LOAC measurements are in agreement with ground-based measurements (lidar, photometer), aircraft measurements (counters), and satellite measurements (CALIOP) in the case of fair spatial and temporal coincidences. LOAC has often detected three modes in the dust particle volume size distributions fitted by lognormal laws at roughly 0.2, 4 and 30 µm in modal diameter. Thanks to the high sensitivity of LOAC, particles larger than 40 µm were observed, with concentrations up to about 10−4 cm−3. Such large particles were lifted several days before and their persistence after transport over long distances is in conflict with calculations of dust sedimentation. We did not observe any significant evolution of the size distribution during the transport from quasi-Lagrangian flights, even for the longest ones (∼ 1 day). Finally, the presence of charged particles is inferred from the LOAC measurements and we speculate that electrical forces might counteract gravitational settling of the coarse particles.