The impact of aerosols on stratiform clouds over southern West Africa: a large-eddy-simulation study

Low-level stratiform clouds (LLSCs) covering a large area appear frequently during the wet monsoon season in southern West Africa. This region is also a place where different types of aerosols coexist, including biomass burning aerosols coming from central and southern Africa and aerosols emitted by local anthropogenic activities. We investigate the indirect and semi-direct effects of these aerosols on the life cycle of LLSCs by conducting a case study based on airborne and ground-based observations from the field campaign of Dynamic-Aerosol-Chemistry-Cloud-Interaction in West Africa (DACCIWA). This case is modeled using a large-eddy-simulation (LES) model with fine resolution and in situ aerosol measurements, including size distribution and chemical composition. The model has successfully reproduced the observed life cycle of the LLSC, from stratus formation to stabilization during the night and to upward development after sunrise until break-up of the cloud deck in the late afternoon. Additional sensitivity simulations using different measured aerosol profiles also suggest that aerosols can affect the cloud life cycle through both the indirect and semi-direct effects. As expected, modeled cloud microphysical features, including cloud droplet number concentration, mean radius, and thus cloud reflectivity, are all controlled by aerosol concentration. However, it is found that the variation in cloud reflectivity induced by different aerosol profiles is not always the only factor in determining the incoming solar radiation at the ground and thus for the cloud life cycle after sunrise. Instead, the difference in cloud fraction brought by dry-air entrainment from above and thus the speed of consequent evaporation – also influenced by aerosol concentration – is another important factor to consider. Clouds influenced by higher aerosol concentrations and thus with a higher number concentration and smaller sizes of cloud droplets are found to evaporate more easily and thus impose a lower cloud fraction. In addition, our sensitivity runs including versus excluding aerosol direct radiative effects have also demonstrated the impacts specifically of solar absorption by black carbon on the cloud life cycle. The semi-direct effect resulting from an excessive atmospheric heating of up to 12 K d−1 by black carbon in our modeled cases is found to lower the cloud top as well as the liquid water path, reducing surface incoming solar radiation and dry entrainment and increasing the cloud fraction.</p

Assessing the role of anthropogenic and biogenic sources on PM₁ over southern West Africa using aircraft measurements

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\u27Ivoire, 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 (<  2000 m), in particular their sources and chemical composition in view of the complex mix of both biogenic and anthropogenic emissions, based on measurements from a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) and ancillary instrumentation. Background concentrations (i.e. outside urban plumes) observed from the ATR42 indicate a fairly polluted region during the time of the campaign, with average concentrations of carbon monoxide of 131 ppb, ozone of 32 ppb, and aerosol particle number concentration ( >  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 (<  0.5 %). Average background PM1 in the region was 5.9 µg m−3 stp. During measurements of urban pollution plumes, mainly focusing on the outflow of Abidjan, Accra and Lomé, pollutants are significantly enhanced (e.g. average concentration of CO of 176 ppb, and aerosol particle number concentration of 6500 cm−3 stp), as well as PM1 concentration (11.9 µg m−3 stp). Two classes of organic aerosols were estimated based on C-ToF-AMS: particulate organic nitrates (pONs) and isoprene epoxydiols secondary organic aerosols (IEPOX–SOA). Both classes are usually associated with the formation of particulate matter through complex interactions of anthropogenic and biogenic sources. During DACCIWA, pONs have a fairly small contribution to OA (around 5 %) and are more associated with long-range transport from central Africa than local formation. Conversely, IEPOX–SOA provides a significant contribution to OA (around 24 and 28 % under background and in-plume conditions). Furthermore, the fractional contribution of IEPOX–SOA is largely unaffected by changes in the aerosol composition (particularly the SO4 concentration), which suggests that IEPOX–SOA concentration is mainly driven by pre-existing aerosol surface, instead of aerosol chemical properties. At times of large in-plume SO4 enhancements (above 5 µg m−3), the fractional contribution of IEPOX–SOA to OA increases above 50 %, suggesting only then a change in the IEPOX–SOA-controlling mechanism. It is important to note that IEPOX–SOA constitutes a lower limit to the contribution of biogenic OA, given that other processes (e.g. non-IEPOX isoprene, monoterpene SOA) are likely in the region. Given the significant contribution to aerosol concentration, it is crucial that such complex biogenic–anthropogenic interactions are taken into account in both present-day and future scenario models of this fast-changing, highly sensitive region

Intercomparison of air quality models in a megacity: Towards an operational ensemble forecasting system for São Paulo

An intercomparison of four air quality models is performed in the tropical megacity of Sao Paulo with the perspective of developing an air quality forecasting system based on a regional model ensemble. During three contrasting periods marked by different types of pollution events, we analyze the concentrations of the main regulated pollutants (Ozone, CO, SO2, NOx, PM2.5 and PM10) compared to observations of a dense air quality monitoring network. The modeled concentrations of CO, PM and NOx are in good agreement with the observations for the temporal variability and the range of variation. However, the transport of pollutants due to biomass burning pollution events can strongly affect the air quality in the metropolitan area of Sao Paulo with increases of CO, PM2.5 and PM10, and is associated with an important inter-model variability. Our results show that each model has periods and pollutants for which it has the best agreement. The observed day-to-day variability of ozone concentration is well reproduced by the models, as well as the average diurnal cycle in terms of timing. Overall the performance for ozone of the median of the regional model ensemble is the best in terms of time and magnitude because it takes advantage of the capabilities of each model. Therefore, an ensemble prediction of regional models is promising for an operational air quality forecasting system for the megacity of Sao Paulo.This article is a direct contribution to the research themes of the Klimapolis Lab-836 oratory (klimapolis.net), which is funded by the German Federal Ministry of Education837 and Research (BMBF)

Transport and Vertical Distribution of Urban Pollutants over the Guinean Gulf

International audienceIn the countries of the Guinean Gulf, the population has been growing rapidly during the last decades. The sustained economic growth is associated with increased emissions from traffic, industries, and households, and with high pollution levels. Particulate matter (PM10) emissions from urbanized areas are analyzed in the Guinean gulf coastal region by both models (WRF and CHIMERE) and observations during the beginning of the monsoon from May to July. From the Guinean gulf coast to the Sahel, the urban PM10 concentrations are highest in June, and they display frequent northward transport events. These urban pollution transport events occurred over the entire Guinean Gulf coastal region with a zonal gradient of low concentration in Abidjan to high concentration in Lagos. The main drivers are the absence of precipitation and low wind associated with the low boundary layer height. The major part of the urban pollution is transported at night in the surface layer (3 m/s), but a significant part of the pollution is caught by the low level jet and transported rapidly (15 m/s). All these results highlight specific atmospheric conditions leading to high urban pollution events along the coast and to pollution transport reaching the Sahel, which may severely impact human health

Impact des aérosols désertiques et du climat sur les épidémies de méningites au Sahel

Les épidémies de méningites sont un problème de santé publique majeur dans plusieurs pays d'Afrique appartenant à "la ceinture" des méningites. De novembre à mai, c est la saison sèche au Sahel qui est alors sous le vent d Harmattan provenant du Sahara et transportant des poussières au niveau du sol. La période du maximum de l incidence de la méningite (i.e. le rapport entre le nombre de cas de méningites et la population respective) coïncide avec la période des plus fortes températures et concentrations de poussières au Sahel. Une base de données rassemblant les relevés épidémiologiques hebdomadaires et les variables géophysiques a été créée à plusieurs échelles spatiales à partir de quatre variables issues d ERA-interim (température, humidité, force et angle du vent) et de l Aerosol Index (AI). Cependant l AI a été préalablement comparé aux concentrations de poussières mesurées au sol dans le contexte des études d impact des poussières sur la santé. A l échelle nationale, un décalage d une semaine a été montré entre l augmentation de l épaisseur optique en aérosols et l augmentation l incidence de la méningite. Ce décalage temporel a été retrouvé avec l AI à l échelle nationale et à l échelle des districts. Pendant les épidémies, un modèle statistique de l incidence a été développé à partir de la température et la concentration de poussières, permettant d expliquer 1/3 de la variance de l incidence observée. De plus, à l échelle nationale, les dates de dépassement de seuil épidémique sont particulièrement bien retrouvées (R = 0,94). Ces résultats montrent que le climat et les poussières du Sahara sont des facteurs à prendre en compte pour expliquer la saisonnalité des épidémies.Meningitis epidemics are a major public health problem in several African countries belonging to the meningitis belt . The dry season in the Sahel lasts from November to May, when the Harmattan wind, coming from the Sahara, carries dust at the ground level into this region. The period of maximum meningitis incidence (i.e. the ratio of the number of meningitis cases and the respective population) coincides with the period of highest temperatures and dust concentrations in the Sahel. A database with weekly epidemiological and geophysical parameters has been created at different spatial scales (district, region, national) based on four ERA-interim variables (temperature, humidity, wind force and wind angle) and the Aerosol Index (AI). However in a first step, the AI has been compared to dust concentration measurements at the ground level and thus been validated in the context of dust impact studies on health. At the national scale, a time-lag of several weeks has been showed between the increase of the aerosol optical thickness and the meningitis incidence. This time-lag was also found using the AI at the national scale and at the district scale. During the epidemic years, a statistical model of the weekly meningitis incidence has been established based on temperature and ground dust concentration. This model explains 1/3 of the variance of meningitis incidence observed. Moreover, the onset dates of the outbreaks are well retrieved at the country scale (R = 0.94). These results show that climate and Saharan dust must be taking into account to explain the seasonality of meningitis epidemics.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Suitability of OMI aerosol index to reflect mineral dust surface conditions: preliminary application for studying the link with meningitis epidemics in the Sahel.

12 pagesInternational audienceThe aimof this study is to analyze the suitability of remotely-sensed aerosol retrievals to progress in the understanding of the influence of desert dust on health, and particularly on meningitis epidemics. In the Sahel, meningitis epidemics are a serious public health issue. Social factors are of prime importance in the dynamics of the epidemics, however climate and environmental factors are also suspected to play an important role. This study focuses on three Sahelian countries (Burkina Faso,Mali and Niger) which are among the most concerned in the "meningitis belt" and affected by strong dust events every year. It investigates the capability of the aerosol index (AI) derived from OMI (ozone monitoring instrument) to represent the aerosol optical thickness (AOT) and the aerosol surface concentration (particulate matter b10 μm; PM10) at different time-steps. The comparison of the OMI-AI with ground-based measurements of AOT shows a good agreement at a daily time-step (R≈0.7). The correlation between OMI-AI and PM10 measurements is lower (R≈0.3) but it increases at a weekly time-step (R≈0.5). The difference in the level of correlation between the AOT and the PM10 is partly related to changes in the altitude of the dust layers, especially from April to June, the period of transition from the dry to the wet season. A temporal shift is observed in the occurrence of themaximum of PM10 concentration (March), of AOT (April) and of OMI-AI (June). Nevertheless, during the core of the dry season (January to March) when dust is transported at low altitude, the OMI-AI is able to correctly detect the dust events and to reproduce the dust variability at the regional scale. For dust impact studies on health, only the surface level is relevant. Thus, we conclude that the OMI-AI is suitable especially at a weekly time-step from January to March. In particular for meningitis impact studies, it appears as suitable from the onset to the maximum of the epidemics. A preliminary investigation of the link between the OMI-AI and the WHO weekly national epidemiological reports reveals a 1-week time-lag between the occurrence of dust and meningitis during the increasing phase of the disease

Impact of vegetation fires on tropospheric chemical composition in the Guinean Gulf and on megacities air quality.

International audienceIn the framework of the preparation of the "Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa" (DACCIWA) project, the tropospheric chemical composition in the megacities along the Guinean Gulf is studied using the WRF and CHIMERE regional models. Two simulations are performed for the May-July 2014 period, without and with biomass burning emissions. The impact of biomass burning from Central Africa is quantified for aerosol optical depth, gaseous species (ozone and carbon monoxide) and particulate matter with a mean mass median of diameter less than 10 μm (PM10, both concentrations and chemical composition). We show that vegetation fires in Central Africa represent an important contribution to air pollution in urbanized areas located in the Guinean Gulf. On average in July 2014, CO and O3 concentrations are increased in Abidjan (Ivory Coast) by 38.5% and 15.4% respectively. In Abidjan and Lagos (Nigeria), two of the biggest megacities in southern West Africa, a net increase of PM10 by 36.5% and 53.5% is quantified. The analysis of the chemical composition of PM10 shows that this increase is mainly related to an increase of Particulate Primary Matter and Particulate Organic Matter in the fine mode of the aerosol size distribution

The role of aerosol–radiation–cloud interactions in linking anthropogenic pollution over southern west Africa and dust emission over the Sahara

International audienceThe aerosol direct and indirect effects are studied over west Africa in the summer of 2016 using the coupled WRF-CHIMERE regional model including aerosol–cloud interaction parameterization. First, a reference simulation is performed and compared with observations acquired during the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) field campaign which took place in June and July 2016. Sensitivity experiments are also designed to gain insights into the impact of the aerosols dominating the atmospheric composition in southern west Africa (one simulation with halved anthropogenic emissions and one with halved mineral dust emissions). The most important effect of aerosol–cloud interactions is found for the mineral dust scenario, and it is shown that halving the emissions of mineral dust decreases the 2 m temperature by 0.5 K and the boundary layer height by 25 m on a monthly average (July 2016) and over the Saharan region. The presence of dust aerosols also increases (decreases) the shortwave (longwave) radiation at the surface by 25 W m−2. It is also shown that the decrease of anthropogenic emissions along the coast has an impact on the mineral dust load over west Africa by increasing their emissions in the Saharan region. It is due to a mechanism where particulate matter concentrations are decreased along the coast, imposing a latitudinal shift of the monsoonal precipitation and, in turn, an increase of the surface wind speed over arid areas, inducing more mineral dust emissions