Impact of the Guinea coast upwelling on atmospheric dynamics, precipitation and pollutant transport over southern West Africa

In West Africa, the zonal band of precipitation is generally located around the southern coast in June before migrating northward towards the Sahel in late June/early July. This gives way to a relative dry season for coastal regions from Côte d'Ivoire to Benin called “little dry season”, which lasts until September–October. Previous studies have noted that the coastal rainfall cessation in early July seems to coincide with the emergence of an upwelling along the Guinea coast. The aim of this study is to investigate the mechanisms by which this upwelling impacts precipitation, using a set of numerical simulations performed with the Weather Research and Forecasting regional atmospheric model (WRF v 3.7.1). Sensitivity experiments highlight the response of the atmospheric circulation to an intensification or reduction of the strength of the coastal upwelling. They clearly show that the coastal upwelling emergence is responsible for the cessation of coastal precipitation by weakening the northward humidity transport, thus decreasing the coastal convergence of the humidity transport and inhibiting the deep atmospheric convection. In addition, the diurnal cycle of the low-level circulation plays a critical role: the land breeze controls the seaward convergence of diurnal anomaly of humidity transport, explaining the late night–early morning peak observed in coastal precipitation. The emergence of the coastal upwelling strongly attenuates this peak because of a reduced land–sea temperature gradient in the night and a weaker land breeze. The impact on the inland transport of anthropogenic pollution is also shown with numerical simulations of aerosols using the CHIMERE chemistry-transport model: warmer (colder) sea surface temperature (SST) increases (decreases) the inland transport of pollutants, especially during the night, suggesting an influence of the upwelling intensity on the coastal low-level jet. The mechanisms described have important consequences for inland humidity transport and the predictability of the West African monsoon precipitation in summer.</p

Retrieval of aerosol optical depth over the Arctic cryosphere during spring and summer using satellite observations

The climate in the Arctic has warmed much more quickly in the last 2 to 3 decades than at the mid-latitudes, i.e., during the Arctic amplification (AA) period. Radiative forcing in the Arctic is influenced both directly and indirectly by aerosols. However, their observation from ground or airborne instruments is challenging, and thus measurements are sparse. In this study, total aerosol optical depth (AOD) is determined from top-of-atmosphere reflectance measurements by the Advanced Along-Track Scanning Radiometer (AATSR) on board ENVISAT over snow and ice in the Arctic using a retrieval called AEROSNOW for the period 2003 to 2011. AEROSNOW incorporates an existing aerosol retrieval algorithm with a cloud-masking algorithm, alongside a novel quality-flagging methodology specifically designed for implementation in the high Arctic region (≥ 72∘ N). We use the dual-viewing capability of the AATSR instrument to accurately determine the contribution of aerosol to the reflection at the top of the atmosphere for observations over the bright surfaces of the cryosphere in the Arctic. The AOD is retrieved assuming that the surface reflectance observed by the satellite can be well parameterized by a bidirectional snow reflectance distribution function (BRDF). The spatial distribution of AOD shows that high values in spring (March, April, May) and lower values in summer (June, July, August) are observed. The AEROSNOW AOD values are consistent with those from collocated Aerosol Robotic Network (AERONET) measurements, with no systematic bias found as a function of time. The AEROSNOW AOD in the high Arctic was validated by comparison with ground-based measurements at the PEARL, OPAL, Hornsund, and Thule stations. The AEROSNOW AOD value is less than 0.15 on average, and the linear regression of AEROSNOW and AERONET total AOD yields a slope of 0.98, a Pearson correlation coefficient of R=0.86, and a root mean square error (RMSE) of =0.01 for the monthly scale in both spring and summer. The AEROSNOW observation of increased AOD values over the high Arctic cryosphere during spring confirms clearly that Arctic haze events were well captured by this dataset. In addition, the AEROSNOW AOD results provide a novel and unique total AOD data product for the springtime and summertime from 2003 to 2011. These AOD values, retrieved from spaceborne observation, provide a unique insight into the high Arctic cryospheric region at high spatial resolution and temporal coverage.</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'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 (< 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

Ozone anomalies in the free troposphere during the COVID-19 pandemic

Using the CAM-chem Model, we simulate the response of chemical species in the free troposphere to scenarios of primary pollutant emission reductions during the COVID-19 pandemic. Zonally averaged ozone in the free troposphere during Northern Hemisphere spring and summer is found to be 5%-15% lower than 19-yr climatological values, in good agreement with observations. About one third of this anomaly is attributed to the reduction scenario of air traffic during the pandemic, another third to the reduction scenario of surface emissions, the remainder to 2020 meteorological conditions, including the exceptional springtime Arctic stratospheric ozone depletion. For the combined emission reductions, the overall COVID-19 reduction in northern hemisphere tropospheric ozone in June is less than 5 ppb below 400 hPa, but reaches 8 ppb at 250 hPa. In the Southern Hemisphere, COVID-19 related ozone reductions by 4%-6% were masked by comparable ozone increases due to other changes in 2020

Adult cardiac myocytes survive and remain excitable during long-term culture on synthetic supports

AbstractObjective: Cardiomyocytes can be transplanted successfully into skeletal and cardiac muscle. Our goal was to determine the feasibility of grafting cardiomyocytes onto various synthetic supports to create an excitable and viable tissue for implantation. Methods: Adult rat cardiomyocytes were cultured over an 8-week period onto different substitutes, including human glutaraldehyde-treated pericardium (n = 3), equine glutaraldehyde-treated pericardium (n = 3), polytetrafluoroethylene (n = 8), Dacron polyester (n = 16), and Vicryl polyglactin (n = 8). Results: Only the cells seeded on the Dacron survived, with the synthetic fibers colonized at 8 weeks. On the other supports, the number of myocytes progressively decreased from the first week, with their density (number of cells per square millimeter) being, after 20 days, 17 ± 2 on the polytetrafluoroethylene and 5 ± 1 on the human or equine pericardium compared with 45 ± 3 on the Dacron. After 8 weeks of culture on Dacron, the sarcomeric protein (sarcomeric α-actinin) was detected in all cells. In addition, the staining was regularly arranged and well aligned in a striated pattern. Spontaneous beating activity was obtained. Moreover, electrical stimulation of the cell preparation resulted in the generation of calcium transients, the frequency of which followed the frequency of the electrical stimulation. Conclusions: These results suggest that adult cardiac myocytes remain viable and excitable during long-term culture on a 3-dimensional Dacron support, which might constitute a new synthetic cardiac tissue. (J Thorac Cardiovasc Surg 2001;121:510-9