Remote biomass burning dominates southern West African air pollution during the monsoon

Abstract

<p><strong>Abstract.</strong> Vast quantities of agricultural land in southern and central Africa are burnt between June and September each year, which releases large concentrations of aerosols 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.</p> <p>Results presented here show that a significant mass of aged accumulation mode aerosol was present in the southern West African boundary layer, over both the ocean and the continent. A median dry aerosol concentration of 6.2 µg m<sup>−3</sup> (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<sup>−3</sup>. This concentration increased to a median of 11.1 µg m<sup>−3</sup> (8.6 to 15.7 µg m<sup>−3</sup>) in the immediate outflow from cities. In the continental air mass away from the cities, the median aerosol loading was 7.5 µg m<sup>−3</sup>, with an interquartile range of 4.2 µg m<sup>−3</sup>. 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 boundary layer of southern West Africa during the monsoon season.</p> <p>As a result, the large and growing emissions from the coastal cities are overlaid on an already substantial aerosol background. On a regional scale this renders cloud properties and precipitation less sensitive to future increases in anthropogenic emissions. 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.</p>

What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

“…Namely, the onset of West African monsoon flow is related to the surface pressure contrast between the SHL, which develops due to the intense surface heating during boreal summer (Lavaysse et al, 2009), and relatively cool waters of the eastern tropical Atlantic. Typically, the monsoon onset occurs around the end of June (Janicot et al, 2008;Sultan and Janicot, 2003), while during 2016 its onset occurred on 22 June (Knippertz et al, 2017). Therefore, the differences in the NLLJ for cloudy and clear nights found in previous work could be related to the analysis of a larger number of nights from the pre-monsoon or late monsoon period compared to our analysis.…”

What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

“…These dry layers most likely originated from a central and southern African dry air mass, which is typically filled with biomass burning aerosol, and is transported to the west over the eastern tropical Atlantic in the layer between 2-4 km a.g.l. (Haslett et al, 2019). Haslett et al (2019) have found that a considerable proportion of the biomass 25 burning aerosol (smoke) is entrained into the marine boundary layer and advected northwards with the southerly trade winds towards the Coast of Guinea.…”

“…(Haslett et al, 2019). Haslett et al (2019) have found that a considerable proportion of the biomass 25 burning aerosol (smoke) is entrained into the marine boundary layer and advected northwards with the southerly trade winds towards the Coast of Guinea. This conclusion is supported by the evidence of aged aerosol particles which were measured by a research aircraft (Flamant et al, 2018;Haslett et al, 2019).…”

“…Haslett et al (2019) have found that a considerable proportion of the biomass 25 burning aerosol (smoke) is entrained into the marine boundary layer and advected northwards with the southerly trade winds towards the Coast of Guinea. This conclusion is supported by the evidence of aged aerosol particles which were measured by a research aircraft (Flamant et al, 2018;Haslett et al, 2019). These studies suggest that reduced moisture over West Africa is a result of intrusions of dry air originating either from the Southern Hemisphere (with the biomass burning aerosol as a tracer) in its southern parts (Haslett et al, 2019) or from dry Saharan air layer in its northern parts (Schrage et al, 2007).…”

“…Furthermore, the near-surface measurements of specific humidity at Savè and from radiosondes at additional five locations 5 in the DACCIWA region suggest that stratus-free nights are on average 1 g kg −1 drier than stratus nights. These decreased values of humidity are observed during the vortex period in mid-July, when a southerly anticyclonic vortex, originating from Gabon in Central Africa, transported a dry air mass into the DACCIWA region (Knippertz et al, 2017;Haslett et al, 2019).…”