Assessing the role of anthropogenic and biogenic sources on PM1 over southern West Africa using aircraft measurements
Authors: Brito Joel , Freney Evelyn , Dominutti Pamela , Borbon Agnes , Haslett Sophie L. , Batenburg Anneke M. , Colomb Aurelie , Dupuy Regis , Denjean Cyrielle , Burnet Frederic , Bourriane Thierry , Deroubaix Adrien , Sellegri Karine , Borrmann Stephan , Coe Hugh , Flamant Cyrille , Knippertz Peter , Schwarzenboeck Alfons 
Published: January 2018

Abstract. 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 (< 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.