Anthropogenic VOCs in Abidjan, southern West Africa: from source quantification to atmospheric impacts
Authors: Dominutti Pamela , Keita Sekou , Bahino Julien , Colomb Aurélie , Liousse Cathy , Yoboué Véronique , Galy-Lacaux Corinne , Morris Eleanor , Bouvier Laëtitia , Sauvage Stéphane , Borbon Agnès 
Published: September 2019

Abstract. Several field campaigns were conducted in the framework of the
Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA)
project to measure a broad range of atmospheric constituents. Here we
present the analysis of an unprecedented and comprehensive dataset
integrating up to 56 volatile organic compounds (VOCs) from ambient
sites and emission sources. VOCs were collected on multi-sorbent tubes in the
coastal city of Abidjan, Côte d'Ivoire, in winter and summer 2016 and
later analysed by gas chromatography coupled with flame ionization and mass
spectrometer detectors (GC-FID and GC-MS) at the laboratory. The comparison between VOC emission source profiles and ambient profiles
suggests the substantial impact of two-stroke motorized two-wheel vehicles
and domestic fires on the composition of Abidjan's atmosphere. However,
despite high VOC concentrations near-source, moderate ambient levels were
observed (by factors of 10 to 4000 lower), similar to the concentrations
observed in northern mid-latitude urban areas. Besides photochemistry, the
reported high wind speeds seem to be an essential factor that regulates air
pollution levels in Abidjan. Emission ratios (ΔVOC∕ΔCO) were established based on
real-world measurements achieved for a selected number of representative
combustion sources. Maximum measured molar mass emissions were observed from
two-wheel vehicles, surpassing other regional sources by 2 orders of
magnitude. Local practices like waste burning also make a significant
contribution to VOC emissions, higher than those from light-duty vehicles by
1.5 to 8 orders of magnitude. These sources also largely govern the VOC's
atmospheric impacts in terms of OH reactivity, secondary organic aerosol
formation (SOAP), and photochemical ozone creation potential (POCP). While
the contribution of aromatics dominates the atmospheric impact, our
measurements reveal the systematic presence of anthropogenic terpenoids in
all residential combustion sectors. Finally, emission factors were used to
retrieve and quantify VOC emissions from the main anthropogenic source
sectors at the national level. Our detailed estimation of VOC emissions
suggests that the road transport sector is the dominant source in Côte
d'Ivoire, emitting around 1200 Gg yr−1 of gas-phase VOCs. These new
estimates are 100 and 160 times larger than global inventory estimations
from MACCity or EDGAR (v4.3.2), respectively. Additionally, the residential
sector is largely underestimated in the global emission inventories, by
factors of 13 to 43. Considering only Côte d'Ivoire, these new estimates
for VOCs are 3 to 6 times higher than the whole of Europe. Given the
significant underestimation of VOC emissions from the transport and residential
sectors in Côte d'Ivoire, there is an urgent need to build more
realistic and region-specific emission inventories for the entire West
African region. This might be true not only for VOCs, but also for all atmospheric
pollutants. The lack of waste burning, wood fuel burning and charcoal
burning, and fabrication representation in regional inventories also needs to
be addressed, particularly in low-income areas where these types of
activities are ubiquitous sources of VOC emissions.