Abstract
Between March 2009 and March 2011 a commercial airliner equipped with a custom built measurement container (IAGOS-CARIBIC observatory) conducted 13 flights between South Africa and Germany at 10–12 km altitude, traversing the African continent north-south. In-situ measurements of trace gases (CO, CH4, H2O) and aerosol particles indicated that strong surface sources (like biomass burning) and rapid vertical transport combine to generate maximum concentrations in the latitudinal range between 10°N and 10°S coincident with the inter-tropical convergence zone (ITCZ). Pressurized air samples collected during these flights were subsequently analyzed for a suite of trace gases including C2-C8 non-methane hydrocarbons (NMHC) and halocarbons. These shorter-lived trace gases, originating from both natural and anthropogenic sources, also showed near equatorial maxima highlighting the effectiveness of convective transport in this region. Two source apportionment methods were used to investigate the specific sources of NMHC: positive matrix factorization (PMF), which is used for the first time for NMHC analysis in the upper troposphere (UT), and enhancement ratios to CO. Using the PMF method three characteristic airmass types were identified based on the different trace gas concentrations they obtained: biomass burning, fossil fuel emissions, and “background” air. The first two sources were defined with reference to previously reported surface source characterizations, while the term “background” was given to air masses in which the concentration ratios approached that of the lifetime ratios. Comparison of enhancement ratios between NMHC and CO for the subset of air samples that had experienced recent contact with the planetary boundary layer (PBL) to literature values showed that the burning of savanna and tropical forest is likely the main source of NMHC in the African upper troposphere (10–12 km). Photochemical aging patterns for the samples with PBL contact revealed that the air had different degradation histories depending on the hemisphere in which they were emitted. In the southern hemisphere (SH) air masses experienced more dilution by clean background air whereas in the northern hemisphere (NH) air masses are less diluted or mixed with background air still containing longer lived NMHC. Using NMHC photochemical clocks ozone production was seen in the BB outflow above Africa in the NH.
Highlights
Only 16% of the world population lives in Africa and industrialization is low, its geographical position across the southern and northern subtropics; the frequent occurrence of lightning and wild fires; its vast savannas and tropical forests and the seasonal migration of the inter-tropical convergence zone (ITCZ), all render this continent of considerable interest and importance for the chemistry of the global atmosphere
Within the trans-African CARIBIC flight dataset taken between March 2009 and March 2011, elevated mixing ratios were observed for CO, CH4, H2O and aerosol particles
Two of them could be attributed to trace gas emission sources, and one to background air
Summary
Only 16% of the world population lives in Africa and industrialization is low, its geographical position across the southern and northern subtropics; the frequent occurrence of lightning and wild fires; its vast savannas and tropical forests and the seasonal migration of the inter-tropical convergence zone (ITCZ), all render this continent of considerable interest and importance for the chemistry of the global atmosphere. TRACE-A (Transport and Atmospheric Chemistry near the Equator-Atlantic) and SAFARI-92 (Southern African Fire-Atmosphere Research Initiative) both investigated high ozone (O3) concentrations above the South Atlantic Ocean between August and October, which were discovered in the mid-1980s by satellite measurements Both campaigns concluded that BB in South Africa was the cause of the seasonal enhanced O3 levels (Fishman et al, 1996; Swap et al, 2003). SAFARI 2000 (the Southern African Regional Science Initiative) focused on the optical and radiative effects of BB plumes from southern Africa (Laakso et al, 2008) whereas the multidisciplinary approach of the AMMA program (African Monsoon Multidisciplinary Analysis) focused on the West African Monsoon Within this program, organic trace gases from BB emission plumes in both the upper and mid troposphere (Bechara et al, 2010; Haywood et al, 2008) were detected and related to O3 photochemistry (Real et al, 2010). The timespan of NMHC removal ranges from regional to intercontinental transport time scales (Baker et al, 2010; Honrath et al, 2008; Parrish et al, 2007, 1992)
Sign-in/Register to view highlights & summary 
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
