Evidence for a gas-flaring source of alkanes leading to elevated ozone in air above West Africa

Abstract

As part of the African Monsoon Multidisciplinary Analysis (AMMA) project, the FAAM BAe-146 research aircraft sampled the lower and mid-troposphere around the West Africa sub-region. Back trajectory analysis of the air parcels sampled on-board during the entire duration of the flights showed the history and fate of the air parcels. Data from flights B228 and B231 showed strongly enhanced carbon monoxide (CO) and ozone levels attributable to emissions of anthropogenic origin from the city of Lagos and gas flaring activities in the Nigeria oil fields. The elevated levels of ozone and CO observed at about 6 km above the sea-level on flight B231 were attributed to long-range transport of biomass burning plume from the East, around Sudan. The strongly enhanced mixing ratios of short-chained alkanes and CO (> 400 ppbv) observed from measurements on flights B228 and B231 are indicative of natural gas/combustion sources. Flight B222 sampled air parcels strongly impacted by emissions from Lagos but not from the Nigeria oil field and measured relatively lower mixing ratios of ozone, CO and short-chained alkanes species. Results from this study strongly suggests gas flaring emissions in the Niger Delta area to be a prominent contributor to the enhanced levels of short-chained alkane species observed in Lagos metropolis, especially during the West Africa Monsoon (WAM) months and, hence, a significant source of atmospheric aerosol in the sub-region. Key words: African Monsoon Multidisciplinary Analysis (AMMA), gas flaring, West Africa Monsoon, alkanes, ozone, Niger Delta.