An Intensive Biomass Burning Aerosol Observation phase in 2022, over Skukuza, South Africa: CO transport and balance over Southern Africa

The Biomass Burning Aerosol Campaign (BiBAC) was conducted in the Kruger National Park (KNP), at Skukuza in South Africa during the 2022 biomass burning season. The campaign included an Intensive Observation Phase (IOP) from September to October, aiming to quantify aerosol optical properties and plume transport.(Ranaivombola et al., 2024). The combination of ground-based (sun-photometer), satellite observations (MODIS, IASI and CALIOP), and CAMS reanalysis show a significant aerosol and carbon monoxide (CO) loading linked to biomass burning activity. Using AOD data from sun-photometer observations, Ranaivombola et al., (2024) define two events of biomass burning plume over the Southwestern Indian Ocean (SWIO) basin: September 18 to 23 and October 9 to 17, called here after event 1 and event 2, respectively.

During Event 1, the plume was transported toward the SWIO basin as a "river of smoke" phenomenon. As reported previously in the literature (Swap et al., 2003 and Flamant et al., 2022), the meteorological conditions were influenced by the passage of westerly waves associated with a cut-off low (COL) that favored the eastern transport pathway. However, it was not the case during Event 1. There were two troughs which supported the formation of two frontal systems and contributed to the transport of aerosols and CO plumes from South America (SAm) towards Southern Africa (SA). This transport was driven by a westerly baroclinic wave through the mid-tropospheric layers.

Event 2 involved a more complex synoptic setup with three frontal systems supported by three distinct troughs, allowing the recirculation of plumes over SA. This dynamic system enhanced the transport of CO plumes from SAm, which merged with African plumes over the Mozambique Channel. The sustained activity of the baroclinic wave generated new troughs, keeping aerosol levels high for an extended period of 1.5 week. The progression of baroclinic waves and frontal system development were essential in driving regional and intercontinental transport of aerosols and CO plumes.

These two events allowed to reveal two transport mechanisms of aerosol plumes and CO between SAm and SA towards the SWIO basin. It shows also that SA is a target region for aerosols and CO from SAm biomass burning. To assess and quantify the contributions of SA and SAm sources to observed CO concentrations over SA, we used the FLEXPART model (version 10.4) coupled with CO emissions database (biomass burning and anthropogenic emission from CAMS: GFAS and CAMS-GLOB-ANT, respectively). Each simulation tracked particles representing CO back in time over a period of 20 days, during the IOP. The setup included daily releases of 20,000 particles over six sites in Southern Africa (Skukuza, Durban, Maun, Upington, Mongu and Gobabeb). Both SA and SAm sources significantly influenced the CO balance over SA. The contribution of biomass burning emissions from SA were higher than those from SAm. Nevertheless, the biomass burning emission from SAm were more variable and could occasionally match or exceed those from SA. This quantification confirmed the predominance of African sources but also highlighted the presence of intercontinental transport which is poorly investigated until now.