Cross-hemispheric transport of central African biomass burning pollutants: implications for downwind ozone production

Abstract

Abstract. Pollutant plumes with enhanced concentrations of trace gases and aerosols were observed over the southern coast of West Africa during August 2006 as part of the AMMA wet season field campaign. Plumes were observed both in the mid and upper troposphere. In this study we examined the origin of these pollutant plumes, and their potential to photochemically produce ozone (O 3 ) downwind over the Atlantic Ocean. Their possible contribution to the Atlantic O 3 maximum is also discussed. Runs using the BOLAM mesoscale model including biomass burning carbon monoxide (CO) tracers were used to confirm an origin from central African biomass burning fires. The plumes measured in the mid troposphere (MT) had significantly higher pollutant concentrations over West Africa compared to the upper tropospheric (UT) plume. The mesoscale modelCorrespondence to: E. Real (elsa.real@aero.jussieu.fr) reproduces these differences and the two different pathways for the plumes at different altitudes: transport to the northeast of the fire region, moist convective uplift and transport to West Africa for the upper tropospheric plume versus north-west transport over the Gulf of Guinea for the mid-tropospheric plume. Lower concentrations in the upper troposphere are mainly due to enhanced mixing during upward transport. Model simulations suggest that MT and UT plumes are 16 and 14 days old respectively when measured over West Africa. The ratio of tracer concentrations at 600 hPa and 250 hPa was estimated for 14-15 August in the region of the observed plumes and compares well with the same ratio derived from observed carbon dioxide (CO 2 ) enhancements in both plumes. It is estimated that, for the period 1-15 August, the ratio of Biomass Burning (BB) tracer concentration transported in the UT to the ones transported in the MT is 0.6 over West Africa and the equatorial South Atlantic.Published by Copernicus Publications on behalf of the European Geosciences Union. E. Real et al.: Cross-hemispheric transport of african biomass burningRuns using a photochemical trajectory model, CiTTy-CAT, initialized with the observations, were used to estimate in-situ net photochemical O 3 production rates in these plumes during transport downwind of West Africa. The mid-troposphere plume spreads over altitude between 1.5 and 6 km over the Atlantic Ocean. Even though the plume was old, it was still very photochemically active (mean net O 3 production rates over 10 days of 2.6 ppbv/day and up to 7 ppbv/day during the first days) above 3 km especially during the first few days of transport westward. It is also shown that the impact of high aerosol loads in the MT plume on photolysis rates serves to delay the peak in modelled O 3 concentrations. These results suggest that a significant fraction of enhanced O 3 in mid-troposphere over the Atlantic comes from BB sources during the summer monsoon period. According to simulated occurrence of such transport, BB may be the main source for O 3 enhancement in the equatorial south Atlantic MT, at least in August 2...

Impact of deep convection in the tropical tropopause layer in West Africa: in-situ observations and mesoscale modelling

“…An interesting extension of this work would be to implement the same analysis considering the whole monsoon season in order to characterize on the longer term (and larger scale) the impact of central/west African convection on the UT. Climatological analysis of METEOSAT data (Laing et al, 2008) shows Sahelian cold clouds often formed over Nigeria, Chad and Sudan, and possibly fed by trace species from biomass burning sources (Mari et al, 2008;Real et al, 2010). Extensive regional model simulations coupled with satellite data analysis would be necessary to quantify their impact on upper tropospheric composition.…”

“…West Africa is characterized by the presence of potentially important and poorly known sources of aerosol and ozone precursors. Large latitudinal variability of nitrogen oxides emissions in West Africa during the wet season was observed during AMMA and is reported by Stewart et al (2008); NO x levels in the Sahelian area are high leading to higher ozone mixing ratios (Saunois et al, 2009) while ozone precursors from biomass burning in the southern hemispheric African continent can be transported by deep convection in the upper troposphere up to 10 • N (Mari et al, 2008;Real et al, 2010).…”

Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility

“…There is a positive correlation between CO and the particle mixing ratio because of the import of pollution from upwind tropospheric sources like Asia and India for SCOUT-AMMA and from Indonesia (in connection with strong convective activity there) for Darwin (Brunner et al, 2009). Real et al (2010) showed that biomass burning activities took place in Central Africa at the time of the SCOUT-AMMA campaign and that these significantly influenced the air over West Africa. Air parcels of a given CO abundance contained higher aerosol loadings (upper "branch" in the middle panel of Fig.…”

“…Fierli et al (2010), used a regional mesoscale model together with the M-55 "Geophysica" observations of water vapour, ozone, CO 2 , and aerosol to demonstrate that signatures of the detrainment from deep convection of mesoscale convective systems can be found as high as 17 km, and possibly higher in some cases. Furthermore, the vertical ozone profiles seem to indicate (Homan et al, 2010) Cross hemispheric transport: Real et al (2010) used BOLAM mesoscale model calculations in conjunction with trace gas measurements to confirm the presence of biomass burning plume remnants from central African fires in the upper troposphere. These were sampled during the M-55 "Geophysica" flight on 13 August 2006 with corresponding signatures in CO, CO 2 , NO, NOy, and submicron aerosols.…”

Developing a portable, autonomous aerosol backscatter lidar for network or remote operations

“…Particulate matter is tiny particles suspended in the air and depending on their size and the meteorological conditions can travel great distances around the globe (Uno et al, 2009;McKendry et al, 2007). Some occur naturally: biogenic substances ), volcanoes (O'Neill et al, 2012Schumann et al, 2011;Winker et al, 2012), dust storms (Ben-Ami et al, 2009;Fischer et al, 2011;Liu et al, 2008;McKendry et al, 2009;Yumimoto et al, 2010), forest and grassland fires (Dirksen et al, 2009;McKendry et al, 2011) and sea spray (Kunz et al, 2002), while others result from human activities (anthropogenic), such as burning fossil fuels (Komppula et al, 2012), industrial by-products (Strawbridge, 2006) and altering natural surface cover (Real et al, 2010). Obtaining vertical profiles of tropospheric aerosols provides critically important information towards understanding climate, air quality and visibility.…”