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

Atmospheric Chemistry and Physics (Impact Factor: 4.88). 01/2009; DOI: 10.5194/acpd-9-17385-2009
Source: DOAJ

ABSTRACT Pollutant plumes with enhanced levels 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 both the origin of these pollutant plumes and their potential to produce O3 downwind over the Atlantic Ocean. Runs using the BOLAM mesoscale model including biomass burning CO tracers were used to confirm an origin from central African fires. The plumes in the mid troposphere had significantly higher pollutant concentrations due to the fact that transport occurred from a region nearer or even over the fire region. In contrast, plumes transported into the upper troposphere over West Africa had been transported to the north-east of the fire region before being uplifted. Modelled tracer results showed that pollutants resided for between 9 and 12 days over Central Africa before being transported for 4 days, in the case of the mid-troposphere plume and 2 days in the case of the upper tropospheric plume to the measurement location over the southern part of West Africa. Around 35% of the biomass burning tracer was transported into the upper troposphere compared to that remaining in the mid troposphere. Runs using a photochemical trajectory model, CiTTyCAT, were used to estimate the net photochemical O3 production potential of these plumes. The mid tropospheric plume was still very photochemically active (up to 7 ppbv/day) especially during the first few days of transport westward over the Atlantic Ocean. The upper tropospheric plume was also still photochemically active, although at a slower rate (1–2 ppbv/day). Trajectories show this plume being recirculated around an upper tropospheric anticyclone back towards the African continent (around 20° S). The potential of theses plumes to produce O3 supports the hypothesis that biomass burning pollutants are contributing to the observed O3 maxima over the southern Atlantic at this time of year.

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Available from: D.J. Stewart, Jan 20, 2014
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    • "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 (Pierce et al., 2012), volcanoes (O'Neill et al., 2012; Schumann 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. "
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    • "In the upper troposphere lightning can produce NO X (LNOx) which can increase ozone mixing ratios [Jenkins et al., 2008; Nalli et al., 2011]. There is strong evidence to support the cross-equatorial transport of enhanced ozone mixing ratios from biomass burning in the Southern Hemisphere to the Guinea region [Real et al., 2010]. "
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    • "sian pollution by the TEJ may also impact the composition of the African UT . Another contribution to enhanced CO may come from BB emissions over Central Africa which can be redistributed to the UT . In fact , occasional plumes were measured over Gulf of Guinea at 100 – 150 hPa and attributed to uplift of Central African BB emissions into the UT ( Real et al . , 2010 ) . In the next section we show that indeed the chemical composition of the UT over West Africa is not only influenced by convec - tive transport of local emissions but also by Asian emissions and Central African BB emissions as discussed in a recent analysis of M55 data collected in the TTL ( Law et al . , 2010 ) . Figure 2 also shows "
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