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Particulate-phase mercury emissions from biomass burning and impact on resulting deposition: a modelling assessment
De Simone, F., P. Artaxo, M. Bencardino, S. Cinnirella, F. Carbone, F. D'Amore, A. Dommergue, X. Bin Feng, C. Gencarelli, I. Hedgecock, M. Landis, F. Sprovieri, N. Suzuki, I. Wangberg, AND N. Pirrone. Particulate-phase mercury emissions from biomass burning and impact on resulting deposition: a modelling assessment. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 17:1881-1899, (2017).
Emissions from biomass burning (BB) are an important source of mercury (Hg) to the atmosphere (De Simone et al., 2015; Friedli et al., 2009), and a major factor in determining the inter-annual variations of its tropospheric concentration (Slemr et al., 2016). Although the Hg released by BB varies from year to year, it can amount to up to roughly one third of the anthropogenic emission estimates (AMAP/UNEP, 2013; Friedli et al., 2009; De Simone et al., 2015). With the eventual implementation of the Minamata Convention (http://www.mercuryconvention.org/) and future curbs on industrial emission, as a by-product of industrial emission abatement measures, its relative importance will increase in the coming years. A previous modelling study (De Simone et al., 2015), used the global Hg chemistry model, ECHMERIT, and three BB inventories to assess the distribution of Hg deposition resulting from BB. A large part of the Hg released from BB deposits over oceans, where its re-emission is driven by sea surface temperature among other factors (Carbone et al., 2016; Andersson et al., 2011), or where it can be converted to toxic methyl mercury (MeHg) compounds, with important implications for the food web, and through fish consumption, also for human health (see Chen et al. (2016) and references therein). The deposition flux of Hg from BB has been shown to be more sensitive to certain factors, in particular the chemical mechanism employed in the model and the choice of emission inventory, than to others such as the vertical profiles of emissions (De Simone et al., 2015). In this previous study all Hg emitted from BB was considered to be Hg0(g). There is, however, evidence that the fraction of Hg emitted bound to particulates (HgP) may be sizeable, up to 30%, especially when the Fuel Moisture Content (FMC) is high (Obrist et al., 2007; Finley et al., 2009; Friedli et al., 2009;Wang et al., 2010). These levels however remain uncertain since different methodologies have led to different conclusions (Zhang et al., 2013; Obrist et al., 2007). Little is known about the mechanisms that control the speciation of Hg in BB emissions, which leads to uncertainties in the Hg deposition patterns, since the atmospheric lifetime of HgP, is significantly shorter than Hg0 (g), leading to greater local deposition. Local Hg deposition due to BB could have important repercussions in regions such as the South-East Asia where there is intensive rice cultivation, and which is subject to major BB events, especially during El Niño periods. Hg deposited to rice paddies can be readily converted to toxic MeHg that can accumulate in the grains (Wang et al., 2015; Feng et al., 2008; Meng et al., 2014; Zhang et al., 2010). Moreover it has been reported that HgP from BB deposited to foliage has the ability to enhance MeHg formation (Witt et al., 2009). The aim of this study is to investigate the effects on simulated deposition fluxes of Hg resulting from BB, when variations in HgP fraction and production processes are considered. The most recent version of the GFED BB emission inventory (van der Werf et al., 2010; Randerson et al., 2012; Mu et al., 2011), has been included in the global online Hg chemical transport model ECHMERIT, to simulate Hg deposition from BB for the year 2013 and to quantify the influence of variations in model inputs, assumptions and parametrisations.
Mercury (Hg) emissions from biomass burning (BB) are an important source of atmospheric Hg and a major factor driving the interannual variation of Hg concentrations in the troposphere. The greatest fraction of Hg from BB is released in the form of elemental Hg (Hg0(g)). However, little is known about the fraction of Hg bound to particulate matter (HgP) released from BB, and the factors controlling this fraction are also uncertain. In light of the aims of the Minamata Convention to reduce intentional Hg use and emissions from anthropogenic activities, the relative importance of Hg emissions from BB will have an increasing impact on Hg deposition fluxes. Hg speciation is one of the most important factors determining the redistribution of Hg in the atmosphere and the geographical distribution of Hg deposition. Using the latest version of the Global Fire Emissions Database (GFEDv4.1s) and the global Hg chemistry transport model, ECHMERIT, the impact of Hg speciation in BB emissions, and the factors which influence speciation, on Hg deposition have been investigated for the year 2013. The role of other uncertainties related to physical and chemical atmospheric processes involving Hg and the influence of model parametrisations were also investigated, since their interactions with Hg speciation are complex. The comparison with atmospheric HgP concentrations observed at two remote sites, Amsterdam Island (AMD) and Manaus (MAN), in the Amazon showed a significant improvement when considering a fraction of HgP from BB. The set of sensitivity runs also showed how the quantity and geographical distribution of HgP emitted from BB has a limited impact on a global scale, although the inclusion of increasing fractions HgP does limit Hg0(g) availability to the global atmospheric pool. This reduces the fraction of Hg from BB which deposits to the world's oceans from 71 to 62 %. The impact locally is, however, significant on northern boreal and tropical forests, where fires are frequent, uncontrolled and lead to notable Hg inputs to local ecosystems. In the light of ongoing climatic changes this effect could be potentially be exacerbated in the future.