Do Regional Aerosols Contribute to the Riverine Export of Dissolved Black Carbon?

Abstract

The fate of black carbon (BC), a stable form of thermally altered organic carbon produced during biomass and fuel combustion, remains an area of uncertainty in the global carbon cycle. The transfer of photosynthetically derived BC into extremely long‐term oceanic storage is of particular significance and rivers are the key linkage between terrestrial sources and oceanic stores. Significant fluvial fluxes of dissolved BC to oceans result from the slow release of BC from degrading charcoal stocks; however, these fluvial fluxes may also include undetermined contributions of aerosol BC, produced by biomass and fossil fuel combustion, which are deposited in river catchments following atmospheric transport. By investigation of the Paraíba do Sul River catchment in Southeast Brazil we show that aerosol deposits can be substantial contributors to fluvial fluxes of BC. We derived spatial distributions of BC stocks within the catchment associated with soil charcoal and with aerosol from both open biomass burning and fuel combustion. We then modeled the fluvial concentrations of dissolved BC (DBC) in scenarios with varying rates of export from each stock. We analyzed the ability of each scenario to reproduce the variability in DBC concentrations measured in four data sets of river water samples collected between 2010 and 2014 and found that the best performing scenarios included a 5–18% (135–486 Mg DBC year−1) aerosol contribution. Our results suggest that aerosol deposits of BC in river catchments have a shorter residence time in catchments than charcoal BC and, therefore, contribute disproportionately (with respect to stock magnitude) toward fluvial fluxes of BC.

Environmental Controls on the Riverine Export of Dissolved Black Carbon

“…The influence of aerosol BC on export has been demonstrated through direct 14 C isotopic methods, which identify DBC derived from fossil fuels (Wang et al, 2016), through spatial evidence for DBC fluxes from catchments without considerable charcoal stocks (Ding et al, 2015), and through the physical modeling of DBC inputs to channels (Jones et al, 2017). This finding follows a number of recent studies that have demonstrated the potential of aerosol BC to contribute nontrivially toward the DBC load of river channels.…”

“…Full details of this procedure are provided in supporting information Text S4 (Reisser et al, 2016;Willmott & Matsuura, 2000). HadGEM2-ES was run with the same setup as reported in detail in our previous work (Jones et al, 2017), including its resolution (1.875°× 1.25°), BC emission grids, and aerosol scheme. Aerosol BC Deposits Aerosol BC deposition was modeled at the scale of South America using the UK Met Office Hadley Centre Global Environment Model version 2 Earth system model (HadGEM2-ES; Figure 2; Collins et al, 2011;Jones et al, 2011).…”

“…3.4.6. Detail regarding the HadGEM2-ES model setup and interpolation procedure is provided in supporting information Text S4 (Akagi et al, 2010;Diehl et al, 2012;Granier et al, 2011;Jones et al, 2017;Lamarque et al, 2010;Olson et al, 2001;Ramankutty et al, 2008;van der Werf et al, 2010;van Leeuwen et al, 2014). HadGEM2-ES represents the life cycle of various aerosol species , including BC from fossil fuel and biofuel emissions and biomass burning aerosol, which comprises a BC component internally mixed with organic carbon (Haywood et al, 2003).…”

“…We previously reported that the contribution of BC aerosol to the riverine DBC load of the Paraíba do Sul River (study region AF1) was most likely to be in the region of 5-18% (Jones et al, 2017), while 15-22% of the riverine DBC load in major Chinese rivers was found to derive from fossil fuel aerosols (Wang et al, 2016). We previously reported that the contribution of BC aerosol to the riverine DBC load of the Paraíba do Sul River (study region AF1) was most likely to be in the region of 5-18% (Jones et al, 2017), while 15-22% of the riverine DBC load in major Chinese rivers was found to derive from fossil fuel aerosols (Wang et al, 2016).…”

“…The DBC exported by rivers derives from stocks of charcoal stored in the soils of river catchments Jaffe et al, 2013;Wagner et al, 2018) and from aerosol (soot) deposited from the atmosphere to river catchments (Jones et al, 2017;Wang et al, 2016). Black carbon (BC) is a product of biomass and fossil fuel combustion and is distinguished from bulk organic carbon (OC) by its condensed aromatic structure and low functionality (Dittmar, 2008;Masiello, 2004).…”