Analysis and fate of combustion residues in soil

Abstract

Combustion residues from biomass and fossil fuel burning, called black carbon (BC), make up a small but very significant part of the global carbon cycle. These residues are universally found in soils, lacustrine and marine sediments and the atmosphere. They can be involved in important global processes, including carbon sequestration, pollutant binding, solar radiation reactions and can also be used to reconstruct fire histories. Black carbon does not have a defined structure, and since it represents combustion residues and condensates from various sources, it is viewed as a combustion continuum, ranging from slightly charred biomass to highly condensed soot, rather than one substance. There are a variety of methods, each quantifying a different part of the combustion continuum and overlapping in detection regions in some cases. Results from these different methods are not comparable, and therefore compiling a composite picture of BC concentrations in the environment is difficult. Reference materials analysed for BC with different methods would assist in drawing conclusions from and elucidating incomparable results. A structured means of achieving such a goal would be an intercomparative study where a number of laboratories employing different methods, all measure these reference materials for black carbon, and compare results. In this study, 12 reference materials from different environments were chosen to serve as reference materials in standardising and validating BC results from various methods. The materials are divided into three sets: (i) laboratory-produced BC-rich materials (n-hexane soot, wood char, grass char); (ii) non-BC materials that could potentially interfere with analysis (melanoidin, shale, bituminous coal, lignite coal); and (iii) environmental matrices that probably contain BC (urban aerosol, harbour marine sediment, two soils, dissolved organic matter). Char reference materials did not exist, and were specifically synthesised. They were chemically characterised, compared to other synthesised and natural chars and found to represent charcoal from typical low temperature fires. All twelve materials were used in a intercomparative study of black carbon quantification techniques, involving 17 international laboratories, employing seven different thermal and chemical oxidation methods. The results for all the materials analysed with the different methods were very disparate. Most methods quantified black carbon in the BC-rich materials, except two, whose harsh oxidation techniques oxidised all the carbon in the charcoals. Many methods found BC in the non-BC materials, thereby failing to exclude biases from these potential interfering materials. There was a large variation in concentrations of BC measured in the environmental samples. The chemical and physical properties of the materials were studied to elucidate how they influenced the BC quantification with different methods. The BC-rich materials were very similar, but the soot had a more condensed structure than the chars. The non-BC potentially interfering materials share properties with the BC-rich materials, which could lead to false positive data from non-BC materials and an overestimation of BC. The environmental matrices have relatively high amounts of inorganic matter and metal oxides, which have the potential to catalyse or inhibit thermal and chemical reactions in black carbon analysis. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. The benzene polycarboxylic acid markers method, employed in the ring trial by our laboratory, gave reasonable results for all the materials, in particular the soil matrices, for which this method was designed. We used this method in a case study to quantify the BC stock in a BC-rich steppe soil in Russia. This soil was sampled twice, 100 years apart, and we determined a BC stock loss of 25% over the whole soil profile (up to 130 cm). We used these stock values to calculate a BC turnover rate of 212-541 years for this soil, according to different assumptions, which is two to five times faster than the turnover (100 years) currently ascribed to inert carbon by the Intergovernmental Panel on Climate Change. Thus, BC in soil is not as stable as is currently assumed. Because fossil fuel burning and vegetation fires will probably increase in future, it will be important to quantify the potential of BC as a carbon sink in soils and sediments. In the foreseeable future, BC method standardisation and calibration will be a continuing process