Global-scale atmospheric modeling of aerosols to assess metal source-receptor relationships for life cycle assessment

PurposeMetals have often been identified as the main contributors to (eco)toxicological impacts in life cycle assessment (LCA) studies. Indeed, environmental fate models are generally unsuitable for these substances as they were developed for organics. Recent work has focused on improving these models by accounting for biogeochemical conditions (e.g., pH, redox potential, organic matter, etc.). These conditions often dictate metal bioavailability and (eco)toxicity. However, biogeochemical conditions cannot be integrated into an LCA framework due to a lack of high-resolution spatially differentiated factors describing the metal atmospheric pathway. This paper aims to derive worldwide source-receptor relationships for aerosol particles to ascertain the atmospheric mechanisms (i.e., dispersion, transport, and deposition) of metals (i.e., copper, cadmium, lead, nickel, chromium, and zinc) at a relatively high resolution.MethodsWe compared black carbon, sulfate, and nitrate aerosols according to the framework developed by Roy et al. Atmos Environ 62:74-81, (2012), which requires the results of a year-long (2005) GEOS-Chem (a three-dimensional global-scale tropospheric model) simulation. These aerosols are used as proxies since metals may sorb with them for short- and long-range transport. Source-receptor matrices (SRMs), whose elements are fate factors, were calculated at a global 2 degrees x2.5 degrees resolution.Results and discussionThe atmospheric fate of metals, as described by black carbon and sulfate aerosols, is similar while the atmospheric fate of nitrate is significantly different: 70% of the black carbon or sulfate emissions deposits within in a radius of less than 2000km while this percentage drops to 40% with nitrate. Nitrate aerosol also showed the lowest agreement with EMEP modeling. Nitrate should not be considered as the optimal proxy. A case can be made for either sulfate or black carbon as proxies for metal; the latter is recommended as it showed the best agreement with EMEP modeling at the source location and similar agreement in the mid/long-range transport.ConclusionsThe SRMs outlined in this paper facilitate further modeling developments without having to run the underlying tropospheric model, thus paving the way for the assessment of the regional life cycle inventories of a global economy.