Mapping the drivers of uncertainty in atmospheric selenium deposition with global sensitivity analysis

• Main Authors: A. Feinberg, M. Maliki, A. Stenke, B. Sudret, T. Peter, L. H. E. Winkel
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/1363/2020/acp-20-1363-2020.pdf

<p>An estimated 0.5–1 billion people globally have inadequate intakes of selenium (<span class="inline-formula">Se</span>), due to a lack of bioavailable <span class="inline-formula">Se</span> in agricultural soils. Deposition from the atmosphere, especially through precipitation, is an important source of <span class="inline-formula">Se</span> to soils. However, very little is known about the atmospheric cycling of <span class="inline-formula">Se</span>. It has therefore been difficult to predict how far <span class="inline-formula">Se</span> travels in the atmosphere and where it deposits. To answer these questions, we have built the first global atmospheric <span class="inline-formula">Se</span> model by implementing <span class="inline-formula">Se</span> chemistry in an aerosol–chemistry–climate model, SOCOL-AER (modeling tools for studies of SOlar Climate Ozone Links – aerosol). In the model, we include information from the literature about the emissions, speciation, and chemical transformation of atmospheric <span class="inline-formula">Se</span>. Natural processes and anthropogenic activities emit volatile <span class="inline-formula">Se</span> compounds, which oxidize quickly and partition to the particulate phase. Our model tracks the transport and deposition of <span class="inline-formula">Se</span> in seven gas-phase species and 41 aerosol tracers. However, there are large uncertainties associated with many of the model's input parameters. In order to identify which model uncertainties are the most important for understanding the atmospheric <span class="inline-formula">Se</span> cycle, we conducted a global sensitivity analysis with 34 input parameters related to <span class="inline-formula">Se</span> chemistry, <span class="inline-formula">Se</span> emissions, and the interaction of <span class="inline-formula">Se</span> with aerosols. In the first bottom-up estimate of its kind, we have calculated a median global atmospheric lifetime of 4.4&thinsp;<span class="inline-formula">d</span> (days), ranging from 2.9 to 6.4&thinsp;<span class="inline-formula">d</span> (2nd–98th percentile range) given the uncertainties of the input parameters. The uncertainty in the <span class="inline-formula">Se</span> lifetime is mainly driven by the uncertainty in the carbonyl selenide (<span class="inline-formula">OCSe</span>) oxidation rate and the lack of tropospheric aerosol species other than sulfate aerosols in SOCOL-AER. In contrast to uncertainties in <span class="inline-formula">Se</span> lifetime, the uncertainty in deposition flux maps are governed by <span class="inline-formula">Se</span> emission factors, with all four <span class="inline-formula">Se</span> sources (volcanic, marine biosphere, terrestrial biosphere, and anthropogenic emissions) contributing equally to the uncertainty in deposition over agricultural areas. We evaluated the simulated <span class="inline-formula">Se</span> wet deposition fluxes from SOCOL-AER with a compiled database of rainwater <span class="inline-formula">Se</span> measurements, since wet deposition contributes around 80&thinsp;% of total <span class="inline-formula">Se</span> deposition. Despite difficulties in comparing a global, coarse-resolution model with local measurements from a range of time periods, past <span class="inline-formula">Se</span> wet deposition measurements are within the range of the model's 2nd–98th percentiles at 79&thinsp;% of background sites. This agreement validates the application of the SOCOL-AER model to identifying regions which are at risk of low atmospheric <span class="inline-formula">Se</span> inputs. In order to constrain the uncertainty in <span class="inline-formula">Se</span> deposition fluxes over agricultural soils, we should prioritize field campaigns measuring <span class="inline-formula">Se</span> emissions, rather than laboratory measurements of <span class="inline-formula">Se</span> rate constants.</p>