Reconstructing recent atmospheric trace gas concentrations from polar firn and bubbly ice data by inverse methods

Abstract

Abstract. We present a method to extract the atmospheric signal of trace gas mixing ratios from firn and bubbly ice measurements. This method, validated using data from Antarctic sites (Vostok and DE08), includes a numerical model that simulates air transport in the firn, and inverse theory. We focus here on atmospheric CH4 reconstruction, but the method can be used to reconstruct recent changes in any trace gas elemental or isotopic composition measured in firn and ice, that has no chemical interaction with solid H20. This study also provides useful information on the effective diffusity-porosity relationship and quantifies the smoothing effect due to gas diffusion in firn and at pore closure, showing the typical time periods of events that are filtered by the firn and therefore not observed in the gas record of ice cores.

Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s

“…Tortuosity of a porous medium represents the complexity of the pathway and is commonly calculated as the ratio of the mean path length to the minimum possible (straight line) path length. We used an inverse method (24,59) to compute tortuosity at all depths in the firn from the CO 2 atmospheric trend and from CO 2 concentrations measured in the firn at Summit. In other words, we obtained a site specific, tortuosity-depth relationship by adjusting diffusivity until the model reproduced the observed CO 2 firn-air profile when driven by the independently derived atmospheric CO 2 history (see Fig.…”

“…No changes in the physical structure of the firn required consideration: surface temperature and accumulation are the main parameters determining firn structure, and their limited evolution at Summit during the last century (22, 23) did not induce significant changes in either density or porosity of the firn. We next applied a one-dimensional firn diffusion model (24) to reconstruct GEM concentrations at all depths in the firn air from different atmospheric histories. A Monte Carlo approach enabled us to optimize agreement between these modeled concentrationdepth profiles and measured GEM levels in the firn (see Materials and Methods).…”

Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

“…Important parameters needed to constrain the model are the site temperature, accumulation rate, depth of the convective layer, and close-off depth, together with profiles of firn density and effective diffusivity. The latter parameter is determined as a function of depth for each firn-drilling site by modeling the mole fractions in firn for trace gases with wellknown atmospheric histories (Buizert et al, 2012;Rommelaere et al, 1997;Trudinger et al, 1997). A multi-gas constrained inverse method is used to calculate the effective diffusivity of each site for each specific gas.…”

“…In this study we use the LGGE-GIPSA firn air transport model to reconstruct the temporal evolution of N 2 O mole fraction and isotopic composition from the measured firn profiles (Allin et al, 2015;Witrant et al, 2012;Wang et al, 2012;Rommelaere et al, 1997).…”

“…We use the multigas firn transport model developed by the Laboratoire de Glaciologie et Géophysique de l'Environnement and Grenoble Image Parole Signal Automatique (LGGE-GIPSA) to obtain an atmospheric scenario that is constrained by and consistent with all individual sites (Allin et al, 2015;Wang et al, 2012;Rommelaere et al, 1997). We then use an isotope mass balance model to infer the changes in the isotopic signature of the N 2 O source over time to investigate possible changes in the source mix.…”