Analyses of firn gas samples from Dronning Maud Land, Antarctica: Study of nonmethane hydrocarbons and methyl chloride

Abstract

Firn air was sampled on the Antarctic plateau in Dronning Maud Land (DML), during the Norwegian Antarctic Research Expedition (NARE) 2000/2001. In this paper, we describe the analyses for methyl chloride and nonmethane hydrocarbons (NMHCs) in these firn air samples. For the first time, the NMHCs ethane, propane, and acetylene have been measured in Antarctic firn air, and concentration profiles for these gases have been derived. A one‐dimensional numerical firn air diffusion model was used to interpret the measured profiles and to derive atmospheric concentrations as a function of time. The atmospheric trends we derived for the NMHC and methyl chloride at DML cover the period from 1975 to 2000. Methyl chloride shows a decreasing trend of 1.2 ± 0.6 ppt per year (annual mean concentration 548 ± 32 ppt). For ethane we found an increasing trend of 1.6 ± 0.6 ppt per year (annual mean concentration 241 ± 12 ppt). The concentrations of propane and acetylene appear to be constant over the period 1975–2000, with annual mean concentrations of 30 ± 4 ppt for propane and 24 ± 2 ppt for acetylene.

Model calculations of the age of firn air across the Antarctic continent

“…The oldest air outside the East Antarctic plateau at PCOD is found on Berkner Island. As an example for trace gas records, we predict a mean age of 187 years at PCOD for methyl chloride at 82 • E, 83 • S. The difference in mean age between methyl chloride and CO 2 is due to the different molecular mass and gas specific diffusion coefficient (D 12 ) of methyl chloride versus CO 2 (Kaspers et al, 2004). Sampling firn air at 82 • E, 83 • S, will therefore significantly lengthen the present longest firn air record from South Pole, being 80 years (Butler et al, 1999).…”

“…The model is described briefly here; a more detailed description is given in Kaspers et al (2004). The model calculates the flux j (z,t) of air into the firn column, which depends on the flux of molecular diffusion j Diffusion and the flux due to gravity j gravity (Eq.…”

“…To determine the accuracy of the diffusion model with all three parameterisations included, we compared the modelled CO 2 age at PCOD with the firn air age described in sitespecific studies (Butler et al, 1999;Sturges et al, 2001;Trudinger et al, 1997Trudinger et al, , 2002Bender et al, 1994, Kaspers et al, 2004. In this modelling experiment we have used the measured meteorological conditions for all ten sites and used the parameterisations to account for the site-specific parameters.…”

“…Brominated methanes were recorded by Sturges et al (2001) in air sampled at Dronning Maud Land, Dome C and Berkner Island. Concentrations of CO 2 and CFCs at Law Dome were measured by Trudinger et al (1997Trudinger et al ( , 2002; non-methane hydrocarbons (ethane, propane and acetylene) were measured by Kaspers et al (2004) for Dronning Maud Land. All the locations of previous firn air analyses are shown in Fig.…”

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

“…Secondly, we measure a contamination indicator of modern air directly with each sample as introduced by Aydin et al (2010). Dichlorodifluoromethane, CCl 2 F 2 , is attractive for this purpose, since it currently has the highest mixing ratio of any man-made halocarbon with about 550 ppt, and was absent in preindustrial times (Kaspers et al, 2004). Our detection limit for CCl 2 F 2 is 1-2 ppt; thus we can identify modern air contamination to the ice sample to a level of ∼ 0.3 % (volume fraction of ambient air relative to the ice core air).…”