Input Estimation from Sparse Measurements in LPV Systems and Isotopic Ratios in Polar Firns

Witrant, Emmanuel; Martinerie, Patricia

doi:10.3182/20130204-3-fr-2033.00201

Cited by 14 publications

(13 citation statements)

References 18 publications

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“…Here we use an improved version of the firn model used in Martinerie et al (2009). A major upgrade is the use of a firn diffusivity profile which optimally fits data from several reference gases with well-known atmospheric histories in the firn (Witrant et al, 2012). This model performed well in an international inter-comparison study (Buizert et al, 2012).…”

Section: Firn Modellingmentioning

confidence: 99%

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Laube¹,

Hanif²,

Martinerie³

et al. 2016

Atmos. Chem. Phys.

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Chlorofluorocarbons (CFCs) are ozone-depleting substances as well as strong greenhouse gases, and the control of their production and use under the Montreal Protocol has had demonstrable benefits to both mitigation of increasing surface UV radiation and climate forcing. A global ban on consumption came into force in 2010, but there is evidence of continuing emissions of certain CFCs from a range of sources. One compound has received little attention in the literature, namely CFC-114 (C2Cl2F4). Of particular interest here is the differentiation between CFC-114 (CClF2CClF2) and its asymmetric isomeric form CFC-114a (CF3CCl2F) as atmospheric long-term measurements in the peer-reviewed literature to date have been assumed to represent the sum of both isomers with a time-invariant isomeric speciation. Here we report the first long-term measurements of the two isomeric forms separately, and find that they have different origins and trends in the atmosphere. Air samples collected at Cape Grim (41 degrees S), Australia, during atmospheric background conditions since 1978, combined with samples collected from deep polar snow (firn) enable us to obtain a near-complete record of both gases since their initial production and release in the 1940s. Both isomers were present in the unpolluted atmosphere in comparably small amounts before 1960. The mixing ratio of CFC-114 doubled from 7.9 to 14.8 parts per trillion (ppt) between the start of the Cape Grim record in 1978 and the end of our record in 2014, while over the same time CFC-114a trebled from 0.35 to 1.03 ppt. Mixing ratios of both isomers are slowly decreasing by the end of this period. This is consistent with measurements of recent aircraft-based samples showing no significant interhemispheric mixing ratio gradient. We also find that the fraction of CFC-114a mixing ratio relative to that of CFC-114 increased from 4.2 to 6.9% over the 37-year period. This contradicts the current tacit assumption used in international climate change and ozone depletion assessments that both isomers have been largely co-emitted and that their atmospheric concentration ratio has remained approximately constant in time. Complementary observations of air collected in Taiwan indicate a persisting source of CFC-114a in South East Asia which may have been contributing to the changing balance between the two isomers. In addition we present top-down global annual emission estimates of CFC-114 and CFC-114a derived from these measurements using a two-dimensional atmospheric chemistry-transport model. In general, the emissions for both compounds grew steadily during the 1980s, followed by a substantial reduction from the late 1980s onwards, which is consistent with the reduction of emission in response to the Montreal Protocol, and broadly consistent with bottom-up estimates derived by industry. However, we find that small but significant emissions of both isomers remain in 2014. Moreover the inferred changes to the ratio of emissions of the two isomers since the 1990s also indicate that the sources...

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Section: Firn Modellingmentioning

confidence: 99%

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Laube¹,

Hanif²,

Martinerie³

et al. 2016

Atmos. Chem. Phys.

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“…Gas age distributions can be calculated for the purpose of estimating firn smoothing. In the case of modern ice cores this may be accomplished by using gas trapping models parameterized by firn air and pore closure data (Buizert et al, 2012;Witrant et al, 2012). However, to estimate the gas age distribution in bubbles it is necessary to use a depth profile of the progressive closure of pores in the firn, quantifying the transformation of open pores into closed bubbles.…”

Section: Introductionmentioning

confidence: 99%

Estimation of gas record alteration in very low-accumulation ice cores

et al. 2020

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Abstract. We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic Plateau. Our work aims to study two effects that alter the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. This means that the gas concentration in an ice layer is the average value over a certain period of time, which removes the fast variability from the record. Here, we focus on the study of East Antarctic Plateau ice cores, as these low-accumulation ice cores are particularly affected by both layering and smoothing. We use high-resolution methane data to test a simple trapping model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic Plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ice equivalent yr−1 the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic Plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to quantify their potential impacts on a hypothetical 1.5-million-year-old ice core from the East Antarctic Plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane using our currently established continuous CH4 analysis and carbon dioxide, respectively). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

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“…A Green function approach, as presented by Rommelaere et al (1997) and used for halocarbon trend reconstruction by Martinerie et al (2009), with an extension for isotopic ratios and revised to take into account the scarcity of the measurements (Allin et al, 2015;Witrant and Martinerie, 2013;Wang et al, 2012), is used to assign a mean age and age distribution to a certain depth.…”

Section: Modeling Trace Gas Transport In Firnmentioning

confidence: 99%

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

et al. 2017

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Abstract. N 2 O is currently the third most important anthropogenic greenhouse gas in terms of radiative forcing and its atmospheric mole fraction is rising steadily. To quantify the growth rate and its causes over the past decades, we performed a multi-site reconstruction of the atmospheric N 2 O mole fraction and isotopic composition using new and previously published firn air data collected from Greenland and Antarctica in combination with a firn diffusion and densification model. The multi-site reconstruction showed that while the global mean N 2 O mole fraction increased from (290 ± 1) nmol mol −1 in 1940 to (322 ± 1) nmol mol −1 in 2008, the isotopic composition of atmospheric N 2 O decreased by (−2.2 ± 0.2) ‰ for δ 15 N av , (−1.0 ± 0.3) ‰ for δ 18 O, (−1.3 ± 0.6) ‰ for δ 15 N α , and (−2.8 ± 0.6) ‰ for δ 15 N β over the same period. The detailed temporal evolution of the mole fraction and isotopic composition derived from the firn air model was then used in a two-box atmospheric model (comprising a stratospheric box and a tropospheric box) to infer changes in the isotopic source signature over time. The precise value of the source strength depends on the choice of the N 2 O lifetime, which we choose to fix at 123 years. The average isotopic composition over the investigated period is δ 15 N av = (−7.6 ± 0.8) ‰ (vs. air-N 2 ), δ 18 O = (32.2 ± 0.2) ‰ (vs. Vienna Standard Mean Ocean Water -VSMOW) for δ 18 O, δ 15 N α = (−3.0 ± 1.9) ‰ and δ 15 N β = (−11.7 ± 2.3) ‰. δ 15 N av , and δ 15 N β show some temporal variability, while for the other signatures the error bars of the reconstruction are too large to retrieve reliable temporal changes. Possible processes that may explain trends in 15 N are discussed. The 15 N site preference (= δ 15 N α − δ 15 N β ) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.

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Input Estimation from Sparse Measurements in LPV Systems and Isotopic Ratios in Polar Firns

Cited by 14 publications

References 18 publications

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Estimation of gas record alteration in very low-accumulation ice cores

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

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Input Estimation from Sparse Measurements in LPV Systems and Isotopic Ratios in Polar Firns

Cited by 14 publications

References 18 publications

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions

Estimation of gas record alteration in very low-accumulation ice cores

Constraining N&lt;sub&gt;2&lt;/sub&gt;O emissions since 1940 using firn air isotope measurements in both hemispheres

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Product

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