ICOS Science Conference, Session 1

1. Efficient sampling of
atmospheric methane for
radiocarbon analysis
G. Zazzeri*, X. Xu and H.D. Graven
Imperial College London and University of California, Irvine
* g.zazzeri@imperial.ac.uk
ICOS 2020

2.  Fossil-derived CH4 is devoid of 14C (14C half life of 5730 years)
 When emitted to the atmosphere, fossil CH4 causes a strong dilution of the atmospheric 14C/C ratio (Δ14C) that can be
used to quantify the fossil fraction of emissions
Fossil CH4 is devoid of 14C: tracerfor partitioning methane sources 2
BIOGENIC SOURCES FOSSIL SOURCES
Devoid of 14C14C level of the atmospheric CO2 + lag time
ICOS 2020

3. Radiocarbon in CO2 and CH4
Lassey et al. ACP 2008; Townsend-Small et al. JGR 2012
Graven et al. GMD 2017
CH4 is more enriched in 14C than CO2 because of nuclearpower plant emissions 3
Only a few Δ14CH4 measurements
reported since 2000!
ICOS 2020
REASONS:
1. SAMPLING CHALLENGES
2. NUCLEAR POWER PLANTS
EMISSIONS

4. For each 10 ppb added,
biogenic CH4 will reduce Δ14CH4 by ~1.5 ‰
fossil CH4 will reduce Δ14CH4 by ~7.5 ‰
Nuclear power plants affect the Δ14CH4 but
not the concentrations
Estimationof fossil CH4 using 14C and a mass balanceapproach 4
Graven et al. 2018
Δ14C notation includes corrections for fractionation and sample age
Δ14CH4
0 ‰-1000 ‰ 350 ‰
ICOS 2020

5. ICOS 2020
Sampling Challenges 5
 Large volumes of air are collected
by pressurization into cylinders
using strong pumps
(Eisma et al. 1995, Townsend-Small
et al. 2012)
 Sample collection into large bags
(Espic et al. 2019)
 CH4 must then be isolated in the
laboratory by cryogens, chemical
traps, or gas chromatography.
The CH4 is combusted to CO2 and
graphitized for 14C measurement
by accelerator mass
spectrometry (AMS)
The new sampling system separates CH4 from air at the point of
sampling.
This minimizes:
 the amount of sample handling needed in the radiocarbon
laboratory;
 additional cost and time before data can be made available.
The system uses sample traps containing molecular sieves, which are
hydrated crystalline aluminosilicates (zeolites) particularly selective
for CO2, allowing for separation of C from the air.
Zazzeri et al. under revisions in Environmental Science & Technology

6. ICOS 2020
The sampling system setup 6
Sample trap: 45 cm length
7 hours to collect ~0.2 mgC at 400 cc/min flow rate!

7. Graphitization Accelerator Mass Spectrometry facility
Sample analysisat UC Irvine 7
ICOS 2020

8. ICOS 2020
Measurements’ reproducibility 8
UCIAMS# ID Δ14C ± Yield
(‰) (mgC)
225028 T4-M6 491.9 4.6 0.12
225029 T4-M8 483.2 3.6 0.16
225030 T4-M9 490.2 3.5 0.16
mean 488.4
St dev 4.6
Table 1: Values of Δ14C of CH4 combustion-derived CO2 from a reference cylinder, and corresponding yields in mgC.
 The reported uncertainty for individual samples includes the error from counting statistics (i.e. the
square root of the sum of the 14C counts), variation of the primary standard OX-I and background
uncertainty
 Blank samples show a background between 1 and 2 µgC
The achieved precision of +/-4.6 ‰ is higher than the uncertainty in the most recent reported
observation: +/-5 to +/-11 ‰ (Townsend-Small et al. 2012) and +/- 12 ‰ Espic et al. 2019

9. ICOS 2020
The influence of NPP on the observations 9
The reference air tank was collected in May 2018 in Norwich
The measured Δ14C value is higher than the
expected atmospheric value of ~ 350 ‰ (Hmiel et
al. 2020)
14C emissions from Sizewell B, a pressurised
water reactor (PWR) sited 40 km southeast of
Norwich, or from European PWR are likely to
explain the large Δ14C enhancement
PWR emit almost all 14C in the form of 14CH4
(Zazzeri et al. 2018)

10. ICOS 2020
The influence of NPP on the observations 10
 Δ14CH4 of 898.8 +/- 3.5 ‰
 Influence from PWR in Belgium and
France
 Sampling days need to be chosen to
avoid emissions from PWR

11. ICOS 2020
CH4 samples in centralLondon 11
No interference from PWR
1) Air from Atlantic
2) Air from North sea or North UK
1
2

12. ICOS 2020
Fossil fraction(FF) of CH4 emissions in centralLondon 12
 No background Δ14CH4 measurements -> fossil fraction of added CH4 calculated using differences
between samples, assuming that the upwind air composition and any influence of NPP are the same
Atlantic air Cm (ppb) Δm (‰) Δb (‰) Δf (‰) Cf (ppb) FF of added CH4 (%)
07/03/2020 1960 315.3 0 -1000
18/03/2020 1992 293.3 0 -1000 32 100
North Sea Cm Δm Δb Δf Cf (ppb) FF of added CH4 (%)
29/05/2020 2032 301.8 0 -1000 64 100
04/06/2020 1986 321.4 0 -1000 29 61
10/07/2020 1968 344 0 -1000
Cf : Added fossil CH4
Cm: Measured CH4
Cbg: Background CH4
Cs: CH4 removed by sinks
Δm: measured Δ14CH4 (‰)
Δb: Δ14C for biogenic CH4 (‰)
Δf: Δ14C for fossil CH4 (‰)
An: 14CH4 added by NPP
Gravenetal.2019

13. ICOS 2020
Conclusions and Future Plan 13
The sampler allows high precision measurements to be made by separating methane
carbon at the point of sampling, thus eliminating the need for transporting and processing
large volumes of air
The calculated FF of emissions is greater than the 30 % estimated by national emissions
inventories (NAEI). Gas leaks in the natural gas distribution network are likely
underestimated by bottom-up inventories
Measuring Δ14CH4 in urban environments could improve the source partitioning of
emissions in these areas
A portable sampling system is being prepared for field Δ14CH4 measurements
Δ14CH4 measurements of background air will be performed
NPP emissions will be simulated