Radon is a useful tracer gas for estimating greenhouse gas emissions and reducing uncertainties in atmospheric transport models. However, current radon measurement techniques have inconsistencies that limit their usefulness. This study proposes a standardized protocol for radon data processing to harmonize measurements across sites and over time. Applying time response corrections to radon detector outputs from two UK sites improved correlations with methane concentrations and allowed more frequent flux estimations in atmospheric transport models. The standardized protocol has potential to better utilize radon measurements for quantifying greenhouse gas emissions.

1. Importance of harmonizing radon
datasets for reducing uncertainty in
greenhouse gas emission estimates
Dafina Kikaj1, Edward Chung1, Alan D. Griffiths2, Grant Forster3,
Chris Rennick1, Jessica Connolly1, Leigh Fleming3, Scott D.
Chambers2, Ute Karstens4, Tim Arnold1,5
1 National Physical Laboratory, Teddington, UK
2 Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC NSW 2232, Australia
3 School of Environmental Sciences, University of East Anglia, Norwich, UK
4 ICOS ERIC Carbon Portal, Lund University, Sweden
5 School of GeoSciences, University of Edinburgh, Edinburgh, UK

2. Introduction
Estimating GHG emission:
• Bottom-up
• Top-down
From the UK’s National Inventory Report to the
UNFCCC 2022.
Uncertainties of top-down methods:
• ambient measurements → low
• atmospheric chemical transport model → high
National scale emissions require high resolution models.
2/23

3. Can we reduce the uncertainties through better
understanding model bias?
Surface-emitted atmospheric tracer:
→ well-defined and similar spatial distributed source,
→ simple sink,
→ responds directly to the physical atmospheric processes,
→ measured at similar temporal resolution as the observable
(GHGs) in question.
Potential candidate for this task is radon (222Rn).
3/23

4. Introduction: what makes radon useful as a
tracer?
• Known source (Earth’s crust)
• Surface flux is fairly uniformly distributed on local to regional scales (Karstens et al. session 18)
• It can easily escape from the unsaturated/unfrozen ground
• It is distributed by vertical and horizontal mixing
• Shows temporal variations: seasonal (1–6 m),
synoptic (1–10 d),
diurnal (24 h);
• Its only atmospheric sink is radioactive decay
• Half-life (t1/2=3.82 d) short enough → to not accumulate in the atmosphere > synoptic
timescales
• Half-life long enough → mixing timescale of the ABL (~1 hour).
4/23

5. Radon measurement techniques
Wetland
PASSIVE
(Not powered, no maintenance)
e.g. charcoal absorption, track-etch,
compact, cheap, long-term
low sensitivity and temporal resolution
DIRECT
Two-filter detectors, electrostatic deposition
detectors, ionization chambers with -spectroscopy
consistent absolute calibration
sensitivity proportional to size, slow time response
ACTIVE
(Need power, require maintenance)
higher sensitivity & temporal resolution
various sizes, costs and maintenance
requirements
DISCRETE
e.g. grab sampling, charcoal traps,
High sensitivity
limited duration, labour intensive, costly
CONTINUOUS
variable size, sensitivity and
duration
expensive
INDIRECT (by progeny)
Single-filter detectors target ambient - or β-activity of
radon progeny, using static, alternating, or moving filters
compact, fast,
inconsistent calibration (disequilibrium assumptions)
(Curcoll et al. Session 18 & Lastra et al Poster 80)
5/23

6. Applications of radon measurement
The atmospheric radon measurements are currently performed worldwide at the
atmospheric GHG monitoring stations.
• characterizing baseline of trace gases (Fleming et al. Session 18)
• independent method to calculate local-to-regional GHG fluxes (Yver-Kwok et al. Poster 68)
• method to identify the errors in chemical transport models (Chung et al. Session 18)
6/23

7. Network of Radon Measurements
ICOS Atmosphere
Non-ICOS
Indirect 222Rn detector
Direct 222Rn detector
(all UK sites)
7/23

8. Need for a standardized protocol
There is not standardized rule or protocol for the radon measurements:
• traceability (Röttger et al. & Curcoll et al. Session 18 & Fuente-Lastra et al Poster 80)
• post-processing:
o correction of slow time response
o harmonizing measurements (compare in space/time)
• uncertainties.
8/23

9. Towards a new protocol for
processing measurements made by
ANSTO dual-flow-loop two-filter
radon detectors

10. Radon Detector: Dual-Flow-Loop Two-Filter
(ANSTO)
• one of the most precise in-situ monitors;
• samples at any height;
• 30-min time resolution;
• 222Rn LLD = 0.025 Bq m–3.
Calibration: once a month (226Ra source NIST);
Background: each three month.
The process takes time → there is a delay between what is happening in the atmosphere and what
the instrument is reporting.
≈ 40% of the signal arrives 1 h after radon pulse delivered [Griffiths et al. 2016].
Comparing radon with GHGs (fast-response instrument) → its difficult.
10/23

11. Importance of response time correction
Deconvolution is a process linked with image sharpening.
11/23

12. Towards a standard protocol of measurement and
processing for harmonising radon data
12/23

13. Measurement sites

14. Measurement sites
Sites:
Heathfield (HFD) → UK DECC, WMO-GAW
Weybourne (WAO)→ UK DECC, WMO-GAW, ICOS
Measurements:
Methane (CH4) – time resolution 1 min
Radon (222Rn) – time resolution 30 min
Period measurement: Sep 2020–August 2021
.
14/23

15. Initial (detector output) and best estimate of radon
HFD WAO
Composite plot based on hourly median.
The shading shows the extent to the 25/75th quantiles.
15/23

16. Assessing accuracy of the estimated radon signal

17. Calibration event
17/23

18. Methane (CH4)
HFD WAO
Composite plot based on hourly median.
The shading shows the extent to the 25/75th quantiles.
18/23

19. Field application of best estimate radon concentration

20. RTM application: one night example at WAO
Using best estimate of radon improves the correlation between radon and CH4 and allows to estimate fluxes more often.
20/23

21. CTM application at HFD
21/23

22. Summary
× =
C
Rn
(mBq
m
−3
)
(Chung et al. Session 18)
(Karstens et al. Session 18)
Yver-Kwok et al Poster 68
(Fuente-Lastra et al. Poster 80)
22/23

23. Thank you for listening!
Questions?

24. npl.co.uk/
measurement-for-our-planet
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