ISO/FDIS 20951

ISO/TC 190/ WG 1
Secretariat: AFNOR DIN
Date: 2025-12-23
Soil Quality quality — Guidance on methods for measuring
greenhouse gases (CO2, N2O, CH4) and ammonia (NH3) fluxes
between soilsoils and the atmosphere

FDIS stage
ISO 20951:2025(E)
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Contents
Foreword . v
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Methods for measuring GHGs and ammonia fluxes between soil and the atmosphere . 2
5 Concentration measurements and air sampling . 4
5.1 General . 4
5.2 Concentration measurement methods . 4
5.3 Air sampling . 7
6 Selection of the appropriate methods . 9
7 Minimum information for reporting . 10
Annex A (informative) Description of the flux measurement methods . 11
Annex B (informative) Description of the concentration measurement methods . 26
Annex C (informative) Description of the air sampling methods . 32
Bibliography . 36

iv
Foreword
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This document was prepared by Technical Committee ISO/TC 190, Soil quality.
This second edition cancels and replaces the first edition (ISO 20951:2019), of which it constitutes a minor
revision. The changes are as follows:
— editorial changes in Table 1 and Table 2;
— adjustment of formatting throughout the text;
— the Bibliography and references in the text have been updated.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
This second edition cancels and replaces the first edition (ISO 20951:2019), which has been editorially revised
(Minor Revision).
The main changes are as follows:
— editorial changes in Table 1 and Table 2;
— — adjustment of formatting throughout the text;
— — the Bibliography and references in the text have been updated.
v
Introduction
Greenhouse gas (GHG) emissions from soils have becomeare a major environmental concern. Global and
national emission inventories have identified soils, in particular agricultural soils, as being a major contributor
to these emissions, in particular nitrous oxide (N O), methane (CH ) and carbon dioxide (CO ) related to loss
2 4 2
of soil organic matter. Agricultural soils are also major emitters of ammonia (NH ), which is a precursor of
N O. Changes in soil management should take account of these emissions as part of efforts to mitigate climate
change.
GHGs and ammonia fluxes from soil are complex to measure. They are variable and heterogeneous as they are
governed by weather/ and meteorological conditions (e.g.,. temperature and moisture regimes), soil
characteristics (e.g.,. soil parental material, pH, clay content, cation exchange capacity) and, for managed soils,
by the agricultural or forestry practices (e.g.,. crop and wood residues management, soil tillage or no-tillage,
inputs of soil conditioner and fertilizers, irrigation). These factors generally interact and their effects on GHG
emissions are still poorly quantified. ItThis results in large uncertainties for the inventories of national and
[1 [1]]
global agricultural emissions. For example, Freibauer (, 2008 ) has estimated an uncertainty at 80 % for
European (EU27) agricultural N O emissions. With the reinforcement of international and regional climate
policies, comparable and reliable information is needed to report on GHG emissions but also to adopt and
verify mitigation options.
No standard covers the measurement of GHGs and ammonia emissions from soils. However, several
measurement methods have been developed. This document provides guidance on the main methods
available to quantify the exchanges of greenhouse gases (CO , N O, CH ) and ammonia (NH ) between soils
2 2 4 3
and the atmosphere. It is intended to help users to select the measurement method or methods most suited to
their purposes by setting out information on the application domain and the main advantages and limitations
of each method.
vi
ISO 20951:2025(E)
Soil Quality quality — Guidance on methods for measuring greenhouse
gases (CO2, N2O, CH4) and ammonia (NH3) fluxes between soilsoils
and the atmosphere
1 Scope
This document gives an overview and provides guidance on the main methods available to quantify the
exchanges of greenhouse gases (CO , N O, CH ) and ammonia (NH ) between soils and the atmosphere.
2 2 4 3
It is intended to help users to select the measurement method or methods most suited to their purposes by
setting out information on the application domain and the main advantages and limitations of each methods.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following
addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
intrusive method
measuring method that can influence the emitting processes
3.2 3.2
mass balance approachmethod
method based on a mass balance consisting of measuring the flux of compounds entering and leaving a volume
of air above the soil surface being studied
3.3 3.3
micrometeorological method
method using analyses of the atmospheric concentration of the gas and meteorological measurements such as
wind speed, wet and dry-bulb air temperatures, net radiation, and heat fluxes.
Note 1 to entry: These techniques are used for determining field-scale fluxes, and include eddy covariance, energy
balance, aerodynamic and mass balance techniquemethod. They do not disturb the environmental conditions.
3.4 3.4
oasis effect
effect arising from the local environment of the field being studied and affecting emissions from a particular
field depending on whether it is in an environment with a high level of emissions or a low level of emissions
Note 1 to entry: the The oasis effect will only affect compounds whose fluxes result from a thermodynamical equilibrium
between the surface and the atmosphere (NH ).
4 Methods for measuring GHGs and ammonia fluxes between soil and the
atmosphere
There are methods for measuring NH and GHG fluxes between soil and the atmosphere for a diversity of
conditions and spatio-temporal resolutions (from less than an hour up to several days, from the soil sample
up to several square kilometres) (this section). The main micrometeorological methods generally involve air
sampling and determination of the concentration of the gas(es) or gases of interest. Several concentration
measurement and air sampling methods are compatible with a given flux measurement strategy (see
Clause 5Clause 5).). The methods used and their combination depend on the purpose for which the
measurements will be used, the operators’ qualifications and the financial resources available (see
Clause 6Clause 6).).
Two main strategies can be used:
a) a) chamber methods measuring the fluxes at source, and;
b) b) atmospheric method used to estimate the fluxes at a distance from the source.
NOTE 1 Chamber methods are intrusive methods based on using static or dynamic flux chambers. Static and dynamic
flux chambers can only quantify emissions for a small area of the source. Fluxes generally vary in time (variations in the
parameters for weather and season) and in space (different soils and climatic conditions). For spatial extrapolation at
field-scale, a sampling strategy using several chambers is required to reflect the variations in emissions over the area.
Flux spatial structure could be determined by exploratory measurements prior to monitoring, or by assuming that flux
will vary according to soil properties (e.g. texture, organic matter content) or landscape features (e.g., position in a slope).
These methods can also be applied under laboratory conditions to determine the emissions of gases from soil samples,
but hard to scale-up to field scale.
NOTE 2 Atmospheric methods are non-disruptive. The net exchange is estimated by measuring concentrations at a
distance from the source together with micrometeorological measurements (e.g.,. wind, air temperature). The fluxes are
then estimated on the basis of these measurements, with a mass-balance approach or with models. Some of these
methods are fairly difficult to implement and are highly dependent on weather conditions. Some knowledge of
micrometeorology is generally required. They can be used to characterize global fluxes from heterogeneous, diffuse
sources within a given area, without being able to distinguish the contribution of each particular source. In particular,
atmospheric methods measure both soil and vegetation contributions, if there is active vegetation.
Table 1Table 1 presents the different methods and their main advantages and limitations.
Table 1 — — Different methods for measuring ammonia and GHGs fluxes from soil and their main
advantages and limitations
Application
Method Main advantages Main limitations
domain
— Applicable to — Easy to implement — Intrusive method modifying
Chambers Static flux
low fluxes emissions conditions. Chambers may
methods chambers
— High sensitivity even alternate between locations (provide
— Mainly used with low multiple chamber bases) to limit the
for instrumental impact of the chamber on the soil
comparison of precision environment and hence emissions.
treatments
— Evaluation of spatial — Spatial extrapolation of
measurements requires a sampling
— Applicable in variability with
field and in several chambers strategy using several chambers with
laboratory spatial and temporal extrapolation
— Most common flux models.
measurement
methods with many
Application
Method Main advantages Main limitations
domain
— Small area methodological
(square references
metres)
— Can be
automated for
monitoring
dynamics
over short
periods
— Non-reactive
gases (CO ,
CH4, N2O)
— Comparison — Possible control of — Intrusive method modifying
Dynamic
of treatments wind speed emissions conditions
flux
chambers
— Applicable in — Preferred for — Very difficult to extrapolate results to
and wind
field and in reactive gas such as representative field emissions
tunnels
laboratory NH3
— The oasis effect can increase or
— Small area decrease the apparent flux
(square
metres)
— Continuous — Easy to implement — Need a large number of
Atmosphe Mass-
monitoring and not expensive measurement points
ric balance
methods methods
— Real — Non-intrusive — Background concentration should be
conditions method measured accurately
— Comparison — Can be used for small
of treatment areas (few tens of
square metres)
— Easy to implement — Suitable for uniform, major
Inverse
and not expensive emissions sources with low and
modellin
fairly constant background
g
— Non-intrusive concentrations
method
— Not suitable for areas with hedges or
— Small numbers of scattered trees or with significant
changes in surface roughness
measurement points
— Can be used to — Deployment over extended periods
estimate emissions with changes in wind direction
requires multiple measurement
from several sources
simultaneously, if points to cover all wind directions.
there is a sufficient
number of sensors
(n + 1 where n is the
number of sources).
Application
Method Main advantages Main limitations
domain
— Continuous — Non-intrusive — Difficult to implement and expensive
Aerodyna
monitoring method
mic
— Assumes a large uniform emitting
gradient
— Real — Measures the flux surface
conditions from the surface
directly
— Allows
measuring — Can be used to
atmospheric monitor multiple
deposition sources
— Applicable to large — Very difficult to implement and
Eddy
areas (>1 ha) expensive
covarianc
e
— Non-intrusive — Assumes a uniform emitting surface.
method Methodologies are in development to
distinguish different emitting
surfaces through flux footprint
analysis (see for example Cowan et
[2] [3]
al. 2016 , Bureau et al. 2017 ).
— Requires high speed sensors for

wind and gas concentrations, but
method derived from eddy
covariance such as disjunct eddy
covariance (DEC) have been
developed for measuring fluxes
without high speed analysers
— In development, for reactive gases

such as NH3
— Flux underestimation when
turbulence is low, particularly during
nights.
5 Concentration measurements and air sampling
5.1 General
Methods for measuring soil-atmosphere exchange of GHGs and ammonia involve the determination of gas
concentrations in volumes of air. There are various methods to determine the concentrations, involving air
sampling if necessary. They are characterized by the chemical species which they can detect and the associated
limits of detection, the acquisition frequency, their precision, their cost and by the ease of use.
5.2 Concentration measurement methods
There are two families of concentration measurement methods (Table 2(Table 2)) that can be used for any
type of gas targeted:
— — Physical methods (absorption spectroscopy). The main characteristics of these methods are their very
short response time, their sensitivity and the possibility of monitoring the concentration dynamics in real
time (possibly monitoring several gases with different levels of concentration at the same time). Open path
technologies exist for measuring integrated gaseous concentrations over a path directly in the
atmosphere.
— — Chemical methods (gas chromatography, laboratory assays, chemiluminescence). These methods are
suitable for ad hoc measurements or for measurements integrated over periods from a few minutes to a
few weeks. They are, therefore, less suitable for monitoring concentration dynamics. Furthermore, most
of these methods are selective and cannot be used to measure several gases at the same time using the
same equipment.
Annex BAnnex B provides concentration measurement methods.
Table 2 — Concentration measurement methods
Application
Method Advantages Limitations
domain
Chemical methods
— Gaseous NH — High sensitivity of a few — — Frequent calibration
Physica Chemilumines
a
ppbppb for NH
l cence
— Suitable with — — Interference maycan
method
both dynamic — — Fast response time be problematic for low
s
chamber and (down to 0,1s1 s) with concentrations in rural
atmospheric rapid data acquisition areas
methods
— — Particularly suitable
for use with eddy
covariance method
+
— NH4 trapped in — High speed analyses (40 to
Laboratory
solution (passive 60 samples per hour)
assay of
diffusion
ammonium
+ sampler, — Robust
(NH4 ) in
denuders and
solution -
trapping of
— Good reproducibility
Continuous
ammonia in acid
flow analysers
solution)
(CFA)
— Can be used to assay all the — Long analysis time
Laboratory
major cations at the same (4 samples per hour)
assay of
time as ammonium
ammonium
— Interference with other
+
(NH ) in
— Reproducible gases if the sample
solution -
contains a large number
Liquid
of cations
chromatograp
hy
— Easy to implement — Long analysis time (5 to
Laboratory
12 samples per hour)
assay of
— Low cost
ammonium
+
(NH ) in
— Small samples
solution -
Conductivity
— Wide measurement range
and low limit of detection
— Good reproducibility
Application
Method Advantages Limitations
domain
— Gaseous CO , — High sensitivity depending — — Risk of interference
Chemic Fourier-
CH4, N2O, NH3 on the instrument and the
al transform and
a
gas (e.g.,. <1 ppbppb for — — Sensitive to ambient
method photoacoustic
N O and CO )
— Closed path and 2 2 conditions
s infrared
open path
absorption
technologies — Can measure several — — Frequent calibration
spectroscopy
compounds at the same for some analysers but
time
— All chamber and low annual drift for
atmospheric others
methods — Fast response time and
rapid data acquisition
— Standard equipment for
greenhouse gases
— Can be used where there
are high concentration
fluctuations
— Standard method for
Laser
quantitative evaluation of
absorption
trace gases
spectroscopy
(TDLAS, OA-
— Fast response time (down
ICOS, CRDS)
to 0,1 s) with rapid data
acquisition
— Very high sensitivity
a
(typically, 0,01 ppb ppb
for N O, 2 ppb for CH , 1,5
2 4
ppb for NH3 at 1 Hz)
— High selectivity and low
risk of interference, in
particular with quantum
cascade laser (QCL)
— Gaseous CO2, — Technique is well — May be used in the field
Gas
CH4, N2O, NH3 understood, commercial for continuous
chromatograp
equipment is available measurements but has
hy
— Suitable with logistical limitations and
chamber and — High sensitivity and low high operating costs
atmospheric limit of detection (of the (bottles of carrier gas,
b
reference gases, daily
methods, except order of ppmppm for CO2
a
eddy covariance and ppbppb for N O, NH intervention by operators
2 3
and CH4) and maintenance of
ambient conditions).
— — Can be used to
quantify several chemical
species at the same time
— Can be used where there
are high concentration
fluctuations
Application
Method Advantages Limitations
domain
— Suitable with small sample
volumes
— Easily automated
— Gaseous CO2, — Fast response time (< 1 s) — Measurements are
Differential
CH , N O, NH with rapid data acquisition affected by poor visibility
4 2 3
optical
(e.g. fog, snow), and
absorption
— Open path — Very high sensitivity clouds if the light source
spectroscopy
a
is the sun
technologies (< 1 ppbppb )
(ambient air)
— Risk of interference
— Atmospheric
methods
a ppb = parts per billion
b ppm = parts per million
5.3 Air sampling
In many cases, the determination of gaseous concentration involves sampling of a volume of air before
analysis. There are various gas sampling methods which depend on the strategies adopted and the methods
used to determine the concentrations.
Table 3Table 3 presents the different methods for measuring NH and GHGs and their main advantages and
limitations.
Table 3 — Air sampling methods
Method Application domain Advantages Limitations
— — Reactive gas — — Easy to implement — — Gives an average
Passive diffusion
(NH ) concentration over the
samplers
— — High sensitivity for NH exposure time
— — Time- in particular in low
integrated concentrations — — Can only be used over
long periods from a few hours
measurement
to a few weeks
— — Inverse
modelling and — Some samplers can be
mass balance affected by dust
method
(atmospheric
methods)
— Total cost of measurement
can be close to that of an
automatic analyser when
measurements have to be
repeated over a long period
— Requires manual intervention
for each measurement
Method Application domain Advantages Limitations
— Reactive gas (NH ) — — Easy to implement — — Gives an average
Denuder tubes
concentration over the
— — Time- — — High sensitivity, can be exposure time
integrated adapted to the
measurement concentrations expected — — Requires manual
intervention for each
— — Chamber — — Preferable to passive measurement
methods, inverse samplers for short sampling
modelling, mass periods (from a few minutes
balance method up to about an hour)
and relaxed Eddy
Accumulation — — Enables a wide variety of
(eddy- compounds to be collected in
accumulation
a relatively short sampling
(eddy covariance- time
derived method)
+
— — NH (i.e. NH — — Robust — — The process is time-
3 4
Trapping NH in
in solution) consuming and difficult to
an acid solution
automate
— — High sensitivity
— — Time-
integrated — — Can be adapted to the — — Not suitable for high
measurement concentrations expected, and temporal resolution
to the sampling period monitoring
— — Chamber
methods — — There maycan be
interference from other
— — Inverse absorbable species
modelling and containing nitrogen (e.g.,.
volatile amines).)
mass balance
method
(atmospheric — — Requires manual
methods) intervention for each
measurement
— — Air sample — — The air samples can be — — It maycan take some time
Continuous
analysed in situ, in real time to set up or move the
sampling methods
— — Continuous and continuously over long sampling system
for real-time
monitoring periods of time (high
analysis
temporal resolution) — — The pipework needs to be
— — High temporal protected against dust and
— — Samples can be taken condensation
resolution
from several different
— — All chamber sampling points in — — For suction systems,
succession at the same site
and atmospheric there should be no leaks
between the sampling point
methods
and the analyser that might
dilute or contaminate the gas
samples
— — Air sample — — Can be used for keeping — — Pressure fluctuations
Sampling tubes or
samples of non-reactive maycan compromise gas-
bags
compounds for days to tightness of the vials
— — Delayed
analysis weeks before analysis.
— — Static chamber — — Applicable when
concentration measurement
method and
Relaxed Eddy
Method Application domain Advantages Limitations
Accumulation is not suitable or possible in
(relaxed eddy- the field conditions
accumulation
(eddy covariance-
derived method)
Description of air sampling methods are given in Annex CAnnex C.
6 Selection of the appropriate methods
Being able to quantify emissions makes it possible to compare probable emissions from various processes,
estimate representative emission factors, monitor and check compliance with emission limits, etc. The
methods used and their combination depend on the purpose for which the measurements will be used, the
operators’ expertise and the financial resources available. For example, research on the underlying processes
mightcan require a different approach than when yearly emission totals are required. As different emission
measurement methods have different characteristics, an appropriate method should be selected, depending
on different criteria: limit of detection and sensitivity required, field or lab measurements, maintenance and
equipment costs, spatial area to cover, comparison of modalities or site monitoring. The sampling strategy
should also be tailored to the gas in question. For example, soil N O flux has different drivers (involving
biological processes) and different dynamics than ammonia (NH ) flux (involving mainly physical and
chemical processes).
Table 4Table 4 presents several published applications of flux measurement methods in relation to their
purposes. Description of these methods are presented in Annex AAnnex A.
Table 4 — Examples of applications of flux measurement methods in relation to their purposes
References Flux measurement method Purpose
de Klein and Harvey (,
To measure field N2O emissions and investigate
[4 [4] ]
2015 ) , ; Loubet et al. (., Static flux chambers
their spatial variabilities
[5 [5]]
2011 )
Minamikawa et al. (., To measure field N2O and CH4 emissions in rice
Static flux chambers
[6 [6]]
2015 ) paddies.
To determine the influence of various factor (e.g.
Sommer et al. (., wind velocity, soil properties, air temperature,
Wind tunnels
[7 [7]]
1991 ) agricultural practices) on ammonia emission
from soils in controlled conditions
LavilleetLaville et al. (.,
[8 [8] ]
1997 ) ; ; Aubinet et al.
To measure soil-atmosphere net exchange of
[9 [9] ]
(., 2012 ) ; ; Eugster and
Eddy -covariance GHGs from field and investigate soil processes
[10 [10]
Merbold (, 2014 ) ;
and agricultural practices involved.
]
; Cowan et al. (.,
[2 [2]]
2016 )
[11 [11] ]
VERA (, 2009 ) , ;
Sintermann et al. (.,
Mass balance method, Inverse To measure ammonia emissions following
[12 [12] ]
2011 ) , ; Carozzi et al.
modelling manure or fertilizer spreading in field conditions
[13 [13] ]
(., 2013 ) , ; Ferrara et
[14 [14]]
al. (., 2014 )
Personne et al. (., To investigate short-time dynamics of ammonia
Aerodynamic gradient
[15 [15]]
2015 ) emission
7 Minimum requirementinformation for reporting
Reporting is important for evaluating the robustness of emission data. It is particularly crucial if they are
collected for meta-analyses or intercomparison purposes, or for the calculation of emission factors.
Furthermore, it is generally accepted that additional measurements are required when monitoring fluxes to
interpret the results with respect to the conditions (surface temperature and humidity, incident radiation,
etc).).
The following minimum information should be reported:
— — Experimental sites including location (e.g.,. latitude, longitude, altitude), soil properties (e.g.,. soil type,
texture, bulk density, pH, organic C, available N, total N), and current and past land uses (e.g.,. pasture,
crop, forest) and managements (e.g.,. fertilization, tillage, irrigation, crop protection, soil cover). Recent
management can strongly affect soil-atmosphere gas exchange. For example, soil tillage and N fertilization
can induce a rapid and transient increase in gaseous N emissions. Fertilizers and soil conditioner
properties should be also reported (e.g,. type, total N, mineral N, pH, organic C, dry matter, C:N ratio).
— — Methodology including treatment details (e.g.,. replicates in the methodology is not spatially
integrative), measurement methods (gas emission and concentration), area of the emitting surface,
equipment (e.g.,. design, precision), duration of the monitoring, sampling scheme and method, background
gas concentration.
NOTE 1 Regarding flux measurement methodology, information needs to be reported on the
equipment used to estimate the variables necessary for the calculation of (1) :
— the rate of accumulation for the chambers methods (headspace volume and air flow
control/measurement devices), (2) );
— the mass balances for the mass balance methods (wind velocity measurement device and design) and
(3) );
— the exchange coefficient for the inverse modelling, aerodynamic gradient and eddy covariance method
(wind velocity measurement device and design).
NOTE 2 Regarding gas concentration measurements and air sampling, information is reported on (1) :
— the equipment used for gas analysis and their performances (e.g.,. detection limit, precision),
(2) );
— the air sampling strategy (e.g.,. direct or indirect, containers, continuous or sporadic,
frequency) and (3) );
— the sample manipulation and storage (e.g.,. duration and conditions of storage) are reported.).
— — Ancillary measurements including soil and weather conditions (e.g.,. soil water content, soil
temperature, air temperature, precipitation, wind speed and direction). These data are essential to analyse
and interpret the results.
— — Data analysis (flux calculation method, estimation of errors, quality control).
Annex A
(informative)
Description of the flux measurement methods
A.1 Chamber methods
A.1.1 Non-steady-state or static flux chambers
A.1.1.1 General
This method is used for measuring non-reactive gas emissions at local scale usually for areas less than a square
metre. Nitrous oxide (N O), carbon dioxide (CO ), methane (CH ) may be measured using static flux chambers.
2 2 4
It is not suitable for compounds whose emissions result from a thermodynamical equilibrium between the
surface and the atmosphere.
A.1.1.2 Operating principles
This method estimates the fluxes from a source based on the accumulation dynamics (dC/dt) of the gases
inside a sealed chamber placed on the surface of the source. The limit of detection depends on the ratio
between the volume, V, and the area, A, of the chamber, the gas analyser and the integration time. The time
taken for the measurement may vary from a few minutes to a few hours. For a system that is perfectly closed
with no outside influences, the accumulation is close to linearity while the chamber is being used and the gas
fluxes, Q, are, therefore, proportional to the accumulation slope (dC/dt or a) as shown in
Formula (A.1)Formula (A.1)::
𝑉𝑉d𝐶𝐶 𝑉𝑉
𝑄𝑄 = = 𝑎𝑎 (A.1)
𝐴𝐴d𝑡𝑡 𝐴𝐴
This method can measure both positive and negative fluxes. Depending on the type of source studied, fairly
large flux chambers should be used (ground area of a significant fraction of a square metre) to take account of
the spatial variability of the fluxes and a number of measurements should be taken to obtain representative
values for fluxes at the scale of the source of emissions considered. Although the static flux chamber method
is one of the most common flux measurement methods for which there are many methodological references
[4[4] ] [6[6] ] [16[16] ]
(de Klein and Harvey, 2015 ; ; Minamikawa et al., 2015 ; ; Hutchinson and Livingston, 2002 ; ;
[17[17] ]
Livingston and Hutchinson, 1995 ), ), it is not standardized.
A.1.1.3 Implementation
Static flux chambers are relatively easy to use depending on the concentration analysis method used (Rochette
[18[ ] ]
and Eriksen-Hamel, 2008 ). ). Static flux chamber can be connected to an in-line gas analyser for real-time
analysis or uses traps for instantaneous or integrative sampling. For real-time analysis, the greenhouse gas
concentrations (CH , N O, CO ) can be detected using, for example, infrared spectroscopy. For systems with
4 2 2
traps, the gases can also be assayed by gas chromatography. If samples are taken from the chamber for
subsequent analysis, they can be stored in small pre-evacuated or flushed vials (a few ml). Chamber opening
and closing can be automated to minimize manipulation, to increase the temporal resolution of the
measurements and to sample specific conditions (e.g.,. soil temperature, rain event).
Static flux chambers are an intrusive method which changes the emission conditions at the surface of the soil,
in particular by changing the turbulence, the pressure fluctuations and the differences in concentration
[19[19] ]
between the soil and the atmosphere (Matthias et al., 1978 ). ). Gas diffusion theory would predict that
the increase inside the chamber during deployment would not be linear and some non-linear models have
[20[20] ]
been proposed (Healy et al., 1996 ). ). To minimize these effects and be able to use a linear approximation,
it is important to limit the measurement time, ensure that the gas is thoroughly mixed inside the chamber
(e.g.,. fan, mixing using a sampling syringe), sink the chambers into the ground or substrate to prevent lateral
diffusion so far as possible, provide a vent that will balance the pressure inside the chamber with the pressure
outside and insert the chamber base into the soil at least 24 h prior to the first sampling. De Klein and Harvey
[4 [4]]
(, 2015 ) give recommendations on the chamber design, implementation and data treatment to maximize
flux detectability and minimize any measurement artefacts.
A.1.1.4 Validation and sources of uncertainty
The measurements can be checked visually or statistically. Plotting the concentration with time enables the
linearity to be checked: the coefficient of determination R is widely used as an estimator and the maximum
slope can also be taken into account. The level of precision of the flux measurement depends directly on the
precision of the analyser and the measurement conditions (leaks, duration) and also on the estimate of the
V/A ratio. The detection threshold of the method can be evaluated from the calculated error on the
determination of the slope. It is inversely proportional to the square root of the number of measurements and
depends on both the sensitivity of the analyser and the V/A ratio. The detection threshold can be minimized
by finding a good compromise between the length for which the flux chamber is deployed, the sensitivity of
the analyser and the height of the chamber.
A.1.2 Dynamic flux chambers and wind tunnels
A.1.2.1 General
Dynamic flux chambers with controlled air circulation and wind tunnels can be used for any gas. They are
generally used to characterize the emissions of reactive compounds such as ammonia, for small areas (of the
order of 1 square metre). They can be used to measure emissions in a laboratory, often in closely controlled
conditions as well as in situ. This is a method suitable for experiments comparing different treatments.
A.1.2.2 Operating principles
A small area is swept with a controlled airflow in a tunnel enclosing the area. The airflow is imposed. It can
−2 −1
simulate wind, one of the major factors controlling volatilization. The volatilization flow Q (µg·m ·s ) for a
s
compound is determined from the difference in concentrations between the input and output of the tunnel, as
given in Formula (A.2)Formula (A.2)::
(𝐶𝐶−𝐶𝐶 )
o i
𝑄𝑄 =𝑞𝑞 × (A.2)
𝑠𝑠
𝐴𝐴
where
−3
C and C (µg·m ) are the concentrations of the compound of interest at the output and input of the
o i
−3
tunnel respectively, in µg·m ;
3 −1 3 −1
q (m ·s ) is the airflow in the tunnel, in m ·s ;
2 2
A (m ) is the area covered by the tunnel, in m .
The turbulent component of the airflow is ignored.
A.1.2.3 Implementation
Although the tunnels and wind chambers are portable, this method maycan be relatively difficult to implement
in the field: a power supply is required for the pump. To include the effects of rain or cultivation on the land,
these should either be simulated manually inside the tunnels, with the risk of not reproducing them correctly,
or the tunnels can be opened automatically for some time. The tunnels can also be moved regularly across the
surface being measured.
The concentrations can be assayed using inline analysers such as infrared, to measure several points at the
same time over integrative periods ranging from a few hours (just after manure spreading, for example), a few
days (for periods after spreading during which the fluxes are expected to be lower), to a few weeks or months.
A.1.2.4 Validation and sources of uncertainty
Positioning the tunnels or chambers directly on the emitting surface maycan require account to be taken of
the unevenness of the distribution of the sources of emissions (e.g.,. manure) which maycan have a major
effect: it is recommended that several tunnels/ or chambers should be used to evaluate the fluxes. Pumping
air maycan create large pressure gradient that maycan strongly affect the fluxes. It is therefore highly
recommended to check the pressure difference between inside and outside the tunnel/ or chamber. The major
source of uncertainty for determining fluxes using wind tunnels or chambers is the measurement of the
[21,22 [21,22] ]
airflow, as was shown by Loubet et al., (1999a;, 1999b. ) . It is important to measure the airflow
accurately, as the airflow not only has a significant direct effect on the measurement but maycan also affect
the emissions themselves. Wind tunnels give results closer to those of the entire surface when the wind speed
in the tunnels is the same as those measured outside but tend to overestimate the real fluxes due to the oasis
effect.
A.2 Atmospheric methods
A.2.1 Mass-balance approaches
These methods which are based on a mass balance consist of measuring the flux of compounds entering and
leaving a volume of air above the soil surface being studied.
A.2.1.1 Integrated Horizontal Fluxhorizontal flux (IHF) method
A.2.1.1.1 General
The Integrated Horizontal Fluxintegrated horizontal flux (IHF) is suitable for measuring the gaseous emissions
from circular sources with a diameter from aboutapproximately 20 m to aboutapproximately 40 m. IHF has
been used mainly for measuring ammonia volatilization from fields.
A.2.1.1.2 Operating principles
The method is based on mass balance by measuring the flux of compounds entering and leaving a volume of
air above the surface being studied (see Figure A.1Figure A.1).). The difference between the input flux, F , and
i
the output flux, F , from the test volume is equal to the flux emitted from the surface, S. The lost flux leaving
o
the top of the test volume F is ignored, and should hence be minimised.
l
Key
1 wind
S is the emissions flux to be measured
Q is the input flux to the test volume (delimited by dashed lines)
i
Q is the output flux
o
Q is the lost flux through the top of the test volume
l
1 wind
S emissions flux to be measured
Q input flux to the test volume (delimited by dashed lines)
i
Q output flux
o
Q lost flux through the top of the test volume
l
NOTE In this case, two masts measure the input and output fluxes. Each mast has an anemometer and a
concentration sensor for the compound at several different heights.
Figure A.1 — Integrated Horizontal Fluxhorizontal flux method
H
The horizontal flux Q (z) of a compound at a concentration C(z) at a given height z is equal to the
c
concentration multiplied by the wind velocity at this height v(z), which, taking the means, gives
Formula (A.3)Formula (A.3)::
(A.3)
H
¯
¯ ¯
𝑄𝑄 (𝑧𝑧) =𝐶𝐶(𝑧𝑧), 𝑣𝑣(𝑧𝑧) =𝐶𝐶(𝑧𝑧) ·𝑣𝑣¯(𝑧𝑧) +𝑣𝑣′𝑐𝑐′(𝑧𝑧)
𝑐𝑐
(A.3)
¯
¯
where the mean of the product is decomposed into the product of the means 𝐶𝐶(𝑧𝑧) and 𝑣𝑣¯(𝑧𝑧) plus a term 𝑣𝑣′𝑐𝑐′(𝑧𝑧)
which represents the horizontal turbulent flux. The bars represent means over the period and the primes
represent the fluctuation around the mean.
Assuming that the term representing the turbulence is negligible, the horizontal flux can be measured with an
averaging concentration sensor and an anemometer. The IHF is estimated by measuring the horizontal flux at
several heights, as given by Formula (A.4)Formula (A.4)::
(A.4)
∞ 𝑧𝑧
¯ max
0→∞
¯ ¯
𝑄𝑄 =� 𝐶𝐶 (𝑧𝑧) ·𝑣𝑣¯(𝑧𝑧)𝑑𝑑𝑧𝑧∼� 𝐶𝐶 (𝑧𝑧) ·𝑣𝑣¯(𝑧𝑧)𝑑𝑑𝑧𝑧
𝑐𝑐
0 0
(A.4)
As taking measurements at great heights is neither easy nor useful, a maximum height z is set, above which
max
the horizontal flux is considered to be negligible (approximately one tenth of the radius of the source). There
are seve
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