Radical chemisty in the Antarctic boundary layer - ACPD

57 Pages·2010·2.24 MB·English

by  W. J. Bloss

Atmos. Chem. Phys. Discuss., 10, 15109–15165, 2010 D
Atmospheric is
www.atmos-chem-phys-discuss.net/10/15109/2010/ Chemistry cu ACPD
s
doi:10.5194/acpd-10-15109-2010 and Physics s
© Author(s) 2010. CC Attribution 3.0 License. Discussions ion 10,15109–15165,2010
P
a
p
e Radical chemisty in
Thisdiscussionpaperis/hasbeenunderreviewforthejournalAtmosphericChemistry r
the Antarctic
andPhysics(ACP).PleaserefertothecorrespondingfinalpaperinACPifavailable. |
boundary layer
D
is
c W.J.Bloss
u
Coupling of HO , NO and halogen s
s
x x io
n
chemistry in the Antarctic boundary layer P
a TitlePage
p
e
r Abstract Introduction
1 1 2 3 3
W. J. Bloss , M. Camredon , J. D. Lee , D. E. Heard , J. M. C. Plane , |
A. Saiz-Lopez4, S. J.-B. Bauguitte5, R. A. Salmon5, and A. E. Jones5 Conclusions References
D
is
1SchoolofGeography,Earth&EnvironmentalSciences,UniversityofBirmingham, cu Tables Figures
s
Edgbaston,Birmingham,B152TT,UK s
io
2DepartmentofChemistry,UniversityofYork,Heslington,York,YO105DD,UK n J I
3SchoolofChemistry,UniversityofLeeds,Leeds,LS29JT,UKandNationalCentrefor Pa
p J I
AtmosphericScience,SchoolofChemistry,UniversityofLeeds,Leeds,LS29JT,UK e
r
4LaboratoryforAtmosphericandClimateScience,ConsejoSuperiordeInvestigaciones
Back Close
|
Cientificas(CSIC),Toledo,Spain
5BritishAntarcticSurvey,NaturalEnvironmentResearchCouncil,HighCross, D FullScreen/Esc
MadingleyRoad,Cambridge,CB30ET,UK isc
u
s
Received: 26May2010–Accepted: 5June2010–Published: 21June2010 sio Printer-friendlyVersion
n
Correspondenceto: W.J.Bloss([email protected]) P InteractiveDiscussion
a
p
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. e
r
15109 |
Abstract D
is
c ACPD
u
A modelling study of radical chemistry in the coastal Antarctic boundary layer, based s
s
upon observations performed in the course of the CHABLIS (Chemistry of the Antarc- ion 10,15109–15165,2010
tic Boundary Layer and the Interface with Snow) campaign at Halley Research Station P
a
p
5 in coastal Antarctica during the austral summer 2004/2005, is described: a detailed er Radical chemisty in
zero-dimensional photochemical box model was used, employing inorganic and or- the Antarctic
|
ganic reaction schemes drawn from the Master Chemical Mechanism, with additional boundary layer
halogen (iodine and bromine) reactions added. The model was constrained to obser- D
is
vations of long-lived chemical species, measured photolysis rates and meteorological c W.J.Bloss
u
s
parameters, and the simulated levels of HO , NO and XO compared with those ob- s
10 x x io
served. The model was able to replicate the mean levels and diurnal variation in the n
P
halogenoxidesIOandBrO,andtoreproduceNO levelsandspeciationverywell. The a TitlePage
x p
NO sourcetermimplementedcomparedwellwiththatdirectlymeasuredinthecourse e
x r Abstract Introduction
of the CHABLIS experiments. The model systematically overestimated OH and HO
2 |
levels, likely a consequence of the combined effects of (a) estimated physical param- Conclusions References
15
D
eters and (b) uncertainties within the halogen, particularly iodine, chemical scheme. is
c Tables Figures
The principal sources of HO radicals were the photolysis and bromine-initiated oxida- u
x s
tion of HCHO, together with O(1D)+H2O. The main sinks for HOx were peroxy radical sion J I
self- and cross-reactions, with the sum of all halogen-mediated HO loss processes
x P
a
20 accountingfor40%ofthetotalsink. Reactionswiththehalogenmonoxidesdominated p J I
e
CH O –HO –OH interconversion, with associated local chemical ozone destruction in r
3 2 2
place of the ozone production which is associated with radical cycling driven by the | Back Close
analogous NO reactions. The analysis highlights the need for observations of physi-
D FullScreen/Esc
calparameterssuchasaerosolsurfaceareaandboundarylayerstructuretoconstrain is
c
u
25 suchcalculations,andthedependenceofsimulatedradicallevelsandozonelossrates ss Printer-friendlyVersion
uponanumberofuncertainkineticandphotochemicalparametersforiodinespecies. io
n
P InteractiveDiscussion
a
p
e
r
15110 |
1 Introduction D
is
c ACPD
u
The chemistry of the sunlit troposphere is dominated by the reactions of the hydroxyl s
s
radical, OH, which is responsible for initiating the degradation of most hydrocarbons ion 10,15109–15165,2010
andotherspeciesemittedtotheatmosphere. Knowledgeofatmospherichydroxyllev- P
a
p
5 els (OH), of related species such as HO2 (collectively HOx), and the chemical pro- er Radical chemisty in
cesses which govern their abundance, is central to explaining current atmospheric the Antarctic
|
trace gas distributions and predicting their likely future evolution. boundary layer
Interest inthe chemistry ofthe polar boundarylayer, and the atmosphericchemistry D
is
abovethesurfaceofthepolaricesheetsandseaice,hasgrowninrecentyears,driven c W.J.Bloss
u
s
in part by interest in understanding atmospheric evolution through measurements of s
10 io
trace gases in air trapped in ice cores and firn. In addition to climatic information de- n
P
rived from long-lived tracers such as CO levels and various isotope ratios, measure- a TitlePage
2 p
ments of sulphur, nitrate and peroxide levels have all been used to infer historic atmo- e
r Abstract Introduction
spheric composition (e.g., Wolff, 1995; Legrand and Mayewski, 1997); understanding
|
the chemical environment in which these species are deposited and incorporated in Conclusions References
15
D
firn/ice, i.e. the background chemistry of the polar boundary layers, is clearly impor- is
c Tables Figures
tant for such analyses. From the perspective of the modern atmosphere, interest in u
s
the polar boundary layer has also been driven by the observation of periodic surface sio
n J I
ozone depletion events linked to bromine chemistry in the Arctic, and the recognition
P
20 that snowpack can act as a source of nitrogen oxides NO and NO2 (collectively NOx), ap J I
e
HONO,HCHOandperoxides,amongstotherspecies,therebymodifyingtheboundary r
layer composition from what might be expected for regions very remote from pollutant Back Close
|
sources (Grannas et al., 2007).
D FullScreen/Esc
This paper presents a model analysis of boundary layer chemistry based upon ob- is
c
25 servations made in the coastal Antarctic boundary layer during the CHABLIS (Chem- us
s Printer-friendlyVersion
istry of the Antarctic Boundary Layer and the Interface with Snow) campaign at Hal- io
n
leyResearchStationincoastalAntarctica.Azero-dimensionaldetailedphotochemical P InteractiveDiscussion
a
boxmodelisemployed,constrainedbyobservedlevelsoflong-livedchemicalspecies, p
e
r
15111 |
meteorological and photochemical parameters, to simulate concentrations of short- D
is
lived radical species. A particular focus is the model’s ability to reproduce the levels c ACPD
u
s
and diurnal profiles of HO and NO measured during the austral summer, and the s
x x io 10,15109–15165,2010
impact of halogen chemistry upon radical cycling and in situ chemical ozone produc- n
P
tion/destruction. a
5 p
e Radical chemisty in
r
the Antarctic
2 Context |
boundary layer
D
The first high latitude boundary layer measurements of OH were performed at Palmer is
c W.J.Bloss
u
Station,acoastalsiteonAnversIsland,locatedapproximatelyhalfwaydownthewest- s
s
ern side of the Antarctic Peninsula (64.0◦W, 64.7◦S), during the 1993/1994 austral io
n
summer using Chemical Ionisation Mass Spectrometry (CIMS; Jefferson et al., 1998). P
10 a TitlePage
OH concentrations were low, ranging from (1–9)×105moleccm−3. Box model simu- p
e
lations, using a model constrained to observed VOC levels (CO, CH and C H were r Abstract Introduction
4 2 6
the only species found to be important) and radiation agreed well with the OH mea- |
Conclusions References
surements, if NO levels of 1–5pmol/mol (ppt) (below the detection limit of the monitor D
deployed) were assumed. Primary production, O(1D)+H O, was found to account for isc Tables Figures
15 2 u
over 70% of the total OH production over the diurnal cycle. ss
io
Subsequent studies in the Arctic and Antarctic identified the snowpack as a source n J I
P
of a number of reactive species: HCHO (Sumner and Shepson, 1999; Hutterli et al., a
p J I
1999),NO (Honrathetal.,1999;Jonesetal.,2000)andpotentiallyHONO(Zhouetal., e
x r
2001) and higher aldehydes (Grannas et al., 2002). Such emissions will significantly
20 Back Close
|
alter the anticipated HO levels: HCHO and higher aldehydes will act as HO sources
x x
upon photolysis (and as OH sinks), while NO will drive radical cycling and HO to D FullScreen/Esc
x 2 is
OH conversion, potentially leading to ozone production, and heterogeneous HONO cu
s
production followed by photolysis will result in a direct source of both NO and OH s Printer-friendlyVersion
io
(Grannas et al., 2007 and references therein). n
25 P InteractiveDiscussion
MeasurementsofOHperformedatSouthPole(altitude=2835m)foundmuchhigher a
p
levels of OH than those observed at Palmer Station; average OH concentrations er
15112 |
6 −3 D
of (2–2.5)×10 cm were reported during the austral summers of 1998 and 2000 is
c ACPD
(Mauldin et al., 1999, 2004). These levels can be compared with the daily maximum u
s
levels (7×105cm−3) from Palmer Station (similar solar zenith angle). The South Pole sio 10,15109–15165,2010
n
OH data were consistent with rapid radical cycling driven by snowpack emissions of
P
NO , the effect of which was enhanced by the low boundary layer height at South a
5 x p
e Radical chemisty in
Pole station (Chen et al., 2001; Davis et al., 2001). While agreement between ob- r
served and modelled OH and HO +RO levels was satisfactory for moderate levels of the Antarctic
2 2 |
boundary layer
NO (ca. 100pmol/mol in this environment), at lower and higher NO levels the model
D
over predicted the observed OH; moreover inclusion of measured HONO levels in the is
c W.J.Bloss
10 model led to a large overestimate of the measured NOx, OH and HO2+RO2 levels uss
(Chen et al., 2004). Subsequent observations of HONO using a photofragmentation- io
n
LIF technique found the HONO abundance to be much lower (by a factor of 7) than P
a TitlePage
mist-chamber/ion chromatography data suggested (Liao et al., 2006). Further mea- p
e
surements were performed during the 2003 ANTCI (Antarctic Tropospheric Chemistry r Abstract Introduction
6 −3
Investigation) project, finding comparable OH levels of typically (1.5–2.5)×10 cm |
15
Conclusions References
(Mauldin et al., 2010), with even higher NO levels (up to 1ppb) than observed previ- D
ously (Eisele et al., 2008). Box model calculations of OH constrained by observations isc Tables Figures
u
of longer-lived species systematically overestimated the measured OH levels by a fac- s
s
tor of 2–3with no clear cause for the discrepancy identified (Mauldin et al., 2010). ion J I
HO have also been measured above snowpack at Summit, Greenland: Sjostedt et P
20 x a
al. (2007) used chemical ionisation mass-spectrometry to measure OH radicals and pe J I
thesumoforganicandinorganicperoxyradicals(ΣHO +RO )attheSummitresearch r
2 2 Back Close
station (altitude=3200m) during summer (June/July) 2003. Median radical levels of |
6.4×106cm−3 (OH)and2.2×108cm−3 (ΣHO +RO )wereobserved,muchhigherthan D FullScreen/Esc
2 2 is
the earlier South Pole observations, which can be explained in part by the lower so- c
25 u
s
lar zenith angles and higher humidity and ozone levels at Summit. Model calculations, s Printer-friendlyVersion
io
constrainedtoobservedradicalsourcesincludingHCHO,H O andHONOwerefound n
2 2
P InteractiveDiscussion
toreplicatetheobservedperoxyradicallevelsverywellwhentheHONOsourcewasre- a
p
moved,withamodestoverestimate(27%)withtheinclusionofHONO.ForOHradicals e
r
15113 |
howeverthemeasuredvalueswerefoundtobeafactorof2–3timeshigherthanthose D
is
modelled,pointingtomissingchemicalprocesses(OHsourcesand/orHO /RO →OH c ACPD
2 2 u
s
conversion processes). Sjostedt et al. noted that the model:observations (M/O) ratio s
io 10,15109–15165,2010
was highest during periods of high wind, and when back-trajectory calculations indi- n
P
cated that air masses had recently originated from the surrounding marine boundary a
5 p
layer,andhypothesisedthathalogenreactionsmayberesponsibleforperoxyradicalto e Radical chemisty in
r
OH conversion and hence the M/O discrepancy, although no halogen measurements the Antarctic
|
wereavailableduringthecampaign. Modelsimulationsoftheseobservations(Chenet boundary layer
al., 2007) were able to reproduce the observed peroxy radical (HO2+RO2) levels, but Dis
c W.J.Bloss
again underestimated OH levels by a factor of 2. Primary HO sources were found to u
10 x s
be O(1D)+H2O and photolysis of HCHO and H2O2, with snowpack emissions forming sio
n
a significant source of these, particularly for H O , while the dominant HO sink was
2 2 x P
a TitlePage
theHO2 self-reaction. AstheNOx levelswereloweratSummitthanSouthPole,overall pe
HOx abundance was driven by the rates of primary production and termination, rather r Abstract Introduction
15 than NOx-driven cycling processes. |
Conclusions References
A wide variation in radical concentrations, and the levels of related species with
D
snowpack sources (NO , H O , HCHO) is thus observed in overtly similar polar en- is
x 2 2 c Tables Figures
vironments, probably driven largely by differences in local dynamical factors (stabil- us
s
ity/boundary layer height) and as a consequence of processes within the snowpack io
n J I
leading to the production of NO , HONO and aldehydes amongst others; it is within P
20 x a
this context that the radical data from the CHABLIS campaign in coastal Antarctica is p J I
e
r
analysed here.
Back Close
Halogen species are also known to be of importance in some polar regions, with |
bromine chemistry in particular involved in the surface ozone depletion events ob- D FullScreen/Esc
is
25 servedinthecoastalareasofbothpolarregions(Simpsonetal.,2007andreferences cu
therein). During CHABLIS, the halogen monoxides IO and BrO were observed by s
s Printer-friendlyVersion
DOAS (Differential Optical Absorption Spectroscopy) at levels of up to 20pmol/mol ion
(Saiz-Lopez et al., 2007a). Halogen chemistry is expected to lead to direct cat- P InteractiveDiscussion
a
p
alytic ozone destruction, and will also perturb the NO and HO cycles giving rise e
x x r
15114 |
to indirect effects upon local oxidising capacity; Saiz-Lopez et al. (2008) used an un- D
is
constrained 1-dimensional model to calculate a chemical ozone loss rate at Halley of c ACPD
u
−1 −1 s
up to 0.55nmol/molh (ppbh ), and substantially increased OH and reduced HO s
2 io 10,15109–15165,2010
levels,byca.50%ineachcase. Subsequently,Bauguitteetal.(2009)haveshownthat n
P
the observed NO levels are consistent with a NO lifetime of the order of 6h, shorter a
5 x x p
than would be anticipated for such a remote location, thought to be a consequence e Radical chemisty in
r
of the hydrolysis of halogen nitrates constituting a significant additional NO sink. In the Antarctic
x |
this paper, we extend these analyses using all the available observations from the boundary layer
D
CHABLIS campaign to constrain the model simulations, with a particular focus upon is
c W.J.Bloss
HO as the species most sensitive to in situ chemical processing within the Antarctic u
10 x s
coastal boundary layer. sio
n
P
a TitlePage
p
3 Measurement overview e
r Abstract Introduction
The CHABLIS (the Chemistry of the Antarctic Boundary layer and the Interface with |
Conclusions References
Snow) measurement campaign was conducted at the British Antarctic Survey’s Hal- D
ley (V) Research Station, located on the Brunt Ice Shelf off Coats Land, at 75◦350S, isc Tables Figures
15 u
26◦390W. A full description of the CHABLIS project, site details and measurements ss
io
performed is given by Jones et al. (2008); pertinent details are summarised here. The n J I
P
base is located on a peninsula of the ice sheet, surrounded by the Weddell Sea to a
p J I
the North around (counter clockwise) to the South West, with the permanent ice front e
r
located15–30kmfromthebasedependingupondirection;thebaselocationisapprox-
20 Back Close
|
imately32ma.s.l. Duringtheperiodoftheobservationsusedhere(January–February
2005), the sea ice cover had almost entirely dissipated. The prevailing wind was from D FullScreen/Esc
is
ca. 80◦, corresponding to a uniform fetch of several hundred km over the ice shelf. cu
s
Themeasurementsite,comprisingashippingcontainerhousingtheLIFinstrumentfor s Printer-friendlyVersion
io
HO measurements, and an adjacent Clean Air Sector Laboratory (CASLab) housing n
25 x P InteractiveDiscussion
the remaining instruments, was located ca. 1km upwind of the other base buildings a
p
(and generators), at the apex of a clean air sector encompassing the prevailing wind er
15115 |
direction, within and above which vehicle and air traffic movements were prohibited. D
is
Measurements were performed over a 13-month period from January 2004, with an c ACPD
u
s
intensive summer measurement campaign, January–February 2005, which included s
io 10,15109–15165,2010
the HO measurements considered here. n
x
P
The observations used in this analysis are summarised in Table 1. OH and HO a
5 2 p
radicals were measured by laser-induced fluorescence (LIF) spectroscopy, using the e Radical chemisty in
r
FluorescenceAssaybyGasExpansionmethodology;fulldetailsofthemeasurements the Antarctic
|
are given in Bloss et al. (2007). The LIF instrument had a sampling height of 4.5–5m boundary layer
D
above the snow surface, and was co-located with inlets for measurement of humidity is
c W.J.Bloss
and ozone levels. NO was detected by chemiluminescence and NO was measured u
10 2 s
following its conversion to NO (photolytic converter) – Cotter et al. (2003)/Bauguitte et sio
n
al. (2009). The NO instrument was housed within the CASLab, and sampled from
x P
the laboratory’s main manifold, with an inlet height approximately 8m above the snow a TitlePage
p
e
surface. Nitrous acid (soluble nitrite) was measured by scrubbing gas-phase HONO r Abstract Introduction
into aqueous solution in a mist chamber, followed by derivatisation to 2,4-DNPH and
15 |
HPLC/absorbance detection (Clemitshaw, 2006). The halogen monoxide radicals IO Conclusions References
D
and BrO were measured by Differential Optical Absorption Spectroscopy (DOAS); the is
c Tables Figures
DOAS instrument and telescope were housed within the CASLab, while a retro re- us
s
flector array was positioned 4km due East of the measurement site, with the beam io
n J I
20 path approximately 5m above the snow surface. Further details of the instrument and P
a
IO/BrO observations are given in Saiz-Lopez et al. (2007a). VOCs (volatile organic p J I
e
compounds) were measured by gas chromatography with flame ionisation detection r
Back Close
(GC-FID):ethane–benzene,DMS(Readetal.,2007;2008). Formaldehydewasmea- |
sured via scrubbing into sulphuric acid followed by the Hantsch reaction/fluorescence D FullScreen/Esc
25 (Aerolaser 4021 monitor; Salmon et al., 2008). VOCs and HCHO were sampled from isc
u
theCASLabinlet,8mabovethesnowsurface. NomeasurementsofoxygenatedVOCs s
s Printer-friendlyVersion
(other than HCHO) were attempted – the potential abundance of these species and io
n
theirinfluenceisconsideredlater. LevelsofCH andH atHalleyinJanuary/February P InteractiveDiscussion
4 2 a
2005 were 1720 and 546nmol/mol, respectively (NOAA Global Monitoring Division pe
r
15116 |
flask analyses). Ozone was measured by UV absorption (Thermo 49C) and carbon D
is
monoxide by VUV fluorescence (Aerolaser AL5001, calibrated from a ±5% accuracy c ACPD
u
s
standard cylinder). s
io 10,15109–15165,2010
Otherenvironmentalparametersused inthisstudyincludehumidity, determinedus- n
P
ing a dew-point hygrometer (General Eastern 1311) and solar radiation: Actinic flux a
5 p
was measured using a 2-π spectral radiometer (Meteorologie Consult GmbH), pe- e Radical chemisty in
r
riodically inverted to determine the upwelling flux. Meteorological parameters were the Antarctic
|
determined using Campbell Scientific weather stations at various locations around the boundary layer
D
measurementsite–thetemperatureusedinthemodelanalysiscorrespondstotheLIF is
c W.J.Bloss
instrument inlet height. Boundary layer structure and stability was to have been deter- u
10 s
mined using a SODAR array which was unfortunately inoperable during the CHABLIS sio
summer measurement period; previous data (Ko¨nig-Langlo et al., 1998; Jones et al., n
P
2008) suggested a typical boundary layer height of 100m, which is adopted as the a TitlePage
p
e
initial estimate in this work; the sensitivity of the model results to this parameter is r Abstract Introduction
evaluated.
15 |
Conclusions References
D
3.1 Data processing/base generator pollution events is
c Tables Figures
u
s
All observations were analysed on a 10-min timebase. In the case of high-resolution s
io
measurements (HO , NO , HCHO, photolysis rates, and meteorological parameters) n J I
x x P
the mean of all observations within each 10-min window was used (where data were a
p J I
available). In the case of low frequency measurements (VOCs; 60–90min) the data e
20 r
were interpolated. A pollution filter was then applied to all data: Halley obtains its Back Close
|
power from a set of generators which were located ca. 1km from the measurement
D FullScreen/Esc
site. On occasion (10.0% of all observation points) the generator exhaust was sam- is
c
pled at the CASLab site (identified from local wind direction and NO levels) and the u
x s
s Printer-friendlyVersion
25 data were flagged as potentially polluted. Such observations were excluded from all io
n
model-observationcomparisonsreportedbelow. Inadditiontothebasesectorpollution
P InteractiveDiscussion
a
events, times at which there were gaps in the dataseries for long-lived species (NOx, pe
O ,CO,HCHO,NMHCs,radiationmeasurements)duetoinstrumentaldifficultieswere r
3
15117 |
excluded from the measurement-model comparison; in total 1963 10-min data points D
is
remained, representing ca. 55% of all HO observations. c ACPD
x u
s
s
io 10,15109–15165,2010
n
4 Model description P
a
p
e Radical chemisty in
TheHO -NO -XO interactionswereinvestigatedusingazero-dimensionalboxmodel, r
x x x the Antarctic
5 comprisingstandardinorganicandorganicreactionkineticsandphotochemistrytaken |
boundary layer
from the Master Chemical Mechanism (MCM) version 3.1 (http://mcm.leeds.ac.uk/). D
The DMS oxidation scheme used by Sommariva et al. (2006)/Carslaw et al. (1999) is
c W.J.Bloss
u
was added. A halogen (iodine and bromine) reaction scheme was implemented as s
s
described below. The model incorporated a simple representation of heterogeneous io
n
processes from the perspective of loss of gas phase species, and dry deposition. The P
10 a TitlePage
model ran within the FACSIMILE integration package, outputting species for compari- p
e
son with observations on a 10-min timestep, with a five-day spin-up period employed r Abstract Introduction
for each day simulated. |
Conclusions References
D
4.1 Photolysis rates isc Tables Figures
u
s
s
15 Photolysisratesusedinthemodelweretakenfromthedirectobservationsinthecases ion J I
of j(O1D) and j(NO ) (Jones et al., 2008). Photolysis rates for other species were cal- P
2 a
culated following the algorithms of Hough et al. (1988) as implemented in the MCM p J I
e
(version3.1),andwerethenscaledbytheratioofthecalculatedandobservedj(NO ) r
2
Back Close
values (Sommariva et al., 2006). For species not included in the MCM (those listed |
20 in Supplementary Table S1 – iodine and bromine compounds) photolysis rates were D FullScreen/Esc
calculated from the measured values of j(NO2), using a scaling factor determined as iscu
the mean ratio of the photolysis rate in question to j(NO ), calculated using clear-sky s
2 s Printer-friendlyVersion
actinicfluxdatafromTUV(MadronichandFlocke,1998)averagedoverthe.SZArange io
n
appropriate to the CHABLIS summer measurement period, employing the cross sec- P InteractiveDiscussion
a
p
25 tions/quantum yields listed in Table S1. The uncertainty introduced by this approach – e
r
15118 |