Sampling systems for isotope-ratio mass spectrometry of atmospheric ammonia

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2006; 20: 81–88

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.2279

Sampling systems for isotope-ratio mass spectrometry

of atmospheric ammonia

Richard Skinner1*, Phil Ineson1, Helen Jones2, Daren Sleep2 and Mark Theobald3

1University of York, Heslington, York YO105DD, UK2CEH Lancaster, Bailrigg, Lancaster LA1 4AP, UK3CEH Edinburgh, Bush Estate, Penicuik EH26 0QB, UK

Received 24 September 2005; Revised 7 November 2005; Accepted 7 November 2005

Passive and active ammonia (NH3) sampling devices have been tested for their nitrogen (N) cap-

ture potential and d15N fractionation effects. Several sampling techniques produced significantly

different d15NH3 signals when sampling the same NH3 source released from field site fumigation

campaigns. Conventional passive NH3-monitoring systems have shown to provide insufficient N

for isotope-ratio mass spectrometry and various modified devices have been developed, based on

existing diffusion tube designs, to overcome this problem. The final sampler design was then

tested in a wind tunnel to verify that sampling NH3 in different environmental conditions did

not significantly fractionate the d15N signal. Copyright # 2005 John Wiley & Sons, Ltd.

Elevated concentrations of reduced nitrogen (NHx) cur-

rently occur over many parts of Europe,1 with deposition

to semi-natural ecosystems being widely regarded as a

cause of significant environmental damage, mostly through

eutrophication and acidification of soils.2,3 The main

sources of NHx are primarily from animal waste4 and,

over recent years, there has been an increase in intensive

agricultural practices and associated increases in feedstock,

resulting in enhanced emission levels.5 Ammonia can exhi-

bit a large degree of spatial variability around emission

sources, with highly elevated concentrations found close

to the source, and reduction to background levels following

approximate exponential decay kinetics.6 The quantifica-

tion of NH3 dispersion is essential when formulating con-

trol policies for regional atmospheric pollutants,5,7 and, in

order to attribute NH3 to a specific emission point, back-

ground values need to be differentiated from those derived

from other sources. Increasing evidence suggests that var-

iations in the d15N signal of N-containing compounds in

vegetation, or as deposited NO3�/NH4

þ, can be used both

in agricultural and forest ecosystems to trace back their

source origins.8,9 The deposition signal from intensive ani-

mal units may also produce significantly different values

from other non-anthropogenic sources.10 The potential

use of d15N values to monitor transfers of NH3 is based on

the fractionation effects between the isotopes 15N and 14N.

The kinetic isotope effect almost always results in 15N

enrichment of the substrate and depletion of the product,

as lighter isotope molecules tend to react faster and volati-

lise more easily than the heavier isotopes.11

Ammonia is primarily emitted from widely distributed

low-level sources, with considerable spatial variation at the

local scale. A large number of monitoring sites are usually

needed to quantify this variation, and monitoring systems

are currently available to assess real-time NH3 fluxes, as

well as producing integrated signals over an extended time

period.12 Ammonia samplers can be either ‘active’, requir-

ing mains or battery power, or ‘passive’, relying on simple

diffusion or meteorological processes. Active systems, such

as denuders,13 annular denuders14,15 and Dreschel bub-

blers,16 use a wet chemistry approach, pulling air through

an acid-coated surface or solution. Systems such as the

rotating annular denuder17 can measure air concentrations

in real time, whereas Dreschel bubblers require subsequent

Kjeldahl distillation and titration to determine NH3

concentrations.

Active sampling is limited by the need for electricity, as

well as the cost of building and running the systems. Passive

devices, such as diffusion tubes18 and ALPHA samplers,19

are subsequently the preferred choice when monitoring the

spatial variation of NH3 over extended time periods. They are

inexpensive, are easily placed in any location and require

minimal maintenance.20

In order to model the atmospheric transfers of NH3,

accurate and reliable concentration data is required, with

both active and passive NH3-sampling systems having been

previously used in various field campaigns.6,21,22 For

conventional NH3 analysis, using wet chemistry, only a very

small amount of N is required and the quantity collected in

standard monitoring systems is frequently too small to

enable analysis using isotope-ratio mass spectrometry

(IRMS). Through comparison of existing NH3 capture

devices a new type of sampler, described here, has been

developed which enables routine passive monitoring of d15N

ratios in atmospheric NH3.

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: R. Skinner, University of York, Heslington,York YO105DD, UK.E-mail: [email protected]/grant sponsor: Natural Environment ResearchCouncil UK (GANE thematic program).

EXPERIMENTAL

Initial field testing of conventionalsampling devicesThree passive and one active system were tested for their

ability to capture N, and for any associated fractionation

effects. The field site was located at the Centre for Ecology

and Hydrology (CEH), Grange over Sands, Cumbria, UK

(Grid Ref. SD 409 797) which consisted of an experimental

area of grassland, 10 m2, with replicate samplers placed in

three randomised blocks within this area. The grass was

watered with 6 L of a 40% w/v urea solution, with subse-

quent hydrolysis of the urea by soil organisms, resulting

in the release of NH3 into the local atmosphere. The

samplers were exposed for a 2-week period (26/06/01 to

9/07/01), with a second and equal application of the urea

solution being made after 1 week of exposure. The sampling

devices tested included moss bags,23 shuttles,24 diffusion

tubes,18 and Dreschel bubblers.16

Moss bagsSphagnum capillifolium (Ehrh.) Hedw was collected from

one site in the North Yorkshire moors (Grid Ref. SE

875 654), and used in moss bag construction as detailed in

Temple et al.23 The bags were made from nylon mesh of

2 mm aperture size with 10 g fresh weight Sphagnum being

placed inside each bag, and the bags subsequently mounted

on a pole 1 m above the soil surface. After exposure, the

moss was washed with deionised water, dried at 508C,

ground and analysed using IRMS.

Diffusion tubeThe device consisted of a polypropylene tube (71 mm long,

11 mm diameter) enclosed by two close fitting polypropy-

lene caps. The top cap contained two round pieces of

stainless steel gauze, trapping an acidified Whatman GFA

filter disk which had been cut to size. Prior to exposure,

the filter disks were washed in Decon 90TM then rinsed in

deionised water before being dried in an NH3-free oven

and then impregnated with 30 mL of 1% v/v sulphuric

acid. The tubes were transported with both end caps in

place and the bottom cap was removed immediately prior

to sampling. Parallel blank tubes were also made, identical

to the field devices but the sampling cap was never

removed, enabling any chemical contamination or tube

leakage to be identified and corrections applied. A detailed

protocol for construction and preparation has been

reported by Hargreaves and Atkins.18

ShuttlesShuttle construction and operating protocols are detailed in

Leuning et al.24 with the shuttle consisting of a cylinder

with a conical inlet orifice and tail fins next to the outlet ori-

fice. Tail fins ensured that the sampler was always orientated

towards the direction of the prevailing wind, with any NH3

flowing inside the device being trapped on the enclosed

coiled corrugated steel shim coated in 3% w/v phosphoric

acid. This NH3 was subsequently removed by washing

with deionised water and then extracted using Kjeldahl

steam distillation.

Dreschel bubblerA pump actively pulled air through a series of two Dreschel

bottles filled with 1% w/v H2SO4 solution, at a flow rate of

6� 10�3 m3 min�1, with neoprene rubber tubing being used

for all connections. The inlet tubes were mounted 1 m above

ground level with atmospheric NH3 being trapped in the

Dreschel bottle by combination with the acidic solution

and then subsequently extracted using Kjeldahl steam dis-

tillation.16

Wind tunnel testingThe wind tunnel consisted of a 40� 200 cm plastic tube con-

nected to a 12 V large air fan, with variable airflows up to

6 ms�1 achieved by fitting a speed controller to the fan. A

1 mm aperture size nylon mesh shield, impregnated with

1% w/v H2SO4, was placed in front of the fan to extract

any atmospheric background NH3 passing over its surface

and thus ensuring that all the air entering the wind tunnel

was NH3-free. A 25 L Tedlar bag (Alltech Associates,

Carnforth, UK) was filled with NH3 from a compressed

gas cylinder and the gas was introduced behind a wind

baffle in front of the mesh shield, via a plastic tube. This

ensured even mixing and dispersal of the NH3 throughout

the tunnel, with NH3 flow rates set to produce final air

concentrations of up to 100 mg m�3. A peristaltic pump con-

nected to the Tedlar bag was used to accurately regulate the

NH3 flow from the Tedlar bag, and sensors were housed on

a shelf towards the end of the wind tunnel designed to

accommodate a variety of NH3 sampler designs. Fractiona-

tion effects associated with each device were investigated

by varying the wind speed and the position of the sampler

in the tunnel.

Testing of conventional sampling devicesat a field experimental ammonia release siteThe ammonia mitigation by enrichment recapture

(AMBER) experiment conducted by the Centre for Ecology

and Hydrology, Edinburgh, UK (Grid Ref. NT 150 499) was

designed to simulate the NH3 emission from a small animal

unit, producing approximately 2900 kg NH3 a�1. This

release site provided a defined experimental field opportu-

nity of testing a variety of other N-sampling systems,

such as ALPHA (adapted low cost passive high absorption)

samplers, throughfall collectors and sampling of on-site

bryophytes, together with active samplers such as ‘filter

packs’.

Ammonia was released parallel to a 50 m section of

woodland where the dominant tree species included Pinus

sylvestris L. and Taxus baccata L. The NH3 release manifold

comprised a 40 m inflated polythene tube with 6 mm

apertures spaced at regular intervals along its length.

Ammonia was introduced into this air stream via a 21 X

Campbell scientific data logger connected to a mass flow

controller. Releases occurred only when the wind direction

was perpendicular to the wood, employing a 308 wind

sector to the normal, ensuring direct fumigation in the

direction of the woodland. Fumigation was also limited to

conditions in which wind speeds were above 0.2 m s�1. The

main features of this site, together with sampler type and

location, are reported in more detail by Theobald et al.21

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2006; 20: 81–88

82 R. Skinner et al.

Throughfall collectorsThroughfall collectors comprised a sloping plastic gutter

0.3 m above the ground connected to a 30 L plastic collection

tank. A nylon mesh filter was placed over the inlet to the col-

lection tank and a thymol biocide25 (100 mg L�1) was added

to the tank to prevent sample deterioration. These collectors

were replicated in triplicate and placed under the tree canopy

at seven locations (1, 4, 9, 16, 23, 30 and 60 m from the edge of

the wood), with sampling on a 2–4 week basis, depending on

rainfall amount. Three collectors were placed in a ‘control’

area 300 m north of the release point, with NH3 concentration

in this area considered to be at background levels.21

Ammonia concentration monitoringAmmonia concentrations were measured using ALPHA

samplers19 at seven locations spaced exponentially from the

source at the same distances as the throughfall collectors,

with one background monitoring location 20 m upwind.

Each location contained three samplers, exposed over a

4-week interval. Captured NH3 was extracted by washing

in deionised water, with NH3 concentration being subse-

quently determined by ammonia flow injection analysis

(AMFIA).26 AMFIA analysis requires only a small aliquot of

the resulting NH4þ solution, enabling the remaining NH4

þ to

be extracted using Kjeldahl distillation16 and analysed using

IRMS.

Filter packsThe filter pack device27 used was an active sampling system

consisting of a series of filters housed in a plastic casing, sepa-

rated by perforated plastic dividers. A pump was attached to

the rear of the device to pull air through these filters at flow

rates between 15 and 25 L min�1 and using sampling periods

of several hours. This campaign took place over a 2-week

period with the devices placed in the same locations as the

ALPHA samplers. Whatman 42 filter papers were cleaned

by soaking in Decon 90TM for 20 min, then re-soaking in deio-

nised water for a further 30 min to remove any traces of NH3.

The papers were then dried in an NH3-free environment and

impregnated with 100 mM citric acid solution prior to

exposure at the field site. Post-exposure filter papers were

stored in sealed plastic bags at 58C before Kjeldahl steam dis-

tillation was used to extract the NH3 for IRMS analysis. Filter

packs were also used to sample NH3 directly from the NH3

release manifold and three separate locations were chosen

to determine fractionation effects associated with NH3 mov-

ing through the manifold.

Vegetation samplesApproximately 5 g of Hypnum spp. was sampled at the same

distances from the source as were used for the ALPHA and

filterpack devices. A 3-cmsection of the shoot tip was removed,

washed in deionised water and oven-dried at 508C. The same

procedure was used to sample the moss 6 months after fumiga-

tion had ceased at the same seven locations. Once dry, the moss

was ground in a liquid-nitrogen mill and subsequently

analysed for foliar %N and d15N using IRMS.

Modification of existing sensor designField testing of adapted NH3 samplers was conducted 20 m

downwind of an intensive pig farm in North Yorkshire. The

modified samplers were based on the diffusion tube design18

with an increased aperture size, allowing increased NH3

capture rates. The filter disks used were cut to size from

Whatman No. 1 filter papers and washed in deionised water

before being dried and impregnated with 2 mL of 1% w/v

H2SO4.

Nine standard diffusion tubes were also exposed to obtain

accurate NH3 concentrations over the same sampling period.

Each system was replicated in triplicate and placed in a

randomised block design. After exposure, all filter papers

were soaked in 5 mL deionised water to extract NH4 and

subsamples were taken for determination of NH4 concentra-

tions using a continuous-flow auto-analyser, The NH4 was

then removed from the solution by Kjeldahl steam distillation

and collected in deionised water with H2SO4 added dropwise

to give efficient sulphate formation. This solution was then

dried down at 808C and the resulting solid analysed using

IRMS.

Field testing was used to determine an optimal filter disk

size to enable sampling of sufficient NH3 for IRMS at

concentrations found close to an intensive animal unit.

Plastic drainage access caps (OsmaTM, Wavin Plastics Ltd.,

Chippenham, UK) were used with each cap containing a

short length of sleeving (80 mm long, 110 mm i.d.) and a

screw lid (125 mm o.d.). A small lip around the underside of

the lid enabled the filter paper to be secured between the

sleeve and the lid when screwed together. For field placement

the cap was secured, usually at 1 m above the ground, by a

pipe bracket mounted to a purpose-built wooden triangle

frame (see Fig. 1).

Other sampler designs were also developed and tested,

based around existing sampling systems. These included

moss bricks, where an inoculum of Hypnum spp. was grown

on an artificial concrete block substrate. These ‘moss bricks’

were devised for d15NH3 sampling in environments devoid

of such bryophyte species. This standardised the vegetation,

with each unit containing the same species and the same

amount of foliar N prior to exposure. On exposure the plants

were placed on a roofed platform 1 m above the ground to

Figure 1. Adapted diffusion tube sampler replicated in

triplicate and mounted on a wooden pole 1m above the

ground.

Sampling atmospheric ammonia for IRMS 83


limit NH3 contamination from the droppings of flying birds.

Drip trays were used to keep the bricks moist and an

automatic watering system allowed unattended monitoring

over an extended period. A. N-free nutrient solution was

used to feed the moss and the platform was covered in

chicken wire to prevent contamination from birds landing on

the stand. Control samples were also used, consisting of the

same moss grown on the substrate concrete but without

exposure.

Chamber measurementsThe closed chamber consisted of a glass container 30 cm in

diameter with an airtight seal around the rim and a rubber

septum for gas introduction at the top. The container was

thoroughly cleaned with Decon 90TM prior to use and the

adapted passive sampler suspended inside the device via a

metal frame. A known quantity of NH3 gas was then intro-

duced into this container via the septum, using a gas syringe,

to achieve the desired atmospheric NH3 concentration. The

adapted passive sampler was then left to sample the NH3

for a 2-week period before being removed to analyse the

NH3 captured and to assess any fractionation effects resulting

from varying the NH3 concentrations.

Sample analysisAll d15N analyses were conducted at CEH Merlewood using a

Eurovector elemental analyser (EA) coupled to an Isoprime

IRMS system (Micromass, Manchester, UK) with an online

Robo-Prep C & N analyser for sample oxidation. Filter papers

from atmospheric NH3 monitoring were soaked in deionised

water, shaken for 20 min, and the NH3 extracted from this

solution by Kjeldahl distillation.16 The distillate produced

was combined with 1 M H2SO4, added dropwise to maintain

the pH of the solution between 4 and 5, ensuring efficient sul-

phate formation. The resulting (NH4)2SO4 solution was then

dried over a water bath and transferred to a 5-mL sample vial

for final dehydration in an oven at 808C. The solid was then

placed in a tin cup for IRMS analysis or, if the amounts were

too small, the solid was resuspended in �10mL of deionised

water and added to an inert support medium such as Ultro-

dexTM for IRMS analysis. Standards were prepared by distil-

ling 1 mL of (NH4)2SO4 standard solution through the same

Kjeldahl apparatus, using the procedure previously detailed.

For the moss bag and brick samples, the initial d15N signal

and %N were determined and subtracted from the final value,

using a two-point mixing model in order to establish the d15N

signal of the added N. The field campaigns in Cumbria and

North Yorkshire both employed a randomised block design,

and all data was tested for normality prior to analysis by

analysis of variance (ANOVA) to test for significant differences

between treatments using SAS.28 All error bars are displayed as

�1 standard error of the mean (s.e.m.).

CalculationsAt a constant temperature, the flux of gas A (FA) through gas

B can be described by Fick’s Law:

FA ¼ �DAB�C=��

where DAB is the bulk gas diffusion coefficient for gas A

diffusing through gas B (cm2 s�1), Dw is the diffusion path

length (cm), and DC is the concentration of gas A in gas B

(mol cm�3);DC is the difference between the average ambient

concentration (CA) and the concentration at the reactive

surface (CS).

It can be assumed for an efficient absorbent that CS is

maintained at zero and that the rate of collection of the gas is

also independent of temperature and pressure under normal

atmospheric conditions.18 Therefore, for a tube of length z

and cross-sectional area A (cm3), the quantity of gas (QA)

diffusing along the tube in time t (s) is given by:

QA ¼ FAAt ¼ ð�DABCAAtÞ=z

The value of DNH3 has been estimated from the theory of

viscosity of pure gases, with a precision of approximately

�9%,30 giving a value of 2.32� 10�7 m3 s�1. Also, at 1 ppm(v),

CA is 4.307� 10�17 mol NH3 m�3. Therefore, the prototype

diffusion tube of diameter 110 mm, and path length 80 mm,

should sample ca. 8.9� 10�3 m3 h�1.

During a 4-week period the amount of air sampled by this

device would be 5.99 m3. The ambient NH3 concentration

around the pig unit was approximately 20 mg m�3, and

�120mg of NH3-N would be captured by the device over

this time period. Research has shown saturation at 10mg for a

device with a sampling area of 154 mm2,29 so the prototype

sampler incorporated an acidified area theoretically capable

of saturating at much higher amounts, in excess of 600 mg

NH3-N. The sampler could therefore be placed in areas with

NH3 concentrations in excess of 100 mg m�3 over a 4-week

period without theoretical significant reduction in the

sampling rate.

RESULTS AND DISCUSSION

Field testing of conventional sampling devicesSamplers tested at the Cumbria site produced different d15N

values from each other and from that of the source urea

(measured at þ2.3 to þ2.6%). The bubbler system produced

a consistently significantly lower d15N signature than any of

the other devices tested, indicating the isotopically light

composition of the NH3 formed (Fig. 2). The shuttle system

Figure 2. d15N values obtained from exposing conventional

NH3 samplers at an artificial fumigation experiment over a

2-week period. Values with the same letter are not signi-

ficantly different and error bars are expressed as 1 s.e.m.



gave the most variable d15N value and had the lowest

N-capture efficiency. The shuttle and diffusion tubes both

sampled very little N, being almost at the limit of detec-

tion for the mass spectrometer. The moss bags did,

however, provide enough N for IRMS analysis and, by

employing a two-component mixing model to remove the

d15N signal from N already present in the shoot tissue, it

was possible to estimate a d15N value for the atmospheric

NH3 sampled.

The Dreschel bubbler gave the lowest standard error but

there was no definitive measure for d15N in the air and it was

impossible to ascertain which sampler gave the correct

isotopic value for atmospheric NH3. The bubbler device

was, however, assumed to be free from fractionation.

It does seem probable, however, that this system would

determine an isotope signature that very closely matched

the actual d15N value of sampled NH3. One further

experiment that could be conducted in the future would

be to use this system in the wind tunnel. A precise

measure of the d15N value for NH3 used in fumigation

could be determined from injection of a small quantity of

gas into a sealed vessel containing an acid absorbent such

as H2SO4. The (NH2)2SO4 could then be analysed and the

result compared with that generated from the capture of

NH3 by the Dreschel bubbler to determine sampler

accuracy.

Wind tunnel testingThe prototype diffusion tube incorporating a 110 mm filter

disk was tested in a wind tunnel to analyse possible fractiona-

tion effects associated with varying atmospheric conditions.

The d15N signal of captured NH3 remained relatively con-

stant throughout all the wind speeds and across all NH3 con-

centrations tested (Fig. 3).

Testing of conventional sampling devicesat a field experimental ammonia release

ALPHA samplersFigure 4 displays the mean monthly NH3 concentrations

obtained from ALPHA samplers for the period 10/9/01 to

12/10/01. The results showed an approximate exponential

decline, with NH3 concentration reducing rapidly with dis-

tance from source. Concentrations at the edge of the wood

were, however, still approximately four times the back-

ground levels. Error bars have not been included to permit

clearer viewing of the figure but all values were low and

have been displayed for reference in Table 1

The d15N values from NH3 captured by ALPHA samplers

(Fig. 5) displayed a similar profile to the concentration data,

but there was a less prominent decline. Values approximately

15 m from source did not significantly differ from those at the

end of the transect, 60 m distant. The upwind value for

ALPHA samplers was �7.8� 2.1%, and, although the errors

for these samples were large, they still showed a significant

difference between the downwind values for the first 9 m

from source (P< 0.05, R2¼ 0.789). A correlation was also

observed between the NH3 concentration and d15N signal

(Fig. 6) displaying a significant positive relationship between

the two variables (P< 0.01, R2¼ 0.916).

Filter packsThe d15N signal from NH3 captured by filter packs (Fig. 5) also

exhibited a similar decrease in value compared with the d15N

values derived from ALPHA samplers. However, the signal

was approximately 3% higher than for NH3 from ALPHA

samplers for all but the last sampling point, 60 m from source.

d15N values sampled downwind of the source from the two

devices were not significantly different from each other,

and a similar positive relationship between d15N value

sampled from filter packs and NH3 concentration derived

from ALPHA samplers was observed (P< 0.05, R2¼ 0.889).

Ammonia sampled at the manifold using the filter pack

system produced a source value of þ2.8� 0.5%. The d15N

values also remained consistent at various points along the

length of the manifold over the entire 2-week sampling

campaign.

Figure 3. Effect of varying wind speed and NH3 concentra-

tion on the d15N values for 110mm acidified filter disks

exposed over a 5-day period to NH3 fumigation in a wind

tunnel. Error bars expressed as 1 s.e.m.

Distance from source (m)

100 20 30 40 50 60 70

Con

cent

ratio

n of

NH

3 (µ

g m

-3)

0

10

20

30

40

50

60

Figure 4. Mean monthly NH3 concentrations obtained from

ALPHA samplers for the period 10/9/01 to 12/10/01. Error

bars expressed as 1 s.e.m.



Throughfall dataFigure 5 also displays the change in d15N signature from

throughfall transecting into the wood with each data point

representing the mean of all the collectors at that distance.

There was a decrease in d15N value with increasing distance

from source showing a similar profile to that reported for

throughfall NH4þ concentration21 and d15N values (Fig. 5).

The largest change occurred 10–15 m from source, with

NH4þ samples nearest the manifold being enriched by

approximately 3–4% compared with source NH3. This signal

rapidly declined, returning to an approximate background

value of �4%. This value was similar to samples taken

from the control site where d15N values were observed of

between �5.8 and �6.9%.

Vegetation dataThe d15N values for foliar N inHypnum spp. along the transect

are not presented here since no significant change in values

was observed with distance from source. A higher %N was,

however, found in those samples close to source relative to

those at the end of the transect, and several foliar N concen-

trations along the length of the transect were significantly lar-

ger than the respective control values.

Modification of existing sensor designThe adapted diffusion tube design was based on the conven-

tional NH3-monitoring diffusion tube18 but the final design

enclosed a filter paper 110 mm in diameter. The d15N values

obtained from these adapted diffusion tubes were all similar,

but varied significantly from the moss brick data. The amount

of N captured by the moss bricks did not differ significantly

from the control samples and the data have not been pre-

sented here. The amount of N captured by the adapted diffu-

sion tubes was, however, significantly greater than the

control samples.

Results from field tests conducted in Cumbria demon-

strated a distinct fractionation effect for each type of

NH3-sampling device. The Dreschel bubbler system gave a

significantly different value from those from the other

systems tested and also had the lowest variance, with the

d15N values produced being very consistent. This compara-

tively low value is unlikely to be due to incomplete recovery

of the NH3 because (i) fractionation would tend to be in the

opposite direction, (ii) NH3 was not found in the secondary

‘breakthrough’ Dreschel bottle, implying that the value

obtained was a true reflection of the d15N in the atmosphere.

This is consistent with the theory that volatilisation from urea

involved a unidirectional reaction, where the lower energy

required for breaking 14N bonds (compared with 15N bonds)

resulted in the product NH3 being depleted in 15N, further

enriching the urea and causing the NH3 released to be

isotopically light.30

The other samplers tested all showed higher variances,

with the shuttles giving the largest standard error, and

Figure 5. d15N values obtained from sampling atmospheric

NH3 using filter packs; ALPHA samplers and throughfall data

all collected at the AMBER field site.

Concentration of NH3 (µg m-3)

200 40 60 80 100 120 140

δ15

N v

alue

(o / oo

)

-8

-6

-4

-2

0

2

4

ALPHA samplerFilter pack

Figure 6. Comparison between the d15N values obtained

from filter pack and ALPHA sampling of atmospheric NH3 and

the NH3 concentrations obtained by these two systems over

the 2-week campaign. Filter packs: P< 0.01, R2¼ 0.889 and

ALPHA samplers: P< 0.01, R2¼ 0.916.

Table 1. Details of the data presented in Fig. 4 (bracketed values represent 1 s.e.m.)

Sampling device

Distance (m)

1 2 4 8 16 32 60 �50

Throughfall 5.0(1.2) 1.1(0.7) �0.1(1.2) �0.4(1.4) �1.9(0.7) �3.1(0.5) �3.9(1.1) —ALPHA 2.6(0.7) �0.7(1.6) �0.6(0.8) �2.1(0.7) �3.8(0.6) �3.6(1.2) �5.1(0.4) �7.7(1.7)Filter pack �0.7(0.6) �2.1(1.1) �2.8(0.7) �4.4(1.3) �4.0(1.1) �5.6(0.6) �4.7(0.3) �8.0(1.4)



appearing unreliable for accurate sampling of isotopic ratios.

This system also exhibited a poor N-sampling rate, which

may partially explain the large range of values obtained.

These small quantities of N were barely sufficient for IRMS

analysis, with the mass spectrometer being at the limit of

detection. Previous studies have indicated that shuttles only

operate efficiently within a narrow range of air speeds; too

slow a wind speed and not enough air enters the device, too

high and the gas travels through too quickly for efficient

deposition.12,24 Therefore, it is possible that the optimal

operating requirements for these devices were not met, with

insufficient wind speed causing a poor capture rate of

atmospheric NH3.

Diffusion tube filters also produced a low N yield and again

any errors could easily compromise the observed signal. Moss

bag data also yielded a wide range of values, primarily due to

the many factors that may affect this technique. For example,

vegetation can occasionally dry out during a sampling period,

so that NH3 sampling is inhibited until rehydration.31

Although all the moss used was taken from the same source,

the foliar concentrations and isotope composition of N for an

individual sampler were unknown and could have varied

significantly. There is also evidence to suggest that plants of the

same species that areN deficient may exhibit a slightly different

d15N signature from those that are not.32

Field test results suggested that only the Dreschel bubbler

was capable of sampling enough NH3 for IRMS analysis at

concentrations found at distances over 50 m from an

intensive animal unit.22 The samplers used had been

primarily designed for monitoring NH3 concentrations

where the amount of N needed for colorimetric analysis

was very small. In contrast, IRMS requires ca. 80 mg of N per

determination.10 Despite showing low variability and mini-

mal fractionation, the main disadvantage of using the bubbler

system was the requirement for mains power, making

monitoring at remote sites across several transects imprac-

tical.

At the AMBER field site both of the tested sampling devices

produced similar background d15N values, but differed in the

d15N values sampled downwind of the release manifold.

Direct comparisons between the two methods are proble-

matic due to the differences in sampling period and this may

account for the discrepancies observed. The ALPHA sampler

required 4 weeks of exposure before analysis, while the filter

pack sampled over only a 2-h time period. Therefore, the filter

packs generated a value that more closely matched a real-

time NH3 measurement and would be expected to be more

variable than the integrated signal sampled over several

weeks. The filter pack d15NH3 values more closely matched

NH3 sampled from the source manifold which was primarily

due to filter packs operating only when NH3 was being

released into the wood during favourable wind conditions.

The ALPHA samplers, however, continued to extract NH3

from the atmosphere whether NH3 fumigation was occurring

or not, resulting in a larger background contribution than for

the filter pack system.

The d15NH4þ signal in throughfall was surprisingly higher

than expected but correlated with NH3 concentration

measurements, suggesting a relatively large capture rate at

the edge of the wood followed by a lower constant rate of

deposition throughout the remainder of the transect. Studies

have shown that the edge of woodlands can also act as a trap

for air-borne pollutants,33–35 and this effect depends on leaf

surface area, wind velocity and structure of the woodland,

with forest edges disturbing the vertical wind profile and

causing higher air turbulence,36 thus increasing deposition.37

Mean d15N values at the end of the transect were not

significantly different from those observed at the control site,

but several individual samples still contained significantly

higher d15N values than from control plots, indicating that

values were not quite consistently at background levels.

The throughfall d15N values at AMBER also showed

enrichment in the d15N signal relative to atmospheric

d15NH3, with similar studies having also observed enrich-

ment of throughfall samples when compared with atmo-

spheric NH3.8 One factor resulting in the d15N signal in

throughfall being positive may be isotopic discrimination

during NH4 uptake by the tree canopy.38 The acidity of the

canopy can also affect deposition, as the solubility of NH3 is

enhanced by low pH. This would favour the more complete

removal of NH3 gas; therefore, the d15NH4 value should be

similar to that of the source NH3.8 This effect can be seen at the

AMBER site where the predominantly acidic Pinus sylvestris

canopy would encourage NH3 deposition resulting in the

enriched positive signal from source NH3 to be observed in

throughfall.

No significant trend was observable in the Hypnum spp.

sampled along the transect. It may be possible that the

fumigation period was not long enough for the moss to

accumulate sufficient N with a significantly different isotopic

signature for it to be observable in the bulk shoot tissue.

Similar studies38 have shown that certain bryophytes,

including Hypnum spp., do exhibit a change in foliar d15N

signature with elevated NH3 levels, although the data in

question were obtained from a site exposed to elevated NH3

levels over much longer time periods.

An acid-coated filter can saturate at high NH3-N loading

due to a change in the mass transfer rate once the filter surface

has fully reacted with NH3. Subsequent molecules that have

to penetrate through a thickened reacted surface take longer

to diffuse and thus yield a false value for the actual air

concentration. Saturation was clearly not reached in these

experiments and the results in Fig. 3 demonstrated consistent

d15N values at all the wind speeds and NH3 concentrations

tested. All the evidence from testing showed that this sampler

could therefore be used to monitor d15N values for NH3 point

sources under a wide variety of atmospheric conditions.

CONCLUSIONS

When the NH3 air sampling devices were tested at the var-

ious field sites they varied both in the actual d15NH3 values

obtained and in the errors associated with each sampler. Pas-

sive systems, including the shuttle and diffusion tubes, pro-

duced very high variance, primarily because the low

quantities of N captured were at the limits of detection of

the IRMS system. The device showing least variance was

the Dreschel bubbler but high unit cost and mains power

requirements largely prohibited its use at future monitoring

sites.



In contrast, the adapted passive sampler, based on the

diffusion tube design, produced consistent values in opera-

tion. It was inexpensive to construct and was shown not to

differentially fractionate sampled NH3 because of changes in

wind speed or NH3 concentration. These devices were

successful in capturing sufficient N for IRMS analysis

(in excess of 80 mg N) at atmospheric NH3 concentrations

present near intensive animal units.

AcknowledgementsWe gratefully acknowledge the large input from staff at the

Centre for Ecology and Hydrology particularly Sim Tang

for assistance with field sites and equipment (CEH

Edinburgh) and the Stable Isotope Facility Laboratory for

analytical provision (CEH Lancaster). This work was

funded by the Natural Environment Research Council UK

under the GANE thematic program.

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Sampling systems for isotope-ratio mass spectrometry of atmospheric ammonia

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