Aerosol isotopic ammonium signatures over the remote Atlantic Ocean

C.T. Lin*, T.D. Jickells, A.R. Baker, A. Marca and M.T. Johnson

Centre for Ocean and Atmospheric Sciences

School of Environmental Sciences

University of East Anglia,

Norwich Research Park, Norwich, NR4 7TJ, UK

* Corresponding author

Now at

Greenhouse Gas Reduction Management Office,

Environmental Protection Administration,

Executive Yuan, Taiwan (R.O.C.)

Abstract

We report aerosol ammonium 15N signatures for samples collected from research cruises on the South Atlantic and Caribbean using a new high sensitivity method. We confirm a pattern of isotopic signals from generally light (δ15N -5 to -10‰), for aerosols with very low (<2nmol m-3) ammonium concentrations from the remote high latitude ocean, to generally heavier values (δ15N +5 to +10‰), for aerosols collected in temperate and tropical latitudes and with higher ammonium concentrations (>2nmol m-3). We discuss whether this reflects a mixing of aerosols from two end-members (polluted continental and remote marine emissions), or isotopic fractionation during aerosol transport.

Keywords

Atlantic Ocean, Ammonium aerosol, 15N

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15161718192021

22

23

24

Introduction

The cycling of ammonia through the atmosphere represents a major component of the global nitrogen

cycle (e.g. Fowler et al., 2013, Gruber and Galloway, 2008). The cycle involves both gaseous

ammonia and aerosol ammonium, neither of which are oxidised in the atmosphere on the time scales

of a few days, and can therefore be transported through the atmosphere and deposited over the oceans.

In remote ocean regions, atmospheric nitrogen deposition represents an important nutrient source

which can increase marine productivity (Duce et al., 2008). However, the net significance of the

atmospheric ammonia/ammonium contribution to this deposition flux depends critically on whether

the ammonia/ammonium source is predominantly terrestrial, and hence a new source of nitrogen to

the marine environment, or from ammonia emissions from seawater that are being recycled and hence

will not result in a net increase in ocean productivity. A net marine ammonia emission source to the

atmosphere has been proposed (e.g. Voss et al., 2013 and refereces therein, Fowler et al., 2013)

although it has been argued that the net direction of this flux may have been reversed in the industrial

period so that it now flows from land to sea as a result of the increases in atmospheric ammonia

emissions from human activity (Galloway et al., 1995). New comparisons of model and field data lead

to a rather lower estimate of the global marine emission than previously suggested (Paulot et al.,

2015). A recent study has suggested that the ocean ammonia emissions can be the dominant source of

ammonium in rainfall on Bermuda, despite this being a region downwind of major terrestrial

anthropogenic sources in North America (Altieri et al., 2014). However, another recent study has

suggested that terrestrial ammonia emissions are the main source for ammonium in rainwater on

Bermuda (Keene et al., 2014). Hence there is still considerable uncertainty over even the direction,

leave alone the magnitude of the air-sea ammonia flux.

Due to the strong temperature dependence of ammonia solubility, it has been argued that marine

ammonia emissions to the atmosphere are likely to be more important from warmer rather than cold

ocean waters, with net emissions at high latitude from the ocean to the atmosphere difficult to achieve

at the low water temperatures, under typical surface ocean and marine atmosphere conditions

(Johnson et al., 2008, Paulot et al., 2015). Gas phase ammonia measurements from the remote marine

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

atmosphere boundary layer would be useful for understanding the marine ammonia cycle. However,

such gas phase ammonia concentrations are very poorly constrained, with few measurements, mostly

with high uncertainties (Paulot et al., 2015). However, high sensitivity measurements suggest that gas

phase ammonia and aerosol phase ammonium concentrations may be of similar magnitude to each

other over the open ocean (Norman and Leck, 2005). Ammonia emissions from seawater can react

rapidly in the marine atmosphere to form ammonium aerosols in the presence of acidic species. Given

the rapid removal of any free nitric acid by reaction with sea-salt in the marine atmosphere (Baker et

al., 2006, Andreae and Crutzen, 1997), the most likely salts to be formed by marine ammonia

emissions are with sulphuric acid. We have previously assumed that these salts would not

disproportionate into gas phase components (Jickells et al., 2003, Paulot et al., 2015) and hence will

not isotopically fractionate once in the aerosol phase. Indeed for dry ammonium sulphate and

ammonium bisulphate salts, the partial pressure of ammonia is very low. However, these salts will

attract water potentially allowing a pH dependent bidirectional exchange of ammonia between aerosol

and gas phases at high humidity in the marine boundary (Johnson et al., 2008, Quinn et al., 1992),

consistent with limited field and laboratory data (Harrison and Kitto, 1992, Scott and Cattell, 1979).

Johnson and Bell (2008) derive the following relationship from the Aerosol Inorganics Model (Clegg

et al., 1998) at high relative humidity appropriate for the marine boundary layer:

pNH3 = 0.00016 e^(0.124T + 4.6R)

where pNH3 is the partial pressure of ammonia over the aerosol in nmol/m3, T is the temperature in

Celsius and R is the molar NH4:nss-SO4 (nss – non-sea-salt sulphate) ratio in the aerosol. This

equation predicts pNH3 comparable to or greater than typically observed marine boundary layer

ammonia concentrations at NH4:nss-SO4 ratios above ~1.2, and such ratios occur at least in parts of

the Atlantic Ocean marine boundary layer (Norman and Leck, 2005).  Thus ammonia-ammonium

partitioning in the marine atmosphere is likely to be very dependent on aerosol pH (Johnson and Bell,

2008), which is also not particularly well known (Pszenny et al., 2004, Kerkweg et al., 2008).

The magnitude of marine ammonia emissions and their significance for ocean productivity is

therefore still rather uncertain and requires further study, particularly given the scale and significance

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

of the human perturbation of the global nitrogen cycle. Isotopic measurements have the potential to

provide additional insight into the marine atmosphere ammonia cycle, and indeed the basis of the

arguments for a major marine ammonia emission source dominating ammonium deposition over the

North Atlantic (Altieri et al., 2014) are predominantly isotopic measurements. In the latter paper

evidence from an earlier study by us of the aerosol ammonium isotopic composition in the remote

marine atmosphere (Jickells et al., 2003) was used to help make the case in support of a marine

source. However, while there have been several studies describing the isotopic signature of ammonia

emissions and the abundance of ammonia/ammonium in rain, aerosol and the gas phase in relatively

polluted areas (Yeatman et al., 2001, David Felix et al., 2013, Fukuzaki and Hayasaka, 2009, Xiao et

al., 2015) there have been less studies in remote areas. In our earlier study (Jickells et al., 2003) we

reported a systematic decrease in the isotopic ammonium signature with decreasing ammonium

concentration which was interpreted as resulting from two end-member mixing of isotopically light

marine emissions of ammonia with isotopically heavy terrestrial ammonia emissions. However, the

nitrogen content of samples from the remote marine atmosphere approached our detection limit for

isotopic analysis in that earlier study, and it is possible that some of the trends in isotopic composition

with concentration reported could reflect uncertainties around detection limits.

We now report here analyses of aerosols collected along the Atlantic Meridional Transect AMT

(Robinson et al., 2006) between the UK and Southern Ocean for ammonium ion isotopic composition

using an improved method with greatly enhanced sensitivity. This allows us to test the validity of the

trends seen in our previous study. The analysis of additional aerosol samples from the Caribbean is

also reported and used to further consider the validity of the two end-member mixing model, between

high concentration and isotopically heavy ammonia from strong terrestrial emission sources regions

and a background low concentration marine emission of isotopically light ammonia.

Methods

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

Aerosol data reported here are based on samples collected on two cruises AMT15, sampling from

approximately 10oN to 40oS in the Atlantic, and JC18 (Figure 1). Our aerosol sampling and

contamination control methods, as well as our procedures for calculating air-mass back trajectories,

have been described in detail in previous publications (Jickells et al., 2003, Baker et al., 2010), and

are therefore only briefly summarised here. Aerosol samples were collected by high volume aerosol

sampling with a Sierra Type cascade impactor and the results presented here are for fine mode aerosol

(<1µm diameter aerosol and where most of ammonium is found Baker et al., 2006) collected on

Whatman 41 filter. Sample filters were subsequently frozen for return to our home laboratory. Filters

were later thawed, major ions water extracted from a fraction of the filter and this extract

subsequently analysed by ion chromatography (anions and ammonium) and ICP-OES (for the other

cations). The ammonium isotopic composition was measured on the water extract of a separate

fraction of the aerosol filter using essentially the same method as Altieri et al. (2014), although

independently developed and described in detail elsewhere (Lin, 2010) 1. This method is much more

sensitive than the method used in our previous study. The method is based on chemical conversion of

ammonium to nitrite and then to N2O which is then cryogenically pre-concentrated and analysed by

isotope-ratio mass spectrometry on a Europa GEO 20-20 (McIlvin and Altabet, 2005, Zhang et al.,

2007, Altieri et al., 2014). The isotope ratios are calibrated against standard reference materials IAEA

N1 and USGS 25 and 26 as well as an internal laboratory N2O gas standard. Results are expressed

here in the standard δ15N notation. The method can successfully measure isotopic ratios on as little as

10nmol of ammonium. Blanks of chemicals and sample preparation yield a beam area of only 10% or

less of samples and are corrected for in the data reported here. The average precision based on

replicate analyses of the standards is +0.2‰ or better.

Results

Figure 2a shows the results for AMT15 samples with the aerosol ammonium δ15N isotopic

composition values plotted against the ammonium concentration. Ammonium concentrations show a

1 https://ueaeprints.uea.ac.uk/19108/1/2010LinCTPhD.pdf

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

1

strong gradient of concentrations with low values at high latitudes consistent with the trend reported

by Norman and Leck (2005). Since ammonia can have both marine and terrestrial sources, it is not an

unambiguous tracer of terrestrial sources, but since the emissions on land are so much larger than

those from the oceans, the gradient in aerosol ammonium is consistent with the mixing of low

concentration ammonium aerosol from remote region air masses with high concentration terrestrial

sources. Nitrate concentrations are an alternative unambiguous tracer of terrestrial emissions. Total

aerosol ammonium and nitrate are well correlated with an essentially zero intercept along the AMT

transect, despite the different aerosol size distributions of nitrate and ammonium due to the reaction of

nitric acid with seasalt (Baker et al., 2006) and this correlation is also seen in the this data set (results

not shown here but available in Lin, 2010). In Figure 2a most of the samples have ammonium δ15N in

the range 0 to +6 ‰ (and occasionally higher), but there are a group of samples with low ammonium

ion concentrations (<2nmolm-3) and much lower ammonium δ15N values (-4 to -6‰). All these latter

samples are from samples collected in air flowing from the remote South Atlantic which had not been

over land masses for the 5 days before sampling (all trajectories available in Lin, 2010 and data are

classified by trajectory in Figure 2c). These air-parcel back-trajectories describe only where the air

sampled has been over the last 5 days and do not preclude a continental source affecting the air prior

to this time. In addition similar air masses may contain very different concentrations due to variations

in precipitation along the trajectory. However, despite these caveats such trajectories have proven a

valuable tool for interpreting data from the Atlantic in many previous studies (Baker et al., 2006). The

overall pattern in Figure 2a is essentially the same as reported previously using a less sensitive δ15N

method (Jickells et al., 2003) from a different AMT cruise 3 years previously. The positive δ15N

values in Figure 2a are all for samples collected in the tropical South Atlantic with 5 day air parcel

back trajectories crossing tropical ocean waters and in most cases approaching, but not reaching, the

African continent, except for one sample where trajectories approached the South American

continent. This latter sample had a very low ammonium concentration but relatively heavy isotopic

signature (0.5nmol m-3 and δ15N +6.2‰).

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

In Figure 2b we report a similar plot of the data from the JC18 cruise in the Caribbean which

encountered air with relatively low ammonium concentrations, and in which we find broadly similar

δ15N values to those seen on AMT15. However, there are two samples that have low δ15N values

despite having relatively higher ammonium concentrations and two samples with low ammonium

concentrations which have particularly heavy δ15N signatures. In the case of one of these samples with

higher δ15N values, the air sampled had relatively recently crossed Guadeloupe. All samples collected

on this cruise were sampling air that may have passed relatively close to a number of Caribbean

islands, although the ammonium and other major ion concentrations suggest little enhancement over

background concentrations from anthropogenic or volcanic sources for any of the JC18 samples.

We therefore conclude that the pattern (Figure 2a & b) of high δ15N ammonium in aerosols which

have relatively high ammonium concentrations due to a stronger influence from terrestrial sources

over the tropical Atlantic, and low δ15N for low concentration ammonium aerosols sampled over the

Southern Ocean reported previously (Jickells et al., 2003) is reproduced in this new data set collected

separately and analysed by the new more sensitive method. The new data from the Caribbean are

broadly consistent with this pattern, although they include some low aerosol ammonium concentration

samples which have heavier isotopic signatures.

In Figure 2c we combine all of the new data with that reported by Jickells et al. (2003) and classify

the samples by trajectory to emphasise the similar overall pattern in both data sets. Thus we suggest

the pattern reported is likely to be real and not an analytical artefact associated with the limited

sensitivity of the methods used for isotopic analysis in the earlier study. This figure illustrates that the

isotopically light samples are all associated with low ammonium concentrations and South American,

Southern Ocean or Caribbean trajectories. Such isotopically light signatures have also been reported

in air masses with low ammonium concentrations crossing the remote high latitude North Atlantic

(Jickells et al., 2003). Heavy isotopic signatures are seen for high ammonium concentration samples

from both hemispheres. There are also some low ammonium concentration aerosol samples that have

heavier signatures. Hence there is no simple linear relationship between isotopic composition and

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

concentration, but the low δ15N ammonium in aerosols appear to be characteristic of high latitude

systems with low ammonium concentrations.

We have previously argued that fine mode aerosol is likely to be acidic and hence to retain

ammonium aerosol (and any gaseous ammonia) effectively as ammonium sulphate (Jickells et al.,

2003, Paulot et al., 2015) as discussed earlier, and hence that the pattern in Figure 2 is unlikely to be a

sampling effect associated with dissociation of aerosol ammonium salts. We also previously

suggested that the low ammonium δ15N values in air over the remote Southern Ocean and North

Atlantic may represent the isotopic signature of marine emissions which would be more readily

detectable in air in this remote region which is less impacted by terrestrial emissions. Hence we

suggested that the pattern in Figure 2 can be interpreted to represent a two end-member mixing series

between high concentration and isotopically heavy terrestrial ammonia sources (consistent with other

published data for terrestrial ammonia/ammonium Altieri et al., 2014, Jickells et al., 2003, Rolff et al.,

2008, Xie et al., 2008, Yeatman et al., 2001) and low concentration isotopically light marine ammonia

emissions (David Felix et al., 2013). However, it has been argued that at low surface-ocean

temperatures there should be little or no emission of ammonia from the oceans (Johnson et al., 2008),

challenging the idea that the light ammonium isotopic signature could be derived from marine

ammonia emissions from high latitude waters. We now use the additional data from the JC18 and

AMT15 cruises to consider this issue further.

The samples collected on JC18 provided the opportunity to sample air with low ammonium aerosol

concentrations with apparently little influence from local terrestrial emissions and underlain by warm

ocean waters from which emission of ammonia is thermodynamically more favourable. We find a

similar range of δ15N values to those sampled on AMT15 (Figure 2), and the values are also broadly

similar to those reported for Bermuda rainwater of -5.5+2.9‰ (Altieri et al., 2014), although aerosol

and rainwater may not be directly comparable if there is isotopic fractionation during rainwater

scavenging.

The results presented here for both cruises do approximate to the two end-member mixing model

described, but this does not readily explain the heavier isotopic signatures at low ammonium

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

concentrations. However, the Henry’s Law for ammonia suggests that marine ammonia emissions

should have a greater impact in low atmospheric ammonia/ammonium marine atmosphere overlying

warm waters, and yet the data from JC18 cruise (Figure 2b) include samples with similar light

isotopic signatures to those seen in samples collected in air arriving from the Southern Ocean. An

alternative explanation may be that the trends in the isotopic signature of ammonium seen along the

AMT transect (Figure 2a&c) indicate isotopic fractionation of aerosol ammonium during long-range

transport resulting in light δ15N signature in the very remote atmosphere overlying the Southern

Ocean.

We can therefore envisage two possible scenarios to explain the observation of isotopically light

ammonium in air of marine origin:

1. Firstly as has been previously postulated that an isotopically light signal represents ammonia

emitted from the ocean to the atmosphere, during which we would expect strong fractionation

compared to marine ammonium favouring emission of the isotopically light form of ammonia

(Jickells et al., 2003, David Felix et al., 2013).

2. The second option is that terrestrial ammonium transported over the ocean at high relative

humidity are able to equilibrate with gas phase ammonia from marine or terrestrial sources

and that this can lead to isotopic fractionation either directly, or via the selective loss of

isotopically heavier aerosol or ammonia gas during long range transport. Substantial

fractionations have been reported between ammonia and ammonium on land (Xiao et al.,

2015, Fukuzaki and Hayasaka, 2009) although in these cases ammonium nitrate is likely to be

present which is more able to dissociate than ammonium sulphate (Paulot et al., 2015). Such

simple dilution and loss of terrestrial ammonia emissions during long range transport with

little substantial impact from marine ammonia emissions is consistent with the strong

correlations between ammonium and nitrate with an intercept close to the origin for aerosol

samples collected along the AMT transect (Baker et al., 2006) discussed earlier.

As outlined in Johnson and Bell (2008), in a situation where ammonia can exchange between the gas

and aerosol phases there is a direct connection through a series of thermodynamic equilibria between

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

ammonium in the ocean surface and ammonia in the gas phase. This implies that any net acidity

transported in the aerosol over the ocean will tend to take up ammonia from the atmosphere and

consequently lead to an ammonia emission from the ocean to compensate; the logical conclusion of

which being that ammonia (and at least some of the ammonium) in the remote marine environment

will be of marine origin, although this in itself does not mean that this marine ammonia will be

isotopically distinguishable from terrestrial sources. Norman and Leck (2005) have estimated the

residence time of ammonia in the marine boundary layer to be only a few hours requiring either a

marine ammonia source or rapid aerosol/gas exchange, or both, to sustain the observed gaseous

marine concentrations in the remote marine atmosphere.

How then can the field observations of Johnson et al 2008, predicting an air to sea flux of ammonia

predominantly at lower latitudes be reconciled with the isotopic evidence here and in Jickells (2003),

which implies a marine ammonia source at temperate high latitudes? Johnson et al (2007) report

observations of high ammonium concentrations in the surface ocean following a phytoplankton bloom

that could drive an ammonia flux out of the ocean even in the relatively cold waters of the Northeast

Atlantic. Hence a seasonal bias in the observations here and in the earlier paper could resolve the

inconsistency. Alternatively, if the major control on atmospheric ammonia concentration is the pH of

the aerosol phase, then this suggests that atmosphere and ocean should tend to being close to

equilibrium, with a small net flux out of the ocean and into the aerosol phase as net acidity is added

from e.g. sulfate produced by DMS emissions (Johnson and Bell, 2008, Lin et al., 2012, Paulot et al.,

2015). This is not necessarily inconsistent with the observations of Johnson et al (2008), as the

uncertainties on gas phase ammonia concentrations can be considerable, as noted earlier.

In conclusion therefore, our results demonstrate a clear geographical range of aerosol ammonium

isotopic signatures. While this can be explained in part by mixing of an isotopically light marine

ammonium source with an isotopically heavier terrestrial source, this requires that a substantial part of

the marine ammonia emission takes place at high latitudes, where thermodynamics predict that this

emission would be smallest. There are mechanisms that can rationalise this dilemma, such as

emissions during strong algal blooms, and we therefore urge more studies of marine ammonia

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

emissions. An alternative explanation for the isotopically light ammonium aerosol signature is that it

arises from fractionation during long range transport, and we therefore also suggest that further

studies of ammonium aerosol cycling and associated isotopic fractionation would be valuable,

although this will not be simple to investigate over long transport times at low concentrations.

The resolution of the causes of the isotopic signatures we see will then open the way for more

effective use of ammonium aerosol isotopic signatures to study ammonium cycling and resolve the

major uncertainty over the significance of marine ammonia emissions.

Acknowledgements

We gratefully acknowledge the NOAA Air Resources Laboratory for the provision of the HYSPLIT

transport and dispersion model and ready website (http://www.arl.noaa.gov/ready.html) used in this

publication. We thank Jenni Barclay and her colleagues for the opportunity to sample on JC18

supported by NERC grant NE/B50461411.We acknowledge the support of the AMT programme for

sampling opportunities and for funding via NERC grant NER/O/S/2001/00680 and thank three

anonymous referees for their constructive and valuable comments on earlier versions of this

manuscript.

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

Figure 1a. Cruise track for AMT15 (17 Sept to 29 Oct 2004) large circles and solid line. Air flow was

from the east until approximately 5oN and then further south from the west based on air-mass back

trajectories which are available in full elsewhere (Lin, 2010). The cruise track for the cruise used for

samples for Jickells et al (2003) sampling is shown as dotted line.

Figure 1b. Sampling stations on JC18, 4-14 Dec 2007. The ship was stationary and head to wind at the

various sampling stations with air flow for essentially all the cruise from the NE except sample 9 from

the SW. Clarify what the yellow and blue lines are

278

279

280

281

282

283

284

285

0.0 2.0 4.0 6.0 8.0 10.0 12.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

Aersol Ammonium Concentration nmolm-3

δ15N

Aer

sol A

mm

oniu

m ‰

Figure 2a Aerosol ammonium isotopic composition plotted against aerosol ammonium concentrations

for AMT15

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

-10.0

-5.0

0.0

5.0

10.0

15.0

Aerosol Ammonium Concentration nmol m-3

δ15N

Aer

osol

Am

mon

ium

‰

Figure 2b Aerosol ammonium isotopic composition plotted against aerosol ammonium concentrations

for JC18

286

287

288

289

290

291

0 5 10 15 20 25 30 35

-15

-10

-5

0

5

10

15

20

δ15N vs. Atmospheric NH4 (by trajectory)

US/EUSaharaAfricaSouth AmericaRemote OceanJC18

Atmospheric NH4 (nmol/m3)

δ15

N (‰

)

Figure 2c Aerosol ammonium data from Figures 2a and 2b plus data previously reported for aerosol

data collected over the Atlantic(Jickells et al., 2003) with samples coded based on the air parcel

trajectory.

292

293

294

295

References

ALTIERI, K. E., et al. 2014. Isotopic evidence for a marine ammonium source in rainwater at Bermuda. Global Biogeochemical Cycles, 28, 2014GB004809.

ANDREAE, M. O. & CRUTZEN, P. J. 1997. Atmospheric aerosols: Biogeochemical sources and role in atmospheric chemistry. Science, 276, 1052-1058.

BAKER, A. R., et al. 2006. Nutrients in atmospheric aerosol particles along the Atlantic Meridional Transect. Deep-Sea Research Part Ii-Topical Studies in Oceanography, 53, 1706-1719.

BAKER, A. R., et al. 2010. Estimation of atmospheric nutrient inputs to the Atlantic Ocean from 50 degrees N to 50 degrees S based on large-scale field sampling: Fixed nitrogen and dry deposition of phosphorus. Global Biogeochemical Cycles, 24.

CLEGG, S. L., et al. 1998. Thermodynamic Model of the System H+−NH4+−SO42-−NO3-−H2O at Tropospheric Temperatures. The Journal of Physical Chemistry A, 102, 2137-2154.

DAVID FELIX, J., et al. 2013. Characterizing the isotopic composition of atmospheric ammonia emission sources using passive samplers and a combined oxidation-bacterial denitrifier approach. Rapid Communications in Mass Spectrometry, 27, 2239-2246.

DUCE, R. A., et al. 2008. Impacts of atmospheric anthropogenic nitrogen on the open ocean. Science, 320, 893-897.

FOWLER, D., et al. 2013. The global nitrogen cycle in the twenty-first century. Philosophical Transactions of the Royal Society B-Biological Sciences, 368.

FUKUZAKI, N. & HAYASAKA, H. 2009. Seasonal Variations of Nitrogen Isotopic Ratios of Ammonium and Nitrate in Precipitations Collected in the Yahiko–Kakuda Mountains Area in Niigata Prefecture, Japan. Water, Air, and Soil Pollution, 203, 391-397.

GALLOWAY, J. N., et al. 1995. Nitrogen fixation: Anthropogenic enhancement - environmental response. Global Biogeochemical Cycles, 9, 235–252.

GRUBER, N. & GALLOWAY, J. N. 2008. An Earth-system perspective of the global nitrogen cycle. Nature, 451, 293-296.

HARRISON, R. & KITTO, A.-M. 1992. Estimation of the rate constant for the reaction of acid sulphate aerosol with NH3 gas from atmospheric measurements. Journal of Atmospheric Chemistry, 15, 133-143.

JICKELLS, T. D., et al. 2003. Isotopic evidence for a marine ammonia source. Geophysical Research Letters, 30.

JOHNSON, M., et al. 2007. Ammonium accumulation during a silicate limited diatom bloom indicates the potential for ammonia emission events. Marine Chemistry, 106, 63-75.

JOHNSON, M. T. & BELL, T. G. 2008. Coupling between dimethylsulfide emissions and the ocean–atmosphere exchange of ammonia. Environmental Chemistry, 5, 259-267.

JOHNSON, M. T., et al. 2008. Field observations of the ocean-atmosphere exchange of ammonia: Fundamental importance of temperature as revealed by a comparison of high and low latitudes. Global Biogeochemical Cycles, 22.

KEENE, W. C., et al. 2014. Long-term trends in aerosol and precipitation composition over the western North Atlantic Ocean at Bermuda. Atmospheric Chemistry and Physics, 14, 8119-8135.

KERKWEG, A., et al. 2008. Consistent simulation of bromine chemistry from the marine boundary layer to the stratosphere – Part 1: Model description, sea salt aerosols and pH. Atmos. Chem. Phys., 8, 5899-5917.

LIN, C.-T. 2010. Isotopic studies of aerosol sulphate and ammonium in the marine atmosphere, Norwich : University of East Angia, 2010.

LIN, C. T., et al. 2012. An assessment of the significance of sulphate sources over the Atlantic Ocean based on sulphur isotope data. Atmospheric Environment, 62, 615-621.

MCILVIN, M. R. & ALTABET, M. A. 2005. Chemical conversion of nitrate and nitrite to nitrous oxide for nitrogen and oxygen isotopic analysis in freshwater and seawater. Analytical Chemistry, 77, 5589-5595.

296

297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346

NORMAN, M. & LECK, C. 2005. Distribution of marine boundary layer ammonia over the Atlantic and Indian Oceans during the Aerosols99 cruise. Journal of Geophysical Research, 110, D16302, doi:10.1029/2005JD005866.

PAULOT, F., et al. 2015. Global oceanic emission of ammonia: Constraints from seawater and atmospheric observations. Global Biogeochemical Cycles, 29, 1165-1178.

PSZENNY, A. A. P., et al. 2004. Halogen cycling and aerosol pH in the Hawaiian marine boundary layer. Atmospheric Chemistry and Physics, 4, 147-168.

QUINN, P. K., et al. 1992. Equilibria of the Marine Multiphase Ammonia System. Journal of Atmospheric Chemistry, 14, 11-30.

ROBINSON, C., et al. 2006. The Atlantic Meridional Transect Programme (AMT): a contextual view 1995 - 2005. Deep-Sea Research Part II, 53, 1485-1515.

ROLFF, C., et al. 2008. Deposition of nitrogen and phosphorus on the Baltic Sea: seasonal patterns and nitrogen isotope composition. Biogeosciences, 5, 1657-1667.

SCOTT, W. D. & CATTELL, F. C. R. 1979. Vapor-Pressure of Ammonium Sulfates. Atmospheric Environment, 13, 307-317.

VOSS, M., et al. 2013. The marine nitrogen cycle: recent discoveries, uncertainties and the potential relevance of climate change. Philosophical Transactions of the Royal Society B-Biological Sciences, 368.

XIAO, H.-W., et al. 2015. δ15N–NH4+ variations of rainwater: Application of the Rayleigh model. Atmospheric Research, 157, 49-55.

XIE, Y. X., et al. 2008. Source of nitrogen in wet deposition to a rice agroecosystem at Tai lake region. Atmospheric Environment, 42, 5182-5192.

YEATMAN, S. G., et al. 2001. Comparisons of aerosol nitrogen isotopic composition at two polluted sites. Atmospheric Environment, 35, 1307-1320.

ZHANG, L., et al. 2007. Sensitive measurement of (NH4+N)-N-15/N-14 (delta(NH4+)-N-15) at natural abundance levels in fresh and saltwaters. Analytical Chemistry, 79, 5297-5303.

347348349350351352353354355356357358359360361362363364365366367368369370371372

373