Organic nitrogen deposition on land and coastal environments: a review of methods and data

Atmospheric Environment 37 (2003) 2173–2191

Organic nitrogen deposition on land and coastal environments: a review of methods and data S.E. Cornella,*, T.D. Jickellsa, J.N. Capeb, A.P. Rowlandc, R.A. Duced a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Centre for Ecology and Hydrology Edinburgh, Bush Estate, Penicuik, Midlothian EH26 0QB, UK c Centre for Ecology and Hydrology Merlewood, Grange-over-Sands, Cumbria LA11 6JU, UK d Departments of Oceanography and Atmospheric Sciences, College of Geosciences, Texas A&M University, 3146 TAMU, College Station, TX 77843-3146, USA b

Received 13 May 2002; accepted 17 February 2003

Abstract Despite over a century of published reports of dissolved organic nitrogen (DON) in precipitation, its implications are still being appraised. The number of studies focusing on atmospheric organic nitrogen deposition has increased steadily in recent years, but comparatively little has been done to draw together this disparate knowledge. This is partly a consequence of valid concerns about the comparability of analysis and sampling methodologies. Given the current global trends in anthropogenic nitrogen fixation, an improved qualitative and quantitative understanding of the organic nitrogen component is needed to complement the well-established knowledge base pertaining to nitrate and ammonium deposition. This global review confirms the quantitative importance of bulk DON in precipitation. This cumulative data set also helps to resolve some of the uncertainty that arises from the generally locally and temporally limited scale of the individual studies. Because of analytical and procedural changes in recent decades, assessments are made of the comparability of the data sets; caution is needed in comparisons of individual studies, but the overall trends in the compiled set are more robust. Despite the large number of reports considered, evidence for long-term temporal changes in rainwater organic nitrogen concentrations is ambiguous. With regard to sources, it is likely that some of the organic material observed is not locally generated, but undergoes extensive or long-range atmospheric transport. The compiled data set shows a land-to-sea gradient in organic nitrogen concentration. Possible precursors, reported data on the most likely component groups, and potential source mechanisms are also outlined. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dissolved organic nitrogen; Rainwater; Nitrogen cycling; Anthropogenic sources; Organic aerosol

1. Introduction Atmospheric nitrogen deposition has been the focus of considerable recent research, particularly in the context of the eutrophication of aquatic systems. This interest is driven by concerns about human modification of the atmospheric nitrogen cycle, a result of rapidly increasing global industrialisation and automotive *Corresponding author. Tel.: +44-1603-593173; fax: +441603-593739. E-mail address: [email protected] (S.E. Cornell).

transport use (Galloway et al., 1995; Vitousek et al., 1997; Smil, 1999). In this context, most nitrogen budgets have focused on inorganic nitrogen, but the relative importance of organic nitrogen species is now recognised. It is clear that organic nitrogen is a ubiquitous yet still poorly characterised component of atmospheric precipitation (e.g., Hammer, 1993; Cornell et al., 1995, 2001; Russell et al., 1998; Seitzinger and Sanders, 1999; Cape et al., 2001; Neff et al., 2002; Mace et al., in press, a, b). The anthropogenic sources and deposition patterns of inorganic nitrogen are comparatively well understood.

1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00133-X

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Applying intentionally fixed nitrogen onto land, as synthetic nitrogenous fertiliser, has had pronounced and widely studied effects on groundwater, rivers and lakes, which trigger local and regional concern (see Goodchild, 1998; Rejesus and Hornbaker, 1999; (Anon., 1999). Ammonia (NH3), nitrous oxide (N2O), and nitric oxide (NO) are the main gas-phase nitrogen species emitted to the atmosphere from agricultural activities (Matthews, 1994; Galloway et al., 1995; Bouwman et al., 1997; Smil, 1999). In terms of total nitrogen deposition, N2O and agricultural NO (which is dwarfed by emissions from combustion sources, discussed below) are very minor contribuents. Ammonia, on the other hand, contributes significantly to atmospheric nitrogen deposition. Ammonia is removed, generally close to its sources, by efficient scavenging in rainfall and by dry deposition (Asman et al., 1998; Flechard and Fowler, 1998). It is an important atmospheric base, readily forming salts with species like nitric and sulphuric acids. Such reactions are an important source of fine atmospheric aerosol (Raes et al., 2000), and result in an enhanced rate of deposition of ammonium compounds from the atmosphere. Interactions between sea salt and polluted continental air masses can also act to enhance nitrogen deposition to coastal seas (Jickells, 1998; Raes et al., 2000). Postdeposition reactions that lead to the release of H+ in the receiving ecosystems mean that ammonia emissions have a very significant role in the acidification of soils and aquatic systems (ApSimon et al., 1987). Fossil fuel combustion fixes dinitrogen to more reactive forms that are released directly into the atmosphere. The main nitrogen oxides, NO and NO2 (together termed NOx), are predominantly anthropogenic in origin (Delmas et al., 1997), and are major pollutants because of their involvement in the formation of photochemical smog and acid rain. These oxides undergo long-range atmospheric transport (e.g., Talbot et al., 1996; Prospero et al., 1996), chemical transformation, and ultimately deposition (e.g., Shepson et al., 1996; Levy et al., 1999), with implications for the acid balance and nutrient supply in regions of the world remote from their land-based sources. Nitrate and ammonium atmospheric deposition budgets have been studied extensively on local and regional scales (e.g., Devenish, 1986; Campbell et al., 1994; NADP, 1997; the European cooperative programme for Monitoring and Evaluation of Pollutants (EMEP), and the World Meteorological Organisation’s Global Atmosphere Watch), and the emission rates and chemistry of their gaseous precursors are reasonably well constrained. Stringent international controls on emissions are reducing the levels of anthropogenic SO2 (the precursor of sulphuric acid and sulphate aerosol; see Newell and Skjelkvale, 1997; Fischer et al., 1998; Lynch et al., 2000), and to some extent this legislation is reducing the emissions of nitrogen species too: in parts

of northwestern Europe and North America, nitrate and ammonium deposition in precipitation has levelled or is declining (Goulding et al., 1998; EPA, 2001). Elsewhere, however, these trends are not observed (e.g., Nilles and Conley, 2001; Fujita et al., 2001; Kopacek et al., 1997), and global atmospheric emissions of nitrogen oxides and ammonia are still rising. The rate of human fixation of nitrogen is predicted to increase by 60% in the next two decades (Galloway, 1989; Galloway et al., 1995; Rodhe, 1994). This increase in nitrogen fixation and consequently its deposition will affect ecosystems, because primary production is often limited by the supply of fixed, bioavailable forms of nitrogen (reviewed in Vitousek et al., 1997). The growing database of marine rain and aerosol organic nitrogen gives stronger indications that the organic nitrogen seen in the remote atmosphere has significant land-based and anthropogenic sources (Cornell et al., 1995, 2001; Russell et al., 1998; Church, 1999; Seitzinger and Sanders, 1999; Cape et al., 2001; Mace et al., in press a, b). It is still unclear what its present and future ecological significance may be. Some forms of organic nitrogen are readily taken up for nutrient use by primary producers in land and marine ecosystems (Antia et al., 1991; Murphy et al., 2000); others are only slowly broken down to yield bioavailable nitrogen, while some organic nitrogen is toxic to some species (e.g., Smidt, 1994). Kuylenstierna et al. (1998) report that large areas of Europe are potentially sensitive to increased nitrogen deposition. Its deleterious impacts include reductions in plant species diversity, and altered biogeochemical functioning, such as nitrogen saturation and reduced methane oxidation (Goulding et al., 1998). Forests may respond to increased nitrogen deposition by increasing carbon uptake and storage, because wood has a wide C:N ratio range (Holland et al., 1997). However, the interactions between the world’s biomass and the increasing levels of nitrogen deposition and of atmospheric CO2 show considerable complexity (Nadelhoffer et al., 1999). Atmospheric nitrogen deposition can also impact aquatic ecosystems. Awareness of human impacts on marine waters is increasing, particularly in coastal regions and enclosed seas (e.g., Owens et al., 1992; Michaels et al., 1993; Spokes et al., 2000). The possibility of the long range transport and deposition of atmospheric nitrogen enhancing the process of eutrophication, with attendant impacts on ecosystems, oceanic CO2 uptake, and on human economic systems, is being monitored more closely now than in the past (Paerl and Whitall, 1999; Paerl and Fogel, 1994; Valigura et al., 2001; Jickells, 2002). Atmospheric organic nitrogen is, of course, a component of the atmospheric organic carbon, a subject that has recently been reviewed (Jacobson et al., 2000). A wide variety of research interests are related to the

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

consideration of atmospheric organic aerosols, particularly cloud condensation processes (Jacobson et al., 2000). In the case of organic nitrogen, we focus here on nitrogen deposition as a key issue because of concerns over nitrogen supply to receiving ecosystems. We consider primarily rainwater dissolved organic nitrogen (DON), which indeed represents most of the published data, but the origin of at least some of this DON is water soluble aerosol organic nitrogen which can also undergo dry deposition, just as aerosol nitrate and ammonium can. Our review will also consider water soluble aerosol organic nitrogen where appropriate to elucidate the composition and sources of DON. A clear starting point for our review is the fact that organic nitrogen in rainwater and aerosols is composed of many different compounds with diverse origins (cf. Saxena and Hildemann, 1996; Jacobson et al., 2000; Neff et al., 2002). We review here the available literature on rainwater DON in coastal, island and inland environments. We first consider what is known about DON, based on analyses for specific compounds and on various indirect deductions. We also assess the comparability of the different analytical methods used, highlighting the methodological differences in sampling and sample preparation. Many land-based records of organic nitrogen deposition pre-date the marine data, but many advances in analytical technology have been made in recent decades. Key questions are whether there is evidence of changing DON fluxes with time, and whether the spatial variation can be used to elucidate sources and impacts.

2. In what form is the organic nitrogen in atmospheric deposition? The organic nitrogen-containing compounds present in rainwater have not routinely been included in budget assessments, yet they are important components of atmospheric nitrogen deposition (e.g., Neff et al., 2002). This is true in all regions of the world where they have been measured (Table 1). Like the inorganic nitrogen ions, organic nitrogen may be incorporated in rainwater by direct dissolution of gaseous species, or by scavenging (in clouds or by falling water droplets) of atmospheric aerosol. It is estimated that B20–50% of continental fine particulate matter is organic (Saxena and Hildemann, 1996; Jacobson et al., 2000; Raes et al., 2000), and this organic material is important in controlling the aerosol physicochemical properties. As with aerosol organic carbon, much of the organic nitrogen is found in the fine mode fraction (Spokes et al., 2000; Cornell et al., 2001), possibly resulting from gas-to-particle reactions and perhaps contributing to aerosol hygroscopicity. Studies of organic nitrogen in fog (USA) and cloudwaters (USA and Chile) support

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Table 1 Organic nitrogen deposition in different regions

Europe North America South/Central America Southeast Asia Oceania Antarctica Islands

ON, % of TDN

Number of data sets

2378 38719 2975

19 51 6

41* 56727 11* 30732

1 6 1 6

Regional DON contributions to total dissolved nitrogen in rainwater are the means of the reported contributions for each region (standard deviations also shown), except for SE Asia and Antarctica, for which only a single value has been reported. Table 4 lists the studies from which these data sets have been obtained.

this idea, and confirm the importance of the contribution of ON to total dissolved nitrogen, of B20–65% (Zhang and Anastasio, 2001; Weathers et al., 2000). Despite their significance in linking the nitrogen and organic carbon cycles, comparatively little qualitative information exists on the compound groups that make up rainwater DON. Only a small percentage of aerosol organic carbon is characterised, despite extensive research (e.g., Facchini et al., 1999). Given the diversity of the organic nitrogen compounds, it has been more difficult to define source-and-sink budgets and describe their atmospheric behaviour than for the major inorganic species. This knowledge demands both the quantitative determination of bulk DON in rainwater, and its characterisation, and success has been constrained by the need to use separate analytical approaches. Currently two distinct bodies of research exist, which may ultimately be expected to converge. One deals with the identification of individual compounds or compound groups, which have typically been targeted because of their bioavailability (e.g., amines and amino acids, urea), or biotoxicity (e.g., N-substituted polycyclic aromatic hydrocarbons). The other focuses on quantifying bulk DON, motivated by the need to constrain fluxes. Here, we attempt to assess the extent to which these two research strands can be merged to give an overall picture of the significance of organic nitrogen. We consider first the groups of organic nitrogen compounds that have been measured (Tables 2 and 3). Peroxyacetyl nitrate (PAN, CH3COOONO2) and related alkyl nitrates (RONO2), have been studied because of their importance in the context of the atmosphere’s oxidative capacity (e.g., Fahey et al., 1986; Buhr et al., 1990), yet little is known about their deposition (Neff et al., 2002). Organic nitrates and other

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Table 2 Gas-phase nitrogen species in the continental boundary layer contributing to DIN and DON in rainwater Gaseous N species:

Estimated annual input to atmosphere

Typical ambient concentrations

Atmospheric residence time

N2O NO and NO2 (NOx)

B9 Tg N yr
1 B39 Tg N yr
1

310 ppbv 5–100 ppbv urban; o20 ppbv rural 5–50 ppbv urban (California); o7 ppbv urban (Europe); B0.5 ppbv rural 3–9 nmol N m
3

120 yr 1 day

B25 ppbv 18 nmol N m
3 (aerosol) o1–14 nmol N m
3

Hours to days o1 day

PAN

—

Alkyl nitrates; other organic nitrates (RONO2)

—

NH3 Urea Aliphatic amines CH3CN

B55 Tg N yr
1 0.2 Tg N yr
1 0.2–2 Tg N yr
1

1 week

1–20 days

Months to years

The gaseous inorganic N species (in bold) are directly emitted into the atmosphere. The direct emission of PAN and the organic nitrates is small compared with atmospheric in situ formation, hence only ambient concentrations are given in the table. Emissions estimates of the inorganic N gases are from Galloway et al. (1995) and references therein; and from Ehhalt (1999). Natural NOx and NH3 account for 15–20% of the total emissions. The oceans also are a source of ammonia (13 Tg N yr
1) and nitrous oxide (2 Tg N yr
1), not included in this continental emissions table. Residence times for the gaseous N species in the atmosphere are highly variable, depending on temperature, pressure, humidity, and the presence of other reactive species. The times given in the table are approximate indicators of the extent to which atmospheric transport can be expected to be a factor in their atmospheric fates. Hence ammonia will have predominantly localised effects, whereas the longer-lived organic nitrates transport N away from the continents, and release it as they themselves undergo photolysis or other reactions.

Table 3 Summary of rainwater concentrations of DON compound classes from continental and coastal sites Compound group

Reported range mmol N l
1

References

Aliphatic amines

o0.002–2.7

Gorzelska et al. (1992); Likens et al. (1983); Mopper and Zika (1987); Van . Neste et al. (1987); Gronberg et al. (1992)

Dissolved free amino acids

o0.002–6.4 3–120 (in fogwaters, including alkyl amines)

Gorzelska et al. (1992) Zhang and Anastasio (2001)

Total hydrolysable amino acids

o0.002–14 7 to >250 (in fogwaters)

Gorzelska et al. (1992); Mopper and Zika (1987); Zhang and Anastasio (2001)

Urea

o0.4–10

Timperley et al. (1985); Cornell et al. (1998); Seitzinger and Sanders (1999); Cornell et al. (2001)

Nitrogen-containing aromatics: N-PAH Heterocyclic compounds PANa Methyl cyanidea

o0.03 p0.2 o0.08 o0.02

Leuenberger et al. (1988); Anastasio and McGregor (2000)

a Upper bounds for the concentrations of these compounds have been estimated from their ambient concentrations and Henry’s Law coefficients (see text).

compounds are formed as a result of photochemical reactions of NOx with volatile anthropogenic or biogenic organic carbon compounds (Atherton and Penner, 1990; W.angberg et al., 1997). Many have

relatively long atmospheric lifetimes (Roberts, 1990). In remote regions of the world, the existence of this reservoir of gaseous reactive nitrogen is important (Atherton, 1989). For example, dissolved PAN is

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

hydrolysed to nitrite and nitrate, raising concentrations of these species in precipitation in regions previously unimpacted by human activity, far removed from the urban and industrial places where NOx originates. In general, the alkyl nitrates are not very soluble in water, and only the larger (>C5) alkyl nitrates are likely to adsorb onto aerosol particles or other surfaces. Thus it is likely that rainwater only removes a small proportion of the alkyl nitrates from the atmosphere, and there are very few published reports of rainwater concentrations. However, aromatic and unsaturated VOC, in particular the biogenic isoprenes and terpenes, are known aerosol precursors, and organic nitrates derived from these compounds may thus contribute more to nitrogen deposition. Taking PAN as an example, simple scavenging calculations allow the estimation of the concentration of organic nitrates in rainwater. Typical PAN concentrations are B20 pptv (or approximately 1 nmol N m
3) in the remote marine boundary layer, and up to 50 ppbv in polluted urban environments. The Henry’s Law constant, KH ; for PAN is temperature dependent; salinity and acidity have only a slight influence. For fresh water, KH is B4 mol l
1 atm
1 at 20 C. Over the tropical oceans, then, the equilibrium rain concentration of PAN would be around 0.1–0.2 nmol l
1, while in industrialised continental regions it is unlikely to exceed 80 nmol l–1. Nitriles (alkyl cyanides) are present in the atmosphere, but they also have comparatively low solubilities. At typical atmospheric concentrations and without further transformation, these compounds are probably not important components of rainwater DON. As with the organic nitrates, research emphasis on gas-phase processes mean there is a paucity of published data on transformations and deposition; however, it is apparent that dry deposition and particle interactions, rather than rainwater scavenging of the gases, is their likely fate (Cicerone and Zellner, 1983; Li et al., 2000; de Laat et al., 2001). Another minor component group is the nitrophenols, formed by aqueous phase nitration reactions of phenol in cloud droplets (Luttke . et al., 1997, 1999). They are of interest because of their phytotoxicity (Rippen et al., 1987; Natangelo et al., 1999), but concentrations of nitrophenols in precipitation and cloud are low, with median values o0.1 mmol C l
1 (Schussler . and Nitschke, 2001). Combustion processes are major sources of nitrogencontaining heterocyclic and polyaromatic hydrocarbons (PAH), which are subject to research attention because of their toxicity and carcinogenicity. Like the organic nitrates, they are trace gases in the atmosphere. Some of these organic compounds are water-soluble. For example, the azaarenes, polycyclic heterocyclic molecules found in urban aerosol, become increasingly watersoluble under acidic solution conditions (laboratory studies, Preston et al., 1997). Precipitation scavenging of

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these gases is again likely only to make a small contribution to total atmospheric nitrogen deposition, with N concentrations generally at sub-picomolar levels (data are available for the USA, e.g., Schuetzle et al., 1975; Mylonas et al., 1991; Nishioka and Lewtas, 1992; Northern Europe, by Nielsen et al., 1984). Soot (elemental carbon) and other carbonaceous aerosol from combustion processes contain nitrogen that may contribute to rainwater DON. About two thirds of carbonaceous aerosol is organic carbon, while soot typically makes up about 1% of aerosol mass, although soot appears to be disproportionately significant in atmospheric chemistry because of its extremely high surface area (Disselkamp et al., 2000). All atmospheric soot is a consequence of combustion processes, with industrial combustion dominating sources in the temperate north, and biomass burning important at lower latitudes. The atmospheric emissions from biomass burning include much higher proportions of reactive organic carbon than those from industrial combustion, which occurs at higher temperatures. Organic carbon emitted during biomass burning evidently rapidly becomes hydrophilic (Ducret and Cachier, 1992; also Hurst et al., 1994), and indeed soots also undergo important solvation processes (Disselkamp et al., 2000). There is increasing evidence that a range of nitrogen species have the potential to alter the hygroscopic properties of these predominantly anthropogenic aerosols, and enhance the processes of particle agglomeration and rainfall scavenging (Saxena et al., 1995). In this way, the transport and chemical transformation of organic nitrogen and carbon appear to be very closely linked, with organic aerosol increasingly accepted as a source of cloud condensation nuclei of similar magnitude to sulphate aerosol (Disselkamp et al., 2000; Saxena and Hildemann, 1996; Ruellan et al., 1999). A crude assessment can be made of whether direct soot emissions contain significant amounts of organic nitrogen relative to the DON observed in rainwater. Cachier et al. (1995) suggest that 4% of aerosol produced by biomass burning is nitrogen, although this figure includes inorganic nitrogen. Our own analysis of vehicle exhaust soot yields an average of 1.6% N (Field, 2001). If we assume that *

*

*

the organic nitrogen in emissions from biomass burning and in higher temperature combustion sources lies in the range 1–4%, global aerosol organic carbon emissions from both combustion sources are 12 Tg C yr
1 (Raes et al., 2000), and soot accounts for 30% of total carbon,

then we estimate an aerosol organic N emission direct from combustion of 0.2–0.7 Tg N yr
1. In comparison,

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Jickells (2001) estimated total N inputs to the oceans alone to be 43–111 Tg N yr
1, with 33–75% of this being soluble organic nitrogen. Since nitrogen deposition on land increases this estimate considerably, and since much of the soot organic N will be insoluble, we conclude that the direct emission of organic nitrogen from combustion processes is unlikely to be an important source. This calculation of the importance of soot considers only the effects of direct emissions of N with soot. In addition, carbonaceous aerosol may be involved in heterogeneous reactions with NH3, NOx, and also with other nitrogen-containing organic compounds (Chang and Novakov, 1975; Wang et al., 1992; W.angberg, 1993). These processes could act to increase the nitrogen content and also the aqueous solubility of this class of organic compounds. As well as organic compounds where nitrogen is present in an oxidised form, there are reduced organic nitrogen compounds (cf. Neff et al., 2002). These include the amino acids and amides, the volatile, basic amines, and ureas. These compounds could make up most of the DON in rainwater (Table 3 shows concentration ranges; Mopper and Zika, 1987; Gorzelska et al., 1992; Cornell et al., 2001; Mace et al., in press a, b; also Saxena and Hildemann’s (1995) review of candidate organic components of aerosol), although there are few data yet available where both compound classes and DON have been determined in the same samples. Sidle (1967) made an early assessment of amino acids in UK coastal rainwater. Individual amino acid concentrations were generally below 0.1 mmol l
1, and total free amino acids accounted for just 2–3% of the total DON. Amino acids contribute 2% of dissolved organic carbon (DOC) in rural US rainwater (Willey et al., 2000), with concentrations of B1.5–5 mmol N l–1. Non-synchronous measurements of DON over a 3-yr period in the same region (Peierls and Paerl, 1997) averaged 5.576 mmol N l
1, so it can be inferred that amino acids could comprise about a quarter of the DON. Free and combined amino compounds are reported to be significant components of DON in fogwaters, contributing 22% of DON, or approximately 3–4% of total dissolved nitrogen, at a site in central California (Zhang and Anastasio, 2001). Zhang et al. (2002) also report that amino acids make up about 20% of water soluble organic nitrogen in aerosol. A further portion of the measured DON could originate from biogenic organic material associated with aeolian soil and dusts. Supporting this idea, Mace et al. (in press b) and Eklund et al. (1997) have reported statistical associations between rainwater DON with crustal material; however, no clear trends were seen in a spatially diverse set of rainwater composition data from several different geographical locations (Cornell, 1996). While soil organic matter in temperate locations has a

C:N ratio ranging from 10 to 14:1 (Holland et al., 1997), it appears that the average nitrogen content of aeolian dust is low, although there are few data. Very large quantities of dust are transported in the atmosphere, principally from the arid regions in North Africa and Southeast Asia. This land-derived particulate mineral material may be kept aloft in the atmosphere for many days, reaching the most remote parts of the world (Prospero et al., 1996; Kawamura et al., 1996). This mobilisation of fine dust could provide a route into the atmosphere for nitrogenous material. Zenchelsky et al. (1976) report that fine aeolian material is enriched in organic matter, relative to the bulk soil from which it is derived. Land use changes in favour of agricultural production lead to increases in seasonal dust incursions into the atmosphere. Combined with the increasing application to these soils of nitrogenous fertilisers such as urea, increased transport and deposition of anthropogenic organic nitrogen by these means is likely (Cornell et al., 2001). These authors speculated that the urea observed in rains from the central Pacific arises from the extensive use of urea as a fertiliser in Asia. Urea emissions could arise either from the fertiliser production processes (direct emission) or its agricultural use (in association with dusts; Mace et al., in press a, b). The preponderant association of urea with fine mode aerosol in the remote atmosphere (Cornell et al., 2001) rather than with larger crustal material suggests that production rather than agricultural use is the more significant source. Amorphous, largely uncharacterised macromolecules like humic material may also contribute significantly to the DON. Gelencser et al. (2000) used pyrolysis gas chromatography/mass spectrometry to characterise about 20% of bulk organic matter in eastern European aerosol. They report that compounds identified in the organic matter are humic-like material, with glycinetype derivatives. Facchini et al. (1999) suggest humiclike soluble macromolecules comprise 40% of water soluble aerosol organic carbon in fogs in the Po Valley, with 50% of the material yet uncharacterised. Likens et al. (1983) reported DOC concentrations in rain in the northeastern USA of 80–170 mmol l
1, of which 50–60% is associated with macromolecules. Humic acid and humin from soils has an atomic C:N ratio of about 20:1 (Schwarzenbach et al., 1993). If we assume that the macromolecules in the Likens et al. study represent soil humic acid or humins, the reported DOC concentrations would correspond to 2–4 mmol l
1 of DON in rain. Scudlark et al. (1998) and Russell et al. (1998) report DON concentrations of 9 mmol l
1 in the northeastern USA (albeit at a different location to the work of Likens et al.). This calculation can be approached in a different way, using precipitation data collected in East Anglia in the UK, where rainwater acid-soluble iron concentrations average about 20 mg l
1 and the iron can be

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

assumed to come entirely from soil material (Kane et al., 1994). Soil material in the region contains about 5% iron, giving an estimate of the rainwater soil content of the order of 400 mg l
1, although this will be a lower bound estimate because of incomplete solubilisation of iron by the techniques used. Assuming the soil material to contain 1–10% humin or humic acid (Alloway, 1999) with a C:N ratio of 20:1 (see above), we predict a rainwater DON concentration from this source of 0.2– 2 mg l
1 or 0.01–0.14 mmol l
1 DON. The tendency to underestimate the significance of soil material that results from the incomplete solubilisation of iron is likely to be countered by a tendency to overestimate because of the assumption of complete solubility for humic material. DON values reported for rain in this location are substantially higher than this (Cornell et al., 1998), suggesting that soil-based humic material, contributing o1%, is a minor source of DON. As with soot material, it is likely that humic acid-like material could be modified during atmospheric transport by reaction with various radical species including NO3 and with ammonia, thereby making humic-like material more nitrogen-rich and more water-soluble. Another potential source of DON is sea-salt aerosol enriched with organic material from the sea-surface microlayer (Mopper and Zika, 1987; Gorzelska and Galloway, 1990). It is unlikely that marine aerosol makes a large contribution to the rainwater DON concentrations observed in land locations, considering the very large enrichment factors that would be required over seawater DON concentrations (Cornell et al., 1995), and given that sea-salt aerosol tends to be relatively large (the mode of the sea-salt aerosol particle diameter size distribution is >1 mm) and hence more rapidly deposited from the atmosphere. Furthermore, water soluble aerosol organic nitrogen (the presumed precursor of rainwater DON) in the remote atmosphere is associated primarily with the fine mode aerosol (Cornell et al., 2001). Weathers et al. (2000), however, suggest that cloudwater ON from southern Chile may have an oceanic source associated with the adjacent upwelling zone. At present, the main conclusion from the direct comparison of the results from the bulk analysis of DON and the individual compound approach is that in most cases, typically half the total DON is still uncharacterised. Isotope techniques may be applied to trace primary organic nitrogen sources (analogous to Cachier et al., 1995, for carbon), or to explore the possibility of secondary DON formation by NOx or NH3 additions, for example, during the atmospheric lifetime of organic material. Yeatman et al. (2001) addressed some of these issues, but the existing d15N data set for DON is still too small to provide clear conclusions (Cornell et al., 1995; Russell et al., 1998). In the remainder of this review, the focus is on studies

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reporting bulk DON in rainwater, rather than individual components.

3. How is bulk DON measured? Nitrate and ammonium dominate dissolved inorganic nitrogen in rainwater. Nitrite is sometimes detected, but under atmospheric oxidising conditions and in the normally acidic pH of rainwater, it is rapidly oxidised to nitrate. In any case, dissolved inorganic nitrogen is the sum of these three ions, which are easily quantitatively analysed at the micromolar concentrations at which they are found in precipitation. However, for organic nitrogen, there are many component species coexisting in solution. It has been a long-standing challenge to analyse the DON with confidence. Measuring total dissolved nitrogen (the inorganic and organic components together) involves releasing the nitrogen from the organic molecules, by chemical oxidation to NO
3 ; thermal/catalytic oxidation to NO; Kjeldahl conversion to NH3; or by photolysis, with or without additional chemical oxidants. These methods will be discussed below. The resulting total nitrogen is measured, and with the initial concentrations of the inorganic ions, bulk DON can be determined by difference: DON ¼ total nitrogen measured
initial inorganic nitrogen concentration: Clearly, for any derivative value, the uncertainties associated with the primary measurements are compounded. This means that less confidence can be held for the DON data reported than for the inorganic nitrogen species, particularly when the initial inorganic nitrogen concentration is high relative to DON. Such a situation is typical for continental rain samples, where the repeatability of the analysis of inorganic N ions and total N might be 75% or better, but the compounded errors result in a much lower precision for the DON analysis. Fig. 1 shows how the precision of the DON measurement is dependent on the inorganic nitrogen concentration, following Hansell’s (1993) treatment of the problem: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sDON ¼ ðs2TDN þ s2DIN Þ: In addition to these concerns about the precision of DON estimates, there are concerns over the accuracy of measurements. Generally speaking, the tendency is for DON to be underestimated. Sampling and analysis methods have long been tested and validated for optimal ammonium, nitrite and nitrate measurement, so the major uncertainties lie with the determination of total nitrogen. The major sources of error in the TDN analysis arise from incomplete conversion of DON to inorganic nitrogen, or from losses during sampling and storage (Scudlark et al., 1998) or during the conversion process, all of which

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

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% standard deviation DON

100

75

50

25

0 0

0.25

0.5 DON/TDN

0.75

1

Fig. 1. Calculated precision and reproducibility of DON analysis. The precision of the DON analysis is a function of the precision of the analysis of total nitrogen and its inorganic components. Where DON is only a small contributor to total N, the calculated precision is low (solid diamonds show calculated precisions for rainwater samples from several locations analysed at the School of Environmental Sciences, UEA 1998–1999). An alternative experimental approach involves the assessment of the repeatability of the analysis of samples covering a wide range of DON/TDN ratios. Open squares show the standard deviation and mean of the analysis of five aliquots each of a suite of samples.

introduce negative bias. Additional sources of DON underestimation (but not of total N) may arise if some organic N components give false positive responses in the analysis for the inorganic ions. For instance, some amine N may be a minor interferent in ammonium determination (Cape et al., 2001). From this perspective, the data reported in the scientific literature can be considered to provide a fairly conservative estimate of the amount of organic nitrogen being deposited in rainfall. The methods used to obtain the total nitrogen concentration have themselves been the subject of quite considerable attention. As expected, newer technologies, particularly the high-temperature catalytic oxidation (HTCO) techniques, have facilitated the determination of organic matter in natural waters. However, the older techniques are still being used, and in many cases adaptations have been made to optimise their use in certain applications, as can be seen in the following paragraphs. The Kjeldahl method is the oldest of the methods in use (Kjeldahl, 1883) and has been widely applied. The nitrogen bound up in organic molecules is converted to ammonium by reaction with hot, concentrated H2SO4. Subsequently, raising the solution pH liberates ammonia, which is distilled and determined by titration. The first, decomposition step of the determination is critical. Nitrogen present in forms other than amines or amides is not efficiently converted to ammonium, but may be released instead as nitrogen oxides or N2, which are not kept in solution. Various reducing agents and catalysts have been proposed to convert more chemically recalcitrant forms, but low total nitrogen yields are still

a shortcoming of the Kjeldahl technique (Doval et al., 1997). A related method, in which DON is thermally converted to ammonia over silver, has been refined by Duursma (1961) and Le Poupon et al. (1997) for seawater DON applications. Distillation at different temperatures means that DON and ammonium–N are separated. This is an attractive method, but applications of the technique for fresh or precipitation water samples have yet been reported. Wet oxidation methods use chemical oxidants such as persulphate, following methods developed for seawater total nitrogen analysis by Valderrama (1981), Koroleff ! (1983), and Solorzano and Sharp (1980). The method lends itself well to adaptation for shipboard analysis, and it is widely used in marine science as a result. Practical improvements to the method have recently been suggested by Dafner et al. (1999) and Hagedorn and Schleppi (2000). There are reports of the application of persulphate oxidation in rainwater analysis by Scudlark et al. (1998), Cornell and Jickells (1999) and Cape et al. (2001). The wet oxidation techniques tend to have fairly high blanks associated with them, and require extensive sample handling, increasing the risk of sample contamination, but with appropriate optimisation, they can give reliable results (Bronk et al., 2000). Low nitrogen recoveries (based on standard solutions of suites of nitrogen-containing compounds) are not as problematic as for the Kjeldahl methods. The final step requires only analysis for nitrate, since all N species appear to be fully oxidised by this procedure (also Sharp et al., 2002), thereby reducing analytical uncertainties. However, the analytical procedure for nitrate needs to be insensitive to the high post-oxidation sulphate concentration. The use of ion chromatography is problematic, so colorimetric techniques are generally used for the nitrate determination. The photolytic breakdown of DON by high-intensity UV light has also been used for total nitrogen measurement. The chemistry of photolysis is complex and poorly understood, and the efficiency of the technique is uncertain (e.g., Scudlark et al., 1998). In theory, the minimal sample handling and reagent addition during analysis confer advantages, but some classes of compounds (e.g., sulphur-containing amino acids) resist UV photolysis (Walsh, 1989; Hopkinson et al., 1993), and standard recoveries of these compounds are low. In order to overcome this, chemical oxidants or photoactive species, such as titanium dioxide, are sometimes added (e.g., Walsh, 1989; Bronk et al., 2000; Takeda and Fujiwara, 1996; Pizzicannella et al., 1996), although such modifications of the method introduce blanks. The UV photolysis technique has been used for rainwater DON analysis (Cornell et al., 1995; Cornell and Jickells, 1999; Scudlark et al., 1998; Russell et al., 1998). The most recent advances have been in the thermal and catalytic oxidation techniques, developed in the

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

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Table 4 Studies included in this analysis Method used

Number of reported studies

References

Kjeldahl

23

Europe: Allen et al. (1968); Carlisle et al. (1967); Gore (1968); Kopacek et al. (1997); Persson and Bromberg (1985); Roberts et al. (1984) North America: Correll and Ford (1982); Dillon et al. (1991); Eriksson (1952); Hendry and Brezonik (1980, 1984); Jassby et al. (1994); Jordan et al. (1983, 1985, 1991); Nicholls and Cox (1978); Peierls and Paerl (1997); Tarrant et al. (1968); Urban and Eisenreich (1988); Valiela and Teal (1979); Valiela et al. (1978); Williams (1967) South East Asia and Oceania: Eriksson (1952); Timperley et al. (1985)

UV photolysis

11

Europe: Cape et al. (2001); Cornell et al. (1995, 1998); Cornell and Jickells (1999); Rendell et al. (1993) North America: Bronk et al. (2000); Grant and Lewis (1982); Scudlark et al. (1998) Islands: Cornell et al. (2001); Spokes et al. (2000)

Wet chemical (persulphate) oxidation

7

North America: Bronk et al. (2000); Eriksson (1952); Russell et al. (1998); Scudlark et al. (1998); Shon (1994) Central and South America: Williams et al. (1997) Islands: Knap et al. (1986)

High temperature catalytic oxidation

6

Europe: Cape et al. (2001) North America: Seitzinger and Sanders (1999) and data cited therein Central and South America: Eklund et al. (1997)

Other or non-specified

23

Europe: Emmett (1989); Reynolds and Edwards (1995) North America: Anastasio and McGregor (2000); Butler and Likens (1995); Jordan et al. (1995); Nixon et al. (1996); Peters and Reese (1995); Prospero et al. (1996); Richter et al. (1983); Seitzinger and Sanders (1999); Shaw et al. (1989); Valiela et al. (1997); Verry and Timmons (1982); Williams (1967) Central and South America: Bowden (1991); Lewis (1981) Oceania: Crockford et al. (1996), Wilson (1959) Antarctica: Vincent and Howard Williams (1994)

1980s. These instruments combust all the organic material completely to CO2 and NO, and the nitrogen is quantitatively detected by chemiluminescence. Practical difficulties still remain, however, and the data set produced using this technique is still comparatively small. While several comparative reviews exist for seawater DOC (e.g., Peltzer et al., 1996; Agatova et al., 1996; Sharp et al., 1993a, b, 1995), and now for seawater DON (Sharp et al., 2002), rather fewer studies have been reported for DON in rain and freshwaters (Bronk et al., 2000; Cornell and Jickells, 1999; Scudlark et al., 1998; Takeda and Fujiwara, 1996), and these do not conclusively prove the superiority of any of the methods. Sharp et al.’s (2002) inter-laboratory study of seawater DON found no systematic difference between the data generated by UV, persulphate and HTCO methods. A previous method comparison study reported that HTCO gave the highest results for rainwater DON, implying

that the other two techniques have underestimated DON (Cape et al., 2001).

4. How can we use the existing global DON data set? In the sections that follow, the broadest possible data set is presented (Table 4). DON is variously reported as single values, means, and ranges, and this presents major interpretative restrictions. Therefore, whenever summary data are presented, it is important to note that the data distributions are narrower than they would be if all the original data were available. 4.1. Analytical methods The Kjeldahl technique was used almost exclusively for rainwater DON analysis prior to 1980. Some of the pioneer data were obtained using unspecified techniques

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S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

Table 5 Summary statistics for Kjeldahl DON, pre-1980 Rainwater DON concentration (mmol N l
1)

Continental Coastal/Island Marine All data inter-quartile rangea, all data minimum reported value maximum reported value

DON as a percent of total dissolved nitrogen

Mean7s.d.

Median

N

Mean7s.d.

Median

N

22713 11710 2 18713 7–24 1.7 53 (90)b

20 7 — 15

12 6 2 20

40717 36721 19 37719 20–50 11 87

40 30 — 32

8 11 1 20

N is the number of DON reports. a The inter-quartile range is the range which includes the middle 50% of the distribution. b The main value is the highest mean concentration reported for a site. The value in parentheses is the highest single concentration reported where concentration ranges were given (i.e., at one site, a DON concentration range of 5–90 mmol l
1 was given).

Table 6 Summary statistics for the modern rainwater DON data set Rainwater DON concentration (mmol N l
1)

Continental Coastal/Island Marine All data inter-quartile rangeb, all data minimum reported value maximum reported value a b

DON as a percent of total dissolved nitrogen

Mean7s.d.

Median

N

Mean7s.d.

Median

N

21715 19721 771 17717 7–23 1.8 89

21 12 6 12

12 24 3 39

36719 33719 67, 6a 35720 21–49 5 88

30 30 — 30

24 28 2 54

The first value is for North Atlantic rain, and the second for polluted North Sea samples. The inter-quartile range is the range which includes the middle 50% of the distribution.

for albuminoid nitrogen, which probably was the Kjeldahl method, although other methods exist. Concerns that the technique under-estimated organic nitrogen, and its reliance on the addition of large amounts of acid reagent meant that it fell out of favour for the analysis of such dilute sample solutions as rainwater. However, this is not a suggestion that we should divide published rainwater nitrogen data into a reliable and an unreliable group, based on the time at which the study was made. While the early studies may lack fine resolution, most of them report nitrogen deposition data that we must consider representative, particularly given the long experimental periods that were typical, of 3–5 yr of precipitation sampling. As Eriksson wrote in 1952, ‘‘yin many cases, no statement concerning the method is given, and in other cases, the data seem quite normal despite less satisfactory methods’’—each case must be taken on its merits. Table 5 summarises the comparable results for Kjeldahl DON from this earlier phase. Since the 1980s, other methods for DON analysis have overtaken the Kjeldahl technique. Table 6 gives

summary statistics for what can be considered the modern DON data set. The striking feature is the similarity of the DON concentration range and distribution to that seen in Table 5. Fig. 2 shows the reported mean concentrations and contributions of DON determined by each technique and classed by sample origin; analysis of variance (ANOVA) confirms that the differences are not significant. As already discussed, there is no systematic published evidence yet of differences in measured rainwater DON in the data generated by Kjeldahl, UV, persulphate or other methods, although the techniques follow very different approaches and may give sub-optimal recoveries for specific organic nitrogen compound groups. This composite data set tends to suggest that concerns about analytical differences have been over-stated. 4.2. Methodological issues As well as the different analytical approaches, there are differences in sampling protocols, and in sample treatments after collection. These have been the subject

100 Continental Coastal/island Marine

Mean DON Concentration, µmol N L

-1

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

75

50

25

0 Kjel

UV

Pers

HTCO

Other

Method

(a)

Mean DON as % of TDN

100

Continental Coastal/island Marine

80

60

40

20

0 Kjel

(b)

UV

Pers

HTCO

Other

Method

Fig. 2. Comparison of precipitation (rain and snow) data sets generated using different DON methods: (a) Mean DON concentrations reported for different locations, (b) mean percent contribution of DON to total dissolved nitrogen. Note that the numbers of points on the two plots are different, because many studies have reported the mean concentration or the mean contribution, and not always both.

of discussion in the atmospheric research community (Church, 1999; Cape et al., 2001; Neff et al., 2002). The rainwater DON results from different studies have to be compared with great caution, complicated by the sparseness of information, particularly in early reports, about sampling, storage and analysis. By the 1960s, precipitation sampling networks were active across the USA and in the UK, using broadly similar methods, but results were not cross-compared. Samples were mainly collected using plastic (polyethylene) sampling vessels, often coarse-filtered through nylon mesh or glass-fibre plugs. Almost all the studies analysed bulk (wet+dry) precipitation, and analysed snow as well as rain. Often in these earlier reports, single values for DON concentration are given. In some cases, given the analytical constraints of the time, it is clear that rain event samples collected over several weeks or months were pooled to give sufficient sample volume for the organic nitrogen analysis (for example, Allen et al. (1968) explicitly state that weekly samples were combined into monthly composites in their study of temporal and spatial rainwater chemistry trends). The most usual storage method was refrigeration at o4 C between sampling and analysis. In a few cases,

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samples were treated with chemical biocides. Iodineimpregnated bottles were used for samples obtained from remote locations by Allen et al. (1968) and by Gore (1968). Tarrant et al. (1968) added toluene after sample collection, and Hendry and Brezonik (1980) used mercuric chloride, but only for bulk samples (wet-only samples were filtered and refrigerated). Cape et al. (2001) have evaluated several storage procedures. They conclude that preservation techniques of some sort are necessary in order to prevent ammonium losses, although there are fewer problems with DON storage. Samples in the early studies were generally not filtered, so the organic nitrogen concentrations reported included some particulate organic nitrogen. Most precipitation samples are now filtered prior to analysis, usually through 0.45 mm membranes or pre-cleaned glass-fibre filters. This may have been expected to cause a decline in measured organic nitrogen (with the exclusion of particulate ON), but there is no systematic evidence in the data set, when viewed in its totality, of discontinuities in the trends as the favoured DON sample preparation method changed. It is possible that particulate ON in rainwater is not quantitatively important. Alternatively, the over-estimation in the early studies as a result of inclusion of particulate ON could counter the under-estimation that was likely in using the Kjeldahl method. As HTCO methods become more established, this is a question that will probably be resolved. 4.3. Temporal changes The long time-series reports of rainwater composition from the Rothamsted Experimental Station in southeast England (Miller, 1905; Russell and Richards, 1919), published almost a 100 yr ago, include reports of organic nitrogen (methods not described) ranging from 2 to 50 mmol N l
1, averaging B14 mmol l
1. This DON made up about a quarter of the total N deposited (1.35 lb per acre per annum, or 0.15 g m
2 yr
1, in more familiar units). Earlier data from New Zealand are also reported, where a deposition rate of 0.05 g m
2 yr
1 (0.45 lb per acre per annum) is said to give an indication of the unpolluted deposition of ON. Precise location and precipitation data are not given, so we cannot be certain of the New Zealand rainwater concentrations, but taking long-term mean annual rainfall of approximately 1000 mm for both countries (data from NIWA and DEFRA websites), we can estimate a rainwater DON concentration of B5 mmol l
1 for turn-of-the-century New Zealand. In another early study, New Zealand snow was found to contain about 8 mmol l
1 albuminoid N, which made up about half the total dissolved nitrogen (Wilson, 1959). Eriksson (1952) reported fairly high concentrations of albuminoid N in the northeastern USA (26 mmol l
1), with lower concentrations in Delhi,

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

4.4. Spatial variability and source regions The early long-term precipitation research networks were initiated at Rothamsted and similar agricultural

-1

100 DON concentration, µmol N L

India (15 mmol l
1) and in Canada (8 mmol l
1). These numbers look typical of what we might expect today. This bulk DON was largely uncharacterised, which is understandable given analytical limitations. In the studies where potential sources for DON were mentioned, these were almost exclusively attributed to biogenic or detrital material. In these early studies, the focus was on the deposition of nutrient nitrogen onto land ecosystems, and the DON in rainwater was considered to be locally recycled material (soil organic matter, pollen, locally generated debris, sea foam), rather than a ‘real’ nitrogen input, despite the emerging evidence of higher DON concentrations in more inhabited areas. In the composite data set, there is no systematic evidence of the types of long-term temporal trends in the DON data that have previously been reported for inorganic nitrogen deposition. This may be due in part to the small numbers of studies prior to 1980 (Fig. 3 shows data from the 1950s to date). Thus, while modern reports indeed show a much larger range in DON concentration, the means are not significantly higher than earlier data. This contrasts with the deposition of nitrate and ammonium that has increased markedly and steadily over this timescale as a result of increased human influence. However, in their recent study of DON in UK rainfall, Cape et al. (2001) report that DON represents about 20% of total N deposition, a very similar proportion to that reported at the beginning of the twentieth century for Rothamsted in Southern England. This observation implies that DON has increased in proportion to nitrate and ammonium deposition (see Fig. 3b). This apparent contradiction probably cannot be readily resolved by considering further historic data, because minor changes in methods may be acting to obscure trends. However, it is clear that for as long as it has been measured, DON has been a significant component of rainwater, suggesting at least that a proportion of the DON is of natural origin. Seasonality of DON is sometimes reported, but the data are conflicting. Anastasio and McGregor (2000) discuss the photodestruction of a number of DON compounds, and outline the types of constraint on the rates of destruction, such as time of year, transport history of the ON, and the nature of the hydrometeors. Zhang et al. (2002) found that water soluble ON in fine aerosol is higher in colder months. Cornell (unpublished data) found no evidence of seasonality in an extensive rainwater data set from East Anglia in the UK. We therefore set aside any further discussion of the question of seasonality and consider only average data.

75

Continental Coastal/Island Marine

50

25

0 1950

1960

1970 1980 Year (of study publication)

1990

2000

(a) 100 Mean % of TDN as DON

2184

75

Continental Coastal/Island Marine

50

25

0 1950

1960

1970 1980 Year (of study publication)

1990

2000

(b) Fig. 3. Time trends of DON in rainwater analysis: (a) shows mean concentrations in micromoles per litre, and (b) shows percent of total nitrogen as DON. Linear regressions fitted to these data show no evidence of a trend (for both (a) and (b), R2 o0:1).

and ecological institutions in the UK (Allen et al., 1968; Carlisle et al., 1967; Gore, 1968), and in North America, where networks were set up in response to acid rain concerns (Canada: Eriksson, 1952; Nicholls and Cox, 1978; and the USA: Tarrant et al., 1968; Bourne, 1976; Valiela et al., 1978; Valiela and Teal, 1979; Hendry and Brezonik, 1980). Many more reports of atmospheric precipitation of organic nitrogen have been published since the 1980s, and consequently we have a clearer picture of DON concentrations and contribution to the total nitrogen deposition, but the spatial distribution is still far from well understood because of the paucity of data from other locations. Tables 5 and 6 show the DON data classed broadly as continental, coastal/island (defined as o100 km from the sea) and marine. Coastal and island sites have both land and marine influence; it is not possible in this study to distinguish these influences, but future DON studies could routinely be accompanied by meteorological (trajectory) analysis to address this problem. There are very few shipboard measurements, but rainwater DON is lower in remote marine locations. The composite data set confirms previous reports that most DON is deposited on land, although DON tends to

S.E. Cornell et al. / Atmospheric Environment 37 (2003) 2173–2191

be a more significant component of total nitrogen in marine areas (Table 1, Fig. 3). This implies preferential removal of inorganic nitrogen, if the DON is of terrestrial origin. On a smaller spatial scale, of tens to hundreds of kilometers, Cape et al. (2001) have published the results of the first systematic network study of DON deposition. They applied consistent sampling and storage methods, and used a network of rural sites across the UK. Nitrate and ammonium concentrations were lowest in rainwater from the sites in the sparsely inhabited mountain regions of Wales and Scotland, and higher at less remote sites. There was considerable variability among the sites, so that a similar trend is not robustly seen for DON, although DON contributed about 20% of total deposited nitrogen overall. In exploring the question of the principle source regions of DON, the data set compiled in this review suggests that the land sources of DON are stronger than the marine sources, although firm conclusions are constrained by the comparative sparseness of the data, the problems in the assumption of direct comparability of the different data sets, and the greater variability and uncertainty in the DON measurements. There is a gradient in DON concentrations from land to more remote marine sites. The marine DON concentration data are lower, while there is no significant difference between the mean concentrations seen in the coastal/ island and continental data sets (ANOVA, mean marine DON significantly lower at the p ¼ 90% significance level). When DON is considered as a proportion of total nitrogen, there are no statistically significant differences between the percent contributions for continents and coasts/islands. Although the mean percent contribution for the marine samples (40%) is the highest value, the data set is by far the smallest of the three, and this has a strong effect on the statistical representativity. Again, there is great variability (as seen in the interquartile range, Table 6), but DON typically contributes about a third of the total wet deposition of nitrogen. Unfortunately, these data do not allow us to conclude whether the main sources of organic nitrogen to the atmosphere are natural or anthropogenic. Isotopic analysis of the DON (e.g., Cornell et al., 1995; Russell et al., 1998) offers a means of assessing the importance of natural and anthropogenic sources, although the current data sets for both rain and aerosol are too small for conclusions to be drawn.

5. Conclusions In summary, the global historic data set of DON is unambiguous in confirming that the atmospheric deposition of organic nitrogen is quantitatively important. Despite about a century of this knowledge,

2185

atmospheric deposition budgets based solely on inorganic nitrogen are likely to be underestimating by about a third. It is increasingly clear that this organic nitrogen is not simply local-scale cycling of biological detritus, which appears to have been the major justification for the exclusion of DON from these budgets since it was first routinely analysed in the 1960s. Since then, our understanding of atmospheric transport has improved, and our impact on the atmospheric nitrogen cycle has very greatly increased. While the evidence for temporal changes in the importance of DON is ambiguous, DON has been seen in remote areas since the turn of the 20th century. Data from remote sites indicate that organic nitrogen from continental sources can be subject to long-range atmospheric transport over distances of hundreds and thousands of kilometers. It is clear that DON is composed of a variety of organic compounds, many of which come from diverse natural and anthropogenic sources. Studies of water soluble aerosol organic nitrogen indicate a very important fine mode aerosol component. Some of the DON may be derived from secondary aerosols formed by gas-to-particle reactions in the atmosphere. In addition, primary organic particles in the atmosphere may be modified by chemical and photochemical reactions during long-range transport to alter their composition and hygroscopic nature. Since we cannot yet describe the chemical composition of the DON, we cannot fully assess the bioavailability of the nitrogen associated with it. However, readily bioavailable compounds, such as the amino acids and urea, have been determined to contribute a significant part of the DON. More of the DON may become bioavailable after chemical or biological processing in a receiving ecosystem. It is therefore important that DON (and dry deposited aerosol) be considered when assessing the nitrogen budgets of ecosystems.

Acknowledgements This work was mainly funded as part of the Global Nitrogen Enrichment thematic programme of the UK Natural Environment Research Council. The collaboration with R. Duce was supported by UK NERC grant GR3/11027.

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