ARTICLE IN PRESS

Continental Shelf Research 23 (2003) 1743–1755

*Correspondin

E-mail addre1 Current addr

0278-4343/$ - see

doi:10.1016/j.csr

Atmospheric nitrogen inputs into the North Sea:effect on productivity

Gerrit de Leeuwa,*, Lucinda Spokesb, Tim Jickellsb, Carsten Ambelas Skj^thc,Ole Hertelc, Elisabetta Vignatic,d,1, Susanne Tamme, Michael Schulze,

Lise-Lotte S^rensend, Britta Pedersenc, Laura Kleinf, K. Heinke Schl .unzenf

a TNO Physics and Electronics Laboratory, P.O. Box 96864, JG The Hague NL-2509, The Netherlandsb School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, Norfolk, UK

c National Environmental Research Institute, P.O. Box 358, 4000 Roskilde, Denmarkd Ris^ National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark

e IAAC, Univ. Hamburg, Martin-Luther-King Platz 6, 20146 Hamburg, FRGf Meteorologisches Institut, Univ. Hamburg, Bundesstr. 55, 20146 Hamburg, FRG

Received 9 April 2002; accepted 30 June 2003

Abstract

The ANICE (Atmospheric Nitrogen Inputs into the Coastal Ecosystem) project addressed the atmospheric deposition

of nitrogen to the North Sea, with emphasis on coastal effects. ANICE focused on quantifying the deposition of inorganic

nitrogen compounds to the North Sea and the governing processes. An overview of the results from modelling and

experimental efforts is presented. They serve to identify the role of the atmosphere as a source of biologically essential

chemical species to the marine biota. Data from the Weybourne Atmospheric Observatory (UK) are used to evaluate the

effect of short episodes with very high atmospheric nitrogen concentrations. One such episode resulted in an average

deposition of 0.8 mmol N m�2 day�1, which has the potential to promote primary productivity of 5.3 mmol C m�2 day�1.

This value is compared to long-term effects determined from model results. The total calculated atmospheric deposition to

the North Sea in 1999 is 948 kg N km�1, i.e. 0.19 mmol N m�2 day�1 which has the potential to promote primary

productivity of 1.2 mmol C m�2 day�1. Detailed results for August 1999 show strong gradients across the North Sea due

to adjacent areas where emissions of NOx and NH3 are among the highest in Europe. The average atmospheric deposition

to the southern part of the North Sea in August 1999 could potentially promote primary production of

2.0 mmol C m�2 day�1, i.e. B5.5% of the total production at this time of the year in this area of the North Sea. For

the entire study area the atmospheric contribution to the primary production per m2 is about two-third of this value. Most

of the deposition occurs during short periods with high atmospheric concentrations. This atmospheric nitrogen is almost

entirely anthropogenic in origin and thus represents a human-induced perturbation of the ecosystem.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Atmospheric nitrogen; Aerosols; Gases; Assessment of atmospheric nitrogen loads; North Sea nutrient fluxes; Source–

receptor relationships

g author. Tel.: +31-70-374-0462; fax: +31-70-374-0654.

ss: deleeuw@fel.tno.nl (G. de Leeuw).

ess: Joint Research Centre, Environment Institute, TP 460, 21020 Ispra, Italy.

front matter r 2003 Elsevier Ltd. All rights reserved.

.2003.06.011

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551744

1. Introduction

The productivity of marine planktonic ecosys-tems is generally considered to be limited by theavailability of nitrogen (Kronvang et al., 1993).The amount of nitrogen available annually toliving organisms has doubled during the pastcenturies due to human activities and in manyestuaries rival or exceed natural inputs (Valiguraet al., 2001). Increased nutrient loadings haveresulted in a higher incidence of harmful algalblooms and other eutrophication phenomena, andcaused deleterious impacts on fisheries and tour-ism in the European coastal zones (Lancelot et al.,1989). Similar pressures exist worldwide (Nixon,1995; Jickells, 1998).

Terrestrial inputs of nitrogen are often domi-nated by riverine transport, especially in the winter(Sanders et al., 1997). However, with an estimatedcontribution of B1 to >40% of ‘‘new’’ nitrogen(Paerl et al., 2001), atmospheric deposition hasbeen shown to be a significant terrestrial input tothe coastal seas. The atmospheric nitrogen load isdirectly available for algae growth, which makesthis contribution more significant because afraction of the river run off contributes nitrogenfixed to biological material. The contribution ofatmospheric inputs to coastal eutrophicationproblems is now beginning to be recognised in anumber of areas around the world (Paerl, 1995).Given that atmospheric emissions of nitrogencompounds are continuing to increase worldwide,this problem can be expected to grow (Gallowayet al., 1995). On the other hand, the EMEPinventories indicate that European emissions arelikely to decrease somewhat in the years to 2010(see www.emep.int). Atmospheric deposition af-fects both coastal marine ecosystems and mid-oceanic regions removed from riverine sources(Galloway et al., 1994), and can be distributedover spatial scales from tens to hundreds ofkilometres (Owens et al., 1992). The effects willbe larger in coastal regions close to the nitrogensources. Indeed, as discussed below, strong off-shore gradients of NH3 have been observed overthe North Sea (Plate et al., 1995; De Leeuw et al.,2001). In contrast, model results and experimentaldata show that the annual mean nitrate concentra-

tions are highest over the middle of the North Sea(De Leeuw et al., 2002).

Atmospheric deposition can be particularlyimportant during episodes with high atmosphericconcentrations, both in coastal regions (Spokeset al., 1993, 2000) and over the open ocean (Owenset al., 1992; Michaels et al., 1993). While small inoverall annual budget terms, short-lived events ofa few hours to a few days (Owens et al., 1992) maybe able to trigger algal blooms under nutrient-depleted conditions in summer and early autumn.Indeed, observations by Plate (2000) indicate thatthe highest nitrogen fluxes in coastal areas occurduring late spring and are accompanied byprimary production. These blooms may be fol-lowed by oxygen depletion due to decay of thealgae when the growth season is over.

Often only wet deposition is considered (Owenset al., 1992; Spokes et al., 1993; Cornell et al.,1995). Dry deposition may represent an input ashigh as 25% of the wet deposition in remotemarine areas (Owens et al., 1992). Similar valueswere obtained over the North Sea, both fromexperiments on research cruises (Rendell et al.,1993) and from model calculations of the annualmean deposition in 1999 (Hertel et al., 2002; DeLeeuw et al., 2002).

This paper focuses on the effects of atmosphericdeposition of nitrogen to the North Sea onprimary production. Atmospheric nitrogen inputsto the North Sea are dominated by nitrate andammonium (Rendell et al., 1993) and these inputsare overwhelmingly anthropogenic and compar-able in size to riverine inputs. The atmosphericcontribution to the total land-based nitrogen inputhas been reported to be on the order of 30% forthe total North Sea (North Sea Task Force, 1993;Rendell et al., 1993). It is in the same order even incoastal, river dominated areas like the GermanBight (Beddig et al., 1997) and still 5%, if the riverand run-off dominated Waddensee area is con-sidered (Schl .unzen, 1994). Calculations presentedby Asman et al. (1994) show that ammoniadeposition to the southern bight of the NorthSea amounts to 7.6 kt N year�1. The atmosphericnitrogen load to the Danish part of the North Seahas been estimated to about 46 kt N year�1 in 1998or 0.93 t N km�2 year�1 (Skov et al., 1999).

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1745

Estimates of nitrogen inputs are often based onmeasurements over land (e.g., Spokes et al., 2000;Valigura et al., 2001) and sometimes on measure-ments at open sea (e.g., Rendell et al., 1993; Ottleyand Harrison, 1992; Schulz et al., 1999; Plate,2000). Coastal effects on the total nitrogendepositions are included in current atmospherictransport and deposition models in a simplisticway, if at all. However, the largest changes in boththe physical and chemical properties of an air massadvected from land over sea, and therefore also inthe resulting chemical transformation and trans-port and removal processes, are expected incoastal regions. Since continental sources of atmo-spheric nitrogen species dominate, sharp gradientsin concentrations and associated fluxes are ex-pected across the coastal zone. These effects aremagnified because also the micro-meteorologicalparameters determining the fluxes change abruptlyat the land–sea transition and sea salt, providing asurface for heterogeneous reaction with, e.g.,HNO3, becomes available over the surf zone inhigh concentrations (De Leeuw et al., 2000).Reduction of ammonia concentrations in theGerman Bight by a factor of 4 after 70 kmtransport out to the sea has been observed (Plateet al., 1995), while 40% of the sea salt istransformed to sodium nitrate (Schulz, 1996).

A focussed study on coastal influences wasundertaken as part of the ANICE project (Atmo-spheric Nitrogen Inputs into the Coastal Ecosys-tem) (De Leeuw et al., 2001, 2002). The overall aimof ANICE was to improve the accuracy andperformance of model tools that estimate atmo-spheric nitrogen deposition to the sea. Specialemphasis was put on chemical and physicalprocesses governing the transformation and de-position of nitrogen compounds to coastal waters.

The ANICE focus was on quantifying thedeposition of atmospheric inputs of inorganicnitrogen compounds (HNO3, NO3

�, NH3 andNH4

+) into the North Sea, and the governingprocesses. Dissolved organic nitrogen (DON) inrain and aerosol received some attention as well,but only during the experiments. Under thestrongly polluted conditions seen in this region,DON is an insignificant component of the totalnitrogen aerosol loading but makes up, on

average, 14% of the total nitrogen in rainwater.In other areas, DON may be much moresignificant, e.g., Cornell et al. (1995) reportcontributions of B20–80%.

Results from the modelling and experimentalstudies undertaken as part of the ANICE projecthave been presented in De Leeuw et al. (2001,2002), Hertel et al. (2002), Tamm and Schulz(2003) and S^rensen et al. (2003). The Lagrangiantransport chemistry model ACDEP (Hertel et al.,1995) was used to compute concentrations ofnitrogen compounds over the whole North Seaand the resulting deposition (Hertel et al., 2002;Ambelas Skj^th et al., 2002). Experimental datafor the open North Sea were collected from 1.5years of automated measurements aboard ferriessailing between Hamburg and Harwich/Newcastle(Tamm and Schulz, 2003). Other experimentalwork included two intensive field experimentscentred on an off-shore platform (MeetpostNoordwijk, or MPN, at 9 km from the Dutchcoast) and a coastal station, the WeybourneAtmospheric Observatory (WAO) in East Anglia(UK). Together, the intensive observational peri-ods and the long-term measurements provideddata for (a) sensitivity studies of a variety ofproblems associated with the coastal region thatare not easily evaluated with larger scale models,(b) to constrain models and (c) to test modelresults. The ANICE results presented in De Leeuwet al. (2001, 2002) are summarised in Section 2.The results serve to identify the role of theatmosphere as a source of biologically essentialspecies to the marine biota which is discussed inSections 3 and 4.

2. Summary of ANICE results

2.1. Experiments

A large experimental data base has beencollected with concentrations and fluxes of gasessuch as HNO3, NH3, O3, NO, NO2, and CO2,aerosol particle size distributions and composition(Na+, Mg++, Ca++, NO3

�, NH4+, SO4

–, and Cl�).Rainwater samples were collected and analysed fortheir chemical composition to determine the

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551746

contribution of wet deposition. In August 1999,measurements of NHx, pH, salinity and chloro-phyll in the water column were also undertaken todetermine air–sea gas exchange rates (S^rensenet al., 2002). Bubble size distributions in the waterwere measured because of their role in sea sprayaerosol production and gas transfer. Micro-me-teorological parameters were measured and a lidarwas used to provide a detailed description of themixing in and evolution of the boundary layer.Both micrometeorological processes and theboundary layer properties influence the transportof aerosols and gases in the boundary layer. Theexperimental details are presented in De Leeuwet al. (2001). Gases and aerosols were sampledaboard a commercial ferry from spring 1999 untilAugust 2000 (Tamm and Schulz, 2003).

Atmospheric deposition of fixed nitrogen to thecoastal zone occurs via wet and dry deposition ofgaseous species (primarily nitric acid and ammo-nia) and aerosol species, such as nitrate andammonium. The concentrations of NH3 decreaserapidly with increasing fetch resulting in a reduc-tion to ‘background’ levels when the air mass istransported across the North Sea, over a distanceof only about 200 km (De Leeuw et al., 2001).Measurements on Meetpost Noordwijk showstrong variations in the ammonia concentrationswith changing wind direction, and thus fetch. Forshort fetch, atmospheric ammonia concentrationsof over 10 mg m�3 were measured, while for thelongest fetches, the concentrations were as low as0.05 mg m�3. Measurements on a ferry sailingbetween Hamburg and Harwich clearly showedthe influence of land at short upwind fetches.

The NH3 dry deposition flux is determined bythe partial pressure difference between the NH3

gas phase species in the water and in the airdirectly above the water. High concentrations inthe water may result in emission of ammonia tothe atmosphere (e.g., Asman et al., 1994). Simul-taneously measured concentrations of ammonia inthe atmosphere and in the water around MPNwere presented in De Leeuw et al. (2001) andS^rensen et al. (2003). Measurements of NHx inthe water around MPN at distances of 400–500 mto the north, south, east and west show stronghorizontal gradients (typical values are on the

order 500 nmol/1000 m) with strong temporalvariations (S^rensen et al., 2003). Because atmo-spheric horizontal gradients are much smaller, thevariations in the water phase have a stronginfluence on the magnitude and the direction ofthe ammonia flux over small spatial scales that arecommonly not accounted for in models. Account-ing for the actual water concentrations and thehorizontal concentration gradients may signifi-cantly influence the ammonia flux. The experi-mental results presented in S^rensen et al. (2003)indicate that the flux may in fact be upward duringperiods with low atmospheric ammonia concen-trations, and that the calculated overall ammoniadry deposition may be overestimated by a factor oftwo or more in the coastal region. A more detailedstudy is needed to quantify how this may influencethe overall deposition to given marine waters. Insome cases the deposition may solely be redis-tributed whereas the total deposition is onlymarginally influenced.

The air–sea exchange of ammonia is highlysensitive to the concentrations of NHx in the waterand the pH. The pH at different locations at theNorth Sea may vary between 7.8 and 8.2 (Asmanet al., 1994). Application of the sensitivity analysisfor the NHx flux parameterisation presented byAsman et al. (1994) to the ANICE study showsthat a change in pH from 7.9 to 8.1 could alter thedeposition flux by more than 30%.

NOx (NO2+NO) is emitted from combustionsources which are mostly on land though shippingis a significant source in the southern North Sea,cf. Hertel et al. (2002) or De Leeuw et al. (2002).NOx is oxidised to nitric acid on a time scale ofdays, so oxidation continues as continental air isadvected over the North Sea. The deposition ofNOx is rather inefficient. Rates of depositiondepend on the reactivity/solubility of gases andin the case of aerosols on the aerosol sizedistribution. Over land ammonia and nitric acidcan react rapidly with each other (and in the caseof ammonia also with sulphuric acid) to formaerosol species. This reaction yields predominantlyfine mode aerosol with relatively low depositionrates (Slinn and Slinn, 1980). Ammonium nitrateformed over land in polluted regions can dissociateas gas phase precursor concentrations decrease

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1747

(for instance over the North Sea) to liberate gasphase ammonia and nitric acid (e.g., Ottley andHarrison, 1992).

HNO3 is highly soluble and is therefore eitherdirectly deposited (dry and wet) to the surface ortaken up by aerosols where they are accommo-dated through chemical reactions. Effects of theuptake by aerosols on the evolution of HNO3

concentrations over sea were estimated by Vignatiet al. (2001), and effects on HNO3 air–sea fluxeshave been presented in, e.g., Geernaert et al.(1998). The dry deposition velocity for gas phaseHNO3 is larger than for particulate nitrate. Thus,formation of aerosols extends the lifetime ofaerosol nitrate. On the other hand, in the marineatmosphere nitrate shifts from the fine to thecoarse fraction (i.e. aerosol particles which aresmaller or larger than 1 mm, respectively), whichincreases the aerosol deposition velocity up to 20-fold and thus reduces the aerosol lifetime over thatpredicted based on the formation of fine modeaerosol nitrate from gas phase precursors. How-ever, most of the importance of the transition fromthe gas phase to the aerosol phase is that theaerosol wet deposition governs the total deposi-tion. Kane et al. (1994) showed estimates of wetscavenging rates that imply the reaction betweennitric acid and sea-salt aerosol can increase wetdeposition rates threefold.

The data obtained during the ANICE experi-ments show that coarse mode nitrate averages62% of total nitrate. Even under very pollutedconditions when air sampled had either not passedover the sea at all or only for a very short period oftime, coarse mode nitrate made up at least 23% oftotal nitrate, suggesting that the sea-salt displace-ment reaction is rapid and may occur over landnear the coast where seabreeze, and other recircu-lation processes can introduce significant amountsof sea-salt aerosol to the air. During long-rangemarine transport from the Arctic up to 87% of thenitrate occurs in the coarse mode.

In all samples, aerosol ammonium is also seen inthe coarse mode, and although fine mode ammo-nium dominates in marine air a significant coarsemode ammonium component is seen in pollutedair. Looking at the amount of coarse nitrate inrelation to the sum of coarse sea spray and coarse

ammonium, all coarse nitrate can be accounted forin terms of chemical ion balances. The formationof coarse mode ammonium is due to diffusivegrowth as discussed by Von Salzen and Schl .unzen(2000). A bi-modal distribution with ammoniummostly in the fine mode but also in the coarsemode has been simulated with the SEMA boxmodel (De Leeuw et al., 2001). The data obtainedduring the ANICE experiments show the occur-rence of a significant coarse mode ammoniumcontribution averaging 31% of total ammonium.The importance of this coarse mode ammoniumaerosol has only recently been recognised in partfrom ANICE and related work at other coastalsites (Yeatman et al., 2001). The phenomena areprimarily seen when polluted air enters the marineatmosphere.

2.2. Modelling

A simple analytical formulation for the varia-tion of the concentrations with the distance fromthe coast is given by (see Seinfeld and Pandis,1998):

C ¼ C0 exp �VdS

HU

� �; ð1Þ

where C is the concentration (mol m�3) evaluatedat the end point of a trajectory after travelling adistance S over water, C0 the initial concentration(mol m�3), Vd the dry deposition velocity (m s�1),S the distance from the coast (m), H the mixingheight (m), U the wind speed (m s�1).

Eq. (1) was applied to a situation with con-nected flow between MPN and WAO on 21 June1998 to calculate the concentrations at Weybournebased on the measured concentrations at MeetpostNoordwijk. The results are in good agreementwith the measured concentrations (De Leeuw et al.,2002). The dry deposition flux Vd used in Eq. (1) ismass-weighted aerosol deposition velocity, seebelow.

Eq. (1) assumes removal by dry deposition only,and constant mixing height and wind speed.Obviously, this is not a realistic situation over anarea as large as the North Sea. Therefore, moresophisticated models were applied. The Lagran-gian transport-chemistry model ACDEP (Hertel

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551748

et al., 1995) was used for the computation of theconcentrations and fluxes of nitrogen compoundsover the North Sea, and to indicate the relativeimportance of wet and dry deposition as well asthe relative contributions of gases and aerosols(De Leeuw et al., 2002; Hertel et al., 2002).ACDEP is based on 96-h backward trajectories.The timestep in the model is 15 min with arrivaltimes at the receptor point for every 6 h for thebasic runs and every hour for more detailed runs,e.g., for model evaluation. The model wasconsiderably improved within the framework ofANICE. In particular, higher resolution inputswere applied for both the meteorological data andthe emission inventories of the relevant gases(Hertel et al., 2002). Gaseous nitrogen compoundsare primarily produced over land and very highconcentrations are observed close to the coast inoffshore winds. Emission inventories show thatNO2 is also produced in large quantities by shiptraffic at the North Sea. The higher resolutionmeteorological data greatly improved the calcula-tions of marine/coastal NH3 concentrations overthe inner Danish waters (Ambelas Skj^th et al.,2002). The mean deposition to the North Seacomputed with ACDEP for 1999 was 948 kgN km�2, with 81% contributed by wet deposition.The model shows the concentration gradients ofgases and aerosols over the North Sea, withhighest concentrations close to the coastal areaswhere production from combustion or agriculturalactivities are highest (De Leeuw et al., 2002).However, for HNO3 the concentrations are highestin the middle of the North Sea. This is due to theslow conversion of NO2 to HNO3 (reaction rate isa few percent per hour), the emission of NO2 byship traffic and the relatively slow gas phasedeposition of NOx. The trends in the ACDEPmodel results are similar to those obtained fromthe ferry measurements (De Leeuw et al., 2002).

To evaluate the effects of aerosols, the CoastalAerosol Transport model (CAT) (Vignati et al.,2001) was developed to describe heterogeneousprocesses in the coastal region, based on a sizesegregated aerosol model for both sea salt andsulphate. A new sea-salt source function wasdeveloped and CAT was used for a number ofindividual studies in the coastal zone. In offshore

wind, a large amount of nitric acid is removed atvery short fetches due to uptake by sea-saltparticles produced over the surf zone. The effectof the surf may even be important at low windspeeds when little sea spray is produced bybreaking wind waves, as the presence of swell stillcauses wave breaking at the coast and thus surf-induced sea spray production. A model thataccounts for effects of non-homogeneity acrossthe coastline was presented in Geernaert andAstrup (1999). High pollution events over theNorth Sea typically arise under high pressureconditions when winds are low and highly pollutedair flows offshore (Spokes et al., 1993; De Leeuwet al., 2001, 2002). Under these conditions, surfzone sea spray may be particularly important(Vignati et al., 2001; De Leeuw et al., 2002).

The mesoscale model METRAS (Schl .unzen,1990; Schl .unzen et al., 1996), the coupled chem-istry transport model MECTM (Klein, 2002), bothwith a spatial resolution of 8� 8 km2 and atemporal resolution of 3–4 min, and the aerosolmodel SEMA (Von Salzen, 1997; Von Salzen andSchl .unzen, 1999a, b) were used for scenario studiesfor a selected period of the first ANICE measuringcampaign in 1998. SEMA was applied to simulatea bi-modal distribution with ammonium mostly inthe fine mode but also in the coarse mode (DeLeeuw et al., 2001).

3. Episodic inputs of nitrogen species to the surface

ocean and their role in supporting productivity in the

southern North Sea

During spring and summer, the water column isstratified, the mixed layer is shallow and watercolumn nutrient levels are depleted. Therefore,during this period, the atmosphere may representthe dominant source of new nitrogen to surfacewaters (e.g., Galloway et al., 1994; Spokes et al.,2000; Plate, 2000). The impact of atmosphericinputs on surface water biogeochemistry may beenhanced if deposition occurs in short, highconcentration, pulses.

The effect of such episodes is illustrated with anexample for the ANICE experiment in August1999, when the southern North Sea experienced a

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1749

short period of strong south-easterly flow. Massconcentrations of ammonium and nitrate aerosolin the fine and coarse fractions measured at WAOduring this period are presented in Fig. 1. Thesemass concentrations were derived from measure-ments using a HiVolume aerosol sampler with sizesegregation using Sierra impactors (cf. De Leeuwet al., 2001 for a brief description and an exampleof the results from these measurements). Timeseries such as presented in Fig. 1 allow for thepresentation of the temporal variation and thedifferences in the size fractions. Converting mea-sured concentrations into fluxes enables determi-nation of the importance of the atmosphere as asource of biologically essential species to surfacewaters. It also allows identification of the role ofthe sea-salt reaction in increasing atmosphericnitrate fluxes. Here, we use the approach of Spokeset al. (2000, 2001). The dry deposition flux isgiven by

Fd ¼ CdVd ; ð2Þ

where Fd is the dry deposition flux (mol m�2 s�1),Cd the measured concentration (mol m�3) and Vd

the dry deposition velocity (m s�1).The concentrations in Fig. 1 were measured over

a range of meteorological conditions. The drydeposition velocity is a function of particle size

0

50

100

150

200

250

300

350

16-Aug-99 21-Aug-99 26-Aug-99 31-Aug-99 05-Sep-99

nitr

ate

nmol

m-3

wao99-coarse nitratewao99-fine nitrate

highly polluted southeasterly winds

-3

Fig. 1. Temporal variation of the concentrations of nitrate and amm

during the ANICE experiments in August/September 1999.

and (micro)meteorological parameters, such as thewind speed, friction velocity and surface roughness(e.g., Slinn and Slinn, 1980), and varies by morethan two orders of magnitude over the particle sizerange of interest (0.1–10 mm) and wind speed andis therefore difficult to define for particle fractionsencompassing a range of sizes. Therefore, deposi-tion velocities of 0.001 m s�1 are used for the finemode, and 0.02 m s�1 for the coarse mode, basedon experimental and model results for aerosolsdepositing to oceanic regions less than 1000 kmfrom land (Duce et al., 1991). It is noted that theuncertainty in the value for the deposition velocityintroduces a great uncertainty in the depositionestimates. The application of models to aerosoldistributions measured with a good size resolution(e.g., using impactors) and the concurrent meteor-ological parameters will greatly reduce this un-certainty, provided that reliable depositionvelocity models are available (cf. the discussionsin Slinn, 1983; Hoppel et al., 2002).

Using the indicated deposition velocities for thefine and coarse fractions, the average aerosol mass-weighted deposition velocities for nitrate andammonium have been calculated as 0.011 and0.006 m s�1, respectively. Using these values, theaverage daily flux of wet+dry nitrate+ammoniumduring the period centred on 26 August 1999

0

100

200

300

400

500

600

16-Aug-99 21-Aug-99 26-Aug-99 31-Aug-99 05-Sep-99

amm

oniu

m n

mol

m

wao99-coarse ammoniumwao99-fine ammonium

unpolluted

onium in the fine and coarse fractions measured at Weybourne

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551750

was calculated as 0.8 mmol N m�2 day�1. Assum-ing that all this nitrogen is available to phyto-plankton as a source of bioavailable nitrogen,this nitrogen flux can be converted into carbonuptake using a Redfield C:N ratio of 6.625. Duringthis short, high concentration, deposition event,the atmosphere provided enough nitrogen to fix5.3 mmol C m�2 day�1.

These values were derived from measurements atWAO and are thus to be regarded as local values.The distribution of the atmospheric nitrogendeposition over the North Sea for August 1999,calculated with ACDEP, is presented in Fig. 2.Deposition values are given in kg N km�2 (scale 25to >225 kg N km�2, corresponding to B1.8 to>16.1 mmol m�2). The deposition value calculatednear Weybourne is 78 or 5.6 mmol N m�2 forthe whole month of August 1999. The averagedaily value is 0.18 mmol N m�2 day�1, as comparedto the episodic value of 0.8 mmol N m�2 day�1. Asseen from the data in Fig. 1, two such episodeswith very high nitrate and ammonium concentra-tions occurred in the second half of August,with a third episode toward the end of thecampaign in early September. The average dailyN deposition is mainly determined by suchepisodes.

The calculated concentrations and experimentalvalues presented above are of similar magnitude.Also, the comparison between model data aver-aged over the whole year of 1999 comparefavourably with experimental data encompassingmore than one and a half year in 1998 and 1999(De Leeuw et al., 2002). This suggests that themodelled deposition values are reasonable.

The ACDEP results to a large degree reflect thedistribution of the source areas around the NorthSea. For the coastal areas in the southernmostpart, i.e. in the English Channel and the areas westand north west of Belgium and the Netherlands,the deposition is typically 100–300 kg N km�2.Strong gradients are observed to the NW fromThe Netherlands. Along the coast of GreatBritain, north of East Anglia, the atmosphericinputs are significantly smaller with values lessthan 50 kg N km�2 and smaller than 20 kg N km�2

north of Scotland. It is striking that in the middleof the southern North Sea the N deposition

values are predicted to be relatively high. Thesehigh deposition values are supported by theelevated concentrations which were also observedduring the long-term ferry measurements (Tammand Schulz, 2003; De Leeuw et al., 2002) and aredue to the slow transformation of NO2 to HNO3.SW of the Norwegian coast the deposition ispredicted to increase due to increasing rainresulting in enhanced wet deposition (De Leeuwet al., 2002).

The average deposition to the North Sea inAugust 1999 was derived from the ACDEP data as76 kg N km�2, i.e. 5.4 mmol N m�2 which corre-sponds to an average deposition of 0.18 mmolN m�2 day�1 for this month. Assuming again thatall this nitrogen is available to phytoplankton as asource of bioavailable nitrogen and using a Red-field C:N ratio of 6.625, this amount of N could fix1.2 mmol C m�2 day�1.

As we do not have corresponding primaryproduction values, these atmospheric fluxvalues are compared with primary productivitydata for the southern North Sea for Augustfrom the NERC North Sea Project in 1991 (e.g.,Rendell et al., 1993; Tett et al., 1994). Thisproject was undertaken to collect data south of56�N. The average production in August was36 mmol C m�2 day�1 in this part of the NorthSea.

The average deposition to this same area duringAugust 1999 was derived from the ACDEPdata as 129 kg N km�2, i.e. 9.2 mmol N m�2 whichcorresponds to an average deposition of 0.30 mmolN m�2 day�1 for this month. With the sameassumptions as above, this amount of N couldfix 2.0 mmol C m�2 day�1. This suggests that theaverage atmospheric deposition could have sup-ported B5.5% of primary production at this timeof year in this area of the North Sea. This numberis in reasonable agreement with earlier estimates ofan atmospheric contribution of 8% for the south-ern part of the North Sea (Rendell et al., 1993). Itis noted that the North Sea south of 56�N is anarea with very high deposition due to the closevicinity of various source regions, which areamong those with the highest NOx and ammoniaemissions in Europe (Hertel et al., 2002; De Leeuwet al., 2002).

ARTICLE IN PRESS

Fig. 2. Total atmospheric nitrogen deposition to the North Sea in August 1999. Deposition values are given in kg N km�2 (scale

25–225). The results to a large degree reflect the distribution of the source areas around the North Sea.

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1751

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551752

4. Discussion and conclusions

The ANICE project has provided a significantcontribution to the advancement of knowledgeconcerning processes contributing to atmosphericinputs of nitrogen to the marine coastal environ-ment, and the contributions of various atmosphericpathways. New models were developed and newexperimental data were used to constrain themodels and to estimate errors introduced byvarious assumptions. Experimental and modelresults show the distribution of concentrationsand deposition patterns of nitrogen compoundsover the North Sea, including the gradients near thecoast and the changes in gaseous and particulateconcentrations in an air mass travelling across theNorth Sea. Integration of the model resultsprovides the deposition to the entire North Sea.

The emphasis of the ANICE project is on theinfluence of coastal zone processes on the input ofnitrogen to the North Sea. The experimental andmodelling results support the hypothesis on theimportance of coastal effects on nitrogen inputs tothe regional seas. Gaseous nitrogen compoundsare primarily produced over land and very highconcentrations were observed close to the coast inoff-shore winds, except for NO2 that is alsoemitted (or quickly formed from emitted NO) inlarge quantities by ship traffic at the North Sea.HNO3 is produced from NOx and because of theslow reaction, relatively high HNO3 concentra-tions occur also in the middle of the North Sea(cf. De Leeuw et al., 2002).

The changes in concentrations and fluxes acrossthe coastal zone must be taken into account becauseof the short residence time of gaseous species such asNH3 and HNO3. Due to their reactivity and due toscavenging of HNO3 by sea-salt aerosol, a significantfraction of these gases is removed from the atmo-sphere in the first 10–20 km. Not only does this affectthe atmospheric inputs of nitrogen near the coast,but the rapid removal of gaseous species such asNH3 will also result in decreasing concentrationswith distance off the coast, and thus the atmosphericinputs to open waters. When the air mass istransported across the North Sea, over a distanceof only about 200 km, the NH3 concentrations arereduced to ‘background’ levels (De Leeuw et al.,

2001, 2002). Fig. 2 shows the spatial distribution ofnitrogen deposition to the North Sea and the stronggradients near the source areas that result from theprocesses described above. They have the effect offocusing atmospheric deposition into coastal areasalready stressed by various other anthropogenicinputs (Jickells, 1998). In large-scale atmospherictransport models such coastal chemical processes aregenerally not included. Without these, it is notpossible to effectively manage nitrogen enrichmentproblems in coastal waters.

Based on measured concentrations at WAO andmass-weighted deposition velocities, the nitrogenflux during a short period with high atmos-pheric concentrations centred on 26 August 1999was calculated as 0.8 mmol N m�2 day�1 for asingle event, which can potentially fix 5.3 mmolC m�2 day�1. The average daily deposition duringthe 1999 experiment at WAO (Fig. 1), includingthese episodes, was 0.3 mmol N m�2 day�1, whichcan potentially fix 2.1 mmol C m�2 day�1. Hence,the episodic fluxes largely determine the totalprimary productivity due to atmospheric N deposi-tion in the area. For the southern North Sea, theatmospheric contribution is estimated at B5.5% ofthe total required new nitrogen, for the entire NorthSea the atmospheric contribution is B3.2%.Although these numbers may not seem impressive,presented results show that most of the nitrogen isdelivered during short episodes. Rendell et al. (1993)estimate that the atmospheric input represents atleast 26% of the terrestrial input. Furthermore,transport trough the atmosphere takes place overlong distances which spreads part of the N fluxesover larger areas than the coastal zones influenced byrun-off. It is further noted that this nitrogen isalmost entirely anthropogenic in origin and thusrepresents a human-induced perturbation of theecosystem. Under such polluted conditions, highlevels of gas phase ammonia and nitric acid wouldfurther increase the importance of the atmosphere asa source of biologically available nitrogen.

Acknowledgements

ANICE is part of ELOISE and is supportedby EC DG XII, contract ENV4-CT97-0526, as

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1753

well as by internal funding of participatinginstitutes. This is ELOISE publication numberNo. 271/18.

References

Ambelas Skj^th, C., Hertel, O., Ellermann, T., 2002. Use of a

trajectory model in the Danish nationwide background

programme. Physics and Chemistry of the Earth 27,

1469–1477.

Asman, W.A.H., Harrison, R.M., Ottley, C.J., 1994. Estima-

tion of the net air–sea flux of ammonia over the southern

bight of the North Sea. Atmospheric Environment 28 (22),

3647–3654.

Beddig, S., Brockmann, U., Dannecker, W., K .orner, D.,

Pohlmann, T., Puls, W., Radach, G., Rebers, A., Rick,

H.-J., Schatzmann, M., Schl .unzen, K.H., Schulz, M., 1997.

Nitrogen fluxes in the German Bight. Marine Pollution

Bulletin 34, 382–394.

Cornell, S., Rendell, A., Jickells, T., 1995. Atmospheric inputs

of dissolved organic nitrogen to the oceans. Nature 376,

243–246.

De Leeuw, G., Neele, F.P., Hill, M., Smith, M.H., Vignati, E.,

2000. Sea spray aerosol production by waves breaking

in the surf zone. Journal of Geophysical Research 105,

29397–29409.

De Leeuw, G., Cohen, L.H., Frohn, L.M., Geernaert, G.,

Hertel, O., Jensen, B., Jickells, T., Klein, L., Kunz, G.J.,

Lund, S., Moerman, M.M., M .uller, F., Pedersen, B., Von

Salzen, K., Schl .unzen, K.H., Schulz, M., Ambelas Skj^th,

C., Sorensen, L.L., Spokes, L., Tamm, S., Vignati, E., 2001.

Atmospheric input of nitrogen into the North Sea: ANICE

project overview. Continental Shelf Research 18–19 (21),

2073–2094.

De Leeuw, G., Ambelas Skj^th, C., Hertel, O., Jickells, T.,

Spokes, L., Vignati, E., Frohn, L.M., Frydendall, J., Schulz,

M., Tamm, S., S^rensen, L.L., Kunz, G.J., 2002. Deposition

of nitrogen into the North Sea. Atmospheric Environment

37 (Suppl. 1), 145–165.

Duce, R.A., Liss, P.S., Merrill, J.T., Atlas, E.L., Buat-Menard,

P., Hicks, B.B., Miller, J.M., Prospero, J.M., Arimoto, R.,

Church, T.M., Ellis, W., Galloway, J.N., Hansen, L.,

Jickells, T.D., Knap, A.H., Reinhardt, K.H., Schneider,

B., Soudine, A., Tokos, J.J., Tsunogai, S., Wollast, R.,

Zhou, M., 1991. The atmospheric input of trace species

to the world ocean. Global Biogeochemical Cycles 5,

193–259.

Galloway, J.N., Levy II, H., Kasibhatla, P.S., 1994. Year

2020: consequences of population growth and develop-

ment on deposition of oxidized nitrogen. Ambio 23,

120–122.

Galloway, J.N., Schlesinger, W.H., Lecy, H., Micheals, A.,

Schnoor, J.L., 1995. Nitrogen fixation: anthropogenic-

environmental response. Global Biogeochemical Cycles 9,

235–252.

Geernaert, G.L., Astrup, P., 1999. Wind profile, drag

coefficient, and radar cross section in the coastal zone for

quasi-homogeneous conditions. In: Banner, M. (Ed.), The

Wind Driven Air–sea Interface. University of New South

Wales, Sydney, Australia, pp. 305–316.

Geernaert, L.L. S^rensen, Geernaert, G.L., Granby, K.,

Asman, W.A.H., 1998. Fluxes of soluble gases in the marine

atmospheric surface layer. Tellus 50B, 111–127.

Hertel, O., Christensen, J., Runge, E.H., Asman, W.A.H.,

Berkowicz, R., Hovmand, M.F., Hov, Ø., 1995. Develop-

ment and testing of a new variable scale air pollu-

tion model—ACDEP. Atmospheric Environment 29,

1267–1290.

Hertel, O., Ambelas Skj^th, C., Frohn, L.M., Vignati, E.,

Frydendall, J., De Leeuw, G., Schwarz, U., Reis, S., 2002.

Assessment of the atmospheric nitrogen inputs into the

North Sea using a Lagrangian model. Physics and

Chemistry of the Earth 27, 1507–1515.

Hoppel, W.A., Frick, G.M., Fitzgerald, J.W., 2002. Surface

source function for sea-salt aerosol and aerosol dry

depostion to the ocean surface. Journal of Geophysical

Research 10.1029/2001JD002014.

Jickells, T.D., 1998. Nutrient biogeochemistry of the coastal

zone. Science 281, 217–222.

Kane, M.M., Rendell, A.R., Jickells, T.D., 1994. Atmospheric

scavenging processes over the North Sea. Atmospheric

Environment 15, 2523–2530.

Klein, L., 2002. Influence of atmospheric processes and

chemical transformations on nitrogen deposition to coastal

regions. Ph.D. Thesis, Fachbereich Geowissenschaften,

Universit.at Hamburg.

Kronvang, B., Ærtebjerg, G., Grant, R., Kristensen, P.,

Hovmand, M., Kirkegaard, J., 1993. Nationwide monitor-

ing of nutrients and their ecological effects: state of the

Danish aquatic environment. Ambio 22, 176–187.

Lancelot, C., Billen, G., Barth, H. (Eds.), 1989. Eutrophication

and algal blooms in the North Sea, the Baltic and adjacent

areas: predictions and assessment of preventive actions.

Report No. 12, Water Pollution Research Progress Series,

EC, Brussels.

Michaels, A.F., Siegel, D.A., Johnson, R.J., Knap, A.H.,

Galloway, J.N., 1993. Episodic inputs of atmospheric

nitrogen to the Sargasso Sea: contributions to new

production and phytoplankton blooms. Global Biogeo-

chemical Cycles 7, 339–351.

Nixon, S.W., 1995. Coastal marine eutrophication: a defini-

tion, social causes and future concerns. Ophelia 41, 199–219.

North Sea Task Force, 1993. North Sea Quality Status Report

1993. Oslo and Paris Commissions, London, Olsen and

Osen, Fredensborg, Denmark.

Ottley, C.J., Harrison, R.M., 1992. The spatial distribution and

particle size of some inorganic nitrogen, sulphur and

chlorine species over the North Sea. Atmospheric Environ-

ment 26A, 1689–1699.

Owens, N.J.P., Galloway, N.J., Duce, R.A., 1992. Episodic

atmospheric nitrogen deposition to oligotrophic oceans.

Nature 357, 397–399.

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–17551754

Paerl, H.W., 1995. Coastal eutrophication in relation to

atmospheric nitrogen deposition: current perspectives.

Ophelia 41, 237–259.

Paerl, H.W., Boynton, W.R., Dennis, R.L., Driscoll, C.T.,

Greening, H.S., Kremer, J.N., Rabalais, N.N., Seitzinger,

S.P., 2001. Atmospheric deposition of nitrogen in coastal

waters: biogeochemical and ecological implications. In:

Valigura, R.A., Alexander, R.B., Castro, M.S.,

Meyers, T.P., Paerl, H.W., Stacey, P.E., Turner, R.E.

(Eds.), Nitrogen Loading in Coastal Water Bodies: An

Atmospheric Perspective. American Geophysical Union,

Washington, DC, USA, pp. 11–52.

Plate, E., 2000. Variabilit.at der Zusammensetzung anoorga-

nischer Aerosole—insbesondere der reaktiven Stickstoffver-

bindungen—in k .ustennahen Gebieten der Nordsee und

Ostsee. Ph.D. Dissertation, Institute of Inorganic and

Applied Chemistry, University of Hamburg.

Plate, E., Stahlschmidt, T., Schulz, M., Dannecker, W., 1995.

The influence of air–sea-exchange on the partitioning

of N-species during transport over sea. In: Jaehne, B.,

Monahan, E.C. (Eds.), Third International Symposium

on Air–Water Gas Transfer. AEON-Verlag, Hanau,

pp. 735–744.

Rendell, A.R., Ottley, C.J., Jickells, T.D., Harrison, R.M.,

1993. The atmospheric input of nitrogen species to the

North Sea. Tellus 45B, 53–63.

Sanders, R.J., Jickells, T., Malcolm, S., Brown, J., Kirkwood,

D., Reeve, A., Taylor, J., Horrobin, T., Ashcroft, C., 1997.

Nutrient fluxes through the Humber estuary. Journal of Sea

Research 37, 3–23.

Schl .unzen, K.H., 1990. Numerical studies on the inland

penetration of sea breeze fronts at a coastline with tidally

flooded mudflats. Contributions to Atmospheric Physics 63,

243–256.

Schl .unzen, K.H., 1994. Atmosph.arische Eintr.age von N.ahr-

und Schadstoffen. In: Loz!an, J.L., Rachor, E., Reise, K.v.,

Westernhagen, H., Lenz, W. (Eds.), Warnsignale aus

dem Wattenmeer–Wissenschaftliche Fakten. Blackwell

Wissenschafts-Verlag, Berlin, pp. 45–48.

Schl .unzen, K.H., Bigalke, K., L .upkes C., Niemeier, U., Von

Salzen, K., 1996. Concept and realization of the mesoscale

transport- and fluid-model ‘METRAS’. METRAS Techni-

cal Report 5, Meteorologisches Institut, Universit.at Ham-

burg, 156pp.

Schulz, M., 1996. Fluxes across marine surfaces. In: Borrell,

P.M., Borrell, P., Kelly, K., Cvita$s, T., Seiler, W. (Eds.),

Transport and Transformation of Pollutants in the Tropo-

sphere, Emissions, Deposition, Laboratory Work and

Instrumentation, Vol. 2. Computational Mechanics Pub-

lications, Southampton, UK, pp. 45–49.

Schulz, M., van Beusekom, J., Bigalke, K., Brockmann, U.,

Dannecker, W., Gerwig, H., Grassl, H., Lenz, C.-J.,

Michaelsen, K., Niemeier, U., Nitz, T., Plate, E.,

Pohlmann, T., Raabe, T., Rebers, A., Reinhardt, V.,

Schatzmann, M., Schl .unzen, K.H., Schmidt-Nia, R.,

Stahlschmidt, T., Steinhoff, G., von Salzen, K., 1999.

The atmospheric impact on fluxes of matter and energy in

the German Bight. Deutsche Hydrographische Zeitschrift

51, 133–154.

Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and

Physics: From Air Pollution to Climate Change. Wiley,

New York, USA.

Skov, H., Hertel, O., Ellermann, T., Ambelas Skj^th, C.,

Heidam, N.Z., 1999. NOVA 2003, Atmospheric deposition

of nitrogen 1998 (In Danish: NOVA 2003 1998, Atmosfær-

isk deposition af kvælstof 1998). Technical Report, No. 289,

NERI, 102pp.

Slinn, W.G.N., 1983. Air-to-sea transfer of particles. In:

Liss, P.S., Slinn, W.G.N. (Eds.), Air–sea exchange of gases

and particles. D. Reidel Publishing Company, Dordrecht,

pp. 299–405.

Slinn, S.A., Slinn, W.G.N., 1980. Predictions of particle

deposition on natural waters. Atmospheric Environment

14, 1013–1016.

S^rensen, L.L., Pedersen, B., Lund, M., Hertel, O., 2003.

Atmospheric NH3 fluxes in the marine coastal area.

Atmospheric Environment 37 (Suppl.), 167–177.

Spokes, L., Jickells, T., Rendell, A., Schulz, M., Rebers, A.,

Dannecker, W., Kr .uger, O., Leermakers, M., Baeyens, W.,

1993. High atmospheric nitrogen deposition events over the

North Sea. Marine Pollution Bulletin 26, 698–703.

Spokes, L.J., Yeatman, S.G., Cornell, S.E., Jickells, T.D., 2000.

Nitrogen deposition to the eastern Atlantic Ocean: the

importance of south-easterly flow. Tellus 52, 37–49.

Spokes, L.J., Jickells, T.D., Jarvis, K.E., 2001. Atmospheric

inputs of trace metals to the northeast Atlantic Ocean: the

importance of southeasterly flow. Marine Chemistry 76,

319–330.

Tamm, S., Schulz, M., 2003. Open ocean aerosol composition

obtained during 15 months on a North Sea ferry. Atmo-

spheric Environment 37 (Suppl.), 133–144.

Tett, P.B., Joint, I.R., Purdie, D.A., Baars, M., Oosterhuis, S.,

Daneri, G., Hannah, F., Mills, D.K., Plummer, D.,

Pomroy, A.J., Walne, A.W., Witte, H.J., 1994. Biological

consequences of tidal stirring in the North Sea. In:

Charnock, H., Dyer, K.R., Huthnance, J.M., Liss, P.S.,

Simpson, J.H., Tett, P.B. (Eds.), Understanding the

North Sea. Royal Society and Chapman-Hall, London,

pp. 115–130.

Valigura, R.A., Alexander, R.B., Castro, M.S., Meyers, T.P.,

Paerl, H.W., Stacey, P.E., Turner, R.E., 2001. In: Nitrogen

Loading in Coastal Water Bodies: An Atmospheric

Perspective. American Geophysical Union, Washington,

DC, USA, pp. 254.

Vignati, E., De Leeuw, G., Berkowicz, R., 2001. Modeling

coastal aerosol transport and effects of surf-produced

aerosols on processes in the marine atmospheric boundary

layer. Journal of Geophysical Research—Atmospheres 106,

20225–20238.

Von Salzen, K., 1997. Entwicklung und Anwendung eines

Modells f .ur die Dynamik und Zusammensetzung des

sekund.aren und marinen Aerosols. Ber. Aus dem Zentrum

f. Meeres- und Klimaforschung, Meteorologisches Institut,

Universit.at Hamburg 30, 167pp.

ARTICLE IN PRESS

G. de Leeuw et al. / Continental Shelf Research 23 (2003) 1743–1755 1755

Von Salzen, K., Schl .unzen, K.H., 1999a. A prognostic physico-

chemical model of secondary and marine inorganic multi-

component aerosols: Part I. Model descriiption. Atmo-

spheric Environment 33, 567–576.

Von Salzen, K., Schl .unzen, K.H., 1999b. A prognostic physico-

chemical model of secondary and marine inorganic multi-

component aerosols: Part II. Model tests. Atmospheric

Environment 33, 1543–1552.

Von Salzen, K., Schl .unzen, K.H., 2000. Simulation of the

dynamics and composition of secondary and marine

inorganic aerosols in the coastal atmosphere. Journal of

Geophysical Research 104, 30201–30217.

Yeatman, S.G., Spokes, L.J., Jickells, T.D., 2001. Comparisons

of coarse-mode aerosol nitrate and ammonium at two

poluted coastal sites. Atmospheric Environment 35,

1321–1335.