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Atmospheric Environment 44 (2010) 1887e1893

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Atmospheric Environment

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Aerosol organic nitrogen over the remote Atlantic Ocean

Timothy Lesworth, Alex R. Baker, Timothy Jickells*

Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK

a r t i c l e i n f o

Article history:Received 7 December 2009Received in revised form12 February 2010Accepted 18 February 2010

Keywords:Organic nitrogenSize-segregated aerosolsNitrateAmmoniumAtlantic Ocean

* Corresponding author. Tel.: þ44 (0) 1603 593117.E-mail address: t.jickells@uea.ac.uk (T. Jickells).

1352-2310/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.atmosenv.2010.02.021

a b s t r a c t

Water soluble organic nitrogen (WSON) has been measured in aerosols collected on three researchcruises on the Atlantic Ocean from approximately 55�N to 45�S. Results are interpreted using air massback trajectories and results for other aerosol components. WSON concentrations range from <1 tow40 (median 5.6) nmol m�3 with significant WSON concentrations in both fine (<1 mm) and coarsemode (>1 mm) aerosol. Concentrations of WSON were highest in samples containing Saharan dust,suggesting a locally significant source associated with soil dust. More generally WSON concentrationswere highest in air which had recently crossed continental areas. In the whole data set, WSON is wellcorrelated to total soluble nitrogen and represents approximately 25% of total nitrogen. This correlationimplies a significant anthropogenic contribution to the organic nitrogen.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

There is now global scale anthropogenic perturbation of thenitrogen cycle, including the atmosphere, with the potential forserious damage to numerous environmental systems and functions(Galloway and Cowling, 2002). The scale and potential impacts ofatmospheric nitrogen deposition by rain and aerosol dry depositionon the oceans have recently been highlighted by Duce et al. (2008).They noted the important contribution of water soluble organicnitrogen (WSON) to total nitrogen deposition, and that we knowrather little about the sources, composition or bioavailability of thismaterial. Gaseous alkyl nitrates play an important role in the longrange transport and cycling of oxidised nitrogen but are unlikely tosignificantly contribute to the deposition flux of organic nitrogen(Cornell et al., 2003), for which species contained in aerosol andrain are more important. A recent study (Altieri et al., 2009) hasdemonstrated that new high resolution mass spectrometry nowhas the ability to identify at least some of the forms of organicnitrogen in precipitation, although to date such techniques haveonly been applied to a few samples from one location on the US eastcoast. Altieri et al. (2009) demonstrate that organic nitrogen inprecipitation is composed of many hundreds of differentcompounds, but reduced forms of nitrogen appear to dominate, atleast in that one area. Another study in the same area (Seitzingerand Sanders, 1999) has also demonstrated that a substantial

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fraction of the atmospheric organic nitrogen is bioavailable.Facchini et al. (2008) have presented data suggesting that dimethyland diethyl ammonium compounds represent a significantcomponent of the aerosol at a coastal site in Ireland, consistent withthe results of Altieri et al. (2009). Mace et al. (2003b, 2003c) foundamino acids to be aminor component of aerosol organic nitrogen atCape Grimm and over the Mediterranean. Kieber et al. (1997) haveproposed a mechanism for the formation of marine humic materialin seawater, via the photo-oxidation of fatty acids and the incor-poration of ammonia into the products, that produces complexorganic compounds containing reduced nitrogen, and similarmechanisms may operate in the atmosphere. More work is clearlyrequired to understand the composition of atmospheric organicnitrogen, as well as its sources.

Several recent studies have considered the significance oforganic nitrogen as a component of total nitrogen in aerosols or wetdeposition, and whether the organic nitrogen is of natural oranthropogenic origin (e.g. Cornell et al., 2001, 2003; Mace et al.,2003a, 2003b, 2003c; Russell et al., 2003; Gonzalez Benitez et al.,2009; Violaki et al., in press), although with the exception ofMace et al. (2003a), Cornell et al. (1995, 2001), these studies havenot considered open ocean environments. Sampling across a widerange of environments allows the relationship between inorganicand organic nitrogen within the atmosphere to be considered ona global scale. Recently Zhang et al. (2008) showed that, while therewas little relationship between organic and inorganic nitrogen inrains sampled at any particular station, on a global scale there doesappear to be a positive correlation between these components. Thislarge scale correlation suggests that the atmospheric organic

Table 1Aerosol collection substrate blank values and limits of detectiona for soluble species.

Cruise Substrate blanks(nmol/filter)

Detection limits(pmol m�3)

<1 mm >1 mm <1 mm >1 mm

NO3� M55 <260 380 100 580

AMT15 600 320 650 110AMT17 230 690 20 1170

NH4þ M55 440 850 600 870

AMT15 1520 590 490 620AMT17 3530 2340 1050 1500

TSN M55 1010 2000 940 1380AMT15 2580 <2200 1780 1560AMT17 4040 2860 1220 1150

Naþ M55 860 2440 550 860AMT15 6640 3010 1370 1270AMT17 1580 2410 1410 1270

Kþ M55 <160 <240 120 280AMT15 <120 <120 90 90AMT17 <350 <350 250 250

Ca2þ M55 <920 <1330 660 1200AMT15 350 190 120 70AMT17 220 500 50 100

SO42� M55 390 210 60 70

AMT15 480 200 170 60AMT17 450 170 530 200

a Calculated using 3s of the blank and an air volume of 1400 m3 (i.e. a samplingtime of w23 h).

T. Lesworth et al. / Atmospheric Environment 44 (2010) 1887e18931888

nitrogen includes a significant anthropogenic component, andrequires therefore that this component be included in estimates ofthe scale of human impacts on the nitrogen cycle.

In this study we present the results of studies of WSON inaerosols collected at sea over the remote Atlantic Ocean and usethese to consider the sources and distribution of organic nitrogen.The advantages of aerosol sampling are that samples can becollected over the whole Atlantic across a wide range of air massessubject to very different emissions. Coupling the analysis of WSONwith air mass back trajectories then allows an assessment of therelative importance of different source regions. Rain samples offera particularly appropriate method to estimate deposition fluxes,but can only be collected during rain events, which are notnecessarily frequent or widely distributed over the remote oceans.We consider only the fraction of the organic nitrogen that is watersoluble, noting that Russell et al. (2003) suggested that there maybe an important additional organic component of the aerosol that isnot readily soluble.

2. Methods

Aerosol samples were collected during 3 cruises over theAtlantic Ocean: M55, AMT15 and AMT17. Cruise tracks and datesare shown in Fig. 1. Our sampling methods have previously beendescribed by Baker et al. (2006, 2007). We used high volumesampling (w1 m3 min�1), typically for periods of approximately24 h, with care taken to avoid contamination from the ship itself.Samples were collected ontoWhatman 41 cellulose filter substratesduring all cruises, using cascade impactors (Tisch Series 230) toseparate the aerosols into coarse (>1 mm) and fine (<1 mm) modes(Baker et al., 2006), and the samples were immediately frozen aftercollection. Analysis for water soluble aerosol components wascarried out after extraction into ultrapure water (Purite 18.2 MU)using 1 h of ultrasonic agitation (Decon ultrasonic bath), followedby filtration at 0.2 mm. Blank filters were routinely analysed andresults presented here are blank corrected (Table 1).

Fig. 1. Approximate cruise tracks for M55 (white, OcteNov 2002), AMT15 (grey,SepeOct 2004) and AMT17 (Dark grey, OcteNov 2005). Markers show the start ofsampling for each sample, which generally continued until the start of the followingsample. Black squares mark the end of collection for each cruise.

Aerosol inorganic components were analysed by ion chroma-tography (Dionex DX 600) using methods described in Baker et al.(2007). Methods used for determination of total soluble nitrogen(TSN) in the M55 samples are also described in Baker et al. (2007).For the other cruises TSN was determined by high temperaturecatalytic oxidation using a Thermalox TOC/TN analyser. The analysisconditions used were the same as those used by Suratman et al.(2008) for seawater samples, where efficient combustion andsubsequent analysis could be confirmed using certified seawaterreference materials with recoveries of 101 � 6% (TSN). Precision ofanalyses was�5% or better. We suggest therefore that the results ofthat analysis represent total water soluble nitrogen in the aerosolextracts. Subtraction of the concentrations of ammonium andnitrate from these measured TSN concentrations allows us todetermine the water soluble organic nitrogen (WSON) concentra-tion. For some aerosol components (see below) we calculated thecontribution to their total concentration from seasalt usingthe aerosol concentration of Naþ and the concentration ratio of thecomponent of interest to Naþ in seawater. We then calculatedthe non-sea-salt (nss) concentration for these components as thedifference between the total concentration and that derived fromseasalt (Baker et al., 2006).

The determination of WSON concentrations by difference, asdescribed above, inevitably leads to relatively large cumulativeerrors on the organic nitrogen determination, as discussed by Maceand Duce (2002) and Cornell et al. (2003). Estimates of organicnitrogen concentrations can therefore be associated with ratherlarge uncertainties, and in some cases yield negative concentra-tions. The latter problem has been addressed by differentresearchers in different ways. Mace et al. (2003a) treated allnegative results as below detection and as zero for averagingpurposes. Gonzalez Benitez et al. (2009) noted the potential for theremoval of negative numbers to bias the data to high values, andaccordingly retained all values, as also suggested byMace and Duce(2002). In this study we have used a different approach. We haveestimated the uncertainty of the organic nitrogen data for each

Table 3Median concentrations of WSON determined in fine, coarse and total aerosol fordifferent air mass classes. Number of observations for each class are given inparentheses, see text Section 2 for air mass classifications.

Air mass WSON (nmol m�3) % fine mode(median)

<1 mm >1 mm Total

NAtl-Rem 2.4 (15) 2.3 (10) 5.3 (8) 51Sahara 4.2 (24) 6.1 (18) 14.6 (14) 49SAfr 2.9 (10) 1.3 (6) 5.6 (5) 70SAfr-BB 2.3 (10) 2.7 (11) 6.9 (7) 65SAtl-Rem 0.9 (11) 1.3 (9) 2.8 (6) 60

T. Lesworth et al. / Atmospheric Environment 44 (2010) 1887e1893 1889

sample using standard cumulative error techniques. We thenarbitrarily eliminate all values where the uncertainty is greaterthan or equal to the estimated concentration (relative uncertaintiesof >100%). This approach does eliminate almost all the negativedata from our data set, suggesting that these arise principally fromthe problem of cumulative errors, but does not necessarily intro-duce a bias to high or low values, since the samples eliminated arethose with highest uncertainty rather than necessarily those withlow organic nitrogen concentrations. However, the highest uncer-tainties do generally arise from samples with a rather lowproportion of organic to total nitrogen and this may introducea bias to the data set. The effect is mainly to reduce extreme valuesand hence to impact the mean more than the median. The effectsare most marked for the coarse mode aerosol since this has a rathersmallWSON fraction compared to the TSN. The final data set we useretains all samples that meet the above criteria (including negativenumbers:w4% of the final data set for both fine and coarse modes).The removal of samples failing the >100% criteria does reduce ourdata set and potentially introduce a bias to our average concen-trations, but yields a data set for which there is high analyticalconfidence (Table 2). This is particularly useful for consideringinter-element relationships and sources, where there is no impactof bias but simply reduced analytical uncertainty.

Air mass back trajectories have been calculated for all samplescollected using the NOAA HYSPLIT model (Draxler and Rolph,2003). In Table 3 we report mean concentrations for samplesclassified according to their air mass origin on the basis of 5 day airmass trajectory analysis. We have previously identified 6 air massclasses in the region covered by the cruises discussed here (Bakeret al., submitted for publication):

1. NAtl. Rem. e Air which has remained over the North Atlanticwithout contact with land for 5 days prior to sample collection.Since the samples were collected in the eastern North Atlanticthis air mass class may contain emissions from North Americasampled after more than 5 days over the ocean.

2. Europe e Air which has crossed Europe within 5 days prior tosample collection.

3. Saharae Air which has crossed the Sahara desert within 5 daysof sample collection.

4. SAfr. e Air which has either crossed or approached theSouthern African continent outside of the biomass burningseasons.

5. SAfr.-BB. e Air which has either crossed or approached theSouthern African continent, whose chemistry is influenced bybiomass burning.

Table 2WSON concentrations in fine and coarse mode aerosols, and their sum, in samplescollected over the Atlantic Ocean (nmol m�3), for A, all measurements, and B, afterremoval of values with relative uncertainties greater than 100%. SD Standarddeviation.

WSON (nmol m�3)

<1 mm >1 mm Total

A. Total data setMedian 2.4 1.4 4.0Mean 3.5 2.8 6.5SD 5.4 5.3 8.7n 72 77 68

B. Data set filtered to remove values with uncertainties >100%Median 2.5 2.5 5.6Mean 3.6 4.1 9.8SD 5.4 5.9 10.0n 71 54 40

6. SAtl. Rem. e Air which has remained over the South Atlanticwithout contact with land for 5 days prior to sample collection.

Within the sampling campaigns discussed here, no samplesfrom class 2 (Europe) were collected.

3. Results and discussion

The median, mean and standard deviation coarse and fine modeand total (coarse þ fine) aerosol WSON concentrations for our dataset are presented in Table 2. This Table also shows the sameparameters for the smaller data set produced as a result of therather stringent data error criteria discussed above. The final dataset consists of 71 fine mode and 54 coarse mode samples, with 40pairs of fine and coarse data from which we calculate total aerosolWSON concentrations. As noted above, the impacts of eliminatingthe >100% uncertainties are most marked on the mean and stan-dard deviation, since extreme values are eliminated, particularly inthe coarse mode. The datasets, as for many aerosol datasets, areskewed (Fig. 2) and we prefer the median as the most appropriateaverage value. All subsequent discussion considers only thereduced data set for WSON meeting the <100% standard deviationcriteria.

The median total WSON concentration in this study(5.6 nmol m�3) is similar to that reported for remote marine airover the North Pacific (3.3 nmol m�3 (Cornell et al., 2001)) andsomewhat higher than reported for the remote Southern Ocean(0.9 nmol m�3 (Mace et al., 2003b)), although there were only2 samples of remote air in the latter study. The median concen-trations in this study for samples which had spent at least 5 daysover the North or South Atlantic without contact with land werelower than the overall median at 5.3 and 2.8 nmol m�3 respectively(Table 3) and closer to the results of Mace et al. (2003b). The lowerconcentration in the SAtl-Rem group probably reflects the muchlower overall nitrogen emissions in the southern hemisphere.

Cornell et al. (2001) reported most (80% average) of WSON to befine mode, while Mace et al. (2003b) reported most to be coarsemode. The results of our study indicate a very wide range in thecoarse/fine mode distribution with a median value of 54% in thefine mode. The results suggest higher fine mode fractions in airfrom the southern hemisphere (Table 3), although more data isrequired to verify this. This wide range of fine/coarse mode ratios isconsistent with theWSON having diverse and multiple sources andsuggests that primary and secondary formation processes forWSON could both be important.

In Fig. 2 the WSON data is shown as box and whisker plots ofconcentration distribution in the different air mass classes togetherwith results from the same samples for other major ions e nitrateand ammonium plus nssCa2þ, nssKþ and nssSO4

2� (used respec-tively as tracers of dust (Mace et al., 2003c), biomass burning(Andreae, 1983; Baker et al., 2006) and anthropogenic activity). Inthe case of nssSO4

2�, the potentially important role of natural

Fig. 2. Box and whisker plots for WSON (a & b), NH4þ (c & d), NO3

� (e & f), nss (non-sea-salt) Kþ (g & h), nssCa2þ (i & j) and nssSO42� (k & l) concentrations in fine and coarse aerosols in

each air mass class observed. Air mass classes are defined in the text Section 2. Boxes show the inter-quartile range of the data, median values are shown by horizontal bars in eachbox. Whiskers show the upper and lower limits of the data range, except for values between 1.5 and 3 box lengths (circles), or greater than 3 box lengths outside the box (stars).

T. Lesworth et al. / Atmospheric Environment 44 (2010) 1887e18931890

marine biogenic sulphur emissions, particularly of DMS needs toconsidered (Brimblecombe et al., 1989). The average concentrationsand the ranges for these other major ions (except nssKþ) are similarto those we have reported previously for the Atlantic (Baker et al.,2006) suggesting that the samples we use for this study of WSONare broadly representative of the region.

Samples from Saharan air masses are readily identified by theircolour and by high concentrations of non-seasalt calcium. This airmass is also characterised by high concentrations of all N species,and nssSO4

2�. As discussed by Baker et al. (2006), the high levels ofmany contaminant components in Saharan air masses probably donot reflect emissions fromwithin the Sahara desert. It is more likelythat these contaminants were introduced to the air mass more than5 days before sampling probably over Europe or North Africa.WSON concentrations in the Saharan air class are markedly higherthan in all other air classes, particularly in the coarse mode. Maceet al. (2003c) have demonstrated in the Eastern Mediterraneanthat dust represents a significant source of organic nitrogen. In ourdata set the correlation between total WSON and nssCa2þ is weak

(r2 ¼ 0.36). However, inspection of Fig. 3 suggests that the data setmay be better described as a mixture of two different relationshipsbetweenWSON and nssCa2þ. The group of samples (A) with a lowerWSON/nssCa2þ ratio has a slope similar to that reported by Maceet al. (2003c) from the western Mediterranean. As Mace et al.(2003c) note, this relationship may reflect organic N associatedwith the desert soils that produce the dust and nssCa2þ, but alter-nately could represent adsorption of organic nitrogen onto dust inthe atmosphere. Sample group B is characterised by a much higherWSON/nssCa2þ ratio. Samples in groups A and B do not havesignificantly different non-sea-salt sulphate concentrations and ifwe use Al as a tracer (data not shown), the two groups are notclearly differentiated. We therefore suggest groups A and B reflectdesert source regions with rather different Ca contents (e.g.Chiapello et al., 1997), rather than regions inwhich the relationshipbetween WSON and dust are different.

Violaki et al. (in press) sampled WSON in the Mediterraneannear Mace's sampling site throughout the whole year, whereasMace only sampled in a season of relatively strong dust transport.

Fig. 2. (continued).

T. Lesworth et al. / Atmospheric Environment 44 (2010) 1887e1893 1891

Violaki et al. (in press) noted difference in the relationship betweenWSON and nssCa2þ between high and low dust regimes, consistentwith Fig. 3. We demonstrate below that there is a significantanthropogenic contribution to the WSON and we therefore believethat the elevated WSON in the Saharan air mass has two compo-nents: organic nitrogen associated with dust sources and thatassociated with emissions from Europe. Bencs et al. (2009) alsoconcluded that organic nitrogen in Northern European rain hada significant anthropogenic source.

The lowest average WSON concentrations were found in airmasses that had the least contact with potential terrestrial sourcesand the highest were associated with the Saharan air flow. Theseresults suggest that terrestrial emissions represent a major sourceof WSON. The modest differences between air masses 4 and 5suggest that biomass burning is not an important source of WSON,although nssKþ concentrations were not significantly differentbetween these air masses. This suggests that, although the sampleswe collected were potentially influenced by biomass burning basedon the seasonality of biomass burning and the trajectories (Bakeret al., submitted for publication), the actual impact of biomassburning on these samples was minor compared to samples with

similar trajectories collected on other cruises in the South Atlantic(Baker et al., 2006). Mace et al. (2003a) have demonstrated thatbiomass burning can be a significant source of WSON based onsampling relatively close to sources.

WSON concentrations were only slightly elevated in air mass 4(Southern Africa without biomass burning influence) compared toother air mass groups and much less than concentrations in theSaharan air masses. This suggests that Africa (apart from theSaharan region) is a weaker source of WSON than Europe, apartfrom the Saharan dust source and possibly biomass burning. Thisimplies that it is terrestrial anthropogenic sources rather thannatural terrestrial emissions that dominate the sources of WSON.Consistent with this argument, there is strong correlation betweentotal (coarse þ fine) WSON and total soluble nitrogen (Fig. 4). Thiscorrelation could reflect the geographical similarity of the sourcesof ammonia, nitrate andWSONwith all beingmore concentrated inhighly populated areas with strong industrial and agriculturalemissions. Alternatively this correlation could reflect the creationof WSON by reactions of organic matter in the atmosphere withammonia or nitric acid (or its precursors). The presence of signifi-cant amounts of WSON in the fine mode, and the inferred

Fig. 3. Total WSON concentrations as a function of total nssCa2þ with air mass backtrajectory classification of samples indicated (diamonds e NAtl. Rem., squares e

Sahara, triangles e SAfr., Circles e SAfr. BB, pluses e SAtl. Rem.).

T. Lesworth et al. / Atmospheric Environment 44 (2010) 1887e18931892

significance of gas to particle conversion, would be consistent withthe latter hypothesis, particularly if the organic matter is of naturalgaseous origin, consistent with its isotopic signature, as discussedby Zhang et al. (2008).

Based on the slope of the relationship in Fig. 4, WSON representson average 24% of total nitrogen (with corresponding values for fineand coarse mode aerosols of 28% and 18% respectively). Duce et al.(2008) reviewed a range of other recent estimates and suggestedthat WSON represents about 30% of total N deposition. Zhang et al.

Fig. 4. WSON concentrations as a function of total soluble nitrogen (TSN) concentra-tions for M55 (squares), AMT15 (triangles) and AMT17 (diamonds). The correlationrelationship for the whole data set is also shown.

(2008) and Gonzalez Benitez et al. (2009) have recently reportedproportions of WSON in total N of 30 and 23% respectively fora global data set and a detailed study at a site in Scotland. Violakiet al. (in press) report an average WSON contribution of 23% attheir sampling site in the Mediterranean although this is variable.Thus our estimates for the remote oceanic environment areconsistent with other estimates, although in all these studies andcompilations there is considerable variability for individualsamples contributing to the average value.

4. Conclusions

We have presented one of the first comprehensive datasets onWSON over the remote oceans. Median aerosol WSON concentra-tions (fine þ coarse mode) over the North and South Atlantic arew6.0 andw3.0 nmolm�3 respectively, with highest concentrationsassociated with Saharan air masses, consistent with a significantdust source for WSON. Background concentrations in the mostremote air masses are about 2 nmol m�3. Overall WSON represents24% of total soluble N in the aerosol. Air masses passing over landwithin a few days of sample collection tend to have higherconcentrations than air masses that have spent the previous 5 daysover the oceans, Fine mode aerosol concentrations are greater thancoarse mode concentrations, but both fractions are important.These results together with a correlation between total nitrogenand organic nitrogen lead us to suggest that much of theWSON is ofanthropogenic and terrestrial origin, probably with multiplesources.

Acknowledgments

We thank the Masters and crews of the FS Meteor and RRSDiscovery for their assistance during cruises and M. Waeles forcollecting samples during AMT15. Our participation in the cruiseswas funded by the UK Natural Environment Research Councilthrough the 36 North and Atlantic Meridional Transect (AMT)Consortia (Grant numbers NER/O/S/2003/00625 and NER/O/S/2001/00680) and grant numbers NER/B/S/2002/00301 and NE/F017359/1. This is contribution 185 of the AMT programme. Wegratefully acknowledge the NOAA Air Resources Laboratory (ARL)for the provision of the HYSPLIT transport and dispersion modeland READY website (http://www.arl.noaa.gov/ready.html) used inthis publication. We are grateful to two anonymous reviewers fortheir helpful comments on the manuscript.

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