Estimation of the net air-sea flux of ammonia over the southern bight of the North Sea

Pergamon

Atmospheric

Enuironmenf

Vol. 28, No. 22, pp. X47-3654, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights ~~~rvcd 1352-2310/94 $7.00 + 0.00

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ESTIM:ATION OF THE NET AIR-SEA FLUX OF AMMONIA OVER THE SOUTHERN BIGHT OF THE NORTH SEA WILLEM A. H. ASMAN National Environmental Research Institute, Frederiksborgvej 399, DK-4000 Roskilde, Denmark

ROY M. HARRISON Institute of Public and Environmental Health, School of Biological Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.

and C. J. OTTLEY Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, U.K. (First received 20 November

1993 and

in final form 22 June 1994)

Abstract-Ivfeasurements of airborne gaseous ammonia and total dissolved ammonium in sea water determined on cruises on the North Sea have been used to calculate net fluxes of ammonia between air and sea. The system is finely balanced with the majority of net fluxes from air to sea, but some periods occur when the sea becomes a net source of ammonia in air. Examination of the field data suggests that the main factor detemrining the direction of flux is the airborne ammonia concentration, which when elevated causes ammonia deposition to the sea. It is calculated that ammonia deposition to the southern bight of the North Sea (below 56”N) amounts to 7.6 x lo3 tonnes N per year, about one-half of an earlier upper limit estimate. Comparison with studies from the Pacific Ocean, in which the sea acts as a source of atmospheric ammonia, reveals that i.he major differences arise from much higher concentrations of airborne ammonia in the North Sea atmosphere caused by advection from adjacent land. Key word k&x:

Ammonia, flux, dry deposition, emission, North Sea.

INTRODUCTION

Gaseous ammonia can represent a substantial proportion of reactive nitrogen in the rural atmosphere over western Europe. Additionally, particulate ammonium salts are a,major component of the inorganic aerosol and thus total ammonia plays a major role in the atmospheric cycle of nitrogen. Recent work (Harrison and Allen, 1991) clearly demonstrates the major importance of deposition of ammonia in contributing to total atmospheric nitrogen inputs to sensitive terrestrial ecosystems. Ammonia also plays an important role in atmospheric chemistry as the only major gaseous base, responsible for neutralisation of airborne acidity. It also influences the atmospheric transport and deposition of nitrate through being a reactant in the atmospheric formation and dissociation of ammonium nitrate and ammonium chloride (Allen et al., 1989). The North Sea is a large and economically important body of water. There is a general concern over pollutant inputs, especially over inputs of nutrient

elements such as nitrogen which have led to an increase in the frequency, intensity and duration of eutrophication incidents in recent years. Estimates of the atmospheric input of nitrogen have indicated that it amounts to some 40% of the input from estuaries (QSR, 1987). The main significance of atmospheric inputs is likely to be in areas remote from estuarine influence. Recent collaborative research has sought to quantify more accurately the atmospheric nitrogen inputs to the North Sea (Rendell et al., 1993). Airborne concentrations of gaseous NH3 and HN03 and and NO; were measured on particulate NH: a number of research cruises and the data used to construct mean concentration fields over the southern bight of the North Sea (Ottley and Harrison, 1992). Dry deposition estimates from these data were combined with estimates of wet deposition of nitrate, ammonium and dissolved organic nitrogen from sampling rainwater on the same cruises to estimate a total nitrogen input of 228 x 10’ tonnes N per year to the southern bight (the area south of 56”N) of the North Sea.

3647

W. A. H. ASMANet al.

3648

One of the major uncertainties in estimating the dry deposition of nitrogen relates to ammonia gas. For HN03 and the aerosol species it is reasonable to assume that the atmospheric concentration which would exist in equilibrium with the sea water is zero

and hence the flux is calculated simply from the product of atmospheric concentration and deposition velocity. In the case of ammonia, however, the sea water contains NHf, and due to the relatively high pH, this can sustain a significant atmospheric equilibrium concentration of ammonia gas: NH:

(aq) + OH-+NH,.H,O NHJ. HZ0 Z$ NH3 (8).

The sum of NH3 (aq) and NH: is called NH, and is analysed in sea water as NH:. If the actual airborne ammonia concentration is less than the gas-phase concentration which is computed to be in Henry’s law equilibrium with the NH, and H+ in sea water, there is a net flux of ammonia out of the sea. If, on the other hand, airborne NH, exceeds the Henry’s law equilibrium gas-phase concentration, there will be a net flux into the sea until equilibrium is restored. The magnitude of the ammonia flux is determined both by the difference between the Henry’s law equilibrium gasbhase concentration of NH3 (g) and the actual atmospheric concentration, and by the exchange velocity, tieritself a function of meteorological conditions in the case of a highly water soluble gas (Duce et al., 1991). In the earlier published work (Ottley and Harrison, 1992; Rendell et al., 1993) only an upper limit value of the assumed downward flux of ammonia was calculated, based upon the assumption of a zero concentration of dissolved ammonia/ammonium (NH,) in the sea in the absence of detailed knowledge of concentrations. Based upon this assumption, the upper limit to the ammonia flux was estimated at 15 x lo3 tonnes N per year. A true estimation of the direction and magnitude of the ammonia flux is, however, important for two reasons: (a) ammonia may comprise a significant input to the sea which requires better quantification, (b) if the sea is a net source of atmospheric ammonia, it will contribute to airborne ammonia advected over land, and to nitrogen inputs to terrestrial systems. In this paper, we use the newly available data upon the sea water composition with respect to dissolved ammonium and other species as well as additional meteorological data to calculate the net flux of ammonia between air and sea.

approximately 2 week duration between February 1989 and October 1989. the research vessel RRS Challenger was used as a sampling platform to measure atmosphehc ammonia and dissolved ammonium within the water column. Each cruise followed the same predefined cruise track (see Ottley and Harrison, 1992) and atmospheric samples were collected under favourable head winds, whilst water samples were collected at set sampling points upon this cruise track, approximately at lo-15 mile intervals. The precise locations of air sampling are given by Harrison et al. (1994). The sampling procedure for atmospheric samples has been described in detail elsewhere (Ottley and Harrison, 1992). Briefly, it is as follows: three-stage filter packs were deployed 6 m from the bow of RRS Challenger, 4 m above deck level some 10 m above the sea level. The inverted filter pack was housed in a simple rain shelter and in order to avoid inadvertently sampling ship-borne contamination this equipment was only operational in a head wind. Each filter pack comprised of three filtration media placed in series. Aerosol species were collected upon a Teflon membrane, whilst gaseous acids were collected upon a secondary nylon-66 filter. Ammonia collection took place upon a third filter (Whatman 41) impregnated with 5% H3P04. A sampling flow rate of approximately 8 L min - 1 was maintained by a small pump with the total sampling volume being recorded upon a gas meter. On completion of sampling after approximately 8-9 h, the filters were transferred to airtight polypropylene bottles using the ship’s cleanroom facilities, and were subsequently refrigerated prior to analysis, which was carried out within one week of arriving back at the base laboratories. During the sampling interval routine meteorological parameters were collected from ship-borne instrumentation. Ammonia collected upon the Whatman 41 filter was desorbed in deionised water with agitation on a shaker, and determined as NH: using a simple flow injection fluorescence system, based on reaction with o-phthalaldehyde (Rapsomanikis et al., 1989).The detection limit for ammonia was determined as 1.5 x lo-* pgrnm3 (defined as 3 sigma of the blank). Water samples were collected from the surface of the water column (defined as the top 0.5-5 m). At each sampling station, the ship’s CTD system was routinely deployed to quantify a wide variety of physical and chemical properties of the water column. Water samples collected with this system were immediately analysed calorimetrically for NH, using an indophenol blue method upon the ship’s auto-analyser system; the detection limit is reported as 0.1 pmol/- ’ (Hydes, 1984).The sea water pH was not routinely measured. For the purpose of the flux calculations a pH value of 8.0 was adopted based upon the work of Richardson (1994) and a limited set of data communicated by the International Council for the Exploration of the Sea (ICES), Copenhagen, Denmark for the Worth Sea. Due to the nature of the cruise programme atmospheric ammonia concentrations provide an integrated average concentration over a section of the cruise track, typically some 60-70 miles in length equivalent to 8-9 h sampling intervals. During this period several surface water samples were collected and analysed for ammonium. These values were subsequently averaged to provide a representative value for the water column above which the atmospheric ammonia samples were collected.

COMPUTATION OF THE NH3 FLUX METHODOLOGY Environmental

sampling

As part of the Natural Environment Research Council’s North Sea project, in a series of seven cruises, each of

NH3 is a highly soluble gas; this means that its exchange is controlled by diffusive resistance at the gas-phase side. The general concepts of air-sea exchange of gases and particles are described by Duce et u/. (1991).

3649

Net air-sea flux of ammonia The NH3 flux between the sea and the atmosphere depends on the following factors: Maximum exchange velocity. This velocity depends only on the meteorological conditions and the diffusivity of the gaseous component and not on the concentradions. The difference in the concentration between the computed NH3 concentration in the air which would be in equilibrium with the pH and NH: concentration in sea water, and the NH3 concentration at the reference height. It is assumed that NH, does not interact with sea spray. For practical reasons it is assumed that the NH, concentration in the upper 200 pm of the sea which determines the NH, flux is the same as the NH, concentration in the upper metre of the sea, where the actual measurements took place. The exchange velocity is, in case of a flux to the surface resistance of zero, equal to the dry deposition velocity and is defined by 11,=

1 r,+rb+r,

where v, is the exchange velocity (m s-l), ra is the aerodynamic resistance (s m- ‘) and rb is the laminar boundary layer resistance (s m - ‘). This is a somewhat artificial resistance, resulting from the fact that the surface roughness 1e:ngth for momentum is different from the surface roughness length for concentration. It can be both positive and negative and is a function of the diffusivity of the gas in question. In this paper rb is computed from both roughness lengths. The surface resistance, r,, the resistance for uptake by the surface itself, for a highly soluble gas like NH3 is negligible compare:d to the other resistances, and for that reason is neglected in the computations. A big difference between land and sea is, that the surface roughness at sea is not only about a factor 1000 less than on land, but also that it is a function of wind speed, because the wave height increases with the wind speed. Joffre (1988) used the following relationship between the surface roughness for wind and the friction velocity:

zone from a smooth to a rough surface, a situation which occurs quite often. The maximum exchange velocity depends on the friction velocity and the atmospheric stability, described by the Monin-Obukhov length. The friction velocity, u*, and the Monin-Obukhov length, L, are found from the shipboard measurements of wind speed, relative humidity and temperature of the air and sea water temperature by a procedure described by Lindfors et al. (1991). Their procedure was followed, taking into account that the meteorological variables and the NH, concentration were measured at different heights on the ship. In the following the heights z,.,,z,, z, and z, indicate the heights at which, respectively, the wind speed, the temperature, the humidity and the NH, concentration are measured on the ship. The values of u., and L are found from an iterative procedure. As an initial estimate u., is found from a relation between the wind speed on the ship and u., for neutral atmospheric conditions computed from equation (2), assuming the normal logarithmic wind profile. Then zO,,,is computed from equation (2). Then the surface roughness for the concentration is computed for smooth conditions (Reynolds number Re = zomu,/v < 0.15) from 3O(v/u,) exp[ - 13.6~ SC~‘~]

Z oc =

and for rough conditions (Rea0.15) from Z OE =

2Oz,, exp[ - 7.3K Re1’4 SC’/~]

ZOill

o.0144u2

4

9

(3b)

where zoC is the surface roughness for concentration (m), ICis the von Karman’s constant (=0.4) and SC is the Schmidt number (= v/D,), where D, is the diffusivity of the gas (m2 s- ‘). Then the surface roughness for temperature and humidity (z,,, respectively zoe) are found from equations (3a) and (3b) by using the Prandtl number instead of the Schmidt number (for temperature) and the Schmidt number with the diffusivity of water vapour (for humidity). Then the temperature scaling 6* (K), the temperature equivalent of u*, is found from KAe

0*= 0.13v =:-+L

(3a)

ln Wo,)

- Y&,l~) + Yh(zo,IL)

(4)

(2)

where zom is the surface roughness length for momentum (m), v is the kinematic viscosity of air (m2 s- ‘), u* is the friction velocity (m s- ‘) and g is the gravitation (m C2). The first part of equation (2) describes the situation of a smooth surface and the second part describes the situation during rough conditions. As can be seen from this equation the first contribution decreases with u* and the second one increases with u*. No good description is, in fact, available for the transition

where 0 is the potential temperature of air at measuring height (K) and A6 is the difference in potential temperature between the air at measuring height and the sea surface (K). The function Y for stable atmospheric conditions is the same for momentum (m), heat (h) and humidity (e) and is found from Y,(z/g=

Yu,(z/Q= Yv,(z/L)= -5z/L

(Sa)

and for unstable atmospheric conditions the Y for momentum (Y,,,) is different from the Y for heat (Y,,)

3650

W.

A. H. ASMANet

and humidity (Y,). They are found from Y.(i)=ln

[(F)

tration in sea water can be found from

(?)‘I

- 2 arctan

Y.(t)=Y.($=2ln

MNH~ W-L1

ceq=

+t

1

(5b)

(y)

where

and is also used in equations (6) and (10). For the value of z, the appropriate value for the parameter in question is substituted. Then the humidity scaling q* is found from

‘* =ln(z,/zo,)-

al.

Y&,/L) + Ye(zo,lL)

(6)

where Aq is the difference in specific humidity between the air and the saturation specific humidity at sea surface temperature. Then the Monin-Obukhov length, corrected for humidity, is found from L=(L,‘+L,‘)-’

(7)

where

(13)

where ceq is the NH3 concentration in air, which would be in equilibrium with [NH,] (pg mm3), MNHJis the molecular mass of NH3 (g mol- ‘), [NH,J is the NH, concentration in sea water (PM), yNHlis the . . activity coefficient of NH3.H20, yNH4is the activity coefficient of NH: in sea water, R is the gas constant (8.2075 x lo-’ atmm3mol-’ K-l), HNHs is the Henry’s law constant for NHJ(M atm-‘), pHs is the pH of sea water, which is a measure of the activity of HC in sea water and KNH4is dissociation constant for NH,+ (M). The values of HNHJ and KNH4used are H NH3=56ew(,,2($w&))

(l4)

(Dasgupta and Dong, 1986) K ..,=5.67x

lo-”

exp( -6286(-:-A)) (15)

(Bates and Pinching, 1950). The NH3 flux is finally found from

Le= +v(cwe*)

(8)

’ = u,(c,p - C.ir)

(16)

and where F is the NH3 flux (pg m-' s- ‘); the flux is positive when the sea emits NH3, and cair is the NH, concentration in air (pg mW3). with TV being the virtual temperature of air (K). The activity coefficient of NH3.H20 is approximThe new value of the friction velocity is then found ated by the following equation, which is valid for an from NaCl solution of 25°C (Garrels and Christ, 1965), and ionic strengths between 0.517 and 1.578 (Randall and KUkv) (10) Failey, 1927): (zw/zo,)- Y&,/L) + Ydzo,IL) L, = u: Tv/(0.61g1c0q,)

(9)

‘* =ln

where u(z,) is the wind speed at measuring height z, (ms-‘). Then the surface roughnesses for wind, concentration, temperature and humidity are computed again with this new value of u*, etc., until the value of II* does not change significantly any more. Then the aerodynamic resistance r, is found from

yNHl= 1+ 0.0851.

(17)

The ionic strength I for sea water as a function of salinity (S(S)) can be found from (Lyman and Fleming, 1940) 1=0.00147+0.019888+2.08357x

10-5S2.

(18)

It should be mentioned that no correction is made in equation (18) for the existence of ion pairing, which [ra=-!-[ln($-)-Y,e)+ YhfT)] (11) would make the ionic strength about 7% lower. Moreover, the relation is not valid if sea water is and the laminar boundary resistance rb from diluted considerably with water for which the relative contribution of the different ions is much different 1 r,=-ln %!!!! . (12) from that in sea water. This is, however, not generally KU* ( zoc> the case in the North Sea, but could occur in extreme situations in the Baltic Sea. The activity coefficient for The maximum exchange velocity u, is now computed NHf in sea water is found from the results of an ion from equation (1). The NH3 concentration in air c_, which would be in pairing model (Miller0 and Schreiber, 1982) to which the following functions fit was derived for a salinity equilibrium with the NH, (NH3 + NH:) concen-

Net air-sea flux of ammonia between 5 and 409t30: yNH; ==0.883 - 0.0768 In S.

(19)

It is of the order o!f 0.61 for a salinity of 35X-+

RESUILTS AND DISCUSSION

The results of the flux calculations appear in Fig. 1 as a time series through 1989. An attempt to resolve the flux data spatially showed no clear pattern, whilst Fig. 1 does show an apparent temporal trend with a predominance of deposition, but a tendency toward emission in May-July and September. Whether the observed tendency is representative of other years is not known. The reasons for this behaviour are not initially obvious. The two extreme results are explicable. The emission flux of 8.51 x 10e3 pg NH, m-' s- ’in September was due to very high NH,+ concentrations in sea water in the Humber estuary; in general, sampIes we.re not derived from estuarine sites as these exhibited higher sea water NH,. The very high deposition flux of 1.48 x 10m2 PgNH, m-2s-1, also in September was due to a high atmospheric concentration of 1.23 pgrnm3. In general, both concentrations of atmospheric NH3 and of sea water NH, are higher close to the coasts (e.g. Ottley and Harrison, 1992) due to land-based and estuarine sources, respectively. It is clear from the flux reversals and the many near-zero fluxes in Fig. 1 that this is a finely balanced system. However, over the study as a whole the net flux is different from zero with a 99% level of

March

April

3651

significance (t-test). The question inevitably arises as to whether the dominant factor in determining the direction of flux is the airborne concentration of NH3 (g) or the sea water concentration of NH: (aq). Time series for the two measurements appear in Figs 2 and 3. These measurements relate to the actual samples used in the flux calculations. Visual examination of the figures suggests that no single factor is dominant; Figs 1 and 2 reveal that, as expected, high concentrations of NH, (g) correspond to deposition of ammonia. Separation of the data sets into periods of emission and deposition reveals concentrations (meanfS.D.) of NH3 (g) as follows: deposition, 0.48 k 0.43 pg m- 3 (n = 52) and emission, 0.10 + 0.08 pgrne3 (n=20). The difference is highly significant (99% in the t-test), suggesting that the atmospheric concentration provides a major determinant of the direction of ammonia flux. Fluctuations in airborne ammonia appear greater than sea water NH: with apparently lower concentrations over the North Sea in mid-summer, perhaps due to higher concentrations of aerosol acidity to remove ammonia into the particle phase (Harrison and Kitto, 1992; Kitto and Harrison, 1992). The average flux of ammonia is downward with magnitude (1.2852.75)x 10-3~gNH3m-2s-1. This corresponds to annual deposition to the southern bight of the North Sea (2.3 x lo5 km2) of 7.6 x lo3 tonnes N per year, significantly smaller than the value of 15 x lo3 tonnes N per year calculated by Rendell et al. (1993) on the basis of maximum potential downward flux and a simple deposition model, or 13 x lo3 tonnes N per year calculated in this work with sea water NH, set to zero. Both estimates were

May/June

July

August

September

l.OE-02 8.OE-03 -Emission 6.OE-03 --

-6.OE-03 -_*,0~~3

__

Deposition

off Kale 1.48E-02

-1.OE-02 -

Fig. 1. Temporal variation in calculated.&monia fluxes.Air samples are characterised by cruise number (CH47, etc.) and a sample number within that cruise.

W. A. H. ASMAN et March

April

Fig. 2. Concentrations

March

April

May/June

al.

July

August

of ammonia in air. Time series as in Fig. 1.

May/June

July

August

September

10 9 -7

6 -y b 5 f E

7 -6-5--

g. 4-8 g 3--

1 0

Fig. 3. Concentrations of total ammonium in sea water, averaged according to air sampling intervals as per Fig. 1.

based upon measurement data covering only the period February to October 1989, and hence do not cover a full year. The uncertainties introduced by these temporal limits are not readily quantifiable. Probably, the greatest uncertainty in these calculations arises from inadequate knowledge of the pH of sea water, which was not measured at the time of air sampling. It appears that this can vary with location and season, with a plausible range of pH, 7.8-8.2. For

the purposes of this study, a median value of pH 8.0 was adopted, and a sensitivity analysis conducted using pH values of 7.9 and 8.1. At pH 7.9 the deposition flux is 14% higher than at pH 8.0, and at pH 8.1, 17% lower. There are few comparative data with which to compare our results. Quinn et al. (1987) have reviewed data upon ammonia in the remote marine troposphere. The more recent data indicate mean

3653

Net air-sea flux of ammonia

concentrations of NH, (g) in marine air of 0.03-l the majority of means being below pgm-‘, 0.3 pg rnm3. Our airborne concentrations measured over the North Sea are thus not greatly at variance with those reported for more remote marine regions. In the same review., Quinn et al. (1987) report total from 0.23 to sea water NH, concentrations 0.55 pmol L- ‘. Thes,e are lower than the majority of the measured North Sea values (Fig. 3). The Henry’s law constant is strongly temperature-dependent (Quinn et al., 1987) as in KNH4, and the relatively modest temperatures of the North Sea compared to Seas at low latitudes will favour deposition relative to emission. Quinn et al. (1988) made measurements on the atmospheric ammo:nia system in the coastal northeast Pacific environment. Their average gaseous ammonia concentrations were 0.02 pg m- 3 aboard ship, and 0.01 pgrne3 on a coastal mountain. Sea water concentrations of total ammonium were 0.45 f 0.35 PmolK l. The ammonia flux estimated using data from an equilibrator which determined the gas-phase concentrations of a:mmonia in equilibrium with surface sea water was in the range (5.1-31) x 10m4 pgrn-‘~-~, from sea to air. In similar work over the mid-Pacific: Ocean, Quinn et al. (1990) measured gaseous am:monia in the range 2 x 1O-4 to 0.058 pg mm3 (mean 0.011 pgmW3) and total sea water ammonium of 0.4 f 0.3 pmol K ‘. The flux of ammonia from sea to air was, on average, 1.4 x 10m3 pgrn -z s - l. Comparison of the data from the above studies with Figs 2 and 3 indicates clearly that whilst sea water NH, concentrations are broadly similar at the North Sea and Pacific Ocean sites studied, which would lead to higher airborne NH, concentrations in equilibrium with NH, for the Pacific because of the higher seawater temperature, there are vast differences in gaseous ammonia. Concentrations of ammonia over the North Sea are greatly elevated due to advection of ammonia generated over land. Because of the higher airborne ammonia concentrations, fluxes of the gas over the North Sea are generally from air to sea. It is interesting to speculate on the behaviour of ammonia in the northern areas of the North Sea. Our earlier work (Ottley and Harrison, 1992) has shown that airborne substances with origins over land show diminished concentrations toward the centre and north of the sea. Thus, lower airborne gaseous ammonia would be expected in these areas and the sea may act as a net source of ammonia in air.

Acknowledgements-The air sampling work was supported by the U.K. Natural Environment Research Council North Sea Programme which, funded a studentship to CJO. Assistance in air sampling was provided by Andrew Rendell, Ged Bradshaw and Jane Merrett. Sea water ammonium data were drawn from the NERC BODC. The authors thank the originators of these data, David Hydes, Helen Edmunds and co-workers. Information on the pH of the North Sea was AE 28:22-H

provided by Dr Katherine Richardson (Danish Institute for Fisheries and Marine Research, Charlottenlund, Denmark) and the International Council on the Exploration of the Seas (ICES). T’he authors also express thanks to the U.K. Department of Environment who supported this work through contract No. PECD 7/12/25 on Marine Deposition of Nitrogen. Full financial support for this work and its presentation is gratefully acknowledged from the Netherlands Foundation for Atmospheric Chemistry (WAHA). Some additional data of the composition of sea water were obtained from the International Council for the Exploration of the Sea, Copenhagen, Denmark, Prof. James J. Morgan, California Institute of Technology, gave information on activity coefficients.

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model to estimate activity coefficients of natural waters. Am. J. Sci. 282, 1508-1540. Ottlev C. J. and Harrison R. M. (1992) The snatial disiribution and particle size of some‘inorganic nitiogen, sulphur and chlorine species over the North Sea. Atmospheric Environment MA, 1689-1699. QSR (1987) Quality status report of the North Sea. In Proc. 2nd Int. Conf: on the Protection ofthe North Sea. HMSO, London. Quinn P. K. Charlson R. J. and Zoller W. H. (1987) Ammonia, the dominant base in the remote marine troposphere: a review. Tellus 39B, 413-425. Quinn P. K., Charlson R. J. and Bates T. S. (1988) Simultaneous observations of ammonia in the atmosphere and ocean. Nature 335, 336-338. Quinn P. K., Bates T. S. and Johnson J. E. (1990)

Interactions between the sulfur and reduced nitrogen cycles over the central Pacific Ocean. .I. geophys. Res. 95, 405-416. Randall M. and Failey C. F. (1927) The activity coefficient of gases in aqueous salt solutions. Chem. Rev. 4, 271-284. Rapsomanikis S., Wake M., Kitto A.-M. N. and Harrison R. M. (1989) Analysis of atmospheric ammonia and particulate ammonium by a sensitive fluorescence method. Envir. Sci. Technol. 22, 948-952. Rendell A. R., Ottley C. J., Jickells T. D. and Harrison R. M. (1993) The atmospheric input of nitrogen species to the North Sea. Tellus 45B, 53-63. Richardson K. (1994) Personal communication, Danish Institute for Fisheries and Marine Research. Charlottenlund, Denmark.