### GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 6, NO. 4, PAGES 459-483, DECEMBER 1992

GASEOUS AMMONIA FLUXES AND BACKGROUND CONCENTRATIONS IN TERRESlRIALECOSYSTEMS OF THE UNITED STATES

A. 

### 0.Langford I
Cooperative Institute for Research in Environmental Science, University ofColorado, Boulder

F. C. Fehsenfeld2

Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado

J. Zachariassen3 and D. S. Schimel4

Natural Resources Ecology Laboratory, Colorado State University, Fort Collins

Abstract. Ammonia (NH3 ) is the dominant gaseous base in the atmosphere and the principal neutralizing agent for atmospheric acids, yetremains one ofthe least well characterized atmospheric trace compounds. In particular, the spatial and temporal distribution ofthe background concentrations in terrestrial ecosystems and the importance of natural emissions from undisturbed soils and vegetation is poorly understood. This situation persists because of experimental difficulties associated with ammonia measurements, the rapid gas-to-particle conversion of ammonia in the atmosphere, and the capacity ofnative soils and vegetation to act as both source and sink for atmospheric ammonia In the present paper, we attempt to summarize the
currentunderstanding ofthe natural sources and sinks for gaseous NH3and the importance ofnatural emissions relative
to anthropogenic emissions in the United States. We briefly review the physical and chemical processes that transform NH3in the atmosphere, the major anthropogenic and potential natural sources ofatmospheric NH3, and the techniques used to measure low concentrations and fluxes of atmospheric NH3

. The available background concentrations

IAlso at Aeronomy Laboratory, NOAA, Boulder, Colorado.

2Also at Cooperative Institute for Research in Environmental Science, Boulder, Colorado.

3Currently at Department ofBiology, University of Stanford, Stanford, California.

4Currently at NCAR, Boulder, Colorado.
Copyright1992 by the American Geophysical Union.

Paper number 92GB02123. 0886-6236/92/92GB-02123$10.00 and flux measurements ofammonia in natural ecosystems are then described and used to infer upper limits for the emissions ofNH3 into the atmosphere from these systems. While the magnitude ofboth anthropogenic and natural emissions of NH3 remain uncertain, itappears that unperturbed terrestrial ecosystems are generally more important as sinks rather than

sources for atmospheric NH3

. However, net emissions are likely from many eastern forests and other ecosystems exposed to large inputs ofatmospheric sulfate.

1. INlRODUCTION

Ammonia (NH3) is the dominant gas-phase base in the atmosphere and the principal neutralizing agent for atmospheric acids. Ammonia thus influences the composition and pH ofaerosols and cloud waterand affects regional airquality and acid deposition patterns [Middleton, 1987]. At low concentrations, atmospheric NH3 may be a valuable source ofnitrogen for the vegetation in nitrogen-limited ecosystems [Hutchinson etal., 1972; Fahey et al., 1985] but is toxic to plants at high concentrations [Haynes, 1986; VanderEerden, 1982] and emissions from anthropogenic sources have been implicated in the acidification ofsoils [Roelofs etal., 1987], forest decline [Van Breemen etal., 1982; NihlgArd, 1985; Schulze, 1989] and the eutrophication ofsurface waters [Hutchinson and Viets, 1969]. Consequently, there is considerable interest in the spatial and temporal distribution ofammonia concentrations andemissions.

There is general agreement that livestock wastes and fertilizers are responSible for most ofthe atmospheric ammonia burden. Detailed emissions inventories have been developed for these and other anthropogenic sources in both Europe [ApSimon etal., 1987; BuijSinan et al., 1987] and North America [Harriss and Michaels, 1982; Heisler etal., 1985; U.S. Environmental Protection Agency, 1989; Wagner460



### et al., 1986; 

### Environmental Protection Service, 1985]. 

### However, there remain large uncertainties about the role of undisturbed terrestrial ecosystems as a source for atmospheric ammonia and even the background concentrations in remote

areas [Bowden, 1986]. 

### There are three reasons for this. First, the plant-soil system 

### can both emit or absorb gaseous NH3 and 

### a given soil or vegetation type may be a net sink in some
areas and a net source in others. Second, scavenging of gaseous NH3by acidic sulfate and nitrate aerosols complicates the interpretation ofconcentration and flux measurements in many areas [Harrison et al., 1989]. 

### Finally, the net emissions ofgaseous NH3 from natural sources and the concentrations in undisturbed ecosystems 

### are generally small compared to anthropogenic sources and automated techniques sufficiently sensitive to make the continuous measurements needed to establish a large database have not been readily available.

The global distribution and behavior ofatmospheric ammonia has 

### been the subjectofseveral reviews and budget analyses [e.g., Galbally and Roy, 1983; 

### National Research Council, 1979; 

### SOderlund and Svensson, 1976; 

### Stedman and Shetter, 1983; 

### Wameck, 1988; 

### Schlesinger and Hartley, 1992]. 

### A concise summary and comparison ofglobal budgets through 1983 

### is given by Wameck [1988] 

### and 

### avery recent global budget analysis publishedby Schlesinger and Hartley [1992]. 

### The purpose ofthe present article is to assess the role ofundisturbed terrestrial ecosystems as a source for atmospheric ammonia in the United States and the spatial and temporal variability ofthe background concentrations ofNH3 in the atmosphere. The techniques for measuring background concentrations and fluxes are reviewed, and the prospects for developing spatially and temporally resolved emission and concentration models discussed.

2. 

### SOURCES OF ATMOSPHERIC AMMONIA

Ammonia is a ubiquitous product ofbiological activity. Sources in the plant-soil system include the mineralization of organic matter and reactions within the cells ofliving plants. Although most soil 

### NJ4+ undergoes nitrification or is immobilized by vegetation, soil colloids, or clays, a small steady state pool is maintained in the soil solution. At sufficiently high pH, some ofthis ammonia exists as NH3 which can be exchanged with the atmosphere. The soil solution will absorb gaseous NH3 when the atmospheric partial 

### pressure exceeds the equilibrium vaporpressure and 

### emit NH3 when the opposite is true. The NH3 vaporpressure (and hence the volatilization rate) is proportional to the dissolvedNH3concentration and increases exponentially with temperature according to Henry's law (cf. section 7.1 

### and Freney et al. [1983]). 

### Thus the soil emissions rate and even the source or sink status can change with temperature and hence season or time ofday.

Inorganic fertilizers and animal excretaenhance soil emissions by directly increasing the NH3 or 

### Nl4+

### contentof the soil solution [Hauck, 1983]. 

### Loss rates from fertilized fields depend on the fertilizer type, soil, meteorological conditions, and the method ofapplication [Fenn and Hossner, 1985]. 

### Surface application ofurea ((NH2hCO) in animal urine or as fertilizerpellets results in the greatest losses since the subsequenthydrolysis Langford et al.: AmmoniaFluxes and Background Concentrations

(I)



### also increases the pH ofthe soil within the urine patch or surrounding the fertilizer pellet. Schimel et al. [1986] 

### found that ammonia N emissions average -=20% 

### ofurine N deposited in pasture grazing systems although this varies substantially with soil type, 

### season, and pasture management practices. Much higher emission rates are likely when high densities oflivestock are confmed in feedlots, barns, and poultry houses which are devoid ofvegetation and therefore have little capacity for absorbing the generated ammonia.

Ammonia is also released directly into the atmosphere through a variety ofindustrial activities. These include wastewatertreatment, manufacturing processes, petroleum refining, coking, coal and oil combustion. Large amounts of ammonia 

### are also volatilized during the burning ofbiomass associated with agricultural or forest managementpractices, paper manufacture, and wildfires. Several field and laboratory studies [Andreae, 1991; 

### Griffith etal., 1989, 1991; 

### Hegg et al., 1988, 1987; 

### LeBel et al., 1988; 

### Lobert et al., 1990, 1991] 

### suggest that pyrolysis ofprotein matter by smoldering fires produces large amounts ofNH3 and CO, while flaming fires produce more oxidized species (NOx and**C~).**

3. THE 

### FATE OF AMMONIA IN THE 

### ATMOSPHERE

The interpretation ofammonia concentration and flux measurements is complicated by the interaction between the atmosphere-biosphere exchange process and transformations in the atmosphere. A simplified diagram illustrating the cycling ofammonia in the atmosphere and between the atmosphere and the biosphere is shown in Figure 1. 

### Gaseous NH3 decreases 

### with altitude above land surfaces and is larger

over the continents than the oceans [Georgii and Gravenhorst, 1977; 

### Georgii and Miiller, 1974; 

### Hoell etal., 1980; 

### LeBel et al., 1985; 

### Lenhard and Gravenhorst, 1980; 

### Levine etal., 1980; 

### Tanner, 1982; 

### Ziereis and 

### Arnold, 1986]. 

### Thus it

seems likely that most ofthe ammonia present in the atmosphere above landmasses originates from terrestrial

sources and that the oceans export little ammonia to the continents (see Quinn et al. [1987] 

### for a review ofthe chemistry ofammonia in the remote marine troposphere).

Gaseous NH3 does not absorb in the actinic ultraviolet and reacts only slowly with OH [DeMore et al., 1990]; 

### the lifetime with respect to this process is -=3 

### months. This, coupled with the low concentrations ofgaseous NH3 above the 

### boundary layer, suggests thatphotochemical destruction is

a relatively minor sink. However, it has been suggested that photooxidation ofNH3may be a potential source or sink for tropospheric NOx (i.e., NO+ N02) in remote regions [Logan, 1983; 

### McConnell, 1973]. 

### Photochemical box model calculations by the authors suggest that the importance of these processes depends heavily on the rate ofreaction between NH2 and**~** 

### and that they become entirely negligible within the boundary layerusing the new upper limit determined in recent laboratory measurements by Tyndall et al. [1991].



### The most likely fate for gaseous NH3escaping the surface layer is incorporation into acidic aerosols and cloud water droplets in the boundary layer and lowertroposphereLangfordet al.: AmmoniaFluxes and Background Concentrations



# . . . . . . ''''' 461



### Wet Deposition

F1g. 1. Schematic diagram illustrating thecycling ofammoniabetween the atmosphere and biosphere. Atmospheric ammonia may bepresentin the gas phase, in fine particulates (solid orliquid), and in liquid droplets (cloud orfog). The dominantprocesses

are shown with solid lines; the dashed lines represent processes that are generally ofsecondary importance: evaporation ofNH3 from ammonium sulfate, escapeofgaseous NH3 the free troposphere, and adsorption ofgaseous NH3 on plant surfaces.

[Seinfeld, 1986]. The incorporation ofammonia into these bquid and/or solid phases displaces H+ as the primarycation and lowers the acidity thereby facilitating the uptake of weakeracids, like nitric acid, hydrochloric acid ororganic ac1ds. The sequestering ofammoniaby aerosols andcloud
wateroccursprimarily through association with H2S04 to form**~HS04** and <NR!hS04. Theequilibrium vapor pressure ofammonia above sulfuric acid is extremely small [Scott and Cattell, 1979; Tang, 1980] so that this process can beconsidered irreversible. Ammonia will bequantitatively Utrated from the gas phase leaving the particulate phase only partially neutralized ifexcess sulfate is present (i.e.,**([NH3]+~+])/** [S04=] 

### < 2. Gaseous NH3 mixing ratios in

excessof=1ppbv are likely only when there is sufficient ammonia to fully neutralize the particulate phase to <NR!hS04(i.e., ([NH3]+**~+])/[S04=] ~**  2).

Ammonia can alsoreact with gas-phase**HN~** to form**~~.** This association is reversible under atmospheric conditions [Stelson and Seinfeld, 1982] andammonium
mtratecan decomposereleasing NH3. Becauseofthe greater affmity ofsulfate for ammonium, little nitrate will be Incorporated into the particulate phase until the sulfate is almostcompletely neutralized [Tang, 1980]. Thus**NJ4N~** IS much moreprevalent in the vicinity ofwestern cities where the**NO,/S~** emission ratio is large (e.g., Denver and Los Angeles) than in the eastern United States [Hidy etal., 1978; Wolff, 1984; Wolffet al., 1981]. In coastal areas, a similar reaction with hydrochloric acid forms ammonium chloride (NH4Cl) [Harrison et al., 1989].

Theaboveprocesses lead to atmospheric lifetimes for gaseous NH3 ofthe orderofseveral hours to a few days [Asman and Janssen, 1987; Georgii and Gravenhorst, 1977]. Long range transport is most important in the free troposphere above the planetary boundary layer. Over continental land masses, transport from the planetary boundary layer to the free troposphere occurs mainly by convectivecloud pumping, frontal upwelling, orographic dynamics, and pumping associated with the diurnal variation in the heightofthe planetary boundary layer. The characteristic time for this transferis ofthe orderof 1week or less [Luecken and Hales, 1986; Wameck, 1988] so that transfer outofthe planetary boundary layer and long-range transport generally occurs after gas-to-particle conversion or cloud water scavenging. Release ofgaseous NH3following the evaporation ofclouds is unlikely since most ofthe dissolved ammoniabecomes associated with sulfate. Hence
wet and dry deposition ofparticulate**~+** is the primary removal mechanism in most areas.

4. TECHNIQUES TO MEASURE AMMONIA CONCENTRATIONS AND FLUXES

4.1. 

### Concentration Measurement Techniques

Much ofthe uncertainty associated with atmospheric ammonia is due to the scarcity ofreliable measurements. This in tum, is due to a lack ofsensitive measurement techniques which are easy to use in the field and can operate automatically and continuously to establish large databases for statistical analysis. A majordifficulty associated with

### 462

ammonia measurements is the presence ofammonia in gaseous (NH3), particulate (e.g., (NJ4)2S04 and**NRtN~** 

### ), and liquid (i.e., NJ40Hin cloud and fog droplets) forms in the abnosphere. The partitioning between these three phases is highly variable and a complete picture generally requires simultaneous but separate measurements ofat least the gaseous andparticulatephases. A second majordifficulty results from the tendency ofammonia to form strong hydrogen bonds with waterand thus adsorb on the surfaces of
most materials exposed to air. This can lead to high backgrounds and memory effects due to retention on sampling tubing and other plumbing.

Most gaseous ammonia measurements reported in the literature 1rap ammonia with an acidic medium: acid bubblers [Breeding et al., 1973;Junge, 1957], acid-coated filters [Shendrikar andLodge, 1975], oracid-coated denudertubes [Ferm, 1979]. The solution from the bubblers orrinsed filters ortubes is then analyzed for ammonium via colorimetry, ion-specific electrode, or ion chromatography. Thediffusion denuders developedby Ferm [1979] separate gaseous NH3 from particulate and droplet

### Nl4+ by exploiting the large difference in diffusion coefficient; gaseous NH3 in

an airstream passing through the tube diffuses to and adheres to the acid-coated wall while the heavierparticles pass straight through. Variants ofthis technique based on both cylindrical and annular designs have become the de facto standards for gaseous ammonia measurements. The detection limit depends

on the sample time and denuder preparation, butlevels below



## ..o.Ql ppbv appear to be possible for a 2-hour sample. Similar limits may be obtained with acid-coated filters which

are also widely used, typically with a Teflon prefilter to collect the particulate phase. Filterpacks are generally considered the standard technique for measuring particulate**~+** and related species although more sophisticated techniques are underdevelopmentandevaluation [Sickles et al., 1990].

Several intercomparisons ofthe diffusion denuderand filter pack techniques for measuring gaseous NH3 have been conducted in recent years. A field comparison ofoxalic acid coated paperfilters and diffusion denuders described by Gras [1984] indicated that volatilization ofparticulate ammonium collected on the Teflon prefilter led to high apparent gaseous NH3in the filter pack technique. This result was also indicated in an intercomparison offilter pack, diffusion denuder, Fouriertransform infrared (FTIR), and tungsten oxide denuder detectors in Claremont, California, during 1986 [Appel et al., 1988; Wiebe et al., 1990]. In this case, ammonia concentration measured with the filter packs were approximately 1.5 times those measured with the oxalic acid denuders. Harrison and K.iitto [1990] also found evidence for particulate volatilization in the filter packs butconcluded that the differences will typically be small compared to the natural variability ofambient ammonia These workers also stressed that the differences depend on theparticular site, operator, and apparatus and thus do notreflect intrinsic limitations ofthe filter pack technique.

Both filter packs and denuders offerthe advantages of simplicity andrelatively low cost, but the preparation and analysis ofthe sampler is labor intensive. Furthermore, great

care mustbe taken to preventcontamination during storage and handling since ammonia is present in human breath and Langford et al.: Ammonia Fluxes and Background Concentrations

perspiration as well as a variety ofcleaning agents found in
most laboratories. Thus the reproducibility and detection limit (and hence the necessary exposure time) obtained by these techniques largely depends on the skill and consistency ofthe operator. Theserestrictions make the acquisition of large data sets with sufficienttemporal resolution to probe the parameters which influence abnospheric concentrations of ammonia difficult

Several groups [Bos, 1980; Keuken etal., 1988] have developed automated diffusion denuder systems in which the acid solution coating is continuously replenished. The system developed atthe Netherlands Energy Research Foundation (ECN)by Keuken etal. [1988] can operate for up to 8 hours without user intervention and detectother species

as well as ammonia. Unfortunately, the detection limit for a 30-min sample time is relatively high**(~.3** ppbv). An alternate approach to automated detection uses "permanent" acid coatings and thermal desorption. McClenny and coworkers [Harward etal., 1982; McClenny and Bennet, 1980] collected ammonia on Teflon beads ortungsten oxide tubes and detected the thermally desorbedproductby photoacoustic spectroscopy with nominal a detection limit of =0.2 ppbv. Braman etal. [1982] developed adenuder tube coated with tungsten oxide/tungstic acid to collect ammonia. The collected ammonia is thermally desorbed and subsequently converted to NO on a hot gold surface. The NO is measured with a standard chemiluminescence detector. Field measurements based on this system are described by McOenny etal. [1982]. LeBel etal. [1985] used a similar instrumentto measure venical profiles ofammonia above coastal Virginia and Maryland from an airplane, and Roberts etal. [1988] used this technique atNiwot Ridge, Colorado, and Point Arena, California. A similar instrument developed by Appel etal. [1984] and tested in the Oaremont inten:omparison described above agreed with the diffusion denuder measurements to within about a factor of2but exhibited a temperature dependent sensitivity.

Thermal denuder systems based on othermetal oxides have also been developed. Keuken et al. [1989] developed a system based on vanadium pentoxide while Langfordetal. [1989] used molybdenum oxide. The former instrument employs an oxidecoated annularquartz denuder while the latteruses the oxidized surface ofa molybdenum rod suspended in a quartz tube. Both vanadium and molybdenum oxide directly oxidize the NH3 to NO orNOx upon desorption thus eliminating the need for a secondary convener. In ambient sampling tests, both ofthese instruments compared well with conventional denuders atmixing ratios above 1-2 ppbv. In general, metal oxide systems are reasonably simple to automate with good reproducibility and detection limits below 0.1 ppbv for 0.3- to 0.5-hour sample times. The major drawbacks are related to the expense and long-term stability ofthe denuders in the field. These systems may also respond to aliphatic amines, although these species are generally present at very low levels relative to ammonia [Hutchinson et al., 1982].

Several otherpromising sampling techniques have been developed to measure gaseous ammonia. Abbas and Tanner [1981] described a fluorescence derivatization technique that collects ammonia in a continuously flowing acidic solution and monitors fluorescence from 1-alkylthiosoindole formed in the subsequentreaction with o-phthahaldehyde. The detectionLangford et al.: Ammonia Fluxes and Background Concentrations

limit is rather high (=0.3 ppbv for a 5-min response time); however, and the specifiCity toward gaseous NH3 has not been established. Rapsomanikis et al. [1988] used this analytical technique to quantify ammonia collected on filters

ordenuders. Farmer and Dawson [1982] used a cold plate to condense ammonia and other soluble gases in water, analyzing the condensate by colorimetry or ion-selective electrode. The extraction ofatmospheric concentrations by this method is rather involved, and the estimated detection limit of0.15 ppbv requires sampling times ofseveral hours. Genfa et al. [1989] described an instrumentin which ammonia diffuses through a porous membrane into water. The ammonium is then reacted with sodium sulfite and o-phthaldialdehyde and the resulting fluorescence from 1-sulfonatoisoindole monitored. The technique offers great potential for general use since it is relatively inexpensive, appears to be highly specific toward ammonia, and has a detection limit ofS0.05 ppbv for a 5-min sample time. At present, this technique has not been compared to other standard methods. Other instruments based on mist chamber techniques [Coferetal., 1985] are under development in several laboratories and offer similar advantages.

Itis clear, however, that direct spectroscopic detection of gaseous NH3 without intervention ofa collecting medium would be most desirable since this could provide both continuous measurements and unequivocal identification of the compound. Long path absorption techniques using C02 differential absorption lidar (DIAL) [Force et al., 1985] and Fourier transform infrared spectroscopy [Biermann et al., 1988] have been used but have very high detection limits (5 and 1.5 ppbv, respectively). Tunable diode laser absorption spectroscopy (TDLAS), an in situ spectroscopic technique [MacKay and Schiff, 1984], has excellent sensitivity and specificity but the application at low concentrations is limited by ammonia losses on absorption cell walls. Recently, a photofragmentation/laser-induced fluorescence technique (PF/LIF) [Schendel etal., 1990] has been used which

overcomes this problem by using very high flow rates (<::1000 L min-I) through the excitation cell. This technique 

### appears to offer detection limits approaching 0.001 ppbv for a 1-min integration time with no known interferences. However, in its present form, the system is large, expensive, andcomplicated. Nevertheless, the technique offers great potential as a transfer standard against which other
measurements can beevaluated.

The photofragmentation/laser-induced fluorescence (PF/LIF) technique was recently compared with citric acid coated diffusion denuders (CAD/IC), a filter pack/colorimetry system (FP/COL), a tungsten oxide denuder (DARE), and a molybdenum oxide denuder system (MOADS) in Boulder, Colorado, during 1989 [Williams et al., 1992]. This study indicated that the PF/LIFand CAD/IC systems agreed within the stated accuracies for all tests with no evidence of interferences. The FP/COL system was systematically lower due to temperature related losses on the Teflon prefilter. The DARE system agreedfor separate inlet ambient sampling, but
not for common inlet sampling due to retention ofNH3 on the (Teflon) transfer line from the main sampling manifold The MOADS system showed a very strong correlation with the PF/LIF data for both common and independent sampling 

### 463

configurations under clearconditions but had a systematic offset due to a calibration error. This system also exhibited a background level of=0.07 ppbv attributed to retention of NH3 on the Teflon inlets and responded to both gaseous NH3 and free 

### NJ4+ dissolved in cloud orfog droplets when the latter were present

A main conclusion ofthe above study is that even short inlet linesretain ammonia and cause memory effects. In particular, Teflon appears to be unsuitable for ammonia sampling inlets. Thus sampling through any type ofinlet should be done using high flow rates (i.e., <::100 L s-1) iflow mixing ratios ofammonia ([NH3] :s; 1ppbv) are to be measmed.

4.2 

### FluxMeasurement Techniques

The only flux measurement techniques presently applicable
to the measurement ofammonia fluxes from terrestrial ecosystems are those based on enclosure methods, gradient diffusion, or aircraft sampling. More sophisticated micrometeorological methods such as eddy correlation await the development ofa sufficiently sensitive and fast (

...0.1 s integration time) NH3 detector. The techniques used to

measure ammonia fluxes from agricultural systems have been reviewed by Harper [1990] andby Denmead [1983].

Enclosure methods are widely used to measure trace gas fluxes from soils and plants because oftheirrelative simplicity. Fluxes are calculated from changes in the gaseous concentration within a small enclosure placed over the ground and purged with air containing a known concentration ofthe compound ofinterest. Similar techniques are widely used in laboratory studies ofgas exchange. Since the enclosure technique measures fluxes from only a small area, replicate measurements spread over a largerplotare required to address the question ofspatial variability in the field (at the other extreme, aircraft sampling interrogates such a large area that it is impossible to exclude nearby anthropogenic emissions). The chiefcriticism ofthe enclosure technique arises from the fact that the microenvironment within the box is not representative ofthe open field. Forammonia, this leads to serious consequences since soil fluxes are known to be sensitive not only to the atmospheric concentrations but also environmental quantities such as wind speed, temperature, evaporation rates, and dew formation [Freney et al., 1983]. Since ammonia is readily adsorbed on the materials used to make enclosure walls and sampling tubing (especially when condensation ispresent), the reliability ofthis technique at the low emission ratesbelieved to be typical ofunmodified field soils is highly questionable. For this reason, field
measurements based on enclosure techniques are not included in the following discussion.

Gradientdiffusion techniques have been widely used to

measure ammonia fluxes from agricultural systems [Denmead, 1983]. In this approach, ammonia fluxes are determined from simultaneous measurements ofan ammonia concentration gradient 6[NH3]/6zand the heat ormomentum flux. The latter is used to derive the eddy diffusivity (KtJ and the flux calculated as

(2)

### 464 Langford

### etal.: AmmoniaFluxes and Background Concentrations



### Pt. Arena Niwot Ridge

WESTERN FORESTS**~** Rocky Mountain 1:·:·:-:·:·1 Pacific Coast**~** Grasslands 

## c::::::J Desert types 

### Pawnee

EASTERN FORESTS

-Northern (::;:::::::::::::1 Central hardwood**~Southern ~Tropical**  

### Harvard Forest

Thoreau's Bog

Oak Ridge

North Inlet

Fig. 2. Distribution ofecosystem types in the contenninous United States. Background ammonia concentrations at 

### sites representing mostofthese divisions are summarized in Table 1. Adapted from Brady [1974].

where a positive flux corresponds to 

### emission from the surface. This approach does not perturb the plant-soil system

or the influence ofenvironmental factors on the exchange process. Gradient diffusion also horizontally averages the large spatial variability ofammonia fluxes over several hundred square meters. This characteristic is also a disadvantage in that it

### necessitates a large fetch and relatively simple terrain [Melillo, 1989]. As 

### a result, most measurements have been made in grassland ecosystems. 1n



### some locations, the interpretation ofgradient measurements is compoundedby gas-to-particle conversion ofammonia in the atmosphere [Harrison et al., 1989]. Nonetheless, this technique appears to bethe mostreliable method available.

5. MEASUREMENTS OF ATMOSPHERIC AMMONIA IN TERRESlRIAL ECOSYSTEMS

Few NH3 concentration measurements have been made at 

### sites thatcan be considered truly representative ofundisturbed natural ecosystems. This is due not only to the difficulties in making NH3 measurements at low concentrations but also the interest in agricultural economy and air quality that 

### motivates most atmospheric NH3 

### measurements. Similarly,

an extensive literature exists for NH3 

### flux measurements in feedlots, cultivated soils, and 

### fertilized and grazedpastures, while measurements ofgaseous fluxes in unmodified grasslands and forests are virtually nonexistent In this section we summarize measurements made in reasonably undisturbed terrestrial ecosystems.

5.1 

### BackgroundConcentraJion Measurements

Acompilation ofsummertime (May to October) measurements ofgaseous NH3 

### at sites representative ofthe broad ecosystem types ofthe 

### United States (cf. Figure 2) is given in Table I. The measurements are listed so as to form

a rough longitudinal transect ofthe United States from west to east coasts. Only sites withoutpermanentbuildings and having relatively intact native vegetation are included. Several ofthe 

### listed sites belong to 

### the Long Term Ecological Research (L 

### TER) network supportedby the National ScienceFoundation (NSF). In comparing these results, it is important to note that the sampling techniques have not all been directly intercompared and that the measurements were made at

### heights ranging from 0.5 to 10m above the surface. Furthermore, the sample sizes vary widely and in somecases may be biased by nonrepresentative meteorological conditions. Finally, not all ofthese studies

### Langford et al.: AmmoniaFluxes and Background Concentrations 

### 465

TABLE I. 

### Background Summer AmmoniaMixing Ratios in Natural Ecosystems

Ecosystem Mean, Range, Sampling Site Type ppbv ppbv Technique Source

Point Arena, Calif. Pacific coast 0.658 

### 0.16-1.25 Tungsten oxide Roberts etal. [1988] denuder system ThurberRanch, Ariz. desat 

### 0.9 na condensation trap 

### ion- Dawson and Farmer selective electrode [1984] Niwot Ridge, Colo. Rocky Mountain 0.42 0.11-1.66 molybdenum oxide Langford and Fehsenfeldc (0.62)b annulardenuder Boulder, Colo. grassland 

### 3.4 0.10-19.3 molybdenum oxide Langford and FehsenfeldC (4.4)b annulardenuder Pawnee, Colo. grassland 11.4 2.5-47.0 molybdenum oxide Langford and Fehsenfeldc (11.8)b annular denuder Pawnee, Colo. grassland 12.2 6.8-17.6 acid trap 

### colorimetry Zachariassend Konza, Kansas grassland 11.0 64.-16.9 acid trap colorimetry Zachariassend Kinterbish, Ala. 

### southern forest 0.69 0.05-1.46 acid<Oated denudt7 

### Norton andFehsenfeldc (0.42)b ion chromatography OakRidge, Tenn. central 

### hardwood 0.30 0.11-1.88 molybdenum oxide Langford and 

### Fehsenfeldc (0.30)b annular denuder Harvard

### Forest, Mass. northern forest 0.24 na acid<Oateddenuder 

### Tjepkemaet al. [1981] ion-selectiveelectrode Thoreau's Bog, Mass. freshwaterwetland 0.33 na acid<Oateddenuder 

### Tjepkemaet al. [1981] ion-selective electrode North Inlet, S. C. saltwaterwetland 0.30 0.14-0.75 molybdenum oxide Langford and

### Fehsenfeldc (0.30)b annular

### denuder

a

### The mean mixing ratio was =0.32 ppbv when winds offthe ocean are included. bMean mixing ratios during periods with T = 

### 25 

### ± 2°C, horizontal wind**speeds~** 

### ms-1, and insolation**~350** W 

### m-2. cPreviously unpublished results, 1992. dZachariassen, J., 

### Ammonia exchange above natural grassland canopies, thesis, Colo. State Univ. Fort Collins, in preparation, 1992.

include nighttime measurements when the concentrations are typically lower (see below). The numbers shown in parentheses under the mean mixing ratios correspond to those periods 

### with a temperature of20t2°C, strong to moderate insolation, and little horizontal wind to advect NH3

### from any nearby 

### sources.

The mean mixing ratios range from =0.3 ppbv at most of the coastal and forest sites to**~10** 

### ppbv at two ofthe three 

### grassland sites. The overall mean mixing ratio at Point Arena [Roberts et al., 1988], a coastal grassland =100 km 

### north ofSan Francisco, decreases from =0.6 ppbv to =0.3 ppbv when periods with westerly winds offthe ocean are included. Dawson and 

### Farmer [1984] measured gaseous NH3
at

### a number ofsites in the desert southwest and inferred the representative background concentration for this 

### region to be 0.9 ppbv. The wide range ofmixing ratios observed at 

### the Bouldergrasslandreflects a dependence on wind direction resulting from the location ofthis site at the boundary between the forested Rocky Mountains and the GreatPlains. The value shown in parentheses is probably most representative.

Gaseous NH3 

### typically exhibits both seasonal and diurnal trends. Figure 3acompares the average summerdiurnal variation at

### Pawnee, Boulder, NiwotRidge, Oak Ridge, and North Inlet and Point Arena. In each case, the NH3

### mixing ratios have been normalized by the appropriate mean mixing ratio from Table 1. The NH3 

### mixing ratios are at a minimum in the early morning, peak

### near midday, and decrease during the night. The day/night ratio is =2 in all three 

### cases. Figure 3b shows the seasonal dependence ofthe diurnal variation at 

### Niwot Ridge in the Colorado mountains. The mean mixing ratio decreases from =0.4 to 0.2 ppbv between summer and winter. Even larger seasonal differences were observedby Tjepkema et al. [1981] at Harvard Forest in Massachusetts, and Cadle etal. [1985] observed very low wintertime values in northern Michigan. However, Dawson and Farmer observed only slight seasonal anddiurnal trends in the desert southwest.

The temporal variability ofgaseous NH3 

### suggests a strong correlation with ambient temperature. This is illustrated in Figure 4 for Boulder, Niwot Ridge, Oak Ridge, and North Inlet. The mean mixing ratios differby more than an order of magnitude, yet similarexponential temperature dependences

are observed atall 

### four ofthese very different.sites. As 

### is discussed below, this observation provides a powerful argumentfor the control ofgaseous NH3 

### in the lower part

### of the boundary layerby air-surface and/or gas-aerosol equilibria.



### The influence ofgas-to-particle conversion on the form of ammonia in the atmosphere can be seen by comparing ambient measurements ofgaseous NH3 

### and particulate 

### Nl4+

### 466 Langford etal.: Ammonia Fluxes and Background Concentrations

(a). 1.75-.--------------------,

1.5

c 0 1.25

### ~ 

# i
8 -:r:." 

### z 0.75
Gl 

# i ;! 0.5

0.25 

# ---

.........

.....__ Pt. Arena, CA 

### --+- oak Ridge, TN

North Inlet, SC 

### -a- Boulder, CO

Niwot Ridge, CO 

### -e- Pawnee, CO**0~~~-r-~-T--~~--~--~~--T-~--~** 0 4 8 12 16 20 24 Local Standard Time
(b). 800,---------------------------------,

700

600 

## c 0 

# = 500 ;: 

# -. a;**~** 400 1:: 

# • .e. 300 -:r:." 

### z 200

100 Niwot Ridge, CO

Winter**0+-----.----~----------~---,----~** 0 4 8 12 16 20 Local Standard Time Fig. 3. (a) Diurnal variation ofgaseous NH3 at several of the grassland, forest, and wetland sites listed in Table 1. The mixing ratios are normalized by the appropriate mean value listed in the table. (b) Seasonal dependence ofthe diurnal variation atNiwot Ridge, a forest site in the Colorado mountains.

and S04=. Table 2 summarizes measurements from rural or suburban (but notnecessarily "background") locations in California, Colorado, Alabama, Tennessee, North Carolina, Michigan, New York, Pennsylvania, and Connecticut made during both summer and winter. These studies represent all ofthe field campaigns atnonurban sites in the United States 24

### to**~----------------------------------,**

J: 

### z

## ... 0 N1wot Ridge, CO 

### e Boulder,CO

•**N.lnle~** SC /j. Oak Ridge, TN**to L---~~~~~~----~~~--~~~~**  240 250 260 270 280 290 300 310

T (K) Fig. 4. Temperature dependence ofgaseous NH3 mixing ratios atfour ofthe background sampling sites. The Boulder andNiwotRidge plots include winter and fall measurements not shown in Table 1. Each symbol represents the average of five individual half-hour measurements spanning the temperature range indicatedby the horizontal bars. The vertical bars represent the 95% confidence limit.

that we are aware ofwith both gaseous and particulate measurements. Table 2 shows that while the total (gaseous 

### + particulate) summertime ammonia concentrations are similar among most ofthe sites, the relative amounts ofgaseous and particulate ammonia differby more than two orders of magnitude. The gaseous and particulate phases ofammonia generally are uncorrelated. However, Figure 

### 5 shows the rapid decrease in the gaseous fraction with increasing particulate sulfate. For sulfate levels below =-20 nmol m-3 (=-2 IJ.g m-3), most ofthe ambient ammonia is in the gas phase during the summer. When sulfate levels exceed =200 nmol m-3 (=20 IJ.g m-3), less than 5% ofthe ambient ammoniaremains in the gas phase. The gaseous fraction decreases more rapidly with increasing [S04 =] in winter due to reduced surface emissions ofNH3and the lower equilibrium vaporpressure ofNH3 above the aerosol.

5.2 

### Gaseous FluxMeasurements

Acomprehensive tabulation ofNH3flux measurements from different terrestrial ecosystems is given by Schlesinger and Hartley [1992]. The measuredemission rates range over
more than 3 orders ofmagnitude with no obvious differences between forest, grassland, desert, and flooded ecosystems. While this result is seemingly at odds with the concentration measurements described above, many ofthese results are not 

### based on direct flux measurements andreflect a widerange of sampling conditions with integration times ranging from minutes to weeks. In the following discussion we focus instead on a small number ofgradientprofile studies that shed light on the environmental and physical factors that influence the variability ofgaseous NH3 fluxes.Langford etal.: AmmoniaFluxes and Background Concentrations 

### 467

TABLE 2. Regional Variability ofAtmospheric Ammonia and Related Species in Rural and Suburban Locations

Site Region NH3 NH4+ HNO:J NO:J· so4= Source

Summer Newtown, Conn. New England 34 112 42 5 65 Keeler et al. [1991] Allegheny Mountain, Middle Atlantic SIS 172 78 9 200 Pierson et al. [1989] Pa. OakRidge, Tenn. Southeast 15 133 48 3 104 Lindberg etal. [1990] OakRidge, Tenn. Southeast 9 240 

### na 24 177 Langford, Norton, andFehsenfeld• Kinterbish, Ala. Southeast 28 87 22 9 56 Norton, andFehsenfeld& Boulder, Colo. Mountain 230 44 16 23 17 Langford, Norton, andFehsenfeld• Niwot Ridge, Colo. Mountain 15 14 9 19 6 Langford, Norton, andFehsenfeld•

Winter
State College, Pa. Middle Atlantic 2 141 40 21 60 Lewin et al. [1986]b State College, Pa. Middle Atlantic 3 65 18 2 26 Lewin et al. [1986]b State College, Pa. Middle Atlantic 11 73 22 4 36 Lewin et al. [1986]b Northern Mich. GreatLakes 2 43 11 19 28 Cadle et al. [1985] Boulder, Colo. Mountain 60 29 13 14 8 Langford, Norton, andFehsenfeld• Niwot Ridge, Colo. Mountain 6 9 3 1 4 Langford, Norton, and Fehsenfeld• Lake Isabella, Calif. West 19 8 16 9 4 Jacob et al. [1986] Tehachapi, Calif. West 34 11 7 11 8 Jacob etal. [1986]

Here 1ppbv NH3= 41 nmol m-3 = 0.7 Jlg m-3 at STP. •Previously unpublished results, 1992. bEntries refer to three measurementperiods between January and March 1984.

Alandmark study in the field is that ofDenmead et al. [1976], who measured NH3 fluxes above an ungrazed ryegrass/subcloverpasture near Canberra in Australia during a 2-day period in the spring. Ammonia was collected using acid traps at eight levels between 0.05 and 1.71 m and exposed for 2 hours. The midday net emission rate was =0.8 Jlg m-2 s-1 justabove the soil surface and =0.06 Jlg m-2 s-1 above the 0.7-m-high canopy. In contrast, an adjacent grazed pasture (50 sheep ha-l) released about 0.36 Jlg m-2 s-1 to the atmosphere during the same period. An earlier study [Denmead et al., 1974] at the latter site found fluxes of0.84 and 0.22 Jlg m-2 s-1 for fall daytime and nighttime measurementperiods, respectively, ora daily mean of0.30 Jlg m-2 s·l.

Harrison etal. [1989] determinedNH3 fluxes from a number ofgrass and crop-covered soils in England. Gradients

were determined from filter packmeasurements (2 to4 hour exposures) at four levels between the surface and 3 m. Gaseous NH3 was collected on phosphoric acid impregnated paper filters, HN03 was collected on nylon filters, and particulate species were collected on Teflon filters. Great care
was taken to account for the sometimes substantial deviations due to the reactions ofNH3 with acidic species. The emission rates from grass-covered soils ranged from -0.03 to 0.11 Jlg m-2 s-1 with no obvious difference between mown and unmown (5-10 em canopy height) surfaces. Diurnal patterns were not determined, but the measured fluxes exhibit

a clear seasonal dependence with the largest emissions during the summer and fall and smallerpositive or even negative 

# .... :I: 

### z + fluxes in the winter (cf. Figure 6). The annualized daytime

mean emission rate for grass-covered surfaces was =0.02 Jlg m-2 s-1 with a mean rate of=0.05 Jlg m-2 s-1 in the summer and fall.**:~:"'** 0.10 

# •**~** :I: 

### z 

# •

•

0.01 +------,.....l..--...,------,---"'T'""------:j 0 50 100 150 200 250 

### so4= (nmol m"

### 3 ) Fig. 5. Dependence ofgaseous/particulate partitioning of ammonia on total atmospheric sulfate. Each point refers to

one ofthe entries in Table 2. The open and closed symbols representwinter and summer values, respectively.

### 468

0.10

0.08 

## .,

E 

## "' .;!,

)( ::J ;;:**~** :1: z

c


# .. ., :;

Month



### Fig. 6. Seasonal variation ofammonia fluxes determined by Harrison etal. [1989] at various grassland sites in England.

Two recent studies by Langford and Fehsenfeldin 1988 and by Zachariassen and Schimel in 1989 examined the gaseous NH3 

### fluxes ata semiarid shortgrass steppe in the Pawnee National Grasslands ofnortheastern Colorado. This site is located on the U.S. DepartmentofAgriculture (USDA) operated Central Plains Experimental Range L TER. The predominantly blue grama ground cover is quite sparse with a
mean canopy height ofonly =0.15 mand frequent patches of exposed soil. The 1988 measurements were made using a molybdenum oxide annular denuder system at 0.5 and 2.0 m [Langford and Fehsenfeld, 1988]. A total of881 half-hour averaged gradientmeasurements were made almost continuously over a 3-weekperiod in June and July. Figure 7a shows atime series for the daily averaged fluxes calculated from the measured gradients and micrometeorological measurements ofheat and momentum flux provided by K. F. Zeller and W. Massman ofthe USDA Forest Service [Massman et al., 1990]. Net emission was observed on only 7 out of24 days, mostly during the first week ofthe study. A strong diurnal pattern was observed with mean emission rates ofabout 0.20 Jlg m-2 s-1 during the afternoons followed by deposition rates ofapproximately -0.10 Jlg m-2 s-1 during the nighttime and early morning hours (Figure 7b). The average (net) flux over the course ofthe study was -0.02 Jlg m-2 s-1 with daytime emissions almost exactly offsetby nocturnal deposition.

The 1989 study at 

### Pawnee by Zachariassen and Schimel [1991] was designed to probe the role ofthe plantcanopy on NH3 

### emissions. Measurements were made on eight days during four different parts ofthe growing season: green up (mid-June), peak

### biomass (late-June), dry 

### down/senescence (early-July), and postsenescence (mid-to-late September). Gradients were determined from acid traps 

### exposed for 2-hour periods at 

### 1and 3 m. The diurnally averaged mean flux rates observed during these periods were 0.004, -0.012, 0.007, and 0.034 Jlg m-2 s-1, respectively. Patterns ofnocturnal deposition and daytime emission consistent with those in Figure 7b were indicated for most ofthe sample periods. Langford et al.: AmmoniaFluxes and Background Concentrations

Zachariassen and Schimel conducted a similar study during the



### summerof 1987 at

### the Konza Prairie Research Natural Area in eastern Kansas. The 

### big bluestem, indiangrass, and switchgrass tallgrass prairie canopy at Konza is much denser than thatatPawnee. The NH3 

### concentrations were measured 1and 10 mabove the soil surface. Samples were collected on 8days encompassing green-up, peak 

### biomass, dry 

### down and
senescence, and postsenescence. The pattern was similarto that 

### observed atPawnee with diurnally averaged mean flux rates of0.003, -0.017, 0.005, and 0.041 Jl& m-2 s-1, respectively, for the four periods. Several studies have examined the effects ofgrazing and fertilizer addition on the NH3 

### flux from grassland soils. A



# ..

E 

# "' 0.2

0.1

.;; 0.0

)( ::J 

### u::



### ... ::r::**~.1** 

### z a; z (a).

CftO'f""NM.IftCD,.._COQtO.-NC"I M---------.,....,....,....,....,....,....,....,....,....,...NNNN**-~~~~~~~~~-------------­** CD**~~,.._,.._,...,...,.._,...,...,...,...,...,...,...** Date

(b).**~~-------------------------------.**



# ..

E

::r:: 

### z 

# "' E.. 200

100

o l 

### T 

## + r..l...U.. 

# 'T' 

# +o+++•

-1oo

# ~.fTTTT

-200

0 1 2 1 6 20 24

Local Standard Time 

### Fig. 7. (a) Time series ofdiurnally averaged ammonia fluxes at the Pawnee Grasslands in northeastern Colorado during July of 1988. {b) 

### Diurnal variation ofthe mean flux over the sampling period. The vertical bars 

### represent the 

### standard deviation ofthe mean.

### Langford et al.: AmmoniaFluxes and Background Concentrations

comprehensive year long study ofNH3 fluxes from a lightly grazed (4 beefcattle ha·l) tropical pasture in Queensland, Australia was described by Harper 

### and coworkers [Harperet al., 1983a,b]. Profiles were determined from measurements at 2 to 

### 5 levels between 0.1 and 0.8 m above the soil surface. NH3was collectedusing acid traps exposed for 2-hour intervals. The pasture was heavily fertilized four times per year with 374 kg N ha·l as urea, but the soil pH**and~+]** 

### levels (and NH3fluxes) returned to nearbackground levels within =4 

### days after spring and fall applications. Diurnal cycling was then observed with much larger fluxes during the day than 

### the night. Net deposition typically occurred during spring and summer nights, while small emissions**(~0.02 ~g**  

### m·2 s·l) were sometimes observed during fall and winter nights. Shortperiods ofdeposition around sunrise and sunset

were observed year-round but were most common during

summer and fall. During the summer, NH3 flux was correlated most strongly with soil temperature, wind speed, 

### and evaporation in that order. During the rest ofthe year, the correlation with evapotranspiration was strongest. Ranges for the net background fluxes were =-0.2 to 0.2, -0.02 to 0.6, 0.06 to 0.8, and 0.2 to 0.3**~g** 

### m·2 s·l during spring,
summer, fall, and winter, respectively.

The grasslandresults are consistent with measurements made by Harperet al. [1987] above a wheat crop grown in northeastern Georgia. Substantial NH3 emissions (=30% of the applied nitrogen) occurred immediately following the application ofammonium nitrate fertilizer, but decreased with time and the plant-soil system became a net sink for NH3 (=-0.06**~g** 

### m·2 s·l) within 30 days ofapplication. The system became a net source again (=0.15 11g m·2 s·l) when the plants reached anthesis and continued through fmal

senescence ofthe leaves, a total period of=40 

### days. More recent studies (L. 

### A. Harper et al., Ammonia transport in a temperate grass system: Seasonal transport in relation to fertility and crop management, submitted to

### Agronomy Journal, 1993a; L. A. Harper 

### etal., Nitrogen cycling in a clover system: Soil, plant, and atmospheric transport, submitted to

### AgronomyJournal, 1993b) show that net deposition also predominates in temperate ryegrass and clover fields once fertilizer inputs are immobilized.

The gradientprofile technique is much more difficult to apply in complex terrain [Fowlerand Duyzer, 1989] and
measurements from forest soils are virtually nonexistent. However, Baldocchi and Meyers [1991] recently described the 

### special requirements for gradientprofile measurements within

an oak-hickory stand in the

### WalkerBranchWatershed near Oak Ridge, Tennessee, and the molybdenum oxide annular denuder system and gradientprofile method was used to

measure the flux ofNH3 there during Septemberof 1988 (A. 

### 0.Langford etal., unpublished results, 1992). Walker Branch (mean elevation =300m) is administered by the Department ofEnergy and was one ofthe sites chosen for the United States/International Biological Program Eastern Deciduous ForestBiome Project supported by the NSF. Johnson and van Hook [1989] have reviewed the biogeochemical cycling atthis site in detail. More than 400 half-hour averaged flux measurements were made over a 3- weekperiod during the summer of 1988. The results ofthese flux measurements are shown in Figure 8. The analysis suggests a pattern similar to that at Pawnee but with very small average emission rates**of~.001 ~g**  

### m·2 s·l during the 

## "'

E

::t: 

### z

Cl 

### .s

)( :I 

### u:: 469


# .f 

# ftt' f f
·1

·2

·3 0 4 1 2 1 6 20 24 Local Standard Time 

### Fig. 8. Diurnal variation ofthe ammonia flux measured above the surface ofan oak-hickory forest within the Walker Branch watershed near OakRidge, Tennessee, during October 1988. The vertical bars represent the standard deviation ofthe

mean.

afternoons followed by deposition rates of=O.OOWg m·2 s-1 during the nighttime and early morning hours. The average (net) flux over the course ofthe sampling period was indistinguishable from zero.

6. MODELING AMMONIA EMISSIONS AND BACKGROUND CONCENTRATIONS IN TERRESTRIAL ECOSYSTEMS

The flux studies described in section 5.2 suggest that most ofthe emissions ofgaseous NH3from terrestrial ecosystems

are offserby local deposition. However, net emissions may
occur under some conditions and the development ofa simple model for predicting the spatial and temporal variability of NH3 emissions and background concentrations is ofinterest for regional airquality and acid deposition models. Several mechanistic models have been developed to estimate NH3 emissions from various soil types [e.g

.. Parton, 1981; Schimel et al., 1990; Fleisher et al., 1987]. Most ofthese models require detailed knowledge ofsoil parameters that are extremely variable and typically unknown for natural ecosystems and are thus not easily extrapolated to large areas. Dawson [1977] circumvented this limitation by relating soil parameters to more readily available climactic information and developed a model thatpredicts emissions from undisturbed soils comparable to those from anthropogenic sources. In this section we examine the soil emissions model developed by Dawson and the prospects for developing similarmodels to predictbackground concentrations ofNH3 in terrestrial ecosystems.

6.1. 

### Processes Controlling Ammonia Exchange

Dawson and others have described the emission ofNH3 from soils in terms ofa simple equilibrium model using a bulk transfer expression ofthe 

### form470

(3)

Herep(NH3)5 is the equilibrium vapor pressure ofNH3in the soil 

### pores andp(NH3)8 is the partial pressure ofNH3 in the atmosphere. The exchange coefficient

### ksa describes the efficiency oftransport ofNH3 through the soil to the surface, through the laminarboundary layer above the soil, and away from the surface. Apositive flux (i.e., F>O) corresponds to emission and occurs whenever the partial pressure in the soil exceeds that in the atmosphere. Similarly, the soil will absorb ammonia when the atmospheric partial pressure exceeds the equilibrium vaporpressure above the soil solution.

Theequilibrium vaporpressure ofNH3above the soil solution is related to the dissolved ammonia concentration by Henry's law

(4)

where [NH3] 5 is the ammonia concentration in the solution, 

### R is the ideal gas constant, and 

### KH is the Henry's law coefficient. Since dissolved ammonia is partitioned according to the equilibrium

(5)

[NH3] 5 will depend on both the total ammoniacal nitrogen and the pH ofthe solution. The NH3fNH4+ratio changes markedly with pH and is virtually zero (S 0.001) in solutions with pH S 6 but approaches one**(~** 0.90) in solutions with pH**~** 

### 10. Both the equilibrium constant for (5) and the Henry coefficientvary with temperature [Dasgupta and Dong, 1986; Emerson et al., 1975]. Hence (5) can be expreSsed in terms ofthe dissolved ammonium concentration**~+**] 5, pH, and temperature ofthe solution
p(NH3)8 

### = 1.325 x 104**(~+] /[H+]** **)** exp(-10,378/D (6)

Equation (6) shows that the equilibrium vaporpressure increases with the solution**[N~+**],pH, and T and predicts an approximate doubling ofthe NH3 vaporpressure for every 5•cincrease in temperature and a tenfold increase with every unit increase in pH.

Qualitatively, (6) implies that soil emissions ofammonia should be greatest from neutral to alkaline soils in warm climates with moderate soil moisb.Jre and adequateporosity that have a steady supply ofammonium provided by rapid turnover oforganic nitrogen (i.e., grasslands and savannahs). Emissions from the relatively acidic soils offorests shouldbe negligible, despite the abundance oforganic nitrogen, while emissions from the alkaline soils ofdesert regions will be limited by the lack ofsoil moisture and the availability of ammonium [Schlesinger and Peterjohn, 1991].

6.2. The Soil Emissions Model of Dawson

The actual calculation ofNH3emissions from natural soils is complicatedby the spatial and temporal variability ofthe pH and**~+**]5 ofthe soil solution and the complex nature ofammonia transport through the soil and subsequently into Langford et al.: Ammonia Fluxes and Background Concentrations

the air [Milchunas et al., 1988]. The populations ofboth decomposing microflora and nitrifying bacteria, and, hence, ammonium levels, depend on the availability oforganic nitrogen, soil moisture, and temperature (Schimel et al., 1986; Haynes, 1986; Paul and Clark, 1989). Soil ammonium may also be taken up (and released) by growing plants, immobiliZedby organic matter, and chemically or physically adsorbed by minerals in clays. In very dry soils, ammonia may be trappedby sites which normally adsorb water. Transport ofammonia through the soil by gaseous diffusion 

### orby convection and liquid diffusion ofdissolved ammonia will be inhibited in waterlogged soils. Finally, significant volatilization requires 

### a low ambientpartial pressure ofNH3 and sufficient wind to transport the NH3 away from the surface to maintain a driving gradient

Dawson [1977] empirically related soil ammonium concentrations to available biomass and the relative rates of decomposition and nitrification. The latter were derived as general functions oftemperab.lre and soil moisture. Evapotranspiration was used 

### as a surrogate forbiomass and soil pH was correlated with annual rainfall. Finally, 

### a soil water dependent gaseous diffusion coefficient was estimated to calculate the emission rates using (3) and the assumption that p(NH3)s >> p(NH3)8 • The model predicts substantial emissions from soils with a maximum at mid-latitudes, negligible polar(>70° latitude) emissions, and lower tropical emissions. The average mid-latitude emission rate of

..0.05 JJ.g m-2 s-1 implies a net annual emission ofabout 4 Tg NH3 

### yrl for the conterminous United States.

Although, the measured midday grasslandemission rates described in section 5.2 are similar to the average mid-latitude emission rate of==0.05 JJ.g m·2 s·l calculated by Dawson, the model implies similar emission rates for forest soils. The model also neglects deposition and canopy uptake and thus predicts emission under all conditions in contrast to the observations. This follows from the assumption that the vaporpressure ofNH3 in the soil always exceeds the atmospheric partial pressure. Dawson noted that the steady state expression 

### used toestimate soil NH4+ was based on measurements from forest soils and underpredicted grassland**~+** levels by about a factor of20. In addition, he was well

aware that the relationship between precipitation and soil pH ignores the influence offactors such as parent material and vegetation type and that arainfall/pH relationship derived from measurements in California and India would not necessary reproduce the soil pH distribution ofthe United States. Dawson's model leads to ecosystem independent

summer**~+] 5**  values of3-5 mM, and mean soil pH values of7.0 and 7.1, respectively, for the 30°-40°N and**40°-50~** belts that contain most ofthe United States. Together with (6), these pH values and**~+] 5**  values imply soil NH3 partial pressures of

..250 ppbv at 20°C, well in excess of ambient concentrations therebyjustifying the assumption that the soil is always a source ofNH3

. However, Hanawalt [1969] observed large soil uptake for atmospheric concentrations as small as 40 ppbv.

Aquantitative test ofthese assumptions can be made using parameters determined from bulk soil samples (==10 cm3) collectedby Williams and Fehsenfeld (1991) ofthe Aeronomy Laboratory at the grassland and forest sites listed in Table 3.Langford etal.: Ammonia Fluxes and Background Concentrations 471

TABLE 3. Calculated Soil Vapor Pressures, Measured Ambient Partial Pressures, and SoilEmission Rates Based on Bulk Soil Samples (200C)



### lNl4tl. Site Ecosystem Type g/kg

Pawnee, Colo. semiarid grassland 3.5 ± 2.0 Boulder, Colo. temperate grassland I4.0 ± 7.0 Niwot Ridge, Colo. montane coniferous I7.3 ± 7.I Kinterbish, Ala. temperate coniferous 5.6 ± 3.1 OakRidge, Tenn. temperate deciduous I7.9 ± 8.5

These samples were analyzed for pH (I:I paste), exchangeable 

### NJ4+ (2 M KCl extract), and otherparameters by the Soil Testing Laboratory ofColorado State University. These results 

### are used along with field measurements ofsoil moisture and temperature to estimate the soil vaporpressure using (6). Exchangeable 

### NRt+ (i.e., the ammonium remaining on 

### the cation exchange complex (CEC) when the soil solution evaporates) was related to 

### £NH4+

### ls using the nonlinearFrewtdlichrelationship ofCameron and Kowalenko [1976]. The calculated mean ammonium concentrations range from 1.2 to 2.4 mM or within a factor of2 ofDawson's estimates.

Amuch largerdiscrepancy arises between the estimated and measured soilpH. The values assumed by Dawson (pH =7)


### are not unreasonable for grassland soils which typically range from

..

### 5.5 to 8.5 but

### are much larger 

### than the typical range of p(NHJ)s, p(NHJ)•• Flux, pH ppbv ppbv ng NH3 m-2s-l

6.05 ± 0.44 II± I8 11.8 ± 4.1 0.03 ± 0.6 5.90 ± 0.22 33 

### ± I5 4.4 

### ± 1.9 1.0 ± 0.5 5.22 

### ±0.38 4.4 ± 3.6 0.62 ± O.I9 O.IO ± 0.09 5.28 

### ±0.26 1.2 

### ±0.7 0.42 ± 0.32 0.04 

### ± 0.05 4.71 

### ±0.26 1.7 

### ± 1.9 0.30 ± O.I5 0.02 

### ± 0.02

forest soil pH**(~.5** to 5.5). The measured pH values in the present study (4.7 to 6.I) 

### are consistently lower

### than those predicted using Dawson's empirical relationship by

.. 0.85 pH uniL ApH increase of0.85 leads to a factor of7 increase in NH3 vaporpressure while a pH increase of2 corresponds to 

### a factor ofIOO increase. Thus Dawson's model underpredicts soil [H+] (and hence overpredicts the soil vaporpressure) for grasslands by about a factorof7 and forests by a factor of40 to400. The values of

### £NH4+]1 and pHdetermined from the soil samples give soil vaporpressures of•I and

..35 ppbv at 20"C for the forest and grassland sites, respectively, which 

### are only slightly larger than the ambient mixing ratios (Figure 9). Table 3 shows the mean fluxes calculated for those five sampling sites using the calculated soil equilibrium vapor pressures, themeasured atmospheric partial pressures, and the gaseous diffusion limited exchange coefficient used by Dawson, 

### are S0.001 JJ.g m-2 s-1.

100.0,-------------------:-1 6.3. 

### The Role 

### ofVegetation in Ammonia Exchange



### >" .D Q. 

## .s.



# ... ::c 

### z 10.0

1.0 0 •

. I ; • I I

• 0 0
• I : •



# . :.

o I o 0 0



# . .

# . 

# .: Pawnee

0 0 0**~t** 0 0 I 1 I I o o

0 0

# t

0

-:

I I

• • Boulder

o o**~** Niwot Ridge

0

# -~ 

# +

Klnterblsh oak Ridge 0.1 +---,---,----..-,----..-.---.--.----..--;

4.00 4.50 5.00 5.50 6.00 6.50 7.00

Soil pH Fig. 9. Calculated soil vapor pressures (points) and mean ambient partial pressures ofNH3 measured at Pawnee, Boulder, NiwotRidge, Kinterbish, and OakRidge (large solid circles). The horizontal bars show the span ofpH values at each site. The vertical bars show the standard deviation ofthe

mean. The most serious limitation ofDawson's model is the omission ofcanopy effects. Vegetation is now known to play an importantrole in ammonia volatilization [Denmead et al., 1976; Lemon and Van Houtte, 1980]. Decaying vegetation is theprimary source ofthe organic nitrogen mineralized to ammonium in native soils and the above ground biomass may lose NH3 directly to the atmosphere following proteolysis [Farquharet al., 1983; Whitehead and Lockyer, I989]. Uptake ofgaseous NH3 by growing plants is well documented [Aneja etal., 1986; Denmead et al., 1976; Hutchinson etal., I9n;Lockyer and Whitehead, I986; Van Hove etal., I987, 1989], and several studies [Denmead et al., I982; Farquhar et al., 1980; Lemon and Van Houtte, I980; Sinclairand Van Houtte, 1982] have shown that growing vegelation will emit NH3 when exposed to air with concentrations below some "compensation point" thought to be related to the partial pressure ofNH3in the substomatal cavities.

The processes controlling the concentration of

### NRt+ within the cells ofgrowing plants is extremely complicated and the dependence ofthecompensation point on species and growing conditions ispoorly understood. Typical values appear to be ofthe orderof**~I** ppbv for trees [Langford and Fehsenfeld, I992],

.. 3-5 ppbv for herbaceous crop plants [Dabney and Bouidin, I990; Farquharet al., I980; Lemon and Van Houtte, I980],·and**~IO** ppbv for wheat and perhaps other

### 472

grain filling grasses [Harpereta!., 1987; Lemon and Van Houtte, 1980; Morgan and Parton, 1989; Parton et al., 1988]. The compensation point has an exponential temperature dependence consistent with the presence ofa steady state concentration ofdissolved ammonia in the waterfilm present

on the mesophyllic surfaces ofthe substomatal cavities [Farquhar etal., 1980]. The compensation point is hypothesized

### tO vary as a function ofthe balance between 

### NJ-4+ releasing reactions (deamination, nitrate reduction, and senescence-inducedproteolysis) and 

### NJ-4+ consuming reactions (N transport, assimilation via glutamine synthetase). The balance shifts to the former with increasing plant maturity leading to higher intracellularNH3 concentrations [Morgan and Parton, 1989; Parton eta!., 1988].

The flux studies described in section 5.2 suggest that NH3 exchange above grasslands is controlledprimarily by canopy uptake and emission during the growing season, and that substantial net emissions are likely only during anthesis, flowering, and senescence ofthe canopy. Similarconclusions have been drawn from a number ofstudies in agricultural systems [Dabney and Bouldin, 1990; Lemon and Van Houtte, 1980; Sinclair and Van Houtte, 1982]. Denmead et al. [1976] found that the emissions were much largerjust above the soil surface(= 0.8 Jlg m-2 s·1) than above the 0.7 m high canopy (= 0.06 Jlg m-2 s·1) indicating that virtually all ofthe NH3 emittedby the soil was reabsorbed by (orredeposited on) the canopy. This result is not unreasonable given the large surface area ofa dense canopy compared to the underlying soil, the corresponding lack ofventilation near the soil surface, and the fact that the transport ofgaseous NH3 through even partially closed stomata is probably much faster than transport through the soil.

6.4. EquilibriumModeling of Background Concentralions

The equilibrium nature ofthe exchange process implies not only that net NH3 emissions from terrestrial ecosystems will generally be small but also that the background concentrations ofgaseous NH3 in many ecosystems should approach the characteristic temperature-dependent equilibrium vapor pressure ofthe soil or compensation point ofthe canopy. Thus it shouldbe possible to use an approach similar to Dawson's to predict ambient concentrations. The general nature ofthis phenomenon is suggested by the similar temperature dependences ofambientNH3 at the different sites shown in Figure 4. This conclusion is also supported by Figure 9 which shows that the calculated vaporpressures at the Boulderand Pawnee grassland sites are very similarto the measured mean atmospheric partial pressures. The ambient mixing ratios at the forest sites are significantly lower than the soil vapor pressures, however, suggesting that the forest canopy compensation point is lower than the soil vapor pressure. The canopy shouid be particularly important as an NH3 regulator in forests and other ecosystems with dense vegetation. This hypothesis is supported by measurements in

a montane/subalpine forest lying between Boulder and Niwot Ridge, Colorado [Langford and Fehsenfeld, 1992], which showed that the ambientconcentration ofNH3 near the top of the canopy approached the same temperature-dependentvalue when exposed to air containing NH3 concentrations differing Langfordet al.: Ammonia Fluxes and Background Concentrations

by more than a factor of20. It seems likely that the temporal variability ofambient NH3 background concentrations can be representedby the temperature dependence for modeling purposes. However, development ofa general model will require additional measurements to identify appropriate surrogates for the spatial variability. Atpresent, we infer typical (to within a factor oftwo) background values of=10 ppbv for grasslands, 0.5 ppbv for forests, and I ppbv for arid lands at 20'C.

7. SIGNIFICANCE OF AMMONIA EMISSIONS FROM TERRESTRIAL ECOSYSTEMS IN THE UNITED STATES

7.1. Magnitude of Natural AmmoniaEmissions

The measurements ofNH3 concentrations and fluxes presented in the preceding sections suggest that the net emissions ofNH3from undisturbed terrestrial ecosystems are relatively small. Upper limits for these emissions from different ecosystems ofthe United States can be estimated using equation (2) and the background concentrations from Table I. Galbally [1975] used this approach to estimate the total flux ofNH3 to the atmosphere from land and ocean surfaces in the northern hemisphere assuming that the NH3 concentration decreases by 50 to 90% between I mand 2km, and long-term climatological average eddy diffusivities of 103 cm2 s-1 at I m, lOS cm2 s-1 at 200m, and 2 x lOS cm2 s-1
at 2 km. A total northern hemisphere flux of 130 Tg N yrl

was obtained using constant surface mixing ratios of7, 18, and 4 ppbv for temperate, tropical, and oceanic regions. In this section, we revise these calculations using more representative NH3 concentrations and estimate seasonal and diurnal effects.

The results tabulated in Table I suggest mean daytime surface summertime mixing ratios of 1, 15, and 0.5 ppbv are representative ofdeserts and shrublands, grasslands, and forests, respectively. Nighttime mixing ratios are set at 50% ofthe daytime values, and wintertime mixing ratios at 50% ofthe corresponding summertime values for the calculations. Itis assumed that gas-to-particle conversion and cloud scavenging reduce the mixing ratios to = 0by I km and longterm average eddy diffusivities between the surface and 1km

are assumed to be 2 x lOS cm2 s-1 and 1 x 105 cm2 s-1 for daytime during the summer and winter, respectively (E.-Y. Hsie, private communication, 1991). Since stable nocturnal inversion layers are often formed, the mean nighttime eddy Jliffusivities are assumed to be=25% ofthe daytime values. This leads to estimated mean summer daytime emission rates of

.. o.ot5, 0.2, and 0.007 Jlg NH3 m-2 s-1 for deserts, grasslands, and forests, respectively. The grassland rate compares favorably to the mean value actually observed at Pawnee. Wintertime emissions rates are 25% ofthe daytime rates, and nocturnal emission rates are

..12% ofthe corresponding daytime rates. Since deserts, grasslands, and forests occupy 1.6, 0.95, and 2.3 x 106 km2 in the United States [Lamb et al., 1987; Olson eta!., 1980], this estimate yields annual emission rates of=0.2, 2.2, and 0.2 Tg NH3 yr1 for deserts, grasslands, and forests, respectively, with two thirds ofthe emissions occurring during summer days.

The above calculations suggest that grasslands are the only

### Langford et al.: AmmoniaFluxes and Background Concentrations

natural ecosystems likely to be a significant source of atmospheric NH3 in the United States. However, the few available directflux measurements suggest that much ofthe NH3emittedby unfertilized and lightly grazed

### grasslands around midday during the warmer months is reabsorbed during the night Assuming a more**~nable** 

### net emission rate of 0.2 Jlg m-2 s-1 for =3 hours per day over a growing season of 100 days reduces the total emissions from U.S. grasslands to only 0.2 Tg NH3yrl. The actual period with net emissions is probably even shorter spanning the period

### between anthesis (flowering), seed

### production, and fmal 

### senescence and subsequentdecomposition ofthe canopy. The Pawnee 

### and Konza studies by Zachariassen [1991] imply netemissions of0.67 and 0.74 kg ha-t during this period for the (temperate tallgrass) Konza and (semiarid shortgrass steppe) Pawnee 

### grasslands, respectively. Extrapolation overthe entire United States yields total grassland emissions ofS0.07 Tg NH3 yr1.

Undisturbed forest ecosystems should emitmuch less NH3

. This situation is perturbed, however, by the addition of

an atmospheric sink for gaseous NH3 in the form of unneutralized sulfate aerosols. As 

### was pointed outby Galbally et al. [1982], the extremely low vaporpressure of gaseous NH3 above unneutralized sulfate aerosols depresses the ambient partial pressure below the compensation point leading to net emissions from 

### the canopy. These processes will be particularly important in the eastern 

### United States where high levels ofunneutralized sulfate 

### are typical 

### (cf. Table 2). Under these conditions, the ambientpartial pressure ofgaseous NH3 above the canopy shouldreach a new "quasi-equilibrium" or steady state value intermediatebetween the compensation point and the equilibrium vaporpressure above the aerosol. Thus forests in the eastern United States



### are probably a net source for gaseous NH3. Similar arguments apply to grasslands exposed to unneutralized sulfate, but most grasslands 

### are located west ofthe main sulfate source regions ofthe United States. 473

An upper limit for the emissions from forests can thus be calculated 

### from the productofthe canopy compensation point (y} 

### and the stomatal conductance (g.). 

### Assuming a representative value ofg5 "' 

### 0.17 mol m-2 s-1 for fully open stomata (equivalent to a deposition velocity of0.4 em s-1 at
sea 

### level) and a mean compensation point of=1ppbv or =40 nmol m-3, implies a volume emission rate of

.. 0.15 nmol m-2 s-1 or 

### =3 Jlg m-2 s-1 at 25 'C. Annual average forest emissions 

### are calculated assuming mean flux rates of3.5 and 1.5 ng NH3 m-2 s-1 for summer and winter days, respectively, where the seasonal difference in emission rates reflects the temperature dependence ofthe compensation point. Winter emissions 

### are probably much smaller, particularly in deciduous forests. Nighttime emissions are estimated to be =10% ofthe daytime rates since stomata typically do not close completely. Since forests occupy 2.34 x 106 km2 or 31% ofthe total land area 

### in the United States, these rates lead to an annualized forest emission of=0.1 Tg NH3yr-1, similar in

### magnitude to the grassland emissions.

7.2. 

### Comparison to Anthropogenic Emissions

Ammonia emissions from livestock wastes, fertilizer production and 

### use, and other anthropogenic sources have been tabulated 

### in detail for the United States [Harriss and Michaels, 1982; Heisler etal., 1985; U.S. EPA, 1989; Wagner etal., 1986] and Europe [ApSimon et al., 1987; Buijsman et al., 1987]. In addition, the Environmental Protection Service (EPS) has

### developed an inventory [EPS, 1985] for Canada. The U.S. inventories 

### are summarized in Table 4. Cattle and other livestock wastes account for 50 

### to 80% ofthe total estimated anthropogenic ammonia emissions in all ofthese inventories.

Total emissions in the published U.S. inventories range from 0.76 to 10.8 Tg NH3yr1. The inventories designated

as National Acid Precipitation AssessmentProgram

TABLE 4. Comparison ofAmmonia Emissions Inventories for the United States

Emissions (Tg NH3 yr1)and Percentage ofTotal

Livestock Combustion Industry Fertilizer application

and land surfaces Wastewater



### Totalc 1980 NAPAP Inventory

0.49

0.23 0.05•

0.76 (64%)

(29%) 

### (7%) 1985 NAPAP Inventory

1.1 

### 0.07 0.25 0.05• (71%) (5%) (16%) (3%)

0.07 (5%)

1.5



### •Includes only the application ofanhydrous ammonia fertilizer. bJncludes emissions from undisturbed land surfaces based 

### on Dawson model. cValues may not sum to totals 

### due to independentrounding. EPRI Inventory

5.5 (50%) 

### 0.7 (6%) 

### 0.02 4.61> (42%)

10.8 

### NASA Inventory

2.1 (62%) 0.7 (21%) 0.2 (6%) 0.4 (12%)

3.4

Sources: Placetetal. [1990], Wagner et al. [1986], U.S. EPA [1989], Heisler et al. [1988], and Harriss 

### and Michaels [1982].474

(NAPAP) [U.S. EPA, 1989; Wagner et al., 1986] and Electric PowerResearch Institute (EPRI) [Heisleretal., 1985] included detailed temporal and spatialresolution for the United States and partSofCanada. Most ofthe disparity results from large differences in animal emission factors and the included source categories. Thus the EPRI inventory is largerthan the NAPAP inventories primarily because the EPRI emission factors for cattle are about 3 times higher than those used in the 1985 NAPAP inventory (40 kg animal-! yr-1 versus 12kg animal-! yr-1) and 7 times higher than those used in the 1980 NAPAP inventory. The EPRI inventory also includes a large emission ofammonia from both croplands and unfertilized land surfaces such as pastures and rangeland.

These disparities reflect the uncertainties inherentin extrapolating a small number oflocalized measurements to large scales and the dependence ofNH3volatilization on environmental effects such as temperature and humidity, as well as animal type, packing density, and surface. Although the emission factors used in the NAPAP inventory were ' chosen based on a careful evaluation ofthe earlier inventories and the reliability ofthe available data bases [Placet, 1990; Warn etal., 1990], the associated uncertainties lead to a low confidence rating from NAPAP. Despite the wide range of values, these inventories suggest that NH3 emissions from grasslands and forests in the United States will be relatively minor compared to the anthropogenic emissions associated with agricultural sources.

7.3. Comparison to Emissions From Biomass Burning

The emission inventories described above do not include NH3 production by biomass burning. Andreae [1991] estimates that biomass burning is responsible for "'12% ofall ammonia emissions on a global basis with most of these emissions are associated with the clearing oftropical rainforests. In the United States, biomass burning is associated primarily with agriculture (e.g., annual burning of grasslands [Hobbs et al., 1991] or crop stubble), as a forest managementprocedure, or wildfires due to natural causes (i.e., spontaneous combustion or lightning initiated) oraccidentally man-caused. According to Andreae [1991], "'80 and 250Tg of drymatter are consumed in the United States and Canada each year through the burning offuel wood and agricultural wastes, respectively. Wildfires probably consume an additional "'100 Tg dm yr·1 in the United States [Logan, 1983; Yamate, 1975]. The relative emissions ofNH3 and**C~** have been examined for a variety offires, and most studies have found**NH /C~**  emission ratios between "'0.1 and 3 mmol mol-l for both wetland and forest fu-es [Griffith etal., 1991] found a much larger value of"'8 mmol moJ·1 for burning sagebrush). Using Andreae's assumptions (cf. section 3) ofa mean emission ratio of 1.3 mmol moJ-1, a carbon content of45%, and that 88% ofthe carbon is volatilized as**C~.** we can estimate that 

### ,5 x 10-4 gm ofnitrogen is emitted as NH3 for each gram ofdry matterburned. Alternatively, we can use the estimate ofLobert et al. [1991] that an average of4% ofthe total N consumed in biomass burning is released as NH3 with

a typical nitrogen content of0.6% for the biomass burned in mid-latitude fu-es [Logan, 1983] to obtain a slightly smaller value of"'2.5 x 10-4 gm/gm. Thus we estimate an upper Langford etal.: Ammonia Fluxes and Background Concentrations

limit of"'0.2 Tg NH3 yr-t for NH3 volatilization from biomass burning in the United States.

7.4. Comparison to Ammonia Deposition

Although itis not possible to separate the contributions of natural and anthropogenic sources, a lowerbound for the total NH3 emissions for the continental United States can be estimated from the amount**of~+** collected in wet deposition. Figure 10 shows the isopleths for**~+** concentrations in precipitation and the subsequent deposition

as determined by the National Acid Deposition Program/National Trends Network (NADP/NTN) [1988] for 1987. These data imply an annual wet deposition of approximately 1.5 Tg**~+** yr-1 for the conterminous United States. The spatial distribution reflects the presence ofstrong NH3 sources in heavily agricultural regions ofthe midwestern United States. The 1980NAPAP inventory places anthropogenic emissions atonly 50% ofthe measured wet deposition while the EPRI inventory (without soil emissions) is a factor offour larger. The 1985 NAPAP emissions are very similar to the wet deposition and the NASA emissions

are a factor of2 higher. Thus we can infer an absolute lower limit of"'1.5 Tg NH3 yr-1 for NH3 emissions in the United States.

Dry deposition inputs ofgaseous and particulate ammonia

are more difficult to estimate but are probably ofsimilar magnitude. Deposition ofexcess gaseous NH3to vegetation is very rapid with apparent deposition velocities ofthe order of2.0 em s-1 [Duyzer et al., 1987]. This quantity is extremely variable, however, and includes contributions from both stomatal uptake and cuticular adsorption when the foliage is wet [Van Hove et al., 1989]. The latter is enhanced when unneutralized sulfate and nitrate are present on the surface [McLeod etal., 1990]. Hanson and Lindberg [1991] recently compiled a comprehensive review ofdrydeposition velocities for ammonia and otherreactive nitrogen species and conclude thatrepresentative values for deposition ofexcess NH3 are 2.0and 0.5 em s·l for forest and crop canopies, respectively. Dry deposition velocities for**particulate~+** appear to be much smaller, ,0.1 to 0.2em s-t.

Arecent modeling study by Asman and Janssen [1987] suggests that 65% ofgaseous NH3deposition occurs within 

### 5 km ofagricultural sources and 90% within 30 km. A subsequentstudy by Asman and Jaarsfeld [1992] indicates that drydeposition ofgaseous NH3 and particulate**~**+is responsible for 44 and 14%,respectively, ofthe total ammonia deposition in western Europe. Washout ofNH3 accounts for only 6% ofthe total since below cloud scavenging ofgaseous NH3 is limited by the rate oftransfer

across the air-waterinterface and, as a consequence NH3is generally notin equilibrium with raindrops [Asman and Janssen, 1987];rainout**of~+** incorporatedinto cloud water accounts for 36%. Harrison and Allen [1991] estimated that dry deposition ofNH3 and**~**+accounts for 64 and 7%,of the total deposition in eastern England, with wet deposition responsible for the remaining 29%. These estimates are based

on gaseous and particulate deposition velocities of2.2 and 0.15 em s·1, respectively, and become 45%, 11%,and 44%, respectively, ifa gaseous deposition velocity of 1.0 em s-1 is assumed.Langford etal.: AmmoniaFluxes andBackgroundConcentrations 

### 475



### 0_1



## 0.2

1987 annual precipitation-weighted mean ammonium ion concentrations (mg/L).

Measured ammonium ion deposition (g/m2 ) 

### for 1987.

Fig. 10. Distribution of

### NJ4+ wetdeposition collected by the National Acid DepositionProgram/National Trends Network during 1987. (a) Sample ammonium concentrations in milligrams per liter. (b) Corresponding deposition in g m·2. Note the preponderance ofhigh ammonium levels in the centralUnited States.476

TABLE 

### 5. Range ofEstimated Ammonia Emissions for the Continental United States

Source

Livestock Fossil fuel combustion Industrial activities Fertilized cropland Biomass burning Native soils and vegetation

Total

Emissions are in Tg NH3 yr-1. Range

1.1 to 

### 5.5 0.07 to 0.7 0.02 to 0.25 0.05 to 0.4
s; 0.2
s; 0.2

1.2 to 7.25

Limits on the total emissions ofNH3 for the continental United States are presented in Table

### 5. Therange of emissions are extracted from the estimates in Table4 and the above discussions. Theseranges were also determined, when appropriate, using the more recent NAPAP 1985 [U.S. EPA, 1989] estimates in favor ofthe older NAPAP 1980 [Wagner
etal., 1986] estimates. Foreach ofthe source categories, the
range ofemission is large, often a factor of 10. Unquestionably, however, thegreatestemissions ofNH3 are associated with the raising oflivestock. The range ofNH3 emission from livestock sources encompasses the NAPAP 1985 [U.S. EPA, 1989] inventory estimate, 1.1 Tg NH3 yr-1, and the EPRI [Heisler et al., 1985] inventory estimate, 5.5 Tg NH3 yr-1. The sum ofthe sources provides a total range ofNH3emission for the United Statesbetween 1.2 IRl 7.3 Tg NH3 yr-1. The contribution from native soils and vegetation emissions is less than 20% ofthe total under the
most favorable circumstances.

8. NATURAL AMMONIA EMISSIONS AND ACID DEPOSmON

Ammonia influences acid deposition both as a neutralizing agentfor atmospheric aerosols and as a regulator ofcloud
waterpHand hence**S~** oxidation rates [Seinfeld, 1986; Behra etal., 1989]. Aircraft measurements by Tanner et al. [1984] show that the acidity ofthe atmospheric aerosol is generally much greater ataltitudes of 1-2 km than at the surface. This results from the emission of**S~** from tall stacks and the oxidation to sulfate occurring in cloud water, while ammonia sources are located on the ground. Thus precipitation is generally more acidic than the aerosol particles found near the surface. This can be seen from Figure 11, which shows the relative amounts ofNR.+ and so4= in both (1) precipitation and (2) particulates collected atthe various rural sites listed in Table 2. In these cases, the particulate phase is morenearly neutralized (1s;[NR.+]/[S04 =] s;2) than precipitation (fNH4+]/[S04=]**~.4).** However, as

was pointedoutby Husar et al. [1989], the [NR.+] to [S04=] ratio in precipitation samples collected in the heavily agricultural upper midwest is typically »2. Langford et al.: AmmoniaFluxes and Background Concentrations

(a).

50 

### , , Precipitation 

### , , ,

,

## , 

# • Summer 40 

### , 1:1 0 Winter 

### , 

## , , , , ,**~** 30 

### , 3 

### ,,

, , 

### , 

# •

, , l: 

### z 20 

## , ,

,

## , 

# • 0
,



## , 

# •

, 0 10 

### , 

# • 

# • 

# ,. , •
,

20 40 6 0 80 100



## so: (J.!M)

(b).

500 Particulates 

# • Summer 400 0 Winter

E 300 0 E 

## .s 200 

# •

+

... l: 

### z 100

50 100 150 200 250**so~** (nmol m· 3 )

Fig. 11. Relative concentrations of ammonium and sulfate in (a) precipitation and (b) aerosols at sites listed in Table 2.

Figures 12 and 13 compare the spatial and seasonal dependences ofthe mean cation and anion concentrations in precipitation collected at 70 NADP/NTN sites in 1987. The data havebeen plotted in 5° longitude bins for clarity. Figure 12a shows that acidic precipitation with H+ concentrations exceeding 10

## J.LM (i.e., pH<5) is typical in the eastern United States, particularly during the summer months. This distribution reflects the preponderance ofsulfuremissions in the east as is reflected in Figure 13a. Thelarge summer/winterdifference in the eastresults primarily from the much slower rate of**S~** oxidation during the winter [Calvert et al., 1985]. In contrast, nitrate concentrations (Figure 13b) are less variable due to the greater dispersion of the sources and the much faster oxidation rates. Figure 12b shows that**N~+** concentrations aregenerally greatest in precipitation collected in the midwest as wouldbeexpected given the large agricultural activity. However, there is very little seasonal variation in this region as mightbeexpected

### :E .z,

+ 

### ... :I: 

### z Langford et al.: AmmoniaAuxes and Background Concentrations

(a).

on 0 on 0 on**~ ~**  '? 

# .. ..

0 on 0 .;. .;



# .. 

# .. .. 

# .. ..
Longitude

(b).

30

20

10

0 0 on on on 0 on 0 

## ... 

# .. 0 0**~ ~**  

# .. '? ':" ";" ";" ";" 0 on .; on 0
0 on 0 

### "' 

# .. 

# .. .. .. 

# ..
.. 0 0 

### "' Longitude Fig. 12. Concentrations ofmajorcations, (a) hydrogen and (b) ammonium, in precipitation as a function oflongitude and

season.

from the temperature dependence ofammoniaemissions. This argues against large emissions from undisturbed grasslands and likely reflects the dominance offeedlot emissions since these often contain more animals during the winter than the summer months and high animal densities keep the surfaces clearofsnow. In contrast, the 

### NJ4+ concentrations in the eastarenotvery different from those in the midwestduring the summer months butare much lower during the winter. This could simply result from the seasonal variability oftransportpatternsbutcould alsoreflect the contribution offorest emissions.

Although the total emissions ofammonia from forests is small on a continental basis, the emissions may be 

### 477

significant in some parts ofthe east. Table 6 shows the forest emission rates calculated using (3) assuming p(NH3)a == 0 and the forested areaby state taken from [Olson etal., 1980]. Also shown are the state-averaged anthropogenic ammoniaemissionrates estimated for domestic animal wastes, fossil fuels, and fertilizers by Harriss and Michaels [1982]. In most states, the calculated forest emissions are only ==10% ofthe anthropogenic emissions. In Ohio and Pennsylvania, the forest contribution is <5% ofthe anthropogenic emissions. By contrast, in New Hampshire and Maine the forest emissions account for 20-40% ofthe total emissions and may play a major role in the neutralization ofatmospheric sulfate in this area. This contribution may beeven greater ifdeposition ofgaseous NH3 is faster than aerosol incorporation and mostofthe ammoniavolatilized from localized anthropogenic sources such as feedlots and fertilized fields is absorbed by the surrounding vegetation.

9. CONCLUDING REMARKS

Ammonia remains oneofthe mostpoorly characterized species involved significantly in processes related to acid deposition. This situation can beattributed to the historical lackofsuitable measurement techniques for gaseous ammonia, the rapid transformations and shortlifetime of
gaseous ammonia in the atmosphere, and the highly variable spatial and temporal nature ofammonia sources and sinks. 

### As pointed outby Harriss and Michaels [1982], the primary

reason for the poorstate ofknowledge is the lack ofhighqualitydata for eitheratmospheric concentrations or fluxes of ammonia. Recentdevelopments in ammoniadetection technology shouldimprove this situation, butthe new techniques are notyetperfected and are notuniversally available. Techniques to measureemission and deposition
rates mustalsobe developed further. However, it seems clear that wetordry deposition and aerosol incorporation will often limit the lifetime ofgaseous ammonia in the atmosphere to

oneday orless with long-range transport confmed to the particulatephase

.

Despite the limited number ofmeasurements, it appears that the native plant-soil system will be a net sink or at most

a relatively minor source ofatmospheric ammoniacompared
to anthropogenic emissions. Net surface emissions from undisturbed soils and vegetation are likely to be negligible in
most western United States forests andgrasslands despite the high ambient ammonia levels found in the latter. In parts of the midwestern United States with strong localized sources of NH3 associated with animal feedlots, the soil and vegetation in surrounding areas will be netNH3 sinks. In the eastern United States and southeastern Canada, both forests and grasslands willbenet sources ofNH3 since unneutralized sulfate aerosols create a net upward gradientnear the surface. However, the total emissions from these sources are probably**~.2** Tg NH3 yr-1, or :5;20% of the total ammonia emissions. Nevertheless, these emissions may significantly impact sulfate neutralization in somelocalities and contribute to the general lackofequilibrium between atmospheric ammonia and sulfate aerosols. Detailed models ofnatural emissions will require additional measurements.478 Langford etal.: Ammonia Fluxes and Background Concentrations

TABLE 6. Comparison ofAnthropogenic and Natw-al Ammonia Emissions in the Eastern United States

Anthropogenic

State Domestic Animals• Fossil Fuels•

Alabama 40.0 11.3 Connecticut 3.4 1.4 

### Delaware 0.9 1.2 Georgia 42.3 19.4 Kentucky 54.4 26.3 Maine 4.8 0.7 Maryland 8.5 9.1 Massachusetts 2.9 3.5 Michigan 32.2 30.2 Mississippi 39.8 3.2 New Hampshire 1.9 1.2 New Jersey 3.1 6.2 New York 42.8 18.7 North Carolina 24.0 20.4 Ohio 45.0 68.8 Pennsylvania 42.7 63.3 Rhode lsland 0.3 0.3 South Carolina 12.0 8.8 

### Tennessee 48.7 25.7 Vennont 8.2 2.3 Virginia 31.8 9.4 West Virginia 9.1 30.4

Emission rate in 1000 metric tons ofNH3 per year. •Estimates from Harriss and Michaels [1982].

50

40

30 

### :! .;!:

II 

# .. 20 0 II)

10

0
0 

### "' 

# ..**~** 

### "' 0**~ ~** **~** Fertilized Fields•

3.2 0.3 0.7 9.0 13.5

0.4 2.4 0.3 10.6



### 5.5 0.1 7.1 2.3 7.0 19.0 4.1 0.1 3.1 3.4 0.3 3.1 0.2

(a).

0 "' 0 0 

# .. ':' .; "' 

### "' 

# .. 

# .. ..
Longitude Natw-al

Forests Anthropogenic

6.8 0.12 0.6 0.12 0.2 0.07 8.0 0.11 3.6 0.04 5.1 0.86 1.0 0.05 1.0 0.15 6.7 0.09 6.6 0.14 1.5 0.47 0.8 0.05 5.0 0.08 

### 6.5 0.13 2.6 0.02 4.9 0.04 0.1 0.14 4.3 0.18 3.1 0.04 1.3 0.12 5.0 0.11 3.4 0.09



### "' 0 

### "' ":' ":' ';"

0 

### "' 0


# .. 

# .. ..

Fig. 13. Concentrations ofmajor anions, (a) sulfate and (b) nitrate in precipitation as a function oflongitude and season.

### i" E.



# .. 0 z Langford etal.: Ammonia Fluxes and Background Concentrations

40

30

20

10 (b).

• Winter

.. 0

.. 

# .. .. ..

0

.. 0 

# .. .. ..
Longitude Fig. 13. (continued) 0

.. 

# .. 

# ..

.. 0


# .. 

# ..

In conclusion, further improvements in ourunderstanding ofammonia sources and sinks in the United States can be made through the following: 1. Ammonia emissions from anthropogenic sources must be bettercharacterized. Future inventories should be made in close collaboration with atmospheric and agricultural scientists to best evaluate animal emissions factors and fertilizer volatilization rates. New measurements in agricultural systems should be initiated to augment this process.

2. Details ofammonia-cloud interactions, mass transport across aqueous droplets atlow concentrations, andgas-toparticle conversion needs to be betterunderstood and incorporated along with surface exchange into large-scale transport models for the United States.

3. Future instrument development should concentrate on both fast response instruments for eddy correlation measurements and simple, inexpensive samplers that can be deployed at a variety ofsites to bettercharacterize the spatial distribution and variability ofatmospheric ammonia.

4. Field campaigns designed to characterize atmospheric ammonia should include supporting measurements of particulate**~+** and S04=, and in some cases gaseous HNO:J

orHCI and particulate N03
- or CJ-. Measurements of gaseous NH3generally cannotbeinterpreted without knowledge ofthese quantities.

Acknowledgments. A.O.L. and F.C.F. were supported by NOAA as partofthe National Acid Precipitation Assessment 

### Program and would like to thank Eric Williams for his soil analysis and assistance in the field measurements. D.S.S. and J.Z. were supported by NASA and NSF and wish to acknowledge the assistance ofBill Parton, Tom Kittel, Gary Peterson,Jack Morgan, and Tony Delany with their measurements and analyses. 479

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gaseous ammonia in the ambient atmosphere using fluorescence derivization,Atnws. Environ.,15, 277-281, 1981. Andreae, M_ 0.,Biomass burning: Its history, use, and

distribution and its impact on environmental quality and global climate, in GlobalBiomassBurning: Atmospheric, Climatic, andBiospheric Implications, pp. 3-21, MIT Press, Cambridge, Mass., 1991. Aneja, V. P., et al., Dry deposition ofammonia at

environmentai concentrations on seiectedpiant species, J. Air Pollut. Control Assoc., 36, 1338-1341, 1986. Appel, B. R., etal., Determination ofacidity in ambient air,

report, Res. Div., Calif. Air Resour. Board, Sacramento, 1984. Appel, B. R., et al., Evaluation ofprocedures fQr measuring

atmospheric nitric acid and ammonia,Atmos. Environ.• 22, 1565-1573, 1988. ApSimon, H. M., et al., Ammonia emissions and their role

in acid deposition, Atnws. Environ., 21, 1939-1946, 1987. Asman, W. A. H., and H. A. v. Jaarsfeld, A variableresolution transport model applied for NHx in Europe, Atmos. Environ., 26A, 445-464, 1992. Asman, 

### W. A. H., and A. J. Janssen, A long-range transport

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A. 0. Langford, Cooperative Institute for Research in Environmental Science, University ofColorado, Boulder,CO 80309.

D. S. Schime1, National Center for Atmospheric Research, Boulder, CO 80307.

J. Zachariassen, DepartmentofBiology, University of Stanford, Stanford, CA 94305.

(Received March 10, 1992; revised August 28, 1992; accepted September9, 1992.)