GEOPHYSICAL RESEARCH LETTERS, VOL. 7, NO. 5, PAGES 317-320, MAY 1980

THE VERTICAL DISTRIBUTION OF TROPOSPHERIC AJ.M>NIA

Joel S. Levine1

, TOIDIIIY R. Augustsson2 and James M. Hoe111

1NASA Langley Research Center, Hampton, Virginia 23665

20ld Dominion University, Norfolk, Virginia 23508

Abstract. Aone-dimensional tropospheric photochemical model is used to simulate measured profiles ofNH3 obtainedwith the Infrared Heterodyne Radiometer. The relative roles of homogeneous loss, heterogeneous loss, and vertical eddy transport are discussed in terms of selecting parameters whichbest fit the
measurements. The best fit was obtained for a vertical eddy diffusion coefficient of 2 x 105 cm-2 sec-1 or greater (corresponding to a characteristic verticaltransport time in excess of about 35 da;ys), and a characteristic heterogeneous loss time in excess of 10 days. The characteristic homogeneous chemical loss time was found tobe about 40da;ys at the surface and decreased to about 180 da;ys at 10 km, andnot very sensitive to model chemical perturbations. Increased ground level concentrations ofNH3 to about 10 ppb, compared to background surface concentrations of about 1 ppb, were measured several weeks after application of ammonium nitrate fertilizer. This suggests that the volatilization of ammonium nitrate fertilizer is rapid, and an important

source ofNH3. Because of the characteristic times for the loss mechanisms, synoptic time scale phenomena ma;y play an important role in determining the tropospheric distribution of NH3 concentrations.

Introduction

Fundamental deficiencies exist in our understanding ofthe sources and sinks, and temporal and spatial variations of tropospheric nitrogen species (see papers on tropospheric chemistry in recent volume editedby Levine and Schryer, 1978). Ammonia (NH3) with a somewhat variable surface mixing ratio around 1 ppb is the thirdmost abundant nitrogen species in the troposphere after nitrogen (N2) and nitrous oxide (NzO) (with a surface mixing ratio of about 320 ppb

### J• Yet ammonia is one of the
most poorly understood andmost difficult to

measure of trace atmospheric gases. The uncertainties associatedwith ourknowledge of tropospheric NH3 have been blamed on the absence of a rapid and reliable instrument to detect the low (parts per billion) and variable levels of tropospheric NH3 (Dawson, 1977). It is interesting to note that the chemical equilibrium concentrations of N20 and NH3 are lo-15.7 and lo-54.5 ppb, respectively (Holland, 1978). This large difference between actual atmospheric levels and calculated chemical equilibrium levels is a testimonial to the ability of microorganisms to maintain drastic disequilibrium conditions in the chemistry ofnatural systems.

This paper is not subject to U.S. copyright. Published in 1980 by the American Geophysical Union.

Paper number 9Ll733. 317NH3 is the only common gaseous base in the atmosphere; it is water soluble and, nence, reacts with acidic aerosols andrain. NH3 ma;y alsoplay a significant role in determining the
rate of formation of sulfate aerosols since it accelerates the oxidation of so2 through various heterogeneous reactions by controlling the acidity ofthe system (Lau and Charlson, 1977). Most of our knowledge concerning the concentration oftropospheric NH3 is due to in situ
measurements of Georgii andMulle;r (1974)

. These measurements indicate that NH3 exhibits great temporal and spatial variability. Georgii and Muller (1974) found that the ground concentration of NH3 was about a factor of 3 higher on warm da;ys than on cool days. In addition, they found that the concentration of NH3 decreased rapidly with altitude, reaching a constant background concentration at an altitude of about 1.

### 5 km on winter days and
at about 3 km on warm da;ys. The surface appears to be the primary

source ofNH3, with different processes controlling the production ofNH3 in different regions. In England, urine from domestic animals ma;y be the dominant source ofNH3 (Healy et al., 1970), while in Western Europe industrial activity ma;y be the major source (Ericksson, 1952). Volatilization ofNH3 from undisturbed (nonfertilized) and fertilized agricultural soils is another source of**NH~** (Dawson, 1977). In addition, coal**convers~on** and combustion processes ma;y also emit significant amounts ofNH3 (Healy et al., 1970; Antizzo, 1978). In the near future the production ofNH3 from anthropogenic activities ma;y be increasing, as the use ofnitrogen agricultural fertilizer to enhance crop yields is increased, andwith a switch to greater dependence on coal. The purpose of this paper is to discuss the factors that control the surface concentration and the vertical distribution ofNH3 through the troposphere. Our study is based on
an interpretation ofthe first routine measurements ofNH3 obtainedwith the Infrared Heterodyne Radiometer (IHR) using a one-dimensional tropospheric photochemical model. The IHR, a passive remote sensing instrument that uses the sun as a source, was developed to demonstrate the feasibility of utilizingheterodyne radiometry to perform
measurements of atmospheric trace gases. The
measurements discussed in this paperwere obtained aroundnoon on clear days from the NASA Langley Research Center, Hampton, Virginia. The IHR instrument, the ground-truth calibration, the data inversion and reduction techniques, and a detailed discussion ofthe
measurements are givenby Hoell et al. (1980a). In this paperwe will interpret the IHR318 Levine et al.: Vertical Distribution of Tropospheric Ammonia

ALTITUDE, km 10

9

8
7

6

5

4

3

2 AUGUST, 1979 MARCH, 1979

NH3MIXING RATIO Fig. 1. Measurement envelope of the vertical distribution of ammonia within the troposphere based on measurements obtainedwith the Infrared Heterodyne Radiometer in March and August, 1979,
at the NASA Langley Research Center in Hampton, Virginia (Hoell et al., 1980a).

measurements of NH3 obtained from March to August 1979 in terms ofboth the source and the atmospheric processes that control the vertical distribution ofNH3 through the troposphere, i.e., atmospheric transport and homogeneous andheterogeneous loss. Measurements obtained in March and early April exhibited a surface mixing ratio of about 10 ppb, which decreased to about 1.5 to 3 ppb
at 10 km. On the otherhand, measurements from early June, and continuing through August,

were characterizedby a lower surface mixing ratio of about 1 ppb that decreased to about 0.4
to 0.7 ppb at 10 km. Typical early spring (March) measurements and late spring/summer (August) measurements are shown in Fig. 1. The IHR measurements are representedby a profile envelope, which includes the uncertainties resulting fromboth the instrument and the inversion technique (Hoell et al., 1980a). A series of ground-based and aircraft in situ NH3 measurements gave excellent agreement with the IHR measurements (Hoell et al., 198Gb). The decrease in the NH3 surface mixing ratio from about 10 ppb to 1 ppb from spring to summer that we report here is contrary to the earlier
measurements of Georgii and Muller (1974), who found that the NH3 surface mixing ratio increasedwith increasing surface temperature with maximum concentrations in the summer. However, it is interesting to note that during the engineering tests of the InfraredHeterodyne Spectrometer (IHS), the predecessor of the IHR,

on New York's Long Islandin 1976, the IHS measured NH3 surface concentrations of about 8-12 ppb in March which decreased to as low as 0.2 ppb during the summer (Peyton et al., 1977). This same trend ofmaximum surface NH3 in the spring decreasing in the summer was measuredby the IHR during 1979.

The Source and Photochemistry of NH3

We believe that the enhanced surface levels of NH3 (10 ppb) that we measured in March resulted from the rapid volatilization of ammonium nitrate fertilizer applied to agricultural fields several weeks prior to the beginning of the measurement program. Within 2 months after the initiation of the measurement program, the surface levels ofNH3 returned to what we

assume tobe local background levels. Amodel
to quantitatively explain the emission of NH3 from uncultivated (nonfertilized) soil to the atmosphere has been developedby Dawson (1977)• In Dawson's model, "exchangeable" NH4+ is supplied to the soil primarily bybacterial decomposition of the biomass, with trace amounts (-1%) suppliedby precipitation and dry deposition. Once in the soil, the NH4+ has two routes. The NH4+ can undergo nitrification forming**NO~-,** which can eitherbe assimilated into the b1omass or can be denitrified forming gaseous N2 andN20, which are returned to the atmosphere. The other route for the NH4+ in the soil is the formation of gaseous NH3 via volatilization which depends on the soil moisture, type, pH, and temperature (Fenn and Escarzaga, 1976). Agricultural nitrogen fertilizer, e.g. ammonium nitrate, is an additional source of "exchangeable" NHh+. Under optimum volatilization conditions, more than 50 percent of the "exchangeable" soilNH4+ can

. form gaseous NH3 (although such a high value

wouldbe an atypical value for efficient agricultural practice)(Dawson, 1977). The production of NH3 via volatilization is rapid, occurringwithin several weeks of the application of nitrogen fertilizer (Dawson, private communication, 1979). Hence, we attribute the high surface levels ofNH3 measured in late March to the volatilization ofNH4+ suppliedby



## ALTITUDE, km 10

9


## 8



## 7
6

5 K

# = 5X

## 104



## z 2 1 em sec



## 1X

## 105


## 2 -1 em sec

AUGUST, 1979

# .., \ 

## \ 

# \ \ \

## \

NH3MIXING RATIO 

# \ 

# \

Fig. 2. The effect of the vertical eddy diffUsion coefficient, Kz (emF sec-1}, on the distribution of tropospheric ammonia. August
measurement is indicatedbybroken line envelope.Levine et al.: Vertical Distribution of Tropospheric Ammonia 

### 319



## ALTITUDE, km 

## 10**~t-T***

## =150DAYS**~-90**



## 10-10 10-9


# NH3MIXINGRATIO· Fig. 3, The effect ofthe characteristic time constant for heterogeneous loss, •* (day),

on the distribution of tropospheric ammonia. August measurement is indicated bybroken line envelope,

the application of ammonium nitrate fertilizer
to the agricultural fields. After the period of rapid volatilization (covering ourmeasurement period from late March through early April), simultaneous IHR and ground-based and aircraft in situNH3 measurements indicatedthat the level of 1 ppb was reachedby the beginning of June (Hoell et al., 1980b). From June until the end ofAugust, the end ofthe present measurement period, the surface level didnot significantly change. From June until the end ofAugust the measured verticalprofiles ofNH3 were characteristic of the August profile shown in Fig. 1. Consequently, we

assume that the August measurements are typical ·ofthe steady-state local tropospherebackgroundprofile to gain some insight into the atmospheric processes that control the vertical distribution ofNH3 through the troposphere. Simulations were performed of the August
measurement using a steady-state troposphere photochemical model that includes more than two dozen oxygen, nitrogen, carbon, and hydrogen species. The photochemical model is based on
more than 60 chemical andphotochemical reactions, similarto the chemical scheme of Chameides and Stedman (1977), with the addition of a detailedNH3 chemistrypackage. The steady-state form of the coupled chemical/ diffusion continuity equation used to calculate the vertical distribution ofNH3 and the other atmospheric species, the numerical scheme, the tridiagonal solver, and the convergence criterion have been described in detail in the appendix of Levine et al, (1979), Following the NH3 chemistry scheme ofMcConnell (1973), we have includedthe following homogeneous loss reactions for NH3 (the loss ofNH3 due to photodissociation 

### (1. 

### < 230 nm) important in the stratosphere is**negligib~e** in the troposphere) (reaction rates taken fromHampson and Garvin, 1977, are in units of cm3 molec-1 sec-1):

NH3 

### + OH + NH2 

### + H 20; k

### ~ 2.3 x lo-12 exp(-8o0/T) (1)

NH3 + 0 + NH2 + OH; k = 6.6 x l0-12 exp(-3300/T) (2)

NH3 

### +

### 0(~) + NH2 

### + 

### OH; k = 2.5 x 10-lO (3)

The homogeneous loss ofNH3 was found to be controlled by (1), with {2) and (3) resulting in a negligible'lO!!S of NH3.**0~** calculated OH profile

### decrea~es from 2 K 10 cm-3 at the surface to 1 x 10°

### ~-3 at 

### 10**~.** .For this OH profil-e, tlle ch"-l"acterist;ip'time 

### for hOWQge:Q.eQus 1oss due to OH varied**~ ab~** ·40 days e:t; the. surface to about 180·days**~:J.O** 

### lan. 

### 'J.'b.e**llltiJ~r** sourae of OR in the**tr.~heie** is the reMtioJ:t 

### between o(1n)**~d** HzO 

### '4at · · . 

# o<ln> +

### ~o + 2Qit;

__

## k.;2.3 x J.o-10 

## (4l

To test the sensitivity ofti1e calcu4te4NH3 profile tothe OH profile, calculations were performed for a standardH20 profile (average of January and July

## ll20 vaporprofile at**30~** from the U. S. StandardAtmosphere Supplements, 1966) and for profiles of 1/2 and 2 times the standard H20 profile. The resulting changes in both the OH andNH3 profiles were negligible. The heterogeneous loss term, representedby k*, includes the loss ofNH3 due to rainout and dry deposition. The exact details ofheterogeneous loss ofNH3 are not known, and hence, all heterogeneous losses are modeledby a single characteristic loss term, k*, which is assumed
tobe constant from the surface to 5 km and to decrease by a factor of e for every kilometer above 5 km. The heterogeneous loss termk* is relatedto the characteristic time constant for heterogeneous loss,**~*by:** k* = 1/T*, Model calculations were made with values of k* corresponding to values of

,. ranging from 1 to 150 dS;vs to determine the sensitivity of the NH3 profile to the choice ofk*. The steady-state continuity equationwas solved for the NH3 mixing ratio subject to the lower Cz = 0 km) and upperboundary ( z

## =10**~)** mixing ratios based

on the August IHR NH3 profile shown in Fig. 1, which 

### was assUJI!ed to represent the steady-state 

### local._ tropospheric background ofNH3. The effect of Kz on the calculatedNH3 profile is shown in Fig. 2. NH3 profiles have beep calculated for values of Kz 

### = 0.5 x 105, 1 X 105, and 5 X 105 cm2 sec-1,**Th~se** calculations were**perfo~ed** for k* corresponding to <* 

## =150 days. Comparison of our calculations with the meas'l,li'ements indicates a Kz value in

excess of 2 x 105 cm-2 sec-1, with a best fit for a**val~** of

## Kz of 5 x ].()5 cm-2 sec-1. The imposedupperboimdary condition unrealistically
turns the NH3 profile back int.o the measurement epvelope for values of

## Kz less than 2 x 105 cm2 sec-1 (which would imply an upper tropospheric

source ofNH3). AKz value of 2 x 105 cxn2 sec-1320 Levine et al.: Vertical Distribution of Tropospheric Ammonia

corresponds to a characteristic transport time of about 35 days, while a Kz of 5 x 105 cm-2 sec-1 corresponds to a transport time of about 15 days. It shouldbe kept in mind, however, that the IHR NH3 measurements may be biased since they are obtained only an clear, smmy days. Increased solar insolation and heating on such days will leadto increased local convective activity, particularly during the spring and smnmer when our measurements were obtained. The effect of T* on the calculatedNH3 profile is shown in Fig. 3. These calculations

were performed for Kz = 5 x 105 emf sec-1. The best fit is for aT* in excess of 10 days, with the lower values of T* resulting in profiles significantlybelowthe NH3 measurement below 6 kilometers. These best fit T* values may alsobe related tothe biasing in the IHR
measurements. Since the measurements are obtained on clear days, we are selectively eliminating from our database periods of precipitation, cloudiness, and poor visibility-- all periods ofpossible enhancedheterogeneous loss. To summarize, the best fit for the August NH3 measurements, whichwe believe represent the steady-state 

### local tropospheric NH3 background, corresponds to a Kz of 2 x 105 cm-2 sec-1 or greater and for a T* of 10 days or greater. Interestingly, these parameters also give the best fit for the March measurements, which we believe are not a steady-state condition and therefore not applicable to a steady-state model calculation. Finally, we believe that the rapid volatilization of ammonium nitrate fertilizer increasedthe surface concentration ofNH3 from thebackground level of about 1 ppb to about lQppb. Because of the characteristic times for the loss mechanisms, synoptic scale phenomena may play an important role in determining the tropospheric distribution of NH3 concentrations.

References

Antizzo, J. (Editor), Health and Environmental

Effects of Coal Gasification and Liquefaction Technologies, Mitre Corporation M78-58, 265 pages, 1978. Chameides

, W. L.

, and D. H. Stedman, Tropospheric Ozone: Coupling Transport and Photochemistry, J. Geoph¥s. Res., 82, 1787

- 1794, 1977. Dawson, G. A., Atmospheric Ammonia from

UndisturbedLand, J. Geophys. Res.

, 82, 3125

- 3133, 1977. Eriksson, E., Composition ofAtmospheric Precipitation, I, Nitrogen Compounds, Tellus, 4, 215

- 232, 1952.

--- Fenn, L. B.

, andR. Escarzaga, Ammonia Volatilization from Surface Applications of Ammonium Compounds on Calcareous Soils, V. SoilWater Content and Method ofNitrogen Application. J. Soil Sci. Soc. Amer., 4o, 537- 541, 1976. Georgii, H. W.

, andW. J. Muller, On the

Distribution ofAmmonia in the Middle and Lower Troposphere, Tellus, 26, 180

- 184, 1974. Hampson, R. G., and D. Garvin-:-Reaction Rate and

Photochemical Data for Atmospheric Chemistry.- 1977, NBS Special Publication 513. Healy, T.V., H. A. C. McKay, A. Pilbeam, and

D. Scargill, Ammonia and Ammonium Sulfate in the Troposphere over the United Kingdom, J. Geophys. Res., 

### ]i, 2317- 2321, 1970. Hoell, J. M., C. N. Harward., and B.S. Williams,

Remote InfraredHeterodyne Radiometer Measurements ofAtmospheric Ammonia Profiles, Geophys. Res. Lett.

, 

## 1. 198oa (this issue)

. Hoell, J. M., 

## c. N. Harward, andW. A. McClenney,

Comparison ofRemote InfraredHeterodyne Radiometer and In SituMeasurements ofAtmospheric Ammonia, Proceedings ofthe Conference

on Laser and Electro-Optical Systems, 198ob. Hollancl, H. D., The Chemistry of the Atmosphere and Oceans, 351pp., Wiley-Interscience, New York, 1978. Lau, N-C., and R. J. Charlson, On the Discrepancy

Between BackgroundAtmospheric Ammonia Gas Measurements and the Existence of AcidSulfates

as a Dominant Atmospheric Aerosol, Atmos. Envir., 11, 475-478, 1977.

-- Le'Viiie,"J.S., P. B. Hays, and J. 

## c. G. Walker,

The Evolution and Variability ofAtmospheric Ozone Over Geological Time,**~. ~.**  295

- 309, 1979. Levine, J. 

## s., and D. R. Schryer (Editors),



### Man's Impact an the Troposphere: Lectures in Tropospheric Chemistry, NASA Reference Publication 1022, 376 pages, 1978. McConnell, J. C., Atmospheric Ammonia, J. Geophys.

Res., 78, 7812- 7821, 1973. Peyton, B. J.• R. A. Lange, M. G. Savage.

R. K. Seals, and F. Allario, Infrared Heterodyne Spectrometer Measurements ofVertical-Profile of Tropospheric Ammonia and Ozone, AIM 15th Aerospace Sciences Meeting, AIM Paper No. 

### .ll;:ll. Jan. 1977. U. S. Government Printing Office, U. S. Standard

Atmosphere Supplements, Washington, D.C., 1966.

(Received November 

### 13, 1979; accepted December 20, 

### 1979.)