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. A one-dimensional tropospheric 

photochemical model is used to simulate measured 

profiles of NH3 obtained with the Infrared 

Heterodyne Radiometer. The relative roles of 

homogeneous loss, heterogeneous loss, and 

vertical eddy transport are discussed in terms 

of selecting parameters which best 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 vertical transport 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 to be about 40 da;ys at the 

surface and decreased to about 180 da;ys at 

10 km, and not very sensitive to model chemical 

perturbations. Increased ground level concentrations of NH 3 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 of NH3. 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 of the sources and sinks, and temporal 

and spatial variations of tropospheric nitrogen 

species (see papers on tropospheric chemistry 

in recent volume edited by Levine and 

Schryer, 1978). Ammonia (NH3) with a somewhat 

variable surface mixing ratio around 1 ppb is 

the third most 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 and most difficult to 

measure of trace atmospheric gases. The 

uncertainties associated with our knowledge 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 of natural systems. 

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

Paper number 9Ll733. 317 

NH3 is the only common gaseous base in the 

atmosphere; it is water soluble and, nence, 

reacts with acidic aerosols and rain. NH3 ma;y 

also play 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 of the system (Lau and Charlson, 

1977). 

Most of our knowledge concerning the concentration of tropospheric NH3 is due to in situ 

measurements of Georgii and Mulle;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 of NH3, with different processes 

controlling the production of NH 3 in different 

regions. In England, urine from domestic 

animals ma;y be the dominant source of NH 3 

(Healy et al., 1970), while in Western Europe 

industrial activity ma;y be the major source 

(Ericksson, 1952). Volatilization of NH3 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 of NH3 (Healy et al., 1970; 

Antizzo, 1978). In the near future the production of NH 3 from anthropogenic activities ma;y 

be increasing, as the use of nitrogen agricultural fertilizer to enhance crop yields is 

increased, and with 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 of NH3 

through the troposphere. Our study is based on 

an interpretation of the first routine measurements of NH3 obtained with 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 

utilizing heterodyne radiometry to perform 

measurements of atmospheric trace gases. The 

measurements discussed in this paper were 

obtained around noon 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 of the 

measurements are given by Hoell et al. (1980a). 

In this paper we will interpret the IHR 
318 Levine et al.: Vertical Distribution of Tropospheric Ammonia 

ALTITUDE, km 

10 

9 

8 

7 

6 

5 

4 

3 

2 AUGUST, 1979 MARCH, 1979 

NH3 MIXING RATIO 

Fig. 1. Measurement envelope of the vertical 

distribution of ammonia within the troposphere 

based on measurements obtained with 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 of both the source and 

the atmospheric processes that control the 

vertical distribution of NH3 through the 

troposphere, i.e., atmospheric transport and 

homogeneous and heterogeneous 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 other hand, measurements 

from early June, and continuing through August, 

were characterized by 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 represented by a profile 

envelope, which includes the uncertainties 

resulting from both 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 

increased with increasing surface temperature 

with maximum concentrations in the summer. 

However, it is interesting to note that during 

the engineering tests of the Infrared Heterodyne 

Spectrometer (IHS), the predecessor of the IHR, 

on New York's Long Island in 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 of maximum surface NH3 in the 

spring decreasing in the summer was measured by 

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 of NH3 returned to what we 

assume to be local background levels. A model 

to quantitatively explain the emission of NH3 

from uncultivated (nonfertilized) soil to the 

atmosphere has been developed by Dawson ( 1977) • 

In Dawson's model, "exchangeable" NH4+ is 

supplied to the soil primarily by bacterial 

decomposition of the biomass, with trace amounts 

(-1%) supplied by precipitation and dry 

deposition. Once in the soil, the NH4+ has two 

routes. The NH4 + can undergo nitrification 

forming NO~-, which can either be assimilated 

into the b1omass or can be denitrified forming 

gaseous N2 and N20, 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" soil NH4 + can 

. form gaseous NH3 (although such a high value 

would be an atypical value for efficient 

agricultural practice)(Dawson, 1977). The 

production of NH3 via volatilization is rapid, 

occurring within several weeks of the application of nitrogen fertilizer (Dawson, private 

communication, 1979). Hence, we attribute the 

high surface levels of NH3 measured in late 

March to the volatilization of NH4+ supplied by 

ALTITUDE, km 

10 

9 

8 

7 

6 

5 

K = 5 X 104 

z 2 1 

em sec 

1 X 105 

2 -1 

em sec 

AUGUST, 1979 

.., 

\ 

\ 

\ 

\ 

\ 

\ 

NH3 MIXING RATIO 

\ 

\ 

Fig. 2. The effect of the vertical eddy diffUsion coefficient, Kz (emF sec-1}, on the 

distribution of tropospheric ammonia. August 

measurement is indicated by broken line envelope. 
Levine et al.: Vertical Distribution of Tropospheric Ammonia 319 

ALTITUDE, km 

10 ~t-T* = 150 DAYS 

~-90 

10-10 10-9 

NH3 MIXING RATIO · 

Fig. 3, The effect of the characteristic 

time constant for heterogeneous loss, •* (day), 

on the distribution of tropospheric ammonia. 

August measurement is indicated by broken line 

envelope, 

the application of ammonium nitrate fertilizer 

to the agricultural fields. 

After the period of rapid volatilization 

(covering our measurement period from late March 

through early April), simultaneous IHR and 

ground-based and aircraft in situ NH3 measurements 

indicated that the level of 1 ppb was reached by 

the beginning of June (Hoell et al., 1980b). From 

June until the end of August, the end of the 

present measurement period, the surface level 

did not significantly change. From June until 

the end of August the measured vertical profiles 

of NH3 were characteristic of the August 

profile shown in Fig. 1. Consequently, we 

assume that the August measurements are typical 

·of the steady-state local troposphere background profile to gain some insight into the 

atmospheric processes that control the vertical 

distribution of NH3 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 and photochemical 

reactions, similar to the chemical scheme of 

Chameides and Stedman (1977), with the addition 

of a detailed NH3 chemistry package. The 

steady-state form of the coupled chemical/ 

diffusion continuity equation used to calculate 

the vertical distribution of NH3 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 of McConnell (1973), 

we have included the following homogeneous loss 

reactions for NH3 (the loss of NH3 due to photodissociation (1. < 230 nm) important in the 

stratosphere is negligib~e in the troposphere) 

(reaction rates taken from Hampson and Garvin, 

1977, are in units of cm3 molec-1 sec-1): 

NH3 + OH + NH2 + H20; 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 of NH3 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~rsourae 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 of ti1e calcu4te4 NH3 

profile to the OH profile, calculations were 

performed for a standard H20 profile (average of 

January and July ll20 vapor profile at 30~ from 

the U. S. Standard Atmosphere Supplements, 1966) 

and for profiles of 1/2 and 2 times the standard 

H20 profile. The resulting changes in both the 

OH and NH3 profiles were negligible. The 

heterogeneous loss term, represented by k*, 

includes the loss of NH3 due to rainout and dry 

deposition. The exact details of heterogeneous 

loss of NH3 are not known, and hence, all 

heterogeneous losses are modeled by a single 

characteristic loss term, k*, which is assumed 

to be constant from the surface to 5 km and to 

decrease by a factor of e for every kilometer 

above 5 km. The heterogeneous loss term k* 

is related to 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 of k*. The steady-state 

continuity equation was solved for the NH3 

mixing ratio subject to the lower Cz = 0 km) and 

upper boundary ( 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 of NH3. 

The effect of Kz on the calculated NH3 

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 

imposed upper boimdary 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 of NH3). A Kz value of 2 x 105 cxn2 sec-1 
320 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 should be 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 lead to increased local convective activity, particularly during the spring 

and smnmer when our measurements were obtained. 

The effect of T* on the calculated NH3 

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 

significantly below the NH3 measurement below 

6 kilometers. These best fit T* values may 

also be related to the biasing in the IHR 

measurements. Since the measurements are 

obtained on clear days, we are selectively 

eliminating from our data base periods of 

precipitation, cloudiness, and poor visibility--

all periods of possible enhanced heterogeneous 

loss. 

To summarize, the best fit for the August 

NH3 measurements, which we 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 

increased the surface concentration of NH3 from 

the background level of about 1 ppb to about 

lQ ppb. Because of the characteristic times 

for the loss mechanisms, synoptic scale 

phenomena may play an important role in 

determining the tropospheric distribution of 

NH 3 concentrations. 

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