Atmospheric Environment 37 (2003) 4419–4424

Dry deposition velocity of PM2.5 ammonium sulfateparticles to a Norway spruce forest on the basis of

S- and N-balance estimations

L!aszl !o Horv!ath

Hungarian Meteorological Service, Gilice t!er 39, Budapest 1181, Hungary

Received 28 February 2003; received in revised form 25 June 2003; accepted 3 July 2003

Abstract

In this paper we make an attempt to estimate the dry deposition velocity of ammonium sulfate particles using the

results of sulfur and nitrogen balance determined between the atmosphere and a forest ecosystem for the years of 1996–

1998. Results of these measurements and of a campaign conducted during summer of 2001 demonstrate that

ammonium and sulfate ions exist in nearly equivalent ratio in particle phase. According to the size distribution

measurements majority ð92%Þ of ammonium sulfate can be found in the fraction of PM2.5. For the balance calculation,

results of throughfall, stemflow and wet-only deposition measurements have been used, together with dry deposition

measurements of gaseous sulfur dioxide and ammonia. Dry deposition velocity of ammonium sulfate particles

determined from sulfur and nitrogen balance were vX0:8270:25 and ¼ 0:8470:25 cm s�1; respectively. The two figures

determined by different ways are in good agreement and they are in accordance with other experimental results found in

the literature. The results suggest the necessity of the revision of the models applied during the theoretical calculation of

dry deposition velocity of PM2.5 particles.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: PM2.5 particles; N-balance; S-balance; Wet deposition; Dry deposition; Throughfall deposition

1. Introduction

The knowledge of the rate of dry deposition velocity

of ammonium sulfate particles is necessary for the

estimation of N and S load or nitrogen and sulfur

balance between the atmosphere and forest ecosystems.

For aerosol particles, limited field studies (Erisman

et al., 1995; Wyers et al., 1995) have obtained system-

atically higher deposition velocities than there were

determined by theoretical calculations and wind tunnel

experiments (Ruijgork et al., 1993; Borrell et al., 1997).

There are still substantial differences between the

experimentally determined and theoretically calculated

dry deposition velocity figures. The deposition velocities

estimated from calculations and laboratory wind tunnel

measurements lies in the order of 0:1 cm s�1; while the

experimental values from the literature are higher with

one order of magnitude ð > 1 cm s�1Þ: Gallagher et al.

(2002) also pointed out the discrepancy between the

modeled and observed dry deposition figures for fine

(0.1–0:2 mm diameter particles).

According to some measurements dry deposition

velocity to forests are: 2:4 cm s�1 for sulfate (S!anchez

et al., 1993); 1.2–1:5 cm s�1 for ammonium particles

over 0:8 mm size (Wyers et al., 1995); 1–2 cm s�1 for fine

and 5 cm s�1 for coarse sulfate and ammonium particles

(Erisman et al., 1995). Different research groups agree

with the high uncertainty of these figures (Lopez, 1994).

Borrell et al. (1997) suggest over 1 cm s�1 deposition

velocity for particles according to the re-evaluation of

theoretical, wind tunnel and field estimations (Ruijgork

et al., 1993).

ARTICLE IN PRESS

AE International – Europe

E-mail address: horvath.l@met.hu (L. Horv!ath).

1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1352-2310(03)00584-3

Sulfur and reduced nitrogen compounds can be

deposited either by wet (WD) or dry deposition (DD)

processes. There are two gas-phase (SO2 and NH3) and

two particulate phase (sulfate, ammonium) components

which have substantial concentrations in the atmo-

sphere, controlling the N- and S-flux to forest ecosys-

tems (there are no important S-sources from the forest,

while ammonia has bi-directional flux).

The dry deposited sulfur and reduced nitrogen

compounds are leached by the precipitation from the

surface of leaves, branches and trunk and can be

detected as sulfate and ammonium ions in the through-

fall (TF) and stemflow (SF) precipitation samples. The

balance of a group of compounds can be written as (see

e.g. Ferm and Hultberg, 1999)

TF þ SF ¼ WD þ DD þ IC � UP; ð1Þ

where IC is the internal circulation or ion leakage, and

UP is the term for the uptake by stomata.

1.1. Sulfur compounds

In the case of sulfur compounds IC is negligible.

Using Eq. (1) the dry deposition of sulfate particles can

be expressed as

DDðSO2�4 Þ ¼TFðSO2�

4 Þ þ SFðSO2�4 Þ � WDðSO2�

4 Þ

� DDðSO2Þ þ UPðSO2Þ: ð2Þ

1.2. Nitrogen compounds

For reduced nitrogen species the stomatal uptake

both of ammonia gas and ammonium are significant,

while IC is negligible, in this case Eq. (1) can be written

as

DDðNHþ4 Þ ¼TFðNHþ

4 Þ þ SFðNHþ4 Þ � WDðNHþ

4 Þ

� DDðNH3Þ þ UPðNH3Þ þ UPðNHþ4 Þ:

ð3Þ

In the dry deposition of ammonia two processes are

dominant, the uptake by stomata (UP) and the

adsorption on wet leaf surface (CU) especially in the

presence of high sulfur dioxide concentrations. Hence

the dry deposition of ammonia can be written as

DDðNH3Þ ¼ UPðNH3Þ þ CUðNH3Þ: ð4Þ

Theoretically, net ammonia emission by plants cannot

be excluded in the case when the net stomatal emission

flux (negative UP) exceeds the magnitude of the

cuticular deposition. Stomata in the function of the

apoplastic pH and ammonium concentration control a

compensation point concentration for ammonia. In

most of cases the compensation point is lower than

atmospheric concentration, i.e. the sign of ammonia flux

is generally negative (i.e. deposition occurs). Compensa-

tion point is highly depend on the atmospheric nitrogen

load to the forest, in our case it is moderately low:

18 kg N ha�1 yr�1 (Horv!ath et al., 2002; Horv!ath,

2003). Dry deposition velocity of ammonia determined

at our forest site is 1.1–3:7 cm s�1 during night and day

hours, respectively (Horv!ath et al., 2001). These deposi-

tion velocity figures suggest a net ammonia deposition.

Eqs. (3) and (4) yield

DDðNHþ4 Þ ¼TFðNHþ

4 Þ þ SFðNHþ4 Þ � WDðNHþ

4 Þ

þ UPðNHþ4 Þ � CUðNH3Þ: ð5Þ

In our estimation Eq. (5) will be used to calculate the dry

deposition rate of ammonium particles.

Throughfall (TF) and stemflow (SF) measurements

are generally carried out on routine basis. Wet deposi-

tion (WD) measurements are also among the basic task

of the monitoring networks. Measurement of dry

deposition (DD) of sulfur dioxide and ammonia is also

well studied, there is a lot of information in the literature

for deposition rate of SO2 and for the net flux of NH3:However, in the estimation of dry deposition of

ammonium and sulfate particles there is one order of

magnitude of uncertainty between the theoretical,

laboratory estimations and experimental results as

mentioned above. There is no generally acceptable

method for the direct measurement of the dry deposition

of fine (PM2.5) aerosol particles.

In this paper we aimed to make an attempt to estimate

the dry deposition velocity of ammonium sulfate

particles, using the results of the sulfur and reduced

nitrogen balance over a forest ecosystem based on

measurements for the years of 1996–1998 in a Norway

spruce stand. For the calculation results of throughfall,

stemflow and wet-only precipitation measurements were

used, together with dry deposition measurements of

sulfur dioxide and ammonia. Another goal of this work

is to give a simple methodology to derive the dry

deposition velocity figures for particles and encourage to

continue the measurements to fill the gap between

theoretical and experimental results.

2. Measurements

Measurements were carried out during 1996–1998 in

the M!atra-Mountains, NW Hungary, in a Norway

spruce forest, planted between 1963 and 1965 in the

frame of the IUFRO international project. The average

height of the trees was 15–17 m (1996–1998), with a leaf

area index of 3.3 (1994). Geographical positions are l ¼19�570E; j ¼ 47�540N; h ¼ 560 m: The station is jointly

operated by the Forest Research Institute and by the

Hungarian Meteorological Service.

Concentration ðCÞ of sulfur dioxide and ammonia as

well as the ammonium and sulfate particles were

determined on the basis of 24 h samplings according

ARTICLE IN PRESSL. Horv !ath / Atmospheric Environment 37 (2003) 4419–44244420

to the EMEP (1996) recommendations by the filter pack

method as sulfate and ammonium ions in the solution of

sampling filters by ion-chromatography and indophe-

nol-blue photometry, respectively.

Wet deposition (WD) of sulfate and ammonium ions

was determined as WD ¼ Cp; where C is the concentra-

tion of the ion measured in the precipitation water, and

p is the precipitation amount. Daily precipitation

samples were taken by a wet-only collector installed

out of the forest canopy. Sulfate and ammonium ions

were determined by the methods mentioned above.

Throughfall (TF) and stemflow (SF) samples were

collected parallel with the precipitation samplings under

the canopy by five wet-only collectors and rims installed

on the trunk of 10 selected trees, respectively. Concen-

trations of sulfate and ammonium ions were determined

by the same way as mentioned.

Beside the concentration and deposition measure-

ments described above aerosol samplings were carried

out in the summer of 2001 to determine the amount

the ammonium and sulfate in different size ranges. A

Ghent-type impactor was used to select the ammonium

and sulfate particles in two ranges (do2:5 mm and

2:5 mmodo10 mm). As a total of 12 samplings were

carried out.

2.1. Uncertainty analysis

According to international inter-calibrations and

parallel samplings the estimated uncertainties of the

different terms are compiled in Table 1. Most critical

term is the dry deposition that has been determined

using results of the gradient flux measurements. Applic-

ability of the gradient method for forests is criticized.

Some authors give estimation on the bias. For example,

Simpson et al. (1998) demonstrated that the ratio of

eddy/gradient fluxes are between 1.1 and 1.24 at

approximately 2 times higher measuring height to

canopy height (our case). Hargreaves et al. (1996)

calculated a ratio of 1.19 for eddy/gradient fluxes.

According to our estimation (M!esz!aros et al., 2000) the

ratio of u� determined by different methods is 1.16.

These figures are below the 730% uncertainty which is

indicated in Table 1.

3. Results

To determine the distribution and concentration of

ammonium sulfate particles in different size ranges

result of 12 Ghent-impactor samplings were used. The

main measured parameters can be seen in Table 2.

As Table 2 shows majority of ammonium and sulfate

exist in the range of PM2.5. There is a strong correlation

between the ammonium and sulfate particles in the

PM2.5 range according to Fig. 1.

Ammonium-to-sulfate ratio expressed in equivalents

are 93 and 104 nequ m�3 i.e. the two ions exist nearly in

stochiometric ratio. From Table 3 the corresponding

figures for years of 1996–1998 are 79 and 94 nequ m�3:These suggest that the two ions exist nearly in equivalent

ratio and can be treated mostly as neutral ammonium

sulfate in the PM2.5 range. Using this close stochio-

metric ratio of ammonium and sulfate one can estimate

that dry deposition velocities determined both for

ammonium and sulfate ions represent the dry deposition

velocity of ammonium sulfate particles.

3.1. Sulfur compounds

In Eq. (2) terms TF, ST, WD and DD were separately

determined. Because UP(SO2) term remains unknown,

only the dry deposition of sulfate minus uptake of sulfur

ARTICLE IN PRESS

Table 1

Uncertainty in determination of different terms

Parameter Uncertainty (%)

NHþ4 and NO�

3 concentration

measurements in precipitation

75

Precipitation amount 710

Overall uncertainty of WD 710

NH3 and SO2 concentration profile 710

Determination of K 725

Overall uncertainty of DD 725

Uncertainty of TF and SF 710

Overall estimated uncertainty of

DD(SO2�4 ) and DD(NHþ

4 )

730

Table 2

Ratio of sulfate to ammonium in different size ranges by 12

cascade impactor samplings in Summer, 2001

Phase NHþ4 ð%Þ SO2�

4 ð%Þ(nequ m�3) (nequ m�3)

do2:5 mm 93.3 96 104 92

2:5 mmodo10 mm 4.1 4 9.0 8

y = 1.0303x + 0.0077 R2= 0.9809

0.00

0.05

0.10

0.15

0.20

0.00 0.05 0.10 0.15 0.20

ammonium

sulf

ate

Fig. 1. Correlation between the concentration of ammonium

and sulfate ions in PM2.5 phase (mequ m�3).

L. Horv !ath / Atmospheric Environment 37 (2003) 4419–4424 4421

dioxide DD(SO2�4 Þ � UPðSO2) can be calculated from

Eq. (2). To derive dry deposition velocity figures the

½DDðSO2�4 Þ � UPðSO2Þ=CðSO2�

4 Þ ¼ vðSO2�4 Þ � f ð6Þ

equation was used. In the case of f > 0; i.e. when the

stomatal uptake of sulfur dioxide is not negligible the

calculated vðSO2�4 Þ � f gives the lower limit for sulfate

deposition. In the extreme case when stomatal uptake is

negligible ðf ¼ 0Þ; vðSO2�4 Þ can be directly calculated

from Eqs. (5) and (6).

The rate of stomatal uptake in the dry deposition of

sulfur dioxide depends among others on the moisture,

leaf wetness, temperature and the ambient ammonia

level. Stomatal uptake of sulfur dioxide can be

important, but in our case it has probably of less

importance in comparison to the surface adsorption.

Despite of the relatively low, 700–750 mm yr�1 rainfall,

the humidity inside the canopy is high (in most cases

> 60%). Though stomatal sulfur dioxide deposition

occurs parallel with surface deposition, several authors

pointed out the great importance of deposition on wet

leaf surfaces involving ammonia (Flechard et al., 1999).

One of the most important factor controlling the canopy

resistance to the dry deposition of sulfur dioxide is the

ratio of ammonia to sulfur dioxide as described by

Fowler and Erisman (2003). For example parallel with

the increase of ammonia to sulfur dioxide, the canopy

resistance of cereal canopy decreased from 130 to

80 s m�1: As one of the main results of the first stage

of the BIATEX project it was demonstrated (Erisman

et al., 1993) that in dry cases, when the humidity is lower

than 60% canopy resistance for SO2 lies between 500

and 1000 s m�1: In other cases except during fog, with

low ammonia level and during negative temperature the

canopy resistance is low ð50 s m�1Þ: At our site the

average dry deposition velocity is 0:64 cm s�1; giving

150 s m�1 for the bulk (Ra; Rb and Rc) resistance.

Roughly, taking into account, that Ra and Rb are

responsible for the half of the bulk resistance, we are in

the range of below 100 s m�1; indicating that surface

deposition is the more important deposition process

than stomatal uptake in our forest. Comparing the 500–

1000 s m�1 canopy resistance with Ro100 s m�1 surface

resistance, as a first approximation, we can say, stomatal

deposition would give the 10%–20% of the total

deposition on yearly basis. In winter when ammonia to

sulfur dioxide ratio is low the estimated surface

resistance can be higher resulting in higher share of the

stomatal deposition.

For the calculation of vðSO2�4 Þ � f the TF, SF and

WD were measured as described above. The results of

wet deposition measurements (WD), throughfall (TF)

and stemflow (SF) deposition estimates are compiled in

Table 4.

Dry deposition fluxes of sulfur dioxide were inferred

from the continuously monitored 3-year concentration

record ðCÞ and dry deposition velocity ðvÞ as DDðSO2Þ ¼vC for different seasons and stratification. Dry deposi-

tion velocity was determined on campaign basis in

different seasons between 1992 and 1994 by the gradient

method described in Horv!ath et al. (1996). Dry flux was

calculated as F ¼ �KH dC=dz; where KH is the

turbulent diffusion coefficient for the sensible heat flux,

dC=dz is the concentration gradient. Concentration

gradient was determined at different heights (28, 23,

and 18 m) above the canopy by a HORIBA gas monitor.

Diffusion coefficient was calculated according to the

Monin–Obukhov’s semi-empirical similarity theory

(Weidinger et al., 2000) for the layer between 28 and

18 m: Dry deposition velocity was derived and calcu-

lated for different seasons and for different stratification

as v ¼ �F=C: The bulk mean dry deposition velocity

averaged for the whole year is 0:64 cm s�1 (variation

according to the season is 0.40–0:96 cm s�1 ranging

0.09–0.34 and 0.60–1:57 cm s�1 during stable and

unstable stratification, respectively). Result of dry

deposition estimate for sulfur dioxide (DD) can be seen

in Table 4.

According to Eq. (2) the yearly dry flux of sulfate is

2578 mequ m�2 yr�1: Bulk dry deposition velocity for

sulfate particles calculated from Eq. (6), using the mean

concentration in Table 3 and taking into account the

ARTICLE IN PRESS

Table 3

Mean concentrations of sulfur and reduced nitrogen species

(1996–1998)

Species Concentration

ðnequ m�3)

Sulfur dioxide 600760

Sulfate 9479

Ammonia 4474

Ammonium 7978

Table 4

Throughfall, stemflow, wet and dry deposition measurements of

sulfur and nitrogen compounds and the calculated dry

deposition flux for sulfate and ammonium particles (1996–1998)

Form of deposition Deposition rate

(mequ m�2 yr�1)

TF(SO2�4 ) measured 200720

SF(SO2�4 ) measured 0.570.05

WD(SO2�4 ) measured 5676

DD(SO2) measured 120730

DD(SO2�4 ) calculated >2578

TF(NHþ4 ) measured 5175

SF(NHþ4 ) measured 5.475

WD(NHþ4 ) measured 3073

DD(NH3) measured 40710

DD(NHþ4 ) calculated 2176

L. Horv !ath / Atmospheric Environment 37 (2003) 4419–44244422

uncertainty of measurements and estimations is v >0:8270:25 cm s�1 or in extreme case (high ammonium

level, high moisture, wet leaves, when stomatal uptake

is negligible to the cuticular adsorption) it is v ¼0:8270:25 cm s�1:

3.2. Nitrogen compounds

TF, SF and WD terms in Eq. (5) can be determined

experimentally. The term UP(NHþ4 Þ � CUðNH3) cannot

be calculated from our measurements. However, it is

possible to make a rough estimation for the magnitude

of these terms. When ammonium ion is taken up by

plants from precipitation the rate of uptake (UP) is

limited as

0oUPðNHþ4 ÞoWDðNHþ

4 Þ: ð7Þ

Wet deposition of ammonium according to Table 4

is 30 mequ m�2 yr�1; therefore the rate of stomatal

ammonium uptake is somewhere between 0 and

30 mequ m�2 yr�1:Cuticular (surface) adsorption of NH3 is limited by

the total measured dry deposition of ammonia, hence

0oCUðNH3ÞoDDðNH3Þ: ð8Þ

Dry deposition fluxes of ammonia were inferred by the

monitored concentrations ðCÞ and dry deposition

velocities ðvÞ; for different seasons and stratification, as

DDðNH3Þ ¼ vC: Dry deposition velocity was measured

on campaign basis by the gradient method (Horv!ath

et al., 2001, 2003). Dry flux was determined by the same

method as described for sulfur dioxide. The bulk mean

dry deposition velocity averaged for the whole year is

2:4 cm s�1 (variations are 1.1 and 3:7 cm s�1 during

stable and unstable stratification, respectively).

The calculated net dry flux for ammonia is

40 mequ m�2 yr�1 therefore the cuticular deposition of

ammonia lies between 0 and 40 mequ m�2 yr�1:Table 5 shows the calculated dry deposition velocities

for ammonium particles using Eqs. (5) and (9),

vðNHþ4 Þ ¼ DDðNHþ

4 Þ=CðNHþ4 Þ: ð9Þ

Because both the stomatal uptake of ammonium and the

cuticular adsorption of ammonia is important the most

probable rate of dry deposition, v ¼ 0:8470:25 can be

inferred when the rate of stomatal and cuticular dry

deposition of ammonia is 1:1 [case of CU(0.5)] and half of

ammonium ions is taken up from the precipitation [case

of UP(0.5)]. Though, these are rough estimations, the

good agreement between deposition velocities from sulfur

balance vX0:8270:25 cm s�1 and from reduced nitrogen

balance v ¼ 0:8470:25 cm s�1 suggests that the assump-

tions for the rate of cuticular adsorption of ammonia and

stomatal uptake of ammonium ion are close to the reality.

4. Conclusions

There are great uncertainties in the estimation of dry

deposition rate of fine aerosol particles to forest

ecosystems. However, the yearly bulk dry deposition

velocity of ammonium and sulfate particles can be

estimated by a simple way using routine wet deposition,

throughfall and stemflow measurements as well as dry

deposition measurements of gases. Since ammonium

and sulfate mostly exist in PM2.5 range and are nearly in

stochiometric ratio dry deposition of ammonium sulfate

particles can be generalized for PM2.5 particles.

The deposition figures determined either from the S-

and N-balance calculations (vX0:8270:25 and ¼ 0:8470:25 cm s�1) are in good agreement and they are in

accordance with other experimental deposition velocities

found in the literature. These results suggest on one

hand the necessity of the revision of the models applied

during the theoretical calculation of dry deposition

velocity for fine particles and on the other, to continue

the simple experimental work described here for as many

places as possible.

Acknowledgements

Investigation were sponsored by: National Committee

for Technological Development, 1996–97, PHARE

TD&QM No. H-9305–02/1033; National Committee

for Technological Development 1996–98, No. 6-97-45-

1047; Ministry for Environment, 1999 No. 063/T; US-

Hungarian Joint Research Fund, 1996–99 No. 608/96;

National Committee for Technological Development,

1998–1999, UNDP-HUN/95/002-0119; Ministry for

Environment, 2000, No. KAC-20834; Ministry for

Environment, 2001, KAC-27822; Ministry for Environ-

ment, 2002, KAC-44146; Hungarian Scientific Research

Fund, 2000–2003, No. OTKA T-31927.

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Table 5

Calculated dry flux (DD) and dry deposition velocity ðvÞ of

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