On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient

Journal of Atmospheric Chemistry29: 17–43, 1998. 17c 1998Kluwer Academic Publishers. Printed in the Netherlands.

On the Exchange of NO3 Radicals with AqueousSolutions: Solubility and Sticking Coefficient

KARIN THOMAS, ANDREAS VOLZ-THOMAS, DJURO MIHELCIC,HERMAN G. J. SMIT and DIETER KLEYInstitut fur Chemie und Dynamik der Geosphare 2: Chemie der Belasteten AtmosphareForschungszentrum Julich, PO Box 1913, D-52425 Julich, Germany

(Received: 18 December 1996; in final form: 10 June 1997)

Abstract. The exchange of NO3 radicals with the aqueous-phase was investigated at room temperature(293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO3 wasdetermined on 0.1 M NaCl solutions and was found to be (NO3) � 2�10�3 in good agreement withrecent studies. The Henry coefficient of NO3 was estimated to beKH(NO3) = 1:8 M � atm�1, with a(2�) uncertainty of�3 M �atm�1. From the upper limit for the Henry coefficient (KH = 5 M �atm�1)and available thermodynamic data, the redox potential of dissolved NO3/NO�

3 is estimated to be inthe range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results arediscussed in the light of a recent publication. Based on our data and a model of the transport andchemistry in the liquid film, an upper limit is derived for the product of the Henry coefficientKH andthe rate coefficientk10 of the potential reaction NO3+H2O! HNO3+OH. ForKH = 0:6 M�atm�1,we findk10 < 0:05 s�1 �atm�1, i.e., about 100 times smaller than what was suggested by Rudich andco-workers. Because of its small solubility, heterogeneous removal of NO3 is only important underconditions where the dissolved NO3 is removed quickly from equilibrium, for example by reactionswith Cl� or HSO�3 ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceedvia N2O5.

Key words: nitrate radical, solubility, sticking coefficient, redox potential, heterogeneous removal.

1. Introduction

The NO3 radical plays an important role in the chemistry of nitrogen oxides atnight. It is produced from the oxidation of NO2 with O3 (R1).

NO2 + O3 ! NO3 +O2 (R1)

NO3 + h� ! NO+ O2

! NO2 +O (R2)

NO3 + NO ! NO2 +NO2 (R3)

During daytime, rapid photolysis (R2) and its fast reaction with NO (R3) serve tomaintain NO3 mixing ratios of<1 ppt. At night, on the other hand, NO3 can reachconcentrations of a few hundred ppt (Noxonet al., 1980; Noxon, 1983; Plattet al.,1980, 1981, 1982, 1984, 1990a). NO3 radicals are, besides O3, the major oxidantfor many hydrocarbons at night (Plattet al., 1990b; Mihelcicet al., 1993).

18 KARIN THOMAS ET AL.

Under conditions of high relative humidity, the effective lifetime of NO3 wasobserved to be relatively short, i.e., on the order of a few minutes (Plattet al., 1981,1984; Heintzet al., 1996), a fact that was associated with heterogeneous losses ofNO3 and/or N2O5, which is present in the atmosphere due to the equilibrium (R4).

NO2 + NO3 + M $ N2O5 + M: (R4)

Heterogeneous losses of NO3 and N2O5 can constitute an efficient sink for NOxand O3 and, in addition, be a major source of nitric acid in fog or rain (Heikes andThompson, 1983, 1984; Chameides, 1986a). Dissolution of N2O5 leads directlyto NO�

3 and H+ (R5). The NO3 radical because of its higher oxidation state will,upon dissolution, react with a variety of reduced species. For example, NO3oxidisesdissolved S(IV) compounds to S(VI) and chloride to chlorine ((R6), (R7); Netaand Huie, 1986). While the reaction with Cl� could play a role in the removal ofNO3 on sea salt aerosols in the clean maritime boundary layer, the fast reaction ofNO3 with dissolved S(IV) is a potential sink for SO2 at night, as was pointed outby Chameides (1986a).

N2O5 + H2O(liq) ! 2NO�

3 + 2H+ (R5)

NO3(aq) + HSO�

3 ! NO�

3 + SO�

3 + H+ (R6)

NO3(aq) + Cl� ! NO�

3 +Cl (R7)

The efficiency of heterogeneous removal of either NO3 or N2O5 depends on

� the sticking coefficient� that, besides molecular diffusion, controls the transferbetween gas and liquid phase (cf. Schwartz and Freiberg, 1981; Schwartz,1986),

� the Henry coefficientKH, that determines the partitioning between gas- andliquid-phase in case of nonreactive species, and

� the concentrations of other dissolved species that undergo chemical reactionswith NO3 and the rate coefficients of the respective reactions.

The sticking coefficient of N2O5 was determined in several studies: Kirchneret al.(1990) determined a lower limit of (N2O5)? � 0.005 to a pure H2O surface andVan Dorenet al.(1990) obtained values between 0.04 and 0.06 for water droplets.Mozurkewich and Calvert (1988) found similar values, i.e., 0:05 < (N2O5) <0:09, for NH4HSO4 aerosols, and Hanson and Ravishankara (1991) determinedvery large values, 0:1 < (N2O5) < 0:14, for sulfuric acid solutions at lowtemperatures (215–230 K). Knowledge of the Henry coefficient of N2O5 is oflesser importance, because the dissolved N2O5 undergoes rapid hydrolysis (R5).

We have studied the exchange of NO3 radicals with aqueous solutions in wet-ted denuders. Pure water was used for determination of the solubility and NaClsolutions for determination of the sticking coefficient. Preliminary results and the

? As is discussed below (Equations (5) and (6)), experiments usually yield the uptake coefficient , which is a lower limit of�.

ON THE EXCHANGE OF NO3 RADICALS WITH AQUEOUS SOLUTIONS 19

implications for atmospheric chemistry were presented in Thomas (1992) andThomaset al. (1993). The only other estimate of (NO3) at that time was anupper limit of 3:6� 10�4, which was based upon changes in the stoichiometry ofthe NOx- and O3-losses in a reaction chamber when humid aerosols were added(Verhees, 1986). Estimates of the Henry coefficient of NO3 covered a wide range.Mozurkewich (1986) calculated a value of 0:03 M � atm�1 from the free energyof hydration determined by Berdnikov and Bazhin (1970) and Chameides (1986b)calculated a value of 12 M� atm�1 from the Gibb’s free energies of formationfor gaseous and dissolved NO3. The much higher values of 2:1� 105 M � atm�1

(Chameides, 1984; Jacob, 1986) and 108 M �atm�1 (Leaitchet al., 1988) that wereused in some model calculations cannot be regarded as estimates of the solubilitybut imply the assumption of rapid removal of the dissolved NO3.

More recently, Rudichet al. (1996) presented measurements ofKH(NO3) =0:6 M � atm�1 and of (NO3) = 6� 10�3 at a temperature of 273 K, from theobserved first-order loss rates of NO3 in a wetted denuder. Although, their results arequite similar to ours, there are a number of principal differences in the experimentalconditions and in the interpretation of the results. In particular, Rudichet al.(1996)suggest a reaction between dissolved NO3 and water and suggest a rate coefficientfor this reaction which is much larger than what can be accounted for by our results,as is discussed below.

2. Experimental

The experimental set-up consisted of a system to generate a continuous flow ofNO3 radicals in air at ambient pressure and a series of three denuders (denotedA,B, andC) in which the uptake of NO3 in water was studied. A schematic diagramof the experiment is shown in Figure 1.

The NO3 radicals were produced by oxidation of NO with excess O3 at 393 K((R8) followed by (R1)).

NO+ O3 ! NO2 + O2 (R8)

The ozone was generated by means of a silent discharge in a flow of O2 (100ml �min�1 STP). This flow was diluted with 2 l�min�1 (STP) of N2. The resultingO3 mixing ratio was (225� 5) ppm. It was regularily measured by its absorptionat 253.7 nm. Both, N2 (99.999%) and O2 (99.998%) were purified with molecularsieve traps at 195 K. A mixture of 90 ppm NO in ultra pure N2 (99.9999%,Messer Griesheim) was added to the gas flow immediately upstream of reactionchamber 2 at flow rates of 2.5 to 10 ml� min�1 (STP), which resulted in NOmixing ratios of 100 to 500 ppb. Reaction chamber 2 was thermostatted at 393 Kin order to suppress formation of N2O5 and was kept dark in order to prohibitphotolysis of NO3. The residence time of the gas mixture in the chamber was40 s, approximately 40 times longer than the time constant for conversion of NO2

to NO3 by (R1), the rate limiting step in NO3 production. Model simulations of


Figure 1. Schematic view of the experimental set-up. The NO3 radicals were generated inreaction chamber 2 from NO and O3. Chamber 1 and the series of traps served to remove reactiveand soluble contaminants. The three denuders served the following purposes: A: removal ofHNO3 or N2O5; B: establishment of phase equilibrium and determination of [NO3]aq; C:quantitative collection of the remaining NO3 and conversion to NO�3 , i.e., determination ofpNO3. The NO�3 concentration was determined photometrically in aliquots collected from theeffluent of the denuders.

the system with FACSIMILE (Curtis and Sweetenham, 1985) predicted that theNO3 should comprise more than 95 percent of the NO oxidation products underall experimental conditions. Formation of N2O5 is negligible because of the largeexcess of O3 and the high temperature used.

From preliminary experiments, it was found necessary to implement a series ofcleaning steps into the ozone flow before being mixed with the NO. In particular,odd nitrogen compounds that are produced in the ozonizer from traces of N2 inthe oxygen add to the NO3 concentration so that a reliable odd nitrogen balancecannot be obtained (see below). At least as important are organic contaminants,since they can react with the NO3 to form HNO3 or organic nitrates. Therefore,reaction chamber 1 was installed behind the ozone generator. Reaction chamber 1was kept at a higher temperature than chamber 2, in order to ensure that any NO orNO2 that might have been present in the gas stream was fully converted to NO3 orHNO3. It also served the purpose to allow reactive contaminants to be oxidised byO3. Behind chamber 1 was a bubbler, filled with 0.1 M NaCl solution, which servedto remove NO3, N2O5 or HNO3 as well as soluble organic compounds, such asaldehydes, that could react with gaseous or dissolved NO3. Finally, the O3/O2/N2

mixture passed through two cold traps at 273 and 195 K, respectively. The last coldtrap served to reduce the water vapour mixing ratio to<1 ppm.

The quantitative conversion of NO to NO3 and the purity of the NO3 mixture wasinvestigated using the method of matrix isolation and ESR spectroscopy (MIESR),which provides a spectroscopic measurement of NO2, NO3, and peroxy radicals


Figure 2. Left panel:ESR spectrum of a cryogenic sample collected before installation of thecleaning procedure. The uppermost trace is the spectrum of the sample, below are the referencespectra of NO3 (2.5 ppb), NO2 (4.4 ppb) and RO2 (1.5 ppb), scaled to the concentrations foundin the sample. The lowest trace shows the residuals after subtraction of the reference spectrafrom the sample spectrum.Right panel:Same for a sample collected after installation of thecleaning procedure. There, the measured NO3 (81 ppb) concentration is in excellent agreementwith the theoretical value, NO2 (0.5 ppb) represents only a minor fraction and RO2 radicalsare below the detection limit of 5 ppt. The vertical line denotes the Lande factor of the freeelectron (g = 2).

(Mihelcic et al., 1990, 1993). Before installation of the cleaning procedure, theNO3 concentration behind reaction chamber 2 as measured by MIESR was muchsmaller than the initial NO concentration and NO2 was the major odd nitrogencompound (see Figure 2). In addition, organic peroxy radicals were found in thesamples at relatively large concentrations, clearly indicating the presence of organiccontaminants. After installation of the cleaning procedure, the concentrations oforganic peroxy radicals were below the detection limit of the MIESR (5 ppt) and themeasured NO3 concentrations were in good agreement with those calculated fromthe NO concentration in the standard mixture and the gas flows (pNO3=pNO > 0:9).The remaining NO2 concentration was<1% of the initial NO, in good agreementwith the model estimate.

The three denuders (Figure 3) consisted of coiled glass tubes of 2 mm innerdiameter (Lazruset al., 1986; Lind and Kok, 1986). A small flow (1 ml� min�1


Figure 3. Schematic outline of the denuders used in this study.

or less) of liquid was added to the gas flow upstream of the coiled part. The liquidflowed as a thin film along the wall of the tube and was separated from the gasin the wider (8 mm ID), vertical section of the tube. Supply and drainage of theliquid was achieved with a peristaltic pump. All experiments were made at roomtemperature (293 K) and atmospheric pressure.

The concentration of the dissolved NO3 was determined after its conversion toNO�

3 by reaction with Cl� (either within the denuder itself or directly thereafter,see below). An automated colorimetric method was used for the measurement ofNO�

3 . It is based on the Saltzman reaction of NO�

2 and photometric detection of theproduct by its absorption at 564 nm with a dual beam spectrometer. Reduction ofthe NO�3 to NO�

2 is achieved on cadmium coated copper granules. Details of thismethod are described in Meixneret al. (1985). The detection limit was between50 and 100 nmol� l�1 and the precision was 2%. The measurements were madeon 5–10 ml samples, which were collected from the effluent of the three denudersafter the system had reached steady state. Calibrations were made with standardsolutions (10�4 M NaNO3).

The three denuders served the following purposes:Denuder A was installed in order to remove traces of HNO3 or N2O5, which

might have formed in chamber 2, from the gas stream. These compounds areextremely well soluble, have large sticking coefficients and dissociate into nitrateions, which makes them indistiguishable from dissolved NO3 in our experiments.The presence of such compounds would lead to an overestimation ofKH and .Denuder A was 100 cm long and was fed with pure water from a Milli-Q-system.


Denuder B was used for the determination of the uptake coefficient and thesolubility of NO3. In each series of experiments, its length was varied between 4and 360 cm in discrete steps. For determination of , a 0.1 M NaCl solution wasused in denuder B in order to ensure rapid conversion of the dissolved NO3 (seebelow). For the determination ofKH, pure water was used as stripping solution.Conversion of the dissolved NO3 was then achieved by mixing the effluent of thedenuder with a known flow of a 0.1 M NaCl solution.

Denuder C was used for the determination of the NO3 that remained in thegaseous phase after passage through denuder B. Denuder C was 100 cm long andwas always fed with a 0.1 M NaCl solution.

For each experiment, the mass balance was calculated by comparing the NOpartial pressure upstream of denuder A (calculated from the mixing ratio quotedfor the gas tank and the gas flow rate) to the NO3 partial pressurepA, as calculatedfrom the measured nitrate concentrations in the three denuders [NO�

3 ]A;B;C, theliquid flow rates through the three denuders (�A,�B, and�C) and the gas flow rate�g (Equation (1)).

pA =RT � (�A � [NO�

3 ]A + � � �B � [NO�

3 ]B +�C � [NO�

3 ]C)

�g; (1)

where� = (�B + �Cl�)=�B is the dilution factor due to admixture of the 0.1 MNaCl solution to the effluent of denuder B.

The comparison provides information on the conversion efficiency of the NO toNO3 and on the presence of contaminants. The presence of contamination was alsodetermined separately during each series of experiments by switching the NO flowoff. The thus determined blank averaged (11� 5) ppb, i.e., 2–10% ofpA. Sinceno systematic differences in the blanks were found between the experiments madewith water or NaCl solution in denuder B, the results are summarised in Table I forall experiments. After subtraction of the blank, about 90% of the initial NO wasfound as nitrate in the three denuders, independent of the NO mixing ratio. Thedeviation from the theoretical value is within the possible error due to uncertaintiesin the various gas and liquid flow rates and the initial NO mixing ratio.

Likewise, the NO3 partial pressurepB upstream of denuder B was determinedfrom the nitrate concentrations in denuders B and C. It ranged from 10 to 70%of pA. While the higher fractions are in accordance with the expected uptake ofNO3 in denuder A (see results forKH below), the lower values clearly indicatethe presence of contaminants in the stripping solution that react with the dissolvedNO3. Since such reactions can lead to an overestimation ofKH, all experimentswherepB was smaller than 50% ofpA were discarded.


Table I. Odd-nitrogen balance as calculatedfrom Equation (1) for all experiments made todetermineKH and (�g = 2060 ml � min�1,�A �= �B �= �C �= 0:92 ml �min�1)

n pNO [ppb] pA [ppb]a pA=pNO

119 0 0� 5 –12 117� 3:3 107� 15 0:91� 0:1392 239� 0:5 212� 18 0:89� 0:0815 476� 2:6 429� 25 0:90� 0:05

a Corrected for a blank of 11 ppb.

2.1. THEORY

Since the experimental conditions for the determination of the sticking coefficientdiffer significantly from those for the determination ofKH, a brief description ofthe relevant theory is given here before the description of the experimental results.

The fluxF+ of a speciesX through a finite surface element dA = 2�r0dl ofthe air/water boundary in a cylindrical denuder with the radiusr0 is obtained fromthe total number of collisions with the surface that lead to a phase transition.

F+ =d2Nx

dAdt=

14� v � � �

px � L

RT= v+ �

px � L

RT; (2)

wherev = (8RT=� Mx)1=2 is the mean molecular velocity,� the sticking or mass

accommodation coefficient,px the partial pressure at the surface,L the Avogadroconstant,R the gas constant andT the temperature. In the absence of chemicalreactions in the aqueous phase, the system will approach phase equilibrium. Theflux of the absorbed gas molecules from the liquid into the gas phase can be definedin analogy to Equation (2),

F� = v� � L � [X(aq)] ; (3)

with [X(aq)] being the concentration ofX in the liquid boundary layer andv� theanlogue tov+. The velocitiesv+ andv� are related through Henry’s law.

KH =[X(aq)]eq

px: (4)

In phase equilibrium ([X(aq)] = [X(aq)]eq), F+ equalsF� and, hence,v� is

v� = 14 � v � � �

1RT �KH

: (5)

Experimentally, the net fluxFnet is determined.

Fnet = F+ � F� = 14 � v � � �

L

RT�

�px �

[Xaq]

KH

�= 1

4 � v � �L

RT� px ; (6)


with

= � �

1�

[Xaq]

[Xaq]eq

!:

Fnet is related to the effective uptake coefficient in the same way asF+ to �. Asis seen from Equation (6), approaches� for [Xaq]� [Xaq]eq. The characteristictime for reaching saturation at the phase boundary (�e, Equation (7); Danckwerts,1970; Schwartz and Freiberg, 1981) depends on�,KH and the diffusion coefficientin the liquid,Daq. It ranges from microseconds for gases with small solubilities toseconds for gases with very large solubilities.

�e = Daq �

�4KH �RT

v � �

�2

(7)

Determination of� on pure water is difficult for species that have a small ratioof KH=�, because saturation at the phase boundary is approached too quickly. Inthe case of NO3 (KH < 5 M � atm�1 and� � 10�3, see below), the residencetime of the water in the denuder would have to be less than 5 ms, in order toremain sufficiently far from equilibrium. The problem is usually solved by addinga reactant to the liquid in large enough concentration so that the chemical removalof NO3(aq) in the liquid boundary layer is fast compared to�e. We have adoptedthis approach by using a 0.1 M NaCl solution instead of pure water in denuderB. The rate coefficient for (R7) was measured by Neta and Huie (1986) to be7� 107 M�1 � s�1 at room temperature and by Exneret al. (1992) to be 1� 107

M�1 � s�1. Using the smaller value, the average chemical lifetime of the dissolvedNO3 is less than 1�s. Larger Cl� concentrations would have disturbed the NO�

3analysis. In addition, the higher ionic strengh could lead to significant changes inthe surface properties of the solution (and, hence,�) as is borne out for exampleby the change in surface tension.

Neglecting the diffusive resistance in the gas phase, the rate of change of NO3

in denuder B is given by Equation (8),

�dpNO3

dt= v+

dAdV

� pNO3 =2 � v+r0

� pNO3 = �v

2r0� pNO3; (8)

where dV = �r20dl is the denuder volume element. Since all experiments were

conducted at room temperature and atmospheric pressure, the partial pressure isproportional to mixing ratio or concentration. Integration of Equation (8) withdt = �r2

0��1g dl yields an expression for the transmittance of the denuder as a

function of

pC

pB= exp(�1

2 � � v � � � r0 � lB � ��1g ) (9)

with pB andpC being the partial pressure of NO3 at the entrance and outlet of thedenuder B, respectively, andlB the length of the denuder.


Because of the neglection of the diffusive resistance, Equation (9) yields alower limit for . The characteristic time for diffusion in the gas phase to thewall of a cylindrical tube is approximately�g = 0:01 s (with r0 = 0:1 cm anda molecular diffusion coefficient ofDg = 0:1 cm2 � s�1). Diffusion becomesrate limiting for > 10�3. The full description of mass transfer in the gas-phase of a cylindrical denuder is given by the Cooney–Kim–Davis solution of thedimensionless characteristic diffusion equation (Cooneyet al., 1974),

pC

pB= B1 � exp(��2

1 � z�) +B2 � exp(��2

2 � z�)+

+B3 � exp(��23 � z

�) + � � �

z� = lB ��

2�D0

g

�g�

�T

T0

��1

;

(10)

wherez� is the normalised cylindrical co-ordinate,D0g the diffusion coefficient at

STP,T the temperature of the gas,T0 = 273 K, and� �= 2 (Murphy and Fahey,1987). The parameters�i are the eigenvalues of the dimensionless characteristicdiffusion equation. Both,�i and the coefficientsBi are functions of the Sherwoodnumber,NShw (Equation (11)), which depends on the diffusion coefficient and .Murphy and Fahey (1987) calculatedBi and�i for different values of the Sherwoodnumber.

NShw=v � r0

4 �Dg�

1�

2

: (11)

For the direct determination ofKH, phase equilibrium must be fully established inthe bulk of the liquid, i.e.,Fnet = 0. (As is discussed later,KH can also be derivedfrom the rate of change of NO3 in a reactive system, provided that the diffusioncoefficient of NO3 in the liquid-phase and the rate constant for the loss of NO3(aq)are known (cf. Rudichet al., 1996).) The time required for establishing saturationin the bulk is largely determined by diffusion. With a diffusion coefficient in theliquid of Daq = 2�10�5 cm2 � s�1 (Schwartz and Freiberg, 1981) and the averagethickness of the liquid film s= 0:03 mm (see below), the characteristic time ofdiffusion in the liquid phase is approximately�a = 0:2 s.

3. Results

3.1. UPTAKE COEFFICIENT

For determination of , 50 experiments were made using 0.1 M NaCl as a scavengerat the NO mixing ratios given in Table I. In each experiment, the length of denuderB (coiled section, see Figure 3) was varied between 4 and 100 cm in discretesteps. The transmittance of denuder B, i.e.,pC=pB, was derived from the nitrate


concentrations that were measured in the effluents of denuders B and C according toEquation (12), with�B and�C being the flow rates of the liquid through denudersB and C, respectively.

pC

pB=

�C � [NO�

3 ]C

�B � [NO�

3 ]B +�C � [NO�

3 ]C: (12)

The largest uncertainty in the residence time of the gas in the denuder (or its effectivelength), which must be known for determination of from Equations (9) or (10),arises from uncertainties in the effective contact time in the gas/liquid separatorbehind the coiled section (see Figure 3). Much better known is the incrementbetween denuders of different length. Therefore, all transmittances obtained fordenuders of different length in a given experiment were normalised to that foundfor the shortest denuder (l = 4 cm) (Equation (13)).

Ti =(pC=pB)i(pC=pB)4 cm

and l0B;i = lB;i � 4 cm: (13)

Ti is the normalized transmittance of denuder B with the lengthlB;i andl0B;i is thecorrected length.

The results are displayed in Figure 4. Each symbol represents the average trans-mittance obtained in all experiments for a given length of denuder B. The errorbars give the standard deviations. Also shown are theoretical curves of the trans-mittance calculated from Equation (9) for different values of the uptake coefficient,assuming that mass transfer in the gas phase is not limited by diffusion, that is byarbitarily assumingD = 1. The experimental data cluster around the theoreticalcurve for (NO3) = 10�3 within the experimental uncertainties. As this simplemodel tends to underestimate , the value of 10�3 is a lower limit of the uptakecoefficient of NO3.

A more refined analysis using Equation (10) requires the knowledge of thediffusion coefficient of NO3, which is not known. We adopted the value for HNO3,because of the similar molecular mass of the two molecules. The diffusivity ofHNO3 in air was determined by a number of investigators (Bramanet al. 1982;Durham and Spiller, 1982; Eatoughet al., 1985; Ferm, 1986; Durham and Stock-burger, 1986; Philips and Dasgupta, 1987; Benneret al., 1988). The results rangefrom 0.089 cm2 � s�1 to 0.121 cm2 � s�1 and average (0:11� 0:011) cm2 � s�1.

The middle panel of Figure 4 shows the experimental data together with the-oretical curves computed with the CKD-solution (10). The values of�i andBi

were interpolated from those tabulated by Murphy and Fahey (1987) to the Sher-wood number that corresponded to the experimental conditions. Interpretation ofthe NO3 transmittances with the theoretical curves leads to a value for of greaterthan unity. A likely explanation for this result is that the gas flow in the denuderis not laminar. Although the Reynold’s number of 1500 is below the critical valueof 2300, it is large enough that small disturbances can cause instabilities in thegas flow leading to turbulence. In addition, establishment of a strictly laminar flow


Figure 4. Transmittance of denuder B for NO3 as a function of its length using 0.1 M NaCl asscavenger. The symbols and error bars are averages and standard deviations for all experimentsmade with a given length of denuder B. The solid lines are calculated for different values of .Upper panel: using Equation (9);middle panel: using Equation (10) and a molecular diffusioncoefficient of 0.1 cm2 �s�1; lower panel: using Equation (10) and an eddy diffusion coefficientof 0.6 cm2 � s�1, as determined from Figure 5.


requires a certain time, which increases with increasing Reynold’s number. Underour conditions, the laminar flow profile is established only after about 15 cm.

In order to determine the effective diffusion coefficient of NO3 under our ex-perimental conditions, we made measurements of the transmittance of HNO3. Thesticking coefficient of HNO3 to aqueous surfaces is large enough that the trans-mittance in our denuders is almost completely determined by diffusive transport inthe gas-phase. For these experiments, the NO was disconnected and HNO3 froma permeation source was added to the gas flow downstream of denuder A (seeFigure 1). A 1 M NaOH was used as stripping solution in denuder B, in order toensure that the absorption was only determined by mass transport in the gas-phase.The results are given in Figure 5. The transmittances derived from the experimentsare corrected for blanks and are normalised to the shortest denuder, as before. Thetheoretical curves show the transmittances obtained with the CKD-solution fordifferent diffusion coefficients and for (HNO3) = 0:01, which is the lower limitreported by Kirchneret al.(1990). The best fit to the experimental transmittances isobtained for an eddy diffusion coefficient of 0.6 cm2 �s�1, about a factor of 5 largerthan the molecular diffusion coefficient. Using (HNO3) = 1, the eddy diffusioncoefficient comes out slightly smaller, i.e., 0.5 cm2 � s�1.

The thus determined eddy diffusion coefficient was then used to re-calculate thetheoretical transmittances for NO3. As is seen in the lower panel of Figure 4, theexperimental data for NO3 are best described with values of = 2� 10�3. Thisvalue must still be regarded a lower limit of the sticking coefficient�. It is alsoseen that the system has virtually no sensitivity to the uptake coefficient for valueslarger than approximately 5� 10�3 under our experimental conditions, becausediffusion becomes the rate limiting process.

3.2. HENRY COEFFICIENT

For determination ofKH, denuder B was fed with pure water from a Milli-Q-system(conductivity< 10�7 cm��1). The dissolved NO3 was converted to NO�3 behindthe gas/liquid separator by mixing the effluent of denuder B with a known flow ofa 0.1 M NaCl solution.

As before, for determination of , the partial pressure of NO3 behind denuder B,pC, was derived from the nitrate concentration, [NO�

3 ]C, measured in the effluentof denuder C (Equation (14))

pC =RT

�g� �C � [NO�

3 ]C (14)

and the Henry coefficient was calculated from the measured quantities using Equa-tion (15),

KHNO3=� � [NO�

3 ]BpC

=�g

�C�� [NO�

3 ]B

RT � [NO�

3 ]C(15)


Figure 5. Transmittance of denuder B for HNO3 as a function of its length using 1 M NaOH asscavenger. The symbols and error bars are averages and standard deviations for all experimentsmade with a given length of denuder B. The solid lines are calculated with Equation (10) fordifferent values of the diffusion coefficient and for = 0:01 (upper panel) and = 1 (lowerpanel).

where� = (�B + �Cl�)=�B�= 2 is the dilution factor due to admixture of the

0.1 M NaCl solution to the effluent of denuder B.For the determination ofKH, the residence timeta of the solution in denuder

B must be long enough that phase equilibrium is fully established, i.e.,ta � �a.However, a highly reactive species such as NO3 can undergo chemical reactionsin the liquid-phase that convert the dissolved NO3 into nitrate. As such reactionslead to an overestimation ofKH, the contact time must be kept as short as possible.


Figure 6. Observed values ofK0

H as a function of the residence time of the water in denuderB. The straight line is a linear fit to the data and the dotted line shows the calculated temporaldevelopment ofK0

H when using Rudich’s values forKH = 0:6 M � atm�1 andk10 = 10M�1 � s�1.

In order to investigate these conflicting conditions, the length of denuder B wasvaried between 4 and 360 cm. In addition, the flow rates of the gas mixture and ofthe water in denuder B were changed.

The results of 70 experiments made to determine the Henry coefficient of NO3

are summarised in Figure 6. The calculated Henry coefficient, Equation (15), isplotted as a function of the residence time of the liquid film (ta) in denuder B.Sinceta is a complex function of both, gas and liquid flow rate, it was determinedexperimentally. After steady state was established for the water flow in denuderB, the peristaltic pump was stopped and the amount of water in the denuder wasweighted. This was done for all the different combinations of gas and liquid flowrates and for all the different denuders employed in our study.

The apparent Henry coefficient is found to increase steadily with increasingresidence time. Since phase equilibrium should be fully established after a fewseconds, conversion of the dissolved NO3 must occur. As stated above, dissolvedNO3 reacts with a number of ionic species as well as organic molecules, in par-ticular carbonyl compounds (Neta and Huie, 1986). Even in the absence of suchcontaminants proceeds the conversion via reaction with hydroxyl ions, OH� ((R9);Exneret al., 1992).

NO3 + OH� ! NO�

3 + OH: (R9)

The hydroxyl radicals that are formed in (R9) recombine to hydrogen peroxide(H2O2) or react with dissolved O3 (see Table II). The radicals (HO2 and O�2 ) thatare formed in the latter reaction can again react with NO3 and thus increase theefficiency of NO3 conversion.


Table II. Liquid-phase reactions used for modelling of NO3 conversion;(a) Exneret al. (1992); (b) Jacob (1986); (c) Graedelet al. (1986)

Reaction k298[l �mole�1 � s�1]

NO3 + OH� ! NO�

3 + OH 8:2� 107 (a)NO3 + HO2 ! NO�

3 + H+ + O2 4:5� 109 (b, estimate)NO3 + H2O2 ! NO�

3 + H+ + HO2 1:0� 106 (b)NO3 + O�

2 ! NO�

3 + O2 109 (b, estimate)

O3 + OH� ! HO2 + O�

2 3:7� 102 (c)O3 + OH! HO2 + O2 2:0� 109 (b)O3 + HO2 ! OH+ 2O2 < 104 (b)O3 + O�

2 + H2O! OH+ 2O2 + OH� 1:5� 109 (b)O3 + HO�

2 ! OH+ O2 + O�

2 2:8� 106 (b)

OH+ OH! H2O2 5:2� 109 (c)OH+ HO2 ! H2O+ O2 7:0� 109 (b)OH+ H2O2 ! H2O+ HO2 2:7� 107 (b)OH+ O�

2 ! OH� + O2 1:0� 1010 (b)HO2 + HO2 ! H2O2 + O2 8:6� 105 (b)HO2 + H2O2 ! OH+ H2O+ O2 5:0� 10�1 (b)HO2 + O�

2 + H2O! H2O2 + O2 + OH� 1:0� 108 (b)HO2 + O�

2 ! HO�

2 + O2 8:5� 107 (b)H2O2 + O�

2 ! OH+ OH� + O2 1:3� 10�1 (b)

Under the assumption that chemical conversion of dissolved NO3 is the causefor the apparent increase in the measured Henry coefficient, the real value forKH

is given by extrapolation tota = 0 (cf. Equation (19) below). This was done byfitting a straight line to the data in Figure 6. The intercept with the ordinate gives abest estimate ofKH = (1:8� 1:5) M � atm�1 for the Henry coefficient of NO3 at293 K.

An additional analysis was made by Thomas (1992) with the help of numericalsimulations of the chemical and physical processes involved in the formation ofNO�

3 in denuder B. The model was formulated on the basis of the characteristictimes required to establish phase equilibrium and included the chemical conversionof the dissolved NO3 to NO�

3 as initiated by OH�-reaction (R9) (for details seeThomas, 1992). For O3, instantaneous phase equilibrium was assumed, using aHenry coefficient of 1:1� 10�2 M � atm�1 (Kozak-Channing and Heltz, 1983).

For each individual experiment, the model was initialised with the NO3 con-centration in the gas-phase. It then calculated the equilibrium NO3 concentrationin the liquid-phase from Henry’s law and integrated the chemical conversion ofdissolved NO3 to NO�

3 over the residence time of the solution in denuder B. There-after, the apparent Henry coefficient was calculated from the NO3 that remained inthe gas-phase and the concentration of NO3 + NO�

3 in the liquid. On the basis ofthe known reactions, the best agreement between the measured and modelled time


evolution of the apparent Henry coefficient was obtained withKH = 2 M � atm�1,in excellent agreement with the value obtained by back extrapolation tota = 0.

4. Discussion

The uptake of NO3 in aqueous solutions was also investigated by Rudichet al.(1996). These authors produced NO3 by thermal decomposition of N2O5 andfollowed the decay of NO3 in the gas-phase in a vertical denuder which wasequipped with a movable injector for the addition of the gas.

Our lower limit for the uptake coefficient of NO3 on aqueous solutions obtainedat 293 K is in good agreement with Rudichet al. (1996), who obtained a value of = 2:73� 10�3 at a temperature of 273 K when using a 0.1 M NaCl solution forremoval of the dissolved NO3. Using more concentrated solutions, however, theseauthors found to increase up to values of 6� 10�3.

From Table III, which compares the relevant experimental parameters of thetwo studies, it is seen that the maximum first order loss coefficient for the dissolvedNO3 (k7 � [Cl�]) is similar in both studies. The five times larger Cl� concentrationused by Rudichet al.(1996) is compensated by the 4 times smaller rate coefficientat the lower temperature. Therefore, the discrepancy is not easily explaind bydifferences in the liquid phase. Another possibility for the apparent increase of with increasing Cl� concentration are changes in the surface conditions of thesolution, which possibly lead to an increase in�. It is also seen in Table III thatRudichet al. (1996), although working at much lower pressure and in Helium, infact had a much longer characteristic diffusion time because of the 10 times largertube diameter. Since the higher values of are, for both experiments, in the rangewhere the uptake is almost exclusively limited by diffusion in the gas phase (cf.Figure 4), the results are extremely sensitive to small errors in the calculation ofDg.

When taken at face value, there is also reasonable agreement for the Henrycoefficient. Rudichet al.(1996) derived a value ofKH = 0:6 M � atm�1 from theirmeasurements, a factor of 3 smaller than our best estimate. The difference is onlyslightly larger than the quoted standard errors and the agreement is sufficient toconclude that the values derived by Chameides (1986b) and Mozurkewich (1986)were significantly over and underestimated, respectively. The situation is somewhatmore complicated, however, since Rudichet al.were unable to explain the uptakerates measured in pure water on the basis of the chemistry used by us and postulatedreaction (R10) to be responsible for the removal of NO3, in addition to (R9).

NO3 + H2O! HNO3 + OH: (R10)

Rudichet al. (1996) measured an uptake coefficient of = 2� 10�4 for purewater, from which they derivedk10 = 23(+30� 13) M�1 � s�1. Thomaset al.(1993) were able to explain the measured uptake in water without reaction (R10)and concluded that it was too endothermic for being of importance. If we analyse


Table III. Experimental conditions and results of this study compared to those of Rudichetal. (1996)

Quantity (units) Rudichet al. Thomaset al. This paper(1996) (1993)

r0 (cm) 0.95 0.1 SameL (cm) 0–18 4–360 SameT (K) 273 293 SameGas mixture (hPa) 6.5–13 He; 800 N2, 200 O2; Same

6 H2O 25 H2Os (cm) 0.025 0.001–0.0035 Sameug (cm/s) 650–1600 1100, 2200 Sametg (s) 0–0.028 0.01–0.33 Samea

ua (cm/s) ca. 20 2, 3, 7 Sameta (s) 0–1.8 1–90 SameDg (cm2/s) 13–17 (calculated) 0.5–0.6 (measured)a SameDa (cm2/s) 1� 10�5 2� 10�5 See text�g (s) 0.007 0.002 See text�a (s)b 30 0.6 See textMethod used d[NO3]=dl gas phase d[NO3]=dl gas phase and Same

d[NO�

3 ]=dl liquid phaseKH (M atm�1) 0:6� 0:3 1:8� 1:5 0.05c

6� 10�3 2� 10�3 2� 10�3

kNO3+H2O (M�1 s�1) 6 Assumed negligible <0.05d

kNO3+OH� (M�1 s�1) Assumed negligible 8:3� 10�7

a Eddy diffusion coefficient as determined with HNO3. The molecular diffusion coefficientwould be around 0.1 cm2 � s�1.b With Da = 10�5 cm2 s�1.c With k10 = 6 M�1 � s�1.d With KH = 0:6 M � atm�1.

our experimental results for pure water in the same way as Rudichet al.(1996), i.e.,from the observed first-order loss of NO3 in the denuder using their Equation (2),we obtain a much smaller uptake coefficient, of approximately 3� 10�5. (A valueof � 2� 10�4 was found by us for a 10�3 M NaCl solution; see Thomaset al.(1993) for details). Using Rudich’s value ofKH = 0:6 M � atm�1 andDaq = 10�5

cm2 � s�1 in combination with our value for on pure water and neglecting allother possible reactions of the dissolved NO3, we obtaink10 � 0:5 M�1 � s�1.The obvious conclusion is that the apparent reaction between dissolved NO3 andH2O discussed by Rudichet al. (1996) is an artefact caused by impurities in thewater, a possibility which they strongly argue against, however. In view of this,other possible explanations are seeked below from the way the experimental datahave been analysed.

A coarse estimate of the influence of (R10) on the time development of the mea-sured Henry coefficientK 0

H in Figure 6 can be made as follows: We assume phase


equilibrium to be established virtually instantaneously compared to the residencetime of the liquid in denuder B. We also neglect the small decrease inp(NO3)between entrance and exit of denuder B. The NO�

3 production rate in denuder B isthen simply given by Equation (16)

d[NO�

3 ]

dt= k10 � [NO3]aq � [H2O] = k10 � p(NO3) �KH � [H2O] (16)

The apparent Henry coefficient,K 0

H, obtained from our data is defined as

K 0

H =[NO�

3 ]

p(NO3): (17)

It’s rate of change is given by

dK 0

H

dt=

1p(NO3)

�d[NO�

3 ]

dt: (18)

Substitution of d[NO�3 ]=dt from Equation (16) and integration yields

K 0

H = k10 � [H2O] �KH � ta +KH (19)

Equation (19) shows that the intercept provides a direct measurement ofKH,whereas the slope of the straight line in Figure 6 is a measure of the product ofKH

and the rate coefficient of (R10). The dotted line in Figure 6 shows the calculatedtemporal development ofK 0

H in our system for Rudich’s value ofKH = 0:6M � atm�1 and their lower limit ofk10 � 10 M�1 � s�1. The slope is more than anorder of magnitude larger than what would be compatible with our experimentaldata.

Since this simple model tends to slightly overestimate dK 0

H=dt, we present inFigure 7 the results of more accurate calculations with a numerical model thatincludes diffusion processes in the liquid and accounts for the small decrease inp(NO3). The model contained 35 discrete layers of the liquid film. The radialdependence of the liquid flow rate was neglected, since the characteristic diffusiontime is much shorter than the residence time of the liquid and since the gas residencetime in the denuder is more than 100 times shorter than that of the liquid film(see Table III). In order to be consistent with Rudichet al. (1996) the diffusioncoefficient was assumed to beDaq = 10�5 cm2 �s�1, rather than 2�10�5 cm2 �s�1

used by Thomaset al. (1993). Phase equilibrium was assumed to be establishedinstantaneously in the first layer and a zero flux condition was used between thelast liquid layer and the glass wall.

The curves in Figure 7 show the dependence of dK 0

H=dt on k10 for differentvalues ofKH between 0.03 and 1.8 M� atm�1. Also shown is the upper limit ofdK 0

H=dt from our measurements and the result of Rudichet al. (1996). Clearly,the combination ofk10 andKH that was determined by Rudichet al. (1996) isabout a factor of 10–20 larger than what can be accounted for from our data. With


Figure 7. Calculated time evolution of the apparent Henry coefficient in our experiment due toa chemical reaction with water (R10) for different values ofKH. The symbol gives the resultobtained by Rudichet al. (1996) and the parallel to the abscissa is the maximum slope thatwould be compatible with our experiments.

KH = 0:6 M � atm�1, our data suggest that reaction (R10) proceeds at a muchsmaller rate than what was proposed by Rudichet al. (1996). If on the other handtheir estimate ofk10 � 10 M�1 � s�1 was valid, thenKH must be very close tothe value of 0.03 M� atm�1 calculated by Mozurkewich (1986) from the heat ofhydration given by Berdnikov and Bazhin (1970).

Rudichet al. (1996) determined the Henry coefficient from the slope of theeffective uptake coefficient versus the Cl� concentration. As is seen from Equation(20), the value obtained depends on a number of parameters, i.e., the diffusionconstant in the liquid phase, the rate coefficient for reaction (R7) and (because isderived from the diffusion corrected first order loss coefficientkcorr, see Equation(2) in Rudichet al., 1996) on the diffusion coefficient in the gas phase. As is seenin Equation (21), the value fork10 as derived by Rudichet al.(1996) depends onlyon the assumed value fork7, whereas the uncertainties inDa andKH cancel in firstapproximation.

KH = CL� � v

4RTqDa � k7 � [Cl�]

(20)

k10 � [H2O] =�

H2O � v

4RT �KH

�2

�1Da

�

H2O

Cl�

!2

� k7 � [Cl�] (21)


The model used by Rudichet al.(1996) for interpretation of their results implicitlyassumes steady state in the liquid phase, which is not valid for a relatively slowreaction. Rudichet al. (1996) argue that, in the absence of liquid-phase chemistry,saturation would be reached within 0.1 ns. This time interval is just sufficient,however, to saturate the first monolayer of the liquid. Saturation in the sense thatF+ equalsF� is only observed when the diffusive flux from the surface intodeeper layers of the liquid becomes small compared toF+ (cf. Equation (7)). Itis important to note that in Rudich’s experiments the characteristic diffusion timein the liquid film is about 20 times longer than the longest residence time of theliquid (see Table III) and, therefore, diffusion into the bulk of the liquid is not easilydistinguished from a reaction with the solvent.

The differences in the experimental results could indeed be explained consis-tently if we assume that the effective diffusion coefficient in Rudich’s experimentswas 100–1000 times larger than the value ofDa = 10�5 cm2 � s�1 used in theirinterpretation. Then, the Henry constant would come out 10–30 times smallerwhile the value fork10 remained unchanged (see Equations (20) and (21)). Withthe smaller value ofKH = 0:03 M � atm�1, our model givesk10 = 10 M�1 � s�1

and the results of both experiments could thus be explained consistently. It must benoted that Rudichet al. (1996) have taken great care to ensure laminar conditionsin the liquid film. However, while the turbulent regime is only reached at Reynoldnumbers>400, strictly laminar conditions are only observed for Reynold numbers<4 (cf. Prandtlet al., 1984).

Another interesting hypothesis that could possibly explain the discrepancies isthat the removal of NO3 in pure water occurs mainly at the surface of the liquidfilm, rather than in the bulk of the liquid phase. Such a mechanism was suggestedby Mertes and Wahner (1995) in order to explain their results for the uptake ofNO2 in H2O. Neglecting the resistance in the gas phase which is very small in theexperiments made with pure water, the relative rate of removal at the surface ofthe liquid film should be proportional to the surface area and the velocity of thewater film and inversely proportional to the gas velocity. When normalised tota, therelative change in the gas phase dlnp(NO3)=dt is approximately 10%/s in Rudich’sexperiment and only 1%/s in our experiments. Since the gas velocity is similar inboth experiments, the 10 times faster removal rate would be in agreement withthe 10 times larger surface area in Rudich’s experiment due to the larger diameterof their denuder. A final answer can only be given on the basis of experiments inwhich the surface to volume ratio of the liquid is changed in a systematic manner.

Both assumptions that could bring the results of Rudichet al. (1996) intoreasonable agreement with ours, implyKH to be much smaller than the apparentvalues derived from either experiment.

The Henry’s law constant can be utilised to calculate the redox potential ofdissolved NO3 (for details see Thomas, 1992) from the reaction sequence

NO3(g) $ NO3(aq) ; (R11)


Table IV. Calculation of�G0f (NO3(aq)) andE0

NO3=NO�3

from the mea-

sured Henry coefficient and thermodynamic data

KH �G0f (NO3(aq)) E0

NO3=NO�3

Source

[M � atm�1] [kJ �mole�1] [V]

0.03 124.8 2.45 Mozurkewich (1986)0.6 117.2 2.38 Rudichet al. (1996)1 116.1 2.362 114.4 2.34 This work5 112.1 2.32

NO3(aq) + e� $ NO�

3(aq) : (R12)

Net:

NO3(g) + e� $ NO�

3(aq) :

Equilibrium constant (K) and Gibb’s free energy (�G) are related according to(22).

�G(R11) = �RT � lnKH : (22)

The free energy of a reaction is given by the difference between the free energiesof the products and those of the educts, i.e.,

�G(R11) = �G0f (NO3(aq))��G0

f (NO3(g)) : (23)

Estimates of the free energy of formation of gaseous NO3 range from 116.11kJ � mole�1 (JANAF, 1971) to 117.15 kJ� mole�1 (Chameides, 1986b). With ourupper limit ofKH < 5 M � atm�1, derived from the intercept in Figure 6 plus 2times the standard error, the free energy of dissolved NO3 is calculated to be>112kJ�mole�1 as is shown in Table IV. The results based on the value of 0.6 M�atm�1

(Rudich et al., 1996) and 0.03 M� atm�1 (Mozurkewich, 1986) are shown forcomparison.

The Gibb’s free energy of the redox reaction (R12) is related to the redoxpotentialE0 by Nernst’s equation

�G(R12) = �jzj � F �E0NO3=NO�

3= �G0

f(NO�

3 )��G0f (NO3(aq)) ; (24)

E0NO3=NO�

3= f�G0

f (NO3(aq))��G0f (NO�

3 )g=F ; (25)

wherez = 1 is the number of electrons andF is the Faraday constant. The freeenergy of formation of dissolved NO�3 is –111.29 kJ�mole�1 (Wagmanet al., 1982;Chameides, 1986b). With this the redox potential of dissolved NO3 is calculated


Table V. Redox potentials of NO3/NO�

3 and other species

Redox pair E0 (V) Source

OH, H+=H2O 2.72 Berdnikov and Bazhin (1970)Schwarz and Dodson (1984)

OH/OH� 1.9 Berdnikov and Bazhin (1970)Schwarz and Dodson (1984)Kl aninget al. (1985)

H2O2/H2O 1.77 Latimer (1952)

SO�4 /SO2�4 2.5–3.1 Estimate by Eberson (1982)

SO�5 /HSO�5 1.1 Huie and Neta (1984)

HSO�5 /HSO�4 1.82 Steele and Appelman (1982)

Cl/Cl� 2.41 Schwarz and Dodson (1984)

Cl�2 /2 Cl� 2.09 Schwarz and Dodson (1984)

O3/O�

3 1.01 Klaninget al. (1985)

NO2/NO�

2 1.03 Wilmarthet al. (1983)

NO3/NO�

3 2.2–2.7 Neta and Huie (1986)2.3 Berdnikov and Bazhin (1970)2.3–2.4 This work

to be between 2.32 and 2.45 V (Table IV). This is at the lower bound of the rangeof 2.2 to 2.7 V that was estimated by Neta and Huie (1986) from kinetic data.Table V shows that dissolved NO3 is a weaker oxidant than SO�4 and OH (in acidicsolutions) but a much stronger oxidant than the more stable compounds H2O2, O3

and NO2. The redox potential is similar to or higher than that of Cl-atoms, sincethe latter recombine readily with Cl� to Cl�2 .

5. Conclusions

From the experiments presented here it is concluded that the sticking coefficient ofNO3 to aqueous solutions is larger than 2� 10�3. It remains to be investigated ifthe three times larger values obtained by Rudichet al.(1996) in more concentratedsolutions are the result of changes in the surface properties at the higher ionicstrengths or if the discrepancies are due to uncertainties in the knowledge of thediffusion coefficient of NO3 in the He/H2O mixture.

The Henry coefficient was determined by us to be (1:8� 1:5) M � atm�1 inreasonable agreement with the value of 0.6 M�atm�1 derived indirectly by Rudichet al. (1996). Both measurements are much smaller than the earlier estimate of 12M �atm�1 by Chameides (1986b). The situation is complicated, however, by the fact


that Rudichet al. (1996) found much larger removal rates in pure water than wereobtained in our study and, hence, suggested a fairly fast reaction between NO3 andwater in order to explain their results. The required rate coefficient is much largerthan would be compatible with our data. The easiest explanation would be that thefaster removal observed by Rudichet al.(1996) was the consequence of impuritiesin the water. Another, very interesting hypothesis is that the removal occurred at thesurface rather than in the bulk of the liquid, due to an enhanced NO3 concentrationat the surface. Yet another possibility is that the results by Rudichet al. (1996)were affected by nonlaminar conditions in the liquid film. Both assumptions couldexplain the differences between both experiments. As a consequence, the Henrycoefficient of NO3 would be much smaller. Such a conclusion is not inconceivable:Mozurkewich (1986) calculated a value ofKH = 0:03 M � atm�1 based on theheat of hydration given by Berdnikov and Bazhin (1970). In addition, there are noobvious reasons from thermodynamic properties, such as the dipole momentum,that the Henry coefficient of NO3 should be much larger than that of NO2, whichwas determined to be in the range 0.007 to 0.012 M� atm�1 (Lee and Schwartz,1981; Schwartz and White, 1981).

As was already discussed by Thomaset al. (1993), removal of NO3 in theatmosphere is only important as compared to that of N2O5 under conditions wherethe NOx concentrations are small and where the dissolved (or adsorbed) NO3 isremoved efficiently by chemical reactions.

Acknowledgements

We wish to thank P. Musgen for the help with the MIESR sampling, K. P. Mullerfor letting us do the photometric NO�3 analysis on his autoanalyser and for hisassistance during the nitrate measurements, and A. Wahner for helpful discussions.The work was partially sponsored by the German Minister for Research (BMBF)as part of the EUROTRAC sub-project TOR (Grant No. 07 EU 723 5A).

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On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient

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