J. Phys. Chem. 1995,99, 14000-14006
14000
Uptake of Nitrogen Dioxide and Nitrous Acid on Aqueous Surfaces Stephan Mertes* and Andreas Wahner Institut f i r Atmosphiirische Chemie, Forschungszentrum Jiilich GmbH, Postfach 1913, 0-52425 Jiilich, G e m n y Received: November IO, 1994; In Final Form: June 21, 1 9 9 P
The uptake of gaseous nitrogen dioxide (N02) and nitrous acid (HN02) on aqueous surfaces is determined using a liquid jet technique. Both the loss of the two species from the gas phase and the formation of nitrite and nitrate in the liquid phase are measured. The experimental results are compared with two-dimensional model calculations simulating transport and chemical processes in the gas and liquid phase. The measured uptake of NO2 is much larger than that calculated from the model and literature values for the solubility of NO2 in water indicating, as all other experimental evidence, a reaction of NO2 on the aqueous surface. The measurements also indicate a partial release of HN02 from the surface to the gas phase. The comparison of experiment and model yields a lower limit of the mass accommodation coefficient of NO2 of a 2 2 x at 278 K surface temperature. For the mass accommodation coefficient of HN02 a range from 4 x to 4 x at 278 K surface temperature is obtained.
Introduction
Gas Phase
Surface
Uptake on aqueous surfaces and subsequent chemical reaction in the bulk liquid or on the surface can play an important role in the removal and conversion of soluble trace gases. One example of such heterogeneous reactions is the conversion of nitrogen dioxide (N02) to nitrous acid (HN02) on wet aerosol surfaces observed at night in the polluted and attributed to reactions 1and 2, since the analogous reactions in the gas phase are too slow to explain the observation^:'.^
HNO,(a)
+
H+ NO3- and HN02(a) H+ (a) aqueous phase; (8) gas phase; (g/a) gas phase or aqueous phase
e
+ NO2-
Investigations of these heterogeneous reactions in smog chamber studies show that they proceed fast on chamber surface^.^,^-" To quantify some possibly rate limiting steps, we investigate in this paper the uptake rate of gaseous NO2 and HN02 into liquid water using a liquid jet technique. At the same time we observe the release of HN02 induced by the uptake of NO?. The uptake can be described as a series of steps (Figure 1). Molecules of type A are transported from the gas phase to the surface by diffusion. Once within a distance of one mean free path they can collide gas kinetically with the surface. A fraction of the collisions result in sticking of molecule A to the surface. The sticking probability is called the mass accommodation coefficient a: The molecules on the surface can be reemitted to the gas phase, react to form another species, B, or enter the liquid phase. Inside the liquid phase diffusive transport and chemical reaction occur. The parameters which control the transfer rate are the diffusion coefficient in the gas phase, the mass accommodation coefficient, the diffusion coefficient in the aqueous phase, aqueous reaction rate, and solubility of the
* Present address: Institut fur Troposphtirenforschung e.V., Permoserstrasse 15, D-04304Leipzig, Germany @Abstractpublished in Advance ACS Absrracrs, August 1, 1995. 0022-3654/95/2099-14OOO$09.00/0
Liquid Phase
'@ I
Figure 1. Illustration of the mass transport between gas and liquid phase as a series of in principal reversible steps: (1) gas-phase diffusion, (2) collisions on the surface, (3) surface reaction, (4) hydration into the liquid phase, ( 5 ) liquid-phase diffusion, and (6) chemical decomposition in the liquid phase.
gas in the liquid phase. SchwartzI2defined characteristic times for each of these steps for drops of spherical geometry. They can be used to estimate the rate-limiting step. Under tropospheric conditions and in the absence of any saturation effect, the uptake of trace gases into cloud droplets depends on the mass accommodation coefficient a as long as a 5 For a> the phase transfer is controlled by diffusion in the gas phase and is independent of a (see SchwartzI2for details). Experimental Setup and Measurements The uptake and reactions of NO2 and HN02 on aqueous surfaces were investigated in a microjet experiment (Figure 2), adapted from the one developed by Kirchner et a l . I 3 It allows the measurement of the loss of NO2 and HN02 from the gas phase as well as the increase of nitrate (NO3-) and nitrite (NO23 in the liquid phase (reactions 1 and 2). The jet emerges from a capillary (i.d. = 100pm) at the lower end of a temperature controlled glass tube and is caught by a second capillary (i.d. = 150 pm) which is connected to a movable tube. The liquid is collected in an attached glass cell. The flow rate through the collecting capillary is precisely regulated by controlling the gas-pressure difference between 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 38, 1995 14001
Uptake of Nitrogen Dioxide and Nitrous Acid
EXPERIMENTAL SETUP
He
-w
Water reservoir
Liquid flow controller
Adjustment
in
x.y-plane
Fitregion
Humidity sensor 1 He+HzO *
1
1
Reactor (thermostated)
340
360
380
400
420
440
Wavelength Cnml Figure 3. Experimental absorption spectrum dominated by NO2 in the wavelength range of 335-445 nm. The horizontal line between 400 and 440 nm indicates the undisturbed fit region of the NO2 reference
spectra.
Shift in z direc
n
ollection cell
Figure 2. Experimental setup. Generation of the water jet and the interaction with the gas-phase molecules occurs in the reactor. The gasphase concentrations are measured by DOAS at the exit of the reactor.
Water samples for liquid-phase analysis are taken from the collection cell below the reactor. For further details see text.
.-
n
reactor and collection cell in order to avoid any water spills or sucking of gas into the capillary. The capillaries are aligned by sliding the reactor dome in x-y directions on a flat joint. The jet length can be varied by moving the collecting tube in the 2 direction. This allows to change the contact time of the jet surface with the gas phase. The fast flow of the jet (424 c d s ) continuously renews the aqueous surface. The experiments are carried out at reduced pressure (310-360 hP). The gas entering the reactor above the jet is a mixture of humidified helium (Messer Griesheim, 99.995%) as canier gas and the trace gas NO2 (Messer Griesheim, 1.O% NO2 in He). HNO2 is formed by reaction of nitrogen dioxide with adsorbed water on the glass surfaces of the reactor and is present in the reactor at a gas phase concentration of 4-6% of the NO2 concentration. By adjusting the water vapor partial pressure of the gas flow to 100% relative humidity (RH)at jet temperature, net condensation or evaporation of water vapor is avoided. By simultaneous measurements of the gaseous H20 concentration at the inlet and outlet of the reactor, the effect of small deviations of the relative humidity (A 2% RH)on the jet temperature is accounted for by calculating the corresponding change of the jet surface temperature. l 4 The gas outlet of the reactor is located below the jet and connected to a White cell with glass walls. The gas is analyzed for gaseous NO2 and HNO2 by differential optical absorption spectroscopy (DOAS). The main components of the optical detection system are a halogen lamp, a White cell (light path of 6 m) and a spectrograph with a photodiode m a y in the focal plane as detector (Figure 2). Absorption spectra are recorded in the wavelength range 335-445 nm with a resolution of 0.36 absorbance results in a nm. The noise level of about 1 x detection limit of approximately 2 x 10l2cm-3 for both NO2
340
360
380
400
420
440
Wavelength Enml Figure 4. Experimental absorption spectrum of HNOz after subtracting the absorption of NO2 from the measured spectrum. The analyzed absorption line at 354.2 nm is indicated by the horizontal line.
and HN02. The determination of the NO2 concentration is carried out by fitting reference spectral5 to the experimental absorption spectra (Figure 3) in the range 400-440 nm. Subtraction of the NO2 component in the spectra reveals the HN02 absorption features between 335 and 390 nm (Figure 4). The strong line at 354 nm is used to determine the concentration of nitrous acid by means of the differential W absorption cross section given by Bongartz et al.16 The detection limit (30) for NO2 is 4 x 10l2 cm-3 and for HNO2 2 x 10l2 cm-3 using a light path of 106 f 2 cm. Samples of collected jet water are analyzed for nitrite and nitrate concentrations. The nitrite concentration is measured after a chemical conversion" with sulfanilamide and naphthylamine to an azotype dye, which is photometrically detected at 544 nm. The detection limit is about 0.07 pmollL and therefore a factor of approximately 10 lower than typical concentrations measured. The nitrate is first reduced to nitrite by a CdCd column and then analyzed as described above. To discriminate the uptake on the jet surface against uptake by the intemal surfaces of the reactor, a differential measurement method is used by switching the jet on and off. The difference of the gas phase concentrations between the on and off phase is attributed to the uptake by the jet, since all other experimental parameters stay unchanged. The periods are repeated 10-15 times and each period lasts for about 5-20 min. Only those absorption spectra are used for the determination of the concentration where the difference of the relative humidity at
Mertes and Wahner
14002 J. Phys. Chem., Vol. 99, No. 38, 1995
2.0 Z
pH=7
N
I
0
NoAs0,
z
i Y-
O v)
L1
1
0
J
L
e
-1 0
L
0
U 0
Experimentnumber
I
10
5
15
20
25
-1
30
Experimentnumber
Figure 5. Relative experimental loss of NO2 from the gas phase plotted for two different NO2 concentrations and different jet water compositions (pH 7, milli-Q; pH 12, 0.01 N NaOH solution; TEA, 33.4 g/Lof triethanolamine solution; NaAsO2, 1 g/L of NaAsO? solution). Dots, 3 mm jet length; squares, 6 mm jet length.
Figure 6. Relative experimental loss of HNO2 from the gas phase plotted for the different NO2 concentrations and different jet water compositions (pH 7, milli-Q; pH 12, 0.01 N NaOH solution; TEA, 33.4 g/L of triethanolamine solution; NaAs02 1 g/L of NaAsO2 solution). Dots, 3 mm jet length; squares, 6 mm jet length.
TABLE 1: Typical Experimental Conditions 24 mm reactor diameter 320-360 hP reactor pressure 298 K reactor temperature 8.5 cm3/s mean gas flow rate mean gas velocity in reactor 2 cm/s mean gas contact time with jet 0.16 and 0.32 s partial pressure of H20 7.0-8.6 hP reynolds number Re (gas phase) 15 8 x 1015~ m - 9~ x; lOI4cm-3 mean NO2 concentrations
TABLE 3: Mean Relative Losses of NO2 and HNOz from the Gas Phase together with the NO2 and HN02 Concentrations [NO*]( ~ m - ~ ' [HNO?](C~-~) AN02 (%) AHNO2 (%) 8 x lOI5 4 x 1014 1.03 f 0.34 3.9 f 3.0 9 x 1014 2 x 1013 0.42 0.26 11.3 f 6.5
TABLE 2: Typical Jet Parameter jet radius jet length mean jet velocity jet temperature jet flow rate mean jet contact time with gas phase Reynolds number Re PH
used solutions milli-Q NaOH triethanolamine (TEA) NaAsO2
50 pm 3 and 6 mm
424 cm/sec 276-282 K 2 mL/min 0.7 and 1.4 ms 320 7 and 12 0.01 N 33.4 g/L 1 g/L
the inlet and outlet of the reactor is less than 2% relative humidity. Great care was taken to avoid any condensation of water on the walls of the reactor or tubing by thermostating these parts. The change of the gas phase concentration of HNO, introduced by the jet surface is also attributable to an uptake by the jet surface, because the gas phase concentration of HN02 is reproducible and shows only slow variations of less than 15% during 1 day whenever the water jet is turned off. The analysis of the liquid phase concentrations enables the calculation of the mass balance of the uptake. Typical experimental conditions and jet parameters are listed in Tables 1 and 2. A total of 150 differential measurements were evaluated. To improve the scatter, successive measurements were averaged. In Figures 5 and 6 the averaged data of the measured loss of NO2 and HN02 from the gas phase is plotted for different experimental conditions such as NO2 concentration, jet water pH, and jet length (6 mm in the experiment number 19, 20, 23, and 24, otherwise 3 mm). The error bars indicate the 30 statistical variation. These statistical variations are larger than the detection limit of NO2 and HN02 due to the reproducibility of the jet during on and off periods. This is mainly due to the small fluctuations of the relative humidity (less than 2%relative humidity). Larger effects on the NO2 gas-phase concentration
*
TABLE 4: Mean Nitrite and Nitrate Concentrationsof the Jet Water Samples for the Two pH Values and a NO2 Concentration of 9 x 1014 ~ m - ~ 7 12
0.84 f 0.28 1.09 f 0.57
0.89 f 0.42 1.22 f 0.57
can be observed if water is spilled in the reactor. These measurements are discarded. Experiments 26 and 27 were carried out with added triethanolamine (TEA), experiment 29 was carried out with added NaAs02 to the water of the jet. These chemicals are used in order to speed up the liquid phase conversion of dissolved NO2 to nitrate.'8s'9 Experimental Results The measured uptake of gaseous NO2 shown in Figure 5 is significant under all experimental conditions and leads to a decrease up to 1.6% of the gaseous NO2 concentration. Within the experimental uncertainty ( 3 4 the observed decrease is independent of the jet water composition and the jet length. On average, the relative loss of NO2 from the gas phase is correlated to the NO2 concentration as can be seen from Table 3: taking the average of all measurements at pH 12 and 7 the decrease in the gas phase rises from 0.42% to 1.03% when the NO2 concentration is increased from 9 x lOI4 cm-3 to 8 x lOI5 ~ m - ~ . Figure 6 shows the measured loss of HN02 from the gas phase. It is significant and amounts up to 25%. Within the large scatter of the HN02 gas-phase concentration, no correlation with the jet water composition and the jet length can be detected. However, the averaged loss of HNO2 from the gas phase is clearly anticorrelated to the NO2 concentration within 2 a errors, i.e., the loss of HN02 is smaller for higher NO2 concentrations (Table 3). The amount of nitrite and nitrate in the liquid phase is found to be equal within the experimental errors (Table 4). This is expected in case of the disproportionation of NO2 via the formation of the intermediate dimer N204 to nitrite and nitrate. The uncertainty of the experimental values does not exclude a
J. Phys. Chem., Vol. 99, No. 38, 1995 14003
Uptake of Nitrogen Dioxide and Nitrous Acid
TABLE 5: Diffusion Coefficients and Henry Constants of NO2 and €IN02 Used in the Model (H(Qas a Function of the Temperature Taken from ChameidesU) NOz 1.8 1.85 x 10-5 2.2 x 10-2 1.2 x exp[2500(1/T- 1/298)]
D,(cm2/s) D, (cm2/s) H (moU(L/atm)) H ( T ) (mol/(L/atm))
surplus of nitrite in the jet water samples possibly caused by the uptake of nitrous acid and its fast dissociation to nitrite. Because the gas-phase concentration of HN02 is more than a factor of 10 lower than the NO2 concentration, the contribution to the liquid-phase concentration of nitrite is expected to be small compared to our error limits. The uptake of gaseous N204 is not significant. Due to the equilibrium constant20of N204 formation (K= 6.84 am-') the gas phase concentration of N2O4 amounts to 0.025% of the NO2 Concentration. Though the Henry constant2' of N204 (H(N204) = 1.4 mol/(L atm)) is a factor 100 larger than that of NO2 (H(N02) = 1.2 x mol/(L atm)), the resulting maximum N2O4 interference in the form of nitrate and nitrite formation is less than 4%. The contribution of gaseous HNO3 uptake to the nitrate concentration can be assumed to be small. According to laboratory experiments, which investigated the kinetics of the H20/N02 system using similar kinds of reactor^,^-^-" no gas-phase HNO3 has been detected. The detection limit for HNO3 is reported to be a factor 100 lower than the measured HN02 concentrations. The nitrogen mass balance is used as a further test of the consistency of the experimental results. The loss rates of NO2 and HN02 and the formation rate of N02- and NO3- in molecules per second are given by RI = F,A[N02], R2 = F,A[HNOz], R3 = Fa[N02-] and R4 = F,[N03-], where F, and Fa are the gas and liquid flow rates. The condition of mass conservation, i.e., RI R2 = R3 R4, is fulfilled in 80% of the measurements within the error limits of each measurement. The average deviation (30) of (RI R2) - (R3 R4) from zero of all measurements is (0.35 f 1.8) x lOI3 molecules s-l. This corresponds to approximately (0.05 f 0 . 2 3 %of the NO2 gas-phase concentration and approximately to (5 25)% of the NO2 gas-phase loss. Although the errors of the N02, HN02, N02-, and NO3- individual concentration measurements are rather large (up to 50%), this result indicates that the major loss processes of NO2 are accounted for.
+
+
+
+
*
Model Calculations The rate of uptake from the gas phase into the liquid jet is controlled by several processes (e.g., diffusion, mass accommodation, chemical reaction). To determine the mass accommodation coefficient from the measured uptake a numerical simulation of the chemistry in the aqueous phase and of the transport and diffusion processes in the gas and aqueous phase is carried out and compared with the experimental data. Other than the mass accommodation, no specific chemical reaction on the surface is included in the model. The numerical model is programmed within FACSIMILE, a 3-dimensional transport and chemistry solver using Gear routines for differential equations. The model treats the water jet as an ideal cylinder with a surface velocity equal to the mean jet velocity and uses the radial grid sizes of 0.0032 mm in the liquid phase and 0.42 mm in the gas phase and the axial grid size of 0.21 mm in both phases. The transport within each of the homogeneous phases is described by diffusion and convection as given by solutions of the generalized diffusion equation (eq 3) in cylindrical geometry with convection along the jet in the z direction only:
HNO:, 1.8 1.85x 10-5 155 4.9 x 10'exp[4781(1/T- 1/298)]
where C is the concentration, D the diffusion coefficient either in the gas phase (D,) or in the liquid phase (D,), r the radius, and v, the vertical velocity in the gas phase or liquid phase. The gas-phase diffusion coefficient D, of NO2 is determined by eq 4, where P H ~ oP, H ~Pr, , and TI denote the partial pressure
of H20 and He, the total pressure in the reactor and the reactor temperature. D N O ~ - Hand ~ O D N O ~ - are H ~ the binary gas-phase diffusion coefficients of NO2 in H20 and He, respectively, which are derived from calculations22of DSo2-n20 and D s o ~ - H For ~. the liquid-phase diffusion coefficient D, a linear temperature dependence is assumed.23 Values of D, and D, which are used in the model for NO2 and HN02 are given in Table 5. Reduced pressure and the use of He as the carrier gas enhances the diffusive transport by one order of magnitude compared to N2 and 1 atm pressure. Relative slow flow velocities are used in the gas phase in order to observe a NO2 gas-phase loss. Due to the different flow velocities in the gas phase and dquid phase, two different flow profiles of the gas phase, ut(r) and vp(r),are used to account for two extreme assumptions (Figure 7). The flow profile vt assumes that the gas at the aqueous surface adopts the jet flow velocity as a result of the friction between the gas and the aqueous surface. It drops linearly to zero with increasing radius r. The flow profile up assumes no friction at the interfaces jet surfacelgas and reactor wall/gas and is therefore independent of r. Both flow profiles are normalized to the mean experimental gas flow rate F,; therefore, ut drops to zero at a distance of 1.4 mm from the surface (Figure 7). The flow profile ut shows larger velocities close to the jet than v,. If the gas-phase transport is the rate-limiting step of the uptake, the calculations using flow profile ut will result in a larger uptake. The real flow profile is very difficult to determine, but it will be shown that the influence of the flow profile in this investigation is small. The rate of exchange between the gas phase through the heterogeneous interface and the liquid phase, F,, is implemented by taking the number of gas kinetic collisions on the surface multiplied by the mass accommodation coefficient a. Equation 5 describes the net flow across the jet surface:
where vmis the mean speed of molecules, H the Henry constant, CS, the gas-phase concentration at the surface, and CS, the liquidphase concentration at the surface. The Henry constants of NO2 and HN02 are calculated using the temperature dependent expression^^^ given in Table 5. The so obtained Henry constant of NO2 at room temperature agrees within a factor of 2 with the results of other authors, who
Mertes and Wahner
14004 J. Phys. Chem., Vol. 99, No. 38, 1995 100 7
- 1 .E
-8
I\
10
I
.........
Y . _.-_ .
"P
1004~
in4u
II
., ..-
P E SI
h
H
Y L
>-
Mass Accommodation Coefficient a Figure 9. Calculated loss of NO1 from the gas phase at 3 mm jet
0 0
0,4
1,2 Radius r [mm]
0.8
1.6
12
Figure 7. Gas flow profiles up (dotted line, right scale) and ut (dashed line, left scale) used in the model calculations as a function of the reactor radius r (jet surface at r = 0 mm). Both profiles are normalized to the mean experimental flow rate.
1000)
I00
t
K
Model Calculations \r
0.0001
0,001
0.Oi
01
!
Mass Accommodation Coefficient a Figure 8. Calculated nitrite formation from HNOz uptake (left scale, lower curves) and loss of HNO2 from the gas phase (right scale, upper curves) as a function of the mass accommodation coefficient using the flow profiles up (dashed line) and ut (dotted line) at 3 mm jet length. These calculations are compared to the maximum surplus of NO2 estimated from the uncertainty of the liquid-phase analysis (right side) and to the observed range of HNOz loss from the gas phase (left side).
determined H e ~ p e r i m e n t a l l yor ~ ~by . ~ thermodynamical ~ calculation~.~',~~,~' No gas-phase or wall reactions are used in the model, because the measurement of the uptake are differential concentration measurements during jet off and on periods and the effect of such reactions would essentially be canceled. The transformation of dissolved NO2 and HN02 to N02- and NO,- in the liquid phase is calculated according to the following reactions:28
2N02
+
N204 H 2 0
HNO,
-
-
-
HNO,
N20,
HNO,
K6 = 6.5 x lo4 M-'
+ HNO,
+
H+ NO2-
-
H++ NO,-
(6)
k7 = 1 x lo3 s-' (7)
K, = 4.4 x low4M
(8)
K9 = 15.4 M
(9)
Comparison of Experimental and Model Results Figure 8 shows the calculated loss of H N 0 2 from the gas phase and the nitrite concentration due to HNO2 uptake as a
length as a function of the mass accommodation coefficient a using the flow profiles up (dashed line) and ut (dotted line) for different modified Henry constants H, given as a multiple of the physical Henry constant. These calculations are compared to the experimentally observed range of NO2 loss from the gas phase.
function of the mass accommodation coefficient a for the two different flow profiles ut (dotted lines) and vp (dashed lines) and a jet length of 3 mm. For a 1 lo-, the slopes of the curves decrease and begin to separate for the different gas flow profiles, reflecting the increasing influence of gas-phase transport (diffusion and convection) on the uptake rate of HNO2. Under the current experimental conditions the liquid-phase concentration of the jet does not reach solution equilibrium with the gas-phase concentration even for a = 1. The surface concentration of HN02 is about 5 orders of magnitude lower than solution equilibrium due to the Henry constant at 278 K and because at the used HN02 concentrations HNO2 is dissociated by &. The calculated loss of HN02 from the gas phase is sensitive to a mass accommodation coefficient for a I0.1. Also shown in Figure 8 are the experimental results: the loss of HN02 from the gas phase as a range between the upper two horizontal lines; the liquid phase nitrite concentration from H N 0 2 uptake as a horizontal line, indicating the maximum surplus of NO2estimated from the uncertainty of liquid-phase analysis. Comparison of the model calculation and the experimental results for the liquid phase shows that only for a mass accommodation coefficient a 5 4 x the calculated nitrite from HN02 uptake agrees with the measured range. The same comparison for the loss of HN02 from the gas phase results in a lower limit of a L 4 x lo-,, taking the differences due to the two flow profiles into account. The obtained range of the mass accommodation coefficient of HN02 on aqueous surfaces at 278 K is consistent with the lower limit of a 1 5 x lo-, determined by Kirchner et al.I3 and with the range measured by Bongartz et alSz9of 0.04 < a < 0.09 at 245 K and 0.03 < a < 0.15 at 297 K. Model calculation of the loss of nitrogen dioxide from the gas phase and of the sum of nitrite and nitrate formation caused by the uptake of NO2 are plotted as a function of the mass accommodation coefficient a in Figure 9 and 10. The calculations are carried out varying the Henry constant Hm (given in multiples of the physical Henry constant H (cf. Table 6)), and using the two different gas flow profiles vt (dotted line) and vp (dashed lines). For small values of a the loss from the gas phase is most sensitive to the rate of interfacial mass transfer. For larger values of a the slope of the curves decreases due to the saturation of the surface. The low physical solubility of NO2 and the resulting saturation limits the transfer from the gas phase and the uptake becomes independent of a. Using a larger Henry constant, Hm,the influence of saturation is apparent
J. Phys. Chem., Vol. 99, No. 38, 1995 14005
Uptake of Nitrogen Dioxide and Nitrous Acid
Discussion
Mass Accommodation Coefficient
There are several possibilities on how to interpret the highvalue of the Henry constant, which is needed to explain the observations: (1) The physical Henry constant is indeed as large as Hm. This is unlikely, because the literature values of the physical Henry constant either determined experimentally or by thermodynamical calculation^*^+^^-^^ agree within a factor of 2. Therefore a 7 times larger H m is outside the uncertainty of the literature values of the physical Henry constant for NO2. (2) The liquid-phase rate constants of reactions 6 and 7 (i.e., the conversion of dissolved NO2 to NO3- and NO27 are much faster than that used in the model. This would increase the NO2 uptake. This is unlikely, because the highest in the literature reported rate constant23of 1 x lo8 M-' s-' for the combination of reactions 6 and 7 is only 54% faster than the value of K& = 6.5 x lo7 M-I s-l used in the model. To reach agreement between the model and the experimental results, an at-least 7 times larger rate constant is needed. Furthermore, the experimental results show within the error limits (approximately a factor of 2) independence of the NO2 uptake from oxidizing substances (TEA, NaAsO2) or pH in the jet water. This indicates that the rate of liquid-phase reactions have no or only a small influence on the uptake of N02. Thus, the interpretation of Schwartz and who assumed unknown reactions of unknown impurities in the liquid phase, can be excluded for our experimental conditions. (3) The discrepancy between the calculated uptake of NO2 using the physical solubility (H) and the measured uptake is due to properties of the surface, which are not accounted for in the calculations. Using the model results obtained with H m = 7H and a = 2 x low4the amount of NO2 molecules taken up by the jet per unit time in excess of the physical solubility can be calculated. Assuming a cross section for each of these NO2 molecules of 6 x cm2 a coverage of the jet surface of approximately 0.001 is obtained. If the reaction of these molecules on the surface is the rate-limiting process, a reaction order of 2 is expected, compared to the uptake process for physical solvation, which is first order in NO2. The observed
a
Figure 10. Calculated sum of nitrite and nitrate formation in the liquid phase due to NO2 uptake as a function of the mass accommodation coefficient a using the flow profiles v, (dashed line) and ut (dotted line) for different modified Henry constants H m given as a multiple of the physical Henry constant.These calculations are compared to the range of the sum of nitrite and nitrate measured in the analyzed jet water samples.
only at higher vhues of a and therefore the loss from the gas phase and the ion concentration in the liquid phase is larger. As long as the disproportionation reaction of NO2 in the liquid phase is the rate-determining step, the uptake is independent of the gas-phase transport. Therefore the results of the model calculations using vt and up are identical unless saturation is not reached due to high solubility, as shown for H m = 100 x H. Comparing the mbdel calculations with the experimental results in the gas and liquid phase (the range is indicated in Figures 9 and 10 by horizontal lines), it becomes obvious that the calculation using the physical Henry constant underestimate the experimental data for all possible a by at least 1 order of magnitude. Calculations using H m 2 7 x H and a mass accommodation coefficient a 2 2 x account for the mean values of the experimental loss of NO2 from the gas phase and for the sum of the measured liquid-phase concentration of nitrite and nitrate. 100%
80%
s
60%
\
C
c
m
t
40%
20%
0Yo 0.0
0.4
0.7
1.1
1.5
1.9
2.2
2.6
3.0
3.3
3.7
4.1
4.4
4.0
5.2
5.6
5.9
jet length I mm
Figure 11. Calculated dependence of the distribution of the nitrogen species in gas (N02(g)) and liquid (NO2(a), N204(a), and N02-(a) iN03-(a)) phase on exposure time normalized to the gas phase loss of N02(g) (=3.05 x lOI3 molecules s-I) within 1.4 ms (6 mm jet length). The N204(g) parameters for the model calculation: H m = 7H,[NO& = 9 x 1014~ m - ~ .
14006 J. Phys. Chem., Vol. 99,No. 38, 1995
Mertes and Wahner
dependence of the loss of NO2 from the gas phase on the NO2 concentration indicates an apparent reaction order of 1.4 f 0.3 (Table 3). Since equal amounts of nitrite and nitrate are measured in the liquid phase the net heterogeneous reaction 10 analogous to reaction 1 is conceivably taking place.
-
2N02(surface) -I- H20(surface) HN02(surface)
+ HNO,(surface)
(10)
The proposed mechanism produces nitric and nitrous acid on an aqueous surface. Both are most likely taken up by the bulk liquid because of their solubility and dissociation into equal amounts of nitrite and nitrate. However, because of the lower dissociation constant and solubility of HNO2 compared to HNO3, HN02 can escape to the gas phase. An indication of this is given by the observed anticorrelation of the relative loss of HNO2 from the gas phase with changing NO2 concentration (cf. Table 3). The heterogeneous uptake of HN02 if undisturbed by saturation effects is a first order process; therefore, the relative loss of HN02 from the gas phase should be independent of the concentration of HN02 and the same in all experiments. The lower relative loss of HN02 from the gas phase observed in the experiments with higher NO2 concentration can be attributed to the formation and release of HN02 on the aqueous surface, partially compensating the loss from the gas phase due to the liquid-phase uptake. This process must be related to the jet surface and cannot be a gas-phase or wall reaction, because of the differential measurements with the jet tumed on and off. Assuming a surface reaction such as reaction 10 the observed difference of the relative loss of HNO2 from the gas phase comparing the experiments at the NO2 concentration of 8 x lOI5 and 9 x l O I 4 cm-, is equal to an emission of 1.8 x loi3 cm-2 s-l of nitrous acid from the surface in the experiments at the NO2 concentration of 8 x l O I 5 cm-3 , which is about 35% of the lost N02. Due to the small Henry constant of NO2 the equilibrium between gas and the surface layer of the liquid phase is reached within very short exposure times (
