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The kinetics of the photolytic production of aerosols from SO2 and NH3 in humid air
ChemicalEngineering Science, Vol.49, No. 24A,pp. 4605--4614,1994
Pergamon
Copyright1~)1995Elsevier Science Lid Printedin GreatBritain.All fightsreserved 0009-2.509/94 $7.00 + 0.00
0009--2509(94)00341-6
T H E KINETICS OF T H E P H O T O L Y T I C P R O D U C T I O N O F A E R O S O L S F R O M SO2 A N D N H 3 IN H U M I D A I R P E T E R SEIER CHRISTENSEN, STIG W E D E L and HANS LIVBJERG* Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark (Received 25 May 1994; accepted.for publication 4 October 1994)
reaction of SO2 and N H 3 in humid air to produce an aerosol of ammonium sulfate is studied in a photolytic laboratory reactor. The reaction is shown to be controlled by ultraviolet irradiation in the wavelength range of 240-330 nm and is initiated by excitation of SO2. It is found that NH3 has a strong catalytic effect on the photolytic oxidation of SO2 and surprisingly high values of the quantum yield are observed with NH 3 concentration in the range of 20-400 ppm. The reaction rate increases with decreasing temperature. An overall mechanism for the formation of aerosol particles is proposed. It is concluded that aerosol formation due to sunlight photolysis by this mechanism explains the high-opacity smoke formed in industrial stack plumes with small amounts of NH3 and SO2.
Abstract--The
INTRODUCTION
The reaction of SO2 and NH3 in humid air to produce an aerosol of ammonium sulfate is investigated in this work_ Aerosols derived from this reaction system have been reported to yield a high-opacity, persistent smoke ("blue smoke") polluting the stack plumes of cement kilns (Deillinger et al_, 1980) and some power plants with ammonia scrubbers for flue gas desulfurization (Moore, 1977; HOvel, 1990; Christensen et al., 1992). In the case of the Walther desulfurization process the formation of blue smoke, according to one theory, is initiated within the scrubber by the homogeneous nucleation of small aerosol particles of an ammonium S(IV)-salt which is subsequently stabilized by oxidation to sulfate particles (Moore, 1977). To avoid nucleation one must control the gas composition in the scrubber to avoid exceeding the threshold of supersaturation which leads to nucleation of new particles. This theory is sustained by some literature references suggesting that ammonium S(IV)-salts may actually nucleate homogeneously from SO2 and NH3 in humid air (Hartley and Matteson, 1975; Vance and Peters, 1976; Rothenberg et al., 1986; Bai et al., 1992) but the precise conditions required for homogeneous nucleation are obscured by the fact that the gas may incidentally become supersaturated with respect to a number of different ammonium S(IV)species under typical scrubber conditions (Christensen et al., 1992). However, recently Christensen (1994) in an experimental study has investigated the rate of homogeneous nucleation of ammonium S(IV)-salts from SO2 and NH3 in humid air in the temperature range 0-50°C. This study showed that in the absence of ultraviolet light irradiation the nucleation rate is negligible even when extreme "To whom correspondence should be addressed.
conditions of supersaturation are imposed. The investigation also has revealed a previously unrecognized catalytic effect of NH3 on the photolytic oxidation of SO2 and this effect has possibly been overlooked in previous investigations. As a consequence of the photolytic influence, an ammonium sulfate aerosol may be formed within a sunlit stack plume with small amounts of NH3 and SO2 even if no nuclei have been formed prior to the stack exit_ PHOTOLYTIC REACTION MECHANISM IN THE GAS PHASE
The only possible photolytic excitation by sunlight of the reactants used in this study is the first allowed excitation of SO2: SO2(~A~) + hv(240 n m < A < 330 rim)---> SO2(IA2, IB2)
(1)
The highly unstable excited singlet state of SO2 partly decays to the triplet state of the lowest energy by intersystem crossing: 502(IA2, IB1)---~502(3BI)
(2)
and partly relaxes to the ground state. Approximately 11% of the SO2(IA2, lBl) produced by eq. (1) is transformed by eq. (2) (Calvert et al., 1978). The triplet SO2(3Bi), which is far more stable than SO2(IA2, *B,) and is believed to be the reactive species in atmospheric photo-oxidation of SO2, initiates a chain of reactions. Even in a system of pure N2, 02, H20, SO2, and NH3 the number of possible reactions is considerable. The network of reaction steps shown in eqs (3)-(21) include all reactions of significance in the proximity of room temperature. They have been selected by Christensen (1994) after a systematic scrutiny of rate constants from the literature and kinetic databases. Some of the reactions given in eqs (3)-(21) are of
4606
PETER SEIERCHRISTENSENel al.
minor importance but are included for their potential importance at other temperatures than 25"C. There are many other possible reaction steps but they are all too slow to influence our measurements. The reactions in the gas mixture of this investigation have been widely studied due to their importance in atmospheric chemistry. The rate constants at 25°C are cited below together with the selected reactions.* The set of rate constants is quite complete at 25°C but far less complete at other temperatures. SO2(3BI) produced in eq. (2) can react by one of the following reactions: SO2(3BI) + SO2---~SO3 + SO
k 3 = 4.2 x 107
(3)
SO2(BBI)-1-O2-->SO2-FO2(IA) k 4 = 9.6x 107 (4) SO2(3BI) + N2--* 502 + N2 k5 = 8.5 × 107
(5)
The species SO is quite stable and the reactions in which it participates are all slow compared with eqs (3)-(21). Thus, SO may be treated as a stable product on a short time scale. Possibly the excited state of O2 primarily formed by eq. (4) is oz(EE~) which will, however, rapidly be transformed to O2(IA) (Sidebottom et al_, 1972). O2(IA) is a low-energy excited state of oxygen which either reacts with SO2 or is quenched to the ground state: O2(IA) + SO2-.o 503 "{-O
k 6 = 1.3 x 105 (6)
O2(lA) + N2---~O2 + N2 k7~<84 O2(zA)+H20--o02+H20
ks = 3x103
O2(1A) + O2"-'>202 k9 -'- 103
(7) (8) (9)
Atomic oxygen produced in eq. (6) reacts further by the following reactions: O + 02---> 03 O+SO2--oSO3
kl0 = 109 kll = 7 x 1 0 1 1
(10) (11)
Although 03 is virtually unreactive it dissociates photolytically by 03 + hv--* O2(IA) + O(ID)
(12)
The extinction coefficient for 03 at h = 284 nm corresponding to eq. (12) is 1800 l/mol/cm (Bauich et al., 1980). O(ID) is a highly reactive excited state of O which yields OH-radicals by reaction with H20 or NH3. Otherwise it is quenched to ground state atomic oxygen.
O(ID) + n20-'* 2OH O(1D)+NH3-->OH+NH2
kl3 = 1.2 x 10 II (13) kl4 = 2 x 1 0 II
(14)
O(ID) + N2---*O + N2 kls = 1.6 x 1010 (15) "All rate constants ki are given in the unit I/mol/s. The data are from Mallard et al. (1993) except for: eqs (3) and (4): Calvert et al. (1978) eq. (5): Sidebottom et al. (1972) eqs (7), (8), (9) and (15): Atkinson et al. (1992)
Numerical calculations show that neither eq. (14) nor the analogous reaction between NH3 and ground state O have any influence at all on the overall oxidation rate. Any NH2 formed by these reactions may participate in a number of further reactions but these reactions are also unimportant. In the presence of OH-radicals SO2 is oxidized to SO3 by the following chain reactions. OH+SO2---,HOSO2 HOSO2 + 02--> HO 2 + 803
kl6 = 3 . 6 x l0 s
(16)
kit = 2.5 x 1"08 (17)
HO 2 + SO2---~ OH + SO3
kzs = 5.2 x 105 (18)
with the major terminating step OH + HO2----~H20 + 02
k19 = 4 x 10 I° (19)
SO3 formed by eqs (3), (6), (11), (17), and (18) reacts fast with H20 or NH3: SO3+H20--~H2SO4 SO3 + NH3-* NH3803
k20~<3.6x 106
(20)
k21 = 4.2 X 10 t° (21)
The final steps leading to aerosol formation are presumably all very fast although their kinetics and precise nature are not known. They may involve the nucleation of the products of eqs (20) and (21) followed by a heterogeneous reaction with H20 and NH3 to yield ammonium sulfate. Alternatively, the products of eqs (20) and (21) may react in the gas phase with H20 and NH3 to yield supersaturaated ammonium salts which then nucleate. To characterize the efficiency by which SO2(IA2,1BI) molecules excited by eq. (1) are transformed to ammonium sulfate the quantum yield Yo shall be defined as the ratio of the overall molar rate of (NH4)2SO4 production F(Nrt~),_s04 to the molar rate of SO2 excitation Fso: by eq. (1) or: YO = FfNH4)'-SO4 Fso.~
(22)
The denominator of eq. (22) is computed from the extinction coefficient for SO2 corresponding to eq. (1) at the given wavelength. Fso~ =
loA (I_10_ECso2L) NAhv
(23)
where N A is Avogadro's number, h Planck's constant, loA the total power of the incident light beam (W), e is the extinction coefficient (l/mol/cm), L the reactor length (cm), Cso~ the molar concentration of SO2 in the gas (tool/l), and u is the frequency of the light. Because approximately 89% of the excited singlet SO2(IA2, ~BI) is quenched to the ground state one would anticipate YO to be less than 0.11. However, due to chain mechanisms like eqs (16)(18), YQ might in principle increase beyond that limit. The rate constants in the chain mechanism are quite small though and actual calculations typically yield YO values in the range 3 x 10-3-10 -5
Photolytic production of aerosols from S02 and in stark contrast to the large experimental values measured in the presence of N H 3 and presented below. EXPERIMENTAL
The overall kinetics of the photolytic aerosol formation is investigated in a laboratory reactor shown in Fig. 1. The reactor is designed to operate at steady state with a continuous flow of a gas passing through a tubular reactor irradiated with light by an optical system. The feed gas is made up by mixing different gas streams, each metered and controlled by a flow controller. NH3 and SO2 as mixtures with N2 are drawn from cylinders and are further diluted with streams of dry N2 and 02. A separate stream of N2 is passed through a humidifier, where it is saturated in a packed column with recycling water at a constant temperature which can be varied to yield different humidities. The streams are thermostatted at 45°C to keep them above the dew point. All gas streams are filtered by absolute filters so that they are completely free from aerosol seeds. Optionally, seed particles of a controllable size and concentration can be added to the feed in a separate stream of N2- The seed particles are produced by a constant output atomizer model 3076, TSI Inc., which yields a high c o n c e n t r a t i o n (>106/cm 3) of small droplets (0.5/zm) of an aqueous salt solution. The aqueous aerosol is dried in a diffusion drier to an aerosol of small solid salt particles the size of which can be varied by changing the concentration of the salt solution. The effluent gas from the reactor is reheated to 45°C, such that only sulfate particles are stable, and passes through a diffusion drier. This is a 600 mm long, cylindrical tube of diameter 10 mm with wire mesh walls surrounded by a molecular sieve (4/~) drying agent. The gas flow is laminar and passage through the drier efficiently removes water, SO2 and NH 3 from the gas and from evaporating, unstable sulfite particles but leaves the stable sulfate particles undisturbed. The aerosol is analyzed using either a differential mobility particle sizer (DMPS) consisting of an electrostatic classifier (EC) model 3071, TSI Inc. and condensation particle counter (CPC) model 3022, TSI Inc. or alternatively a Berner-type low pressure cascade impactor, Model LPI 25/0.018/2, Hauke G m b H & Co. KG. The DMPS-system measures the particle size distribution in the range 0.019--0.931/~m in 28 intervals. The impactor samples the particles in 10 size channels in the range 0.010--12.0/.tin. The gas phase concentrations of N2, 02, H20 and SO2 are measured before and after the reactor with a Balzers Q M G 420 mass spectrometer (MS). The reactor consists of a 120 mm long stainlesssteel tube with an internal diameter of 36 mm. The end walls of the reactor are quartz lenses transparent to the UV-light beam. The gas enters and leaves the reactor through a ring of small holes
NH 3
4607
evenly spaced in the wall close to each of the lenses. The reactor is surrounded by a heating jacket with water pumped from a thermostat bath for temperature control. To check against the possibility that the stainless steel wall of the reactor exerts any catalytic or other kinetic influence special shielding inserts of pyrex glass and quartz glass were applied in some runs. No effect on the observed kinetics could be detected. The optical system consists of a 500W superquiet mercury-xenon lamp type L2424, Hamamatsu Corporation. Light in the IR range is removed by reflecting the beam by a UV-reflecting and IR-transmitting planar mirror. The beam passes a high frequency chopper for control of the beam intensity followed by a UV-bandpass filter type U V - K M Z 20-3, Schott Glaswerke. The bandpass filter is a thin plate which must not be heated above 70°C. A special arrangement of air cooling through small nozzles directed at both sides of the filter has to be applied to avoid superheating. Bandpass filters with nominal wave lengths of 284 and 309 nm are used in this study. The transmittance spectra of these filters yield quasi-monochromatic beams. The spectra, at the half peak height, have widths of 20nm for the 284nm filter and 7.5nm for the 309 nm filter. The filtered light beam is collected by the first lens to form nearly parallel rays within the reactor. The second lens focuses the beam on the UV-energy meter type AC25UV, Scientech. The UV-radiation in the studied range can damage the skin and the eyes. Direct and secondary irradiation of body parts must hence be avoided. The whole reactor and lamp assembly is therefore built into an effectively shielding cupboard. As reagents high-purity nitrogen, high-purity oxygen, 2% NH3 in N2 and 1% SO2 in N 2 a r e used. In the humidifier high-purity water is used. To check against the possibility that trace impurities affect the observed kinetics the purity of the gases and the water were changed in some runs with no observable effect on the measurements.
RESULTS
Procedure An experimental run is made by turning on the gas feed flows and checking the gas feed composition. It is checked with the DMPS that there are no aerosol nuclei in the effluent gas when the UV-light is turned off. The UV-lamp is then turned on and the particle size distribution of the aerosol produced is measured repeatedly until steady state is achieved. The beam power is measured by the UV-detector without the presence of SO2 in the reactor. The duration of one run is typically 1 h. The size distributions measured by the DMPS are normally monodisperse with a quasi-lognormal shape (see Fig. 6 below). The particle size varies over a wide range. To ensure a correct measurement of the quantum yield it is necessary to keep
PETER SEIER CHRISTENSEN et al_
4608
Hg-Xe-Lamp Humidifier
NH 3
502
Mirror Mirror
Purge
(F
Chopper i
Bandpass Filter
N2
\
Lens
IOlO
N2
I_
Atomizer
olo¢11 ~ m
Healing Jacket rjr~t
| •
02 Lens /
/
~Purge
I~
Impactor I
UV Energymeter Fig. 1. Monochromatic photolytic reactor, schematical. FC, flow controller; PC, pressure controller; EC, electrostatic classifier; MS, mass spectrometer; TC, temperature controller; DD, diffusion drier; CPC, condensation particle counter; R, restrictor. E~, Reducing valve; B~, filter (prefilter and absolute filter); [], heating tape; 121,thermal insulation.
Photolytic production of aerosols from SO2 and NH3 all particles within the size limits of the DMPS, a constraint which limits the applicable range of operating conditions_ For the ensuing measuremerits this is reflected in the choice of conditions which may appear slightly illogical for some of the series. If a small fraction of the size distribution is outside the measuring range (see Fig. 6 below) the measurement can be corrected for the missing data by fitting a log-normal distribution to the measured values. Data points adjusted accordingly are marked in the following. In some runs the particles were sampled by the Berner low pressure impactor for elemental analysis and for microscopic investigation. The particles consisted of (NH4)2SO4 and appeared compact and spherical in a scanning electron microscope. For that reason the total volume of particles calculated from the DMPS distribution data is believed to yield a quite precise estimate of the ammonium sulfate production rate, i.e. F(NH4)2SO J of eq. (22). This is verified by the fact that the measurements of the total production rate by the DMPS and the impactor were in a satisfactory agreement. Hence, the DMPS distribution data can be used to characterize the kinetics of the overall rate of photolytic SO2 oxidation as well as the kinetics of the nucleation and growth of the aerosol particles. The considerable decline in the intensity of the UV-lamp over some hundreds of hours of operation also constrains the maximum intensity which can be used. The intensity applied in each measurement is shown together with other operating conditions in the ensuing data. Since the intensity influences the quantum yield it must be kept constant when the influence of other parameters is being studied. Wavelength
The observed photo-kinetics of the aerosol production agrees well with the UV-absorption spectrum of eq. (1). The wavelengths 284 and 309 nm yield qualitatively the same effect. This is seen, for example, in Fig. 4 below by comparison of the two series with different wavelengths but otherwise nearly identical conditions (filled circles and open triangles, respectively). Sunlight effectively photolyzes the aerosol formation because the UVabsorption spectrum of eq. (1) overlaps with the sea-level sunlight spectrum in the range of approximately 300---330 nm. In the laboratory, experiments must hence be shielded from ambient light because glass is translucent down to approximately 300310nm. By replacing the bandpass filter with a UV-absorption sharp cut filter it was shown that light with wavelength larger than 340nm yields negligible photolytic activity for the aerosol formation. Influence o f NH3
The results show that the quantum yield is highly and somewhat surprisingly affected by the NH3
"O
T25°C~ ~H2
1
05~H20_4 5 ~ ~C .
.p,-I
0.1
~ (2¢
4609
0.0 1
:::~Calculated,
25°C
0.001
0 zr0 260 360 400 NHa c o n c e n t r a t i o n (pprn) Fig. 2. The influence of the NH3 concentration on the quantum yield at different temperatures and humidities. 10% O2, wavelength 284 nm. Gas composition, temperature, light power, and residence time for individual series: (0) 200ppm SO2, 0.5% H20, 25°C, 18mW, 3.7s. Quantum yield of 1 corresponds to 0.86% conversion of SOz. (11) 200ppm SO2, 1% H20, 25"C, 13roW, 3.7s. Quantum yield of 1 corresponds to 0_62% conversion of SOz. (A) 100ppm SO2, 2% H20, 45°C, 18mW, 12.2s. Quantum yield of 1 corresponds to 2.9% conversion of SO2. (O) Quantum yield at conditions of (0) calculated from reactions (1)-(21). (UI) Quantum yield at conditions of (11) calculated from reactions (1)-(21).
concentration, the temperature and the humidity. Figure 2 shows the variation of YO with NH3content at three different conditions. In all series, but especially at the low temperature, 25°C, even small amounts of ammonia very significantly accelerate the overall rate of sulfate formation. The effect of ammonia is surprising because it cannot be predicted from the reaction mechanism shown in eqs (1)-(21) as seen by comparison with the simulated data at 25°C in Fig. 2. The simulated points are calculated by solving the following equations for a plug flow model of the experimental reactor: u-
dc~ dz
=
ri;
Ci = Cio for
z = 0
(24)
Here, u is the mean gas velocity in the reactor, z the axial distance from the inlet, ci the molar concentration of component Ai and r i is the net production rate of component Ai- Equation (24) constitutes a set of 17 equations which is integrated for T = 25°C, where all rate constants are known, to yield the effluent concentrations cie of all components at z = L. For this calculation it assumed as an approximation that a constant fraction equal to 89% of the singlet state produced by (1) decays to the ground state while 11% is transformed by (2) to the triplet state. If possible model deficiencies and experimental errors are taken into account the simulated data agree comparatively well with the experimental in the limit of zero NHrconcentration. The calculated
PETER SEIER CHRISTENSEN el 12[.
4610
~ 1o7!
zO
i0-3 ~i:
0
I >~
0
1o-4 "~ >
,~. 1 0 6
r~ q) (9
~
0. I
09
0
C.9 10 5
I00 260 0 NHa c o n c e n t r a t i o n
300 4~d 0-5 (ppm)
Fig. 3. The influence of the NH3 concentration on the particle number concentration and the particle mean volume. Conditions as for (A) data in Fig. 2: 100ppm SO2, 2% H20, 10% O2, 45°C, wavelength 284 nm, light power 18 roW, residence time 12.2 s.
quantum yields are, however, almost independent of the NHrconcentration and completely miss the observed remarkable, "catalytic" influence of ammonia. The effect of ammonia is not observed if the gas stream with ammonia is led to the reaction mixture after the reactor, immediately downstream from the irradiated zone. In the absence of ammonia in the irradiated zone a sulfuric acid aerosol is formed by the photolytic oxidation of SO2 and the acid is transformed to ammonium sulfate by the secondary injection of ammonia. However, no additional SO2 oxidation is observed by the ammonia injection. Hence, the catalytic effect of ammonia obviously requires the simultaneous presence of ammonia and UV-irradiation. A further indication of an unusual mechanism is the very large quantum yield at high NH 3concentration, especially noteworthy at 250C, where it even exceeds unity.
Temperature The inverse temperature variation observed in the presence of NH 3 in Fig. 2 is further analyzed in Fig. 4. These data show an irregular dependence on the temperature, characterized by a quite dramatic shift from a high quantum yield at low temperature to a much lower quantum yield at high temperature. The shift occurs over a limited temperature range of approximately 10°C within which the quantum yield is extremely sensitive to even small variations in the temperature. The temperature sensitivity is much less in both the low temperature and the high temperature ranges outside the transition range. The transition temperature depends on the overall gas composition and, for example, shifts to lower temperatures when the gas humidity is reduced. It is believed that the transition temperature range depends on the dew point temperature of the gas To and always occurs above, but in the close
o.o 10
2'0
3'0
4'0
Temperature (°C)
5'0
Fig. 4. The influence of temperature on the quantum yield at different H20 concentrations. 10% 02, residence time 12_2s. Gas composition, wavelength and light power for individual series: (11) 300 ppm SO2, 200 ppm NH3, <20ppm H20, 284nm, 10mW_ Quantum yield of 1 corresponds to 1.6% conversion of SO2. (A) 100ppm SO,_, 50 ppm NH~, 0.5% H20, 284 nm, 4.5 mW. Quantum yield of 1 corresponds to 0.73% conversion of SO2. Dew point temperature is 13°C. (0) 100ppm SO2, 50 ppm NH3, 1% H20, 284 nm, 18 mW. Quantum yield of 1 corresponds to 2.9% conversion of SO2. Dew point temperature is 18°C. (A) 100 ppm SO2, 50ppm NH3, 1% H20,309 nm, 14 mW. Quantum yield of 1 corresponds to 2.3% conversion of SO2. Dew point temperature is 18"C_ (Fq) 100ppm SO,,, 50ppm NH3, 2% H20, 284nm, 22 mW. Quantum yield of 1 corresponds to 3_6% conversion of SO_,. Dew point temperature is 25"C.
vicinity of To. The dew point temperatures calculated from the NH3, SO2 vapor pressure relations of Johnstone (1952) (cf. Christensen, 1994) confirm this hypothesis (cf_ text Fig. 4). On the basis of Fig. 4 and similar results we propose that the low temperature kinetics is influenced by a separate heterogeneous SO2 oxidation mechanism which occurs in parallel with the homogeneous gas phase mechanism. The low temperature heterogeneous path requires the presence of (NH4)2SO4 aerosol particles and takes place in a condensed phase which is either a physisorbed layer of molecules on the surface of a solid (NH4)2SO4 particle or a liquid droplet of an aqueous (NH4)2SO4 solution. Both these condensed phases will tend to form in the vicinity of the dew point temperature. The data in Fig. 4 with very low humidity only show a comparatively slight enhancement of the quantum yield at low temperature. The actually observed, although weak, enhancement for these data cannot be explained. They are measured quite far above the dew point and, hence, should not be influenced by the heterogeneous reaction. The heterogeneous path is apparently a photolytic mechanism which, like the homogeneous mechanism, requires UV-irradiation, because no oxidation takes place in the absence of UV-
Photolytic production of aerosols from SO2 and NH-~ ~.,,107 ~
]~10-1 ,._,
4611
~ 4r.)
"-"
--ql0-
1
I0 I /
\
,
15
40"C
,
tlO
.
20 25 30 35 Temperature (°C)
o
°
40
Fig. 5. The influence of the temperature on the particle number concentration and the particle mean volume. Conditions as for (0) data in Fig. 4: 100ppm SO2, 50 ppm NH3, i% H20, 10% 02, wavelength 284 nm, light power 18 mW, residence time 12.2 s. irradiation, when seed particles of (NH4)2504 from the aerosol generator are added to the gas. Because the heterogeneous mechanism occurs in the condensed phase it merely contributes to the growth of existing seed particles and cannot be expected to create new aerosol particles. They are produced exclusively by the homogeneous mechanism_ Therefore the variation with temperature of the size and concentration of effluent aerosol particles shown in Fig. 5 confirms the theory of a parallel heterogeneous path. In the temperature range from 15-30°C the particle size and concentration varies inversely in agreement with the theory that the heterogeneous path becomes dominant at the low temperature and the large particle surface created by the heterogeneous path depletes the gas of condensable components and thereby reduces the rate of homogeneous nucleation. However, it appears from the data obtained with very low water, concentration that the homogeneous mechanism in itself has a negative temperature dependence (curve marked • in Fig. 4). This explains that both particle size and concentration in Fig. 5 decline with increasing temperature above 300C, a temperature range in which the overall quantum yield is presumably dominated by the homogeneous mechanism. Figures 2 and 3 show that the increased quantum yield does not lead to an increased rate of nucleation, i.e. an increased particle number concentration, and therefore lend credence to the hypothesis of a surface or volume reaction_ For the series with low humidity the homogeneous mechanism is dominant in the whole temperature range and therefore the particle concentration increases slightly with increasing quantum yield throughout this range.
Influence of humidity The existence of two kinetic regions is reflected in the variation of the quantum yield with humidity depicted in Fig. 7. At low humidity, where we
.~0 0'30.01 Particle
O.l diameter
(~m)
Fig. 6. The logarithmic size distribution at two different temperatures. Conditions as for (0) data in Fig. 4" 100ppm SO,_, 50ppm NHj, 1% H20, 10% 02, wavelength 284 nm, light power 18 mW, residence time 12.2 s.
"O ,...-i
0.) >,,
..4--a
0.1 5°
(3'
0.01
0.
. . . . . . .
i
HzO concentration (~)
Fig. 7. The influence of the HzO concentration on the quantum yield at different temperatures. 10% O2, wavelength 284 rim, residence time 12.2 s. Gas composition, temperature and light power for individual series: (11) 100ppm SO> 100ppm NH> 15"C, 6.5 mW. Quantum yield of 1 corresponds to 0.62% conversion of SO,,. Dew point at 0.49% H20. (A) 200ppm SO> 200ppm NH3, 45"C, 17 mW. Quantum yield of 1 corresponds to 2.9% conversion of SO> Dew point at 7.5% H20.
believe the reaction is dominated by the homogeneous mechanism, the observed influence of humidity on the quantum yield is small. When the humidity is increased a shift in the mechanism to a much more water sensitive kinetics obviously occurs. The shift occurs at a humidity somewhat less than, but apparently related to the dew point humidity at the given temperature. The watersensitive range corresponds to the transition temperature ranges in Fig. 4 and thus can be explained by an increasing influence of the heterogeneous mechanism when the humidity increases to levels where the reactions start to take place in a condensed phase.
PETER SEIER CHRISTENSEN et al.
4612
low for the feed composition which only sustains the homogeneous mechanism because it is without aerosol seeds. As aerosol nuclei are formed by ,-==1 homogeneous nucleation the heterogeneous ~0.15 mechanism takes over and the quantum yield ~, - gradually increases as the aerosol surface/volume increases. ~0.10 The corresponding, predicted interaction between seed nucleation and seed growth kinetics is confirmed by the measured aerosol characteristics depicted in Fig. 9. The number concentration of 0"0.05 particles, assumed to be formed initially at the entrance of the reactor, is practically constant, while the increased conversion at high residence 0.00 times manifests itself mainly in a growth of the 0 5 lb 1'5 20 R e s i d e n c e t i m e (s) initially formed particles_ The particle number concentration is, however, affected by conditions Fig. 8_ The influence of the residence time on the quanwhich influence the homogeneous rate. Increasing tum yield at different temperatures and intensities. 10% the light power, for example, increases the rate of 02, wavelength 284 nm. Gas composition, temperature and light power for individual series: (11) 100 ppm SO2, homogeneous nucleation which yields a larger 100ppm NH3, 0.25% H20, 15"C, 6.5 roW. ( l ) 2 5 0 p p m effluent particle concentration (Christensen, 1994). SO2, 250 ppm NH3, 2% H20, 45°C, 6 roW. (O) 250 ppm SO2, 250 ppm NHs, 2% H20 45°C, 25 mW. if-l) 250 ppm DISCUSSION SO2, 250 ppm NH3, 2% H20, 45"C, 30 mW, Residence The observed acceleration of the photolytic oxtime is varied by changing the reactor volume. idation of SO2 in the presence of NHs cannot be explained by any known mechanism of gas reactions. A somewhat remote possibility is that the Residence time excited singlet molecule 502(IA2, IB1) is stabilized Normally, the drop in the gas phase concentraby the formation of complexes (exciplexes) with tions of the reactants and the extinction of light by NH3. Formation of exciplexes with various gas passage through the reactor are both small and the molecules has been observed to increase the oxidareactor in that respect behaves differentially. tion rate of SO2(IA2, 'Bl) (Richards et al., 1976) Under these conditions one would immediately but NH3 has not previously been mentioned in this anticipate the quantum yield to be independent of context. the residence time of the gas in the reactor, The photolytic oxidation of S(IV) in the aerosol provided all other imposed conditions such as the phase probably occurs via absorption of reactive gas feed composition and the irradiation are kept oxidants from the gas phase and not by photolytic constant. Actually the measurements deviate conexcitation in the aerosol phase. Neither HSO~ nor siderably from this anticipation. In several runs the SO~- absorbs radiation in the wavelength range residence time was varied, either by changing the feed flow rate with constant reactor volume or by changing the reactor volume with constant flow rate. The latter procedure was adapted by using a 10-3 10 modified reactor design in which the rear end of the g--. I reactor can be moved axially like a piston to change E :& the reactor volume. In all these runs, shown in Fig. 0 v 8, the quantum yield increases with increasing residence time in a manner characteristic for reaco tions with auto-catalytic kinetics. Accordingly, the '~10 6 lo-42 0 reactant conversion does not increase linearly with residence time but the slope of conversion versus residence time increases with increasing residence time. If the measurement in Fig.-8 were extended o to larger values of the residence time one would 10 5 expect the quantum yield to go through a maximum 2010-5 6 tb f5 and decrease when the reactant conversion beR e s i d e n c e time (s) comes large as usual for autocatalytic reactions. Fig. 9. The influence of the residence time on the particle This range, however, exceeds the measuring range number concentration and the particle mean volume. of the DMPS-system. Conditions as for (I-I) data in Fig. 8" 250ppm SO2, The autocatalytic behavior agrees well with the 250ppm NH3, 2% H20, 10% 02, 45°C, wavelength proposed homogeneous-heterogeneous, parallel 284 nm, light power 30 mW. Residence time is varied by mechanisms which predict that the quantum yield is changing the reactor volume. 0.20
Photolytic production of aerosols from SO2 and NH 3 used (Deister et al., 1986). Both H202 and 03 are powerful condensed-phase S(IV) oxidants (Schwartz, 1984)_ However, their computed production rates in the gas phase are very low and cannot explain the very high quantum yields unless they initiate chain reactions in the liquid--a theory which cannot be substantiated. A more likely key species is the SO~-radical which initiates a very fast SO 2- oxidation chain mechanism (Deister and Warneck, 1990). The SO~-radical can be formed by UV-excitation of SO 2- at wavelengths below 260 nm (Deister and Warneck, 1990), but it cannot be formed at the higher wavelength of the sunlight and in the present investigation (Christensen, 1994). However, a probable theory is that the SO3-radical is formed by absorption and dissociation of H O S O 2 [formed by eq. (16)] by the following reaction:
HOSO2.aq--~ SO3,aq + H +
4613
• Step 3: Aerosol condensation or nucleation: (NH4)2SO4(g)--~ (NH4)2SO4(s) All evidence shows that the steps prior to condensation produce vapor components which readily nucleate homogeneously. Hence, these components must be produced into a state with a high degree of supersaturation. However, the precise identity of the nucleating component is not certain. It is still possible that either H2SO 4 or NH3SO3 nucleate homogeneously, in which case (NH4)2SO4 is formed subsequently by reaction with NH3 and H20. The proposed steps 2 and 3 are judged to be representative of the chemical and physical processes which must occur in any ammonium sulfate aerosol formation. • Step 4: Absorption of reaction components by the aerosol:
(SO 2 + H20 + 2NH3)(g) --~ (SO2 + H20 + 2NH3)(s,I)
CONCLUSION When humid air with small amounts of SO2 and NH3 is irradiated by UV sunlight an ammonium sulfate aerosol is rapidly formed. This reaction can significantly influence the opacity of stack-plumes from various industrial processes_ The kinetics of this process, which has been studied in the present investigation, shows some distinct features. The photolytic oxidation of SO2, which is one step in the overall pattern, is strongly catalyzed by NH3 and the rate increases significantly with decreasing temperature in the range 15-45°C_ The overall process is very complicated and many important details are still unknown. However, the kinetic observations strongly indicate that two different, parallel mechanisms are involved. A gas phase homogeneous mechanism and a heterogeneous mechanism involving reactions in or at the surface of the aerosol phase. The overall process can be visualized to occur by the following steps: • Step 1: Photolytic excitation in the gas phase: SO2(IA 0 + h~,(240 nm < ;t < 330 nm)
SO2(IA2, IBt)---~ SO2(3B1) The triplet molecule SO2(3Bj) is the usual initiator of many atmospheric reactions involving SO2. All photo-kinetic observations in our study agree with this step. • Step 2: Homogeneous ammoxidation of SO2:
502(3BI) + 102 -I- H20 + 2NH3--~ (NH4)2SO4(g) This step occurs by an intricate mechanism, the details of which are still obscure. The results exclude the possibility that the oxidation of SO2 to S(VI)-species strictly precedes the addition of ammonia, but the way in which ammonia significantly enhances the overall rate of steps 1 and 2 over that expected from the usual kinetics of photolytic SO2-oxidation is unclear.
In the vicinity of the dew point H20, 502, and NH3 condenses onto the aerosol formed by step 3. The species are either physisorbed on the surface of an aerosol particle or dissolved in a liquid aerosol droplet of an aqueous ammonium sulfate solution. • Step 5: Heterogeneous photolytic ammoxidation of 502: (SO3 + H20 + 2NH3)(s3) + 102(g) + hv--* (NH4)2SO4(s,I)
This step is similar in the stoichiometry to step 2, but it occurs on or within the aerosol particles and therefore does not lead to the nucleation of new particles but rather to the growth of existing particles. The step requires a photolytic excitation by UV-irradiation, probably the same excitation of SO2 as that of step 1. The rate of this step is very fast compared to step 2. Details of the underlying mechanism are open for discussion, but it is believed that a heterogeneous step akin to step 5 is essential for the explanation of the high quantum yields observed. Due to the interaction between steps 3, 4 and 5, the overall reaction behaves distinctly autocatalytic. As a consequence hereof, the laboratory reactor in certain ranges of the operating conditions yields sustained oscillations of the aerosol production rate. The quantitative background for the reactor behavior and the exact nature of the kinetic mechanisms are presently under further investigation_ REFERENCES
Atkinson, R., Baulch, D. L., Hampson, R. F. Jr, Kerr, J. A..and Troe, J., 1992, Evaluated kinetic and photochemical data for atmospheric chemistry. Supplement IV. IUPAC subcommittee on gas kinetic data evaluation for atmospheric chemistry. J. Phys. Chem. Ref. Data 21, 1125-1568.
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Bai, H., Biswas. P. and Keener, T. C., 1992, Particle formation by NH3-SO2 reactions at trace water conditions. Ind. Engng Chem. Res. 31, 88--94. Baulch, D. L., Cox, R. A., Hampson, R. F. Jr, Kerr, J. A., Troe, J. and Watson, R. T., 1980, Evaluated kinetic and photochemical data for atmospheric chemistry. J. Phys. Chem. Ref. Data 9(2), 295--471. Calvert, J. G., Su, F., Bottenheim, J_ W. and Strausz, O. P., 1978, Mechanism of the homogeneous oxidation of sulfur dioxide in the troposphere. Atmos. Environ_ 12, 197-226. Christensen, P. S., 1994, The formation of aerosol from SO2 and NH 3 in humid air, Ph.D. thesis, Technical University of Denmark, Chemical Engineering Department. Christensen, P. S., Madsen, N. M. and Livbjerg, H., 1992, The formation of aerosols from SO2 and NH3 in humid air. J. Aerosol Sci. 23(S1), $261-265. Deillinger, B., Grotecioss, G., Fortune, C. R_, Cheney, J_ L. and Homolya, J. B., 1980, Sulfur dioxide oxidation and plume formation at cement kilns. Environ. Sci. Technol. 14(10), 1244--1249. Deister, U. and Warneck, P., 1990, Photooxidation of SO~ in aqueous solution, J. Phys. Chem. 94, 21912198. Deister, U., Neeb, R., Helas, G. and Warneck, P., 1986, Temperature dependence of the equilibrium CH2(OH)2 + HSO~ = CH2(OH)SO3 + H20 in aqueous solution. J. Phys. Chem. 90, 3213-3217. Hartley, E. M_ and Matteson, M. J., 1975, Sulfur dioxide reactions with ammonia in humid air. Ind. Engng Chem. Fundam. 14(1), 67-72.
HOvel, Von B., 1990, Kombination einer Waltherentschwefelung mit einer SCR-entstickung als nachriistung hinter einer kohleschmelzfeuerung. VBG Kraft. werkstechnik 70(7), 602--607. Johnstone, H. F., 1952, Recovery of sulfur dioxide from waste gases. Ind. Engng Chem. 27(5), 587-593. Mallard, W. G., Westley, F., Herron, J. T., Hampson, R. F. and Frizzell, D_ H., 1993, NIST Chemical Kinetics Database--Ver 5.0, NIST Standard Reference Data, Gaithersburg, MD. Moore, N. D., 1977, Fume formation in ammonia scrubbers. A S M E Ann. Meeting Atlanta, pp. 1-11_ Richards, J. R., Fox, D. L. and Reist, P. C., 1976, The influence of molecular complexes on the photooxidation of sulfur dioxide. Atmos. Environ. 10,211-217. Rothenberg, S. J., Dahl, A. R., Barr, E. B. and Wolff, R. K., 1986, Generation, behavior, and toxicity of ammonium sulfite aerosols. J. APCA 36, 55-59. Schwartz, S. E., 1984, Gas-aqueous reactions of sulfur and nitrogen oxides in liquid-water clouds, in S02, NO and NO,_ Oxidation Mechanisms: Atmospheric Considerations (Edited by J. G. Calvert). Butterworth Publishers. Sidebottom, H. W., Badcock, C. C_, Jackson, G. E_, Calvert, J. G., Reinhardt, G. W. and Damon, E. K., 1972, Photooxidation of sulfur dioxide. Environ. Sci. Technol. 6(1), 72-79. Vance, J. L. and Peters, L. K., 1976, Aerosol formation resulting from the reaction of ammonia and sulfur dioxide. Ind. Engng Chem. Fundam. 15(3), 202-206.
Pergamon
Copyright1~)1995Elsevier Science Lid Printedin GreatBritain.All fightsreserved 0009-2.509/94 $7.00 + 0.00
0009--2509(94)00341-6
T H E KINETICS OF T H E P H O T O L Y T I C P R O D U C T I O N O F A E R O S O L S F R O M SO2 A N D N H 3 IN H U M I D A I R P E T E R SEIER CHRISTENSEN, STIG W E D E L and HANS LIVBJERG* Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark (Received 25 May 1994; accepted.for publication 4 October 1994)
reaction of SO2 and N H 3 in humid air to produce an aerosol of ammonium sulfate is studied in a photolytic laboratory reactor. The reaction is shown to be controlled by ultraviolet irradiation in the wavelength range of 240-330 nm and is initiated by excitation of SO2. It is found that NH3 has a strong catalytic effect on the photolytic oxidation of SO2 and surprisingly high values of the quantum yield are observed with NH 3 concentration in the range of 20-400 ppm. The reaction rate increases with decreasing temperature. An overall mechanism for the formation of aerosol particles is proposed. It is concluded that aerosol formation due to sunlight photolysis by this mechanism explains the high-opacity smoke formed in industrial stack plumes with small amounts of NH3 and SO2.
Abstract--The
INTRODUCTION
The reaction of SO2 and NH3 in humid air to produce an aerosol of ammonium sulfate is investigated in this work_ Aerosols derived from this reaction system have been reported to yield a high-opacity, persistent smoke ("blue smoke") polluting the stack plumes of cement kilns (Deillinger et al_, 1980) and some power plants with ammonia scrubbers for flue gas desulfurization (Moore, 1977; HOvel, 1990; Christensen et al., 1992). In the case of the Walther desulfurization process the formation of blue smoke, according to one theory, is initiated within the scrubber by the homogeneous nucleation of small aerosol particles of an ammonium S(IV)-salt which is subsequently stabilized by oxidation to sulfate particles (Moore, 1977). To avoid nucleation one must control the gas composition in the scrubber to avoid exceeding the threshold of supersaturation which leads to nucleation of new particles. This theory is sustained by some literature references suggesting that ammonium S(IV)-salts may actually nucleate homogeneously from SO2 and NH3 in humid air (Hartley and Matteson, 1975; Vance and Peters, 1976; Rothenberg et al., 1986; Bai et al., 1992) but the precise conditions required for homogeneous nucleation are obscured by the fact that the gas may incidentally become supersaturated with respect to a number of different ammonium S(IV)species under typical scrubber conditions (Christensen et al., 1992). However, recently Christensen (1994) in an experimental study has investigated the rate of homogeneous nucleation of ammonium S(IV)-salts from SO2 and NH3 in humid air in the temperature range 0-50°C. This study showed that in the absence of ultraviolet light irradiation the nucleation rate is negligible even when extreme "To whom correspondence should be addressed.
conditions of supersaturation are imposed. The investigation also has revealed a previously unrecognized catalytic effect of NH3 on the photolytic oxidation of SO2 and this effect has possibly been overlooked in previous investigations. As a consequence of the photolytic influence, an ammonium sulfate aerosol may be formed within a sunlit stack plume with small amounts of NH3 and SO2 even if no nuclei have been formed prior to the stack exit_ PHOTOLYTIC REACTION MECHANISM IN THE GAS PHASE
The only possible photolytic excitation by sunlight of the reactants used in this study is the first allowed excitation of SO2: SO2(~A~) + hv(240 n m < A < 330 rim)---> SO2(IA2, IB2)
(1)
The highly unstable excited singlet state of SO2 partly decays to the triplet state of the lowest energy by intersystem crossing: 502(IA2, IB1)---~502(3BI)
(2)
and partly relaxes to the ground state. Approximately 11% of the SO2(IA2, lBl) produced by eq. (1) is transformed by eq. (2) (Calvert et al., 1978). The triplet SO2(3Bi), which is far more stable than SO2(IA2, *B,) and is believed to be the reactive species in atmospheric photo-oxidation of SO2, initiates a chain of reactions. Even in a system of pure N2, 02, H20, SO2, and NH3 the number of possible reactions is considerable. The network of reaction steps shown in eqs (3)-(21) include all reactions of significance in the proximity of room temperature. They have been selected by Christensen (1994) after a systematic scrutiny of rate constants from the literature and kinetic databases. Some of the reactions given in eqs (3)-(21) are of
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PETER SEIERCHRISTENSENel al.
minor importance but are included for their potential importance at other temperatures than 25"C. There are many other possible reaction steps but they are all too slow to influence our measurements. The reactions in the gas mixture of this investigation have been widely studied due to their importance in atmospheric chemistry. The rate constants at 25°C are cited below together with the selected reactions.* The set of rate constants is quite complete at 25°C but far less complete at other temperatures. SO2(3BI) produced in eq. (2) can react by one of the following reactions: SO2(3BI) + SO2---~SO3 + SO
k 3 = 4.2 x 107
(3)
SO2(BBI)-1-O2-->SO2-FO2(IA) k 4 = 9.6x 107 (4) SO2(3BI) + N2--* 502 + N2 k5 = 8.5 × 107
(5)
The species SO is quite stable and the reactions in which it participates are all slow compared with eqs (3)-(21). Thus, SO may be treated as a stable product on a short time scale. Possibly the excited state of O2 primarily formed by eq. (4) is oz(EE~) which will, however, rapidly be transformed to O2(IA) (Sidebottom et al_, 1972). O2(IA) is a low-energy excited state of oxygen which either reacts with SO2 or is quenched to the ground state: O2(IA) + SO2-.o 503 "{-O
k 6 = 1.3 x 105 (6)
O2(lA) + N2---~O2 + N2 k7~<84 O2(zA)+H20--o02+H20
ks = 3x103
O2(1A) + O2"-'>202 k9 -'- 103
(7) (8) (9)
Atomic oxygen produced in eq. (6) reacts further by the following reactions: O + 02---> 03 O+SO2--oSO3
kl0 = 109 kll = 7 x 1 0 1 1
(10) (11)
Although 03 is virtually unreactive it dissociates photolytically by 03 + hv--* O2(IA) + O(ID)
(12)
The extinction coefficient for 03 at h = 284 nm corresponding to eq. (12) is 1800 l/mol/cm (Bauich et al., 1980). O(ID) is a highly reactive excited state of O which yields OH-radicals by reaction with H20 or NH3. Otherwise it is quenched to ground state atomic oxygen.
O(ID) + n20-'* 2OH O(1D)+NH3-->OH+NH2
kl3 = 1.2 x 10 II (13) kl4 = 2 x 1 0 II
(14)
O(ID) + N2---*O + N2 kls = 1.6 x 1010 (15) "All rate constants ki are given in the unit I/mol/s. The data are from Mallard et al. (1993) except for: eqs (3) and (4): Calvert et al. (1978) eq. (5): Sidebottom et al. (1972) eqs (7), (8), (9) and (15): Atkinson et al. (1992)
Numerical calculations show that neither eq. (14) nor the analogous reaction between NH3 and ground state O have any influence at all on the overall oxidation rate. Any NH2 formed by these reactions may participate in a number of further reactions but these reactions are also unimportant. In the presence of OH-radicals SO2 is oxidized to SO3 by the following chain reactions. OH+SO2---,HOSO2 HOSO2 + 02--> HO 2 + 803
kl6 = 3 . 6 x l0 s
(16)
kit = 2.5 x 1"08 (17)
HO 2 + SO2---~ OH + SO3
kzs = 5.2 x 105 (18)
with the major terminating step OH + HO2----~H20 + 02
k19 = 4 x 10 I° (19)
SO3 formed by eqs (3), (6), (11), (17), and (18) reacts fast with H20 or NH3: SO3+H20--~H2SO4 SO3 + NH3-* NH3803
k20~<3.6x 106
(20)
k21 = 4.2 X 10 t° (21)
The final steps leading to aerosol formation are presumably all very fast although their kinetics and precise nature are not known. They may involve the nucleation of the products of eqs (20) and (21) followed by a heterogeneous reaction with H20 and NH3 to yield ammonium sulfate. Alternatively, the products of eqs (20) and (21) may react in the gas phase with H20 and NH3 to yield supersaturaated ammonium salts which then nucleate. To characterize the efficiency by which SO2(IA2,1BI) molecules excited by eq. (1) are transformed to ammonium sulfate the quantum yield Yo shall be defined as the ratio of the overall molar rate of (NH4)2SO4 production F(Nrt~),_s04 to the molar rate of SO2 excitation Fso: by eq. (1) or: YO = FfNH4)'-SO4 Fso.~
(22)
The denominator of eq. (22) is computed from the extinction coefficient for SO2 corresponding to eq. (1) at the given wavelength. Fso~ =
loA (I_10_ECso2L) NAhv
(23)
where N A is Avogadro's number, h Planck's constant, loA the total power of the incident light beam (W), e is the extinction coefficient (l/mol/cm), L the reactor length (cm), Cso~ the molar concentration of SO2 in the gas (tool/l), and u is the frequency of the light. Because approximately 89% of the excited singlet SO2(IA2, ~BI) is quenched to the ground state one would anticipate YO to be less than 0.11. However, due to chain mechanisms like eqs (16)(18), YQ might in principle increase beyond that limit. The rate constants in the chain mechanism are quite small though and actual calculations typically yield YO values in the range 3 x 10-3-10 -5
Photolytic production of aerosols from S02 and in stark contrast to the large experimental values measured in the presence of N H 3 and presented below. EXPERIMENTAL
The overall kinetics of the photolytic aerosol formation is investigated in a laboratory reactor shown in Fig. 1. The reactor is designed to operate at steady state with a continuous flow of a gas passing through a tubular reactor irradiated with light by an optical system. The feed gas is made up by mixing different gas streams, each metered and controlled by a flow controller. NH3 and SO2 as mixtures with N2 are drawn from cylinders and are further diluted with streams of dry N2 and 02. A separate stream of N2 is passed through a humidifier, where it is saturated in a packed column with recycling water at a constant temperature which can be varied to yield different humidities. The streams are thermostatted at 45°C to keep them above the dew point. All gas streams are filtered by absolute filters so that they are completely free from aerosol seeds. Optionally, seed particles of a controllable size and concentration can be added to the feed in a separate stream of N2- The seed particles are produced by a constant output atomizer model 3076, TSI Inc., which yields a high c o n c e n t r a t i o n (>106/cm 3) of small droplets (0.5/zm) of an aqueous salt solution. The aqueous aerosol is dried in a diffusion drier to an aerosol of small solid salt particles the size of which can be varied by changing the concentration of the salt solution. The effluent gas from the reactor is reheated to 45°C, such that only sulfate particles are stable, and passes through a diffusion drier. This is a 600 mm long, cylindrical tube of diameter 10 mm with wire mesh walls surrounded by a molecular sieve (4/~) drying agent. The gas flow is laminar and passage through the drier efficiently removes water, SO2 and NH 3 from the gas and from evaporating, unstable sulfite particles but leaves the stable sulfate particles undisturbed. The aerosol is analyzed using either a differential mobility particle sizer (DMPS) consisting of an electrostatic classifier (EC) model 3071, TSI Inc. and condensation particle counter (CPC) model 3022, TSI Inc. or alternatively a Berner-type low pressure cascade impactor, Model LPI 25/0.018/2, Hauke G m b H & Co. KG. The DMPS-system measures the particle size distribution in the range 0.019--0.931/~m in 28 intervals. The impactor samples the particles in 10 size channels in the range 0.010--12.0/.tin. The gas phase concentrations of N2, 02, H20 and SO2 are measured before and after the reactor with a Balzers Q M G 420 mass spectrometer (MS). The reactor consists of a 120 mm long stainlesssteel tube with an internal diameter of 36 mm. The end walls of the reactor are quartz lenses transparent to the UV-light beam. The gas enters and leaves the reactor through a ring of small holes
NH 3
4607
evenly spaced in the wall close to each of the lenses. The reactor is surrounded by a heating jacket with water pumped from a thermostat bath for temperature control. To check against the possibility that the stainless steel wall of the reactor exerts any catalytic or other kinetic influence special shielding inserts of pyrex glass and quartz glass were applied in some runs. No effect on the observed kinetics could be detected. The optical system consists of a 500W superquiet mercury-xenon lamp type L2424, Hamamatsu Corporation. Light in the IR range is removed by reflecting the beam by a UV-reflecting and IR-transmitting planar mirror. The beam passes a high frequency chopper for control of the beam intensity followed by a UV-bandpass filter type U V - K M Z 20-3, Schott Glaswerke. The bandpass filter is a thin plate which must not be heated above 70°C. A special arrangement of air cooling through small nozzles directed at both sides of the filter has to be applied to avoid superheating. Bandpass filters with nominal wave lengths of 284 and 309 nm are used in this study. The transmittance spectra of these filters yield quasi-monochromatic beams. The spectra, at the half peak height, have widths of 20nm for the 284nm filter and 7.5nm for the 309 nm filter. The filtered light beam is collected by the first lens to form nearly parallel rays within the reactor. The second lens focuses the beam on the UV-energy meter type AC25UV, Scientech. The UV-radiation in the studied range can damage the skin and the eyes. Direct and secondary irradiation of body parts must hence be avoided. The whole reactor and lamp assembly is therefore built into an effectively shielding cupboard. As reagents high-purity nitrogen, high-purity oxygen, 2% NH3 in N2 and 1% SO2 in N 2 a r e used. In the humidifier high-purity water is used. To check against the possibility that trace impurities affect the observed kinetics the purity of the gases and the water were changed in some runs with no observable effect on the measurements.
RESULTS
Procedure An experimental run is made by turning on the gas feed flows and checking the gas feed composition. It is checked with the DMPS that there are no aerosol nuclei in the effluent gas when the UV-light is turned off. The UV-lamp is then turned on and the particle size distribution of the aerosol produced is measured repeatedly until steady state is achieved. The beam power is measured by the UV-detector without the presence of SO2 in the reactor. The duration of one run is typically 1 h. The size distributions measured by the DMPS are normally monodisperse with a quasi-lognormal shape (see Fig. 6 below). The particle size varies over a wide range. To ensure a correct measurement of the quantum yield it is necessary to keep
PETER SEIER CHRISTENSEN et al_
4608
Hg-Xe-Lamp Humidifier
NH 3
502
Mirror Mirror
Purge
(F
Chopper i
Bandpass Filter
N2
\
Lens
IOlO
N2
I_
Atomizer
olo¢11 ~ m
Healing Jacket rjr~t
| •
02 Lens /
/
~Purge
I~
Impactor I
UV Energymeter Fig. 1. Monochromatic photolytic reactor, schematical. FC, flow controller; PC, pressure controller; EC, electrostatic classifier; MS, mass spectrometer; TC, temperature controller; DD, diffusion drier; CPC, condensation particle counter; R, restrictor. E~, Reducing valve; B~, filter (prefilter and absolute filter); [], heating tape; 121,thermal insulation.
Photolytic production of aerosols from SO2 and NH3 all particles within the size limits of the DMPS, a constraint which limits the applicable range of operating conditions_ For the ensuing measuremerits this is reflected in the choice of conditions which may appear slightly illogical for some of the series. If a small fraction of the size distribution is outside the measuring range (see Fig. 6 below) the measurement can be corrected for the missing data by fitting a log-normal distribution to the measured values. Data points adjusted accordingly are marked in the following. In some runs the particles were sampled by the Berner low pressure impactor for elemental analysis and for microscopic investigation. The particles consisted of (NH4)2SO4 and appeared compact and spherical in a scanning electron microscope. For that reason the total volume of particles calculated from the DMPS distribution data is believed to yield a quite precise estimate of the ammonium sulfate production rate, i.e. F(NH4)2SO J of eq. (22). This is verified by the fact that the measurements of the total production rate by the DMPS and the impactor were in a satisfactory agreement. Hence, the DMPS distribution data can be used to characterize the kinetics of the overall rate of photolytic SO2 oxidation as well as the kinetics of the nucleation and growth of the aerosol particles. The considerable decline in the intensity of the UV-lamp over some hundreds of hours of operation also constrains the maximum intensity which can be used. The intensity applied in each measurement is shown together with other operating conditions in the ensuing data. Since the intensity influences the quantum yield it must be kept constant when the influence of other parameters is being studied. Wavelength
The observed photo-kinetics of the aerosol production agrees well with the UV-absorption spectrum of eq. (1). The wavelengths 284 and 309 nm yield qualitatively the same effect. This is seen, for example, in Fig. 4 below by comparison of the two series with different wavelengths but otherwise nearly identical conditions (filled circles and open triangles, respectively). Sunlight effectively photolyzes the aerosol formation because the UVabsorption spectrum of eq. (1) overlaps with the sea-level sunlight spectrum in the range of approximately 300---330 nm. In the laboratory, experiments must hence be shielded from ambient light because glass is translucent down to approximately 300310nm. By replacing the bandpass filter with a UV-absorption sharp cut filter it was shown that light with wavelength larger than 340nm yields negligible photolytic activity for the aerosol formation. Influence o f NH3
The results show that the quantum yield is highly and somewhat surprisingly affected by the NH3
"O
T25°C~ ~H2
1
05~H20_4 5 ~ ~C .
.p,-I
0.1
~ (2¢
4609
0.0 1
:::~Calculated,
25°C
0.001
0 zr0 260 360 400 NHa c o n c e n t r a t i o n (pprn) Fig. 2. The influence of the NH3 concentration on the quantum yield at different temperatures and humidities. 10% O2, wavelength 284 nm. Gas composition, temperature, light power, and residence time for individual series: (0) 200ppm SO2, 0.5% H20, 25°C, 18mW, 3.7s. Quantum yield of 1 corresponds to 0.86% conversion of SOz. (11) 200ppm SO2, 1% H20, 25"C, 13roW, 3.7s. Quantum yield of 1 corresponds to 0_62% conversion of SOz. (A) 100ppm SO2, 2% H20, 45°C, 18mW, 12.2s. Quantum yield of 1 corresponds to 2.9% conversion of SO2. (O) Quantum yield at conditions of (0) calculated from reactions (1)-(21). (UI) Quantum yield at conditions of (11) calculated from reactions (1)-(21).
concentration, the temperature and the humidity. Figure 2 shows the variation of YO with NH3content at three different conditions. In all series, but especially at the low temperature, 25°C, even small amounts of ammonia very significantly accelerate the overall rate of sulfate formation. The effect of ammonia is surprising because it cannot be predicted from the reaction mechanism shown in eqs (1)-(21) as seen by comparison with the simulated data at 25°C in Fig. 2. The simulated points are calculated by solving the following equations for a plug flow model of the experimental reactor: u-
dc~ dz
=
ri;
Ci = Cio for
z = 0
(24)
Here, u is the mean gas velocity in the reactor, z the axial distance from the inlet, ci the molar concentration of component Ai and r i is the net production rate of component Ai- Equation (24) constitutes a set of 17 equations which is integrated for T = 25°C, where all rate constants are known, to yield the effluent concentrations cie of all components at z = L. For this calculation it assumed as an approximation that a constant fraction equal to 89% of the singlet state produced by (1) decays to the ground state while 11% is transformed by (2) to the triplet state. If possible model deficiencies and experimental errors are taken into account the simulated data agree comparatively well with the experimental in the limit of zero NHrconcentration. The calculated
PETER SEIER CHRISTENSEN el 12[.
4610
~ 1o7!
zO
i0-3 ~i:
0
I >~
0
1o-4 "~ >
,~. 1 0 6
r~ q) (9
~
0. I
09
0
C.9 10 5
I00 260 0 NHa c o n c e n t r a t i o n
300 4~d 0-5 (ppm)
Fig. 3. The influence of the NH3 concentration on the particle number concentration and the particle mean volume. Conditions as for (A) data in Fig. 2: 100ppm SO2, 2% H20, 10% O2, 45°C, wavelength 284 nm, light power 18 roW, residence time 12.2 s.
quantum yields are, however, almost independent of the NHrconcentration and completely miss the observed remarkable, "catalytic" influence of ammonia. The effect of ammonia is not observed if the gas stream with ammonia is led to the reaction mixture after the reactor, immediately downstream from the irradiated zone. In the absence of ammonia in the irradiated zone a sulfuric acid aerosol is formed by the photolytic oxidation of SO2 and the acid is transformed to ammonium sulfate by the secondary injection of ammonia. However, no additional SO2 oxidation is observed by the ammonia injection. Hence, the catalytic effect of ammonia obviously requires the simultaneous presence of ammonia and UV-irradiation. A further indication of an unusual mechanism is the very large quantum yield at high NH 3concentration, especially noteworthy at 250C, where it even exceeds unity.
Temperature The inverse temperature variation observed in the presence of NH 3 in Fig. 2 is further analyzed in Fig. 4. These data show an irregular dependence on the temperature, characterized by a quite dramatic shift from a high quantum yield at low temperature to a much lower quantum yield at high temperature. The shift occurs over a limited temperature range of approximately 10°C within which the quantum yield is extremely sensitive to even small variations in the temperature. The temperature sensitivity is much less in both the low temperature and the high temperature ranges outside the transition range. The transition temperature depends on the overall gas composition and, for example, shifts to lower temperatures when the gas humidity is reduced. It is believed that the transition temperature range depends on the dew point temperature of the gas To and always occurs above, but in the close
o.o 10
2'0
3'0
4'0
Temperature (°C)
5'0
Fig. 4. The influence of temperature on the quantum yield at different H20 concentrations. 10% 02, residence time 12_2s. Gas composition, wavelength and light power for individual series: (11) 300 ppm SO2, 200 ppm NH3, <20ppm H20, 284nm, 10mW_ Quantum yield of 1 corresponds to 1.6% conversion of SO2. (A) 100ppm SO,_, 50 ppm NH~, 0.5% H20, 284 nm, 4.5 mW. Quantum yield of 1 corresponds to 0.73% conversion of SO2. Dew point temperature is 13°C. (0) 100ppm SO2, 50 ppm NH3, 1% H20, 284 nm, 18 mW. Quantum yield of 1 corresponds to 2.9% conversion of SO2. Dew point temperature is 18°C. (A) 100 ppm SO2, 50ppm NH3, 1% H20,309 nm, 14 mW. Quantum yield of 1 corresponds to 2.3% conversion of SO2. Dew point temperature is 18"C_ (Fq) 100ppm SO,,, 50ppm NH3, 2% H20, 284nm, 22 mW. Quantum yield of 1 corresponds to 3_6% conversion of SO_,. Dew point temperature is 25"C.
vicinity of To. The dew point temperatures calculated from the NH3, SO2 vapor pressure relations of Johnstone (1952) (cf. Christensen, 1994) confirm this hypothesis (cf_ text Fig. 4). On the basis of Fig. 4 and similar results we propose that the low temperature kinetics is influenced by a separate heterogeneous SO2 oxidation mechanism which occurs in parallel with the homogeneous gas phase mechanism. The low temperature heterogeneous path requires the presence of (NH4)2SO4 aerosol particles and takes place in a condensed phase which is either a physisorbed layer of molecules on the surface of a solid (NH4)2SO4 particle or a liquid droplet of an aqueous (NH4)2SO4 solution. Both these condensed phases will tend to form in the vicinity of the dew point temperature. The data in Fig. 4 with very low humidity only show a comparatively slight enhancement of the quantum yield at low temperature. The actually observed, although weak, enhancement for these data cannot be explained. They are measured quite far above the dew point and, hence, should not be influenced by the heterogeneous reaction. The heterogeneous path is apparently a photolytic mechanism which, like the homogeneous mechanism, requires UV-irradiation, because no oxidation takes place in the absence of UV-
Photolytic production of aerosols from SO2 and NH-~ ~.,,107 ~
]~10-1 ,._,
4611
~ 4r.)
"-"
--ql0-
1
I0 I /
\
,
15
40"C
,
tlO
.
20 25 30 35 Temperature (°C)
o
°
40
Fig. 5. The influence of the temperature on the particle number concentration and the particle mean volume. Conditions as for (0) data in Fig. 4: 100ppm SO2, 50 ppm NH3, i% H20, 10% 02, wavelength 284 nm, light power 18 mW, residence time 12.2 s. irradiation, when seed particles of (NH4)2504 from the aerosol generator are added to the gas. Because the heterogeneous mechanism occurs in the condensed phase it merely contributes to the growth of existing seed particles and cannot be expected to create new aerosol particles. They are produced exclusively by the homogeneous mechanism_ Therefore the variation with temperature of the size and concentration of effluent aerosol particles shown in Fig. 5 confirms the theory of a parallel heterogeneous path. In the temperature range from 15-30°C the particle size and concentration varies inversely in agreement with the theory that the heterogeneous path becomes dominant at the low temperature and the large particle surface created by the heterogeneous path depletes the gas of condensable components and thereby reduces the rate of homogeneous nucleation. However, it appears from the data obtained with very low water, concentration that the homogeneous mechanism in itself has a negative temperature dependence (curve marked • in Fig. 4). This explains that both particle size and concentration in Fig. 5 decline with increasing temperature above 300C, a temperature range in which the overall quantum yield is presumably dominated by the homogeneous mechanism. Figures 2 and 3 show that the increased quantum yield does not lead to an increased rate of nucleation, i.e. an increased particle number concentration, and therefore lend credence to the hypothesis of a surface or volume reaction_ For the series with low humidity the homogeneous mechanism is dominant in the whole temperature range and therefore the particle concentration increases slightly with increasing quantum yield throughout this range.
Influence of humidity The existence of two kinetic regions is reflected in the variation of the quantum yield with humidity depicted in Fig. 7. At low humidity, where we
.~0 0'30.01 Particle
O.l diameter
(~m)
Fig. 6. The logarithmic size distribution at two different temperatures. Conditions as for (0) data in Fig. 4" 100ppm SO,_, 50ppm NHj, 1% H20, 10% 02, wavelength 284 nm, light power 18 mW, residence time 12.2 s.
"O ,...-i
0.) >,,
..4--a
0.1 5°
(3'
0.01
0.
. . . . . . .
i
HzO concentration (~)
Fig. 7. The influence of the HzO concentration on the quantum yield at different temperatures. 10% O2, wavelength 284 rim, residence time 12.2 s. Gas composition, temperature and light power for individual series: (11) 100ppm SO> 100ppm NH> 15"C, 6.5 mW. Quantum yield of 1 corresponds to 0.62% conversion of SO,,. Dew point at 0.49% H20. (A) 200ppm SO> 200ppm NH3, 45"C, 17 mW. Quantum yield of 1 corresponds to 2.9% conversion of SO> Dew point at 7.5% H20.
believe the reaction is dominated by the homogeneous mechanism, the observed influence of humidity on the quantum yield is small. When the humidity is increased a shift in the mechanism to a much more water sensitive kinetics obviously occurs. The shift occurs at a humidity somewhat less than, but apparently related to the dew point humidity at the given temperature. The watersensitive range corresponds to the transition temperature ranges in Fig. 4 and thus can be explained by an increasing influence of the heterogeneous mechanism when the humidity increases to levels where the reactions start to take place in a condensed phase.
PETER SEIER CHRISTENSEN et al.
4612
low for the feed composition which only sustains the homogeneous mechanism because it is without aerosol seeds. As aerosol nuclei are formed by ,-==1 homogeneous nucleation the heterogeneous ~0.15 mechanism takes over and the quantum yield ~, - gradually increases as the aerosol surface/volume increases. ~0.10 The corresponding, predicted interaction between seed nucleation and seed growth kinetics is confirmed by the measured aerosol characteristics depicted in Fig. 9. The number concentration of 0"0.05 particles, assumed to be formed initially at the entrance of the reactor, is practically constant, while the increased conversion at high residence 0.00 times manifests itself mainly in a growth of the 0 5 lb 1'5 20 R e s i d e n c e t i m e (s) initially formed particles_ The particle number concentration is, however, affected by conditions Fig. 8_ The influence of the residence time on the quanwhich influence the homogeneous rate. Increasing tum yield at different temperatures and intensities. 10% the light power, for example, increases the rate of 02, wavelength 284 nm. Gas composition, temperature and light power for individual series: (11) 100 ppm SO2, homogeneous nucleation which yields a larger 100ppm NH3, 0.25% H20, 15"C, 6.5 roW. ( l ) 2 5 0 p p m effluent particle concentration (Christensen, 1994). SO2, 250 ppm NH3, 2% H20, 45°C, 6 roW. (O) 250 ppm SO2, 250 ppm NHs, 2% H20 45°C, 25 mW. if-l) 250 ppm DISCUSSION SO2, 250 ppm NH3, 2% H20, 45"C, 30 mW, Residence The observed acceleration of the photolytic oxtime is varied by changing the reactor volume. idation of SO2 in the presence of NHs cannot be explained by any known mechanism of gas reactions. A somewhat remote possibility is that the Residence time excited singlet molecule 502(IA2, IB1) is stabilized Normally, the drop in the gas phase concentraby the formation of complexes (exciplexes) with tions of the reactants and the extinction of light by NH3. Formation of exciplexes with various gas passage through the reactor are both small and the molecules has been observed to increase the oxidareactor in that respect behaves differentially. tion rate of SO2(IA2, 'Bl) (Richards et al., 1976) Under these conditions one would immediately but NH3 has not previously been mentioned in this anticipate the quantum yield to be independent of context. the residence time of the gas in the reactor, The photolytic oxidation of S(IV) in the aerosol provided all other imposed conditions such as the phase probably occurs via absorption of reactive gas feed composition and the irradiation are kept oxidants from the gas phase and not by photolytic constant. Actually the measurements deviate conexcitation in the aerosol phase. Neither HSO~ nor siderably from this anticipation. In several runs the SO~- absorbs radiation in the wavelength range residence time was varied, either by changing the feed flow rate with constant reactor volume or by changing the reactor volume with constant flow rate. The latter procedure was adapted by using a 10-3 10 modified reactor design in which the rear end of the g--. I reactor can be moved axially like a piston to change E :& the reactor volume. In all these runs, shown in Fig. 0 v 8, the quantum yield increases with increasing residence time in a manner characteristic for reaco tions with auto-catalytic kinetics. Accordingly, the '~10 6 lo-42 0 reactant conversion does not increase linearly with residence time but the slope of conversion versus residence time increases with increasing residence time. If the measurement in Fig.-8 were extended o to larger values of the residence time one would 10 5 expect the quantum yield to go through a maximum 2010-5 6 tb f5 and decrease when the reactant conversion beR e s i d e n c e time (s) comes large as usual for autocatalytic reactions. Fig. 9. The influence of the residence time on the particle This range, however, exceeds the measuring range number concentration and the particle mean volume. of the DMPS-system. Conditions as for (I-I) data in Fig. 8" 250ppm SO2, The autocatalytic behavior agrees well with the 250ppm NH3, 2% H20, 10% 02, 45°C, wavelength proposed homogeneous-heterogeneous, parallel 284 nm, light power 30 mW. Residence time is varied by mechanisms which predict that the quantum yield is changing the reactor volume. 0.20
Photolytic production of aerosols from SO2 and NH 3 used (Deister et al., 1986). Both H202 and 03 are powerful condensed-phase S(IV) oxidants (Schwartz, 1984)_ However, their computed production rates in the gas phase are very low and cannot explain the very high quantum yields unless they initiate chain reactions in the liquid--a theory which cannot be substantiated. A more likely key species is the SO~-radical which initiates a very fast SO 2- oxidation chain mechanism (Deister and Warneck, 1990). The SO~-radical can be formed by UV-excitation of SO 2- at wavelengths below 260 nm (Deister and Warneck, 1990), but it cannot be formed at the higher wavelength of the sunlight and in the present investigation (Christensen, 1994). However, a probable theory is that the SO3-radical is formed by absorption and dissociation of H O S O 2 [formed by eq. (16)] by the following reaction:
HOSO2.aq--~ SO3,aq + H +
4613
• Step 3: Aerosol condensation or nucleation: (NH4)2SO4(g)--~ (NH4)2SO4(s) All evidence shows that the steps prior to condensation produce vapor components which readily nucleate homogeneously. Hence, these components must be produced into a state with a high degree of supersaturation. However, the precise identity of the nucleating component is not certain. It is still possible that either H2SO 4 or NH3SO3 nucleate homogeneously, in which case (NH4)2SO4 is formed subsequently by reaction with NH3 and H20. The proposed steps 2 and 3 are judged to be representative of the chemical and physical processes which must occur in any ammonium sulfate aerosol formation. • Step 4: Absorption of reaction components by the aerosol:
(SO 2 + H20 + 2NH3)(g) --~ (SO2 + H20 + 2NH3)(s,I)
CONCLUSION When humid air with small amounts of SO2 and NH3 is irradiated by UV sunlight an ammonium sulfate aerosol is rapidly formed. This reaction can significantly influence the opacity of stack-plumes from various industrial processes_ The kinetics of this process, which has been studied in the present investigation, shows some distinct features. The photolytic oxidation of SO2, which is one step in the overall pattern, is strongly catalyzed by NH3 and the rate increases significantly with decreasing temperature in the range 15-45°C_ The overall process is very complicated and many important details are still unknown. However, the kinetic observations strongly indicate that two different, parallel mechanisms are involved. A gas phase homogeneous mechanism and a heterogeneous mechanism involving reactions in or at the surface of the aerosol phase. The overall process can be visualized to occur by the following steps: • Step 1: Photolytic excitation in the gas phase: SO2(IA 0 + h~,(240 nm < ;t < 330 nm)
SO2(IA2, IBt)---~ SO2(3B1) The triplet molecule SO2(3Bj) is the usual initiator of many atmospheric reactions involving SO2. All photo-kinetic observations in our study agree with this step. • Step 2: Homogeneous ammoxidation of SO2:
502(3BI) + 102 -I- H20 + 2NH3--~ (NH4)2SO4(g) This step occurs by an intricate mechanism, the details of which are still obscure. The results exclude the possibility that the oxidation of SO2 to S(VI)-species strictly precedes the addition of ammonia, but the way in which ammonia significantly enhances the overall rate of steps 1 and 2 over that expected from the usual kinetics of photolytic SO2-oxidation is unclear.
In the vicinity of the dew point H20, 502, and NH3 condenses onto the aerosol formed by step 3. The species are either physisorbed on the surface of an aerosol particle or dissolved in a liquid aerosol droplet of an aqueous ammonium sulfate solution. • Step 5: Heterogeneous photolytic ammoxidation of 502: (SO3 + H20 + 2NH3)(s3) + 102(g) + hv--* (NH4)2SO4(s,I)
This step is similar in the stoichiometry to step 2, but it occurs on or within the aerosol particles and therefore does not lead to the nucleation of new particles but rather to the growth of existing particles. The step requires a photolytic excitation by UV-irradiation, probably the same excitation of SO2 as that of step 1. The rate of this step is very fast compared to step 2. Details of the underlying mechanism are open for discussion, but it is believed that a heterogeneous step akin to step 5 is essential for the explanation of the high quantum yields observed. Due to the interaction between steps 3, 4 and 5, the overall reaction behaves distinctly autocatalytic. As a consequence hereof, the laboratory reactor in certain ranges of the operating conditions yields sustained oscillations of the aerosol production rate. The quantitative background for the reactor behavior and the exact nature of the kinetic mechanisms are presently under further investigation_ REFERENCES
Atkinson, R., Baulch, D. L., Hampson, R. F. Jr, Kerr, J. A..and Troe, J., 1992, Evaluated kinetic and photochemical data for atmospheric chemistry. Supplement IV. IUPAC subcommittee on gas kinetic data evaluation for atmospheric chemistry. J. Phys. Chem. Ref. Data 21, 1125-1568.
4614
PETER SEIER CHRISTENSEN e t al.
Bai, H., Biswas. P. and Keener, T. C., 1992, Particle formation by NH3-SO2 reactions at trace water conditions. Ind. Engng Chem. Res. 31, 88--94. Baulch, D. L., Cox, R. A., Hampson, R. F. Jr, Kerr, J. A., Troe, J. and Watson, R. T., 1980, Evaluated kinetic and photochemical data for atmospheric chemistry. J. Phys. Chem. Ref. Data 9(2), 295--471. Calvert, J. G., Su, F., Bottenheim, J_ W. and Strausz, O. P., 1978, Mechanism of the homogeneous oxidation of sulfur dioxide in the troposphere. Atmos. Environ_ 12, 197-226. Christensen, P. S., 1994, The formation of aerosol from SO2 and NH 3 in humid air, Ph.D. thesis, Technical University of Denmark, Chemical Engineering Department. Christensen, P. S., Madsen, N. M. and Livbjerg, H., 1992, The formation of aerosols from SO2 and NH3 in humid air. J. Aerosol Sci. 23(S1), $261-265. Deillinger, B., Grotecioss, G., Fortune, C. R_, Cheney, J_ L. and Homolya, J. B., 1980, Sulfur dioxide oxidation and plume formation at cement kilns. Environ. Sci. Technol. 14(10), 1244--1249. Deister, U. and Warneck, P., 1990, Photooxidation of SO~ in aqueous solution, J. Phys. Chem. 94, 21912198. Deister, U., Neeb, R., Helas, G. and Warneck, P., 1986, Temperature dependence of the equilibrium CH2(OH)2 + HSO~ = CH2(OH)SO3 + H20 in aqueous solution. J. Phys. Chem. 90, 3213-3217. Hartley, E. M_ and Matteson, M. J., 1975, Sulfur dioxide reactions with ammonia in humid air. Ind. Engng Chem. Fundam. 14(1), 67-72.
HOvel, Von B., 1990, Kombination einer Waltherentschwefelung mit einer SCR-entstickung als nachriistung hinter einer kohleschmelzfeuerung. VBG Kraft. werkstechnik 70(7), 602--607. Johnstone, H. F., 1952, Recovery of sulfur dioxide from waste gases. Ind. Engng Chem. 27(5), 587-593. Mallard, W. G., Westley, F., Herron, J. T., Hampson, R. F. and Frizzell, D_ H., 1993, NIST Chemical Kinetics Database--Ver 5.0, NIST Standard Reference Data, Gaithersburg, MD. Moore, N. D., 1977, Fume formation in ammonia scrubbers. A S M E Ann. Meeting Atlanta, pp. 1-11_ Richards, J. R., Fox, D. L. and Reist, P. C., 1976, The influence of molecular complexes on the photooxidation of sulfur dioxide. Atmos. Environ. 10,211-217. Rothenberg, S. J., Dahl, A. R., Barr, E. B. and Wolff, R. K., 1986, Generation, behavior, and toxicity of ammonium sulfite aerosols. J. APCA 36, 55-59. Schwartz, S. E., 1984, Gas-aqueous reactions of sulfur and nitrogen oxides in liquid-water clouds, in S02, NO and NO,_ Oxidation Mechanisms: Atmospheric Considerations (Edited by J. G. Calvert). Butterworth Publishers. Sidebottom, H. W., Badcock, C. C_, Jackson, G. E_, Calvert, J. G., Reinhardt, G. W. and Damon, E. K., 1972, Photooxidation of sulfur dioxide. Environ. Sci. Technol. 6(1), 72-79. Vance, J. L. and Peters, L. K., 1976, Aerosol formation resulting from the reaction of ammonia and sulfur dioxide. Ind. Engng Chem. Fundam. 15(3), 202-206.