J. Phys. Chem. 1995, 99, 2080-2087
2080
Measurement of the Kinetics of Reactive Uptake by Submicron Sulfuric Acid Particles Edward R. Lovejoy" and David R. Hanson National Oceanic and Atmospheric Administration, Aeronomy Laboratory, Boulder, Colorado 80303, and the Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado Received: July 28, 1994; In Final Form: October 4, 1994@
An experimental apparatus for measuring the kinetics of the reactive uptake of gas phase species by sulfuric acid aerosol is described. The reactive uptake is studied by monitoring the concentration of the gas phase species at the exit of a laminar flow reactor as a function of the position at which the gas is added to the aerosol flow. The gas phase species concentration is measured by chemical ionization mass spectrometry. Sulfuric acid aerosol is generated by the gas phase reaction between SO3 and H20. The aerosol have lognormal radius distributions with peak radii in the range 0.05-0.15 p m with a radial dispersion of about f30%. The particle number density (5 x 104-1 x lo6 particles cm-3 in the reactor) is measured in a separate apparatus by monitoring the extinction of visible light by the growing particles following a rapid expansion at high relative humidity. The size distribution of the aerosol in the reactor is extracted from measurements of the UV transmission between 200 and 400 nm by using Mie theory. Kinetic results for the reaction of N2O5 with 70 wt % sulfuric acid aerosol at room temperature are presented.
Introduction Gas-particle interactions play critical roles in the development of the ozone hole over the Antarctic' and may be responsible for the recent global decline of stratospheric ~ z o n e . ~Laboratory -~ studies of stratospheric heterogeneous chemistry have focused mostly on reactions on bulk liquid and solid substrates.6-l3 Notable exceptions are studies by Mozurkewich et al.,14 Mozurkewich and Calvert,15 and Fried et a1.I6 which have examined the reactions of N2O5 and HO2 with various submicron aerosol substrates found in the atmosphere. In this work, an experimental apparatus designed for the measurement of the kinetics of the reactive uptake of gas phase species by sulfuric acid aerosol is described. Results for the kinetics of the reactive uptake of N2O5 by submicron sulfuric acid aerosol are presented.
Experimental Section The experimental apparatus for the measurement of the kinetics of the reactive uptake of gas phase species by sulfuric acid aerosol is shown in Figure la-c. The apparatus consists of an S03/H20 reactor for aerosol production, an expansion aerosol counter, a UV transmission cellllaminar flow reactor, and a chemical ionization mass spectrometer (CIMS). Sulfuric Acid Aerosol Production. Sulfuric acid aerosol is generated by the gas phase reaction of SO3 and H20. The mechanism of particle formation in the presence of SO3 and H20 is poorly understood. It is believed that the reaction of so3 and H20 produces H2S04 in the gas phase, HzS04 and H20 nucleate new particle formation, and the particles grow by further accommodation of H2SO4, SO3,and Other nucleation mechanisms involving the formation of SO3/H2O clusters may also be important but are not well characterized. In the present experiments, the aerosol contains negligible quantities of SO3 due to the high partial pressures of H20 in the gas phase. The H2SOfi20 aerosol composition is determined by the relative humidity and temperature.22 @
Abstract published in Advance ACS Abstracrs, January 15, 1995.
The aerosol generator is shown in Figure la. SO3is admitted to a Pyrex reactor (20 cm long x 4 cm i.d.) by passing N2 over solid SO3 held in a temperature-regulated trap (T = 220-230 K, AT < 0.5 K). The SO3/N2 gas mixture enters the reactor through a 6 mm 0.d. Pyrex injector on the axis of the reactor and contacts a sheath flow of N2 and H20. The H20 stream is produced by bubbling N2 through liquid H20 at room temperature. The reactor is operated at atmospheric pressure (620 f 10 Torr) and room temperature. Measured flows of N2 are added to both the SO3 and H20 streams to control the concentrations and the residence time in the reactor. A column of particles forms at the exit of the SO3 injector and remains intact until the exit of the reactor, spreading radially as the aerosol moves away from the tip of the injector. The total flow through the reactor is 1-10 STP cm3 s-l (STP = 273 K and 1 atm), giving average residence times of about 20-200 s. The longer residence times enhance particle coagulation and yield larger aerosol. The reactor effluent enters a 25 cm3 round bottom flask where the gas mixture is stirred with a Teflon-coated magnetic bar to distribute the aerosol homogeneously in the carrier gas. Additional gas is added to the mixture to vary the concentration of the aerosol and the humidity (i.e., the aerosol composition). The well-mixed aerosol stream is conditioned by flowing it through a horizontal jacketed Pyrex cylinder (3 cm i.d. x 50 cm long) filled to a depth of about 1.5 cm with a sulfuric acid/ water solution (Figure IC). The cooling jacket of the conditioner is connected in series to the exit of the cooling jacket of the laminar flow reactor. The temperature difference between the conditioner and the reactor is less than 1 K for temperatures ranging from 230 to 300 K. Model calculations indicate that the aerosol leaving the conditioner has an acid composition within 1 wt % of the liquid solution in the conditioner for typical operating conditions: 60-80 wt % liquid, 230-300 K, 20 s residence time, volume fraction aerosol 1 5 x (particle volume per unit gas volume). The difference in the composition of the aerosol ( r > 0.03 um) and the bulk solution due to the Kelvin effect is smaller than the uncertainty in the composition of the bulk solution (0.2 wt %). The acid solution in the conditioner is analyzed by using standard titration techniques.
0022-365419512099-2080$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 7,1995 2081
Reactive Uptake by Submicron Sulfuric Acid Particles N2
+N2
FILTER
A
so3
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i TO CONDITIONER AEROSOL IN 675 nm DIODE LASER
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ment of particle number density with this technique have been described by Szymanksi and Wagner.23 The counter apparatus is shown in Figure lb. A small fraction (0.01-0.10) of the total aerosol stream is sampled from the W transmission cell/ laminar flow reactor and mixed with a humidified particle-free recycle stream to dilute the particles by 10-lo00 times. The dilution is performed close ( < l o cm) to the aerosol sampling point to minimize particle losses. The humidified particle-free stream is generated by recycling (Teflon and stainless steel diaphragm pump) gas through a filter (Teflon, 0.2 pm), mass flow meter, and a humidifier. The flow of aerosol into the counter is controlled by a needle valve and measured with a mass flow meter, which are both located at the exit of the apparatus, upstream from a vacuum pump. Aliquots of humidified diluted aerosol are isolated in the expansion cell, and the gas is expanded rapidly by opening a solenoid actuated valve connecting the cell to a small evacuated volume (about 15% of the volume of the expansion cell). The expansion cell is constructed of Pyrex (2.5 cm i.d. x 10 cm long) and equipped with quartz windows mounted at Brewster's angle. The rapid expansion cools the gas mixture and creates an H20 supersaturation which activates the small aerosol to grow by condensation of water. The growing aerosol is illuminated with a 3 mW, 675 nm diode laser, and the transmitted light is detected with a silicon photodiode. The amplified photodiode output is processed with a transient digitizer triggered simultaneously with the expansion valve. Measurements of the cell pressure vs time indicate that the expansion is 90% complete in about 5 ms. The light transmitted through the aerosol is described by Beer's law:
I = I, exp{ -o(r,A,n)Nl}
*
VACUUM PUMP
(1)
where I is the transmitted light intensity, 10 the incident light intensity, o(rJ,n) the extinction cross section as a function of particle radius (r), wavelength (A), and particle refractive index (n),N the particle number density, and 1 the path length. The extinction cross section (a(rjl,n))includes contributions from absorption and scattering, and for homogeneous spherical particles it is given by Mie theory.24 An experimental trace of the transmittance of the aerosol as a function of time following the rapid expansion is shown in Figure 2. The data exhibit the well-defined step structure due to interference between the phase shifted light which has passed directly through the particle and the radiation which has not interacted with the aerosol. An experimental trace of the first interference step with higher aerosol concentration is shown in Figure 3. These data display the fine ripple structure which is superimposed on the steps. The fine structure arises from higher order resonances within the spherical particle.24 The particle number density in the expansion cell is determined by measuring the extinction at the first and second interference steps and using Beer's law with extinction cross sections of 7.7 x (first step: 0.80 r < 1.2 pm) and 2.9 x lo-' cm2 particle-' (second step: 1.8 < r < 2.3 pm) for water particles and 675 nm radiation (n = 1.33 Oi). The influence of the sulfuric acid on the refractive index is negligible, since the mole fraction of H2SO4 in the growing particle is less than 1 x at the first interference step (initial sulfuric acid aerosol radius c0.2 pm and growth by condensation of water). The number density of aerosol in the W transmission cell/ laminar flow reactor is related to the expansion cell concentra-
""-***--
REACTANT
>.
-&
/ II tI
I
/
SOLUTION
1 I
+
Figure 1. (a, top) Aerosol generator. (b, middle) Expansion particle counter. (c, bottom) UV transmission celVlaminar flow reactor.
Measurement of the Particle Number Density. The particle number density in the laminar flow reactor is measured with an extinctiodexpansioncounter. The principles of the measure-
2082 J. Phys. Chem., Vol. 99, No. 7, 1995
Lovejoy and Hanson
n
0 -
'IW
1
0.98
0.00
,
0.02
I
I
0.04
0.06
0.08
0.10
time (s) Figure 2. Aerosol transmittance as a function of time. The lower
trace is experimental data from one expansion. The upper trace is calculated from Mie theory by assuming single-exponential growth, r ( f ) = r, (1 - exp(-br)), of nearly monodisperse aerosol (log s = 0.004). I
I
I
growth so that the first extinction step is not observed and the number density is not defined accurately. With the present apparatus, the minimum detection limit (signdnoise at the second step = 2 for one expansion) is estimated to be about IO0 particles ~ m - ~ . Measurement of the Particle Size Distribution. Mie theory describes the interaction of electromagnetic radiation with homogeneous spherical particles as a function of the wavelength of incident light and the size, number density, and composition (refractive index) of the aerosol.24 The inversion of transmission data to determine a particle size distribution has been discussed in the In the present work, the size distribution of sulfuric acid aerosol is determined by using Mie theory coupled with measurements of the total particle number density and the UV transmission of the aerosol between 200 and 400 nm. The UV aerosol transmission apparatus is shown in Figure IC. The aerosol stream exiting the conditioner enters a jacketed Pyrex transmission cell/flow reactor (50 or 95 cm long x 3.1 cm i.d.) fitted with quartz windows. The temperature of the cell is regulated by flowing thermostated fluid through a jacket surrounding the cell. The collimated output of a D2 lamp traverses the length of the cell and is focused onto the entrance slit of a 0.25 m crossed Czerny-Turner monochromator (spectral resolution = l nm at 300 nm) equipped with a photomultiplier detector. Transmission spectra are measured by scanning the monochromator grating with a stepper motor (typically 5 nm steps) and monitoring the light transmission vs wavelength with and without particles. Data are collected with an A/D interface and a personal computer. The wavelength scale is calibrated to f0.5 nm by measuring spectra from low-pressure Hg and Zn lamps. The particle size distribution is extracted from the UV transmission spectra by using Beer's law (eq 1) and aerosol extinction cross sections o(r,il,n) calculated with the Bohren and Huffman Mie scattering algorithm.24 The UV spectra are fit by minimizing chi-squared by the Levenberg-Marquardt method.29 The radial dispersion is modeled with a log-normal distribution where the number of particles (dN)in the interval d log r is given as a function of the radius by30
cw= 0.000
0.005
0.010
0.01 5
0.020
time ( s ) Figure 3. Aerosol transmittance as a function of time. The lower trace is the average of data from five expansions. The upper trace is calculated as described in the Figure 2 caption.
tion by
where Nexpis the particle number density in the expansion cell (from Beer's law), Rexp the expansion ratio (volume after expansion divided by the volume before expansion), Texpthe temperature in the expansion cell, Tu, the temperature in the UV transmission cell, Fret the flow rate of the recycle stream in the expansion apparatus, and Finthe flow rate into (and out of) the expansion apparatus. The maximum aerosol concentration which can be measured with this method is about 3 x lo5 particles cm-3 in the expansion cell (or greater than IO8 particles cm-3 before dilution). At higher number densities the latent heat release and depletion of gas phase water inhibit the aerosol
N
& log s
dlog r
(3)
where N is the total number density of particles, rpk the radius at the peak of the distribution, and s the geometric standard deviation. The calculation of the size and radial dispersion of the aerosol from the UV spectrum requires knowledge of the refractive index of the aerosol. The refractive index is a function of the composition of the aerosol, the wavelength, and the temperature. The Mie fitting algorithm utilizes a parametrization of the wavelength and composition dependence of the refractive index, based on measured indices for a range of sulfuric acid water solutions at wavelengths between about 210 and 370 nm at room t e m ~ e r a t u r e . ~The ~ parametrization reproduces the experimental refractive indices to better than 0.5%. The temperature dependence of the refractive index is calculated with the empirical r e l a t i ~ n s h i p ~ ~ (4)
where n(il,T)is the real part of the refractive index at wavelength il and temperature T, e is the density, and TOis the reference temperature. The imaginary part of the refractive index of the
Reactive Uptake by Submicron Sulfuric Acid Particles 1 .oo
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0.88 200
250
300
350
wavelength ( n m ) Figure 4. W transmission spectra of several 70 wt % sulfuric acid aerosol samples (path length = 83.4 cm). The solid lines are leastsquares regression fits to the data giving the following parameters: (open circle) f-pk = 0.044 pm, log s = 0.054, N = 3.1 x lo6 particles cm-3 (measured);(triangle) f-pk = 0.101 pm, log s = 0.094, N = 5.4 x lo5 particles cm-3 (measured); (filled circle) f-pk = 0.168 pm, log s = 0.078, N = 3.4 x lo5 particles cm-3 (measured).
sulfuric acid-water system is negligible for the conditions of this The temperature dependence of the density is calculated by using a parametrization of experimental data.22%31 The density parametrization reproduces the experimental densities to better than 1%. Experimental UV spectra of three sulfuric acid aerosol samples are shown in Figure 4. The solid lines are least-squares regression fits to the data using the procedure described above. The fits are performed with the number density fixed at the value measured with the expansion counter. Laminar Flow Reactor. The kinetics of the reaction of a gas phase reactant with sulfuric acid aerosol are measured by monitoring the reactant concentration as a function of the contact distance with well-characterized aerosol in a laminar flow reactor. Two reactors are used in this work. Both reactors are constructed of Pyrex, jacketed for temperature regulation, and have i.d.'s of 3.1 cm. The first reactor is 50 cm long and has a movable Pyrex inlet which travels down the center of the reactor. The injector is guided with a thin stainless steel cage positioned near the middle of the reactor. Quartz windows are sealed to the ends of the reactor with Viton O-rings. The second reactor is 95 cm long and has a movable inlet traveling down the side of the reactor inside of a 6 mm 0.d. Pyrex tube fixed to the reactor wall by two thin stainless steel bands (Figure IC). The injector has a short offset section at the exit which is rotated into the center of the reactor during kinetic measurements and moved against the wall for the UV transmission measurements. A small Teflon cap with six radial holes is attached to the end of the injector to enhance the mixing of the reactant with the aerosol. The movable inlet is connected to a piece of l/g in. 0.d. Teflon tubing which exits the reactor through a tube aligned with the reactor wall. The reactors are positioned vertically, with gas flow from top to bottom to reduce the possible disturbance of the flow by convection. Most (about 90%) of the gas from the flow reactor enters the CIMS through a small Teflon capillary (0.4 mm i.d. x 2 cm long) located in a port at the bottom of the reactor. The capillary creates a pressure drop between the reactor (p RZ 620
J. Phys. Chem., Vol. 99, No. 7, 1995 2083 Torr) and the flowing afterglow section of the CIMS ( % O S Torr). The aerosol passing through the capillary impacts on the wall opposite to the capillary. A small fraction of the reactor flow (%O.l) is vented to the atmosphere via a port at the bottom of the reactor, across from the mass spectrometer capillary, to maintain a constant pressure in the reactor. Aerosol is sampled from the vent flow to measure the particle number density. The total flow in the reactors ranges from 5 to 10 STP cm3 s-l (230 < T < 300 K, p = 620 Torr of N2), giving average flow velocities of about 1 cm s-l and a Reynolds number near 20. For these conditions, the flow is well developed and laminar after about 5 cm.33 The time for diffusional mixing of the N205 into the aerosol stream is estimated to be about 5 s, or 5 cm of travel down the reactor.33 Data taken within the mixing region are not used in the kinetic analysis. N2O5 is synthesized using the procedure outlined by Davidson et al.34 N205 is admitted to the laminar flow reactor by passing N2 over solid N205 held in a temperature-regulated (-78 to -50 "C, AT = 0.5 "C)) Pyrex trap. The concentration of N2O5 in the reactor ranges from 3 x 10l2 to 20 x 10l2 molecules ~ m - ~Nitric , acid is an impurity in the N2O5 sample and is also generated by N205 hydrolysis in the lines leading to the reactor. As a result, the nitric acid concentration in the reactor is several times higher than N2O5. High-purity N2 (99.9995%), which is used for all the flows, is passed through a molecular sieve trap at -78 "C. Gases and aerosol are transported via Teflon PFA tubing connected with Teflon PFA fittings. Gas flow rates are measured with mass flow meters which are calibrated with a wet-test meter or by the rate of change of pressure in a known volume. Pressures are measured with capacitance manometers and a mercury manometer (ambient atmospheric pressure). The cross-sectional area of the laminar flow reactor was determined by measuring the mass (volume) of water required to fill the reactor to a specific height. Kinetic Data Analysis. In a reactor with laminar flow and reaction faster than diffusion, radial gradients develop in the reactant concentration due to enhanced loss near the walls of the reactor where the flow is slower and species are lost on the wall. Therefore, a larger fraction of the reactant travels down the center of the reactor, and the average reaction time is less than the reaction distance divided by the average flow velocity. In this work, the formalism presented by Brown3s is used to extract the kinetic parameters from measurements of reactant concentration vs distance in the laminar flow reactor. This approach is tested by measuring the kinetics with pulsed techniques (vide infra). The procedure used in this work to determine the reaction probability from measurements in a laminar flow reactor is similar to that described by Fried et al.,16 but modified to account for a non-monodisperse aerosol reactant. The first-order rate coefficient for the loss of a gas phase species to monodisperse particles is given by36
k*(r)=
+
yNcnr2 y[o.750 0 . 2 8 3 4 3
+
K,,K
(5)
+ 1)
where c is the mean molecular speed of the reactant gas, r the particle radius, K,, the Knudsen number, y the reaction probability, and N the particle number density. The Knudsen number (K,,) is the mean free path of the gas phase reactant divided by the particle radius
K,, = 3Dlcr
(6)
2084 J. Phys. Chem., Vol. 99, No. 7, 1995
Lovejoy and Hanson
The diffusion constant for N205 in NZ is given by37,38
D ~ , (cm2 ~ , s - ~= ) 63 92 (T (K))'.'' P(Torr) 293
(7)
When the Knudsen number is much larger than the reaction probability, diffusion does not limit the loss, and the first-order loss rate constant (eq 5 ) reduces to the gas kinetic rate constant, kl(r) = ycNx?. Conversely, if the Knudsen number is much smaller than the reaction probability, the loss is diffusion controlled, and the first-order loss rate coefficient becomes k l ( r ) = 4xrND. For the conditions of this study ( K , rc lOy), diffusion reduces the first-order loss rate constant by less than 10% below the gas kinetic value. In the present work, the aerosol is not monodisperse, and the first-order rate coefficient for loss is an average over the distribution of radii
(k'> = f
@wmdr
(8)
kl(r) is given by eq 5 , and fir) is the normalized radius distribution function. For a log-normal distribution of radii, the average first-order loss rate coefficient is yNczr2 0.750+0.283Kn
K,(K"+ 1)
X
]}&log
s
enp{ - [log(rpJr)l
'}
d log r (9)
2(log s), The reaction probability ( y ) is determined by solving (k') = kIexpnumerically, where kIeXpis the experimental first-order loss rate coefficient from the Brown analysis.35 Chemical Ionization Mass Spectrometer. The chemical ionization mass spectrometer used in this work has been described previously.ll Ions are produced at the upstream end of a flowing afterglow (FA) reactor by passing neutral precursors over an electron source. The ions are entrained in a large flow of He (about 100 STP cm3 s-l, p 0.3 Torr) traveling along a stainless steel FA reactor (7.5 cm i.d. x 1.2 m long). The bulk of the FA flow is pumped away by a Roots pump. A small portion of the flow is sampled through a pinhole into a differentially pumped quadrupole mass spectrometer where the ions are mass filtered and counted. The effluent of the laminar flow aerosol reactor enters the FA reactor about 65 cm upstream from the mass spectrometer sampling point. Neutrals added to the FA reactor react with the ions and produce new ions characteristic of the neutrals. The product ion signal is proportional to the neutral precursor concentration at the exit of the laminar flow aerosol reactor. N205 is detected by monitoring the NO3- produced in the following ion-molecule reactions
+ SF,- - NO3- + products N,O, + I- - NO3- + products
N,O,
(10) (11)
SF6- and I- are produced by adding SF6 and CF31, respectively, to the FA upstream from the source of electrons. Nitric acid is detected with SF6-
HNO,
+ SF,-
-
-
HF.N03NO3-
+ SF,
+ products
(12) (13)
The CIMS N2O5 detection limit (S/N = 2, 1 s integration time) is about 2 x lo6 molecules cm-3 in the FA reactor or about 2 x 1Olo molecules cm-, in the aerosol reactor.
Discussion of Errors Particle Number Density Measurement. Particle loss in the inlet to the counter apparatus was evaluated by establishing a constant number density of aerosol in the UV transmission cell and measuring the expansion cell number density as a function of the flow rate of the aerosol in the inlet. The expansion cell number density increased linearly with flow rate, indicating that loss in the inlet was negligible (
