Neutralization of Sulfuric Acid Aerosol by Ammonia James J. Huntzicker', Robert A. Cary, and Chaur-Sun Ling Department of Environmental Science, Oregon Graduate Center, 19600 N.W. Walker Road, Beaverton, Oreg. 97006
The rate of neutralization of sulfuric acid aerosol by ammonia gas has been measured in a laboratory flow reactor for particle diameters between 0.3 and 1.4 pm and for relative humidities between 8 and 80%.The rates were between 21 and 70% of the rates expected if diffusion of NH3 to the aerosol droplets was the rate-determining factor, and the reaction coefficient, which is the fraction of gas-particle collisions resulting in a chemical reaction, increased with increasing particle size. A simple model of H2SO4 aerosol generation and neutralization in the atmosphere showed that sulfate aerosol more acidic than NH4HS04 should only be present during periods of rapid oxidation of the precursor SO2 and high ratios of [SO21 to [NH3] or when the equilibrium vapor pressure of NH3 over the partially neutralized H2S04 droplet exceeds the ambient NH3 partial pressure. The oxidation of SO2 either by homogeneous gas-phase reactions or by heterogeneous gas-particle reactions results in the production of sulfate aerosols. Although the end product of those processes is (NH4)2S04, sulfate aerosols as acidic as H2SO4 can exist in the atmosphere, and the existence of such aerosols has been reported by many investigators (1-10). Such acidic aerosols could result either because of insufficient NH3 to neutralize the sulfuric acid or because of a rate-limiting step in the neutralization. (Unless otherwise indicated, the term sulfuric acid aerosol will denote a sulfate aerosol more acidic than NH4HS04, i.e., an aerosol specified empirically as (NH4),HYS04 where 0 < x < 1and 1 < y 6 2.) The goal of this reaearch was to measure the rate of neutralization of H2S04 aerosol by NH3 under simulated atmospheric conditions in a laboratory reactor. The NH3-H2SO4 reaction was first investigated by Robbins and Cadle (11, 12) under the following conditions: [NHs] = 2-10 ppm, [H2S04] = 1930 pg/m3, particle diameter (d,) = 0.2-0.9 pm, and relative humidities of 0 and 93%. In their experiment neutralization rates were measurable only when the relative humidity was essentially zero. In this case it was found that in the initial stage of the neutralization about 10%of the NH3 molecules colliding with the surface of the sulfuric acid droplet reacted, and over the whole course of the neutralization the data suggested that the rate was controlled by diffusion of a reaction product in the droplet. At the high relative humidity the rate was too fast to measure, and it was speculated that every collision of an NH3 molecule with a H2SO4 droplet produced (a reaction and that the rate of neutralization was limited by gas-phase diffusion of NH3 to the droplet. In the years since the pioneering work of Robbins and Cadle, considerable advances in measurement technology have occurred. Specifically, the advent of the highly sensitive flame photometric detector (13) and the adaptation of the flame photometric detector to the real time measurement of sulfur-containing aerosols (14-18) have made possible the measurement of the neutralization kinetics for conditions more closely approximating actual atmospheric conditions. Moreover, because of the role of sulfate aerosol in visibility reduction and the importance of sulfate as a component of the respirable aerosol, extensive field measurements of sulfate aerosol have been undertaken (see, e.g., 1 9 , 2 0 ) .Because an understanding of the atmospheric chemistry of sulfate aerosol requires a knowledge of the kinetics of neutralization of sulfuric acid aerosol, it was decided to reinvestigate this reaction. 0013-936X/80/0914-0819$01 .OO/O
While this study was in progress, two independent experiments relevant to the NH3-HzS04 reaction were reported. Using the tandem differential mobility analyzer (also called the aerosol mobility chromatograph ( 2 1 ) ) ,McMurry and Liu ( 2 2 ) measured the rate of reaction between NH3 and HzSO4 for particle diameters between 0.05 and 0.2 pm, NH3 concentrations between 0.01 and 300 ppb, and a relative humidity of about 10%.Their results indicated that a t least one-third of NH3-HzS04 collisions resulted in a neutralization reaction and that the reaction was first order in "3. In the most recent study, Baldwin and Golden (23) studied the reaction of various gases including NH3 with the surface of bulk H2SO4. to the fraction of They were able to set a lower limit of NH3-HzS04 collisions resulting in reaction. Experimental In this experiment the rate of reaction between NH3 and monodisperse H2S04 aerosol was measured in a flow reactor. In practice this required the measurement of the NH4+ fraction of the aerosol as a function of reactor residence time. This was accomplished in an experimental system which consisted of three major parts: the thermal speciation particulate sulfur analyzer, the flow reactor, and the aerosol generator. The system is shown in Figure 1. Particulate Sulfur Analyzer. The particulate sulfur analyzer is depicted in Figure 2. The actual sulfur measurement was accomplished by a flame photometric detector which for this experiment was the Meloy SA-285. Earlier work (14)had shown that with appropriate modifications this instrument was capable of measuring particulate sulfur concentrations in the low pg/m3 range. The modifications included replumbing of the inlet sample line to improve aerosol transmission into the flame and use of an external electrometer to measure the phototube current. The latter was necessary because of instabilities in the response of the Meloy electrometer-linearizer electronics at sulfur concentrations near zero. Aerosol entering the flame photometer was conditioned in four ways: (1)NH3 was first removed in the sample probe to stop any further neutralization; (2) the aerosol was separated into acidic and ammonium fractions in the thermal speciation section; (3) sulfur-containing gases were removed from the air stream in the diffusion stripper (denuder);and (4) NH3 was added to the air stream just before it entered the flame photometer. NH3 removal in step 1 was accomplished in the sample probe, which was a 250 cm long X 0.4 cm i.d. glass tube beveled a t the entry. The inside of the tube was coated with phosphorous acid (H3P03) to strip NH3 from the air and stop the reaction between NH3 and H2SO4. The length of the coating was variable depending on whether or not the H3P03 deliquesced. The minimum length was about 80 cm, and the maximum was the full length of the probe. Stevens et al. (24) found that the efficiency of NH3 removal in such a stripper agreed well with the theoretically calculated value. In our system this implied that the NH3 concentration was reduced to 10%of its initial value within 0.1 s after entering the probe and to essentially zero at the end of the coated section. At some point in the stripper the NH3 concentration can fall below the equilibrium vapor pressure of NH3 over the drop depending on the degree of neutralization. If this occurs, then and the aerosol will appear to be less the drop will lose "3,
@ 1980 American Chemical Society
Volume 14, Number 7, July 1980 819
InSuIoting locket
-
F l o w reoctor
1"s Permeation tube S a m p l e Probe Anolyzer
Flame Photometric Oetecior
Moveoble sample probe w i t h NH, strtpper
\
'/4'
Swogelock union cross
Figure 2. Thermal speciation particulate sulfur analyzer. Chemical speciation of the sulfate aerosol is accomplished in the 25, 130, and 250 "C tubes Aging flask
In channel 2 the acid fraction of the aerosol is volatilized as shown in Equation 1,and the ammonium fraction is measured. Figure 1. Experimental setup. The flow reactor was 248 cm long with The 250 "C tube is used for the measurement of refractory a main section diameter of 10.5 cm. The neck of the reactor was 4.1 sulfates and in this experiment was equivalent to the base line. cm in diameter and flared up to the full diameter through a 6" angle. The speciation capability was tested by preparing aerosols The i.d. of the insulating jacket was 14.3 cm from known mixtures of H2S04 and ("&SO4 and measurneutralized than was actually the case a t the point of entry to ing the apparent [NH4+]/[S042-] ratio with the particulate the probe. An estimate of this effect can be determined by sulfur analyzer. Excellent agreement between the measured assuming NH4HS04 stoichiometry, 40% relative humidity, and expected results was obtained, as noted above. Henry's law, 0.3-pm diameter particles, and a wall which is The air leaving the thermal speciation section flowed into perfectly adsorbing for "3. Under these conditions, the a diffusion stripper (denuder) in which sulfur-containing gases maximum NH4+ loss which could occur in our system is 10%; were removed (14, 15, 17, 1 8 ) , thereby making the flame Le., a t the end of the sample probe where the air enters the photometer specific to particulate sulfur. The stripper was thermal speciation section, an aerosol which was initially simply a stainless steel tube into which was inserted a cylinder NH4HSO4 would have an [NH4+]/[S042-] ratio of 0.9 instead of Whatman 41 filter paper impregnated with Pb(CH3COZ)Z of 1.0. An aerosol initially more acidic than NH4HS04 would and AgNOS. The filter paper adhered to the wall of the tube lose NH3 less rapidly-the rate decreasing approximately as and provided a reactive surface for the removal of SO;?and [NHd+]/[H+].Because of this and because the fractional loss H2S. The calculated removal efficiency of the stripper was rate of NH4+ is inversely proportional to dp2,the above estigreater than 99.9% for SO;?but less than 0.3% for 0.3-pm dimate represents a worst case. Moreover, an experiment was ameter particles, the smallest used in this research. conducted in which [NH4+]/[S042-] ratios measured by the The final phase of the sample conditioning was the addition particulate sulfur analyzer were compared with [NH4+]/ of "3. This was necessary because previous research (14,15) [S04*-] ratios in the solution from which the aerosol was had shown a significantly lower response of the Meloy SA-285 generated. Excellent agreement between the two was found for HzSO4 in comparison to NH4HS04or (NH&S04. The and indicated that NH3 loss from the particles was not a addition of NH3 at a concentration of about 7 ppm was suffiproblem. cient to neutralize any H2S04to at least an NH4HS04 stoiSeparation of the aerosol into acidic and ammonium fracchiometry and thereby ensured that all particles-regardless tions was accomplished in the thermal speciation section. This of their acidity-were analyzed on an equivalent hasis. NH3 speciation relied on the difference in volatilization temperawas added from a permeation tube as shown in Figure 2. This tures between H2SO4 and NH4HS04 aerosols and the fact that NH3 did not interfere with the thermal speciation process the acid fraction of an internally mixed acid ammonium suldiscussed above. fate aerosol could be separately volatilized (14). This process Calibration of the particulate sulfur analyzer was accomis depicted in Equation 1: plished by measuring the flame photometer response as a a(2Hf S 0 4 V bNH3 (2a - b)H+ bNH4+ U S O ~ ~ - function of sulfate aerosol concentration. The latter was determined by collecting filter samples of the aerosol, extracting 130 "C the filters in water, and measuring the extracted sulfate by our ( U - b)H2S04 bNH4HS04 (1) version of the flash volatilization-flame photometric method a > b vapor solid Vibrating o r i f i c e o e r o s o l generalo!
+
+
-
+ +
+
(14).
The first step occurs in the flow reactor and is the neutralization reaction. The second step occurs in the thermal speciation section. The H2SO4 vapor which is produced is sorbed on the heater wall and does not reach the flame photometer. Thus, passage of the aerosol through a 130 "C tube results only in the measurement of that amount of so42-which had been neutralized by "3. Because NH4HS04 and (NH&S04 have the same thermal behavior as far as sulfate is concerned, separation of the two is not possible in this manner, and the neutralization rate can be measured only to NH4HS04 stoichiometry (14). The thermal speciation section is shown in Figure 2. Air entering the five-port ball valve (Whitey SS-43ZF2) is routed into one of four channels. In channel 4 the air passes first through a glass fiber filter and then through the room temperature (25 "C) tube and into the flame photometer. This establishes the analyzer base line. In channel 3 air flows directly through the 25 "C tube, and total sulfate is measured. 820
Environmental Science 8 Technology
Flow Reactor. The flow reactor was of the tubular type shown in Figure 1. I t was surrounded by an insulating jacket whose ends were sealed to the reactor with Styrofoam. This jacket provided a dead air space and was intended to minimize thermal convection in the reactor. Both the reactor and the insulating jacket were fabricated from Pyrex glass. NH3 from permeation tubes containing liquefied NH3 was injected into the neck of the reactor through a 1-cm i.d. Pyrex tube. A number of different possibilities were explored for the configuration a t the injector, the principal criterion of ac. ceptability being rapidity of mixing. On the basis of smoke tests the best configuration was found to be a ring of nine holes, each about 1mm in diameter, located near the tip of the injection tube. Under typical operating conditions, 3 L/min of air containing the NH3 flowed through the injector and was mixed with 21.5 L/min of air from the aerosol generator. The smoke tests indicated that mixing was essentially complete in less than 1 s for these flow conditions.
NH3 concentrations in the reactor were measured with a Thermoelectron Model 14 NO, monitor. When the monitor is operated in the NO, mode, NH3 is quantitatively oxidized to NO in the 750 "C stainless steel converter. The instrument was calibrated against known concentrations of N H j using a Monitor Laboratories Model 8500 calibrator with a gravimetrically standardized permeation tube. T o avoid contamination of the NO, monitor, NH3 measurements were made only when no H2SO4 aerosol was present in the reactor. I t is estimated that the uncertainty in [NH3] was f10% when [NHB]was 70 ppb. Residence times in the reactor were determined by introducing a pulse of (NH4)2S0paerosolthrough the NH3 injector. Simultaneously with the introduction of the aerosol, a strip chart recorder connected to the output of the flame photometer was turned on. The time increment between the injection and the peak of the flame photometer response consisted of two components: the residence time of the aerosol in the flow reactor and the residence time of the aerosol in the sulfate analyzer. The latter was determined in a separate experiment by injecting ("&SO4 directly into the sample probe. This was then subtracted from the measured time increment to give the reactor residence time. The residence times determined in this manner were estimated to have an uncertainty of &lo%. Fully developed laminar flow did not occur until more than halfway down the reactor. Aerosol Generator. The aerosol generator used in this experiment was the commercial version (Thermosystems Model 3050) of the Berglund-Liu (25) vibrating orifice type modified in one important respect. In this instrument filtered liquid is forced through a small vibrating orifice by a syringe pump. The vibration of the orifice causes the liquid jet to break into uniform-size droplets, which are dispersed and diluted with pre!;surized air. In the initial stage of the experiment considerable instability in the aerosol output was observed and was traced to the syringe pump. The principal difficulty was the inability of the syringe pump to work against the back pressure created by the liquid flow through the filter and orifice. T o remedy this situation, the syringe pump was replaced by a pressurized nitrogen drive system. A nitrogen cylinder with a two-stage regulator was connected through a ballast tank to a '25-mL graduated pipet. The ends of the pipet had been converted to a precision 0.25-in. (0.635 cm) 0.d. tube to accommodate Swagelok fittings. The flow rate through the ,system could be set by adjusting the output pressure of the nitrogen regulator. Because pressures as high as 620 kPa (90 psi) were required when using the 5-pm orifice, the pipet was enclosed in a thick tygon tube as a safety precaution. As a result of these changes the sulfuric acid aerosol concentration was usually stable to better than f3%. Monodispersity of the primary aerosol was checked a t the beginning and end of each experiment by the tranverse air jet method of Strom (26). The aerosol flowing out of the vibrating orifice was carried through a s5Kr charge neutralizer and into a humidification chamber which was a 1000-mL Erlenmeyer flask containing about 100 mL of water. The final relative humidity depended on the temperature of the water in the humidification chamber and was determined by measuring the ambient and dewpoint temperatures with an EG&G Model 880 dew-point hygrometer. After the humidifier, the aerosol flowed into a 5-L aging flash in which the dilute aqueous droplets lost water by evaporation until equilibration of the water content of the aerosol with the relative humidity occurred. Calculations based on the Maxwell diffusion formula (27) indicated that the final particle size was attained in less than 1 s. The air supply for the aerosol generator and the permeation tubes was house compressed air, which was filtered, chemically scrubbed, and filtered again. The chemical scrubbers co_nsisted of Cr203 on silica gel, silica gel, and activated charcoal. This
combination was intended to minimize NH3, NO,, and organic vapors, and to produce a low initial relative humidity. The activated charcoal and silica gel canisters were changed on a daily basis. The final particle size of the aqueous H2S04 was calculated from the operating parameters of the aerosol generator ( 2 5 ) and the assumption of thermodynamic equilibrium between the air and the droplets after evaporation of excess water. Thermodynamic equilibrium requires that the vapor pressure of water a t the surface of the droplet be equal to the partial pressure of water in the air. This, in turn, specifies the relative amounts of H20 and HzSO4 in the droplets and therefore the droplet density. Because H2S04 is nonvolatile and conserved during the evaporation of the water, the final droplet diameter can be determined from the calculated initial diameter and the compositions and densities in the initial and final states. In a study of dioctyl phthalate (DOP) aerosol generated from a DOP-ethanol solution, Berglund and Liu (25) found the calculated diameter to be within 4 2 % of the diameter measured with an electrical mobility analyzer. Similar accuracies can be expected in this experiment if the concentration of impurities is low relative to the sulfate concentration. As discussed below, this was the case, and the particle diameter was calculated assuming no impurities. If undetected impurities were present, however, the actual diameter would be larger than the calculated diameter. The net uncertainty in the diameter is taken to be f 5 % and includes contributions from the primary generation process and the relative humidity. In a typical run HzS04 aerosol of a known size was generated and allowed to flow through the reactor. Before adding "3, the sulfate fraction not volatilized in the 130 "C heater was measured. A nonzero value was indicative of some form of contamination resulting in partial neutralization of the HzS04. Possible sources of contamination included NH3 gas not removed in the scrubbers and NHJ+ initially present in the liquid feed water to the aerosol generator. Cations such as Na+ which might precipitate S042-were not the likely source of the contamination because of lack of response in the 250 "C channel. The level of such contamination was usually less than 10%. The NH3 permeation tubes were then put in line and sufficient time was allowed for the NHs concentration to reach a steady state in the reactor. This was evidenced by a stable response in the 130 "C channel of the sulfate analyzer. When the steady state was reached, the [NH4f]/[S042-] ratio was measured a t various points along the center line of the reactor. T o minimize end effects, most measurements were made in the front half of the reactor. All data reported herein correspond to an NH3 concentration of about 70 ppb, although a number of preliminary runs were performed a t 15 ppb. The particle size range covered in this study was from 0.3 to 1.4 pm.
Results The rate of neutralization of HzS04 aerosol was measured for a variety of particle sizes and relative humidities a t an average temperature of 23 "C. Representative results are shown in Figures 3 and 4 in which the ratio [NH4+]/[S042-] is plotted as a function of residence time in the reactor. Also plotted (as the dashed line) is the [NH4+]/[S042-]ratio expected if the neutralization rate was limited only by diffusional mass transport of NH3 to the surface of the H2S04 droplet. This ratio is calculated in the following manner: The rate of mass transfer of a gaseous species to a stationary spherical droplet is given by the well-known Maxwell formula multiplied by the Fuchs-Sutugin interpolation factor (28). The latter accounts for noncontinuum effects in the mass transfer. For the NH3-HzS04 system, the rate of NH4+ proVolume 14, Number 7, July 1980
821
I
0
-I d, = 0.32pm
RH= 11%
O 2 V
ooy
I
I
5
0
10
00
15
Figure 3. [NH4+]/[S042-] vs. time. dD= 1.03 pm, relative humidity = 46%, [NH3] = 73 ppb, [SO4*-] = 65 pg/m3.The dashed line is the diffusion limited case and the solid line the fit for a = 0.69 (Equation
8)
duction in the aerosol is:
-d[NH4+1- N - dn"4+ dt
dt
f(Kn) =
+
1
- 2~d$[NH3] f(Kn)N 1+Kn 1.71Kn 1.333Kn2
+
=
["do
- P[NHI+ICL
(3)
(4)
where [ N H ~ + ] cis L the concentration of NH4+ a t the reactor center line and [ N H B ]is~the initial NH3 concentration. The factor /3 can have a value between 1 and 2 and is dependent on the details of the flow. For plug flow each particle regardless of its position on a particular cross-sectional plane of the reactor has the same residence time and therefore the same NH4+concentration. Conservation of mass therefore requires that @ = 1. For laminar flow, which is fully developed a t the point of mixing between the NH3 and H2S04, the residence time of a particle traveling along the reactor center line is half the average residence time for particles in a particula; crosssectional plane of the reactor (assuming no radial diffusion for the particles). If it is assumed that radial diffusion of NH3 is sufficiently rapid to produce an essentially uniform [NHs] across the plane a t all times, then p would have the value 2 . The actual situation is between these two extremes. As is shown below, the value of p has only a second-order effect on the calculation. If Equation 4 is substituted into Equation 2 , and the differential equation solved, the result is: ~
[NH4+1- -___ lNH310 [ l - e x p ( - 2 d P P N D f(Kn)t)] ( 5 )
[so42-l P [S042-l 822
Environmental Science & Technology
15
10
Figure 4. [NH4+]/[S04*-] vs. time. dp = 0.32 pm, relative humidity = 11%, [NH3] = 70 ppb, [S042-] = 22 pg/m3. The dashed line is the diffusion limited case and the solid line the fit for a = 0.28(Equation 8). The nonzero intercept on the ordinate is the result of NH4+ present in the aerosol at t = 0
(2)
where the parameters are defined as follows: [NH4+], the NH4+ concentration (mol/cm3) in the aerosol; N, the number concentration ( ~ m - of ~ )particles in the aerosol; ~ N H ~ the + , number of NH4+ ions in one droplet; [NH3], the NH3 concentration (mol/cm3); d,, particle diameter; D, diffusion coefficient of NH3 in air = 0.23 cm s-I(29); Kn = 2X/d,, the Knudsen number, where X is the mean free path of NH3 in air. N0.11 H ~pm, The use of Equation 3 requires that = ~ D / ~ = where is the average velocity of the NH3 molecules (8hT/TmNH3)1/2 and mNH3is the mass of an NH3 molecule (28). In Equation 2 the equilibrium vapor pressure of NH3 over the partially neutralized drop was neglected in comparison to [NH3]. Effects due to the heat of reaction were also neglected. For the conditions of this experiment these are valid assumptions. Because of the conversion of NH3 to NH4+, the concentration of NH3 varies with time:
"[SI
5
0
t (SI
t (51
after dividing by [S042-] (the concentration of S042- in mol/cm3 of air). The number concentration N can be determined from the measured mass concentration of S042-(i.e., 96[S042-]) and the operating parameters of the aerosol generator: 96[S042-]v (6) C~ 0 4 2 - Q l i q where v is the vibration frequency of the vibrating orifice aerosol generator, C s o p is the S042-mass concentration in the liquid feed to the aerosol generator, and Qliq is the flow rate (cm3/s) of liquid to the aerosol generator. Equation 6 is essentially the number of particles generated multiplied by the transmission efficiency into the flow reactor. The Taylor series expansion of Equation 5 gives: N=
["4+1
[so42-l =
192~dpDf ( K n b t t second-order terms] ( 7 ) C~042-Qliq Note that both [S042-] and /3 drop out in the first-order approximation. Thus, when the argument of the exponential is significantly less than 1, as was the case for most of the measurements, the absolute values of P and [S04*-] have little consequence. The approach which we have taken is to calculate Equation 5 for ,8 = 1 and P = 2 and average the results. As is evident from Figures 3 and 4, the measured neutralization rates are slower than would be predicted simply on the basis of diffusional mass transfer. If the flux ratio a is defined as the ratio of the measured flux to the diffusion-controlled flux, Equation 8 results. ( a is assumed to be independent of both time and [NHs].) ~[NH4+1- i["310[I
lso42-l P [so42-l
- exp(-27rrdpcupND f(Kn)t)] (8)
The flux ratio a is a function of cyr, the reaction coefficient (Le., the fraction of NH3 molecules striking the surface which react). From the treatment of Fuchs and Sutugin (28, Equation 3.33) the following relationship can be derived: 3 1--Ly = (4Kn f(Kn) '1-l
(7) +
The reaction coefficient is analogous to the condensation coefficient.
The values of CY were determined by fitting Equation 8 to the data, and the solid lines in Figures 3 and 4 represent the best fits. For the whole set of data. the values of cy were in the range 0.21-0.70. The decreasing quality of the fit in Figure 4 as [NH4+]/[S042-]increases indicates that one value of cy is not always appropriate and suggests that the buildup of reaction products in the droplets can inhibit the reaction rate. The uncertainty in the values of cy is estimated to be &17% and includes contributions from uncertainties in the ammonia concentration, the particle diameter, the reactor residence time, and the curve fitting procedure. In the determination of the flux ratios, no account was taken of the finite but small (
