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The effect of atmospheric SO2 photochemistry upon observed nitrate concentrations in aerosols
Atmospheric
Enurronment
Vol. 11, pp. 87-91. Pergamon
Press 1977. Printed
in Great Bntain.
THE EFFECT OF ATMOSPHERIC SO2 PHOTOCHEMISTRY UPON OBSERVED NITRATE CONCENTRATIONS IN AEROSOLS A. B.
HARKER,
L. W. RICHARLX and W. E. CLARK
Science Center, Rockwell International,
P.O. Box 1085, Thousand Oaks, CA 91360, U.S.A. 11 June
(First received 2 April 1976 and in final form
1976)
Abstract-In this study, X-ray photoelectron spectroscopy was used to observe the relationship between the nitrate and sulfate content of photochemically formed aerosol particles which were produced by exposing initially particle-free ambient air in a 14 m3 transparent Teflon chamber to sunlight with various amounts of NOz, S02, and C3Hs added to the reaction mixture. It was observed that the photochemical oxidation of SO2 to SOi- decreased the amount of nitrates present in the aerosol significantly below that observed in the absence of SO,, It was also found that nitrate could be removed from an aerosol sample on a glass fiber filter by passing air with sulfate-containing aerosols through the filter. These observations indicate that HzS04 formed from the photochemical oxidation of SO, in the reaction chamber released NO; from the surface of the aerosol particles, probably in the form of HNOB. An examination of analyses of ambient aerosols from the Los Angeles area showed a number of cases where a similar inverse relationship between particulate sulfate and nitrate content existed. INTRODUCTION
particle-free ambient air doped with specific gaseous poliutants and exposed to natural sunlight. The Teflon chamber previously described by Clark, Landis and Harker (1976), was constructed of 0.051 mm thick Teflon. which has a
Because of the concern
over the visibility reduction and adverse health effects associated with sulfates and nitrates in atmospheric aerosols, the concentrations of these species have been monitored in a number of field programs in order to study their diurnal fluctuations and to determine the relative importance of contributions from various emission sources. Examination of data from two studies sponsored by
:
E
Particulate goWest Covina 6O _ 23-24 July 1973
sulfate
s
7
the California Air Resources Board, the Aerosol Characterization Study (Hidy, 1974) and a power generating station plume study (Richards, 1976), showed that in a number of instances there was an inverse relationship between the levels of sulfate and nitrate in particulate samples. This inverse relationship is shown in two examples in Fig. 1, where the samples were 2-h filters collected in West Covina (15 miles east of Los Angeles) in August 1973 and analyzed by wet chemical techniques (Hidy, 1974). This same pattern was found in the ARB plume study where the nitrate levels observed when the sulfate containing plume was present were often below those measured nearby in either time or distance in the absence of the plume. These ambient observations motivated a series of controlled experiments to confirm that the variations were due to a real chemical effect rather than some other cause such as advection of air masses from different sources, and to obtain information on the chemical mechanism which decreases the observed nitrate levels when processes involving sulfur compounds are important. EXPERIMENTAL
E T
m ‘E
$,
,
1 ,
x;
,
9 22 24
2 4
6
8
Time,
IO 12 14 16 18 20 22 PST
R
‘E s ” ‘E 9
‘:
~~~~lc~+~
, , ,
E s 22 24 2
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6 8 Time,
IO 12 14 16 I8 20 22 PST
Fig. 1. Diurnal patterns of particulate sulfate, carbon, and mtrate observed during the ACHEX study in July 1973 (Hidy, 1974).
In this study, photochemical aerosols were generated in a flexible 14m3 Teflon reaction chamber using initially 87
88
A. B.
HARKER,
L. W. RICHARDSand W. E. CLARK
high transmittance for ultraviolet light and is chemically inert to atmospheric gases. The chamber was filled by pumping air through a HEPA filter to provide the initially particle-free air for the reaction mixture. During filling, the chamber was shielded from light by an opaque polyethylene sheet, and known amounts of specific reactants such as NOZ, SO2 or CsH, were injected into the mixture. During the experiment, the chamber was exposed to natural sunlight for -90 min periods. The reaction mixture was monitored for Os, SOZ, N02, NO, CH4, CsH,, and total hydrocarbon levels. Simultaneously, particle formation and growth were observed and filter samples of the aerosol material were collected upon completion of the experimental runs. For these measurements, the SOZ, was monitored by a Meloy SA 16D-2 Sulfur Gas Analyzer, the NO, NO2 and total NO, by a Bendix NO, analyzer, and the 0, by a REM Chemiluminescent Ozone Monitor. Methane, propylene and total hydrocarbon were determined by gas chromatography. The particle formation and growth rates were observed with a Thermo-Systems Model 3030 Electrical Aerosol Analyzer, operated with nominal charging conditions of N x t = 1 x 10’ (ions cmm3) (s). In most of the experimental runs, after 6&90 minutes of irradiation, the aerosol material was collected by pumping the reaction mixture through 47 mm Gelman Spectra Grade A glass fiber filters which had been baked at 400°C for 24 h before use. The aerosol material was then analyzed for chemical state information by X-ray photoelectron spectroscopy (XPS) looking primarily at the sulfur, nitrogen and carbon species using a Hewlett-Packard model 595OA XPS unit. Most of the XPS analyses were done at room temperature and at 10m9 Torr vacuum. However some of the samples were temperature programmed from 25 to 260°C to observe the volatility of the aerosol components. The XPS technique allows the identification of the individual chemical states of the various components of the aerosol material by using the chemical shifts of the electron binding energies away from the electron levels of the atoms in their elemental form (Hollander and Jolly, 1970). Some of the filter samples were subjected to wet chemical analyses to determine quantitatively the amount of sulfate and nitrate formed during the experimental runs. RESULTS AND DISCUSSION Twelve experimental runs were conducted on the roof of the Rockwell Science Center facility in Thousand Oaks, CA during the summer months of 1975. Additional data were obtained from two experiments carried out at a location 25 m north of the San Diego Freeway in West Los Angeles, CA in August of 1974. The experiments were designed to produce photochemical aerosols containing nitrate and sulfate. Hence all runs except for background studies and those performed at the freeway site were initially doped with 0.14.2 ppm by volume of NOa to stimulate aerosol formation. Propylene was also added to a number of the runs at concentrations ranging from 0.3 to 3 ppm to further enhance the aerosol formation rate. When higher sulfate levels were desired in the aerosol, SO, was injected into the mixture at concentrations from 30 to 1OOppb. The basic pattern of aerosol formation and growth, ozone formation, and non-methane hydrocarbon decay was similar for all experimental runs. Figure 2 shows an example with particle-free ambient air doped with 0.15 ppm of NO,. No particle growth was
V Surface tot01 x 10~3~~m’ cmm3) cme3 1 0 Number total x 6’(prn3 0 Volume total x 10%2m-3)
Elapsed time,
min
Fig. 2. Typical photochemical aerosol formation and growth curves from a reaction mixture containing initially particle-free ambient air doped with NOz.
observed before the reaction mixture was exposed to sunlight. Immediately after exposure to sunlight, there was a rapid rise in the number concentration of particulate nuclei reaching a maximum and then showing a uniform decrease with time as condensation and coagulation became significant. The particles continued to grow throughout the experiment by condensation as shown by the steady increase in the volume concentration. The steady increase in the surface area curve indicates that nucleation and condensation provided more aerosol surface area throughout the run than coagulation of existing particles could remove. If either propylene or sulfur dioxide was added to the reaction mixture after irradiation had begun, a new episode of particle formation would be observed which was again followed by condensation and coagulation. During all experiments, ozone showed monotonic growth throughout the irradiation period and propylene, when present, displayed uniform first order decay after injection into the chamber. These aerosol formation and growth characteristics are very similar to those described by McNelis (1974) and Clark et al. (1976). The photochemistry of mixtures containing NO,., hydrocarbons, and SO2 is being studied in many laboratories and recent discussions of the chemistry involved are available in the literature (Hecht and Seinfeld, 1972; Demerjian et al., 1974). In the present work, attention is focused on the effect of the SO;! photochemistry on the amount of NO; observed in the aerosol. The XPS analyses of ‘the filter samples collected during the experimental runs showed the primary aerosol chemical constituents to be sulfate, nitrate, ammonium, parafinic hydrocarbon, and an amine-like nitrogen species. A nitrogen 1s electron XPS spectrum
Atmospheric SOz photochemistry
I
89
in aerosols
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Fig. 3. Nitrogen 1s photoelectron spectrum of aerosol material formed from sunlight irradiated, initially particle-free ambient air doped with 0.16 ppm by volume NO,.
for an aerosol formed in a reaction mixture containing 0.16 ppm NOz in the particle-free ambient air is shown in Fig. 3. The spectrum shows that nitrate was the primary nitrogen species with the small peak between 400 and 402 eV indicating the presence of NH: and an amine-like species. Figure 4 shows the same spectral region for an aerosol sample collected from a reaction mixture identical to that used in Fig. 3 except for the addition of 37ppb of SOz. Note that the small reduced nitrogen peak from 400 to 402 eV remained about the same amplitude. However, the amplitude of the nitrate peak was dramatically lowered. This behavior was typical for the experiments, with the nitrate concentrations in the aerosols formed in the presence of SO2 being well below that of the reaction mixtures without added SO*. In a set of experiments conducted at the Los Angeles freeway site, two chambers were irradiated side by side, one containing particle-free ambient air, and the other particle-free ambient air doped with 36 ppb of SO*. The aerosols were collected on filters and subjected to wet chemical analyses for sulfate and nitrate formation. As shown Table 1. Initial conditions and results from freeway site experiments Run SOz concentration Aerosol volumetric conversion rate Sulfate formation rate Nitrate formation rate Nitrite formation rate SO1 conversion rate
9A
9B
0.018
0.054 ppm
5.29 0.72 3.2 0.037 1.1
10.4~m3cm-3h-i 2.6~gmm-3h-i 1.5~gm-3h-1 0.038 pg mm3 h-l 1.3% h-’
Initial concentrations in both 9A and 9B, ppm by vol. O3 = 0.022, NO = 0.141, NO2 = 0.175, CO = 3.47, CHL = 1.94, non-methane hydrocarbon = 2.09, U.V. light intensity - 295-385 mn = 28.7 W m-*.
in Table 1, the ambient air chamber had an SOz concentration of 18 ppb and the doped chamber 54 ppb, and this 36 ppb by volume increase in SO* was sufficient to suppress the nitrate level in the photochemical aerosol in the doped bag by a factor of two. In order to clarify the mechanism by which the presence of SO2 in the reaction mixture lowered the NO; content of the photochemical aerosol, a series of experiments were carried out under a variety of conditions. Experiments were conducted to determine if the nitrate removal process could occur during collection on the glass fiber filters. A previously collected filter sample of nitrate-containing aerosol had 10m3 of a non-irradiated reaction mixture of 0.08 ppm SOz, 0.18 ppm NOz and l.Oppm C,H, in air pumped through it. A comparison of the XPS spectra taken before and after the exposure to the non-irradiated gases showed that there was no significant change in the nitrate content of the aerosol sample. This experiment was repeated using a sunlight irradiated mixture at the same concentrations of SOz, NOz and C3H6. In this case, the comparison of the XPS nitrogen spectra showed that pumping the irradiated mixture of gases and aerosol through the filter had lowered the amplitude of the NO; signal by about a factor of three. The possibility that this decrease in the NO; signal was caused by a coating of the existing particles by H2S04 rather than an actual removal of the nitrate is ruled out on the basis that the coverage of the filter was quite light. Typically less than 50 pg of total aerosol material was collected on a 47 mm dia. filter. At this light a coverage, the aerosol particles could not collect to a depth sufficient to significantly attenuate the photoelectron signal from the NO;. An attempt was made to minimize the amount of time the aerosol material was exposed to the reaction mixture while on the filter. This Cas done by taking
A. B. HARKER.L. W. RICHARDSand W. E. CLARK
90
smaller samples of the aerosol material, requiring less pumping time, and analyzing the samples by XPS immediately after collection. In each of these experiments, initially particle-free air doped with NOz was irradiated with sunlight for 30 min after which a 3 m3 filter sample was rapidly collected. Then SO2 was added to the mixture with another small volume filter sample collected after 30min of further irradiation. Comparison of the XPS spectrum of the samples taken before and after SOZ addition showed that for a SOZ concentration of 200 ppb, the NO; level on the aerosol surface was lowered by a factor of about 5. The results from these experimental studies demonstrate that it requires photochemical oxidation of SO* to. decrease the nitrate concentration in aerosol particles, and that this nitrate removal process can occur on particles previously collected on a filter substrate. The evidence does not specifically prove that the process can proceed in aerosols suspended in the atmosphere, but it is reasonable to assume that it does. Most atmospheric chemical phenomena occur through the combined effects of many reactions, and it is likely that more than one chemical process is involved in the present case. However, one of the more important reactions is probably that of photochemically formed sulfuric acid reacting with nitrate to form volatile nitric acid as shown by
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This is consistent with the observation that the nitrate decrease induced by SO2 was not accompanied by a similar disappearance of the ammonium and amine from the aerosols, as shown in Figs. 3 and 4. As nitrate leaves the aerosol, either in the atmosphere or on a filter, the sulfate ions would form salts with
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Atmospheric SO2 photochemistry the available cations thus retaining the reduced nitrogen species in the aerosols. Volatility measurements were made on a sample of aerosol produced in a SO2 doped mixture by heating the sample to 260°C in the vacuum of the XPS spectrometer. These experiments showed that the NH: peak and the SO:- peak volatilized at the same rate, disappearing from the sample at a l/l atomic ratio as shown in Fig. 5. This indicates that the HzS04 formed a bisulfate salt with ammonium on the aerosols thus preventing any loss of the ammonium while the NO; was released as HNO,. The remaining amine-like nitrogen peak was nonvolatile at 260°C and at 10m9 torr vacuum as was the bulk of fhe sulfate peak, indicating that these species are present as either non-volatile salts or as strongly bound surface species. The results of this study indicate that areas with high ambient SO2 levels, such as eastern urban centers, would be expected to have a significant suppression of the NO; content of their photochemical aerosols. This study also indicates that techniques for measuring ambient aerosol nitrate concentrations which rely upon time-averaged sampling of particles on a filter substrate can produce artificially low results. This could occur if during the sampling process a mass of air containing H2S04, either as a primary emission or from the photochemical oxidation
in aerosols
91
of S02, passed over the filter, releasing the NO; to the gas phase. In view of this possibility care should be taken in drawing conclusions from ambient particulate nitrate data, and more studies should be conducted to further quantify this effect.
REFERENCES
Clark W. E., Landis D. A. and Harker A. B. (1976) Measurements of the photochemical production of aerosols in ambient air near a freeway for a range of SO2 concentrations.. Atmospheric Environment 10, 637-644. Demerjian K. L., Kerr J. A. and Calvert J. G. (1974) In Advances
in
Environmental
Science
and
Technology
(Edited by Pitts J. N., Jr. and Metcalf R. L.) Vol. 4, pp. l-262. Hecht T. A. and Seinfeld J. H. (1972) Development and validation of a generalized mechanism for nhotochemi_ cal smog. Environ Sci. Technol. 6, 47-57. Hidv G. M. (1974) Characterization of aerosols in Califorma. Final Report, State of California Ai,r Resources Board Contract No. 358, pp. 4-58. Hollander J. M. and Jolly W. L. (1970) X-ray Photoelectron Spectroscopy. Accounts Chem. Res. 3, 193-200. McNelis D. N. (1974) Aerosol formation from gas-phase reactions of ozone and olefins in the presence of sulfurdioxide. E.P.A. publication No. EPA-650/4-74-039. Richards L. W. (1976) The chemistry, dispersion and transport of air pollutants emitted from fossil fuel power plants in California: ground level pollutant measurements and analysis. Final Report State of California Air Resources Board Contract No. 3-916.
Enurronment
Vol. 11, pp. 87-91. Pergamon
Press 1977. Printed
in Great Bntain.
THE EFFECT OF ATMOSPHERIC SO2 PHOTOCHEMISTRY UPON OBSERVED NITRATE CONCENTRATIONS IN AEROSOLS A. B.
HARKER,
L. W. RICHARLX and W. E. CLARK
Science Center, Rockwell International,
P.O. Box 1085, Thousand Oaks, CA 91360, U.S.A. 11 June
(First received 2 April 1976 and in final form
1976)
Abstract-In this study, X-ray photoelectron spectroscopy was used to observe the relationship between the nitrate and sulfate content of photochemically formed aerosol particles which were produced by exposing initially particle-free ambient air in a 14 m3 transparent Teflon chamber to sunlight with various amounts of NOz, S02, and C3Hs added to the reaction mixture. It was observed that the photochemical oxidation of SO2 to SOi- decreased the amount of nitrates present in the aerosol significantly below that observed in the absence of SO,, It was also found that nitrate could be removed from an aerosol sample on a glass fiber filter by passing air with sulfate-containing aerosols through the filter. These observations indicate that HzS04 formed from the photochemical oxidation of SO, in the reaction chamber released NO; from the surface of the aerosol particles, probably in the form of HNOB. An examination of analyses of ambient aerosols from the Los Angeles area showed a number of cases where a similar inverse relationship between particulate sulfate and nitrate content existed. INTRODUCTION
particle-free ambient air doped with specific gaseous poliutants and exposed to natural sunlight. The Teflon chamber previously described by Clark, Landis and Harker (1976), was constructed of 0.051 mm thick Teflon. which has a
Because of the concern
over the visibility reduction and adverse health effects associated with sulfates and nitrates in atmospheric aerosols, the concentrations of these species have been monitored in a number of field programs in order to study their diurnal fluctuations and to determine the relative importance of contributions from various emission sources. Examination of data from two studies sponsored by
:
E
Particulate goWest Covina 6O _ 23-24 July 1973
sulfate
s
7
the California Air Resources Board, the Aerosol Characterization Study (Hidy, 1974) and a power generating station plume study (Richards, 1976), showed that in a number of instances there was an inverse relationship between the levels of sulfate and nitrate in particulate samples. This inverse relationship is shown in two examples in Fig. 1, where the samples were 2-h filters collected in West Covina (15 miles east of Los Angeles) in August 1973 and analyzed by wet chemical techniques (Hidy, 1974). This same pattern was found in the ARB plume study where the nitrate levels observed when the sulfate containing plume was present were often below those measured nearby in either time or distance in the absence of the plume. These ambient observations motivated a series of controlled experiments to confirm that the variations were due to a real chemical effect rather than some other cause such as advection of air masses from different sources, and to obtain information on the chemical mechanism which decreases the observed nitrate levels when processes involving sulfur compounds are important. EXPERIMENTAL
E T
m ‘E
$,
,
1 ,
x;
,
9 22 24
2 4
6
8
Time,
IO 12 14 16 18 20 22 PST
R
‘E s ” ‘E 9
‘:
~~~~lc~+~
, , ,
E s 22 24 2
4
6 8 Time,
IO 12 14 16 I8 20 22 PST
Fig. 1. Diurnal patterns of particulate sulfate, carbon, and mtrate observed during the ACHEX study in July 1973 (Hidy, 1974).
In this study, photochemical aerosols were generated in a flexible 14m3 Teflon reaction chamber using initially 87
88
A. B.
HARKER,
L. W. RICHARDSand W. E. CLARK
high transmittance for ultraviolet light and is chemically inert to atmospheric gases. The chamber was filled by pumping air through a HEPA filter to provide the initially particle-free air for the reaction mixture. During filling, the chamber was shielded from light by an opaque polyethylene sheet, and known amounts of specific reactants such as NOZ, SO2 or CsH, were injected into the mixture. During the experiment, the chamber was exposed to natural sunlight for -90 min periods. The reaction mixture was monitored for Os, SOZ, N02, NO, CH4, CsH,, and total hydrocarbon levels. Simultaneously, particle formation and growth were observed and filter samples of the aerosol material were collected upon completion of the experimental runs. For these measurements, the SOZ, was monitored by a Meloy SA 16D-2 Sulfur Gas Analyzer, the NO, NO2 and total NO, by a Bendix NO, analyzer, and the 0, by a REM Chemiluminescent Ozone Monitor. Methane, propylene and total hydrocarbon were determined by gas chromatography. The particle formation and growth rates were observed with a Thermo-Systems Model 3030 Electrical Aerosol Analyzer, operated with nominal charging conditions of N x t = 1 x 10’ (ions cmm3) (s). In most of the experimental runs, after 6&90 minutes of irradiation, the aerosol material was collected by pumping the reaction mixture through 47 mm Gelman Spectra Grade A glass fiber filters which had been baked at 400°C for 24 h before use. The aerosol material was then analyzed for chemical state information by X-ray photoelectron spectroscopy (XPS) looking primarily at the sulfur, nitrogen and carbon species using a Hewlett-Packard model 595OA XPS unit. Most of the XPS analyses were done at room temperature and at 10m9 Torr vacuum. However some of the samples were temperature programmed from 25 to 260°C to observe the volatility of the aerosol components. The XPS technique allows the identification of the individual chemical states of the various components of the aerosol material by using the chemical shifts of the electron binding energies away from the electron levels of the atoms in their elemental form (Hollander and Jolly, 1970). Some of the filter samples were subjected to wet chemical analyses to determine quantitatively the amount of sulfate and nitrate formed during the experimental runs. RESULTS AND DISCUSSION Twelve experimental runs were conducted on the roof of the Rockwell Science Center facility in Thousand Oaks, CA during the summer months of 1975. Additional data were obtained from two experiments carried out at a location 25 m north of the San Diego Freeway in West Los Angeles, CA in August of 1974. The experiments were designed to produce photochemical aerosols containing nitrate and sulfate. Hence all runs except for background studies and those performed at the freeway site were initially doped with 0.14.2 ppm by volume of NOa to stimulate aerosol formation. Propylene was also added to a number of the runs at concentrations ranging from 0.3 to 3 ppm to further enhance the aerosol formation rate. When higher sulfate levels were desired in the aerosol, SO, was injected into the mixture at concentrations from 30 to 1OOppb. The basic pattern of aerosol formation and growth, ozone formation, and non-methane hydrocarbon decay was similar for all experimental runs. Figure 2 shows an example with particle-free ambient air doped with 0.15 ppm of NO,. No particle growth was
V Surface tot01 x 10~3~~m’ cmm3) cme3 1 0 Number total x 6’(prn3 0 Volume total x 10%2m-3)
Elapsed time,
min
Fig. 2. Typical photochemical aerosol formation and growth curves from a reaction mixture containing initially particle-free ambient air doped with NOz.
observed before the reaction mixture was exposed to sunlight. Immediately after exposure to sunlight, there was a rapid rise in the number concentration of particulate nuclei reaching a maximum and then showing a uniform decrease with time as condensation and coagulation became significant. The particles continued to grow throughout the experiment by condensation as shown by the steady increase in the volume concentration. The steady increase in the surface area curve indicates that nucleation and condensation provided more aerosol surface area throughout the run than coagulation of existing particles could remove. If either propylene or sulfur dioxide was added to the reaction mixture after irradiation had begun, a new episode of particle formation would be observed which was again followed by condensation and coagulation. During all experiments, ozone showed monotonic growth throughout the irradiation period and propylene, when present, displayed uniform first order decay after injection into the chamber. These aerosol formation and growth characteristics are very similar to those described by McNelis (1974) and Clark et al. (1976). The photochemistry of mixtures containing NO,., hydrocarbons, and SO2 is being studied in many laboratories and recent discussions of the chemistry involved are available in the literature (Hecht and Seinfeld, 1972; Demerjian et al., 1974). In the present work, attention is focused on the effect of the SO;! photochemistry on the amount of NO; observed in the aerosol. The XPS analyses of ‘the filter samples collected during the experimental runs showed the primary aerosol chemical constituents to be sulfate, nitrate, ammonium, parafinic hydrocarbon, and an amine-like nitrogen species. A nitrogen 1s electron XPS spectrum
Atmospheric SOz photochemistry
I
89
in aerosols
I
I
I
I
I
I
I
I
I
414
412
410
408
406
404
402
400
398
Bindingenergy,
eV
Fig. 3. Nitrogen 1s photoelectron spectrum of aerosol material formed from sunlight irradiated, initially particle-free ambient air doped with 0.16 ppm by volume NO,.
for an aerosol formed in a reaction mixture containing 0.16 ppm NOz in the particle-free ambient air is shown in Fig. 3. The spectrum shows that nitrate was the primary nitrogen species with the small peak between 400 and 402 eV indicating the presence of NH: and an amine-like species. Figure 4 shows the same spectral region for an aerosol sample collected from a reaction mixture identical to that used in Fig. 3 except for the addition of 37ppb of SOz. Note that the small reduced nitrogen peak from 400 to 402 eV remained about the same amplitude. However, the amplitude of the nitrate peak was dramatically lowered. This behavior was typical for the experiments, with the nitrate concentrations in the aerosols formed in the presence of SO2 being well below that of the reaction mixtures without added SO*. In a set of experiments conducted at the Los Angeles freeway site, two chambers were irradiated side by side, one containing particle-free ambient air, and the other particle-free ambient air doped with 36 ppb of SO*. The aerosols were collected on filters and subjected to wet chemical analyses for sulfate and nitrate formation. As shown Table 1. Initial conditions and results from freeway site experiments Run SOz concentration Aerosol volumetric conversion rate Sulfate formation rate Nitrate formation rate Nitrite formation rate SO1 conversion rate
9A
9B
0.018
0.054 ppm
5.29 0.72 3.2 0.037 1.1
10.4~m3cm-3h-i 2.6~gmm-3h-i 1.5~gm-3h-1 0.038 pg mm3 h-l 1.3% h-’
Initial concentrations in both 9A and 9B, ppm by vol. O3 = 0.022, NO = 0.141, NO2 = 0.175, CO = 3.47, CHL = 1.94, non-methane hydrocarbon = 2.09, U.V. light intensity - 295-385 mn = 28.7 W m-*.
in Table 1, the ambient air chamber had an SOz concentration of 18 ppb and the doped chamber 54 ppb, and this 36 ppb by volume increase in SO* was sufficient to suppress the nitrate level in the photochemical aerosol in the doped bag by a factor of two. In order to clarify the mechanism by which the presence of SO2 in the reaction mixture lowered the NO; content of the photochemical aerosol, a series of experiments were carried out under a variety of conditions. Experiments were conducted to determine if the nitrate removal process could occur during collection on the glass fiber filters. A previously collected filter sample of nitrate-containing aerosol had 10m3 of a non-irradiated reaction mixture of 0.08 ppm SOz, 0.18 ppm NOz and l.Oppm C,H, in air pumped through it. A comparison of the XPS spectra taken before and after the exposure to the non-irradiated gases showed that there was no significant change in the nitrate content of the aerosol sample. This experiment was repeated using a sunlight irradiated mixture at the same concentrations of SOz, NOz and C3H6. In this case, the comparison of the XPS nitrogen spectra showed that pumping the irradiated mixture of gases and aerosol through the filter had lowered the amplitude of the NO; signal by about a factor of three. The possibility that this decrease in the NO; signal was caused by a coating of the existing particles by H2S04 rather than an actual removal of the nitrate is ruled out on the basis that the coverage of the filter was quite light. Typically less than 50 pg of total aerosol material was collected on a 47 mm dia. filter. At this light a coverage, the aerosol particles could not collect to a depth sufficient to significantly attenuate the photoelectron signal from the NO;. An attempt was made to minimize the amount of time the aerosol material was exposed to the reaction mixture while on the filter. This Cas done by taking
A. B. HARKER.L. W. RICHARDSand W. E. CLARK
90
smaller samples of the aerosol material, requiring less pumping time, and analyzing the samples by XPS immediately after collection. In each of these experiments, initially particle-free air doped with NOz was irradiated with sunlight for 30 min after which a 3 m3 filter sample was rapidly collected. Then SO2 was added to the mixture with another small volume filter sample collected after 30min of further irradiation. Comparison of the XPS spectrum of the samples taken before and after SOZ addition showed that for a SOZ concentration of 200 ppb, the NO; level on the aerosol surface was lowered by a factor of about 5. The results from these experimental studies demonstrate that it requires photochemical oxidation of SO* to. decrease the nitrate concentration in aerosol particles, and that this nitrate removal process can occur on particles previously collected on a filter substrate. The evidence does not specifically prove that the process can proceed in aerosols suspended in the atmosphere, but it is reasonable to assume that it does. Most atmospheric chemical phenomena occur through the combined effects of many reactions, and it is likely that more than one chemical process is involved in the present case. However, one of the more important reactions is probably that of photochemically formed sulfuric acid reacting with nitrate to form volatile nitric acid as shown by
Electron binding energy, 2
eV
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z :
12‘. -225°C
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.’
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-
_ .,.’ I A , I : I 1,’
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:/
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This is consistent with the observation that the nitrate decrease induced by SO2 was not accompanied by a similar disappearance of the ammonium and amine from the aerosols, as shown in Figs. 3 and 4. As nitrate leaves the aerosol, either in the atmosphere or on a filter, the sulfate ions would form salts with
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Fig. 5. X-ray photoelectron spectra of nitrogen 1s and sulfur 2p electrons in a photochemically produced aerosol subjected to heating in vacuum. The loss in area under the electron peaks during heating showed SO; and NH: to be volatilized at 1: 1 atomic ratio.
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Fig. 4. Nitrogen 1s photoelectron spectrum of aerosol material formed from sunlight irradiated, initially particle-free ambient air doped with 0.16 ppm NO2 and 37 ppb SO, by volume.
Atmospheric SO2 photochemistry the available cations thus retaining the reduced nitrogen species in the aerosols. Volatility measurements were made on a sample of aerosol produced in a SO2 doped mixture by heating the sample to 260°C in the vacuum of the XPS spectrometer. These experiments showed that the NH: peak and the SO:- peak volatilized at the same rate, disappearing from the sample at a l/l atomic ratio as shown in Fig. 5. This indicates that the HzS04 formed a bisulfate salt with ammonium on the aerosols thus preventing any loss of the ammonium while the NO; was released as HNO,. The remaining amine-like nitrogen peak was nonvolatile at 260°C and at 10m9 torr vacuum as was the bulk of fhe sulfate peak, indicating that these species are present as either non-volatile salts or as strongly bound surface species. The results of this study indicate that areas with high ambient SO2 levels, such as eastern urban centers, would be expected to have a significant suppression of the NO; content of their photochemical aerosols. This study also indicates that techniques for measuring ambient aerosol nitrate concentrations which rely upon time-averaged sampling of particles on a filter substrate can produce artificially low results. This could occur if during the sampling process a mass of air containing H2S04, either as a primary emission or from the photochemical oxidation
in aerosols
91
of S02, passed over the filter, releasing the NO; to the gas phase. In view of this possibility care should be taken in drawing conclusions from ambient particulate nitrate data, and more studies should be conducted to further quantify this effect.
REFERENCES
Clark W. E., Landis D. A. and Harker A. B. (1976) Measurements of the photochemical production of aerosols in ambient air near a freeway for a range of SO2 concentrations.. Atmospheric Environment 10, 637-644. Demerjian K. L., Kerr J. A. and Calvert J. G. (1974) In Advances
in
Environmental
Science
and
Technology
(Edited by Pitts J. N., Jr. and Metcalf R. L.) Vol. 4, pp. l-262. Hecht T. A. and Seinfeld J. H. (1972) Development and validation of a generalized mechanism for nhotochemi_ cal smog. Environ Sci. Technol. 6, 47-57. Hidv G. M. (1974) Characterization of aerosols in Califorma. Final Report, State of California Ai,r Resources Board Contract No. 358, pp. 4-58. Hollander J. M. and Jolly W. L. (1970) X-ray Photoelectron Spectroscopy. Accounts Chem. Res. 3, 193-200. McNelis D. N. (1974) Aerosol formation from gas-phase reactions of ozone and olefins in the presence of sulfurdioxide. E.P.A. publication No. EPA-650/4-74-039. Richards L. W. (1976) The chemistry, dispersion and transport of air pollutants emitted from fossil fuel power plants in California: ground level pollutant measurements and analysis. Final Report State of California Air Resources Board Contract No. 3-916.