Photochemical Smog and Ozone Reactions - ACS Publications

7 Formation and Destruction of Ozone in a Simulated Natural System (Nitrogen

Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: June 1, 1972 | doi: 10.1021/ba-1972-0113.ch007

Dioxide +α-Pinene+ hv) DANIEL LILLIAN Department of Environmental Sciences, Rutgers, The State University, New Brunswick, N . J.

The hypothesis that the naturally occurring system N O + α-pinene + hv (3000-4000 A), would show analogous be havior to simplified systems of photochemical smog was tested statistically and verified. During the reaction ozone and organic oxidants were formed and consumed; aerosol (condensation nuclei) was formed. These data indicate that the naturally occurring photo-oxidation of α-pinene may serve as a sink for the significant quantities of terpenes that are emitted globally, a source and sink for ozone, and a source of the naturally occurring light scattering aerosol (blue haze). 2

Tprying to account for the fate of an estimated 10 tons of terpenes emitted annually from plants, Went ( I ) hypothesized that terpenes underwent reactions similar to those of olefins i n photochemical smog. H e suggested that the photo-oxidation of the terpenes was responsible for forming the blue haze observed over densely vegetated areas. O b serving a Tyndall beam when ozone was allowed to react with a terpene ( 1 ) and detecting condensation nuclei when a blend of N 0 and a-pinene was irradiated with sunlight (2) supported this hypothesis. 8

A

2

Here, Went's hypothesis, that the naturally occuring system N 0 + a-pinene -\- hv behaves analogously to simplified models of photochemical smog is tested. Besides suggesting the mode of natural aerosol formation, the proof of this hypothesis has important implications in the atmospheric chemistry of other nonurban trace constituents, particularly ozone. 2

211 Photochemical Smog and Ozone Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

7.

LILLIAN

Formation and Destruction of Ozone

217

than 100% absorption efficiency was obtained, or nitric oxide was synthe sized i n the Saltzman scrubbing reagent (19): 3 N 0 + H 0 -> 2 H N 0 2

2

3

+ NO

Both artifacts may have been concurrently operative.


Conclusion

The data presented have important implications i n the behavior of tropospheric nonanthropogenic ozone, aerosol, and other trace constitu ents. Observational and experimental data have been reported b y R i p perton et al. (20) indicating the natural synthesis of ozone i n the tropo sphere. Considering this study, the ubiquitous presence of various terpenes (21), isoprene (22), and oxides of nitrogen (20) suggest that some ozone is synthesized in the lower troposphere by the reaction N 0 + α-pinene + hv. Conversely, the destruction of ozone i n the tropo sphere is partially ascribed to reactions with the terpenes and intermedi ates of the photochemical mixture. 2

During the photooxidative reactions, aerosols are formed which are undoubtedly similar to the aerosols forming the blue haze over densely vegetated areas. This aerosol may also account for a significant quantity of the natural organic continental aerosol. There is basically little difference between the mechanism of photo chemical smog formation and the naturally occurring photo-oxidation of terpenes. The former, associated with a greater emission intensity of ozone precursors, however, leads to higher concentrations of those inter mediates responsible for the undesirable effects of photochemical air pollution. Literature Cited 1. 2. 3. 4. 5. 6. 7.

Went, F. W., Proc. Nat. Acad. Sci. (1960) 46, 212. Went, F. W., Tellus (1966) 28, 549. Scheffe, H., "The Analysis of Variance," Wiley, New York, 1963. Mast, G. H., Sanders, H . E . , I.S.A. Trans. (1962) 1, 325. Rich, Τ. Α., Goefis. Pura, Appl. (1955) 31, 60. Regener, V. H.,J.Geophys. Res. (1964) 69, 3795. U. S. Public Health Service, "Selected Methods for the Measurement of Air Pollutants," U. S. Government Printing Office, Washington, D. C., 1965. 8. Ripperton, L. Α., Lillian, D., 63rd Meeting, Air Pollution Control Associa tion, Saint Louis (June, 1970). 9. Leighton, P. Α., "Photochemistry of Air Pollution," Academic Press, New York, 1961. 10. Altshuller, A. P., Bufalini, J. J., Photochem. Photobiol. (1965) 4, 97.

Photochemical Smog and Ozone Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

212

PHOTOCHEMICAL

SMOG AND OZONE REACTIONS

Experimental

Methods. Blends of 10 pphm nitrogen dioxide and 50 pphm a-pinene in dry air (absolute humidity 0.0005 gram H 0 / g r a m dry air) were prepared i n 150-liter transparent bags and irradiated at 25 °C for 120 minutes. A l l experiments were performed i n duplicate and blanks were run according to a complete factorial design. Statistical comparisons were made of the mean value of a given variable with the mean value of the appropriate blank using Tukey's (3) method of multiple comparisons. The results are reported at a .05 confidence level. During the course of the irradiations, the concentrations of the fol lowing variables were monitored: oxidants (Mast coulometric ozone me ter) (4), condensation nuclei with radii greater than 10" cm ( G . E . Type C N small particle detector) (5), ozone (Regener chemiluminescent ozone meter (6), nitrogen dioxide and nitric oxide (Saltzman method) (7), and α-pinene ( Perkin Elmer model 800 gas chromatograph ). The gas chromatograph was equipped with a flame ionization de tector. A 50-foot length of 0.020 inch i.d. stainless steel open tubular capillary column coated with Carbowax 1540 served as the main column. A freeze out trapping technique was used to concentrate the a-pinene before entering the main column. The pre-column trap consisted of an in-line capillary column, identical to the main one, inserted between the injector and inlet of the main column. The trap was located outside the oven and cooled with a dry ice-ethanol bath before injection of a 5 cc sample. A 80 °C hot water bath was used to release the α-pinene. The operating conditions of the gas chromatograph were as follows: 2


7

Helium (carrier) flow rate 3.1 cc/minute Hydrogen flow rate 50 cc/minute Air flow rate 850 cc/minute Helium (make up) flow rate 40 cc/minute Oven temperature (isothermal) 100°C Reaction Bags and Irradiation Chamber. Teflon (50-mil F E P Type C ) bags of 32 inches X 48 inches were fabricated by impulse heat sealing. The bags were fitted with glass ball joints to connect them to the T F E Teflon sampling lines. A Hotpack controlled environmental room was used as an irradiation and constant temperature chamber. By maintaining the chamber tempera ture at 21 ± 1°C, a temperature of 25 ± 2°C was achieved in the Teflon reaction bag. Radiation simulating solar radiation was given by a bank of four G . E . 40-watt cool white fluorescent lamps and two Westinghouse 400watt E H 1 mercury vapor lamps mounted on one wall of the chamber. The walls of the chamber were covered with aluminized Mylar which provided a reflecting surface. k for N 0 was 2.8 hr" . Chemicals Used. Listed below are the specifications of the chemicals used in this study. Common laboratory reagents used for the various standard analyses met with the specifications prescribed i n the cited methods and are not listed. A l l gases were supplied by the Matheson Company, East Rutherford, N . J . a

2

1


7.

LILLIAN



•

213

OXIDANTS

TIME (MINUTES)

Figure 1.

Concentration-time profiles of the indicated variables for the system N0

2

+ a-pinene + hv

Air, zero gas: maximum dew point — 78 °F, less than 0.5 ppm hydro carbon equivalent to methane Hydrogen: pre-purified grade, 99.95% minimum purity Nitrogen dioxide: 99.5% minimum purity Helium: ultra-high purity grade, minimum purity 99.999% Oxygen: extra-dry grade, maximum dew point — 79 °F, minimum purity 99.6% a-Pinene: minimum purity 99.9%, supplied by Glidden Chemical Company, Jacksonville, F l a . Results

The concentration-time profiles obtained upon irradiating synthetic blends of nitrogen dioxide and α-pinene i n air, absolute humidity N O + Ο

2

0 + 0 0

3

+ M->0

2

3

(1)

+ M

(2)

0

(3)

+ N O -> N 0 + 2

2

[NO] [O,] [N0 ]

ΦΚ k z

(4)

2

M represents a third body—e.g., nitrogen, φ is the quantum yield for N 0 photolysis, k is its specific absorption rate, and k is the bimolecular reaction rate constant for Reaction 3. 2

a

3

Since Reactions 1, 2, and 3 are much faster than competing reactions involving olefins, the equilibrium relationship 4 must hold even i n the presence of a-pinene (15). The buildup of ozone i n Figure 1 above the steady state concentration it shows when no α-pinene is present is there fore accompanied by an increase i n the N 0 : N O ratio. This increase is effected by reactions which convert N O to N 0 as shown by a few reac tions from Wayne's (11) mechanism for the N 0 - i n i t i a t e d photo-oxidation of an olefin: 2

2

2

ocpO*

0 -> οφ0 * 3

(6)

αρ + 0 —* αρ0 *

(7)

apO*

+

2

3

3


7.

215


LILLIAN

οφ0 * -> Aldehydes, + R O

4- R C O

3

RO

+ N O + 0 -> R 0 + N 0 2

R0

2

+ NO -* RO + N 0

2

(8) (9)

2

(10)

2

ap refers to α-pinene and the asterisk designates an unstable intermediate. As pointed out by Leighton (9), synthesis of ozone by free radical reac tions with molecular oxygen may similarly lead to a buildup of ozone above steady state.


R0

2

+ 0 -> R O - + 0 2

(11)

3

The difference between simultaneous oxidant and ozone readings (Figure 1) are attributed to organic oxidants formed from reactions of free radicals, the oxides of nitrogen, and the allotropes of oxygen. The positive 10% response of the Mast instrument to N 0 accounts for only a small part of this difference, particularly in the latter stages of irradiation when the N 0 concentration is low. The nature of the organic oxidants is speculative. However Stephens observed that the system N 0 + α-pinene + hv formed P A N (16), indicating that part of the organic oxi dant is attributable to this well known lachrymator and phytotoxicant. 2

2

2

Condensation Nuclei. M a n y mechanisms have been proposed (9) involving free radical polymerizations of various radicals which could lead to formation of condensation nuclei. It seems that if condensation nuclei are formed by such reactions, the myriad different radicals in a given system would lead to formation of a highly mixed polymer. Noting this, an oversimplified mechanism by which the system N 0 + a-pinene + hv may form condensation nuclei ( Figure 1 ) is for example, reactions of the alkyl peroxy radical formed in Reaction 9 with α-pinene and molecular oxygen: 2

R00-

+ ap

R O O ap. + 0

2

R O O apOO- + ap

R O O ap.

(12)

R O O apOO-

(13)

R O O apOO ap-

(14)

The leveling off of the condensation nuclei concentration after the first few minutes of the irradiation indicates that the size distribution is shifting to larger particles as oxygenated olefin is incorporated into the aerocolloidal mass. Ripperton et al. (17) and Groblicki and Nebel (18) have shown that the dark-phase reaction of ozone and α-pinene leads to rapid formation of condensation nuclei. Since relatively high concentrations of ozone are produced by the photochemical system N 0 + α-pinene -f- hv, the ozone— 2


216

PHOTOCHEMICAL

SMOG AND OZONE REACTIONS

α-pinene reaction probably is responsible for a significant fraction of the condensation nuclei observed here. The relative importance of this mech anism of condensation nuclei production to one involving a peroxy free radical polymerization initiated by an atomic-oxygen-a-pinene reaction, however, cannot be assessed from the data available. Using experimental techniques and conditions identical to those used for the controlled irradiations of gaseous mixtures i n Teflon bags, a fifteen minute irradiation of a blend of 1 ppm N 0 and 1 ppm α-pinene yielded a barely perceptible bluish haze i n a Tyndall beam. Adding more ozone with a six-inch ultraviolet (uv) Penray lamp fitted into the top ball-joint of the 50-liter flask intensified this haze within seconds. Figure 2 is a picture of the Tyndall beam taken two minutes after the uv lamp had been activated for ten seconds. Over a substantially longer path length, the system N 0 + α-pinene + hv at concentrations near natural concen trations should be capable of forming the haze observed over densely vegetated areas (blue haze).


2

2

Figure 2.

Light scattering aerosol as viewed in a Tyndall beam

N i t r i c Oxide. Since photolysis of N 0 did not proceed measurably before irradiation, as indicated by zero ozone readings for systems con taining N 0 in zero air, the N O readings obtained before irradiation for these systems and for the system of Figure 1 are artifacts of the analytical method. The positive error is probably attributable to the scrubbing column used to remove N 0 before the N O oxidation step. Either less 2

2

2



218

PHOTOCHEMICAL SMOG AND OZONE REACTIONS

11. Haagen-Smith, A. J., Wayne, G., "Air Pollution," A. C. Stern, Ed., Vol. 1, Academic, New York, 1968. 12. Pitts, J. N., Jr., "Photochemical Air Pollution: Singlet Molecular Oxygen as an Environmental Oxidant," Advan. Environ. Sci. (1969). 13. Altshuller, A. P., Bufalini, J. J., Environ. Sci. Technol. (1971) 1, 39. 14. Jaffee, S., Loudon, R., Advan. Chem. Ser. (1972) 113, 264. 15. Shuck, Ε. Α., Stephens, E. R., "Oxides of Nitrogen," J. N. Pitts and R. L. Metcalf, Eds., Advan. Environ. Sci., Wiley, New York, 1969. 16. Stephens, E. R., Proc. Amer. Petrol. Inst. (1962) 42, 665. 17. Ripperton, L. Α., Jeffries, Η. E., White, O., Advan. Chem. Ser. (1972) 113, 219. 18. Groblicki, P. J., Nebel, G. J., "The Photochemical Formation of Aerosols in Urban Atmospheres," General Motors Research Symposium on Chemical Reactions in Urban Atmospheres, Warren, Michigan (October, 1969). 19. Mueller, P. K., Transah, N. O., Tokiwa, Y., Kothny, E. L., "Series vs. Par allel Continuous Analysis for NO, N 0 , and NO . II. Laboratory Data," 9th Conference, Methods in Air Pollution and Industrial Hygiene Studies, Pasadena (February, 1968). 20. Ripperton, L. Α., Jeffries, H., Worth, J. J. B., Environ. Sci. Technol. (1971) 5, 246. 21. Rasmussen, R. Α., Went, F. W., Proc. Nat. Acad. Sci. (1965) 53, 215. 22. Rasmussen, R. Α., Environ. Sci. Technol. (1970) 4, 667. 2

X

RECEIVED May 10, 1971.


Photochemical Smog and Ozone Reactions - ACS Publications

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