have usually involved reacting NO2 with water to produce “ 0 2 which decomposes to NO which is then quantified (Green and Pust, 1958). Oxygen must be eliminated since NO will react with 0 2 to reproduce N o s . This has necessitated the removal of oxygen from the system, usually by condensing out the NO2 formed (Smith et al., 1960). The reduction of NO2 to NO has also occurred on column supports if sufficient moisture is present (Levaggi et al., 1972). The columns and carrier gas used in this work were sufficiently dry to prevent the reduction of absorbed KO2 to NO in detectable amounts. The use of gas chromatography for the separation of inorganic gases has become increasingly important in environmental studies, particularly those concerned with the fate and effects of air pollutants in the ecosystem. However, problems of gaseous contamination have frequently prevented accurate measurements of inorganic gases in
closed, experimental systems. The experimental design reported herein is satisfactory for the quantitative analysis of Nz, NO, N 2 0 , 0 2 , and Hz.
Literature Cited Green, A . , Pust, H., Anal. Chem., 39 (61, 1039 (1958). Levaggi, D . A , , Sin, W., Feldstein, M.; Kothny, E. L., Enuiron. Sci. Technol., 6, 260 (1972). Matsubara, T., Mori. T., J . Biochem., 64 (61, 863 (1968). Smith, D. H., Kakayama, F. S., Clark, F. E., Soil Sci. Soc. Amer. Proc., 24, 145 (1960). Stevenson, F. J . , Harrison, R. M., ibid., 30, 609 (1966). Wullstein. L. H., Bruening, M. L., Chemosphere. 1 ( 2 ) , 71 (1972).
Receiced for reuieic June 25, 1973. Accepted October 12, 1973 This study was supported by EPA Grant N o . R-800535 awarded to L. H.W .
NOTES
Ozone Measurements in Smoke from Forest Fires Leslie F. Evans,l Nicholas K. King, David I?.Packham, and Edwin T. Stephens Division of Applied Chemistry,
CSIRO,Melbourne, Australia
Airborne measurements of the ozone concentration in the smoke plumes produced by the burning of forest debris confirm t h a t ozone is generated when these plumes are exposed to sunlight. Close to the fire, the ozone concentration is zero due to reaction of ozone with combustion products, but as the plume drifts downwind, ozone develops in the upper layer of the plume. In one case, the ozone concentration had reached 0.1 ppm after 45 min of irradiation, thus warranting some concern if the ozonecontaining layer should reach ground level.
It has been known for some time (Darley et al., 1966) that the smoke produced by the burning of agricultural wastes contains ingredients liable to react under the influence of sunlight to form “Los Angeles-type” photochemical smog, typified by an ozone concentration several times higher than the ambient level of 0.03 ppm. We report here what appear to be the first field measurements to detect an excessive ozone concentration in photolyzed rural smoke, the source of the smoke, in the present case, being the burning of forest debris. These measurements were carried out as part of a general investigation into the pollution aspects of the controlled burning of forest litter (Vines et al., 1971), a practice which has become widespread in Australia as a means of reducing the fire hazard. Our general procedure has been to traverse the smoke plume in an a i k r a f t (Piper PA23 AZTEC), equipped to monitor continuously the integrated light-scattering coefficient, the temperature, the altitude, and, most recently, the concentration of carbon dioxide by a nondispersive infrared analyzer (UNOR 2 , Maihak AG, Hamburg). The ozone concentration was measured by a Mast detector (Mast Development Co., Davenport, Iowa). Other reactants liable to interfere with the Mast detector were To whom correspondence should be addressed.
apparently absent, since a prior passage of the smoke through a CrOa absorber failed to affect the readings (Salzman et al., 1965). To study the generation of ozone by sunlight and its decay in the dark, it was sometimes desirable to store samples of smoke in containers. Bags of polyester film (Melinex, ICI) proved to be suitable containers, since they transmitted most of the sun’s radiation longer than 300 nm and, after conditioning in ozonized air (1000 ppm 0 3 ) , they were virtually unreactive toward ozone. The several smoke plumes studied have all risen to a n inversion level (about 2000 meters above ground), then drifted downwind a t between 15 and 22 km/hr. The top of the plume was well defined and remained at an almost constant altitude; the base of the “horizontal” plume, though ill defined, was estimated to be initially about 1000 meters above ground. Excessive ozone was found only in the top layer of the plume and only when the sun was shining. In a typical controlled burn, an area of 4000 hectares, carrying a fuel density of about 4 tonnes/hectare, largely in the form of forest litter, was ignited by about 2000 airborne incendiaries. By traversing the plume at various altitudes and at various distances downwind of the fire, it was established that an ozone-containing layer developed along the top of the plume, its thickness and concentration increasing with distance downwind in accord with the increased duration of irradiation by sunlight. Five km downwind of the fire (after about 15 min of irradiation), the layer was 100 meters thick; 29 km downwind of the fire (after about 75 min of irradiation), the layer was 300 meters thick, and a t this point the ozone concentration had reached 0.065 ppm. The COz concentration a t the same point was 30 ppm above ambient and the meteorological range, as defined by Middleton (1952) was 2.5 km. On another occasion, a somewhat hotter fire was generated by burning 64 kilotonnes of windrows of felled eucalyptus trees distributed over 70 hectares. Figure 1 is a section of the in-flight record taken during a study of this Volume 8,Number 1 , January 1974
75
NEPHELOMETER 05 OUTPUT
0 150 I00 c 02 ( P P M above 50 ambient)
0
_-_-_ 1113
1118
......... 1123
_._ 1128
1133
LOCAL TIME (hours)
Figure 1. Tracing of in-flight record of ozone concentration, carbon dioxide concentration, and light scattering Aircraft speed = 90 knots, altitude 2000 meters, wind from north at 20 knots. Visual range is deduced from the nephelometer output volts, V , according to Vis. range = 3.9 X 104/(54.5V - 0.5) meters
plume. Between 11:13 and 11:18 hr the aircraft flew downwind along the plume a t 100 meters below the nominal plume top. The trace shows the overall rise in ozone concentration with distance downwind, the smoke density remaining relatively constant. At 11:19 hr the aircraft reached the downwind end of the plume, turned, climbed 80 meters, and flew upwind along the upper edge of the plume from 11:21 to 11:32 hr. Figure 1 shows how four billows of plume were penetrated, then the aircraft emerged into clear air upwind of the fire. There is good correlation in time between the smoke density and the ozone concentration in each billow. The Mast detector has a time constant too long to portray peak ozone concentrations in each billow, but measures correctly the ambient atmospheric concentration of about 0.025 ppm after 11:33 hr. A bag of smoke sampled a t 11:22 hr had reached a concentration of 0.11 ppm ozone after a n estimated irradiation time of 45 min, and after the bag of smoke was irradiated for a further 50 min in sunlight, the ozone concentration reached 0.20 ppm.
76
Environmental Science & Technology
To determine whether the absence of excess ozone in the lower levels of the “horizontal” plume was due to the absence of ultraviolet light or to the consumption of ozone by its reaction with other compounds of the smoke, a bag of smoke was sampled from deep within the plume where there was no excess ozone, then irradiated in sunlight for 50 min. The concentration of ozone rose to 0.16 ppm, and furthermore, when the bag was stored in the dark, the ozone concentration fell by only 20%/hr. It seemed therefore that the depth of the ozone layer was governed primarily by the depth to which the ultraviolet radiation penetrated the plume. It is planned, in subsequent flights, to correlate the ozone concentration with the ultraviolet intensity at various levels in the plume. Nevertheless, these preliminary observations are sufficient to show that in spite of the presence of ozone “sinks” in smoke, there is a net generation of ozone, the peak concentration of ozone being several times that of the ambient atmosphere. It is indeed fortunate that the excess ozone is confined to a shallow layer at the top of the plume and that the plume from controlled burning rises above 1000 meters, thereby ensuring sufficient dilution of the ozone layer to eliminate any health hazard a t ground level. Nevertheless, one should keep in mind that low-level smoke may well constitute a hazard, particularly if it drifts over urban areas and merges with other urban pollutants such as nitrogen oxides, the effects of which cannot yet be predicted (Altshuller et al., 1967). We feel that it is strange that such high ozone concentrations have not been obvious elsewhere in the world. Perhaps the indigenous eucalypt fuels used in the present work yield smokes which are particularly photosensitive.
Literature Cited Altshuller, A. P., Kopezynski, S. L.. Lonneman, W. A , , Becker, T. L., Slater, R., Enciron. Sei. Technol. 1,899 (1967). Darley, E. F.. Burleson, F. R., Mateer, E. H., Middleton, J. T., Osterli. V. P . , J .Air Pollut. Contr. Ass.. 11, 685 (1966). Middleton, \V. E. K., “Vision through the Atmosphere,“ p 105, Toronto Univ. Press, 1952. Saltzman, B. E., Wartburg, A. F., Taft, R. A , ; dnai. L‘hem.. 37, ’779 (1965j . Vines, R. G., Gibson, L., Hatch, A. B., King, X . K., MacArthur, D. A,. Packham, D. R., Taylor, R. J., C‘SIRO Aust. Dip. rlppi. Chem Tech. Paper No. l ( l 9 7 1 ) .
Received for revleu M a > , 18, 197:j. Accepted September 1 I , 1973.
