Amospheric Encmnmmt Vol. 17. No. 7. pp. 1331 -1337, 1983 Gm4981,83 13.00 + 0.00

Prmted in Great Bntam. Q 1983 Pergamon Press Ltd.

SOME ENVIRONMENTAL EFFECTS OF FOREST FIRES IN INTERIOR ALASKA

FRANK EATON and GERD WENDLER

Geophysical Institute, University of Alaska, Fairbanks, AK 99701, U.S.A.

(First received 21 June 1982 and in~~al~orrn 3 Deeember 1982)

Abstract-The high variability of burning conditions and fuels, found in Alaskan forest fires, produces an associated complex emission of particulate matter. Histological evidence of some large particles has been found in the forest fire plumes as well as aerosols resulting apparently from gas-to-particle conversion. Particles analyzed with a scanning electron microscope and X-ray energy dispersive techniques show large variability in both physical and chemical characteristics. Optical measurements show forest fire smoke affects atmospheric turbidity regionally. Turbidity values presented which were measured in the plume from a forest fire 400 km from Fairbanks show values in excess of those found for heavily polluted urban regions. The particulate matter analysis showing irregular shapes and highly varied chemical composition displays the difficulty in radiative transfer calculations due to the assumptions of Mie theory. The nature of the aerosol size concentrations (non-Junge power law distributions) found in forest fire plumes also violates the assumption necessary for application of Angstrom’s classic method of defining the turbidity coefficient and wavelength exponent. Consequences of such particulate matter may affect the temperature structure of the atmosphere, radiation balance as well as visibiIity. In addition, the burnt over forest regions display a reduction of surface albedo and roughness parameter which will have prolonged influence on the heat exchange at the earth’s surface.

I. INTRODUCTION

In the past wildland fires have burned a larger area of temperate forests than is generally appreciated (Holbrook, 1943). A cultural history of wildland and rural fires in America from the American Indian to the present is described by Pyne (1982).

In Alaska alone, fires still burn approximately 2000 km’ of forest annually and over 20,ooO km2 have burned in interior Alaska on individual years within the past two decades (Barney, 1971). Such fires are within northern coniferous forests characterized by slow decay resulting in heavy accumulation of ground litter and are occasionally associated with periods of summer drought. Intense convective activity often resulting in thunderstorms with associated lightning is found in interior Alaska during the fire season. Barney (1971) presented results of wildland fires over a 30-y period in interior Alaska which show over 70% were man-caused, but lightning-induced fires accounted for 78 ‘? of the area burned.

I%ring mid-summer 1977 a series of fires on the Seward peninsula in Alaska covered almost 3800 km2. These lightning-caused fires produced smoke patterns which were seen from the NOAA-5 polar-orbiting satellite (Ernst and Matson, 1977) for 31 July 1977. Convection appears to be inhibited over large areas covered by smoke and it is suggested that absorption by solar radiation by the smoke aerosol may be responsible.

In order to assess the potential impact of forest fire smoke on regional or global climate, estimates must be examined of the quantity of emissions of particulate

matter from wildland fire smoke. Results of a literature survey, showing estimates of particles smaller than 20 pm radius emitted into or formed in the atmosphere from various sources, are shown by Wilson (1971). Forest fires and slash-burning debris estimated to emit or produce 3-lSOMty_‘. Flohn (1971) made calculations for the man-ignited bush fires in African savannahs and found an annual particulate input of 80 Mt. Unfortunately the un- certainty of emissions from forest fires and slash burning are greater than for other sources due to the limited number of studies made on wildland fires to date.

2. OPTICAL MEASUREMENTS OF FOREST FIRE SMOKE

Forest fire smoke from prescribed burning has been documented to seriously reduce visibility (Packham and Vines, 1978). Three prescribed burns of conifer slash were also examined by Stith ef al. (198 1) and by neglecting absorption of solar radiation by the par- ticles, the light-scattering coefficients measured in the plume showed visibilities only about 3 7” of that of the background air. Episodes of forest fire smoke observed in interior Alaska have reduced visibility so severely as

to require officially closing several of the remote “bush” airports and the Fairbanks International Airport on at least one occasion.

Interior Alaska is bound by the Alaska Range to the south and the Brooks Range to the north. Wind speeds are light to moderate and associated with a high frequency of inversions; conditions conducive to

1331

1332 FRANK EATON and GERD WENDLER

severe smoke episodes. During mid-summer 1977, several large fires burned in interior Alaska, encircling Fairbanks. Five fires ranging in size from 42 to 300 km’ ignited during the last week of July alone. On 6 August 1977 the Bear Creek fire ignited from lightning near Farewell, Alaska, about 400 km south- west of Fairbanks. Conditions were ideal for a large fire since dry, warm air was being funneled down

nearby Windy Pass to the point of ignition. Two days later the fire was enhanced by winds of 10-15 ms- ’ and a relative humidity in the teens, causing the fire to spread approximately 32 km in a few hours. Experienced fire fighters reported that the Bear Creek fire was the only fire of this type they had ever encountered in Alaska. Fire personnel en route from Fairbanks to the fire also reported the dense, continu-

Fig. 1. A portion of the Landsat (MSS 7) image of 28 August 1977 showing the lowered albedo as a resutt of the Bear Creek fire.

Same env~ronmcota~ &ects of forest fires in interior Alaska 1333

ous smoke throughout the entire flight. The final Figure 2 shows the wind speed and direction profiles

inventory showed 1500 km2 were burnt. During the as determined from RAOBs taken at the Fairbanks fire itself, no Landsat imagery was available but the International Airport. At 2:OO a.m. on 11 August, area which was burnt can be seen fairly clearly on the 2:Otl p.m. on 11 August and at 2:OO a.m. on 12 August a Landsat image of 28 August 1977, shown in Fig. 1, southwest wind direction is shown in the lower part of except for some cloud cover on the southeasterly the atmosphere, agreeing with the transport of smoke corner of the fire zone. When measuring the cloud free as witnessed by the fire teams. Visibility was reported burnt area from this image, we obtained 1320 km*. Of by the U.S. Weather Service at the Fairbanks particular interest is the reduction of albedo in the International Airport as 3.2 km during the initiation of burnt area which will affect the heat exchange at the the smoke reports and increased to 9.6 km during the surface until the area is revegetated. afternoon of 12 August.

0 5 10 15 20 25

‘SINI) DIRECTION WIN0 SPEED !MPS)

Fig. 2. Wind speed and direction vs height (geopotential meters) determined by radiosonde observations at Fairbanks International Airport for 11 and 12 August

1977.

Approximately every 2 h on 12 August (with the exception of two close sets at 237 and 751) measure- ments were taken with a Linke-Feussner pyrheli- ometer. Fluxes of the integral direct solar radiation were measured as well as the spectral fluxes by using Schott filters OGf, RG2 and RG8 and taking into consideration the proper Davos reduction factor for each filter. Figure 3 shows a photograph facing south from the Geophysical Institute during the 12 August smoke episode.

In Figure 4, the turbidity factor T as proposed by Linke (1922) is shown. The quantity T is a simple measure of the haze and water vapor content of the atmosphere and is equivalent to the number ofatmos- pheres of pure air that would produce the same depletion of direct solar radiation as actually measured. The measurements of forest fire smoke show a turbidity factor T value greater than 9 for the morning hours and a T value of about 6 for the afternoon. This decrease in turbidity agrees with the reported surface improved visibilities.

The turbidity coefficient fi and wavelength exponent tl as derived by Angstrom (1929. 1930) were also

Fig. 3. Photograph of view facing south from the Geophysical Institute showing pall of smoke on 12 August 1977.

1334 FRANK EATON and GERD WENDLER

IO

8

T 6 /

. .*

. . .

5 6 7 8 9 IO II 12 13 14 15 16 17

TimeChours)

Fig. 4. Linke turbidity factor vs time for 12 August 1977 measured at the Geophysical Institute.

evaluated from the measurements. The wavelength exponent a is expected to vary between 0 and 4 and knowledge of a is a good indication of the number of haze particles as a function of their size. However, our results were inconclusive. There are two ways to explain the erratic results; (a) the measurements were not of high enough accuracy, due to filter constant changes and (b) a Junge power law number con- centration-size distribution of the particulates is as- sumed. However, under conditions of excessive input to the atmosphere from large fires or maritime sources, power law concentration-size distributions are not necessarily found.

No particle size measurements were taken during the Bear Creek fire but Radke et al. (1978) found a multimodal character of sizes for particles from several prescribed forest fires. Stith et nl. (1981) found similar results in the plumes from prescribed burns of conifer slash with the number concentration-size spectra to be bimodal with peaks at 0.1 and 0.5 pm. The forest fires in Alaska show large variability of burning conditions as well as types of fuels consumed. Such variety, even found on individual fires, is expected to explain the multimodal character of the size distribution found in forest fire plumes.

The smallest aerosol material originates from com- bustion processes (Bullrich, 1964), originating from gases which cool producing smoke and residue through condensation and sublimation processes. In forest fires, soil and partially cornbusted organic matter will also be introduced into the plume.

Peterson and Drury (1967) measured solar radiation from an airplane under smoke that drifted great distances over the Canadian tundra. They found reductions of solar radiation due to smoke up to 25 % under these conditions.

3. PHYSICAL AND CHEMICAL PROPERTIES OF FOREST

FIRE SMOKE

Several studies have been carried out to examine the fate of nutrients as a result of wildland fires. For example, DeBell and Ralston (1970) examined burning litter, green needles and fuel of the loblolly pine and found that most of the nitrogen in the organic matter

was presumably volatilized. Lloyd (1971) found high losses of potassium, phosphorus and nitrogen from burned herbaceous vegetation while Evans and Allen (1971) investigated nutrient losses in smoke during heather burning and found heavy losses of nitrogen and sulfur. Several other nutrients were also examined with losses attributed to the burning of l&20%.

Some nutrients are expected to be returned down- wind from wildland fires. Clayton (1976) found con- centrations of sodium, potassium, calcium, magnesium and nitrogen in precipitation falling through smoke to be 2&70 times greater than in normal precipitation. An examination of fly ash in fields downwind of wildland fires was carried out by Smith and Bowes (1974). They accounted for 30 % of the nutrients lost from burns in the downwind deposits.

Although much attention has been devoted to such nutrients in wildland fire smoke, Sandberg et al. (1979) pointed out that two products of complete combus- tion, carbon dioxide and water will contribute over 90% of the total mass emission of wildland fires. He estimated that by cornbusting 1 t of wood, 1670 kg of carbon dioxide and 490 kg of water will be produced. Sandberg et al. (1979) also discussed production of carbon monoxide, sulfur oxides, oxidants, nitrogen oxides and hydrocarbons under different combustion conditions.

Since the absorption bands of carbon dioxide within the solar spectrum are rather weak and in regions where the radiance of solar radiation is weak, increased carbon dioxide from wildland fires will have negligible impact on Linke’s turbidity factor.

However, of great importance to the turbidity factor values is production of water vapor since water vapor displays a number of intensive and wide water-vapor bands in the near-infrared region of the solar spec- trum.

In order to examine the physical and chemical characteristics of particulate matter emitted from forest fires in interior Alaska, air samples were taken using Nuclepore membranes. These samples were analyzed using a JSM-35U scanning electron micro- scope (SEM) manufactured by JEOL, Inc. with an effective magnification ranging from 10 x to 180,000 x. A Kevex 7000 X-ray energy dispersive micro-analyzer is also coupled to the SEM which allows chemical analysis of SEM specimens for ele- ments such as sodium and heavier. Preparation of the samples involved mounting portions of the mem- branes (Nuclepore) on carbon stubs coated with evaporated carbon or carbon and palladium. Other investigators have used electron microscopy coupled with various associated techniques for elemental analysis of airborne particulate matter. Among some of the more recent such studies displaying a spectrum of interests are included those of Adams et nl. (1980), Del Monte et al. (1981), Dillard et al. (1980) and Flocchini et al. (1981).

Particulate matter sampled on 6 June 1980 during a forest fire occurring about 64 km south of Fairbanks

were examined from samples taken on the roof of the taining particles were also found in ‘background Geol jhysical Institute. Many of the near spherical aerosols. Bigg (1979) found sulfuric acid to be the partit cles seen were 0.2 pm or smaller and sulfur was dominant winter aerosol in Barrow, Alaska, on the ident ified by the X-ray energy dispersive analysis. coast of the Arctic Ocean. The spring aerosols con- Althc Jugh high losses of sulfur from wildland fires have tained a variable and greater proportion of ammonium been documented as previously discussed, sulfur con- sulfate than the winter particulate matter. Therefore

Some environmental effects of forest fires in interior Alaska 1335

Fig. S.(a) Scanning electron ticroseope micrograph of burned wood showing structure typical of a hardwood tree. (b) The calcium as seen in the X-ray energy dispersive microanalysis is believed to be

from decomposition of calcium oxalate in the wood. Pore size equals 0.4 pm.

our samples probably contain a mixture of sulfur- wiidland fire smoke on modulating the thermal struc- containing aerosols originating from the forest fire and ture of the atmosphere as well as on the radiation the natural ‘background’ aerosol. balance. The same understanding of the true physical

Figure 5 shows a large particle on the SEM micro- and chemical nature of the smoke’s particulate matter graph of what appears to be the structure of wood is needed to properly interpret data obtained by optical from a hardwood tree after burning. The peak for methods used for probing the atmosphere such as with calcium is probably from the decomposition of the lidar. calcium oxalate which was in the wood. A similar example is shown by McCrone et al. (1980) with a similar chemical signature except for the silicon, possibly due to some ~on~mination of a clay particle Acknow~e~ge~nt-This study was supported by the in our sample. Other examples found showed large, Department of Energy Grant EY-77-G-~1~0. We are

long irregular particles appearing to be wood fibers thankfuf to Mary Ann Borchert, Electron Microscopist and

from conifer wood, some heavily ‘singed’ on one or Edith Curry for preparing the manuscript.

both ends. The chemical composition of such fibers was found to show similarity to that in the previous example of presumed residue from bardwood burning.

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