forest fire enhanced photochemical air pollution. a case study

Pergamon

PII: S1352-2310(97)00319-1

Atmospheric Environment Vol. 32, No. 4, pp. 673-681, 1998 © 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 1352-2310/98 $19.00 + 0.00

FOREST FIRE ENHANCED PHOTOCHEMICAL AIR POLLUTION. A CASE STUDY

L. C H E N G , * ' t K. M. MCDONALD, :~ R. P. A N G L E * and H. S. S A N D H U * *Alberta Department of Environmental Protection, 9820-106 Street, Edmonton, Alberta, Canada T5K 2J6;

artd :~ Environment Canada, 4999-98 Avenue, Edmonton Alberta, Canada T6B 2X3

(Firsi: received 18 February 1997 and in final form 16 June 1997. Published February 1998)

Al~tract--A large forest fire occurred about 300 km to the northeast of the Edmonton area in early summer 1995. The forest fire produced nitrogen oxides, hydrocarbons and ozone which were transported downwind. Continuous monitoring of 03, NO and NO2 and integrated measurements of volatile organic compounds, together with air trajectories, during the period of 1--6 June indicate that air pollutant concentrations were enhanced by the forest fire emissions. In the rural environment the influence of the forest fire on air quality could be easily detected; significantly higher NO2 and 03 concentrations were observed when air came from the direction of the forest fire area. Hourly NO2 and 03 concentration were 50-150% higher than the seasonal median values. The influence of the forest fire on air quality was also noticeable in the urban center even though local emissions are much higher than in the rural area. Maximum hourly ozone concentrations at the urban air quality monitoring stations in Edmonton on 4 June 1995 were above the 82 ppbv national and provincial air quality objectives. © 1998 Elsevier Science Ltd. All rights reserved.

Key word index: Forest fire, ozone, photochemical smog, urban and rural pollution, air quality.

1. INTRODUCTION

Burning of forests can produce substantial increases in the concentrations of carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and ozone (03) downwind of the fire. In recent years, there has been much interest in studying the contribution of forest fires to enhartce the production of air pollutants in the atmosphere (Andreae et al., 1988; Kirchhoff and Rasmussen, 1990; Delany et al., 1985; Kaufman et al., 1992; Kirchhoff et al., 1992; Kirchhoff and Marinho, 1994; Dixon and Krankina, 1993 and Auclair and Carter, 1993). At mid-latitudes, emission plumes from prescribed (deliberate) forest fires (Radke et al., 1978; Stith et al., 1982) reached one to two thousand metres above the ground and could be detected at a great distance downwind from the fires. Particulates (most of their mass was contained in particles a few tenths of a micrometre in diameter), ozone and nitrogen oxides were found in these plumes. It is known that biogenic emissions of volatile organic compounds (VOCs) can affect urban and regional ozone concentrations (Rao and Sistla, 1993; (;hock et al., 1995; Roselle, 1994). Biomass burning also contributes in an important way to the global budgets of several major atmospheric trace gases,, including NOx, CO and CH3C1

(Crutzen et al., 1979). In Alberta, Peake et al. (1983) suggested that forest fire smoke might have contributed significantly to high peroxyacetyl nitrate measured in southwestern Alberta in the summer of 1982.

In the summer of 1995, a large forest fire occurred in northeastern Alberta. At Edmonton, approximately 300 km south of the forest fire location, smoke and forest fire odour were detected for days. The fire lasted, from fire ignition to fire extinction, for a total of 43 days from 28 May-10 July 1995. On 4 June ozone concentrations measured at the monitoring stations around Edmonton were relatively high. The ozone 1 h average Alberta Ambient Air Quality Guideline of 82 ppbv was exceeded at three monitoring stations, with a total of nine exceedances. The physical and chemical aspects of photochemical formation in Alberta have been characterized by Angle and Sandhu (1986, 1989), Cheng et al. (1997), Gladstone et al. (1991), Peake et al. (1988a, b). This study was undertaken to investigate the contribution of this fire in the boreal forest to photochemical air pollution in the northern continental climate of Alberta.

2. FOREST FIRE LOCATION AND AIR QUALITY STATIONS

Edmonton, the capital of the province of Alberta t Author to whom correspondence should be addressed, with a metropolitan population of about 800,000, is

673

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Fig. 1. Locations of monitoring stations (solid dots) and forest fire area (shaded area with fire symbol). Trajectory envelopes at 925 mb (solid line) and 850 mb (dashed line) levels for the period of 1 June 1800 MDST through 5 June 1200 with sample path (arrow line) through the northeastern forest fire region into

Edmonton showing 6 h time step.

located at 5315°N latitude and 113.5°W longitude and is the most northern major city in Canada. On 28 May 1995 a large forest fire burning in northeast Alberta was started by lightning. Figure 1 gives the location of the forest fire which was about 80 km southwest of Fort McMurray and 300km to the northeast of Edmonton. The fire lasted for more than a month. It was controlled on 17 June and eventually extinguished on 10 July. Total area burned was about 135,000ha estimated from the aerial photography taken on 14 June. Based on Provincial volume tables, more than 4 million cubic metres of forest were destroyed: 2,844,684 m a of coniferous sawlog, 1,308,718 m 3 of coniferous smallwood and 92,071 m 3 of deciduous.

Surface winds and continuous measurements of CO, NOx (NO + NO2) and 03 using Bendix 8501, Teco 14B and Teco 48 analyzers, respectively, are made at five locations in and around Edmonton: Edmonton Central (downtown), Edmonton North- west (residential), Edmonton East (industrial), Fort Saskatchewan (approximately 30 km northeast of Edmonton) and Royal Park (approximately 85 km east of Edmonton) sites (Fig. 1). The separations of Edmonton Northwest and Edmonton East sites from

the Edmonton Central are about 6 and 9 km, respectively. The concentration data are reported as 1 h averages, which form the basis of this study. Intermit- tent monitoring of volatile organic compounds is con- ducted jointly by Alberta Environmental Protection and Environment Canada at the Edmonton Central and Edmonton East stations over a 24 h sampling period (midnight to midnight) every six days. Details of the sampling procedures, the basic analy- tical system and procedures are given in Cheng et aL (1997), Dann and Wang (1992) and Winberry et al. (1988).

3. RESULTS

3.1. Air trajectories and meteorology

Backward air trajectories ending at Edmonton from the forest fire region were calculated for the period of 1-10 June, using the Lagrangian advection model operated by the Atmospheric Environment Service, Environment Canada. The model solves the trajectory equations by the trapezoidal rule (semi- implicit method) using three-dimensional wind fields at 1000, 850, 700 and 500 mb levels at 6 h intervals.

Photochemical air pollution 675

The horizontal wind components are routinely analyzed at the Canadian Meteorological Centre. The vertical motion fields are computed using mountain and friction surface effects and from the Haltiner omega equations at the upper levels. It also has an ageostrophic turning term at the 1000mb level. Details of the model have been described in other studies (Rutherfor,:l, 1977; Olson et al., 1978).

Figure 1 shows the outermost extent of the trajectories at the 925 mb (approximately 750 m above ground) and 850mb level (approximately 1500m above ground). These pressure levels were chosen because they were levels likely to affect the surface air quality in Edmonton. A sample trajectory (925 mb, 0600 MST, 4 June) which passes over the fire region is also shown. (All time hereafter is referred to in Moun- tain Standard Time, MST). Each symbol on the trajectory represents a 6 h step backward in the time indicating that the air originating at the fire region reached Edmonton in 3-4 days. However, the trajectories over the time period imply a consistent transport from the northeast. Air arriving in Edmonton from 1 June 1800 through 5 June 1200 had trajectories passing through the northeastern Alberta forest fire area. Of the 16 trajectories considered, 8 passed dir- ectly over the fire region at 850 mb level and 12 at 925 mb level. From 5 June 1200, the low-level air trajectory changed its direction reaching Edmonton from the east to northeast. Air reaching Edmonton at higher levels originated from an area different from that of the lower levels because the 700 mb level (approximately 3(300 m above ground) and 500 mb level (approximately 5500 m above ground) trajectories were from the southwest between 1 and 6 June.

Prevailing upper-level winds at Edmonton are west-northwesterly and daylight ranges from 7.5 h in the winter to 17 h in the summer. Early morning ground-based inversions are frequent throughout the year, with median mixing height less than 100 m even in the summer (Myrick et al., 1994). During the analyzed period, surface wind measured at Royal Park, Edmonton Northwest, Edmonton East, and Fort Saskatchewan monitoring stations show consistent direction and speed. Wind was southeasterly to northwesterly about 5-10 km h - 1 on the first day of June but was quite variable on the next two days. On 4 June the wind was very calm up to early afternoon, and became quite gusty on 6 June. Temperature and radiation measurements obtained at Stony Plain, located approximately 32 km west, and Royal Park, 100 km east of Edmonton were similar to seasonal values. Radiation measurements were low on 3-5 June. The temperature varied in a similar manner as radiation on 1-5 June. Maximum temperature, which usually peaks at about 1500, reached 26.6°C on 2 June, but dropped down to 19.6°C on 3 June. On 4 June, the temperature went up again with maximum of 23.5°C. In the following few days, maximum temperature was below 20°C and temperature variation differed from that of radiation. Both temperature and radiation measurements suggest cloudy skies on 3-5 June.

3.2. Ambient air concentrations

Figure 2 gives the seasonal median (May-July) observations of each hour at stations around Edmonton. Ground-level ozone concentration peaks at about 1500 at all stations. Edmonton Central, in general,

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Fig. 2. Seas,anal (May-July) median of hourly ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2) and carbon motcoxide (CO) concentrations at Edmonton Central (thick line), Edmonton Northwest (open

diamond), Edmonton East (open square), Fort Saskatchewan (solid dot), and Royal Park (thin line).

676 L. CHENG et al.

shows the lowest daytime ozone concentration and Fort Saskatchewan, a suburban station, observes the highest. The Edmonton downtown and suburban stations show distinct peaks in NO and CO concentrations in the morning and late afternoon, concurrent with the automobile traffic increases. After sunset, lower levels of the atmosphere become stable and continuous emissions lead to the late night hour peak. In contrast to this, the rural station at Royal Park shows that there is very little diurnal variation in NO. The NO2 concentrations closely follow NO at the urban stations, but the magnitudes of the late night maxima are higher. Both suburban and rural stations also show NO2 peaks at the late night hours and minima at about 1500. The late night NO2 maxima may be the result of lower mixing height and the oxidation of NO by 03.

Seasonal medians derived from 1990 to 1994 data are given in Table 1. Both CO and NOx concentrations are highest in downtown Edmonton. CO and NO2 concentrations are higher at the Edmonton Northwest station, than at the outskirts (Edmonton East and Fort Saskatchewan). Nitrogen oxides concentrations at the rural Royal Park site are higher than at the suburban Fort Saskatchewan station. The NO2-to-NO ratio is lowest in the rural area, with a value close to one. In the city and in the suburban area, the NO2-to-NO ratio of median ranges from 1.43 to 1.71. Seasonal median ozone is lowest in downtown Edmonton (24 ppbv) and highest in the suburban station of Fort Saskatchewan (30 ppbv).

The CO, NO, NO2 and O3 normalized anomaly from the seasonal median concentration of the hours for 1-6 June at the Edmonton downtown station is shown in Fig. 3. Observations at other urban and suburban stations in the Edmonton area behaved very similarly. The coefficient of variation (standard deviation divided by the mean) is also shown. There were times within the period of 1-3 June when concentrations of these gases were higher than the 1990-1994 seasonal medians. These higher-than-seasonal values were observed at times different from the normal traffic rush hours of the day (0700 and 1600). However, these concentrations were within the normal variability (i.e. less than one standard deviation above or below the mean) most of the time. On 4 June from 0000 to 0700, CO, NO and NO2 were very much

above the seasonal normal. From that time on to midnight, CO and NO were slightly below the seasonal median values of the hours, while NOz followed the same pattern in the afternoon but showed higher values in the evening. During this period ozone behaved completely opposite to the CO and NO, showing significantly lower-than-seasonal values in the first-half of the day and significantly higher in the other half. On 5 June after the air trajectory shifted, NO2 concentration showed a considerable reduction, as did ozone concentration.

Figure 4 gives the continuous monitoring results of NO, NO2 and 03 at Royal Park for the period of 1-6 June. The hourly NO concentrations during these few days were below the seasonal median values. On the other hand, the NO2 was higher for 1-5 June but closer to the seasonal median on 6 June. NO2 also showed a different diurnal pattern from 1-5 June, with highest concentrations further after midnight. Ozone concentrations at Royal Park were higher than the seasonal median values of the hours on 1-4 June. On 5-6 June ozone concentrations were close to seasonal values. Table 2 summarizes the NOx concentrations and the averaged NO2 to NO ratio on 4 June, 1995. NOx concentrations, as well as the NO2 to NO ratio, were significantly higher at all stations. The NO2 concentration was about double, and the NO2 to NO ratio was more than double that of the seasonal medians at the suburban and rural stations.

Integrated measurement of volatile organic compounds are taken every six days at Edmonton Central and Edmonton East stations. 2 June was the only day that a VOC sample was collected and analyzed during the period of 1-6 June. The VOC concentrations on 2 June 1995 were compared with the median VOC concentrations for May and June of 1991 and 1993. Samples of three years were used to deduce the median values because samples were obtained once every other six days and 1994 data were not available. Table 3 compares the VOC measurements at Edmonton Central. Data from Edmonton East monitoring station are not included. It has been shown that variation of VOCs is dominated by local industrial emissions (Cheng e t al., 1997). The total VOC concentration on 2 June 1995 was slightly higher than the seasonal value, though the difference is still within one standard deviation about the mean. Alkanes and

Table 1. Air quality and meteorological parameters and their seasonal (May-July) median values measured at continuous monitoring stations around Edmonton

CO NO NO2 03 VOC a WDR WSP Monitoring station (ppbv) (ppbv) (ppbv) NO2/NO (ppbv) (ppbvC) (deg) (km h - 1)

Edmonton Central 800 13 21 1.62 24 157 - - - - Edmonton Northwest 500 7 12 1.71 26 - - 192 4.1 Edmonton East 300 7 10 1.43 28 457 183 4.6 Fort Saskatchewan 300 3 5 1.67 30 - - 197 3.4 Royal Park - - 6 6 1.00 26 - - 216 10.5

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1~5 June 1996. The dashed lines give the range of coefficient of variation.

aromatics conformed with the results of total VOC. Alkenes and alkynes/dienes, on the other hand, showed completely opposite behavior, i.e. slightly lower than seasonal values. Alicyclic hydrocarbons on 2 June 1995 were more than one standard

deviation higher than the median of May/June 1992/93. Except for alicyclic hydrocarbons, analysis of the VOC sample collected on 2 June did not show a considerable difference from the seasonal concentrations.

678 L. CHENG et al.

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give the range of coefficient of variation.

Table 2. 4 June 1995 median and maximum (in parentheses) hourly values for ozone and its precursors

O 3 NO NO 2 Stations (ppbv) (ppbv) (ppbv) NO~,IO

Edmonton Central 38 (92) 11 (84) 25 (44) 2.4 (4.3) Edmonton Northwest 41 (85) 1 (41) 15 (28) 7.5 (24) Edmonton East 49 (77) 3 (47) 15 (36) 4.6 (14) Fort Saskatchewan 48 (92) 2 (21) 10 (27) 5.0 (13) Royal Park 42 (66) 4 (6) 12 (14) 2.5 (3.5)

4. DISCUSSION

Trajectory analysis revealed that the air which ar- rived in the Edmonton area passed through the forest fire area in northeast Alberta during the period of 1-5 June 1995. Air concentration measurements in the urban and suburban stations, however, showed values

similar to seasonal medians on 1-3 June. Although higher than seasonal values were observed at times different from the traffic rush hours, most of the times these high concentrations were within the seasonal variability. Nitrogen dioxide showed more occasions of higher-than-seasonal values than other pollutants. These incidents are the result of transported forest fire

Photochemical air pollution

Table 3. Comparison of VOCs' ozone formation potential at Emonton Central Monitoring Station

679

Concentration MIR ~ MOR ~ (ppbvC) (#g of 03) (#g of O3)

2 June 1!996 Alkane 125.2 68.8 Alkene 7.6 36.2 Alkyne and Diene 7.9 11.7 Alicyctics 7.1 8.6 Aromatics 42.2 105.3

Total 190.0 230.6

May/June Median (1992-93) Alkane 104.5 (39.1) ~ 52.4 Alkene 10.9 (5.8) 46.3 Alkyne and Diene 8.3 (7.9) 11.0 Alicycilics 4.6 (1.5) 5.0 Aromatics 41.3 (22.0) 111.8

Total 169.6 (58.2) 226.5

43.8 14.2 4.4 4.6

29.7 96.7

(21.0) 33.5 (13.8) (18.7) 18.8 (8.2) (4.5) 4.6 (2.1) (1.5) 2.8 (0.9) (59.8) 34.3 (22.2) (93.0) 91.0 (42.4)

"Carter (1991). b Standard deviations in parentheses.

emissions. On 4 June, concentrations of all four air pollutants monitored at Edmonton Central were much higher than the seasonal medians. The high concentrations period, which was outside of the traffic rush hours, coincided with the time when wind speed at Edmonton was very low. Thus, the high concentration of these air pollutants are caused by weak dispersion of transported forest fire emissions. From 0000 to 0700 ozone at downtown Edmonton was almost totally scavenged by the high concentrations of nitric oxide. Du:ring daytime hours with enhanced nitrogen dioxide available from the forest fire, ozone concentration steadily reached a maximum of 92ppbv at 1600, 180% higher than the seasonal median values. Maximum ozone concentrations were 85 ppbv recorded at Edmonton Northwest, 77 ppbv at Edmonton East ;and 92 ppbv at Fort Saskatchewan on that day.

Under steady-state conditions, ozone concentrations can be approximated by the NO2-to-NO ratio (03 oc NO2/NO). In more common unsteady conditions, the ultimate., amount of 03 depends on the nature and concenl:ration of VOCs present as well as the concentration of NOx. In a VOC-deficient and NOx-rich atmosphere, 03 formation is limited by ozone- and radical-scavenging reactions. Ozone formation is favored in VOC-rich atmospheres, where every NO molecule oxidized into NO2 for each VOC molecule consumed could result in a molecule of 03. Table 2 shows a significant increase in the NOz-to- NO ratio on 4 June when the emissions of the forest fire appeared to a~,~ct the Edmonton area. Except for the Edmonton Central station, considerable decreases in NO and increases in NO2 were recorded at all the stations in the Edmonton area. As a result, the ratios of NO2 concentration to NO concentration were at least one standard deviation higher than the seasonal medians. (Standard deviations of the seasonal NO2 to NO ratio are 1.4, 21.3, 2.6, 2.8 and 0.9 for Edmonton

Central, Edmonton Northwest, Edmonton East, Fort Saskatchewan and Royal Park stations, respectively.) Although NO maxima during the day could be well above average, the daily median of NO was significantly lower than the seasonal normal. Neither NO nor NO2 maximum concentrations were measured during the normal peak hours (see Fig. 2). They occurred at other hours of the day. The increase in downtown Edmonton ozone median concentration on 4 June is proportional to the increase in the median NO2-to-NO ratio. At other locations, the increase in median ozone concentration is less than that predicted from the NO2-to-NO ratio by 35-70%, suggesting that excess NO2 was transported into the area.

Emissions of NOx are much lower in the rural environment than in the urban and less variable. This is the reason that the coefficients of variation for NO and NO2 at Royal Park are relatively smooth when compared to that observed at downtown Edmonton (Figs 2 and 3). This also makes the influence of the forest fire on the air quality much more obvious at Royal Park than at Edmonton Central. The forest fire increased NO2 as much as 50-150% above seasonal medians. Consequently, ozone concentrations were 50-100% higher during this period. Ozone concentrations were higher during daytime hours as well as nighttime hours, suggesting that ozone was also formed in the plume and transported downwind. Maximum hourly ozone concentration of 69 ppbv occurred on 2 June. On 4 June the ozone concentration reached a maximum of 66 ppbv. The ratio of NO2-to-NO at Royal Park was 2.50 on 2 June and 2.51 on 4 June. Ozone concentrations on the next two days were similar to seasonal values because of low radiation on 5 June and seasonal NO2 levels on 6 June. Trajectory calculations showed that the air arriving in the Edmonton area on 6 June no longer originated from the forest fire area.

680 L. CHENG et al.

All VOC species are not equally important in urban- and regional-scale photochemical oxidant production. Two sets of reactivity factors developed by Carter (1991) were examined. Total "Maximum Incremental Reactivity" (MIR) and "Maximum Ozone Reactivity" (MOR) on 2 June were slightly higher than the May/June seasonal medians. At these long travel distances, VOC concentrations from the forest fire would be small, and differences in composi- tion would not be detectable in the larger concentrations contributed by automobiles. Nevertheless, alicyclic hydrocarbons were significantly higher than normal. Sampling closer to the fire is needed for the determination of organic compounds in the forest fire plumes.

5. CONCLUSION

Forest fires can have a significant effect on photochemical pollutant concentrations 300 km away. In a rural environment, influence of a forest fire on air quality was easily detected because of lower local emissions. Significantly higher NO2 and 03 concentrations were observed when air came from the direction of the forest fire. Hourly NO2 and 03 concentrations were as much as 50-150% higher than the seasonal median values. In the urban center, influence of the forest fire on air quality was also noticeable. A high ozone and high nitrogen oxides incident which occurred on 4 June 1995 in Edmonton was attributed to the forest fire. Under stagnant meteorological conditions and high solar radiation, photochemical ozone formation was enhanced in the city. Hourly ozone concentrations at the urban and suburban stations in Edmonton were above the 82 ppbv Ambient Air Quality Guideline.

Acknowledgements--The authors would like to acknowledge John Torneby and Don Kupina of Alberta Environmental Protection for managing and operating the monitoring pro- gram, Tom Dann of River Road Environmental Technology Center, Environment Canada for the VOC analyses, and Dave Fox and Jim Ross of Prairie and Northern Region, Environment Canada for the plotting and the upper air data, respectively. We are grateful to the senior management of Alberta Department of Environmental Protection for their support of this work.

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