2
Fairbanks Case Study

The committee was charged to develop this interim report with a focus on Fairbanks, Alaska, one of the most challenging carbon monoxide (CO) nonattainment areas in the country. The Fairbanks North Star Borough stands out as the only serious CO nonattainment area with a population under 100,000 and little industry. In Fairbanks, vehicle emissions, meteorology, and topography combine to produce conditions conducive to high ambient CO concentrations.1 First, the low winter temperatures increase CO emissions from passenger cars, vans, sport utility vehicles, and light-duty trucks, especially during cold starts. Second, Fairbanks experiences strong and long-lasting ground-level temperature inversions during the winter, trapping pollutants close to the ground. Third, Fairbanks is in a river valley, which exacerbates the strength of inversions and decreases windspeeds, further reducing pollutant dispersion.

As discussed in later sections, it is not the coldest days that have the highest CO concentrations. At very low temperatures (down to −50 °F), the use of engine-block heaters, which reduce cold-start emissions, and the presence of

1  

In the Fairbanks case, the committee concluded that large stationary sources, such as power plants and refineries, do not contribute substantially to high CO concentrations in the areas around the CO monitors. Thus, the committee did not consider stationary-source controls in this interim report. However, the importance of stationary sources and their controls will be considered as the committee produces its final report.



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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report 2 Fairbanks Case Study The committee was charged to develop this interim report with a focus on Fairbanks, Alaska, one of the most challenging carbon monoxide (CO) nonattainment areas in the country. The Fairbanks North Star Borough stands out as the only serious CO nonattainment area with a population under 100,000 and little industry. In Fairbanks, vehicle emissions, meteorology, and topography combine to produce conditions conducive to high ambient CO concentrations.1 First, the low winter temperatures increase CO emissions from passenger cars, vans, sport utility vehicles, and light-duty trucks, especially during cold starts. Second, Fairbanks experiences strong and long-lasting ground-level temperature inversions during the winter, trapping pollutants close to the ground. Third, Fairbanks is in a river valley, which exacerbates the strength of inversions and decreases windspeeds, further reducing pollutant dispersion. As discussed in later sections, it is not the coldest days that have the highest CO concentrations. At very low temperatures (down to −50 °F), the use of engine-block heaters, which reduce cold-start emissions, and the presence of 1   In the Fairbanks case, the committee concluded that large stationary sources, such as power plants and refineries, do not contribute substantially to high CO concentrations in the areas around the CO monitors. Thus, the committee did not consider stationary-source controls in this interim report. However, the importance of stationary sources and their controls will be considered as the committee produces its final report.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report ice fog (from water vapor that is emitted and freezes), which raises the inversion height, help prevent accumulation of high ambient CO concentrations. The current problem with high ambient CO concentrations tends to occur (five out of the last six exceedances of the 8-h National Ambient Air Quality Standard [NAAQS]) at temperatures of roughly 0–20°F, when ice fog is not present and the use of engine-block heaters is not necessary for starting. This chapter discusses the CO problem in Fairbanks, beginning with the physical and demographic characteristics of the area; the management strategies used to control the CO problem; and a simple air quality model for coupling meteorology and vehicle emissions. Lessons learned from the Fairbanks case are summarized in the committee’s findings and recommendations in the Summary of this interim report and serve as a basis for the development of a final report that will include assessments of other CO problem areas. The specific health effects of exposure to CO in Fairbanks are not discussed, because no human exposure or epidemiological data were available. GEOGRAPHICAL, METEOROLOGICAL, AND SOCIETAL CONTEXT Physical Setting Fairbanks is in a floodplain on the north shore of the Tanana River, just upstream of its confluence with the Chena River. The city is open to the south and southeast, with a very gradual slope in that general direction from the Tanana River up to the foothills of the Alaska Range, some 75 km away. To the west and north, residential areas extend into the Yukon-Tanana uplands a few hundred meters above the city. Level ground extends some 35 km to the east except for Birch Hill northeast of town. Figure 2–1 shows a topographical representation of Fairbanks and the surrounding area. The meteorology is fairly typical of interior Alaska and other continental high latitudes. In the typical winter pattern, anticyclones (high-pressure systems), weak lows, and other weak pressure fields dominate the weather, all with moderate windspeeds. Strong pressure gradients and cyclonic (low-pressure) systems are unusual in winter. For 1996 through 2001, all exceedances of the 8-h CO health standard in Fairbanks have occurred with southeasterly winds aloft. These winds, which travel over the Alaska Range, are associated with counterclockwise geostrophic flow around a low-pressure system in or near the Gulf of Alaska.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–1 Map of topography in the Fairbanks area. Ground-based inversions of considerable strength (typically a few degrees Celsius per 100m but sometimes much stronger) topped by weaker inversions reaching as high as about 1–2 km are normal in winter and can occur anytime during the year. A surface inversion due to net energy loss from the surface occurs in the few meters closest to the ground, although the weaker inversion topping it may be caused by subsidence or transport of warmer air aloft. The combination of high albedo (reflection of sunlight due to snow cover) and the low solar elevation (failure of the sun to rise high in the sky) characteristic of northern latitudes in winter creates little heating of the ground and weak vertical mixing between the surface and overlying air. With clear skies and low absolute water-vapor content, the ground loses considerably more energy by radiation to space than it is able to absorb from the sun. Those surface conditions may persist in Fairbanks for days, and the situation is exacerbated by the

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report insulation provided by high-albedo snow cover. Although such an inversion may weaken or even dissipate during the middle of the day, it tends to become reestablished or strengthened throughout the late afternoon and into the night. The upper part of the inversion appears to be associated with subsiding (downward) southeasterly flow crossing the Alaska Range. Although the lack of surface warming in winter is common, it now appears that recent exceedances occurred with the upper-level inversion also in place. Observations in Fairbanks and Anchorage suggest that solar heating of the ground alone is insufficient to produce any substantial mixing layer with solar elevations less than 6° over bare ground or 12–15° over deep snow (Bowling 1985). As shown in Figure 2–2, solar elevations in winter are very low in Fairbanks, even at solar noon.2 Thus, solar heating seldom breaks the ground inversion from November to February, even without snow cover. Clear skies are much more likely in January through March than in November (Figure 2– 3); the low incoming solar heating may help to explain the greater frequency of exceedances during these months. Inversion measurements (temperature as a function of height) provide an indication of the strength of a ground-level inversion. Ordinary sounding measurements are not sufficient to resolve strong inversions near the ground, although they do at times record inversion strengths larger than 10°C/100 m. In most areas, inversions of several degrees per 100 m would be considered strong. Three limited datasets support such high inversion strengths: Traverse data of temperature differences between vehicle-mounted thermistors 0.5 m and 2 m above the ground showed inversions of about 1°C/1.5 m (67°C/100 m) in ice fog and as much as 3°C/1.5 m (200°C/100 m) in clear air (Bowling and Benson 1978). Background inversions in the first 2 m are obviously extremely strong, but nonlinearity in inversion strength makes it difficult to extrapolate this observation to higher altitudes. Such strong low-level ground inversions are unlikely to persist in the downtown area, because heat is normally added with CO in automobile exhaust plumes and mechanical turbulence associated with vehicles can cause further mixing. Three tethered-balloon measurements in the downtown area during high-CO conditions indicated a “mixing layer” from the ground up to 6–30 m 2   At 40°N latitude (running from the middle of New Jersey to northern California), the sun is 6° above the horizon about a half-hour after sunrise; in Fairbanks, at 65° N latitude, the sun does not reach this elevation from the middle of November to late January, even at solar noon. The Arctic Circle is at 66.5°N latitude.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–2 Solar elevation for Fairbanks in degrees as a function of time of day and day of year. (Bowling 1986). Background measurements made outside the downtown area with the same tethered balloons showed ground-based inversions of 10–40°C/ 100 m. Borough measurements of temperatures 3, 10, and 23 m above the ground are taken on a meteorological tower in the downtown area (Figure 2– 4). Inversion strengths of as much as about 30–40°C/100 m have been measured on the tower during episodes. For example, Figure 2–5 shows the inversion strengths measured on the tower during an exceedance of the CO standard in November 1999. The inversion, measured between 10 and 23 m altitude, averaged 21°C/100 m on November 19, 1999, with maximum hourly values of about 30°C/100 m. Inversions of the strength observed in Fairbanks cause the buildup of CO in several ways, with stronger inversions associated with higher CO concentrations. First, such inversions strongly inhibit vertical mixing. Second, inversions inhibit momentum transfer, thereby preventing winds aloft from reach-

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–3 Fractions of clear (0–2 oktas [eighths of sky]), partly cloudy (3–6 oktas), and cloudy (7–8 oktas) by month, based on NOAA 1961–1990 averages. ing the surface—a particular problem when warm air advected from the south across the Alaska Range does not reach the ground in Fairbanks. The surface winds that exist in Fairbanks during winter are probably controlled by gravity drainage from higher elevations to the north, often in a direction opposite to that of winds just a few hundred meters higher. These surface winds are difficult to characterize in the downtown area; 90% of the time, winter windspeeds are at or below the minimal starting speed (3 mph) of a standard anemometer. Finally, the combination of strong ground-level inversions with surface temperatures and winds that probably vary widely over horizontal distances of 0.1 km suggests that inversion strength (and probably mixing height) may be highly variable spatially (Holmgren et al. 1975). February has been the month with the most exceedances over the last 5 y. In February, the reestablishment (or strengthening) of ground inversions occurs rapidly near 5:00 p.m., trapping pollutants emitted when sunset and maximal traffic occur near the same time (Bowling 1984, 1986). The reestablish-

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–4 Map of downtown Fairbanks. Asterisks indicate CO monitoring sites; triangles indicate where vehicles are counted. The meteorological tower is indicated by a dot. ment of the ground-level inversion in late afternoon and early evening is clearly visible in the inversion-strength measurements made from the meteorological tower in downtown Fairbanks (S.A.Bowling, Fairbanks Meteorology, in preparation, 2002).

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–5 CO concentrations (ppm) and inversion strengths (degrees Celsius/100 m) measured during an exceedance of the CO standard on November 19, 1999. The inversion strengths were calculated from temperature measurements taken on the meteorological tower. The upper inversion was measured between 10m and 23 m altitude and the lower inversion was measured between 3 m and 10m. Demographics As pointed out earlier, Fairbanks is a small city to have such a severe air pollution problem. Its 2000 population of only 30,224 was spread over an area of 32.8 mi2. The entire Fairbanks North Star Borough had a population of 83,632 in an area of 7,361 mi2, growing from 77,720 in 1990. Over the last decade, the city population has been nearly constant, most of the growth being in the outlying parts of the borough. The borough’s population can be greatly affected by large construction projects and government spending. For example, the construction of the Trans-Alaskan Oil Pipeline caused the borough’s population to rise from 45,864 in 1970 to 72,037 in 1976. After completion of the pipeline, the population fell to 51,659 in 1981. Spending of state oil revenue by local and state government helped the borough’s population rebound to 75,079 in 1985. The economy of the borough depends heavily on federal, state, and local government sectors, which combine to account for 30% of jobs. Some 20%

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report of the borough’s population are military or military dependents. Retail trade and services account for 18% and 26%, respectively, reflecting the importance of Fairbanks as a tourist center and service center for most of the state north of the Alaska Range. The population is relatively young. In 2000, the median age of the borough’s residents was 29.5 y, compared with 32.4 y in the state and 35.3 y in the United States as a whole. Only 9.5% of households had members 65 y old or older, compared with 11.9% in the state and 23.4% in the United States as a whole. Male residents made up 52.2% of the population in 2000, compared with 51.7% in the state and 49.1% in the United States (U.S. Census Bureau 2000). Local transportation is largely by motor vehicle. Traffic counts show that vehicle-miles traveled (VMT) in the area increased from an estimated average of 665,398 mi/day (d) in 1990 to 752,992 mi/d in 2000 (ADEC 2001a). The VMT growth rate exceeds the growth in population of the borough, indicating that residents are driving more on a per capita basis, a trend observed throughout the United States. The average hourly traffic counts observed at three locations in Fairbanks (see Figure 2–4) are shown in Table 2–1 for the CO seasons of 1995–1996 through 2000–2001. Traffic has increased by 6–7% on the Steese Expressway, decreased by 3% on Airport Way, and remained fairly constant at Cushman Street and the Chena River in downtown Fairbanks. Increasing residential, commercial, and retail development outside the city of Fairbanks probably contributes to the increase in VMT and in traffic on the Steese Expressway; however, little additional data are available to confirm this hypothesis. Changing traffic patterns in the city will change the spatial distribution of emissions and ambient CO. CO DATA The borough has measured CO in downtown Fairbanks since 1972. The locations of the three monitoring sites currently in operation are shown in Figure 2–4. Although the sites were run year-round in the past, they are now operated only from October 1 through March 31 (ADEC 2001a). The measurements are made with commercially available monitors that have an uncertainty on the order of 0.1 ppm and a detection limit of around 0.1 ppm. Hourly average concentrations are recorded. The borough provided these hourly data to the committee for the CO seasons (November through

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report TABLE 2–1 Average Hourly Traffic Counts for Fairbanks, Alaska, During Winter Months   Airport Way Steese Expressway Cushman St. and Chena River Nov. 1995–Feb. 1996 644 758 557 Nov. 1996–Feb. 1997 641 747 558 Nov. 1997–Feb. 1998 648 778 568 Nov. 1998–Feb. 1999 636 779 536 Nov. 1999–Feb. 2000 615 786 539 Nov. 2000–Feb. 2001 626 814 575 % change 95–96 to 00–01 −2.75 7.31 3.16 % change 95–97 to 99–01a −3.35 6.27 −0.18 aReferring to the average of the last two seasons (Nov. 1999–Feb.2000 and Nov. 2000– Feb. 2001) compared with the first two seasons (Nov. 1995–Feb. 1996 and Nov. 1996– Feb. 1997), reducing the possible effect of one unusual year. February) of 1995–1996 through 2000–2001. As described below, a detailed trend analysis of the running 8-h average concentrations computed from these data (a total of 47,980 data points) was conducted by the committee to illustrate statistical techniques for assessing progress in reducing CO concentrations. Although the three monitoring sites satisfy EPA’s monitoring guidelines, they are located quite close together and therefore do not provide much information about the spatial distribution of CO concentrations across the borough. The following analysis of observations at these three sites is limited in that respect, and similar analyses should be conducted in the future with data that are more spatially representative. The Alaska Department of Environmental Conservation (ADEC) has conducted two saturation studies—during the winters of 1999–2000 and 2000–2001—to begin improving the characterization of CO on the regional scale (Guay 2001). Exceedances in Fairbanks As in most areas, the number of exceedances of the 8-h NAAQS for CO

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report in Fairbanks has decreased over the last 25 y (Figure 2–6).3 During the early years of regular CO monitoring in Fairbanks, there were more than 130 d with exceedances per year, distributed over all months. The numbers dropped rapidly until 1987–1988 after which they decreased more slowly. Exceedances since then have occurred entirely during the winter months of November through March. As of January 2002, there have been no exceedances since February 2000. As shown in Figures 2–7 and 2–8, exceedances vary by day of the week and hour of the day. Figure 2–7 shows that substantially fewer exceedances occur on weekend days; this reflects the decreased use of vehicles on these days and possibly the timing of vehicle operations. Figure 2–8 shows the number of monitor hours4 with 1-h CO concentrations over 9 ppm for the six most recent exceedance days for each hour of the day. This figure shows that high CO concentrations have recently been most likely around 5:00–6:00 p.m. Five of the six most recent exceedance days have been in February. As described in the previous section, strong temperature inversions tend to occur around sundown when people are leaving work and starting their cars. Temperatures on seven of the eight most recent exceedance days ranged from −4.2°F to 28.6°F. The potential importance of exceedances occurring in the “relative warmth” of about 0–20°F is that, at this temperature range, vehicles do not usually need to be plugged in to start easily. It is thought that the high emissions from cold starts of vehicles that had not been plugged in contributed to the exceedances. CO exceedances in the borough are a function of location. Of the last six exceedances, four had the highest measured CO concentrations at the Post Office monitor (Courthouse Square), one at the Hunter School, and one at the State Office Building. Studies by the ADEC Air Monitoring Section, which deployed temporary monitors over a wider area, indicate that CO concentra- 3   The year here is defined to cover the winter season and is the period from July 1 of one calendar year to June 30 of the following year. A violation of the NAAQS occurs when the standard is exceeded for the second time and all subsequent times during such a year. 4   If the 1-h average CO concentration was over 9.0 ppm at two of the three monitors in downtown Fairbanks at a particular hour, that would count as two monitor hours.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–10 Illustration of the depth and extent of mixing that wind machines can achieve. Solid lines indicate temperature change (in degrees Fahrenheit) due to induced mixing. Source: Crawford 1965. Reprinted with permission from Meteorological Monographs; copyright 1965, American Meteorological Society. surrogate gas, could be input at the lowest level of the physical model. Alternatively, the spatial scale of the downtown area is small enough to use detailed fluid-dynamics models (such as large-eddy simulation) to conduct numerical experiments on the effect of wind turbines in mixing air and the effect of buildings in dissipating air movement. As discussed previously, the spatial extent of the high CO concentrations found in Fairbanks needs to be known to evaluate the value of such an approach. Furthermore, cost, noise, and safety considerations could be important in such a strategy and must be assessed. Because of these concerns, the committee feels that other emissions-control strategies should be pursued before further investigating a strategy of mechanical vertical mixing. SIMPLE BOX MODEL OF THE BOROUGH Various models that have been used for attainment demonstrations, research purposes, and air quality forecasting were discussed in Chapter 1. In

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report its most recent SIP, Fairbanks used a statistical rollback model to estimate the emissions reductions needed to reach attainment (ADEC 2001a). Although more sophisticated tools (such as dispersion and urban-airshed models) are available, Fairbanks claimed in its SIP that it did not have sufficient information about its meteorology or emissions inventories to use them. Nevertheless, the borough acknowledges the limitations of the rollback approach and is committed to pursuing the development of dispersion modeling capabilities. In addition to improving the meteorological data and emissions inventory, the borough will also have to address the limitations of widely available dispersion models under conditions of severe temperature inversions and very low windspeeds (Bowling 1985). In the meantime, the committee has developed a simple box model for the borough nonattainment area to gain some insight into the roles played by emissions and meteorological variables. This type of model can also be used to test various emissions scenarios and possibly for attainment demonstration. The model solves for the time-varying CO concentration as a function of CO emissions and meteorological conditions in the nonattainment area, about 88 km2—6.4 km north-south by 13.8 km east-west, centered on downtown Fairbanks. Uniform conditions are assumed for the volume of the box. Specifically, CO concentrations are assumed to be well mixed, and meteorological variables—windspeed and wind direction, temperature, pressure, and mixinglayer height—are constant throughout the box. It is known from observations that those quantities vary over the nonattainment area and probably have a substantial effect on local CO concentrations, but the variations are not resolved here. In addition, it is assumed that there is negligible CO in the air entering the box—a reasonable assumption for a box of this size and for a city far from other CO sources. To demonstrate how a simple box model works, the results are presented for an average 72-h CO event in which the 9-ppm standard is exceeded during the middle day (hours 24–48). The event was calculated by averaging hourly observations of CO concentrations, vehicle counts, and meteorological variables during five of the last six exceedances in Fairbanks. The five events averaged were centered on 2/23/98, 2/11/99, 2/16/99, 11/19/99, and 2/8/00; an exceedance of the 8-h CO standard also occurred on 2/24/98. The highest 8-h average was observed four of the five times at the Post Office and once (2/11/99) at the Hunter School. A more rigorous application of the model would be to simulate a single exceedance.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–11 Windspeed (km/h) measured at 10 m height at the meteorological tower in downtown Fairbanks. The hourly values are averaged over the five exceedance events. The Data The model inputs include average hourly values of meteorological variables and vehicle counts and average daily emissions rates. Temperature, pressure, and windspeed (Figure 2–11) were measured at a 10-m altitude at the meteorological tower in downtown Fairbanks.12 The temperature-inversion strength (−∆T/∆z) is computed from temperature measurements at 3 and 10m (Figure 2–12). The air density is calculated from the hourly average temperature and pressure, assuming that air is an ideal gas. Variations in these meteorological parameters might be much larger for the original measurements than for the averages shown here. Emissions of CO in the box are assumed to be 8.4 tpd from stationary sources and 16.5 tpd from mobile sources, fol- 12   The CO, meteorological, and vehicle-count data were kindly supplied by Paul Rossow of the borough Air Quality Office and Paul Prusak of the Alaska State Department of Transportation.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–12 Inversion strength (°C/100 m) computed as the temperature difference between 3 and 10 m heights measured at the meteorological tower in downtown Fairbanks. The hourly values are averaged over the five exceedance events. lowing the values estimated by Sierra Research in a 1999 CO emissions inventory.13 Mobile sources in that inventory are estimated to be 7.0 tpd from cold-start and initial-idle emissions and 9.5 tpd from traveling emissions. Observed vehicle counts in downtown Fairbanks (at locations shown in Figure 2–4) are used to convert the daily mobile-source emissions rate to hourly emissions (Figure 2–13). The model performance is evaluated on the basis of CO measurements made at the three monitoring sites in downtown Fairbanks. The CO observations are averaged hourly and over the three monitoring sites. The Model The mass of CO in a box representing the nonattainment area can be determined with a mass-balance expression (assuming that CO concentrations 13   Data provided by Robert Dulla, Sierra Research.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–13 CO emissions (E[t]) in tons per hour (tph) for the average exceedance event. outside the box are negligible): (1) where t=time (h); m(t)=mass of CO in box (ton); E(t)=emissions of CO in box (tph); S(t)=windspeed (km/h); and L=length of box (km) in direction of wind. Using the relationship between mass and concentration: m=c(t)ρ(t)V(t), (2) where c(t)=concentration (ppm);

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report ρ(t)=air density in the box (g/km3); and V(t)=volume of the box (km3). The mass-balance equation can be expressed in units of concentration: (3) Finally, the concentration of CO at any time c(t+∆t) can be estimated with Equation 3 as follows: (4) We solved Equation 4 by using a time increment, ∆t, of 1 h. Solving Equation 4 requires knowing or assuming time-dependent values of the variables on the right side. As discussed previously, observed values were available for windspeed, S(t); air density, ρ(t) (computed from temperature and pressure); emissions, E(t); and an initial value for CO concentration, c(t). The box length, L, was assumed to be 6.4 km, the length of the box from north to south, in the direction of the incoming wind. The major challenge was devising a function for the volume V(t) that is reasonable and yields a calculated CO concentration that varies with time in a way similar to the observed hourly average. Because the area of the box is constant, the variations in the volume are based on variations in the height of the mixing layer, H(t). The committee assumes that H(t) depends on the observed inversion strength ∆T/∆z) in the following way: Stable conditions: ∆T/∆z<0°C/100 m H=8 m Mild inversion conditions: 0°C/100m<∆T/∆z<11°C/100 m H=5 m

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report Severe inversion conditions: ∆T/∆z>11°C/100 m H=2.25 m The mixing heights assigned that way are shown in Figure 2–14. They are highest in the early afternoon, when the atmosphere can become unstable (positive lapse rate, negative inversion) because of solar heating. Mixing height during severe inversion conditions is determined by a combination of turbulent mixing (mostly from high-speed exhaust gas leaving vehicle tailpipes and vehicle motion under low-windspeed conditions) and convective mixing from the rising, initially hot exhaust gas. Thus, the mixing height is rather heterogeneous over the nonattainment area, and it is difficult to determine directly from observations a single height to use in the box model. For these reasons, the mixing heights used in the model were derived to best match the observed CO concentrations. As such, the model provides a useful tool for estimating an effective mixing height based on the constraints of estimated emissions and observed surface CO concentrations. Results and Discussion Concentrations of CO predicted by the model are compared with those of the average observed event in Figure 2–15. In general, the model does a good job of reproducing the magnitude of the CO concentration and the diurnal variation over the 72-h event. To simulate the high CO concentrations observed during the exceedance, the mixed-layer height needed to be low (2.25 m, about 7.4 ft) during severe inversions. Marginally less wind and more traffic on the average exceedance day (see Figures 2–11 and 2–13) did not yield the observed doubling of CO concentrations compared with the days before and after the exceedance. A box height of 2.25 m may be too low if the spatial variability in CO emissions invalidates the assumption of a well-mixed box. In particular, if CO emissions are greater in the downtown area (where the three CO monitors are) than elsewhere in the modeled area, the mixing height downtown could be higher. The lowest CO concentrations (about 1 ppm) are observed in the early morning hours, between about 2 and 6 a.m. The model shows concentrations during those times about twice as high (about 2 ppm), suggesting that the stationary-source CO emissions used in the model (0.35 tph) are too high during these hours. The discrepancy could be due to insufficient horizontal dispersion in the model, but that does not appear to be the case. With an

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–14 The box heights (m) used for the model fit shown in Figure 2–15. assumed box length in the direction of the wind of 6.4 km and an average windspeed of 2.6 km/h, the time constant for horizontal dispersion in the model is about 2.5 h, certainly fast enough to deplete CO to low concentrations during the early morning hours, when emissions are low. Although the model generally yields the correct shape of the diurnal variations of CO concentration, the calculated afternoon maximums lag those observed at about 5 p.m. The highest CO emissions in the model are between 5 and 6 p.m., when vehicle traffic is maximal. However, idling emissions in the downtown area before people leave work to go home have the effect of shifting the observed emissions maximum 30–60 min earlier than would be expected on the basis of vehicle counts. Improving the model requires an emissions inventory more detailed in time and space, particularly with respect to plug-in and idling behavior. The box model described here provides an illustrative example of how this technique can be applied to better understand the roles of emissions and meteorology in high-CO episodes. The model could be adapted in a number of ways. As mentioned before, a specific exceedance event, rather than averaged events as shown here, could be analyzed. Another modification could be to consider a different surface area of the control volume. For example, the

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report FIGURE 2–15 Comparison of simulated and observed CO concentrations (ppm) for the average exceedance event. model could simulate only downtown Fairbanks. For this case, the assumption that no CO is transported into the box would need to be reassessed. A vertically resolved, one-dimensional model also could be used. That approach could help improve the understanding of how stationary sources affect surface CO concentrations. In the short term, the committee recommends that the borough use a box-model approach for air quality planning and for demonstrating attainment. Unlike less physically comprehensive rollback models, a box model can provide information on how temporal changes in emissions and meteorology affect CO concentrations. Unlike more physically comprehensive airshed modeling, the monitoring data to implement a box model are readily available. The box-model approach used by the borough need not be identical to the one developed here; some of the modifications described above should be considered. The borough may want to use a box model in conjunction with a model that focuses on an area immediately surrounding a monitoring site, such as the probabilistic rollback models described in Chapter 1; however, the time and effort needed to develop inventories for these local modeling approaches may be better spent developing more physically comprehensive regional modeling

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report capabilities for the Fairbanks area. For the future, the committee recommends that the data needed to initiate and validate airshed modeling be gathered and that more physically comprehensive models be used. BOX 2–9 Recommendations: Improving Ambient-CO Modeling in the Borough In the near term, Alaska should use a simple box-model approach, which simulates the effects of emissions and meteorology in a well-mixed control volume, for air quality planning purposes in Fairbanks. Such an approach could provide greater insights into the effects of the timing of CO emissions and meteorological variables. The relative contributions of mobile and stationary sources to CO episodes could also be assessed with this type of model. Enhanced data-collection efforts are required to support this and more sophisticated modeling efforts. Improvements in the statistical forecasting approach used by the borough might help in forecasting episodes of high CO concentrations. More work is also needed to develop, apply, and evaluate more sophisticated, physically comprehensive models that would simulate how CO concentrations vary with time and space over the entire borough. Such models could be used for planning, forecasting, and assessing human exposure to high CO concentrations. It is important that model development and testing be specific to the extreme conditions that occur in Fairbanks. Model development must occur in conceit with improved monitoring to enable model evaluation. SUMMARY Fairbanks presents a challenge for air quality management. It constitutes an extreme example of the roles of topography and meteorology in producing air quality problems. In winter, the area is subject to extreme ground-level inversions. Although no industries in the region emit large amounts of CO or other pollutants, the inversions are extremely effective in trapping the products of incomplete combustion that are emitted near ground level, particularly CO from vehicles. Monitors in the downtown area have measured as many as 130 d/y with exceedances of the 8-h CO standard. The situation has greatly improved over the last 30 y. No exceedances have occurred over the last 2 y.

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The Ongoing Challenge of Managing Carbon Monoxide Pollution in Fairbanks, Alaska: Interim Report The days currently most susceptible to high CO concentrations in Fairbanks are not the coldest days. Meteorological conditions in Fairbanks lead to exceedances of the CO health standard primarily when the ambient temperatures typically are −20 to 20°F. The combination of human behavior and motor-vehicle technology further narrows the primary temperatures of concern to 0–20°F. Air quality planning and controls should focus on such days. The committee has identified a number of options available to the borough and Alaska to reduce CO emissions further. Among them, improving the vehicle I/M program has high priority, as does an enhanced plug-in program. Use of low-sulfur fuel would also help. An enhanced alert-day program might provide needed reductions on critical days. And there is a need to inform and educate the community better about the health effects of exposure to air pollutants and ways to improve air quality. Improved monitoring and characterization of CO concentrations in the area are needed. Modeling CO in the borough is a serious challenge, but it can be helpful in planning and in forecasting possible exceedance days. For now, a simple box model is suggested for planning purposes; more sophisticated models either are inappropriate for the conditions or require much more extensive monitoring data. Statistical models can be used, for now, to help in forecasting, but work is needed to improve them. More research with comprehensive models is needed for future application for both forecasting and planning. These more comprehensive models will also require improvements in the emissions inventory, including the nonroad, area, and point sources that contribute to high-CO episodes.