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The Columbia River Gorge (“the Gorge”) is a narrow gap in the Cascade Mountains of Washington and Oregon with outstand-ing natural features, including steep for-ested mountainsides and numerous high waterfalls. Cut by the flow of the Columbia River, the Gorge is approximately 190 km in length, 5 km wide (at river level), and 1 km deep. The elevations at river level are less than 100 m above mean sea level. The Portland, OR/Vancouver, WA metro-politan area, with a population of approxi-mately 2 million, is located at the western terminus of the Gorge.

This article briefly describes a several-year study of the causes of visibility impairment in the Gorge and the devel-opment of a strategy to improve visibility in the area. For detailed information, the reader is encouraged to refer to the journal articles and project reports referenced in this article.

In 1986, the U.S. Congress afforded special protection to this area when it passed the Columbia River Gorge National Scenic Area Act. Pursuant to this act, the Columbia River Gorge Commission was established to develop and enforce policies and programs that protect and enhance the scenic, natural, cultural, and recreational resources of the Gorge, while encouraging compatible growth within existing urban areas of the Gorge and allowing economic development outside urban areas consistent with resource protection. It is noteworthy that the people of the Pacific Northwest have decided to take steps beyond what is required by any law or regulation to understand and improve visibility in the Gorge. In 2001, a technical study plan focusing on atmospheric visibility (haze) was developed that included comprehensive data collection and analysis and modeling components.

Haze is caused mainly by scattering and absorption of light by particles suspended in the atmosphere (i.e., aero-sols). Numerous urban, industrial, and natural sources in the region emit gaseous or particulate air pollution that may contribute to light scattering and absorption in the Gorge. These sources include the Portland, OR/Vancou-ver, WA metropolitan area; numerous pulp and paper mills (mainly west of the Gorge); coal-fired electric gener-ating units (the largest regional source of sulfur dioxide is the Boardman plant, east of the Gorge); highway, railroad, and barge vehicles that travel through the Gorge; and

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Mark C. Green, Ph.D., is a research professor at the Desert Research Institute, Reno, NV; Marc L. Pitchford, Ph.D., is a meteorologist with the National Oceanic and Atmospheric Administration’s Air Resources Laboratory, Las Vegas, NV; and Chris Emery is a manager with ENVIRON, Novato, CA. E-mail: mark.green@dri.edu.

The Columbia River Gorge Air Quality and Visibility Studyby Mark Green, Marc Pitchford, and Chris Emery

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small cities in the Gorge, such as Hood River and The Dalles. These sources emit substantial quantities of primary particulate mat-ter, which scatters light as well as significant quantities of sulfur dioxide and nitro-gen oxides that react with other compounds to form light-scattering secondary particles (e.g., sulfate and nitrate). Other significant sources of haze include forest fires, biogenic emis-sions, and ammonia from confined animal feeding operations, agricultural fertilizer application, and other sources. The main objective of the technical study was to identify sources, source categories, and/or geographical regions that contributed to haze in the Gorge and, to the extent practicable, quantify those impacts.

Overview of Study MethodsThe initial phase of the study was the haze gradient study,1,2 which was conducted from July 2003 to Febru-ary 2005. The study used hourly measurements of light

scattering and surface meteorology at nine locations through the Gorge (Figure 1 shows the locations of study monitoring sites) to look at spatial and temporal patterns of light scattering and how it varied with wind speed and direction. Winds within the Gorge are channeled by the terrain so they blow either upriver (west-to-east) or downriver (east-to-west), depending on seasonal forc-ings (i.e., winds blow from high to low pressure). Thus, much information about the transport of haze through the Gorge can be ascertained by having monitors along a line through the Gorge.

Figure 1. Location of study monitoring sites. Note: The Portland metropolitan area is located at the bottom left edge of the figure.

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The second phase of the study was conducted con-current to the initial phase. Within the 20-month haze gradient study were embedded three periods with addi-tional monitoring: December 2003–February 2004, July–November 2004, and December 2004–February 2005. The additional measurements included high time resolved measurements of sulfate, nitrate, and carbon aerosol at the two Interagency Monitoring of Protected Visual Environments (IMPROVE) sites in the Gorge (Mt.

Zion and Wishram) and at a mid-Gorge site at Bonn-eville Dam. The additional measurements were used for qualitative assessment of episodes and for evaluation of the chemical transport model simulations.

Following the field study, chemical transport model-ing was conducted using the Comprehensive Air Qual-ity Model with Extensions (CAMx)3 for two haze epi-sodes (August and November 2004). The modeling used a 2004 emissions inventory prepared for the study and a 2018 emissions projection to consider current source contributions to haze and to project future haze levels.4,5

Summary of Study ResultsThe study design provided for in-depth monitoring and

wide geographical coverage for emissions inventory and modeling analyses for the Gorge area. The study provided insight on both man-made and natural sources of haze. Of most interest are the man-made emission contributions, as those are the focus of emission reduction strategies that can lead to reduced haze in the Gorge. During certain periods, natural and very distant sources contributed significantly to haze in the Gorge.

The haze gradient study found that the highest haze conditions occurred in winter when flow was easterly (i.e., from the east) and that haze levels decreased with distance into the Gorge. This suggests that sources east of the Gorge were mainly responsible for the haze. In summer, winds are nearly always from the west and haze levels are highest in the western part of the Gorge and decrease with distance eastward due to vertical and horizontal dilution. The summer data clearly show an influence of the Portland, OR/Vancou-ver, WA metropolitan area as a “slug” of hazy air gets transported from west to east through the Gorge when the winds increase in mid-to-late

Figure 2. Diurnal variation in particle light scattering (bsp) for four western and central Gorge sites and one eastern Gorge site, summer (June–August).

Note: The eastern propagation of higher bsp levels late morning to early afternoon at the western and central Gorge sites. Sites noted in legend are ordered from west to east.

Figure 3. Daily average light scattering by particles (Mm-1) at the Gorge study monitoring sites for the November 2004 episode.

Note: Sites in legend are ordered from west to east.

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11/1 11/3 11/5 11/7 11/9 11/11 11/13 11/15 11/17 11/19

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Sauvie IsSteigerwaldMt ZionStrunk RdBonnevilleMemaloose7 mileWishramTowal Rd

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Figure 4. View looking west into the gorge from Wishram for (a) pristine condi-tions and (b) November 2004 episode day (1200 PST 10 November 2004, bsp= 267 Mm-1). The episode was characterized by daily average bsp over 200 Mm-1, high relative humidity, and high concentrations of nitrate, sulfate, and organic carbon aerosols.

morning in summer (see Figure 2). On some occasions when winds shifted in direction during the day, analysis of monitoring data indicated sources within the Gorge contributing to haze.

The August 2004 episode saw elevated levels of organic carbon and sulfate aerosols. The November 2004 episode saw high levels of nitrate, sulfate, and organic carbon. Figure 3 shows a time series of daily average particle light scattering (bsp) at the nephelometer sites. Haze levels were highest at the eastern sites and decreased to the west. Figure 4 shows photographs from Wishram at the height of the episode and during pristine conditions.

The CAMx chemical transport model results indicated that for the November episode most of the haze was caused by man-made sources within the region (Oregon and Washington), while for the August episode a majority of the haze was due to natural (vegetation and wildfires) and more distant sources. Figure 5 summarizes the modeled source impacts at the western Gorge Mt. Zion site for the August episode and the eastern Gorge Wishram site for the November episode. Of the 41% impact at Mt. Zion from local man-made sources in August, approximately half was due to the Portland area, a finding consistent with the summer diurnal haze pattern shown in Figure 2. At Wishram for the November episode, 57% of the haze came from sources east of the Gorge with 32% from electric

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generating units (EGUs). The dominant EGU east of the Gorge is the Portland General Electric (PGE) coal-fired plant near Boardman, OR. Based on a weight of evidence approach considering the modeling, data analysis, and emissions inventories, the PGE Boardman plant is respon-sible for a substantial fraction of the haze in the Gorge during certain winter conditions with easterly winds. It should be noted that ammonia emissions are required to achieve the high levels of winter time nitrate particulate. Confined animal feeding operations (CAFOs), including dairies, are expected to comprise much of the ammonia emissions east of the Gorge. Substantially reducing emis-sions from the PGE Boardman plant and CAFOs would be expected to quite noticeably improve visibility in the Gorge on some winter days with easterly winds.

The modeling for 2018, which accounts for future state and federal emission reduction strategies, suggested little change in summer haze (1–3% improvement) and approximately 10% improvement in winter haze in spite of a projection of increased population. Receptor modeling using Positive Matrix Factorization (PMF) for

Figure 5. Source contributions to haze during August and November 2004 episodes at Mt. Zion (August) and Wishram (November) from CAMx modeling.

all 2003–2004 samples6,7 showed broadly similar results to the CAMx modeling.5 PMF attributed much of the haze to two broad categories—secondary sulfate and secondary nitrate factors—potentially containing many source types. PMF did specifically identify paper mills, responsible for 7% of haze at Mt. Zion and oil combustion (mainly shipping) responsible for 10% of Mt. Zion haze averaged over the two years.

Strategy for Accomplishing Visibility ImprovementsThe Oregon Department of Environmental Quality (ODEQ) and the Southwest (Washington) Clean Air Agency (SWCAA) are developing a strategy to manage visibility in the Gorge.8 The proposed strat-egy is based on a four-pronged approach: continued implementation of existing strategies; development of new strategies based upon current regulations; development of future strategies based upon future regulations; and a consensus-based Gorge Solutions Initiative. The agencies propose to report back to the Columbia River Gorge Commission on progress in improving Gorge visibility in 2013. The Regional Haze Rule will provide some benefit because there are Federal Class I areas near the Gorge, including Mt. Hood Wilderness (30 km south) and Mt. Adams Wilderness (50 km north). However, in winter, the Class I areas are by and large located at a higher elevation than the haze that affects the Gorge; thus, emissions affecting the Gorge may not affect the Class I areas at times. Also of note is a commitment by ODEQ to recommend emission controls for the PGE Boardman plant in 2008 as part of the Best Available Retrofit Technology analysis for regional haze. Furthermore, the Oregon Dairy Task Force is exploring options for reducing emissions and will issue a report by July 2008. em

References1. Green, M.; Xu, J. Transport of Atmospheric Aerosol by Gap Winds in the

Columbia River Gorge; J. Climate Appl. Meteorol. 2008, 47, 15-26.2. Green, M.; Adhikari, N.; Xu, J.; Nikolich, G. Columbia River Gorge Haze Gra-

dient Study. Prepared for the Southwest Clean Air Agency, Vancouver, WA, 2006; available online at www.swcleanair.org/gorgereports.html (accessed March 2008).

3. User’s Guide: Comprehensive Air quality Model with Extensions, Version 4.40; ENVIRON International Corporation Inc., Novato, CA, 2006; available online at www.camx.com.

4. Emery, C.; Tai, E.; Nopmongcol, U.; Johnson, J.; Morris, R. Modeling Analyses Conducted for the Columbia River Gorge National Scenic Area Air Quality Study: Final Report (2007); available online at www.swcleanair.org/gorgereports.html (accessed March 2008).

5. Pitchford, M.; Green, M.; Morris, R.; Emery, C.; Sakata, R.; Swab, C.; Mai-rose, P. Columbia River Gorge Air Quality Study: Science Summary Report (2008); available online at www.swcleanair.org/gorgereports.html (accessed March 2008).

6. Green, M.; Xu, J.; Adhikari, N.; Nikolich, G. Causes of Haze in the Gorge (CoHaGo). Prepared for the Southwest Clean Air Agency, Vancouver, WA, 2006; available online at www.swcleanair.org/gorgereports.html (accessed March 2008).

7. Green, M.; Xu, J. Causes of Haze in the Columbia River Gorge; J. Air & Waste Manage. Assoc. 2007, 57, 947-958.

8. Columbia River Gorge Air Study and Strategy, Draft Report, February 29, 2008; Oregon Department of Environmental Quality and Southwest Clean Air Agency, 2008; available online at www.deq.state.or.us/aq/gorgeair (accessed March 2008).