Now you see it, now you don't: impact of temporary closures of a coal-fired power plant on air quality in the Columbia River Gorge National Scenic Area

Abstract

Abstract. We have analyzed 14 years of aerosol data spanning 1993–2006 from the IMPROVE site at Wishram, Washington (45.66° N, 121.00° W; 178 m above sea level) in the Columbia River Gorge (CRG) National Scenic Area (http://www.fs.fed.us/r6/columbia/) of the Pacific Northwest of the US. Two types of analyses were conducted. First, we examined the transport for days with the highest fine mass concentrations (particulate matter with diameter <2.5μm or, PM2.5) using HYSPLIT back-trajectories. We found that the highest PM2.5 concentrations occurred during autumn and were associated with easterly flow, down the CRG. Such flow transports emissions from a large coal power plant and a large agricultural facility into the CRG. This transport was found on 20 out of the 50 worst PM2.5 days and resulted in an average daily concentration of 20.1 μg/m3, compared with an average of 18.8 μg/m3 for the 50 highest days and 5.9 μg/m3 for all days. These airmasses contain not only high PM2.5 concentrations but also elevated aerosol NO3− concentrations. These results suggest that emissions from large industrial and agricultural sources on the east end of the CRG, including the coal-fired power plant at Boardman, Oregon, have a significant impact on air quality in the region. In the second analysis, we examined PM2.5 concentrations in the CRG during periods when the Boardman power plant was shut down due to repairs and compared these values with concentrations when the facility was operating at near full capacity. We also examined this relationship on the days when trajectories suggested the greatest influence from the power plant on air quality in the CRG. From this analysis, we found significantly higher PM concentrations when the power plant was operating at or near full capacity. We use these data to calculate that the contribution to PM2.5 mass in the CRG from the Boardman plant was 0.90 μg/m3 averaged over the entire year, 3.94 μg/m3 if only the month of November is considered and 7.40 ug/m3 if only November days when the airflow is "down-gorge" (from east to west). This represents 15–56% of PM2.5 mass in the CRG. In all 3 cases the difference in PM2.5 concentrations are statistically significant at a >95% confidence interval for the comparison of normal plant emissions vs shutdown conditions. We, therefore, find that the coal-fired power plant at Boardman, Oregon is a significant contributor to PM2.5 concentrations in the CRG.