INRr.Pl.YRKFFR TO 



July 8, 1994 

 N3615 (475) 



197 



United States Department of the Interior 



NATIONAL PARK SERVICE 



Air Quality Division - Ft Collins Office 



Cira-Foothills Campus 



Colorado State University 



Ft. Collins, CO 80523 



Ruth Fleischer, Esq. 



Counsel, Subcommittee on the Environment, Energy, and Natural Resources 



Committee on Government Operations 



U.S. House of Representatives 



B3719 Rayburn Office Building 



Washington, D.C. 20515-6145 



Dear Ms. Fleischer: 



This letter is to correct an error in my testimony during the hearing on air quality in national park 

 and wilderness areas before the Subcommittee on Environment, Energy, and Natural Resources 

 of the House Government Operations Committee on April 29, 1994. In responding to Mr. Synar's 

 invitation to comment upon the earlier testimony of Patrick Michaels, I made statements that I 

 have realized, in retrospect, incorrectly characterized the 1940's visibility conditions in the 

 Shenandoah National Park area. While the corrections set forth below show the changes in 

 visibility at Shenandoah between the 1940's and 1990's to be of lesser magnitude than my 

 statements at the hearing had suggested, the changes are nevertheless significant and dramatic. 

 The visibility conditions at Shenandoah National Park in the 1990's are substantially degraded 

 in comparison to the visibility conditions of the 1940's. 



With respect to my specific error, I incorrectly stated that the average visual range in the 

 Shenandoah National Park are in the 1940's was 120 to 150 miles. In fact, Trijonis has estimated 

 natural background visual range in the eastern United States as 64 to 120 miles (State of Science 

 and Technology Report No. 24, National Acid Precipitation Assessment Program, 1990). The 

 "clear" photograph of Shenandoah National Park displayed at the hearing probably approaches 

 natural conditions. Husar's study of airport visibility data suggests that average summertime 

 visual range in the southeastern United States in the late 1940's had already deteriorated to about 

 13 miles, and that it further decreased to about 6 miles by the early 1970's. The National Park 

 Service data for Shenandoah National Park during the 1990's shows average summertime visual 

 range of about 6 miles. 



My statements, as corrected, continue to attest to the substantially degraded visibility conditions 

 at Shenandoah National Park. 



In addition, I stand by my fundamental criticism of Dr. Michaels' testimony, as follows: Dr. 

 Michaels' implicit attempt in his testimony to discredit certain trend analysis performed by the 

 Shenandoah National Park staff was flawed, because Dr. Michaels failed to use the same period 



