94 



COMPARISON TO OTHER DATA 



Transmissometer Data 



Automated visibility instruments have been in place at several National Parks, Monuments, 

 and Wilderness Areas since 1987. Figure 3 details a "typical" transmissometer summary 

 of this data, published by Air Resources Specialists (ARS), of Fort Collins, Colorado, 

 who were contracted by the Interior Department for this purpose. 



Figure 4 reproduces the entire Shenandoah Park summer ("excluding weather-affected" 

 data) record published by ARS. While the document states that it is for Winter, 1987, 

 through Spring, 1993, the Shenandoah summer data is published only for 1989 through 

 1992. 



It is obvious that there is very little data, and that no trends can be deduced from such a 

 spotty record. One could calculate a trend, but it would not be statistically valid. For 

 example, figure 5 details the trend in mean haziness from these sparse records. There is an 

 apparent decline in haziness (improvement in visibility), but it is not statistically significant 

 because of the very poor quality of the input data. 



Survey Reports 



Park visitors may report an increasing number of days with perceived poor visibility. 

 However, there is no trend in the mean airport visibility during any period of survey. 

 Mathematically, this must mean that the number of days with above mean visibility is also 

 increasing; otherwise there would be a decline in the mean overall visibility. 



Causes 



Several variables may be involved in the visibility issue. While it is commonly assumed 

 that only the emission rates are responsible, other factors-such as climate and land use- 

 can be important factors. 



Sloane (1983) determined that relatively high visibility noted in the early 1950s over this 

 region occurred during relatively hot, dry summers, and that by the late 1960s we were 

 experiencing fewer hot days and the airmasses had loger residence times. Thus seasonal 

 and annual climatic variation is an important factor in visibility trends. 



Robert Davis of the University of Virginia recently (1991) analyzed regional airport 

 visiblity trends for the Mid-Atlantic after explicit removal of "weather" variability. The 

 only statistically significant trends that he found since the mid- 1950s were for improving 

 visibility after making the weather adjustment. 



Stenger and Michaels (1989, 1992) found that the most important pattern of airflow into 

 northwestern Virginia was from the northwest (from the emission source regions of the 



