### IMPROVE MONITORING UPDATE

Preliminary data collection statistics for the Winter 1993 season (December 1992 - February 1993) are:

Figure 1 is a map of the current IMPROVE and IMPROVE Protocol sites. Network changes in the last quarter included the installation of NGN-2 ambient nephelometers at Okefenokee, Upper Buffalo, Mammoth Cave, and Mount Rainier. Eight more nephelometers are scheduled for installation during the next three months. Aerosol data for the Summer 1992 season is complete and seasonal summaries have been submitted to the NPS. Analyses of Fall 1992 and Winter 1993 data are under

way. The recovery rate of aerosol data for the Winter 1993 season was 96%, the highest to date. Reprocessing of all transmissometer optical data from Spring 1991 through Fall 1992 was completed and deliv

ered in March 1993. An effort is now underway to reprocess and replot all IMPROVE transmissometer data from December 1987 through December 1992 to incorporate newly refined lamp drift correction factors. A comprehensive data report will be delivered in June 1993. Winter 1993 optical data (December 1992 - February 1993) is currently being compiled and will be reported in May 1993. Feature Article 

### DECIVIEW, A STANDARD VISIBILITY INDEX

INTRODUCTION An easily understood visibility index (Pitchford and Malm, 1993) has been developed to uniformly describe levels of monitored and modeled visibility impairment and to assess the progress of visibility protection
programs. The scale of this visibility index, expressed in deciview ( dv ), is linear with respect to perceived visual changes over its entire range, analogous to the decibel scale for sound. This article explains this new index and provides examples of its application.

BACKGROUND Visibility is traditionally described by visual range (the greatest distance that a large, dark object can be seen) or by the light extinction coefficient (the attenuation of light per unit distance due to scattering and absorption by gases and particles in the atmosphere). Visual range (expressed in kilometers or miles) was developed and continues to function well as an aid in military operations and transportation safety. It is likely to remain a popular measure of atmospheric visibility because if its familiar distance units, simple definition, and the fact that any sighted person can use it to charac

terize visual conditions without instruments. Extinction coefficient (expressed in km -1 ) is used by scientists studying the causes of reduced visibility. Direct relationships exist betwee n concentrations of atmospheric constituents and their contribution to extinc

tion coefficient. Apportioning extinction coefficient to atmospheric constituents provides a method to estimate change in visibility caused by change in constituent concentrations. This methodology, known as extinction budget analysis, is important for assessing visibility consequences of proposed pollutant emission sources, or for determining the extent of pollution control required to meet a desired visibility condition. Neither visual range nor extinction coefficient is linear to perceived visual scene changes caused by uniform haze. For example, a 5km

change in visual range or a 0.01km -1 change in extinction coefficient can result in a scene change that is either imperceptible or very obvious, depending on the baseline visibility conditions. 

### Volume 2, No. 1 Winter 1993 April 1993

82% Scene (photographic) Data 76% Optical (transmissometer) Data 96% Aerosol Data Collection Percentage Data Type

Figure 1. IMPROVE and IMPROVE Protocol Sites April 1993 DECIVIEW continued on page 2

### DECIVIEW (continued from page 1)

Presentation of visibility data or model results in terms of visual range or extinction coefficient creates the poten

tial for misinterpretation by those who are not aware of the non-linear

relationship, and requires the inconven

iences of further analysis for those who are aware.

VISIBILITY INDEX The newly-developed visibility index's dv scale is linear to humanly-perceived changes in visual air quality. A

one dv change is approximately a 10% change in the extinction coefficient, which is a small, but usually perceptible scenic change. The dv scale is near zero for a pristine atmosphere ( dv = 0 for Rayleigh conditions at approximately 1.8km

elevation) and increases as visibil

ity degrades. Because the index increases as haze increases, it is characterized as a haziness index. Expressed in terms of extinction coefficient (b ex ) and visual range (vr):

haziness ( dv ) = 10 ln (b ext /0.01 km -1 ) = 10 ln (391 km/vr)

The name deciview was chosen because of the similarity of the decibel scale in acoustics. Both use 10 times the logarithm of a ratio of a measured physical quantity to a reference value to create scales that are approximately linear with respect to changes as perceived by human senses. Figure 2 shows the relationship between haziness expressed in the dv scale and extinction coefficient or visual range. Ideally, a just noticeable change (JNC) in scene visibility should be approximately a one or two dv change in the deciview scale (i.e., a 10% to 20% fractional change in extinction coefficient) regardless of the baseline visibil

ity level. Similarly a change of any specific number of dv should appear to have approximately the same magnitude of visual change on any scene. The degree to which the deciview scale meets these ideals will depend on the extent to which any scene departs from the assumptions required to develop the scale (Pitchford and Malm, 1993).

APPLICATIONS
The dv scale provides a convenient, n umerical method for presentation of visibility values. Any visibility monitor

ing data or model predictions that are available in visual range or extinction coefficient are easily converted to the new visibility index expressed in deciview. A few examples were selected to illustrate the value of the dv scale. Perhaps the most convincing evidence that the dv scale is approximately linear with changes in visibility comesfrom viewing scenic photographs generated using imageprocessing techniques to display results from radiative transfer modeling (Molenar, et al., 1992). Photographs have been generated which show the effects of varying the extinction coefficient without changes to lighting or scene conditions (Trijonis, et al., 1990). The dv scale seems to adequately characterize perceived changes in visual conditions except when the assumption of suffi

cient targets at sensitive distances is viola ted.

Use of the dv scale is an appropriate way to compare and combine data from different visibility perception and valuation studies. When results from multiple studies are presented in terms of a common perception index, the effects of survey approach and other factors influential to

the results can be evaluated. Figure 3 is a contour map of median summer visibility for the continental United States. This map was origi

nally produced as a visual range (miles) contour map (Trijonis and Shapland, 1978), and is perhaps the most popular display of visibility conditions ever produced (Environ

mental Protection Agency, 1979; Trijonis, et al., 1990). The labels on the contour intervals have been converted to dv to produce Figure 3. The contour lines show an unbiased 

### Page 2 IMPROVE Newsletter April 1993

Figure 2. Visual Range and Extinction Coefficient as a Function of Haziness Expressed in Deciview.representation of the spatial gradient of perceived visibil

ity. The median summer visibility in the relatively clean Colorado Plateau has a dv value of approximately 12, and median summer visibility in the Eastern U.S. has a dv value of approximately 32. The use of an index which is linearly related to perceived visibility improves appropriate interpretation of study results when the issue concerns visual effects.

CONCLUSIONS A constant, fractional change in the extinction coefficient or visual range is shown to correspond to a similar change in perceived visibility regardless of the baseline visibility levels with the assumption that scenic elements are visible at sensitive distances. Based upon this relationship, a new visibility index is defined in terms of the extinction coefficient (or visual range). A haziness index using the dv scale has the following properties:

v deciview ( dv ) units are linear with respect to perceived visibility changes;

v a 1 to 2 dv difference corresponds to a small, visibly perceptible change in scene appearance where the assumptions used to develop the dv scale are met;

v zero (0) dv represents extremely good visibility (Rayleigh atmosphere where b ext

= 0.01km -1 ); and

v dv scale increases correspond to greater visibility impairment.
It is appropriate to use the dv scale to quantify visibility whenever the perceptibility of visibility is at issue. Example uses include assessing the visibility effects of proposed changes in emissions, investigating relation

ships between societal economic values and visibility, and displaying predicted or measured visibility levels which may influence public policy to decision-makers and the public. REFERENCES Molenar, J.V., Malm, W.C. and Johnson, C.E. (1992) Visual Air Quality Simulation Techniques. Transactions: Conference on Visibility and Fine Particles - Vienna Austria

, in press Atmospheric Environment. Pitchford, M.L. and Malm, W.C. (1992) Development and Applications of a Standard Visual Index. Presented at the Conference on Visibility and Fine Particles, Vienna, Austria. Submitted to Atmospheric Environment

. Pitchford, M.L., Polkowsky, B.V., McGown, M.R., Malm, W.C., Molenar, J.V., and Mauch, L. (1990) Percent Change in Extinction Coefficient: A Proposed Approach for Federal Visibility Protection Strategy. Transactions: Visibility and Fine Particles edited by C.V. Mathai, p. 37-49. Air and Waste Management Association, Pittsburgh, PA. Trijonis, J.C., Malm, W.C., Pi tchford, M.L., White, W.H., Charlson, R., and Husar, R. (1990) Visibility: Existing Conditions and Historical Conditions - Causes and Effects. National Acid Precipitation Assessment Program State of the Science and Technology Volume III , Report 24. Trijonis, J.C. and Shapland D. (1978) Existing Visibility Levels in the U.S. Technology Services Corporation Report to the U.S. Environmental Protection Agency, Grant 802815, Research Triangle Park, NC. U.S. Environmental Protection Agency (1979) Protecting Visibility: An EPA Report to Congress. Report EPA 450/5-79-008, Washington, D.C.



### V ISIBILITY NEWS

.......
PROTECTING VISIBILITY IN WESTERN CANADA AND THE PACIFIC NORTHWEST A workshop titled, Protecting Visibility in Western Canada and the Pacific Northwest , was held in Harrison Hot Springs, British Columbia, from March 14-17, 1993. Contact Peter Reid, BC Environment at 604/371-6296 if you are interested in a synopsis of the workshop.

USDA FOREST SERVICE INITIATES IMPROVE PROTOCOL MONITORING SITES The USDA Forest Service is currently planning to install and operate as many as five IMPROVE Protocol sites by fall 1993. The sites will include scene (camera), optical (NGN-2 nephelometer), aerosol (four-stage IMPROVE modular aerosol sampler), and meteorology (temperature and relative humidity) components. Several sites will also collect wind speed and wind direction data. Over the next several years the Forest Service will install and operate additional IMPROVE Protocol sites. 

### April 1993 IMPROVE Newsletter Page 3

Figure 3. Median Summer Visibility for Suburban/Non-Urban

Areas, 1974-1976, Displayed using the Deciview Scale.

### Page4 IMPROVE Newsletter April1993

IMPROVE STEERING COMMITTEE IMPROVE Steering Committee members represent their respective agencies and meet periodically t o establish and evaluate program goals and actions. IMPROVE-related questions within agencies should be directed to the agency's Steering Committee representative. Steering Committee representatives are :

WESTAR John Core Executive Director 1001 S.W. 5th Ave., Suite 1000 Portland, OR 97204 503/220-1660 (Phone) 503/220-1651 (Fax) STAPPA Dan Ely Colorado Dept. of Health Air Pollution Control Div. 4300 Cherry Creek Drive S. Denver, CO 80222-1530 303/692-3228 (Phone) 303/692-5493 (Fax) NESCAUM Rich Poirot VT Agency of Nat. Res. 103 South Main Street Building 3 South Waterbury, VT 05676 802/244-8731 (Phone) 802/244-5141 (Fax) FWS Sandra Silva Fish and Wildlife Service P.O. Box 25287 Denver, CO 80225 303/969-2814 (Phone ) 303/969-2822 (Fax) USFS Rich Fisher Air Specialist, Wash. Office Rocky Mtn. Experiment Sta. 240 W. Prospect Fort Collins, CO 80526 303/498-1232 (Phone) 303/323-1010 (Fax) B LM Scott Archer Service Center (SC-212A) P.O. Box 25047 Denver, CO 80225-0047 303/236-6400 (Phone) 303/236-3508 (Fax) NPS William Malm NPS-AIR Colorado State University CIRA - Foothills Campus

Fort Collins, CO 80523 303 /491-8292 (Phone) 303/491-8598 (Fax) U.S. EPA Joe Elkins MD-14 OAQPS Research Triangle Park, NC 27711 919/541-5653 (Phone) 919/541-2357 (Fax) U.S. EPA Marc Pitchford Environmental Monitoring Systems Lab P.O. Box 93478 Las Vegas, NV 89193-3478 702/895-0432 (Phone) 702/895-0496 (Fax)

PREVIEW OF UPCOMING ISSUE

. . . The next IMPROVE Newsletter will be published in July 1993, and will include:

v Network Status for the Spring 1993 Season

v FEATURE ARTICLE : Highlights from the NAS report "Protecting Visibility in National Parks and Wildernesses." PUBLISHED BY:

1901 Sharp Point Drive Suite E Fort Collins, CO 80525

The IMPROVE Newsletter is published four times a year (April, July, October, & January) under NPS Contract CX-0001-1-0025.

Your input to the IMPROVE Newsletter is always welcome. For more information, address corrections, or to receive the IMPROVE Newsletter,
contact:

Air Resource Specialists, Inc.

303/484-7941 Phone 303/484-3423 Fax

IMPROVE Newsletter text is also available on the EPA AMTIC Electronic Bulletin Board:

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