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 underway. 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 delivered 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 characterize visual conditions without instruments.

Extinction coefficient (expressed in km-1) is used by

scientists studying the causes of reduced visibility.

Direct relationships exist between concentrations of

atmospheric constituents and their contribution to extinction 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

Scene (photographic) Data 82%

Optical (transmissometer) Data 76%

Aerosol Data 96%

Collection 

Data Type Percentage

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 potential for misinterpretation by those who are not aware of

the non-linear relationship, and requires the inconveniences 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 visibility degrades. Because the index increases as haze

increases, it is characterized as a haziness index.

Expressed in terms of extinction coefficient (bex) and

visual range (vr):

haziness (dv) = 10 ln (bext/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 visibility 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, numerical method for

presentation of visibility values. Any visibility monitoring 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 comes

from 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 sufficient targets at sensitive distances is violated. 

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 originally produced as a visual range

(miles) contour map (Trijonis and

Shapland, 1978), and is perhaps the

most popular display of visibility

conditions ever produced (Environmental 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 visibility. 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 2dv 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 bext = 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 relationships 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., Pitchford, 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.

VISIBILITY 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 to 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) 

BLM

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."

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Newsletter is always welcome.

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