APPENDIX B

Developing Values for Natural Background and Progress Goals for the Class I Area of the 

Virgin Islands National Park, St. John, Virgin Islands

The Federal Implementation Plan for Regional Haze for the Virgin Islands is designed to take 

the first steps in improving visibility of the view in the Virgin Islands National Park by

reducing emissions of substances that scatter light. Emitting more light scattering aerosols in 

the air make it harder to see distant objects; these reductions are caused by human-caused 

emissions and emissions from natural causes. This Plan will include the first ten years of 

visibility improvements through 2018 that will lead, by 2064, to no visibility impairment due 

to human emissions.

The prospect of clear skies - scenic vistas unmarred by human interference- go along with the 

Virgin Islands’ reputation for clear water with magnificent coral reefs and underwater marine 

life. The FIP has a goal of protecting scenic views visible to the people of the Islands and 

their many visitors. The Clean Air Act gives special protection to the pristine areas like the 

Virgin Islands National Park on St. John.

Overall, a goal in the Clean Air Act is to develop a plan that reduces human interference with 

seeing the view. Sometimes dust from human activities reduces the ability to see the view. 

Motor traffic, especially on dirt roads, throws dust into the air. Making concrete or working 

with gravel emits dust. Emissions from combustion- electric generators, water heaters, 

industrial boilers, ships, power plants, open burning and refineries add sulfate, nitrates and 

carbon to the hazy mix. 

Sometimes, particles from nature spoil our view. Dust travels from the Sahara Desert, 

plankton can give off sulfur particles; and once in a while, the Montserrat Volcano sends ash 

toward the Islands. We can’t control the sources of these particles. Some days, their effects 

on visibility can be very noticeable, especially because there are not as many human sources 

of air pollution in the Islands as opposed to the more built-up megalopolises in the 

continental United States. 

Natural Background and Baseline Data

The Interagency Monitoring of Protected Visual Environments (IMPROVE) program was 

initiated in 1985 to establish current visibility conditions, track changes in visibility, and help 

determined the causes of visibility impairment in Class I Areas

We can use information from the types and amounts of particles captured in particulate 

samplers to calculate the effect the particles have on visibility. The IMPROVE network

collects particles at the air monitoring site in the Virgin Islands National Park every three 

days. Since different particles scatter light in different ways, they have different effects on 

visibility. Scientists have developed equations to quantify the obscuration of visibility from 

the particulates. These calculations also take into account the humidity in the air, which can 

make some particles swell up and increase their ability to reduce visibility. Then modelers 

can project the effect of reducing particle emissions on visibility by calculating the change in 

the amounts of the various particles and the amount of change in visibility that will occur.
The Clean Air Act stipulates that states should improve visibility to reach natural conditions. 

The Act says that states should do this by improving the visibility on the worst days while 

making sure visibility does not get worse on the best days. To address this requirement, we 

calculate the average visibility impairment over the worst one-fifth (top twenty percent) days 

and the best one-fifth (lowest twenty percent) days. 

The Table below compares the average visibility impairment for the best and worst days for 

Class I areas that represent various parts of the country. Brigantine and Acadia represent the 

‘pristine’ areas in the urban northeastern corridor of the United States. Everglades has a 

tropical climate like the Virgin Islands, but has more sources of pollution nearby than the 

Virgin Islands. Grand Canyon is representative of the dry deserts of the western United 

States, remote from major urban areas and sources of pollution. While the Virgin Islands are 

remote from major urban areas and have less industry nearby, like the western United States, 

its more humid climate allows particulates, especially sulfates, to have a greater effect on 

reducing visibility. 

Figure 1. Visibility impairment for selected National Parks and Wilderness Areas

Virgin 

Islands

Brigantine, 

NJ

Acadia, ME Everglades, 

FL

Grand 

Canyon, AZ

Worst 20% 17.02 29.01 22.89 22.30 11.66

Average 12.20 21.14 14.59 16.04 6.58

Best 20% 8.54 14.33 8.77 11.69 2.16

Notes: Units are amount of visibility impairment in deciviews. Lower values of deciviews 

mean better visibility. Higher numbers indicate more impairment to visibility. Brigantine is 

representative of sites in the urban corridor of the eastern United States. Acadia is a more 

rural location on the east coast. The Everglades are representative of tropical conditions in 

the Continental United States. Grand Canyon is a representative of the dry desert regime in 

the western United States. 

Sidebar: What is a deciview? 

A deciview is a unit of visibility reduction. More deciviews indicate lower 

visibility, or, a decrease in how far away you can see objects. The deciview scale 

was developed as a measure of visibility that is linear; to make it easier to do 

calculations needed to compare improvements to visibility. Typically, a change of 

one deciview is large enough of a change to the noticeable to the human eye. The 

following figure shows the different ways visibility is quantified, with worst 

visibility on the right side and clearer air on the left.

Virgin Islands’ visibility is better than typical locations in the eastern United States. But 

even the best days in the Virgin Islands have more obscuration than most days in dry western 

locations like the Grand Canyon. Higher humidity in the tropics results in more obscuration 
when sulfates are in the air, since sulfates scatter more light, and decrease visibility, as the 

humidity increases. Thus sites in the western United States have much better visibility than 

more humid places in the eastern United States and the Virgin Islands. 

We can use information from the types and amounts of particles captured in particulate 

samplers to calculate the effect the particles have on visibility. The IMPROVE network 

collects particles at air monitoring sites in the Virgin Islands National Park and other Class I 

areas every three days. Since different particles scatter light in different ways, they have 

different effects on visibility. Scientists have developed equations to quantify the 

obscuration of visibility from the particulates. These calculations also take into account the 

humidity in the air, which can make some particles swell up and increase their ability to 

reduce visibility. Then modelers can project the effect of reducing particle emissions on 

visibility by calculating the change in the amounts of the various particles and the amount of 

change in visibility that will occur.

The Clean Air Act stipulates that states should improve visibility to reach natural conditions. 

The Act says that states should do this by improving the visibility on the worst days while 

making sure visibility does not get worse on the best days. To address this requirement, we 

calculate the average visibility impairment over the worst one-fifth (top twenty percent) days 

and the best one-fifth (best twenty percent) days. 

Baseline and Natural Visibility

In September 2003, EPA issued guidance for the calculation of natural background and 

baseline visibility conditions. The guidance provided a default method and describes certain 

refinements that states may wish to evaluate to tailor these estimates to a specific Class I area 

if it is poorly represented by the default method. 

EPA’s Region 2 office met with representatives of the Federal Land Manager for the Virgin 

Islands National Park in 2008 to discuss the calculation of baseline and natural background 

conditions in the Virgin Islands National Park. The FLM agreed with the calculation of the 

baseline conditions and the estimate of progress needed to reach natural conditions. EPA 

noted that the default calculation of natural background conditions did not include the large 

impact of non-anthropogenic impacts from Sahara Dust and biogenic emissions. The FLM 

agreed that the information presented (and described in this Plan) showed the large influence 

of Sahara Dust on visibility in the Park. However, the FLM noted that more work would 

need to be done on more precisely calculate the impact on natural sources on visibility in the 

Park. They suggested that a Plan that strove for implementing reasonable controls on sources 

that affect visibility in the Park would be the most important element of this Plan that covers 

the first ten years of improving visibility for the Park. The issues of the impact of natural 

sources on visibility in the Virgin Islands National Park needs additional research the needs 

to be done before the next iteration of this Plan, due in 2018, is prepared. 

Natural visibility represents the visibility for each Class I area representative of the 

conditions before human activities affected air quality in the area. Certain natural phenomena 
can reduce visibility. The Clean Air Act goal is to remedy visibility impairment resulting 

from human activity.

Natural visibility, without anthropogenic emissions, is calculated to provide a target level to 

meet the Act’s goal of no anthropogenic obscuration of visibility. This natural visibility is 

based on formulas that are specific to various parts of the country. It does not include the 

impact of locally important natural obscurers of visibility. In the Virgin Islands, for example, 

Saharan Dust can noticeably reduce visibility. Less obvious are biologically produced 

sulfates from marine life, which may also contribute to obscuring the view.

Using data from the IMPROVE program, we can calculate the baseline (2000-2004) twenty 

percent worst visibility and natural visibility conditions for the IMPROVE monitoring site in 

the Class I area in Virgin Islands National Park. These values are computed from the 

Visibility Information Exchange Web System (VIEWS) operated by the Regional Planning 

Organizations (available online at http://vista.cira.colostate.edu/views/). The VIEWS site 

lists the worst visibility days as a five-year average for 2000-2004 using the alternative 

IMPROVE algorithm approved in 2006 by the IMPROVE Steering Committee. Also listed 

are the natural background visibility for the twenty percent worst and the twenty percent best 

visibility days.

Since one of the baseline years, 2000, has incomplete data, EPA guidance allows the use of 

calculations based on four year of data, from 2001-20041. The resulting data are listed in the

table below.

Table 2. Calculations of visibility for natural conditions for selected National Parks and 

Wilderness Areas.

Virgin 

Islands

Brigantine, 

NJ

Acadia, ME Everglades, 

FL

Grand 

Canyon, AZ

Worst 20% 10.68 12.24 12.43 12.15 7.04

Average 7.10 8.48 7.88 7.77 3.20

Best 20% 4.41 5.51 4.65 5.22 0.31

This Table lists the amount of visibility reduction due to natural causes only, calculated 

based on typical background concentrations of pollutants, with assumed average local 

meteorological conditions. 

Note that natural visibility calculations for the Virgin Islands are numerically similar to 

natural visibility calculations for eastern United States sites. 

 

1 According to the EPA document entitled, Guidance for Tracking Progress Under the 

Regional Haze Rule, states that, “...a minimum of 3 years of data meeting these 

completeness requirements is sufficient to calculate the 5-year averages within each 5 year 

period...the 3 year completeness criterion allows for the calculation of baseline conditions at 

sites with less than 5 years of data.”
Figure 1. Actual and natural visibility statistics from selected National Parks and Wilderness 

Areas compared with the Virgin Islands. Note that the goal for these areas is for all days to

achieve the natural background by 2064.

To calculate the natural and basecase actual visibility for the twenty percent worst days, we 

used the data from the IMPROVE site to calculate the impairment for each portion of the 

particulate data. 

Table 3. Calculations of Natural and Actual Visibility Reductions for the Virgin Islands 

National Park, by Parameter

Parameter

Natural 

worst 20% 

estimated 

by formula

Actual 

worst 20% 

calculated 

from data

Description

aBext 18.52803 44.85217

Bext is the amount of visibility reduction for 

all particles

bCM 3.27672 15.70808

Visibility reduction from coarse particulate 

matter

bEC 0.27781 1.545539 Visibility reduction from elemental carbon 

bNO3 0.947763 2.075003 Visibility reduction from nitrates

bOMC 1.744017 1.836166 Visibility reduction from organic carbon

bSO4 1.483411 10.30606 Visibility reduction from sulfates

bSoil 1.394714 5.901773 Visibility reduction from fine soil

bSs 9.403591 7.479546 Visibility reduction from sea salt

dv 10.68113 17.02366

Total visibility reduction from all particles, 

converted to deciviews.

Source: VIEWS (http://vista.cira.colostate.edu/views/), prepared on 05/12/08

Calculated from IMPROVE data using the most recent set of calculation equations.

Data, except for the final row, is in Mm-1, which is light extinction per million meters, 

commonly said as units of ‘inverse megameters’
Then we combined all the visibility impairment from each contributor to the reduced 

visibility – the result is in the last row of Table 3. To reach the calculated natural background 

of 10.68 dv by 2064, visibility will need to improve by 6.34dv from the present actual 

17.02dv. 

According to EPA’s Regional Haze Rule, a reduction of 6.34 dv by 2064 is to be 

accomplished by reducing visibility impairment by at least the rate of uniform reduction in 

each ten-year period covered by the haze plans. A target for 2018 results from 6.34 divided 

by the number of years from 2004 to 2064 and multiplied by the number of years from 2004 

to 2018 to get the target of visibility improvement for each ten-year period of at least 1.48dv.

Are the Calculations Accurate for the Virgin Islands?

Based on the work of Tombach, concentrations of fine soil in summer average 7.54 ug/m3 

(likely to be mostly Saharan Dust in the Virgin Islands’ summer – which is prime season for 

Sahara Dust). This calculated natural background of fine soil of 7.54 ug/m3 is much higher 

than the 0.5ug/m3 assumed in the calculations of natural visibility or even the recorded 

concentrations of about 3 ug/m3 in the southeastern United States.

This is important to note that in order to be on track to improve visibility to the Clean Air Act 

goal of reaching natural visibility levels, this plan estimates the amount of progress needed 

by 2018 based on the calculated natural level of visibility. If possible, emission reductions 

should meet the reasonable progress goal line for 2018. However, the amount of progress 

estimate for 2018 may be an overestimate due to the underestimation of natural visibility

conditions. This calculation does not include the full impact of Saharan Dust and other 

natural emissions that interfere with visibility.

 

While there is uncertainty in estimating how much of the Virgin Islands’ visibility

impairment is due to natural sources, the amount of emission reductions for the first ten 

years’ plan will still be based on the application of reasonably available emission reductions 

from sources that affect visibility in the Virgin Islands National Park on St. John, U. S. 

Virgin Islands.