Phase I Visibility Effects Analysis






Prepared for the Southern Appalachian Mountains Initiative









Scott Copeland



CIRA/Foothills Campus

Colorado State University

Fort Collins, CO 80523








The Effects Subcommittee of the Southern Appalachian Mountains Initiative (SAMI) is examining approaches to predict effects of pollutant emission changes on Air Quality Related Values (AQRVs).  The Subcommittee has divided effects analysis into two parts, Phase I and Phase II.  Phase I is intended to produce response curves that describe changes in visibility given corresponding changes in the concentration of aerosol species which affect visibility.  PHASE I ANALYSIS HAS NO EMISSION-EXPOSURE LINK.  Phase I does not address the relationship between pollutant emissions and resultant aerosol concentrations at the Class I area.  This is left to Phase II Analysis, which will involve development of models or post-processors to the atmospheric model to describe visibility response to changes in pollutant emissions.

Phase I analysis was completed through in-kind services from the USDA Forest Service.  Scott Copeland, visibility analyst for the USDA Forest Service (located at the Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado), has constructed visibility response curves with input from Bill Malm (National Park Service), Peter McMurry (University of Minnesota), and Pradeep Saxena (Electric Power Research Institute); all are visibility research scientists involved in the Southeastern Aerosol Visibility Study (SEAVS).  Hereafter, Malm, McMurry, and Saxena will be referred to as the “scientists”.


Data and Methodology


Two data sets currently exist for visibility analyses at Class I areas in the SAMI region: IMPROVE (Interagency Monitoring of PROtected Visual Environments) and SEAVS (Southeastern Aerosol and Visibility Study).  IMPROVE has the advantage of up to 8 years of data collected in a uniform manner over many Class I areas, however aerosol sampling is limited to two 24-hour periods per week.  SEAVS emphasized measurement of the contribution of water and organics to aerosols at one site, the Great Smoky Mountains National Park.  The study was conducted over a six week period in July and August, 1995, and sampling was done every day.  The study also included 6-hour, 12-hour, and 24-hour sampling periods, using the same aerosol sampler as used in the IMPROVE network.  SEAVS was designed to provide information to fill gaps in the knowledge of atmospheric fine particle characteristics under humid conditions typical of the southeastern United States.  Both SEAVS and IMPROVE data will be used in Phase I of the effects analysis. 

IMPROVE and IMPROVE-equivalent SEAVS data from Class I areas in the Southern Appalachians will be used to construct visibility response curves that relate a visibility surrogate (deciview) to aerosol species concentrations (for an example see Figure 7).  Visibility is presented in terms of the deciview (dv), calculated using the following equation: dv=10ln(Dbext/0.01 km-1).  Differing response curves will be developed to illustrate differences in calculation parameters and interpretation of the data.

The five similar curves in Figure 7 are all “response curves”.  The difference or similarity between the curves shows variations which currently exist in scientific opinion.  This reflects limitations in knowledge and understanding, sometimes called epistemic uncertainty, and is not to be confused with other types of uncertainty arising from actual randomness and imprecise measurements.  However, at SAMI’s December 13, 1996 visibility workshop, effects subcommittee members and scientists agreed that the difference, or similarity, between response curves for a given scenario could be used as a surrogate for overall uncertainty.

In order to generate the response curves, extinction efficiencies are needed.  Extinction efficiencies are a measure of how well a given type of aerosol interferes with light, and hence reduces visibility.  Higher extinction efficiencies mean less visibility for a given aerosol mass.  It will be assumed throughout this report that extinction efficiencies are effectively constant over the range of concentrations of interest.  Given this assumption, changes in extinction can be calculated by the following equation.



Where, Ei dry is the dry extinction efficiency of species i calculated using Mie theory for the specified change in concentration Dci,  f(RH) is a factor which accounts for the effects of relative humidity, and Dbext is the change in extinction caused by the change in concentration.


For this report, up to five sets of extinction efficiencies and corresponding f(RH) curves will be used to generate response curves.  The parameters for each set, and a brief description of each is provided below in Table 1.

Table 1


Extinction Efficiency


A 1

B 2

C 3

D 3

E 3

Ammonium Sulfate

3 x fi(RH)

5.0 x fn(RH)

4.2 x fa(RH)

3.7 x fb(RH)

3.9 x fc(RH)

Ammonium Nitrate

3 x fi(RH)

3 x fi(RH)

3 x fi(RH)

3 x fi(RH)

3 x fi(RH)

Organic Mass





5.4 x fco(RH)

Elemental Carbon




















1)  A - The efficiencies used by Sisler, et al in the 1996 IMPROVE report.

2)  B - NPS scientists recommendations for scattering efficiencies based on National Park Service monitoring results during the SEAVS study.

3)  C, D, E - Three sets of efficiencies and f(RH) curves based on EPRI monitoring data from the SEAVS study, and three different sets of assumptions, including E where organic aerosols are also assumed to be affected by RH (in addition to the sulfates and nitrates).  Set E was intended to provide a realistic upper bound on organic extinction efficiency.

Figure 1 shows the f(RH) curves used to generate the response curves.  The IMPROVE f(RH) curve applies to hourly RH observations, whereas the other five curves are intended for use with 12 hour average RHs.  Set E includes a second curve for organics (Set E Organics).  In this set, unlike the other 4 sets, it is assumed that organics are hygroscopic, like sulfates and nitrates, but with somewhat less growth than sulfates and nitrates.

The scientists are in agreement that the methodology used in this report to relate visibility to aerosol concentrations is a reasonable for the purposes of Phase I, and that the five response curves represent reasonable limits to the differences of opinion regarding dry scattering efficiencies and f(RH) curves.  These differences result from the fact that available data are incomplete and are consistent with varying theoretical interpretations.  It should be understood that the assumptions used in Phase I may or may not be applied in Phase II as SEAVS analyses progress and new information becomes available.  For instance the results of SEAVS work on the relationship between light extinction and perception will be completed and available for consideration in Phase II.

The scientists are not in agreement with respect to fine mass closure.  Each of the fine mass species described in this report is measured via a different technique, and in some cases the measurements are made from different types of filters sampled at the same time.  In addition, the total weight of all of the fine mass is determined independently.  Closure for fine mass would mean that the sum of individual species’ masses add to the measured total fine mass.  In general this is not the case for measurements in the SAMI region.  The degree to which this is significant depends on the assumptions underlying the accounting process.

Figure 2 shows fine mass budgets based on two sets of assumptions for the same data set.  The data are averages of IMPROVE-equivalent measurements on all SEAVS days. The left-hand pie shows the budget based on NPS assumptions.  In this version, 5% of measured fine mass is unaccounted for and 23% of the total fine mass is assumed to be water associated with sulfates.  (Water is not directly measured from any of the filters, but some water probably remains on the filters through the weighing process.)  The right-hand pie shows the total fine mass budget based on EPRI assumptions. In this version an unknown quantity of water is included in the 33% of the measured fine mass left after summing all of the measured species.  

All of the visibility calculations in this report are based on extinction coefficients reconstructed from measured aerosol species, according to the various sets of assumed efficiencies  in Table 1.  The calculations thus ignore any extinction from fine-particle material other than the measured species and their associated waters of hydration.  For the NPS interpretation of the mass budget, with only 5% of the fine mass “unaccounted for”, there is little potential for impact on the analyses done in this report.  For the EPRI interpretation, attributing even a fraction of the “unaccounted for” mass to unmeasured organic material could significantly affect several of the response curves in this report.  In general, more organic mass would increase the effect of organic mass removal, while decreasing the effect of sulfate removal.  More details regarding the issue of fine mass closure will be made available in a separate report.


Visibility Response Curve Scenarios


Response curves will be generated for various episodes/scenarios using the extinction formula described above with IMPROVE and SEAVS aerosol data.  The scenarios are divided into three groups: 

Group 1 represents the most specific look at visibility response: individual days representative of particular episodes during SEAVS in the Great Smoky Mountains National Park.  There are three separate objectives for Group 1.

Objective 1 is to examine the change in visibility condition in response to changes in concentration for four aerosol species during specific episodes from SEAVS.

Objective 2 is to interpret the representativeness of the selected episodes by comparing them to the IMPROVE record.

Objective 3 is to understand the importance of relative humidity with respect to visibility conditions and response curves.

Group 2 examines two days during SEAVS, but at five Class I areas in the SAMI region. 

Group 3 steps away from episodes and examines visibility conditions for different ranges of visibility conditions (summer, winter and annual clear, median, and hazy) at the same five Class I areas used in Group 2.  Response curves are also presented for summer, winter, and annual clear, median, and hazy days at Great Smoky Mountains National Park.


Visibility Response Curves


Before presenting the results of the analyses, it is appropriate to describe a basic response curve.  Figure 3 shows a single response curve with no other information.  This curve tells us how large a change in haziness  (Y-Axis) occurs for a given percentage change in aerosol concentration (X-Axis).  The X-Axis varies from –100% (complete removal of all of the species in question) to +100% (doubling of the species’ concentration).  While these ranges of change may never be considered by SAMI, they are included for completeness. 

Looking at the sample curve, a 40% decrease in this hypothetical species’ concentration yields a roughly -3 deciview change in haziness (that is, a typical scene would become less hazy).   While the term “deciview” (abbreviated “dv”) is unfamiliar to most people, it is a very practical unit.  A one to two deciview change corresponds to a “just noticeable change” in a scene’s haziness.  Also, a one deciview change in a pristine scene is equally perceptible to a one deciview change in a hazy scene.  The use of the deciview scale allows us to examine how changes in aerosol concentrations would be perceived across a wide range of visibility conditions.


Since the information in each figure is relatively confusing at first, Figure 4 was created to describe the layout of the 55 response curve figures.  The following features are included with some or all of the response curve figures.


Response Curve Legend – This legend identifies the plotted curves.  The legend either lists the five extinction efficiency sets, or the four species being considered.  The Response Curve Legend only applies to the curves, not the pie chart or cumulative frequency plot.


Current Condition Indicator  This bar displays the haziness assuming no change to any species’ concentration. The vertical line marks  0% change, and where the curves cross the 0% line is 0 deciview  change.  On days where there is a range of deciviews given for the current condition, the range represents the range calculated by the five different sets of assumptions.


Extinction Budget Legend - This is the legend for the extinction budget pie chart to the right.


Extinction Budget Pie Chart - This pie chart tells what fraction of the total extinction was caused by each aerosol species.  Extinction includes the effects of humidity.  “Rayleigh” is that portion of the total extinction caused by natural atmospheric gases such as oxygen and nitrogen.  An atmosphere with no other pollutants (100% removal of all species) would still scatter light because of Rayleigh scattering.  Rayleigh scattering varies as a function of elevation, but was taken to be 10Mm-1 (a reasonable approximation) for all plots in this report.


Relative Humidity - This white box contains the average RH used to determine extinction and response curves.


Cumulative Frequency Statistics - This bar chart displays the cumulative frequency statistics(based on historical data at the monitoring site) associated with the conditions used to generate this figure.  The number plotted is the fraction of observations less than or equal to the value used to generate this figure.  If the Measured Fine Mass bar is about 10%, then only 10% of days in the IMPROVE database for the site had measured fine masses less than or equal to the measured fine mass represented by this figure.  (i.e. it was a clear day.)


Similarly, the fractions indicate how often measured fractions were less than or equal to the fractions used to generate this figure.  A high cumulative frequency statistic here means that the figure represents an unusually high fraction of the specific species.


Winter, Summer, and Annual Cumulative Frequency values consider only observations from their respective season(s).


These statistics are provided specifically to address Objective 2 of Group 1, which was to describe the representativeness of the selected episodes by comparing them to the longer IMPROVE record.  These frequency indicators are provided for Group 2 and Group 3 plots for continuity.


Error Bars - On many of the figures, error bars are used as a surrogate for the five sets of parameters.  This allows all four species’ response curves to be plotted on one figure.


Haziness Scale - These three arrows indicate the locations on the Y-Axis which correspond to clear, median, and hazy days at the site and for the season represented in the plot.  The arrows indicate, for example, how much of a given species would need to be removed to change the days which are currently “median” days, to be the equivalent of current “clear” days.



Group 1 Objective 1

Figure 5 shows a timeline of several variables based on SEAVS data.  This figure was used in conjunction with a meteorological report by Sherman, et al. to select days for this analysis.  The five days chosen are as follows.


Julian Day



Reason This Day Was Chosen




Arbitrary Day




Highest Organic Mass Fraction




Dust Event (High Soil Fraction)




Hurricane Erin (Clear Day)




Stagnation Period (Hazy Day)


For each of the five days selected, four figures were prepared, (Figures 6-25) each with five response curves (one for each set of efficiencies and f(RH) curves). A set of response curves for the summertime median condition at Great Smoky Mountains considering summer data from 3/88 – 5/96 was also generated (Figures 26-29).  Each of the four figures for each day describes the removal of a single aerosol species: sulfate, nitrate, organics, or soil.  Response curves for elemental carbon were not generated, and coarse mass is included as a subset of soil for these analyses. 


Group 1 Objective 1 Results

The selected days represent a broad range of summertime conditions at Great Smoky Mountains.  During SEAVS, extremes were seen with respect to several of the key variables; including measured fine mass, sulfate mass fraction, nitrate mass fraction, soil mass fraction, and organic mass fraction.  This means that the Group 1 response curves should be representative of the range of possible responses to changes in aerosol concentrations. 

Day 1 of the study, although high in terms of fine mass is somewhat typical (see Figures 26-29) in terms of sulfate, organic, and soil fraction.  The response curves (Figures 6 and 8) show virtually no effect from complete removal of nitrates or soil.  Because they contribute such a small fraction of the total extinction, removing them is inconsequential.  Removal of organic mass (Figure 7) has some effect, with four of the five sets of assumptions producing nearly overlapping curves.  The fifth set, Set E shows more effects from organic mass removal.  This set was designed, in essence, to maximize the effect of organic removal, so this is expected.  The sulfate removal curves (Figure 9) show the same relationship as the organics removal curves (virtually overlapping curves) except that a higher total deciview change is predicted for a given percentage decrease in sulfate compared to that seen for organic mass removal (Figure 7).  The Set E shown in Figure 9 provides a minimum bounding condition. In this context, “minimum bounding condition” means that removal of sulfates would most likely yield a larger deciview change (closer to the other curves)  than predicted by the Set E curve.  This is because Set E is designed to maximize the effects of organic mass removal, which automatically minimizes the effects of sulfate removal.

On Day 1, which was a roughly 55th percentile sulfate fraction day, removal of 40% of sulfate mass would improve visibility by roughly 3.5dv.  Day 1 was a 35th percentile organic fraction day, and removal of 40% of organic mass would improve visibility by roughly 1dv.

The “High Organic Fraction” day shows similar results (Figures 10-13), when compared to  Day 1, except that because this day has a quite high organic mass fraction, removing a percentage of the organic mass produces a greater deciview decrease (Figure 11).  Because sulfate makes up a smaller part of the extinction budget, removal of a percentage of sulfate mass now produces a smaller deciview change (Figure 13) than for Day 1 (Figure 9).

On the high organic fraction day, which was a roughly 25th percentile sulfate fraction day, removal of 40% of sulfate mass would improve visibility by roughly 2.5dv.  On the same day, which was a 90th percentile organic fraction day, removal of 40% of organic mass would improve visibility by roughly 2dv.

The Dust Event day (Figures 14-17) is unique because soil is a significant fraction of the extinction budget (“Soil”, in all of these response curves, is the sum of the soil and coarse slices of the pie).  The effect of soil removal on this day (Figure 16) is larger than that for organics removal(Figure 15).  The relatively large spread between the curves for soil removal is tied to the differences in assumed extinction efficiency for soils.

On the Dust Event day, which was a roughly 5th percentile sulfate fraction day, removal of 40% of sulfate mass would improve visibility by roughly 2dv.  On the same day, which was a 1st  percentile organic fraction day, removal of 40% of organic mass would improve visibility by roughly 0.5dv.  Removal of 40% of soils (95th percentile mass fraction) would yield a roughly 1.5dv improvement.

The relatively high RH on the Hurricane Erin day  (Figures 18-21), coupled with the assumed hygroscopicity of organics in Set E is the cause for the separation seen in the response curves for organic (Figure 19) and sulfate (Figure 21) removal.  Because Set E assumes organics to have a very high extinction efficiency at this RH, the Set E curve is significantly different from the other four curves.  Again the Set E curve is an upper bound on organics removal effects (Figure 19) and a lower bound on sulfate removal effects (Figure 21).  The Summer Cumulative Frequency plots in figures 18-21 show that this was a very low sulfate and organic mass fraction day.  As a result, removal of either soils or nitrates produces modest deciview changes.

On the Hurricane Erin day, which was a roughly 1st  percentile sulfate fraction day, removal of 40% of sulfate mass would improve visibility by roughly 2dv.  The Hurricane Erin day was a 3rd percentile organic fraction day, and removal of 40% of organic mass would improve visibility by roughly 1.5 - 2dv.

The Stagnation Episode curves (Figures 22-25) demonstrate one extreme; an extinction budget almost completely dominated by sulfate.  As a result, there is virtually no effect due to removal of soil (Figure 24), nitrate (Figure 22), or organics (Figure 23), and a dramatic change resulting from removal of sulfate (Figure 25).

On the stagnation day, which was a roughly 90th percentile sulfate fraction day, removal of 40% of sulfate mass would improve visibility by roughly 4dv.  The stagnation day was a 1st percentile organic fraction day, and removal of 40% of organic mass would improve visibility by less than 0.5dv.

Summer median conditions (considering all summer data from 3/88 – 8/96) at Great Smoky Mountains are characterized by extinction dominated by sulfates and organics (see Figure 26), with sulfates accounting for roughly 70% of extinction, and organics perhaps 10%.  Under these conditions, again soils (Figure 28) and nitrate  (Figure 26) removal has very little effect.  Organics removal produces a modest change with Set E providing an upper bound (Figure 27).  Sulfate removal produces the largest effect with Set E providing a lower bound on the magnitude of change (Figure 29).

Based on these results, in subsequent figures (Figures 30-41 and 55-63), the five curves (each representing one set of assumptions) will be replaced with four curves each representing one of the species being removed.  The Set C assumptions will be used to generate the curves because the Set C curves tended to be in the middle of the five curves.  Error bars are added to roughly simulate the differences typical of the five sets of assumptions.  In this way, it is possible to display four figures’ worth of information on one figure with little loss of information.  The extinction budgets displayed are based on IMPROVE assumptions since they are similar to Set C, and they were much easier to generate.


Group 1 Objective 2

            Objective 2 was to interpret the representativeness of the selected SEAVS days by comparing to summer days in the longer IMPROVE record at Great Smoky Mountains.  The cumulative frequency values plotted on each curve are intended to provide this information.  To determine the representativeness of Day 1 relative to the IMPROVE record, note the height of each of the five bars in the Summer Cumulative Frequency plot (Figure 6).  The first bar is roughly 95 percentile, which means that 95% of all days had fine mass lower than this day.  So in terms of fine mass, this was a high day.  The middle bar, representing nitrate mass fraction, is roughly 5 percentile, which means that only 5% of days have lower nitrate mass fractions.  In other words, this day had relatively little nitrate.  Sulfate, organic, and soil fractions are between roughly 35 and 55 percentile, so they are relatively close to median conditions; that is, not particularly high or low.

            These statistics are provided for comparison on all of the response curve plots.


Group 1 Objective 2 Results

            No one episode from SEAVS would be considered common.  In fact, each episode represents an extreme of some kind;  from the highest fine mass ever recorded at Great Smoky Mountains on 8/18/95 (Figure 22) to days with roughly 1 percentile organic mass fractions on 7/26 (Figure 14) and 8/3 (Figure 19).  As a result, the response curves for these days should not be considered typical, but rather a good demonstration of the effects of five different sets of assumptions on wildly varying conditions.  In other words, if the five response curves (A-E) agree as well as they do under this range of conditions, then we can be confident that they should agree under the more typical conditions examined later.


Group 1 Objective 3

            Relative humidity plays a key role in Appalachian Region visibility science.  Some aerosols, specifically sulfates, nitrates, and some organics, absorb water from the atmosphere under high relative humidity conditions (greater than roughly 50%).  As a given particle absorbs water it grows in size and mass.  When a group of these particles grows in the atmosphere, they become more efficient scatterers of light, and hence visibility conditions deteriorate.  Thermodynamically, this water absorption occurs very quickly (equilibrium conditions are reached in less than 1 second), so that these effects are seen almost immediately.  It should be noted that relative humidity alone does not cause visibility impairment, but rather augments the impairment due to sulfates, nitrates, and perhaps organics that are present.

Objective 3 of Group 1 is to understand the effects of RH on the response curves.  To this end, one day’s (the High Organic Fraction day) concentrations were modeled under conditions of 50%, 70%, and 90% relative humidity.  The results are shown in Figures 30-32.


Group 1 Objective 3 Results

            At 50% RH, since organics and sulfates comprise a roughly the same fraction of the extinction pie, they have similar response curves (Figure 30). 

At 70% RH, the “current condition” is roughly 1 deciview hazier, and sulfates begin to dominate the IMPROVE extinction budget.  For the Set E assumptions, sulfates and organics remain roughly equal fractions of the extinction budget (this is not displayed on the plot).  The error bars in this figure and figure 32 are changed to one-sided since under these conditions, all four curves are very close except the Set E which is represented by the end of the error bar.  The ends of the error bars, again representing a bounding condition, stay fairly close together for sulfates and organics (Figure 31).

At 90% RH, the “current condition” has become much hazier (from 28 dv at 50% to 35 dv at 90%), and sulfates now dominate the IMPROVE budget.  Organics and sulfates are still nearly equal fractions of the Set E extinction budget.  The sets of curves (Set C and Set E) diverge dramatically, as shown by the large error bars, with the curves (ends of the error bars) still showing nearly the same response to organic and sulfate removal (Figure 32).


Group 2

            The question being answered here is “are the response curves from Great Smoky Mountains representative of the other Class I areas in the SAMI region during the same episodes?”  This group examines two days during SEAVS at five sites across the SAMI region.  The two days used for this group are 8/16/95 and 8/23/95.  These days were chosen because they are regular IMPROVE sample dates, and there is valid aerosol and RH data available for four of the sites on 8/16/95 and all five sites on 8/23/95.  Figures 33-41 show the response curves for the two days.


Group 2 Results

            8/16/95 was two days before the height of the stagnation episode.  As a result, it was a very high measured fine mass day at all four sites considered.  Sulfate fractions were high (Figures 33 & 34) to median (Figures 35 & 36) at all four sites, with organic fractions being relatively low at all four sites.  As a result, the response curves across the region are very similar (Figures 33-36).

            8/23/95 was a day with the SAMI region split into two distinct groups: north and south.  The northern sites, Shenandoah National Park (Figure 37), Dolly Sods Wilderness(Figure 38), and James River Face Wilderness (Figure 39) had median fine mass concentrations with very high organics fraction.  As a result the organics and sulfate removal curves are fairly close together.  The southern sites, Great Smoky Mountains National Park (Figure 40) and Shining Rock Wilderness (Figure 41), were still experiencing conditions similar to those of the stagnation episode.

            Analysis of these two days confirms a somewhat intuitive result:  there are days when the Class I areas in the SAMI region experience nearly identical visibility conditions, and there are days when unique visibility conditions are experienced at one or more of the Class I areas.  One way to minimize the complication caused by this result is to consider “typical” conditions at each site, which is done in Group 3.


Group 3

            The previous two Groups of analyses focused on SEAVS episodes.   This Group examines the longer term record at five of the IMPROVE sites in the SAMI region (Figure 42).  For purposes of this analysis, the data at each site is sorted into three categories: “clear”, median, and “hazy”.  Clear day conditions are calculated by taking the average concentrations for each species for the sample dates with the lowest 20% of all measured fine mass concentrations.  For example, at Dolly Sods, there were 104 samples taken over the last 6 sample years during the summer season.  The 20 samples with the lowest measured fine mass were averaged to create the “clear” day condition.

Similarly, median and hazy days were generated by averaging the samples with the median 20% and highest 20% of measured fine mass, respectively.  The extinction budgets shown are generated from IMPROVE assumptions only.

            The following table lists the number of samples available for each site for each period considered.


Table 2

Site  Abbreviation



Data Record

Annual Samples

Winter Samples

Summer Samples



9/91 – 5/96






3/88 – 5/96






9/94 – 5/96






3/88 – 5/96






7/94 – 5/96





            Note that for Figures 55-63, the cumulative frequency statistics are generated from the historical record of the period shown in the figure.  That is, the cumulative frequency statistics in the bars charts in Figures 55-57, which depict winter conditions, were generated from winter data only.  The same is true for the summer (Figures 58-60) and annual(Figures 61-63) figures.


Group 3 Results

            Figure 42 shows the location of the five IMPROVE monitoring sites used in this analysis.  Figures 43-48 and 49-54 show the fine mass and extinction budgets for these five Class I areas.  The pie charts are scaled linearly by diameter to the scale circle in the legend. 

For winter fine mass budgets (Figures 43-45), the mass fractions are similar across all sites except James River Face.  James River face is the lowest elevation sampler site in the region and appears to be frequently below the wintertime inversion, whereas the other sites are 2000 or more feet higher, and evidently stay above the inversion more often.  In the summertime (Figures 46-48), when strong inversions are uncommon, the effect is not seen, and James River Face has fine mass concentrations and budgets very similar to the other four sites.

In general, although fine mass concentrations increase from clear to median to hazy days, the fine mass fractions stay relatively constant for each site for the winter (Figures 43-45).  In the summer, as days move from clear to hazy, the sulfate fraction increases from roughly 55% on clear days to roughly 75% on hazy days (Figures 46-48), with the other fractions generally decreasing.  With the exception of James River Face in the winter, all five sites experience similar conditions.

The patterns seen in fine mass budgets, not unexpectedly, translate into similar patterns in the extinction budgets (Figures 49-54).  James River Face has somewhat higher wintertime clear and hazy extinctions (Figures 49 & 51) and a uniquely high fraction of extinction from organics (roughly 25%).  Summertime extinction is dominated (50%-80%) by sulfates, particularly on hazy days, across all sites (Figures 52-54).

The important result from these maps is that the extinction fractions at all sites for the winter and summer and each haziness condition are fairly similar.  As a result, response curves generated for one site will be nearly identical to those for the other four sites.  The last analysis in this report (Figures 55-63) considers response curves generated for Great Smoky Mountains for the winter, the summer, and the annual time periods on clear, median, and hazy days.

Figures 55-63 demonstrate as closely as can be done for the Phase I analysis what will happen to visibility conditions in the SAMI region if an arbitrary amount of an aerosol species is removed.  For example a 40% reduction in sulfate concentrations would yield a roughly 2 dv improvement in visibility conditions during the winter (Figures 55-57), and a 2.5 to 4 dv improvement in visibility conditions during the summer (Figures 58-60).  A 40% reduction in organics would yield a roughly 0.5 dv change in visibility conditions across clear, median, and hazy days during both seasons (Figures 55-60).  A 40% reduction in soils or nitrates would result in changes of roughly 0.1 to 0.2 dv across clear, median, and hazy days during both seasons (Figures 55-60). 

These changes would make days that are hazy now move closer to current median conditions, while days that are median now would move closer to current clear conditions, and current clear days would become even more clear. The net effect is to shift the entire distribution of visibility conditions toward the clear end of the spectrum in proportion to the magnitude (number of dv) of the decrease.


The following general conclusions can be made based on the results of the Phase I visibility analysis.  The results apply across summer, winter, and annual clear, median, and hazy days at all five Class I areas with IMPROVE monitoring stations.

Ø      Sulfates comprise the majority of fine mass.  Sulfates also comprise the majority of extinction.  Sulfates’ contribution to visibility impairment is relatively well understood, and agreed upon, including it’s dry scattering efficiency and the effects of relative humidity on its scattering efficiency.

Ø      Organics are the second most prevalent category of fine aerosols.  Organics also contribute the second largest fraction of extinction.  Organics’ contribution to visibility impairment is less well understood and agreed upon than sulfates’, including dry scattering efficiency and the effects of relative humidity.

Ø      Soils and nitrates are the two smallest contributors to extinction considered in this report.  Soils and nitrates under normal conditions do not substantively influence visibility conditions.  There are exceptional events when soils and nitrates do influence visibility conditions.  Soils’ and nitrates’ impacts on visibility impairment is nominally well understood for purposes of this report.

Ø      On any given day, visibility conditions can be quite similar at Class I areas across the SAMI region, or the conditions can be different at one or more of the Class I areas.

Ø      When measured visibility conditions are averaged over time, the Class I areas considered in the SAMI region are fairly uniform and can be reasonably characterized by choosing one representative site.

Ø      Estimates of the effects on visibility conditions caused by removing visibility reducing aerosol species from the atmosphere, on an annual average basis, can be made from the final three sets of response curves in this report.  These curves use Great Smoky Mountains as a surrogate for Class I areas in the SAMI region.