Appendix O.3 

South Carolina’s Response to EPA 

Comments on Sea Salt
Appendix O.3 - 1
EPA Region 4 Review Comments on the Document Titled: 

Evaluation of Alternative Methods for Estimating Sea Salt 

Concentrations at Cape Romain, dated March 5, 2007 

EPA Region 4 appreciates the opportunity to review the Sea Salt Report developed by 

Ivar Tombach through the Visibility Improvement - State and Tribal Association of the 

Southeast (VISTAS). EPA Region 4 continues to endorse the recommendation made by 

the IMPROVE Committee for use of chloride ion to estimate sea salt concentration. 

However based on our preliminary review of this report, it appears that there could be a 

technical basis to support the use of other methods for determining the sea salt 

concentration for use in the new IMPROVE algorithm. EPA Region 4 recommends that 

the following additional comments be considered by the State in making this decision. A 

rationale for any approach that differs from our recommended procedure of using 

chloride ion concentration should be provided. 

EPA Comment 1: 

The goal of this analysis is to find the best method for estimating natural 

background levels during modeled point source impacts at Class I areas for the 

BART visibility assessment. That said, it may be more important to select a sea 

salt level for periods of high point source impacts than to select a different sea salt 

algorithm. We recommend that the rationale for how the sea salt term is used in 

the new IMPROVE algorithm include a discussion of the 

connection/comparison/correlation of sea salt, meteorology and times of high 

point source impacts. 

SC DHEC Response: 

When determining the relative impact that an individual BART source may have 

at coastal Class I areas as compared to estimated natural conditions, it is very 

important to select a modeling algorithm that properly depicts the estimated 

natural visibility background by including the effects due to sea salt 

concentrations in the air. As noted in the comments provided by several BARTeligible sources and in a report provided by Dr. Ivar Tombach (Enclosure 1 dated 

5 March, 2007), the use of the chloride ion approach to estimate annual sea salt 

concentrations will significantly underestimate this contribution to natural 

background. We support the use of the sodium ion analysis by ion 

chromatography (IC) approach to estimate annual sea salt concentrations at the 

Cape Romain Class I area as discussed in Dr. Tombach’s report. This alternative 

approach, using sodium ion measurements analyzed by ion chromatography (IC), 

provides a more reliable estimate of sea salt concentrations and is still a 

conservative estimate of background sea salt concentrations at Cape Romain. 

In the case of coastal Class I areas like Cape Romain, much of the area occupied 

by the Class I area is actually salt water. Consequently, there is likely ample salt 

1

Appendix O.3 - 2
water available to generate salt aerosol from virtually all wind directions at Cape 

Romain. This is shown in the figure provided below, obtained from 

http://www.fws.gov/caperomain/cape_romain_map.gif. 

For BART-eligible sources situated along the coast (e.g., International Paper and 

Santee Cooper-Winyah to the northeast in Georgetown and MeadWestvaco to the 

southwest in North Charleston), flow along the Atlantic coast is exposed to fresh 

salt aerosol particles from spray generated by coastal waves and would be 

prominent for the trajectories required to advect the source plumes to Cape 

Romain from either direction. Therefore, sea salt concentrations would likely 

remain consistent. 

Although the DAK Americas facility is located inland to the west-northwest of 

Cape Romain, many cases of northwest winds (offshore in the case of most of the 

Atlantic coast) are preceded by several hours of onshore flow during frontal 

passage and within a normal diurnal sea-breeze circulation. In such cases, the 

sea salt advected inland prior to a wind shift would then be advected back out to 

sea. When a plume blown out to sea is advected back toward shore by a 

recirculation regime, there would be fresh emissions of sea salt aerosol. 

For all wind regimes in the vicinity of Cape Romain, sea salt is expected to be 

present as a naturally occurring aerosol. VISTAS states, including South 

Carolina, are using annual average natural background conditions for visibility 

impairment caused by the species (e.g., sulfates, nitrates, organic and inorganic 

PM) represented in the IMPROVE light extinction equation. As such, long-term 

average sea salt concentrations should be applied for consistent application as a 

visibility-impairing component of natural background conditions. 

2

Appendix O.3 - 3
An attempt to create a directionally-dependent function of just one term of the 

IMPROVE equation due to a source of the aerosol being identifiable is 

complicated by recirculation regimes, the availability of the sea salt within the 

Class I area itself, and the fact that the sampling was conducted only every third 

day. Other particulate components of the IMPROVE equation could also have 

directionally-dependent sources, so if the implementation of the new IMPROVE 

equation for sea salt is complicated by the introduction of a directionallydependent analysis, why not extend this to sulfates, nitrates, organics, etc.? 

Consider the possibly important directional dependence of organic aerosols on 

forest fire plume trajectories, for example. The same point could be made 

regarding windblown dust from various sources. The implementation would be 

too complicated to start considering directional influences for each Class I area 

for the various terms of the new IMPROVE equation. Also, as is the case for 

sodium, data for the other components is available (at best) only every third day. 

A daily (or more frequent) analysis of airborne particulate during various 

wind/meteorological regimes is impractical, if not impossible. We have 

concluded that it is best to keep the implementation reasonably simple by using 

annual averages for each component of the new IMPROVE equation, including 

the sea salt term. 

EPA Comment 2: 

It appears that uncertainty exists in the collection and analysis of sea salt data. It 

is unclear how the submitted report for determining the appropriate sea salt 

concentration would actually be implemented and used in a specific BART 

Assessment. Information should be provided that discusses how data from 

specific years will be used in the BART modeling to determine visibility impacts. 

SC DHEC Response: 

SC DHEC reviewed the September 2002 through December 2004 sodium ion data 

provided by VISTAS and the calculated sea salt values were very consistent yearto-year at 0.79 (2002 partial year), 0.69 (2003), and 0.77 (2004) ug/m3

. In 

addition, long term elemental sodium and strontium data was analyzed for 

insights into how representative the VISTAS sodium ion data set may be. SC 

DHEC compared several data sets in the spreadsheet named “Cape Romain Sea 

Salt Analysis 2007-04-10.xls” that is included with this submittal. First, the 

3

Appendix O.3 - 4
average sea salt concentration estimates for the longer data periods (1991 to 

2001), whether using sodium or strontium as the marker, compare favorably to 

the shorter term VISTAS sodium ion marker average (1.1 ug/m3

 vs. 0.77 ug/m3). 

In addition, the short term 2002 to 2004 strontium marker average also compares 

very favorably to the VISTAS sodium ion marker data for the same period (0.78 

ug/m3

 vs. 0.77 ug/m3). Since the VISTAS data encompasses a period of a little 

over two years, SC DHEC analyzed the data for rolling averages of 242 samples 

(or approximately two years worth of samples). That data, for all periods and all 

markers, showed fairly good stability in the sea salt concentration estimated, with 

a minimum sea salt estimate of 0.745 ug/m3 and a maximum estimate of 1.535 

ug/m3, both when compared to the other rolling averages and when compared to 

the average concentration for the entire applicable period. While there is, 

understandably, more variability in the sea salt concentrations on a sample to 

sample basis, the data suggests that the ambient sea salt concentration is 

relatively stable over periods of two years or more. It is important to add that the 

exemption decision would be the same for all of the facilities in question no 

matter what two year (or longer) period of data is used. Finally, it is worth 

noting that the sea salt estimated using the VISTAS sodium ion data is lower and 

approximately mid point between the estimates using the long term data sets and 

the IMPROVE default chloride marker estimate. All of this suggests that sea salt 

concentrations are fairly consistent over time and that the sea salt estimate using 

the VISTAS sodium ion marker data is a very representative, but still reasonably 

conservative, estimate for the ambient sea salt concentration at Cape Romain. 

Therefore, for the specific application of the BART exemption analyses for Cape 

Romain, we support the use of the sea salt estimate calculated using the VISTAS 

2002 to 2004 sodium ion data as described by Dr. Tombach. 

EPA Comment 3: 

We recommend that the relationship between sea salt and sulfate concentrations 

be reviewed and discussed in the documentation. This discussion should be 

submitted for review by EPA and the FLMs. 

SC DHEC Response: 

Na+

 ion was measured by ion chromatography for IMPROVE samples collected 

at Cape Romain for the period September 2002 through December 2004. Na+

from soil was estimated using the accepted methodology as outlined by Dr. 

Tombach in Attachment 1 (0.57 times the Fe concentration). The remaining nonsoil Na+

 ion, again using the accepted methodology outlined by Dr. Tombach, has 

been attributed to sea salt, NaCl, and used to calculate the estimated sea salt 

concentration ([Sea Salt] = 3.27[ Na+

non-soil], where [ Na+non-soil] = [ Na+

] – 

0.57[Fe]. 

SO4 ions are routinely measured by ion chromatography by IMPROVE. SO4 may 

replace Cl in current samples to form Na2SO4. To the extent that Cl is replaced 

4

Appendix O.3 - 5
by SO4, total current mass may be overestimated by using non-soil Na+

 and SO4

mass without correction. Unfortunately, available IMPROVE data are not 

sufficient to estimate the extent of replacement under current conditions. 

Several SC BART facilities used non-soil Na+

 ion to represent sea salt under 

natural conditions. As natural conditions are approached at each Class I area, 

the sulfate concentration component will drop considerably and will be much less 

than the sea salt-related sodium concentrations in shoreline environments. 

Therefore, non-soil Na will be an accurate estimate of sea salt under natural 

conditions because SO2 emissions and SO4 ambient levels will be much lower 

than current conditions and any interference or “double counting” of the sodium 

for sea salt vs. natural sulfates (which can only now be estimated, not measured) 

will be minimized when the goal of natural conditions is attained. Also, it is 

worth noting again that the sea salt estimates calculated using the Na+ ion 

marker are consistent with, but below those estimates that were calculated using 

the elemental strontium and sodium markers. This suggests that the Na+ ion 

derived sea salt estimate is a conservative estimate even if there is any SO4

replacement occurring in the 2002 to 2004 IMPROVE samples. 

 

EPA Comment 4:

Blank problems on the nylon filters with respect to the analysis of chloride data 

was discussed in the report. No discussion of sodium ion (Na+) blank data is 

included in the report. It appears that the standard operating procedure for 

IMPROVE monitors doesn’t call for analysis of Na+, so there has never been a 

reason to look at Na+ blank values on those filters. VISTAS may have asked for 

analysis of blanks when they commissioned the cation analysis, but no 

information is provided in the report. The Na+ blank values for the nylon filters 

should be subtracted from the measured Na+ values. If it’s not being subtracted 

but is relatively large, the sea salt estimates would be biased high. If the data are 

available, we recommend they be assessed and included in the documentation 

provided in the VISTAS report. 

SC DHEC Response: 

VISTAS provided funding to the Research Triangle Institute (RTI), the laboratory 

that analyzes the IMPROVE samples under contract to the National Park Service, 

for ion chromatographic (IC) analysis of IMPROVE module B Nylon filter 

samples for cations -- originally ammonium (NH4

+

) and later sodium (Na+) -- for 

samples collected at nine IMPROVE locations, including Cape Romain, from 2 

September 2002 through 29 December 2004. Sodium was analyzed following 

IMPROVE protocol. These cation analyses by ion chromatography were 

analyzed at the same time as the routine IMPROVE analyses. As part of this 

analysis, 80 field blanks were analyzed for sodium ion loading. As detailed in the 

explanation provided by the VISTAS consultant, Dr. Ivar Tombach (Enclosure 2), 

Air Resource Specialists (ARS) used this data to calculate a median blank loading 

5

Appendix O.3 - 6
for each of the three filter batches and the result was subtracted from each sample 

in the batch to provide the blank-corrected results that were used in the BART 

exemption analysis. The spreadsheet SC DHEC received from VISTAS that was 

used to calculate the sodium concentrations from those nine IMPROVE locations 

is included as a separate file (VISTASNa_NH4Data.xls). On the “DATA” tab of 

this spreadsheet, cells M2 through M81 contain the filter blank measurements and 

column P contains the blank mass for each batch. A second spreadsheet is also 

included (VISTAS sodium_chloride_comparisons_fal 2005.xls). Column Z of the 

“data calculations” tab on this spreadsheet calculates the blank corrected sodium 

mass for each sample. Column AA of this worksheet calculates the blank 

corrected sodium concentration in ug/m3. 

EPA Comment 5: 

EPA Region 4’s position on use of the new IMPROVE Equation and the 

procedure developed by VISTAS for implementing it with the CALPUFF model 

has been previously shared with South Carolina and the other Region 4 States via 

email in October 2006. This position indicates that when the new IMPROVE 

algorithm is used, documentation explaining the rationale and implementation 

should be provided to EPA. For a case where a method other than the default 

procedure of using chloride ion concentration for estimating sea salt is used, we 

recommend that the rationale for using an alternate method should be provided in 

the facility-specific BART modeling protocol or modeling report. 

SC DHEC Response: 

SC DHEC has been working with the VISTAS RPO and EPA to implement the 

Best Available Retrofit Technology (BART) requirements of the Regional Haze 

Rule. The VISTAS RPO retained Dr. Ivar Tombach to develop a methodology to 

post-process CALPUFF/CALPOST modeling data to implement the new 

IMPROVE equation. The post processor spreadsheet developed by Dr. Tombach 

has been reviewed by EPA Region 4, the National Park Service, and the US Fish 

& Wildlife Service. Subsequent to their review, EPA Region 4 staff commented to 

SC DHEC that they consider this post-processing methodology a non-regulatory 

modeling application requiring compliance with section 3.2 of Appendix W Part 

51: Guideline on Air Quality Models and thus, SC DHEC must obtain formal 

approval for its use from the EPA Regional Administrator. In response to these 

comments, SC DHEC requested approval for the use of the new IMPROVE 

equation in a letter dated September 10, 2007. In a letter dated October 5, 2007, 

the EPA Regional Administrator approved the use of the new IMPROVE equation 

for BART modeling in South Carolina. 

Regarding sea salt estimates, the sodium ion measurements taken by VISTAS were 

not available to the public until March 2007. Prior to that data availability, it 

was still evident that the use of the chloride ion concentrations for determining 

the sea salt concentration term of the new IMPROVE equation has serious 

drawbacks, as documented in Dr. Ivar Tombach’s March 5, 2007 white paper 

6

Appendix O.3 - 7
(Enclosure 1). The chloride ion from sea salt reacts with naturally-occurring and 

anthropogenic acids (primarily sulfuric and nitric) in the atmosphere. This 

reaction converts the chloride to several gaseous forms such that it is no longer 

captured on the filter media. As such, technical/scientific experts (including 

VISTAS’ expert consultant) readily acknowledge that the chloride ion is a very 

imperfect method of obtaining an estimate of naturally occurring sea salt 

concentrations. In addition, as noted by Dr. Tombach in his March 5 white 

paper, “Cl-

 measurements from summer 2000 to the end of 2003 were influenced 

by excessively high blank levels in the Nylon filters, as a result of which the blankcorrected Cl- concentrations were too small and often negative.” This means 

that, in addition to the chloride depletion bias, the blank-corrected Cl- based sea 

salt concentrations for the 2000-2005 baseline are significantly biased low 

because of the excessively high blank levels. 

In lieu of the chloride derived sea salt estimate default, the facility-specific BART 

submittals for several coastal emission sources in South Carolina used the more 

reliable sodium ion derived sea salt estimate for their submittals. These facilities 

also provided information on estimates based on strontium and elemental sodium 

markers, the use of which has been supported in the scientific literature surveyed 

on estimating sea salt concentrations. Enclosure 3 provides a detailed survey of 

methods for estimating sea salt concentrations using the available data at Cape 

Romain. The results of the survey found that the strontium approach generally 

estimates average concentrations very close to average concentrations using the 

VISTAS consultant’s preferred method for applying measurements of sodium ion. 

A long history of reliable observations are available from Cape Romain, 

including a continuous record of strontium measurements from September 1994 

through September 2004. This data corroborates the 2002 to 2004 average value 

of 0.77 ug/m3

 based on the sodium ion approach from the VISTAS measurements, 

and illustrate a long-term average sea salt concentration of 1.1 ug/m3

 based on 

continuous strontium measurements that correlate well with sea salt 

concentrations estimated from elemental sodium and sodium ion measurements. 

After review of the available VISTAS sodium ion data for the 2002-2004 period 

and the long term strontium and elemental sodium data, SC DHEC has 

concluded, as suggested by Dr. Tombach, that the use of a calculated average 

concentration of 0.77 ug/m3

 is a better estimate of sea salt concentrations at Cape 

Romain for those facilities requesting an alternative to the default IMPROVE 

chloride based estimate for their BART exemption modeling. 

EPA Comment 6: 

Consultation with the Federal Land Managers is recommended in the BART 

assessment and ultimately in the development of a reasonable progress long term 

strategy for a class I area. We encourage the State to share the VISTAS report 

and our comments with the appropriate FLM for their review and comment on 

7

Appendix O.3 - 8
this sea salt issue. The FLMs have a great deal of expertise in the area of 

visibility impairment and EPA Region 4 would value their input. 

SC DHEC Response: 

SC DHEC has been in consultation for several months concerning the issue of the 

proper method for estimating the sea salt term in the new IMPROVE equation for 

several SC BART facilities that have requested the use of an alternative to the 

IMPROVE default. We are sharing the VISTAS report, as well as these responses 

and other documents and data that have been used in coming to a decision on this 

issue, with the Cape Romain FLM. We value the opinion of our FLM partners 

and will continue to consider their input into our BART and Regional Haze 

decisions. 

 

EPA Comment 7: 

It appears that the report has undergone a limited review as compared to the 

lengthy and varied discussions that were held with the Regional Haze Data 

Workgroup to develop recommendations on the use of the chloride ion for 

assessing sea salt in the new IMPROVE algorithm. These workgroup 

recommendations have been used by EPA Region 4 in our comments on the 

BART protocols for the Region 4 States and are also a part of the EPA modeling 

guidance for assessing the uniform rate of progress (i.e., glidepath) modeling for 

Regional Haze. 

SC DHEC Response: 

SC DHEC has engaged in a considerable number of discussions with, and 

gathered information from, many different persons and sources and considered 

input from varied points of view on this subject, including federal and state 

officials, consultants for both VISTAS and industry, and leading technical experts 

in the field of regional haze. We have found that there is a considerable variety of 

opinion on this subject and have come to the understanding that there is probably 

no one best method for estimating sea salt concentrations in ambient air. We 

have learned that the decision to use the chloride marker for estimating sea salt at 

the Class I areas was made as a compromise among many factors. Of which, one 

important factor was the routine availability of chloride data from IMPROVE 

samples vs. the lack of routinely available sodium ion data. It has become clear, 

however, that for estimating sea salt in the ambient air for the Cape Romain 

Class I area, where a good set of sodium ion data analyzed by ion 

chromatography exists, estimating sea salt with the sodium ion marker is the most 

reasonable method for estimating visibility impairment caused by coastal BART 

sources. 

EPA Comment 8: 

8

Appendix O.3 - 9
9

We continue to remind the States that the BART assessment of a source is only 

one requirement for assessing a source for controls and compliance with the 

Regional Haze Rule. An exemption from controls through BART does not equate 

to an exemption through the Reasonable Progress assessment. 

SC DHEC Response: 

SC DHEC understands that the BART assessment is only one part of the Regional 

Haze Rule. In fact, three of the four facilities that exempt from BART only if the 

alternative sodium method is used have been requested to submit a four factor 

analysis so that SC DHEC can assess whether or not controls are reasonable 

under the Regional Haze Rule. The only facility that has not been requested to 

submit a four factor analysis is an electric generating facility covered under the 

Clean Air Interstate Rule (CAIR). SC DHEC has decided that compliance with 

CAIR satisfies sources’ requirements under the Regional Haze Rule for this first 

planning period. We are confident that the end result for all these facilities will 

be the same as if these facilities were required to submit a BART determination 

analysis for consideration of controls. 

Appendix O.3 - 10
Ivar Tombach, Ph.D.

 Environmental Consulting

753 Grada Ave.

Camarillo, CA 93010

805 388-2341

805 445-9424 fax

itombach@aol.com

EVALUATION OF ALTERNATIVE METHODS FOR ESTIMATING 

SEA SALT CONCENTRATIONS AT CAPE ROMAIN

5 March 2007

1. Background

The new IMPROVE algorithm for estimating light extinction from concentrations of 

various components of particulate matter includes a term for the light scattering by fine 

sea salt. The document that presents the new IMPROVE algorithm1 recommends that the 

mass concentration of fine sea salt be calculated as 1.8 times the mass concentration of 

the chloride ion (Cl-) or, if a valid chloride measurement is not available, as 1.8 times the 

mass concentration of elemental chlorine (Cl). The IMPROVE document describes the 

result as “a reasonable lower-limit to the fine sea salt mass.”

The principal component of sea salt is ordinary salt, NaCl, but sea salt also contains other 

chemicals. The fraction of the sea salt mass that is represented by the sodium ion (Na+) 

and by the chloride ion (Cl-) has been determined experimentally. For fresh sea salt, 

recent measurements put the sodium fraction at 0.308 of the dry sea salt mass and the 

chlorine fraction at 0.554.2 This means that, if you have a reliable measurement of 

sodium or chlorine in fresh sea salt, then the mass concentration of the sea salt is 3.25 

[Na] or 1.8 [Cl], where the concentration can represent either the ion or the element.3

This simplistic approach seldom works in practice because the needed measurements are 

not available or reliable and actual sea salt is seldom fresh, which means that its chemical 

composition has changed by an unknown amount from that which was used to calculate 

these factors.

The rest of this report is in three sections. In Section 2, I review various methods for 

estimating sea salt concentrations from IMPROVE chemical and elemental 

measurements. In Section 3, I apply sea salt concentrations determined by several of 

these methods to calculate the reconstructed PM2.5 mass concentration and the light 

scattering calculated by the new IMPROVE algorithm and compare the results against 

 1

 “Revised IMPROVE Algorithm for Estimating Light Extinction for Particle Speciation Data” by 

Marc Pitchford, November 2006.

2

 “Physicochemical controls on seawater” by F.J. Millero. Chapter 6.01 of Treatise on 

Geochemistry. H.D. Holland and K.K. Turkian, editors. Elsevier Ltd., Amsterdam. 2004

3

 Square brackets represent concentrations. The factors used for sea salt vary slightly. We will use 

3.27 [Na] later in this report

Enclosure 1

Appendix O.3 - 11
2

gravimetric measurements of PM2.5 mass and nephelometer-measured light scattering at 

the Cape Romain, South Carolina, IMPROVE site (ROMA). In Section 4, I draw 

conclusions from the results of the first two parts. Although the focus of the analyses is 

on conditions at ROMA, the conclusions of this report should be judiciously applicable at 

other VISTAS locations near the coast, such as Okefenokee (OKEF).

2. Methods of Estimating the Mass Concentration of Sea Salt

The qualification that was given above about the 1.8 [Cl-] formula producing a lower 

bound estimate of the sea salt mass concentration arises because chloride in ambient 

particulate matter samples can be depleted in atmospheric reactions with nitric acid and 

sulfuric acid. In such circumstances, some of the chlorine in the particles is released in 

the gas phase as hydrochloric acid and some of the particulate NaCl is replaced by 

sodium sulfate or sodium nitrate. This depletion of chlorine from the particles means that 

estimates of sea salt concentrations from the chlorine that is left will be low.

The depletion of chlorine varies with location. Recently, Warren White observed that the 

Cl- concentrations determined by ion chromatography (IC) on IMPROVE Nylon filters 

collected at several Pacific coast sites in northern California and in Alaska are in seawater 

ratios with non-crustal calcium, strontium (Sr), and potassium,4 i.e., the measured Clconcentration is consistent with that of fresh sea salt and doesn’t reflect any depletion, a 

consequence of the coastal locations of the sites and the prevailing onshore flow of clean 

air at those Pacific coast locations. Despite the quantitative relationship he observed 

between Cl- and conservative sea salt tracers at the sites he studied, White concluded 

that Cl - would not serve as a quantitative tracer for aged sea salt away from the coast or 

in polluted air, where atmospheric reactions can deplete the Cl- in situ.

As another example, Lowenthal and Kumar compared sea salt concentrations estimated 

from elemental Cl and Sr (strontium) measurements at the IMPROVE site in the Virgin 

Islands.5 They estimated that, on average, more than 50% of the chlorine had been lost 

from the sea salt, even though the Virgin Islands site is virtually surrounded by ocean and 

is relatively unpolluted.

White also hypothesized that, independent of the depletion of Cl in the atmosphere, 

elemental Cl concentrations measured by XRF on Teflon filters are typically less than the 

ionic Cl- concentrations measured on Nylon filters because the Cl collected on the Teflon 

filters is further depleted, while on the filter, due to atmospheric sulfate droplets that 

reach the filter and react with the NaCl there, releasing gaseous HCl that escapes from the 

filter. The same droplets also make it through the denuder to the NaCl on the Nylon filter, 

but the Nylon filter captures the gaseous chloride that is released (in the same way that it 

 4

 “Chemical markers for sea salt in IMPROVE aerosol data,” by W.H. White. Presentation at the 

Conference on Visibility, Aerosols, and Atmospheric Optics, Vienna, 3-6 September 2006.

5 “Light Scattering from Sea-Salt Aerosols at Interagency Monitoring of Protected Visual 

Environments (IMPROVE) Sites,” by D. Lowenthal and N. Kumar. Journal of the Air & Waste 

Management Association, Vol. 56, pp. 636-642. 2006.

Enclosure 1

Appendix O.3 - 12
3

captures gaseous nitrate), so that the measured Cl- concentration is not affected. Hence, it 

appears that the Cl- concentration on Nylon filters probably accurately represents the Clthat remains in the sea salt at the time of sampling, while the Cl measured on Teflon 

filters is further depleted by reactions that take place on the filter.

The Cl- concentration is determined routinely in IMPROVE by ion chromatography (IC) 

of samples collected on Nylon filters. The data are usually of high quality, except that

Cl- measurements from summer 2000 to the end of 2003 were influenced by excessively 

high blank levels in the Nylon filters, as a result of which the blank-corrected Clconcentrations were too small and often negative. Since the beginning of 2004 the 

chloride blank levels on the Nylon filters have returned to minimal levels, so the quality 

of the chloride concentration data has improved. Reliable elemental Cl concentration 

measurements are also available routinely. 

For ROMA, this Cl- blank level problem means that the annual average sea salt 

concentration (= 1.8 [Cl-]) reported in the IMPROVE data base ranges from 0.16 to 0.20 

μg/m3 over the four years from 2000 through 2003, but leaps to 0.345 μg/m3 in 2004. A 

similar response is seen at Everglades National Park (EVER), where sea salt values 

during 2000 to 2003 range from 0.14 to 0.32 μg/m3, while the 2004 sea salt concentration 

is reported as 0.56 μg/m3. This means that the baseline (2000-2004 average) sea salt 

concentrations are biased low, apparently by roughly a factor of two at both ROMA and 

EVER.

Because of the bias introduced by the depletion of chlorine in sea salt, it would seem 

preferable to derive sea salt concentrations from measurements of Na+ ion or elemental 

Na, and this is indeed the case. Unfortunately, the Na+ concentration is not measured 

routinely by IMPROVE, although measurements were made by the VISTAS RPO at nine 

IMPROVE sites in the VISTAS region from September 2002 through December 2004. 

As for elemental Na, since December 2001 the IMPROVE measurements of elements 

have been via X-ray Fluorescence (XRF) spectroscopy, which is unable to quantify Na 

reliably. 

To work around some of the problems with recent Na and Cl- measurements and the bias 

caused by chlorine depletion, use of elemental sodium data taken before December 2001 

has been proposed. Elemental Na analyses were made then using Particle-Induced X-ray 

Emission (PIXE) spectroscopy, which produces substantially more reliable measurements 

of Na than does XRF, although the PIXE spectral peak for Na is still appreciably smaller 

(and therefore more uncertain) than that of Cl.

Marc Pitchford of NOAA (assigned to the US EPA) has prepared a memo that 

summarizes the merits and deficiencies of various approaches to estimating sea salt 

concentrations.6 His recommendations are, in his order of preference:

 6

 “Thoughts on Sea Salt Concentration Estimates for Southeastern US IMPROVE Sites,” by Marc 

Pitchford. 18 January 2007.

Enclosure 1

Appendix O.3 - 13
4

1. Na+ on Nylon filters, analyzed by IC. The Na+ in soil in the sample has to be 

subtracted from the total Na+ concentration to arrive at the Na+ associated 

with sea salt. From the crustal elemental composition data that was used to 

construct the IMPROVE formula for soil, the typical Na+ due to soil is 

estimated at 0.57 [Fe], although it can vary a bit from place to place, which 

adds some uncertainty at low sea salt concentrations.7 Unfortunately, Na+ is 

not routinely measured in IMPROVE, so this approach cannot be used 

generally. As noted earlier, Na+ was specially measured at several VISTAS 

sites (including ROMA) from September 2002 through December 2004. The 

formula for this approach is [Sea Salt] = 3.27 [Na+

non-soil], where [Na+non-soil] = 

[Na+] - 0.57 [Fe].

2. Cl- ion on Nylon filters, analyzed by IC. Blank problems from mid 2000 to the 

end of 2003 mean that the Cl- levels are biased low during that period. The 

formula is [Sea Salt] = 1.8 [Cl-].

3. Elemental Cl on Teflon filters, analyzed by XRF or PIXE. Because all of the 

gaseous HCl is lost from these filters, the resulting estimates of sea salt 

concentration will be biased lower than those derived by method 2 above. The 

formula is [Sea Salt] = 1.8 [Cl].

4. Elemental Na on Teflon filters, analyzed by PIXE, not by XRF. (The quality 

of Na analyses by XRF is unacceptable and values are not given in the 

IMPROVE data base.) Because of uncertainties in the correction for selfabsorption of the x-rays by the particles on the filter, the Na concentrations 

may be uncertain by up to a factor of 2. The measured Na has to be corrected 

for Na in soil, as in method 1. The formula is [Sea Salt] = 3.27 [Nanon-soil], 

where [Nanon-soil] = [Na] - 0.57 [Fe].

An additional method, which was not ranked, was that of using the element strontium 

(Sr), which is measured routinely on Teflon filters by PIXE and XRF. Because a 

somewhat uncertain correction is required to adjust for the Sr due to soil, this method 

works best when the sea salt Sr is much greater than the soil Sr (as is the case at the 

Virgin Islands and Point Reyes, but not at Acadia, Maine, for example). The method is 

described by Lowenthal and Kumar in the article mentioned earlier. 

 7

 There are indications that 0.57 [Fe] overstates the amount of Na in some soil. Soil sample 

analyses by Raed Labban et al. (“Size and Geographical Variation in PM1, PM2.5, and PM10: 

Source Profiles from Soils in the Western United States,” Water, Air, and Soil Pollution, Vol. 

157, pp. 13-31. 2004) and Judy Chow et al. (“Similarities and differences in PM10 chemical 

source profiles for geological dust from the San Joaquin Valley, California,” Atmospheric 

Environment, Vol. 37, p. 1317-1340. 2003) both find much smaller values of the [Na]/[Fe] ratio 

for soil, typically more than an order of magnitude smaller than 0.57. Both these studies were 

performed in the West, while 0.57 is a global crustal average; we don’t know if either represents 

the soil in the Southeast. If a lower soil Na+ value can be established, that will decrease the 

influence of soil adjustment on the uncertainty in the sea salt concentration estimates.

Enclosure 1

Appendix O.3 - 14
5

There is some debate about where PIXE-based elemental Na should be in the above list. 

Analyses by White (in the paper mentioned earlier) show that, for the Pacific coast 

locations he analyzed, sea salt concentrations derived from PIXE elemental Na 

measurements account better for the difference between gravimetric fine mass and the 

reconstructed non-sea-salt fine mass than do sea salt concentrations derived from Clmeasurements. Thus, in some cases, Na from PIXE may be a better indicator of sea salt 

than Cl-, and is even more likely to be a better indicator than elemental Cl. However, the 

Na+ ion remains a much better indicator than any of these.

In summary, the only method that produces unbiased and relatively precise estimates of 

sea salt concentrations is the Na+ method, although it may have some uncertainties when 

the measured concentrations approach those of Na+ in soil, as might occur inland. The 

Na+ method can be applied at ROMA and other VISTAS region IMPROVE sites from 

September 2002 through December 2004, when Na+ concentrations were analyzed by 

VISTAS.

Sea salt mass calculations based on Cl- ion, as advocated by IMPROVE, are likely to be 

biased low, especially at locations in Southeast. Sea salt estimates from elemental Cl 

measurements can reflect large negative biases. Calculations based on PIXE Na 

measurements, despite potential uncertainties, may be a usable option, but only if data for 

better approaches are not available. XRF Na measurements are always too uncertain to be 

usable.

3. Application to Cape Romain

To evaluate the practical implications of using the Na+, Cl-, and PIXE Na concentrations 

for sea salt estimates at Cape Romain, this section provides comparisons of the mass 

concentrations of fine particles and the light scattering calculated by the new and old 

IMPROVE algorithms. Unfortunately direct comparisons of all three methods are not 

possible, because Na+ ion measurements started September 2002 and ended at the end of 

2004; credible Cl- ion measurements were not available from summer 2000 to the end of 

2003; and PIXE elemental Na measurements ended at the end of November 2001. 

Therefore the Na element method, using PIXE analyses of samples, cannot be compared 

with the Na+ method used in 2004, because there is no overlapping time period, There is 

yet another data limitation when light scattering performance is evaluated – nephelometer 

data at ROMA are only available starting at the beginning of 2004.

Therefore, we will evaluate the different methods by (1) intercomparing sea salt mass 

concentration results separately for 2004 and 1999; (2) with sea salt computed by various 

methods, comparing reconstructed PM2.5 mass, against gravimetric PM2.5 mass in 2004 

and 1999, and (3) with sea salt computed by various methods, comparing light scattering 

by particles, as calculated by the relevant terms in the new IMPROVE algorithm, against 

light scattering measured by integrating nephelometer in 2004. 

Sea Salt Concentration Intercomparisons. Figure 1 shows scatter plots that compare 

daily sea salt concentrations calculated by pairs of the four methods ranked above by 

Enclosure 1

Appendix O.3 - 15
6

Pitchford. The left column compares sea salt concentrations calculated from Na+, Cl-, and 

elemental Cl (by XRF) during 2004; the right column compares sea salt concentrations 

calculated from elemental Na and Cl (by PIXE) and by Cl- in 1999. (For example, the 

notation, “Sea Salt (Cl-)” denotes the sea salt concentration derived from the 

concentration of the chloride ion.) The solid lines are linear regression lines that have 

been forced through the origin, which means they are defined by the origin and the 

intersection of the averages in the x and y directions. Consequently the slope of the best 

fit line is just the ratio of the two averages.

The upper pair of plots in Figure 1 shows that the sea salt concentration calculated from 

elemental Cl is always less than the concentration calculated from Cl- (except for one 

outlier on the 1999 plot), although the two sets of sea salt estimates are highly correlated. 

The relative deficit in the elemental Cl concentrations was greater in 2004 than in 1999. 

The middle pair of plots shows that sea salt concentrations calculated from non-soil 

sodium concentrations (whether in the form of Na+ in 2004 or elemental Na from PIXE 

in 1999) always exceed sea salt concentrations calculated from Cl- concentrations. The 

correlation was strong for 2004 and relatively weak in 1999 (which may reflect the larger 

uncertainty in the elemental Na determinations). For seawater, the slope Na/Cl should be 

unity, so both plots suggest considerable chlorine depletion. Although the ratio of Na+ 

(or Na, as appropriate) to Cl- was greater in 1999 than 2004, one shouldn’t draw any 

conclusions from this because a few high-concentration points govern the slope and the 

scatter at lower concentrations is considerable. 

The bottom pair of plots shows, not surprisingly, that sea salt concentrations calculated 

from Na+ or Na exceed estimates derived from elemental Cl concentrations by an even 

larger margin than they do when compared to estimates from Cl- concentrations. The 

correlation was relatively weak in both years, though.

In 2004 the mean sea salt concentration at Cape Romain from non-soil Na+

measurements was 0.77 μg/m3, that from Cl- measurements was 0.35 μg/m3, and the 

value from elemental Cl measurements was 0.11 μg/m3. The differences between the 

methods are substantial.8 These averages are strongly influenced by a few high values, so 

the more-representative medians (the values of the middle concentrations in lists of 

ranked concentrations) of 0.41, 0.11, and 0.00 μg/m3, based on Na+, Cl-, and Cl 

respectively, are substantially smaller than the means. (For comparison, the 

corresponding means for the full September 2002 through December 2004 period when 

Na+ was measured are 0.74, 0.27, and 0.07 and the medians are 0.45, 0.03, and 0.00.)

 8

 The ratios of these values do not match up exactly with the slopes in Figure 1 because these 

averages are based on all available values for each method while the data in Figure 1 represent 

only those days where values are available for both of the variables in a given plot. The 

qualitative conclusions are identical, however.

Enclosure 1

Appendix O.3 - 16
7

2004 1999

Figure 1. Relationships between estimates of sea salt concentrations at Cape 

Romain, using Na+, Na, Cl-, and Cl measurements available in 2004 (left) and 1999 

(right).

Enclosure 1

Appendix O.3 - 17
8

Figure 2 compares the same sea salt estimates by the various methods from day to day 

during 2004 and 1999. Some of the higher concentration days neatly show the hierarchy 

of estimates, with the estimate from Na+ (in 2004, or Na in 1999) at the top, the one from 

Cl- below it (although the difference was small for a sample in early September 2004), 

and the one from Cl well below the others. 

In both 2004 and 1999, many Cl concentrations were reported as zero (i.e., below the 

minimum detection limit) while the corresponding Cl- concentrations were reported as 

zero less frequently, which accounts for some of the difference between concentrations 

based on the Cl- and Cl measurements. Since the sea salt concentrations are proportional 

to the concentration of Cl-, Cl, non-soil Na+, and non-soil Na, Figure 1 showed in effect 

that there are many cases where the Cl concentration was zero while the Cl- concentration 

had a positive value and also many cases where the Cl- concentration was zero and the 

non-soil Na or non-soil Na+ concentration had a non-zero value.

Reconstructed vs. Measured Mass Concentrations. Figure 3 shows comparisons 

between the reconstructed fine particle mass concentrations and the gravimetric fine mass 

concentrations, for three different sea salt concentration methods. Data are for 2004 and 

1999, and the new IMPROVE approach to calculating the concentration of particulate 

organic matter (POM) as 1.8 times the OC concentration was used. The sea salt mass 

concentration was based on the Na+ measurements in the top panel; on the Clmeasurements on the second panel; and on elemental Cl measurements in the third panel, 

all using the formulas given earlier in Pitchford’s recommendations. The diagonal lines 

are the linear regression lines with zero forced through the origin. As always, the data 

used for these plots are only for days that have measurements of all three sea salt 

indicators, and thus the data sets are missing a few dates in each year.

All 6 graphs in Figure 3 show that every one of the reconstructed fine mass 

concentrations is highly correlated with the gravimetric fine mass concentrations. Since 

sea salt is a minor contributor to the fine mass concentration on average, the high 

correlation primarily reflects the contributions of more prevalent components than seasalt 

mass.

The 2004 reconstructed mass concentrations are biased high relative to the gravimetric 

mass concentrations while the 1999 reconstructed mass concentrations are biased low. 

The annual averages of all of the component concentrations were very similar in both 

years, except that the ammonium sulfate average was about 0.5 μg/m3 higher in 1999, so 

that the reconstructed mass concentration using sea salt from Cl- (the official IMPROVE 

procedure) was slightly greater, by about 0.7 μg/m3, in 1999. Of greater import, the 

average gravimetric fine particle mass concentration was 9.03 μg/m3 in 2004 and 12.22 

μg/m3 in 1999, which accounts for the lower slopes in the 1999 plots than in the 2004 

ones.9

 9

 The behavior displayed by all of these figures is substantiated by the official IMPROVE data set 

on the VIEWS web site. In particular, the difference in gravimetric fine mass concentration also 

Enclosure 1

Appendix O.3 - 18
9

Figure 2. Time series of estimates of sea salt concentrations based on Na+, Na, Cl-, 

and Cl measurements in 2004 (top) and 1999 (bottom) at Cape Romain.

 

appears there, with the average fine mass concentration being 9.47 μg/m3 in 2004 and 12.51 

μg/m3 in 1999 for data sets that are somewhat more complete than the ones used here.

Enclosure 1

Appendix O.3 - 19
10

2004 1999

 Figure 3. Comparisons of 2004 (left) and 1999 (right) Reconstructed Fine Mass 

Concentrations (New IMPROVE Formulas, with Sea Salt Calculated from Na+, Na, 

Cl-, and Cl Concentrations) vs. Gravimetric Fine Mass Concentrations.

Enclosure 1

Appendix O.3 - 20
11

In both 2004 and 1999, when compared with the case where Cl- was used to calculate sea 

salt, the slopes of the best fit lines when elemental Cl was used have a slope that is, not 

surprisingly, 1-2% less and those when Na+ or Na was used have a slope that is about 4% 

greater. Because sea salt is a minor contributor to the fine mass concentration, on 

average, and is a particularly small contributor on the highest concentration days that are 

dominated by anthropogenic and land-based aerosol, it is not possible to infer from this 

comparison which of the sea salt indicators is the best one.

Aerosol vs. Measured Light Scattering. Figure 4 displays the temporal variation of 

daily light scattering measured by an integrating nephelometer and calculated by three 

sea salt variants of the IMPROVE equations, using daily RH, at Cape Romain in 2004. 

The calculated bsp values are labeled “Adj bsp” because the calculated scattering by 

coarse matter has been cut in half to correspond with the inability of the nephelometer to 

account for all of the coarse particle scattering. The solid line connects the nephelometer 

measurements on the days for which sufficient data existed for all the analyses. The dots 

represent four calculations of light scattering: one using the old IMPROVE algorithm 

(oia), which does not include fine sea salt; and three using the new IMPROVE algorithm 

(nia) with sea salt estimated from, in turn, non-soil Na+ ion, Cl- ion (the official 

IMPROVE approach), and elemental Cl concentrations.10

Figure 4 shows the expected result that the new IMPROVE formula calculates greater 

aerosol light scattering than the old one on many (but not all) days, especially on days 

with high scattering. On the other hand, the estimates by all four methods are virtually 

identical on some other days, and on a few days the old algorithm produces a higher 

estimate than the new one. Setting aside the old IMPROVE algorithm, the differences 

due to the various sea salt methods with the new algorithm are quite small relative to the 

total particle scattering.

The same points are reinforced by the scatter plot in Figure 5, which compares the 

various aerosol bsp values with the nephelometer measurements. There is a tight 

correlation between all the values for measured scattering in the range below about 40 

Mm-1, with a slope slightly greater than 1. The scatter increases substantially above that 

value and the various estimates diverge more.

 10 An egregious outlier on 3/13/03 was deleted from this figure and from Figure 6 because the 

scattering calculated by the new IMPROVE algorithm was an unreasonable 3.5 times as large as 

the measured scattering and the corresponding factor for the old algorithm was 2.4. That day’s 

POM concentration in the IMPROVE data base was the largest in 2004, by far, and was followed 

by the second largest POM concentration on the next sampling day, 3/16/04, but all other 

concentrations were typical.

Enclosure 1

Appendix O.3 - 21
12

Figure 4. Comparison of Different Measures of Daily Light Scattering at ROMA in 

2004.

Figure 5. Comparison of Aerosol Light Scattering Calculated by Various Methods 

vs. Nephelometer Measurements.

Enclosure 1

Appendix O.3 - 22
13

The data in Figure 5 are distributed over four graphs in Figure 6, so that the output of 

each aerosol light scattering approach can be compared separately to measurements. Each 

panel shows both the linear regression line (solid) and the regression line through the 

origin (dashed), and their formulas. The correlation of every data set with the 

nephelometer measurements is very good and is the same for all four sets (R2 = 0.94), 

which implies that the choice of sea salt calculation method has minimal effect compared 

to the light scattering from the other components of light scattering. Even the choice 

between old and new algorithms has no effect on the correlation. 

Figure 6. Relationships Between Aerosol Scattering and Measured Light Scattering 

in 2004 for the old IMPROVE Algorithm with no sea salt (top left), and the New 

IMPROVE Algorithm with Sea Salt Concentrations Based on Non-soil Na+ (top 

right), Cl- (bottom left), and Elemental Cl (bottom right).

Enclosure 1

Appendix O.3 - 23
14

The choice of approach slightly influences the slope of the best fit line. Much shouldn’t 

be made of this comparison, however, because the light scattering is dominated by 

components other than sea salt and uncertainties in their contributions can overwhelm the 

differences between the various methods for estimating light scattering by sea salt.

Conclusion

Analysis of the reconstructed mass and aerosol light scattering provides little insight into 

the preferred method for calculating sea salt concentrations when using the new 

IMPROVE algorithm. All approaches with the new algorithm slightly (by 12-17%) 

overstate the measured light scattering on average at Cape Romain, but the differences 

between them are small. Our findings are inconsistent for reconstructed fine mass 

concentration, with the reconstructed values slightly exceeding gravimetric 

concentrations (by 10-16%) at Cape Romain in 2004 and slight falling short of them (by 

5 to 11%) in 1999. 

Since the Na+ and Na methods produce the highest sea salt concentrations, reconstructed 

mass is greatest when that method is used. At Cape Romain, the analyses presented here 

show that the IMPROVE-sanctioned Cl- method produces sea salt concentrations that 

average roughly 40% less than those from Na+ or Na, while those from Cl are roughly 

60% smaller than those from the two sodium measures and are poorly correlated with 

them.11

Sea salt concentrations calculated using the Na+ ion are better correlated with values from 

the IMPROVE Cl- method than sea salt calculated using elemental Na, which could be a 

consequence of greater uncertainty in the Na method or, as the points piled up along the 

vertical axes in the middle panels in Figure 1 show, may be a consequence of a higher 

fraction of the Cl- concentrations being reported as very near zero in 1999 than 2004. 

In light of this behavior, the data analyses described in this report were unable to show 

that sea salt estimates produced from PIXE Na measurements were as good as those from 

Na+ measurements, and they suggest rather (based on incomplete evidence) that the 

elemental Na measurements may be inferior to those of the Na+ ion. Furthermore, based 

on analytical considerations, one should expect determinations based on Na+ to be 

superior to those based on Na by PIXE. 

Turning to Cl-, the general appearance of the concentration data, with large numbers of 

very low or zero concentrations for Cl-, implies that Cl- will provide a less reliable 

assessment of sea salt concentration than either Na (by PIXE, before mid 2000) or Na+. 

 11 These statistics consider only those days for which both of the values being compared were 

available. As Footnote 8 points out, grand averages that consider all available values without 

regard for pairing give slightly different ratios. The conclusion remains the same -- sea salt 

concentrations at Cape Romain based on Cl+ are substantially less than those estimated from Na+

or Na, and those based on elemental Cl are even smaller yet.

Enclosure 1

Appendix O.3 - 24
15

Therefore the ranking developed by Pitchford, with sea salt from Na+ ranked first, that 

from Cl- second, and that from elemental Cl third, appears to be a sound one. The fourth 

place rank assigned by Pitchford for estimates from elemental Na may be too pessimistic. 

However, since we don’t have a good handle on the uncertainty of PIXE Na 

determinations from IMPROVE filters and have no concurrent valid Na+ and Na 

measurements, we cannot establish from current information where the sea salt 

determinations based on elemental Na should rank in this hierarchy, although it is clear 

that the rank should be below that of determinations based on the Na+ ion.

Therefore, since Na+ concentration data are available for part of the regional haze 

baseline period at nine VISTAS Class I areas, using that Na+ information to estimate sea 

salt concentrations will produce the most reliable results and should be the preferred 

approach.

Enclosure 1

Appendix O.3 - 25
Ivar Tombach, Ph.D.

 Environmental Consulting

753 Grada Ave.

Camarillo, CA 93010

805 388-2341

805 445-9424 fax

itombach@aol.com

Comments Concerning the VISTAS Sodium Ion Data 

26 March 2007 

Sodium Ion Concentration Measurements 

VISTAS funded the Research Triangle Institute (RTI) for ion chromatographic (IC) 

analysis of IMPROVE module B Nylon filter samples for cations – originally ammonium 

(NH4

+

) and later sodium (Na+) -- for samples collected at nine IMPROVE locations from 

2 September 2002 through 29 December 2004. The standard IMPROVE analysis is for 

anions – SO4

=

, NO3

-

, and Cl- -- so VISTAS’ analyses augmented the IMPROVE data set. 

RTI had calibrated for Na+ when it analyzed the filters for NH4

+

 (VISTAS' original 

request), so it was easily able to return to the IC data to also determine the sodium ion 

loadings on the VISTAS samples and on 80 field blanks. 

Using this data, Air Resource Specialists (ARS) calculated blank-corrected filter 

loadings. It turns out that the amount of Na+ in the field blanks was reasonably consistent 

for each of the three filter batches that were used, so ARS used each batch's median blank 

loading to represent that batch. The median blank loading were 11.2 and then 9.2 μg per 

filter in 2002 and 2003, and improved greatly to 0.0 μg per filter in 2004. 

ARS then applied IMPROVE’s flow rates and sampling durations to calculate the mass 

concentrations of Na+

. 

Negative Na+

 Concentrations 

After the field blank loading was subtracted and flow rates and durations were used to 

calculate the Na+ concentration, the resulting concentration in 2002 and 2003 was 

negative for some samples, especially at inland sites where the ambient Na+

concentration is very small because the site is far from the ocean (the main source of Na+

in the atmosphere). Since three median concentrations were used to represent all of the 

samples over a 28-month period, the actual blank value for a given day could have been 

above or below that median, and so every blank-corrected Na+

 concentration could have 

an error that is related to the range in the blank measurements. In particular, the actual 

blank could have been overestimated at times, which would mean that too large a 

correction was subtracted from the ambient filter loading, possibly producing a negative 

blank-corrected result. 

Enclosure 2

Appendix O.3 - 26
There were also a few calculated negative Na+

 concentrations at coastal sites. For 

example, at Cape Romain (ROMA), there were four negative values, all in 2003, which 

occurred when the ambient concentration was low. These negative values ranged from -

0.010 to -0.015 μg/m3

, very small quantities compared to the average blank-corrected 

Na+

 concentration there of 0.239 μg/m3. 

The question arises as how to deal with the physically-impossible negative values of 

concentrations. There is a temptation to set the value to zero or to delete it. Neither of 

these is mathematically sound. If a concentration is negative, it is the result of errors in 

the measurement and/or the blank correction. Unless you know one of them is biased in 

only one direction, these errors can go in either direction. So, just because a resulting 

blank-corrected concentration is too low (negative in this case), doesn't mean that another 

value at some other time won't be too high by the same amount, and so the average is 

likely to be correct. To produce an unbiased data set, it would be necessary to delete or 

alter values that have positive errors in addition to those that are negative. The only 

trouble is that we can't identify the values that are too high, and thus deleting or altering 

negative values takes care of some erroneous values at the low end of the range but we 

are unable to handle correspondingly erroneous values from the high end of the range. 

Therefore, setting a negative concentration value to zero is incorrect. The effect of 

replacing a negative number with a zero is to increase the average so that it is too high. 

Deleting all negative values (i.e., not using the data for that day) also has the same effect, 

but to even a greater degree, because now only values greater than zero are included in 

the average. 

Negative Sea Salt Concentrations 

In order to determine the sea salt concentration from the Na+

 concentration, it is 

necessary to subtract the concentration of Na+ originating from soil. The actual soil-based 

sodium concentration was not measured at IMPROVE sites, nor is it practical to do so, so 

an estimate is made based on the typical composition of the earth’s crust. Using the same

global crustal composition tables in the Handbook of Chemistry and Physics that were 

used to construct the IMPROVE formula for fine soil concentration as the sum of the 

concentrations of Al, Si, Ca, Fe, Ti, and (indirectly) soil-based-K, the soil-based Na+

concentration is 0.57 times the Fe concentration. This amount is subtracted from the total 

Na+ concentration to arrive at the portion that is related to sea salt. 

Using a single value of 0.57xFe to represent soil-based-sodium at locations, under all 

wind speeds and directions, is a simplification. As with the blank correction, sometimes 

the correction may overshoot the true value and sometimes it may fall short. When it 

overshoots, the amount of Na+ attributed to soil will be too large and negative sea salt 

sodium concentrations may then result at times when there is little sea salt in the air. 

Since the sea salt concentration is a simple multiple of the non-soil Na+

 concentration 

(3.27 x non-soil-Na+

), the corresponding sea salt concentration will then also be negative. 

As was the case with the blank corrected Na+

, summarily deleting such negative values or 

setting them to zero is not recommended. 

2

Enclosure 2

Appendix O.3 - 27
3

For the 2004 ROMA sea salt calculations, there was only one negative Na+ and sea salt 

value, on June 29th, at -0.073 μg/m3 of Na+ or -0.237 μg/m3 of sea salt. The 

corresponding averages for that year were 0.236 μg/m3

 of Na+ and 0.771 μg/m3 of sea 

salt, so the magnitude of the negative value is about a third of the average. It may be that 

careful investigation of all of the IMPROVE measurements on that day might provide 

grounds for discarding the data from that day, either because of error or contamination, 

but such a conclusion is not certain. However, the negative value on June 29th, being only 

one sample out of a hundred or so samples for the year, doesn't affect the average by 

much. Also, deleting it or setting it to zero would also have a negligible result on the 

average for the year. 

Ten negative Na+ and sea salt concentration values occurred at ROMA in the 16 months 

of sampling in 2002- 2003 (interestingly all in the winter and late autumn), but they were 

all much smaller than the negative value in 2004, with non-soil Na+ concentrations 

ranging from -0.002 to -0.030 μg/m3

 and resulting sea salt concentrations ranging from -

0.007 to -0.097 μg/m3. The corresponding average concentrations over the 16-month 

period were 0.22 μg/m3

 for Na+ and 0.73 μg/m3 for sea salt. The negative values just 

mentioned are so close to zero that statistics such as averages are going to be virtually 

identical whether you use the numbers as is or set them to zero (but you would get higher 

averages if you deleted the negative values). 

Enclosure 2

Appendix O.3 - 28
Enclosure 3 

BACKGROUND SEA SALT AEROSOL CONCENTRATIONS

Prepared by Trinity Consultants, Inc. 

The 2005 IMPROVE Report identified sea salt aerosols as an important component of natural 

visibility conditions, particularly in humid coastal environments such as the Class I areas 

potentially affected by BART eligible sources. The 2006 IMPROVE Report identifies sea salt as 

a statistically significant component of fine particle mass at Cape Romain (which is considered 

representative of Wolf Island) and Okefenokee. 

Measurement and estimation techniques for naturally-occurring sea salt concentrations continue 

to be a focus within the scientific community in general and stakeholders in the VISTAS region 

in particular. The IMPROVE workgroup has developed a dry extinction efficiency and 

hygroscopic growth curve (fSS(RH)) for sea salt aerosols based on observations in remote marine 

environments and thermodynamic models. Although there is reasonable confidence in these 

components of the calculation of light extinction due to sea salt aerosols, there is greater 

uncertainty in the average sea salt mass concentration. Specific questions remain about how 

ambient sea salt concentrations are reliably inferred from measurements of elemental markers and 

the period of record for which reliable data are available to compute a representative natural 

background level. VISTAS states, including South Carolina, are using annual average natural 

background conditions for visibility impairment caused by the species (e.g., sulfates, nitrates, 

organic and inorganic PM) represented in the IMPROVE light extinction equation. The 

following analysis reviews available techniques for estimating the long-term average sea salt 

concentration for consistent application as a visibility-impairing component of the natural 

background. 

The 2006 IMPROVE Report describes challenges in measuring sea salt aerosols in ambient 

environments due to imprecise analysis techniques and various investigators have surveyed 

methods for estimating sea salt concentration to compute an average value for Cape Romain. 

Sodium (Na+

) and Chloride (Cl-) ions are generally accepted as the best markers for the presence 

of sea salt. Difficulties in computing sea salt from data from the IMPROVE network arise 

because positive ions have historically not analyzed; therefore, sodium ion (the strongest 

indicator of sea salt) data are not readily available, except for a brief period between September 

2002 and December 2004 during which time VISTAS measured Na+

 at Cape Romain and other 

Southeastern Class I areas. This Na+

 approach computes the sea salt concentration as: 

[Sea Salt] = 3.27[Na+

non-soil], (5) 

where [Na+

non-soil] = [Na+

] – 0.57[Fe] and [Fe] is the measured iron concentration. 

Issues arise when using the chloride ion or elemental chlorine to estimate sea salt because 

reaction of gaseous nitric acid with sea salt produces sodium nitrate particles and the release of 

gaseous hydrochloric acid. The depletion of chlorine during this reaction results in an 

underestimation of sea salt when using chlorine as the marker. The sea salt concentration would 

be computed as: 

Appendix O.3 - 29
Enclosure 3 

[Sea Salt] = 1.8[Cl] or [Sea Salt] = 1.8[Cl-

], (6) 

based on reliable measurements of [Cl-

] using ion chromatography or [Cl]. The VISTAS 

Technical Contractor originally computed an average sea salt concentration of 0.20 μg/m3

 using 

Cl-

 as the marker, which is recognized to underestimate the sea salt aerosol concentration.1 

Appendix 1 of the 2006 IMPROVE Report lists an average sea salt concentration of 0.377 μg/m3

, 

which is also based on Cl-

 measurements that may underestimate sea salt concentrations. 

Elemental sodium (Na) data are available from X-ray Fluorescence (XRF) analyses; however, 

sensitivity issues regarding poor detection of Na result in large uncertainties corresponding to Na 

from XRF (White et al., 2004).2

 Sea salt concentrations inferred from Na measured by Proton 

Induced X-ray Emission (PIXE) are considered reliable; however, such measurements have not 

been made at Cape Romain since 2001. Paine et al. (2006) computed an average sea salt 

concentration of 1.123 μg/m3

 at Cape Romain using Na as the marker from measurement 

collected between 1994 and 2001, during which time the more reliable PIXE analysis method for 

sodium was in use.3

 This sodium approach uses ambient measurements of sodium and fine soil 

concentrations and a reference concentration for soil-derived sodium provided as: 

[ ] = [Soil0283.0Na ] Soil , (7) 

where 0.0283 is the crustal reference composition of sodium. Using this value, the ambient 

sodium attributable to sea salt is computed as: 

[ ] [ ] [ ] Na Salt Sea = − NaNa Soil , (8) 

from which the sea salt concentration is computed as: 

[ ] [ ]

0.306

Na Salt Sea Salt Sea = , (9) 

where 0.306 is the average sodium content of fresh sea salt. An alternative estimate of sea salt 

concentration using [Na] can be constructed in a similar manner to the Na+

 approach as: 

[Sea Salt] = 3.27[Nanon-soil], (10) 

 

1

 Tombach, I. Estimating the Role of Sea Salt when Determining the Visibility Impact of a Source with 

CALPUFF, prepared for VISTAS, June 13, 2006. 

2

 White, W. H., R. A. Eldred, P. J. Feeney, C. E. McDade, B. P. Perley, D. J. Shadoan, and P. H. 

Wakabayashi, Behavior of fine-particle elemental data near the detection limit, Paper # 24, A&WMA Specialty 

conference: Regional and Global Perspectives on Haze: Causes, Consequences and Controversies, Asheville, NC, 

October 25-29, 2004. 

3

 Paine, R., J. Connors, and S. Heisler. Computation of Average Sea Salt Concentrations for Consideration 

in Determination of Natural background Levels at Eastern Class I Areas, August 2, 2006. 

Appendix O.3 - 30
Enclosure 3 

where [Nanon-soil] = [Na] – 0.57[Fe] and [Fe] is the measured iron concentration. 

The VISTAS Technical Contractor4

 summarized a preferred hierarchy for estimating sea salt 

concentrations as envisioned by Pitchford (2006)5

: 

1. [Sea Salt] = 3.27[Na+

non-soil], where [Na+non-soil] = [Na+

] – 0.57[Fe] and Na+ is collected on 

Nylon filters and analyzed using ion chromatography 

2. [Sea Salt] = 1.8[Cl-

], where Cl- is collected on Nylon filters and analyzed using ion 

chromatography 

3. [Sea Salt] = 1.8[Cl], where Cl is analyzed by XRF or PIXE, despite a known bias toward 

underestimates due to loss of HCl on the filter media 

4. [Sea Salt] = 3.27[Nanon-soil], where [Nanon-soil] = [Na] – 0.57[Fe] and Na is analyzed by 

PIXE, which presents an alternative method to that used by Paine et al. (2006) 

The VISTAS Technical Contractor evaluated available data at Cape Romain to assess the 

practical application of these methods given variations in measurement techniques over time and 

concluded that the Na+

 method is superior, but is limited by the brief period of time (September 

2002 through December 2004) when such measurements were taken. The report identifies an 

annual average sea salt concentration in 2004 of 0.77 μg/m3

 using the Na+ method, but does not 

recommend a longer-term average value. The VISTAS Technical Contractor demonstrates that 

the 2004 annual average sea salt concentrations of 0.35 μg/m3

 estimated using [Cl-] and 

0.11 μg/m3

 estimated using [Cl] exhibit the expected underestimation bias due to the analysis 

technique. The elemental sodium (Na) approach could not be compared for this time period 

because Na measurements are available only from XRF, which is excluded as an unreliable 

method. 

To assess a long-term average natural background sea salt concentration, reliable estimation 

techniques were applied over the period of record when such measurements were taken. 

Additionally, a fifth estimation technique using strontium (Sr) as the elemental marker for sea salt 

was applied to corroborate sea salt estimates using other techniques. Lowenthal and Kumar 

(2006) suggest the following approach using Sr, which is measured accurately using XRF and for 

which measurements are available over the long-term time period (1994 through 2004) of 

interest.6

 This approach computes a non-crustal, marine-derived Sr concentration (xSr) as: 

[ ][] SrxSr −= [Si 0.0013528 ], (11) 

 

4

 Tombach, I. Evaluation of Alternative Methods for Estimating Sea Salt Concentrations at Cape Romain, 

prepared for VISTAS, March 5, 2007. 

5

 Pitchford, M. Revised IMPROVE Algorithm for Estimating Light Extinction for Particle Speciation Data, 

November 2006. 

6

 Lowenthal, D. and N. Kumar. “Light Scattering from Sea-Salt Aerosols at Interagency Monitoring of 

Protected Visual Environments (IMPROVE) Sites.” J. Air & Waste Management Association, 56: 636-642, May 2006. 

Appendix O.3 - 31
Enclosure 3 

where 0.0013528 is the crustal reference concentration of Sr and Si represents the silicon 

composition that is presumably entirely representative of entrained crustal material.7

 The sea salt 

concentration is computed from this value using the average strontium composition of sea water, 

0.000234. 

[ ] [ ]

0.000234

xSr Salt Sea = (12) 

The VISTAS Technical Contractor acknowledges Lowenthal and Kumar’s Sr approach as a 

newer method that has some uncertainty, but does not discount its application at Class I areas in 

the Southeastern U.S. This approach is appealing to apply at Cape Romain because Sr 

measurements have been made consistently since September 1994 using the same analytical 

techniques that are considered reliable for this marker throughout the period. 

Electronic files on the DVD enclosed with this report contain an analysis of average ambient sea 

salt concentrations using the Paine et al. (2006) and Pitchford (2006) methods using elemental 

sodium derived from PIXE measurements for the available data period September 1994 through 

November 2001, the VISTAS-preferred sodium ion approach for the available data period 

September 2002 through December 2004, and the Lowenthal and Kumar (2006) method using 

strontium for the available data period September 1994 through December 2004. Table 4-1 

summarizes data from each approach over the September 1994 through December 2004 time 

period during which reliable analytical techniques were in use for each species. 

TABLE 4-1. SUMMARY OF AVERAGE SEA SALT CONCENTRATIONS COMPUTED USING 

VARIOUS METHODS, SEPTEMBER 1994 – DECEMBER 2004 

 

Average [Sea Salt] (μg/m3)

Period Start Period End 

Paine et al. (2006) 

[Na] Method 

Pitchford (2006) 

[Na] Method 

VISTAS (2007) 

[Na+

] Method 

Lowenthal & Kumar 

(2006) [Sr] Method 

September 1994 November 2001 1.1 1.1 -- 1.1 

December 2001 August 2002 -- -- -- 1.3 

September 2002 December 2004 -- -- 0.77 0.78 

Long-term Average (September 1994-December 2004) 1.1 

The summary results presented in Table 4-1 indicate that the Sr method for calculating sea salt 

concentrations replicates estimates of sea salt concentrations calculated by preferred methods that 

use Na or Na+

 as the sea salt marker. When applied for Sr measurements at Cape Romain over a 

continuous, long-term (1994 through 2004) time period, the average background concentration of 

sea salt is estimated to be 1.1 μg/m3

. 

 

7

 Mason, B. Principles of Geochemistry, 4th ed. Wiley & Sons, Inc.: New York, 1982, p.344. 

Appendix O.3 - 32