# Appendix O.3 South Carolina’s Response to EPA
Comments on Sea SaltEPA Region 4 Review Commen ts 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 Improveme nt - State and Tribal Association of the Southeast (VISTAS). EPA Region 4 conti nues to endorse the recommendation made by
the IMPROVE Committee for use of chloride io n 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 ot her 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 concentrat ion 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 s ource 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 algorith m that properly depi cts the estimated natural visibility background by incl uding the effects due to sea saltconcentrations 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 concentrati ons 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

1water available to generate salt aerosol fr om 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 th e Atlantic coast is exposed to fresh salt aerosol particles from spray gene rated by coastal waves and would be prominent for the trajectories required to advect the source plumes to Cape Romain from either direction. Therefor e, sea salt concentr ations would likely

remain consistent.

Although the DAK Americas faci lity 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-br eeze 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, se a salt is expected to be present as a naturally occurring aeroso l. VISTAS states, including South Carolina, are using annual average natura l background conditions for visibility impairment caused by the species (e.g., sulfates, nitrates, organic and inorganic PM) represented in the IMPROVE light ext inction equation. As such, long-term average sea salt concentrations should be applied for consistent application as a visibility-impairing component of natural background conditions.

2An attempt to create a directionally-depe ndent 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 examp le. The same point could be made regarding windblown dust from various s ources. The implementation would be too complicated to start considering dire ctional 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 av ailable (at best) only every third day. A daily (or more frequent) analysis of airborne particulate during various wind/meteorological regimes is imprac tical, if not impossible. We have concluded that it is best to keep the im plementation 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 co llection and analysis of sea salt data. It is unclear how the submitted report fo r determining the appropriate sea salt concentration would actually be implem ented and used in a specific BART Assessment. Information should be pr ovided that discusses how data from specific years will be used in the BART m odeling 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 yea r), 0.69 (2003), and 0.77 (2004) ug/m 3 . In addition, long term elemental sodium and strontium data was analyzed for insights into how represen tative 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

3average sea salt concentration estimates for the longer data periods (1991 to 2001), whether using sodium or strontiu m as the marker, compare favorably to the shorter term VISTAS sodium ion marker average (1.1 ug/m 3 vs. 0.77 ug/m 3 ). In addition, the short term 2002 to 2004 str ontium marker average also compares very favorably to the VISTAS sodium i on marker data for the same period (0.78 ug/m 3 vs. 0.77 ug/m 3 ). Since the VISTAS data en compasses 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 samp les). That data, for all periods and all markers, showed fairly good stability in th e sea salt concentra tion estimated, with a minimum sea salt estimate of 0.745 ug/m 3 and a maximum estimate of 1.535 ug/m 3

, both when compared to the other ro lling averages and when compared to the average concentration for the entire applicable period. While there is, understandably, more variability in the se a salt concentrations on a sample to sample basis, the data suggests that th e ambient sea salt concentration is relatively stable over periods of two years or more. It is import ant to add that the exemption decision would be the same fo r all of the faci lities 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 estim ates using the long term data sets and the IMPROVE default chloride marker estimate . All of this s uggests that sea salt concentrations are fairly c onsistent 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 sa lt estimate calculated using the VISTAS 2002 to 2004 sodium ion data as described by Dr. Tombach.

EPA Comment 3 :

We recommend that the relationship betw een sea salt and sulfate concentrations
be reviewed and discussed in the doc umentation. 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 met hodology outlined by Dr. Tombach, has been attributed to sea salt, NaCl, and used to calculate th e estimated sea salt concentration ([Sea Salt] = 3.27[ Na + non-soil ], where [ Na + non-soil ] = [ Na + ] – 0.57[Fe].

SO 4 ions are routinely measured by ion chromatography by IMPROVE. SO 4 may replace Cl in current samples to form Na 2 SO 4 . To the extent that Cl is replaced

4by SO 4 , total current mass may be overestimated by using non-soil Na + and SO 4 mass without correction. Unfortunatel y, available IMPROVE data are not sufficient to estimate th e 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 s odium concentrations in shoreline environments. Therefore, non-soil Na will be an accurate estimate of sea salt under natural
conditions because SO 2 emissions and SO 4 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 thos e estimates that were calculated using
the elemental strontium a nd sodium markers. This suggests that the Na + ion
derived sea salt estimate is a conserva tive estimate even if there is any SO 4 replacement occurring in the 2002 to 2004 IMPROVE samples.

EPA Comment 4:

Blank problems on the nylon filters with resp ect to the analysis of chloride data was discussed in the report. No discu ssion of sodium ion (Na+) blank data is included in the report. It appears th at the standard operating procedure for IMPROVE monitors doesn’t call for analys is of Na+, so there has never been a
reason to look at Na+ blank values on thos e filters. VISTAS may have asked for analysis of blanks when they co mmissioned 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 bi ased high. If the data are available, we recommend they be as sessed 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 cont ract to the National Park Service, for ion chromatographic (IC) analysis of IMPROVE module B Nylon filter samples for cations -- or iginally ammonium (NH 4 + ) and later sodium (Na + ) -- for samples collected at nine IMPROVE loca tions, including Cape Romain, from 2 September 2002 through 29 December 2004. Sodium was analyzed following IMPROVE protocol. These cation anal yses 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 s odium ion loading. As detailed in the explanation provided by the VISTAS consu ltant, Dr. Ivar Tombach (Enclosure 2), Air Resource Specialists (ARS) used this data to calculate a median blank loading

5for each of the three filter batches and th e result was subtracted from each sample in the batch to provide th e 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_N H4Data.xls). On the “DATA” tab of this spreadsheet, cells M2 through M81 c ontain the filter blank measurements and column P contains the blank mass for each batch. A second sp readsheet is also included (VISTAS sodium_chloride_compar isons_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 c oncentration in ug/m 3

.

EPA Comment 5 :

EPA Region 4’s position on use of the new IMPROVE Equation and the procedure developed by VISTAS for im plementing 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 i ndicates that when the new IMPROVE algorithm is used, documentation explai ning the rationale and implementation should be provided to EPA. For a case where a method other than the default procedure of using chloride ion concentra tion for estimating sea salt is used, we
recommend that the rationale for using an alternate method should be provided in the facility-specific BART modeli ng 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. Iv ar Tombach to develop a methodology to post-process CALPUFF/CALPOST mode ling 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. Subse quent to their review, EPA Re gion 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 Qua lity 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 approva l 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 i on 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(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 chlori de to several gaseous form s such that it is no longer captured on the filter media. As such, technical/scientific experts (including VISTAS’ expert consultant) readily acknow ledge that the chloride ion is a very imperfect method of obtaining an estimat e 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 Nyl on filters, as a result of which the blankcorrected Cl - concentrations were too small and often negative.” This means that, in addition to the chloride de pletion bias, the blank-corrected Cl - based sea salt concentrations for the 2000-2005 bas eline are significa ntly 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 estimat e for their submittals. These facilities also provided information on estimates based on strontium and elemental sodium markers, the use of which has been suppor ted in the scientific literature surveyed on estimating sea salt co ncentrations. Enclosure 3 provides a detailed survey of methods for estimating sea salt concentrati ons using the available data at Cape Romain. The results of the survey fo und that the strontium approach generally estimates average concentrations very cl ose to average concentrations using the VISTAS consultant’s preferred method fo r applying measurements of sodium ion. A long history of reliable observation s are available from Cape Romain, including a continuous record of stro ntium measurements from September 1994 through September 2004. This data corroborates the 2002 to 2004 average value of 0.77 ug/m 3 based on the sodium ion approach from the VISTAS measurements, and illustrate a long-term average sea salt concentration of 1.1 ug/m 3 based on

continuous strontium measurements that correlate well with sea salt

concentrations estimat ed from elemental sodium and sodi um 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/m 3 is a better estimate of sea salt concentrations at Cape Romain for those facilities requestin g an alternative to the default IMPROVE chloride based estimate for their BART exemption modeling.

EPA Comment 6:

Consultation with the Federal Land Ma nagers is recommended in the BART assessment and ultimately in the developm ent 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

7this sea salt issue. The FLMs have a gr eat 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 reque sted the use of an alternative to the IMPROVE default. We are sharing the VIST AS 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 repor t has undergone a limited revi ew 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 IM PROVE algorithm. These workgroup recommendations have been used by EPA Region 4 in our comments on the BART protocols for the Region 4 States a nd 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 considerabl e number of discussions with, and gathered information from, many differ ent persons and sources and considered input from varied points of view on th is subject, including federal and state officials, consultants for both VISTAS and industry, and leading technical experts in the field of regional haze. We have f ound 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 ch loride 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 avail ability 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 wi th the sodium ion marker is the most reasonable method for estimating visibility impairment caused by coastal BART

sources.

EPA Comment 8:

89 We continue to remind the States that the BART assessment of a source is only one requirement for assessing a source fo r 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 th e 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 b een requested to submit a four factor analysis so that SC DHEC can assess wh ether or not controls are reasonable under the Regional Haze Rule. The only fa cility that has not been requested to submit a four factor analysis is an electr ic generating facility covered under the Clean Air Interstate Rule (CAIR). SC DHEC has decided that compliance with CAIR satisfies sources’ requirements under th e Regional Haze Rule for this first planning period. We are confident that th e end result for all these facilities will be the same as if these fa cilities were required to submit a BART determination analysis for consideratio n of controls.

### 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 te rm for the light scattering by fine sea salt. The document that presents the new IMPROVE algorithm 1 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, bu t 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 0.3 08 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 unkno wn 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 ap ply sea salt concentrations determined by several of
these methods to calculate the reconstructed PM 2.5 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 factor s used for sea salt vary slightly. We will use 3.27 [Na] later in this report2 gravimetric measurements of PM 2.5 ma ss 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 seawat er ratios with non

- crustal calcium, strontium (Sr), and potassium, 4 i.e., the measured Cl concentration 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 on shore 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

, in dependent 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 deple ted, 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 t he 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 20 06.

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. 2006.3 captures gaseous nitrate), so that the measured Cl - concentration is not affected. Hence, it appears that the Cl - concentration on Nylon filters probably accurately rep resents the Cl that 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 (I C) 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 Cl concentrations 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 concent ration 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

μ g/m 3 over the four years from 2000 thr ough 2003, but leaps to 0.345 μ g/m 3 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 μ g/m 3 , while the 2004 sea salt concentration is reported as 0.56 μ g/m 3 . 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 sal t 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 reg ion 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 Sal t Concentration Estimates for Southeastern US IMPROVE Sites,” by Marc Pitchford. 18 January 2007.4 1. Na + on Nylon filters, analyzed by IC. The Na + in soil in the sample has to be
subtracted from t he 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 (inc luding 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 me an 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

concen tration 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
IMPRO VE data base.) Because of uncertainties in the correction for self

absorption 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. T he formula is [Sea Salt] = 3.27 [Na
non

- soil ], where [Na
non

- 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
somewha t 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
d escribed 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 PM 1 , PM 2.5 , and PM 10 : 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 PM 10 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 estim ates.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 analy zed, 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 Cl measurements. Thus, in so me 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 un biased 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 o ther 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 South east. 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 ma ss 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 th e 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 wil l 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 PM 2.5 mass, against gravimetric PM 2.5 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 Inter comparisons. Figure 1 shows scatter plots that compare daily sea salt concentrations calculated by pairs of the four methods ranked above by6 Pitchford. The left column compares sea salt concentrations calculated from Na + , Cl - , and elemental Cl (by XRF) dur ing 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 rati o 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 fo rm 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 det erminations). 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 eleme ntal 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 +

measuremen ts was 0.77 μ g/m 3 , that from Cl - measurements was 0.35 μ g/m 3 , and the
value from elemental Cl measurements was 0.11 μ g/m 3 . The differences between the
methods are substantial. 8 These averages are strongly influenced by a few high values, so
the more

- repres entative medians (the values of the middle concentrations in lists of ranked concentrations) of 0.41, 0.11, and 0.00 μ g/m 3 , based on Na + , Cl - , and Cl respectively, are substantially smaller than the means. (For comparison, the corresponding means for the f ull 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.7 2004 1999

Figure 1. Relationships between estimates of sea salt concentrations at Cape Romain, usin g Na+, Na, Cl - , and Cl measurements available in 2004 (left) and 1999 (right).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 es timates, 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 concent rations 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 concentra tions 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 concen tration was used. The sea salt mass
concentration was based on the Na + measurements in the top panel; on the Cl measurements on the second panel; and on elemental Cl measurements in the third panel, all using the formulas given earlier in Pitchford’s reco mmendations. 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 f ew 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 a verage, 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 μ g/m 3 higher in 1999, so that the reconstructed mass concentration using sea s alt from Cl

- (the official IMPROVE procedure) was slightly greater, by about 0.7 μ g/m 3 , in 1999. Of greater import, the average gravimetric fine particle mass concentration was 9.03 μ g/m 3 in 2004 and 12.22

μ g/m 3 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 also9 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 μ g/m 3 in 2004 and 12.51

μ g/m 3 in 1999 for data sets that are somewhat more complete than the ones used here.10 2004 1999

Figure 3. Compariso ns 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.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 co ntributor 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 sa lt 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 dail y RH, at Cape Romain in 2004.
The calculated b sp 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 calcu lates 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 algorith m 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 b sp 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 tha n 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 figur e 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 dat a 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.12 Figure 4. Comparison of Different Measures of Daily Light Scattering at ROMA in
2004.

Figure 5. Comparison of Aerosol Light Scattering Calculated by Var ious Methods vs. Nephelometer Measurements.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 regress ion 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 (R 2 = 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 Aeroso l 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).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 overwhe lm 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 concentratio ns 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 reconstruct ed 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 c oncentrations, 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 re ported 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.

T urning 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 co mpared 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 substan tially less than those estimated from Na + or Na, and those based on elemental Cl are even smaller yet.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 sou nd one. The fourth place rank assigned by Pitchford for estimates from elemental Na may be too p essimistic. 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 ba sed 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.

### Ivar Tombach, Ph.D. 

## Enviro nme nta l Co ns

ulting 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 Inst itute (RTI) for ion chromatographic (IC) analysis of IMPROVE module B Nylon filter samples for cati ons – originally ammonium (NH 4 + ) 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 – SO 4 = , NO 3 - , and Cl - -- so VISTAS’ analyses augmented the IMPROVE data set.
RTI had calibrated for Na + when it analyzed the filters for NH 4 + (VISTAS' original request), so it was easily able to return to th e 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 gr eatly to 0.0 μg per filter in 2004.

ARS then applied IMPROVE’s flow rates a nd 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 c oncentration 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 concen trations were used to represent all of the samples over a 28-month period, the actual blan k 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 ti mes, which would mean that too large a
correction was subtracted from the ambient filter loading, possibly producing a negative
blank-corrected result.There were also a few calculated negative Na + concentrations at coastal sites. For example, at Cape Romain (ROMA), there we re four negative values, all in 2003, which
occurred when the ambient concentration wa s low. These negative values ranged from - 0.010 to -0.015 μg/m 3 , very small quantities compared to the average blank-corrected

Na + concentration there of 0.239 μg/m 3

.

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 concentratio n is negative, it is th e 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 ei ther direction. So, just because a resulting
blank-corrected concentration is too low (negative in this cas e), 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 neces sary 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 th at 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 inco rrect. The effect of replacing a negative number with a zero is to increase the aver age 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 onl y 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 IMPR OVE sites, nor is it practical to do so, so an estimate is made based on the typical com position 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 IMPROV E 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 concentrati on. This amount is subtracted from the total

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

Using a single value of 0.57xFe to represen t soil-based-sodium at locations, under all wind speeds and directions, is a simplificati on. As with the blank correction, sometimes the correction may overshoot th e 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 concentr ation 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.

23 For the 2004 ROMA sea salt cal culations, there was only one negative Na+ and sea salt value, on June 29 th , at -0.073 μg/m 3 of Na + or -0.237 μg/m 3 of sea salt. The corresponding averages for that year were 0.236 μg/m 3 of Na + and 0.771 μg/m 3 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 IMPROV E 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. Ho wever, the negative value on June 29 th , 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 zer o would also have a ne gligible 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 (interes tingly all in the winter and la te autumn), but they were all much smaller than the negati ve value in 2004, with non-soil Na + concentrations ranging from -0.002 to -0.030 μg/m 3 and resulting sea salt concentrations ranging from - 0.007 to -0.097 μg/m 3 . The corresponding average con centrations over the 16-month period were 0.22 μg/m 3 for Na + and 0.73 μg/m 3 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 3



### B ACKGROUND 

### S EA 

### S ALT 

### A EROSOL 

### C ONCENTRATIONS

Prepared by Trinity Consultants, Inc.

The 2005 IMPROVE Report identified sea salt aeros ols as an important component of natural visibility conditions, particularly in humid co astal 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 partic le mass at Cape Romain (which is considered representative of Wolf Island) and Okefenokee.

Measurement and estimation techniques for natura lly-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 ( f

SS (RH)) for sea salt aerosols based on observations in remote marine environments and thermodynamic models. Alt hough 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 inferre d from measurements of elemental markers and the period of record for which reliable data ar e available to compute a representative natural background level. VISTAS states, including Sout h 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 th e IMPROVE light extinction equation. The following analysis reviews available techniqu es 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 techni ques 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 sa lt from data from the IMPROVE network arise because positive ions have historically not anal yzed; therefore, sodium ion (the strongest indicator of sea salt) data are not readily availa ble, 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 produ ces 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: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 μ g/m 3 using

Cl
- as the marker, which is recognized to unde restimate the sea salt aerosol concentration. 1
Appendix 1 of the 2006 IMPROVE Report lists an average sea salt concentration of 0.377 μ g/m 3

, 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 infe rred 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. Pain e et al. (2006) computed an average sea salt

concentration of 1.123 μ g/m 3 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 concentra tion 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:



### [ ] [ ] [ ]

Soil Salt Sea NaNa Na

− = , (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 cons tructed in a similar manner to the Na + approach as:

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

1 Tombach, I. Estimating the Role of Sea Salt when Determin ing 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-parti cle elemental data near the detection limit, Paper # 24, A&WMA Specialty conference: Regional and Global Perspectives on Haze: Ca uses, Consequences and Cont roversies, Asheville, NC, October 25-29, 2004.

3 Paine, R., J. Connors, and S. Heisler. Computation of Average Sea Salt C oncentrations for Consideration in Determination of Natural backgr ound Levels at Eastern Class I Areas , August 2, 2006.Enclosure 3

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

The VISTAS Technical Contractor 4 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[Na non-soil ], where [Na non-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 va riations 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 measurem ents were taken. The report identifies an annual average sea salt concentration in 2004 of 0.77 μ g/m 3 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 μ g/m 3 estimated using [Cl
- ] and

0.11 μ g/m 3 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 fro m XRF, which is excluded as an unreliable

method.

To assess a long-term average natural backgroun d sea salt concentration, reliable estimation techniques were applied over the period of reco rd when such measurements were taken. Additionally, a fifth estimation technique using st rontium (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, wh ich is measured accurately using XRF and for

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

interest. 6 This approach computes a non-crustal, marine-derived Sr concentration (xSr) as:



### [ ][] [ ] Si 0.0013528 SrxSr −= , (11)

4 Tombach, I. Evaluation of Alternative Met hods for Estimating Sea Salt Con centrations 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. “L ight Scattering from Sea-Salt Aeroso ls at Interagenc y Monitoring of Protected Visual Environments (IMPROVE) Sites.” J. Air & Waste Management Association

, 56 : 636-642, May 2006.Enclosure 3

where 0.0013528 is the crustal reference concen tration 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 app ealing 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 repor t 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 th e available data peri od September 1994 through November 2001, the VISTAS-preferred sodium i on approach for the available data period September 2002 through December 2004, and th e Lowenthal and Kumar (2006) method using strontium for the available data period Se ptember 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.

T ABLE 4-1. S UMMARY OF A VERAGE S EA S ALT C ONCENTRATIONS C OMPUTED USING V ARIOUS M ETHODS , S EPTEMBER 1994

– D ECEMBER 2004

Average [Sea Salt] ( μ g/m 3 )

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 in dicate 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 pe riod, the average background concentration of

sea salt is estimated to be 1.1 μ g/m 3

.

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