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16 · HAZE IN THE GR4ND CANYON Quality3, the U.S. Department of Energy, the U.S. Environmental Protection Agency, and the Arizona Salt River Project. The committee's final report, which will address this charge, will be issued in 1991. In addition to the final report, the committee was also asked to provide this special report evaluating WHITEX, the recent site-specif~c study conducted by NPS. This report evaluates the scientific evidence relevant to EPA's recent- ly proposed finding that NGS contributes to impairment of visibility in GCNP. Specifically, the committee was asked to review NPS data and analyses upon which the EPA determination was based and other data and analyses related to source apportionment for Grand Canyon haze. It was also asked to evalu- ate the contribution of the WHITEX study toward the science of source ap- portionment. The committee reviewed the December 1989 NPS-WHITEX re- port, other relevant published materials, and some unpublished information. In March 1990, the committee conducted site visits to the GCNP and NGS near Page, Arizona, and it heard technical presentations from NPS, SRP, and their scientific consultants. This information was used as part of the basis for the committee's evaluation. The Committee's Specific Interpretation of Its Charge The committee focused on assessing the methodology and design of WHITEX and the validity of the conclusions. The committee based its evalu- ation solely on the scientific aspects of WHITEX. The committee is aware that its assessment is relevant to the regulatory matters currently before EPA. The committee wishes to emphasize that it has not considered regulatory issues relating to NGS and expresses no opinion on them. Such issues involve policy considerations; these matters lie outside the committee's purview. This review of WHITEX u ill be considered by the committee in the prepa- ration of its final report. WHITEX is an example of approaches to source attribution in Class I areas. Because WHITEX is one of the most recent studies of this type, it will provide useful insight for the final report. EVALUATION OF W1IITEX WHITEX Overview WHITEX was conducted on the Colorado Plateau in Northern Arizona and Southern Utah between January 7 and February 18, 1987 (days 7-49 in 1987~.
EVALUATION OF WHITEX · 17 The objective of the WHITEX research program was to evaluate the su~tabili- ty of various receptor modeling methods for attributing haze in specific Class I areas (i.e., Grand Canyon and Canyonlands) to emissions from an isolated point source (i.e., NGS). Previous investigations demonstrated that wintertime regional hazes occur during periods of air stagnation and that these hazes are largely attributable to submicrometer particles composed primarily of SO4=, organic carbon compounds, and black carbon (soot) (Maim and Walther, 1979; Walther and Maim, 1979; Chinkin et al., 1986~. A significant part of WHITEX focused on SO4=, because it is the dominant light-scattering species during the most severe haze episodes. NGS emissions include primary SO4= particles, as well as gaseous SO2 which is converted to SO4= in the atmo- sphere. The NPS-WHITEX report contains the following major elements: . A discussion of the experimental setting, including a review of regional emissions and climatology; · A description of the optical, particle, SO2, and tracer measurements, as well as discussions of data quality; ~ A description of some preliminary prognostic transport modeling for one 2-day period; and . Various analyses supporting the attribution of SO4= concentrations and haze to NGS. ~ distinctive and novel feature of the WHITEX experiment was the use of CD4. CD4 is nearly inert; its concentration during WHITEX would not have been significantly affected by chemical reactions, precipitation scavenging, or dry deposition to the surface. The concentration of CD4 in an air parcel can be reduced only through dilution with air that does not contain CD4. WHITEX used CD4 as a tracer to identify air parcels that contained NGS emissions, to estimate the dilution that had occurred during transit, and to estimate the amount of sulfur species that were originally injected into the air parcel by NGS. Knowing the ratio of SO2: CD4 in the stack emissions at NGS and knowing the concentration of CD4 at GCNP, the concentration of NGS SO2 that would have been present in the air parcel in the absence of deposition or conversion can be calculated, assuming the CD4 and sulfur species from NGS travelled by the same trajectory. This calculated upper limit on NGS-derived sulfur is referred to as S(CD4~. The analyses in the NPS-WHITEX report focused on data acquired at Hopi Point, because higher CD4 concentrations were found there than at the other sampling stations (except for Page, which is immediately adjacent to NGS). NPS estimates of NGS effects on haze at GCNP depended on the
18 · HAZE IN THE GRAND MUON measurements of CD4 at Hopi Point. However, measurements of CD4 by themselves could not provide information on the fraction of SO2 that was converted to SO4= in transit, nor could they account for the quantities of these species that were deposited during transit. Thus, S(CD4) yielded only an upper-limit estimate of possible effects of NGS SO2 emissions at GCNP. For this reason, statistical and modeling tools were needed to make quantita- tive estimates of NGS impacts. The attribution analysis of the NPS-WHITEX report was carried out in two stages: (1) the observed light extinction first was apportioned to fine-particle SO4= and other atmospheric species, and (2) the observed SO4= concentra- tions then were attributed to NGS and other sources. The apportionment of extinction among chemical species (extinction budgeting) was largely based on literature and statistical values for the extinction: mass ratios (extinction efficiencies) of the various species. The quantitative attribution of SO4= to specific sources rested primarily on semi-empirical statistical models, highly Simplified physical models fit to the data through least-squares procedures. WHITEX Source-Attribution Models The quantitative attribution of SO4= to NGS rested on two empirical mod- els: Tracer Mass Balance Regression (TMBR) and Differential Mass Balance (DMB). T-MBR employs multiple linear regression (MLR) of the SO4= concentration on selected source-tracer concentrations to estimate the ambient SO4=: tracer ratios attributable to individual sources. MLR has been used since the mid-1970is to apportion primary (directly emitted as particles) source contributions to ambient aerosol mass, although it has been subjected only to limited testing and verification. The literature does not contain convincing evidence that MLR applications can successfully apportion a predominantly secondary (particles formed in the atmosphere) species, such as SO4=, among several source types. In the ~'S-WHITEX report, some of the tracer concentrations are multi- plied by relative humidity (RH) in an attempt to account for the increased rate at which SO2 is converted to SO4= in liquid-phase reactions ~ clouds or fogs. RH scaling as used in the NPS-WHITEX report appears to be previous- ly untested. DMB is an elaboration of TMBR in which the regression variable for the target source is adjusted to reflect the varying ages of emissions at the recep- tor. The expected proportion of SO4=: tracer is calculated based on the following factors: 1) assumed and constant values of SO2 and SO4= deposi- tion rates, 2) an SO2 conversion rate assumed to be in constant proportion to
EVALUATION OF WHITEX · 19 RH, and 3) plume ages estimated from u~nd data. Because these factors were not measured during WHITEX, NPS selected them from within a range that they believed to be physically reasonable to maximize the correlation coeff~- cient of the multiple linear regression relating SO4= to the NGS tracer term and other source tracer terms. The net effect is one of nonlinear multiple regression. The use of DMB appears to be unprecedented in the source-apportionment literature. Because it ultimately relies on MLR, its statistical assumptions are similar to those of TMBR. As with TMBR, DMB requires that SO4= from untraced sources be only negligibly correlated with the source-tracer terms used in the regressions. The statistical assumptions used In the NPS- WHITEX report are accurately identified in the report (Appendix 2~. Critical Aspects of WHITEX Techniques and Design The committee assessed the qualitative and quantitative aspects of the NPS- WHITEX conclusions; these two aspects are addressed separately. Qualitative Assessment The committee concludes that a properly executed experiment using a methodology and design similar to those used in WHITEX could provide qualitative information as to whether NGS emissions con- tribute to 5O4= aerosol and resultant haze in GCNP. The WHITEX protocol included measurements of ambient optical proper- ties, concentrations of key gaseous and particulate species, tracers for con- tributing SO4= sources (including CD43, wind-flow patterns and other meteo- rological data, and time-lapse photography. WHITEX analyses included dynamic meteorological modeling of air movements, tracer mass-balance calculations, and multiple-regression analyses for apportioning SO4= among sources. Such information should be adequate to support a qualitative assess- ment of whether NGS emissions reach GCNP and whether these emissions contribute to haze in GCNP. The use of CD4 in NGS emissions could provide definitive evidence of the transport of NGS emissions to the GCNP. Dynamic meteorological modeling could provide supplemental information that could be used to help evaluate the extent to which the emissions are transported into and distributed throughout the Grand Canyon. For any particular site, the contributions of the various aerosol species to optical extinction could be reasonably estimated.
20 · HAZE IN THE GRAND CANYON Multiple-regression techniques, such as those used in TMBR and DMB, have a long history of success in many areas of science; they are widely ac- cepted in epidemiology, econometrics, and other disciplines for which cause- effect relationships are complex and for which extraneous factors cannot be controlled. Such techniques clearly could be used in a WHITEX-type experi- ment if 1) satisfactory tracers were available for all major sources that might affect GCNP, and 2) there were a strong correlation between the NGS tracer and the fraction of haze-form~ng aerosol (i.e., SO4=) that was not accounted for by the tracers for all other sources. Under these conditions, the results of multiple-regression analyses would constitute persuasive qualitative evidence that NGS emissions had a detectable effect on haze at GCNP. However, the literature does not demonstrate that previous MLR applications can success- fully apportion a predominantly secondary species, such as SO4=, among several source types. Therefore, it would not seem advisable to rely solely on such models for the success of a major field experiment. The committee concludes that WHITEX qualitatively showed that, at some times during the study period, NGS emissions significantly contributed to SO4- aerosol and resultant haze at Hopi Point in GCNP. This conclusion is based on the data presented in the NPS-WHITEX re- port. The conclusion is not based on the results of the TMBR and DMB analyses, which contained various shortcomings that are discussed in more detail later in this report. Instead, the committee's qualitative assessment is based on the following observations and measurements made during WHITEX. Meteorological Evidence. Meteorological analyses support the NPS conclu- sion that NGS emissions can be transported to GCNP during the wintertime when the air stagnates. The NPS-WHITEX report focused on February 11- 14, 1987 (days 42-45 in 1987), for its intensive analyses of the NGS contribu- tion to haze in GCNP. During this time, the region was dominated by a polar high-pressure system that resulted in low-speed surface winds. Under such conditions, surface winds at Page usually alternate between northeasterly during the day and southwesterly during the night (Balling and Sutherland, 1988), and observations at Page during the study period showed this pattern. Furthermore, the upper-air winds measured at Page confirmed the NPS con- clusion that winds at the expected plume height flowed from the northeast during much of this period. Deterministic meteorological modeling performed in WHITEX also indicated that winds at the height of NGS stacks could have
EVALUATION OF WHITEX · 21 carried emissions to the GCNP area; however, these simulations did not re- produce the diurnal fluctuations in wind flow observed at the surface at Page. The meteorological data and the deterministic meteorological modeling do not allow quantification of the contribution that NGS might have made to haze at GCNP. The deterministic meteorological modeling cannot pinpoint the location of the NGS plume nor its entrainment into the canyon. The model uses a grid size of 5 km; hence, it cannot reproduce the complex topog- raphy of GC~P (Fig. 2) nor the associated small-scale meteorological effects, such as gravity flows. (For example, the mode! could not be elected to quantify the mass of sulfur entering the Grand Canyon from the rim versus that transported directly dour the Grand Canyon.) Thus, the meteorological studies provide only qualitative evidence of transport. Photographic Evidence. The wind-field analyses are supported by time- lapse photography and still photographs of cloud, fog, and haze conditions. Photographic evidence was obtained on the rim of the Grand Canyon and elsewhere in the region. The time-lapse images provided particularly striking evidence of the complex meteorological conditions that are due in part to the complicated topography (Fig. 2~. Time-lapse video sequences taken on the east end of the south rim during the early part of the period showed well- developed wind flow into the Grand Canyon from the east; in contrast, aloft and at relatively low altitudes, winds flowed strongly from the west. Still photographs provided additional information on the meteorological context of February 11-14. Photographs from Echo Cliffs looking northeast tower-d NGSabout 24 km away~howed noticeable haze on February 8 that dissipated on February 9. On the 9th, a brown plume was seen moving in a westerly direction from NGS. On February 10-12, the plume was embedded In fog; when the fog rose, the plume appeared to move to the west. On the afternoon of February 12, skies were clear and visibility was improved, except In the Lake Powell valley, where a haze was obvious. These photographs are evidence that the NGS plume was entrained into a cloudy environment with winds traveling toward the GCNP most of the time during February 11-14. The presence of cloud water within the plume has important implications regarding the conversion rate of SO2 to SO4=, because the heterogeneous conversion rate in cloud droplets can be much faster than that for homoge- neous gas-phase conversion. Chemical and Physical Evidence. The chemical and physical evidence are summarized by the following statements, most of which are based on data obtained during WHITEX.
22 · HAZE IN THE GRAND CANYON 1. Averaged over the WHITEX study period, SO4= aerosol was a signifi- cant contributor to non-Rayleigh light extinction at Hopi Point in GCNP. During certain episodes, SO4= was the predominant contributor to non-Ray- leigh light extinction. These conclusions regarding the contribution of SO4= aerosol to haze at Hopi Point are not sensitive to uncertainties in the WHIT EX data (including substantial uncertainties regarding the carbon data). These conclusions are based on visibility and aerosol data taken dunag WHITEX and on literature values for sulfate light extinction efficiencies and are consistent with prior studies linking SO4= to haze ~ the Southwest (Tri- jonis et al., 1989~. 2. NGS is one of the largest single SO2 sources in the United States west of the 100th meridian, and during WHITEX, it was also among the largest. Although other large SO2 sources could affect Hopi Point (e.g., the smelters in southeast Arizona and Mexico, other power plants, and urban areas), NGS is the source closest to Grand Canyon 25 km from the GCNP boundary and 110 km northeast of Grand Canyon Village- while the other major sources are 300-500 km distant. Mass-balance considerations suggest that the rate of SO2 emissions from NGS during WHITEX was large enough to produce sulfur concentrations at GCNP much greater than those measured at Hopi Point. 3. The SO4= measured at Hopi Point during haze episodes probably in- cluded contributions from sources within the region. These episodes tended to occur during stagnant wind conditions, which could lead to the accumula- tion of emissions from sources in the region, as evidenced by the significant spatial inhomogeneities in SO4= concentrations. The modeling studies of transport winds during the major stagnation episodes showed that NGS emis- sions could affect GCNP. However, these findings do not preclude the possi- bility of significant contributions from other local and regional sources, such as copper smelters, urban areas, and other power plants. 4. During the periods selected for tracer analysis, the tracer data showed that Page and Hopi Point were affected significantly by the NGS plume. During three episodes, average CD4 tracer concentrations were generally much higher at Page and substantially higher at Hopi Point than at the other six sampling locations. The CD4 tracer indicated that NGS contributions could account for total sulfur concentrations 2.5 times greater than those actually measured at Hopi Point. However, these tracer studies cannot ac- count for losses in transit, nor can they reduce the large uncertainties regard- ing the conversion rates of SO2 to SO4=. 5. Cloudy conditions were observed during WHITEX haze episodes. These conditions favor the higher conversion rates required to generate significant SO4= contributions from NGS at Hopi Point.
EVALUATION OF WHITEX Quantitative Assessment The committee concludes that a properly conceived and executed expenment, using a methodology and design similar to those used in WHITEX, might be useful to determine qua~ti~tively the fraction of SO4- aerosol and resultant haze in GCNP that is attributable to NGS emissions. · 23 The committee concludes that an experiment based on the WHITEX meth- odology could provide a quantitative determination of the fractional con- tribution of NGS to haze in GCNP in simple- but highly improbable~ases, such as: · If no CD4 were measured in GCNP, then the unambiguous conclusion would be that NGS made no contribution to GCNP haze. The absence of measurable CD4 would be evidence that no material of any kind was trans- ported from NGS to GULP. · If CD4 were measured in GCNP and background measurements showed that SO4= from other sources were insignificant at that time, then all SO4= detected could be attributed to NGS. Beyond simple cases such as these, there is little consensus among those in the source apportionment field about which methods might be appropriate for apportioning haze due to secondary SO4=. Labeling the sulfur or omen might provide a definitive test. However, because of the large background S and the radioactivity of Us, use of these isotopes is impractical. Some believe that extensions of receptor-oriented techniques similar to those used In WHITEX, if applied with better tracers and better temporal and spatial reso- lution, might provide quantitative estimates. Others believe that alternative analyses would provide more reliable quanti- tative estimates. For example, a mass balance might be developed to explain measured SO2 and SO4= concentrations across the sampling stations. The mass balance would incorporate emissions from all sources in the region, calculations of convective fluxes based on dynamic meteorological modeling, and wet and dry deposition (using measured values where possible). Others feel that source apportionment can best be achieved using deterministic mod- els that couple transport, deposition, and known SO2-to-SO4= conversion mechanisms. The validity of the models would be tested by comparing simu- lations with measurements from the sampling stations. This lack of consensus among experts is evidence of the need for further efforts to validate or other- wise evaluate methods used for source apportionment of secondary aerosols.
24 · HAZE IN THE GRAND CANYON The committee concludes that WHITEN did not quantitatively deter- mine the fraction of SO4= aerosol and resultant haze ir' GCNP that is attributable to NGS emissions. The committee found that the data analyses described In the NPS- WHITEX report contain weaknesses that preclude quantitative source appor- tionment. The report did not attempt to quantify the effects of departures from model assumptions on the analysis, nor did it establish an objective and quantitative rationale for selecting among various statistical models. In addi- tion, the conceptual framework for DMB involves physically unrealistic s~mpli- fications, and their impact on quantitative assessments was not addressed. These points are elaborated in the following section. Limitations of the WHITED Study .1 Weaknesses in the Data Base Uncertainties about Tracer Data. DMB and TMBR require that emissions from specific sources or source types be associated with unique tracers. In WHITEX, these tracers were CD4 for NGS, As for copper smelters, and Se for coal-fired power plants (although the latter two sources each emit some Se and As, respectively). No tracer in WHITEX was used to evaluate urban -emissions; therefore, the fraction of haze attributable to these sources is im- possible to calculate. Furthermore, the source profile for power plants was based on limited aircraft measurements of NGS emissions downwind from the stacks. The copper-smelter profile was based on old and uncertain data from the literature. Variabilities and uncertainties in NGS CD4 emission rates (which ranged from 2 to 5 mg CD4 per MW during the study (Appendix 2, p. 75~) led to substantial uncertainties in the day-to-day relationship between CD4 and NGS sulfur emissions. Moreover, at Hopi Point, CD4 concentrations were determined for only 36 samples, an undesirably small data set for the types and large numbers of statistical analyses performed on the data. Several questions have been raised about the accuracy of the data regarding CD4 emissions from NGS and, specifically, the ratio of CD4: SO2. The rate of injection of CD4, normalized to power output, was known to vary during the experiment by a factor of 2.5, and these changes were factored into the WHITEX data analyses. However, the ratio of CD4: SO2 was not measured in the stack (samples apparently were collected but not analyzed). The ratio was measured in the plume using samples collected from aircraft. In addition, a small leak was discovered in the CD4 injection line after the experiment was completed.
EVALUATION OF WH17~EX · 25 The MPS-WHITEX report provides little documentation of procedures and quality assurance for the sampling and analysis of ambient CD4. Despite the known problems with emissions and the lack of documentation on ambient measurements, the committee concluded that the CD4 data are among the most useful data obtained In WHITEX, because CD4 is the most specific tracer available for NGS emissions. In addition, most of the difficulties with the CD4 data pertain to daily variations in the concentrations, not the overall average concentrations. Nonetheless, deficiencies in the expenmental design precluded quantitative results regardless of CD4 data quality. Absence of Measurements within the Grand Canyon. One of the greatest weaknesses of the study was that no measurements were made below the rim of the Grand Canyon, within the canyon itself. As suggested by meteorologi- cal considerations and supported by still photographs and a time-lapse video of the February 11-14 period, a strong shallow wind flowed over the Colorado plateau and cascaded into the eastern end of the canyon at Desert View. This uggests that sulfur concentrations in the canyon might have been considerably greater than was observed on the rim farther away at Hopi Point. Inadequacy of Background Measurements. Because the WHITEX study originally focused on Canyonlands National Park, too few sampling stations were located In the area surrounding GCNP. Without data from additional stations, the effect of NGS emissions is difficult to differentiate from those of other sources in the region. These considerations are important for a thor- ough evaluation of the sources of SO4= in GCNP. This issue is addressed in more detail below. Departures from Statistical Assumptions The statistical assumptions underlying TMBR are accurately identified in Appendix 6B of the NPS-WHITEX report. Analogous assumptions underlie DMB, because it too is based on regression analysis. The discussion of the TMBR assumptions concludes, "Ideally, if there was a constant background pollutant concentration . . . and if the tracer release was directly proportional to emissions, and emissions were conservative, the reported estimated average NGS contribution should be a reliable estimate of the actual value for the time period in question.. Each of the quoted conditions appears to have been violated by the WHITEX data. The nonproportionality of the CD4 release rate and the nonconservation of SO2 emissions are discussed at length in the NPS-
26 · HAZEIN THE GRAND CANYON WHITEX report and this report. Less is Clown about the behavior of the background SO4=, because insufficient attention was devoted to it in the placement of sampling sites and the selection of CD4 samples for analysis. However, the concentration of non-NGS SO4= in the region clearly vaned significantly. For example, concentrations at Monticello increased from 0.27 fig S/m3 from late February 9 to 0.45 ,ug S/m3 early February 10 (Fig. 3~; during this period, CD4 was 9 x 10-s ppt, correspondin~to a maximum possi- ble contribution from NGS of only about 0.05 fig S/m (Appendix 2, pp. 76, 77, 85 (eq. 6-10~. Samples collected at Green River, Canyonlands, and BulEfrog during this period were not analyzed for CD4. The NPS-WHITEX report did not attempt to quantify the effects on its analyses of departures from the statistical assumptions that it identified. However, the potential magnitude of such effects is substantial. Unfortunate- ly, the WHITEX design did not provide the data needed for a definitive reso- lution of this issue. Formulation of Statistical Models The SO4- contribution attributed to NGS depends strongly on the model chosen, the tracers included in the model, and the criteria by which the model is fit to the data. The MPS-WHITEX report attached most significance to the TMBR and DMB models using the variables of CD4 concentration x RH for .NGS am1 As concentration x RH for copper smelters. Variable selection was critical to the interpretation of the results, because CD4 is clearly not the only tracer correlating with GCNP SO4=. Indeed, NPS noted in its reply to SRP's comments that two-thirds of the SO4= variance can be accounted for by RH and As alone. To establish a more rational basis for quantitative attribution, more attention must be given to alternative formulations for TMBR and DMB and to criteria for selecting among them. However, even if these criteria were adequately considered, the statistical results would most likely remain non- robust in the sense that the source attributions generated by the various statis- tical models would probably still differ substantially from one another. One difficulty is that the number of plausible alternative models is substantial relative to the number of samples for which CD4 data are available. As the number of models increases, so does the likelihood that one of them will test significant purely by chance. The NPS-WHITEX report assumed SO4= yields from NGS and smelter emissions to have been proportional to the ambient RH, as an index of their exposure to liquid water. This is a simple and indirect assumption, which scales intermittent processes along the entire trajectory at cloud level directly
EVALUATION OF WHITEX · 27 'TV _ __. 0.2°' GREEN RIVER ·~` I~NYO~ HIT£28 ~0. 19 10 BRYOt:£22 Buffing ~30 - ,IJT CO Age * / AZ NO 0.26/ N9VR'JO J Hi POINT 0.29 _ ~ Dorm r DAY 40 BOO AbI (FED 9) 0.30 GREEN R I VER C-~ rE 29 t'ON r I CE {3RTCE BULLFROG \ 0.38. ~ 1C 0.29 ~TT ~XICRN t~~ · ~ A~ Ct 1 0.35 ~I POl~T 10.29 0 . 3 ~ DAY 40 800 PM (FEB 9) FIGURE 3 Fine particulate sulfur (pg/rn3) measured at WHITEX sampling sites, February 9-12, 1987. (Multiply by 3 to get sulfate.) Source: Malm, 1990.
28 · HAZEIN THE GRAND CANON .~ - 0.34 GSt£EN RIVER CP~ ~(~°rl'T ~0~2~3ULF34 / ~: I \t'EOi 33 m/JT CO 144~ ~ Flop! ~IN' ( 0.12 al - are DAY 41 800 AN (FEB 10) 1 W~ ~ .~ :( AVID 0.39 GR££~t RIVER I E 3 r I CE ts T ~XICRh - ~ · ,7 AD L FIGURE 3 (continued). 0.52 U ~T~! - DAY 41 500 Pb6 . (FE8 10)
EVALUATION OF ITEM -' ~ ;f\OV I EM 0.56 GR£~R ~ Vim Q f J ~ C. 67 or 75 - t/~ m~T a;| LO CO NO 0.75 DAY 42 800 AD (FED 11) _ . -,.,0 _ o.~o ~JP9lY I _ DAY 42 800 PA (FED 11) FIGURE 3 (continued). 29
30 · HAZEIN THE GRAND CANYON ) off' J hITE BRAWL/ 0.87 GREEN R I YER !~ , on' STIR - K!,-' HOPI PO<N / to CO Nk 'it 0.33 I "Parw' DAY 43 800 AN (FEB 12) 0.67 . G2£FN R ~ Vie I ~ ~T0~6 ice. I ~ :~HEOl'S8Y-pT~C Jr :: AZgi t - I l°O _ 0.16 spoor DAY 43 800 P31 (FEB 12) FIGURE 3 (continued).
EVALUATION OF WHITEX · 31 to a continuous variable measured locally at ground level. The RH factor is critical to the explanatory power of the statistical models: without it, CD4 alone can account for only 3% of the observed variance in SO4= at Hopi Point and only 6% of the observed variance in total sulfur. Given the over- riding importance of the RH scaling factor, the committee believes that the sensitivity of results to alternative assumptions should have been explored in formulating the models used for the TMBR and DMB analyses. The NPS- WHITEX report also assumes that the contributions of background sources, such as other power plants and urban areas, were unaffected by RH. No effort was made to justify this assumption. The committee believes the report should have considered the possibility that yields from other sources were also affected by RH. Simplifications in the DMB Model The DMB analyses are dependent on unique Plume ages." The validity of these ages is questionable, given that travel times from NGS to Hopi Point were estimated to be 12-48 h on February 11-12. Slow-moving air parcels typically contain a mixture of materials (possibly more than one plume) emit- ted from a variety of sources. Furthermore, plume ages were estimated only for NGS emissions and not for other contributing SO4= sources. The effects of these simplifications on quantitative apportionment are unknown. The DMB -approach is based on linear models for the oxidation of SO2 to SO4= and for the deposition of SO2 and SO4=. In reality, both processes are likely to occur at rates that can vary greatly in time and space. The major transformation process for SO2 during wintertime conditions is probably oxidation by hydrogen peroxide (H2O2) ~ in clouds. Oxidation rates by this process theoretically can exceed several percent per minute. Such high rates are maintained for only short periods due to rapid depletion of either H2O2 or SO2. In the absence of clouds, the photochemical conversion rate of SO2 is very slow~lose to 0%/min under wintertime conditions at GCNP. The result is that under cloudy conditions, a significant portion of the SO2 in an air parcel is rapidly transformed to SO4= each time the parcel is entrained into a cloud; otherwise, the SO2 remains essentially unconverted and, hence, cannot contribute significantly to haze conditions in GCNP. Furthermore, deposition and oxidation are coupled processes. Because of HA more realistic model should include the heterogeneous oxidation of SO2 in cloud water by ozone, oxygen (carbon or metal ion catalyzed), and other oxidants.
32 · HEREIN THE GRAND MOON the nonuniform transformation rate of SO2 to SO4=, dry deposition rates also are nonuniform. Rainfall was measured between February 10-12 at the Grand Canyon Airport weather station; consequently, wet deposition occurred. Because rainfall in complex terrain is seldom spatially or temporally uniform, wet-deposition rates were probably not uniform. These nonuniformities in conversion and deposition rates lead to variabilities ~ the relationship be- tween SO4= concentrations measured at the receptor sites and tracer concen- trations used In the regression analyses. Because these nonuniformities were not taken into account in the DMB formulation, the DMB results are of questionable applicability. Potential Covariance of NGS and Other Source Contributions Even if CD4, Se, and As were accepted as satisfactory tracers for ad major sources that could potentially affect GCNP, a critical gap remains in the chain of evidence - D4 was not shown to add anything to the explanatory power of Se and As. In other words, the NGS tracer was not shown to correlate with any of the SO4= variability that is not already accounted for by generic source tracers. One reason for this might be that CD4 and Se are themselves corre- lated, with a correlation coefficient of 0.6. This suggests that the effects from NGS emissions and those from other Se sources affecting the Grand Canyon were correlated (and perhaps highly correlated considering imprecisions in the -data set). It is true, as stated by NPS, that a high degree of collinearity between CD4 and Se is consistent with the conclusion that the emissions from other plants did not reach GCNP and that all Se came from NGS. However, the observed degree of collinearity is also consistent with the hypothesis that emissions from other plants did reach GCNP and that their SO4= contributions were correlat- ed with those of NGS. The latter hypothesis is not unreasonable, given that most other power-plant emissions occur also to the east and north in the Colorado River drainage basin, and that RH could have had a similar effect on those emissions. Under such conditions, it is difficult to distinguish statisti- cally the relative effect of NGS from those of other coal-fired power plants in the region, given the limited number of data. The committee concludes that GCNP haze due to NGS emissions cannot be quantitatively estimated solely on the basis of TMBR and DMB analyses.
EVALUATION OF WHITEX · 33 Estimates of the Range of Possible Impacts of NGS Emissions at Hopi Point The WHITEX data can be used to estimate ranges of possible SO4= ef- fects from NGS. These estimates consist of a series of mass-balance calcula- tions made on the basis of simplifying assumptions. These calculations are made for iDustrative~ purposes and cannot, In themselves, prove or disprove that NGS emissions were responsible for GCNP haze, because measurements needed to confirm some of the assumptions were not made during WAX. The committee's estimates are summarized below. In discussing these esti- mates, the committee devised a set of three questions to address specific concerns. Assuming that all NGS emissions are camed into GCNP under typical wind conditions, is the rate of NGS SO2 emissions sufficient- large to produce total sulfur concentrations in GCNP that are comparable in magnitude to those measured during WHITEX at Hopi Point? This is the simplest quantitative question that can be asked about the po- tential impact of NGS on haze in GCNP. This case ignores all complicating questions and focuses on the worst case~hat all NGS sulfur emissions are carried into the Grand Canyon and distributed uniformly throughout it. Using the NGS emission rate during 1987 reported in the NPS-WHl I OX report (163 tons SO2/day) (Appendix 2, p. 80), assuming the width and depth of the Grand Canyon range generally from 8 to 16 km and 0.9 to 1.2 km respectively, and assuming a mean wind speed of 2-4 m/see (pers. comm., K. Gebhart, NPS, May 25, 1990), the total sulfur concentration within the canyon would be about 10-60 ~g/m3. This is much greater than the NPS-WHITEX-estimat- ed upper limit of total sulfur attributed to NGS at Hopi Point over the period of the CD4 analyses. This total sulfur concentration is also significantly great- er than the total sulfur measured at this site during this period, when values were typically in the range of 0.2-1 ~g/m3 (absolute range, 0.07-1.50 1lg/m3, excluding the single value of 4.4 ~g/m3) (Appendix 2, pp. 84, 85~. Although crude, this estimate suggests that under appropriate conditions, the rate of SO2 emissions from NGS is easily large enough to serve as the source of the sulfur measured in GCNP. The implicit assumptions in this upper-limit calculation are: 1) that the meteorological conditions enable the NGS plume to be transported into the Grand Canyon with little dispersion (i.e., that a substantial fraction of the NGS output actually enters the canyon), and 2) that there is relatively little loss of sulfur during transit. It is clear that
34 · lIAZE IN THE GRAND CANYON both assumptions often were not true to some degree during the experiment. Furthermore, this estimate does not address the question of the degree to which SO2 is converted to SO4=, a factor that is critically important to haze effects. If the NPS-WHITEX estimates of sulfur transport from NGS to GCNP are correct, how n~uc1' SO2 might be expected to be converted to 5O4~ aerosol during transit from NGS to GCNP under winter meteorological conditions such as those observed during WHITEX? To address this question, the committee estimated upper and lower limits for the amount of conversion that could take place using data for the haze episode on February 11-12. Data were used from this episode because it is the focus of much of the WHITEX analysis. At Hopi Point on February 11- 12, a maximum of about 2 ~g/m3 S(CD4) could have come from NGS (Appendix 2, p. 85~. This is based on the CD4 concentrations at Hopi Point and the total sulfur: CD4 ratio in the NGS plume. The total measured concentration of sulfur at Hopi Point was about 0.5 ~g/m3 on February 11, and 0.25-0.4 ~g/m3 on February 12 (Appendix 2, p. 84~. Minimum Conversion of SO2 to SO4=. In the absence of clouds, SO2 conversion is~controlled by homogeneous gas-phase photochemistry, and conversion-rates are at a minimum. The NPS-WHITEX report provided estimates of the wintertime 12-h average daytime conversion rate (about 0.06%/h) and the 24-h average rate (about 0.03~o/h) (SAI, 1985; Appendix 2, p. 81~. The NPS-WHITEX-estimated transport times from NGS to Hopi Point during February 11-12 ranged from 12 h to 48 h (Appendix 2, p. 82~. 2 The committee assumed that the maximum 2 ~g/m3 S(CD4) at Hopi Point began its transit from NGS as SO2 and that there was no loss from the plume due to wet or dry deposition. For a 12-h transit time and an omdation rate of 0.06%/h, the maximum amount of secondary SO4= aerosol generated during transit would be only 0.043 ~g/m3 SO4=. For a 48-h transit and a daily average homogeneous SO2 oblation rate of 0.03%/h, the concentration i2Figure 6.10 of the NPS-WHIT~X report provides conversion rate estimates for December and March, two periods that bracket the February period of interest. Because the present objective is to estimate a lower limit on the SO2 conversion, the conversion rate estimates for December are used. These conversion rates are consistent with experimental data on the SO2 conversion rate that was observed to occur in the NGS plume during another visibility experiment (Richards et al., 1981~.
EVALUATION OF ITEM · 35 would be only 0.086 ~g/m3 SO4=. Aerosol concentrations of this magnitude should have little effect on haze at Hopi Point. This conclusion Is based on field measurements and on the relationship between SO4= concentrations and haze (Trijonis et al., 1989~. From data in the NPS-WHITEX report on the primary-particle emission rate from NGS and the primary-particle concentration in the NGS plume, the committee also estimated an upper limit for the transport of pr~mary-particle emissions from NGS to Hopi Point and concluded that NGS primary particles should not play an important role in GCNP haze. The calculations above assume that the only effective processes are the generation of secondary aerosol through homogeneous gas-phase chemistry and the transport of primary aerosol. In reality, some depositional loss of SO2 and aerosol during transport to Hopi Point is inevitable. Consequently, the actual contribution of NGS emissions would be lower than that calculated here. Maximum Conversion of SO2 to SO4=. The maximum conversion rate would occur through heterogeneous oxidation of SO2 by H2O2 (and 03) to form SO4= within cloud droplets. Video tapes show that clouds were present in the vicinity of the Grand Canyon during much of the study period. Meas- urements of H2O2 rarely are made in the atmosphere except in connection with a specific experimental program. None were made at or near the Grand Canyon during WHITEX. The nearest temporal and spatial measurements appear- to be those of Van Valin et al. (1987~. They found that for cloud-free conditions, H2O2 concentrations ranged between 0.1 and 0.5 ppb in February 1987 near Memphis, Tennessee, approximately the latitude of the Grand Canyon. Because H2O2 concentrations in the Grand Canyon were not meas- ured, the committee assumed that, for the purpose of estimating the maximum oxidation rate, these data were representative of the NGS plume. If 0.1-0.5 ppb H2O2 reacts completely with SO2 in an oxidant-limited system, about 0.~ 2 ~g/m3 of SO4= is formed. This concentration range includes the maximum 12-h average total SO4= concentration measured at Hopi Point, 1.3 ~g/m3 (derived from Fig. 3) during February 11-12, and is below the limit of the maximum amount of NGS-sulfur that potentially could be presents ,ug/m3 SO4= (2 ~g/m3 S(CD4~. This estimate suggests that the heterogeneous conversion of NGS-emitted SO2 could account for virtually all of the SO4= measured at Hopi Point on February 11-12. If all of the SO4= measured at Hopi Point over February 11- 12 were due to NGS emissions, then NGS definitely would have contributed to haze at Hopi Point. However, the validity of the assumptions regarding heterogeneous conversion are unknown.
36 · RAZE IN THE GRAND CANYON Was there evidence that regional background SO4- could have ac- counted for a significant fraction of the 1.3 ~g/m3 5O4. at Hopi Point on February 11? This question can be addressed by examining the WHITEX data obtained at all sampling stations. Late on February 9, SO4= levels were relatives uniform throughout the area northeast of NGS, ~ the range of 0.~1 Gym (Fig. 3~. The same was true early on February 10, at ad sites northeast of NGS except Monticello, where the concentration was about 1.4 ~g/m3. (Data from Page were excluded from this background determination, because this site clearly was too close to NOS.) Thus, values In the range of 0.6-1 fig SO4= /m3 could be concluded to constitute the regional background for Febru- ary 9, and early February 10. Subsequent NGS emissions can be added to the background as the air mass passes over NGS and proceeds to GCNP. At Hopi Point on February 11, SO4= concentrations were near 1.3 ~g/m3. On February 12, the total SO4= measured at Hopi Point was about the same as the initial regional SO4= background estimate. If the committee's estimate of background SO4= Is correct, then the SO4= increment above regional background that might be attributed to recent NGS emissions would be in the range of 0.3-0.7 ~g/m3 out of the total 1.3 ~g/m3 measured at Hopi Point on February 11. The committee's assumption of the existence of background SO4= concen- trations;says nothing about the possible sources of that background SO4=. It does n lot preclude the possibility that a significant portion of background SO4= was derived from NGS emissions in the days preceding February 11. CD4 concentrations at Mexican Hat and Monticello on February 9-10 were 8-9 x As ppt (Appendix 2, p. 76-77~. Samples collected at Green River, Canyon- lands, and Bullfrog during this period were not analyzed for CD4. The measured concentrations imply an upper-bound NGS contribution of 0.15 ~g/m3 SO4= to the regional background, suggesting that on this occasion, most of the regional background SO4= was actually not derived from NGS. This illustration obviously is inexact; its primary purpose is to show the importance of accurate data on background concentrations for each air-parcel trajectory. Unfortunately, background SO4= was not adequately addressed in the NPS-WHITEX report. A further analysis of the WHITEX data is war- ranted to assess the effect of regional background SO4= on the amount of SO4= measured at Hopi Point. Even if this analysis were pursued, back- ground estimates at GCNP would remain uncertain, because the number of sampling stations was inadequate to evaluate this aspect. The existence of significant background SO4= concentrations implies that, if NGS emissions were controlled, wintertime haze at GCNP likely would be reduced but not eliminated.
