Haze in the Grand Canyon An Evaluation of the Winter Haze Intensive Tracer Experiment (1990) / Chapter Skim
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Evaluation of Whitex
Evaluation of Whitex
Pages 16-36
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From page 16... ...
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 evaluate the contribution of the WHITEX study toward the science of source apportionment.
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From page 17... ...
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.
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From page 18... ...
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)
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From page 19... ...
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.
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From page 20... ...
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=)
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From page 21... ...
2~. Time-lapse video sequences taken on the east end of the south rim during the early part of the period showed welldeveloped wind flow into the Grand Canyon from the east; in contrast, aloft and at relatively low altitudes, winds flowed strongly from the west.
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From page 22... ...
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.
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From page 23... ...
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)
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From page 24... ...
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)
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From page 25... ...
As suggested by meteorological 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.
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From page 26... ...
However, even if these criteria were adequately considered, the statistical results would most likely remain nonrobust in the sense that the source attributions generated by the various statistical 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.
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From page 28... ...
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!
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From page 29... ...
EVALUATION OF ITEM -' ~ ;f\OV I EM 0.56 GR£~R ~ Vim Q f J ~ C
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From page 31... ...
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.
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From page 32... ...
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.
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From page 33... ...
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)
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From page 34... ...
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.
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From page 35... ...
Because H2O2 concentrations in the Grand Canyon were not measured, 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.
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From page 36... ...
Even if this analysis were pursued, background 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.
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