2) Agent Decay. The following equation approximates the agents’ linear decay effects:

(Equation A-42)

where is the decay time scale (s) and Q is the agent mass (kg) in a puff. Based on the analysis in Tab A-I, a sinusoidal variation for the decay time scale,
GB (s), characterizes GB's daytime hydroxyl reactivity:

(Equation A-43)

where t is the time of day, tsunrise is the time of sunrise, and tsunset is the time of sunset. Hence, GB's decay time scale is about 1.5x104 seconds, or 4 hours at noon.

During nighttime hours, nitration is the primary decay mechanism. However, since no nitrate data were available, the nighttime decay is simply ignored so the predicted agent concentrations are higher.

Based on the same analysis, the GF decay time scale, GF, is 60% of the GB decay time scale, reflecting GF's higher reactivity from hydroxylation. Thus,

GF = 0.6 GB,

(Equation A-44)

so GF's decay time scale is about 3 hours at noon.

4. Refined Unit Location and Personnel Data

The USASCURR maintains two data sets essential for determining which veterans might have been exposed to the Khamisiyah Pit plumes: the first is a registry of Gulf War veterans with their Gulf War unit identification; the second is a registry of daily unit locations during the war. By associating information from these two data sets, a list of veterans possibly exposed over March 10-13, 1991 was developed. Since 1997, continuing DoD efforts have improved both the quantity and quality of the unit location and personnel data. For example, the original unit location and personnel data depend primarily on paper records, which were later found to be incomplete and in some cases erroneous. Consequently, beginning in 1997 and continuing through mid-1998, a series of meetings were held with the operations officers of various troop units to further refine the data. Moreover, the 1997 data contained no information about Air Force personnel, a shortcoming the 2000 data addressed. The result is more comprehensive data with higher quality.

5. Results

As in 1997, to partly account for the uncertainty in meteorological and dispersion modeling, DoD considered the model combinations in 2000 described earlier. The GPL and FNE contours for each model combination and each day were calculated and the union of the contours given by all model combinations was interpreted as the possible hazard area. In the 1997 Khamisiyah analysis, VLSTRACK was not run with the MM5 and OMEGA fields because the data conversion software was not developed in time. Therefore, these runs were not included for the 2000 analyses either for the sake of consistency. This is not likely to affect the size of the union hazard area because the hazard areas VLSTRACK predicts are almost always inside those HPAC/SCIPUFF predicts, due to the fact that VLSTRACK predicts median (50%) concentrations whereas HPAC/SCIPUFF's results present concentrations with a 1% probability of exceeding the nominal value.

The union approach is also called the ensemble approach in this report. However, in other literature, the ensemble approach sometimes also indicates the average of a set (ensemble) of model predictions (e.g., Gates et al., 1999 and Weisse et al., 2000).

The following four combinations of modeling assumptions were considered to study the incremental changes in exposure assessments due to dry deposition and decay:

(1) without deposition, without decay (the most inclusive case-has largest hazard areas);
(2) with deposition, without decay;
(3) without deposition, with decay;
(4) with deposition, with decay.

The second case is most similar to the 1997 results; however, different values for the deposition velocity were assumed in HPAC/SCIPUFF (0.3 cm/s in 1997 and 0.1 cm/s in 2000). Moreover, VLSTRACK did not account for dry deposition in 1997. However, this is inconsequential with the union method, since the contours generated by HPAC/SCIPUFF coupled with COAMPS, MM5, and OMEGA always masked the contours generated by VLSTRACK coupled with COAMPS. The fourth case represents our best, most realistic estimate of possible troop exposure.

Figures A-76 through A-79 show the GPL and FNE contours predicted by the union of the four model combinations overlaid with unit locations (shown as brown dots) for March 10-13, 1991, respectively. The upper panel shows the 2000 results, assuming dry deposition and agent decay (the 4th case), whereas the lower panel shows the 1997 results (CIA and DoD, 1997). The 1997 possible hazard areas, previously shown in Figures A-53 through A-57, are redrawn so they have the same style and scale as the 2000 results.

For Day 1 (March 10, 1991), during which the material was airborne for only 10.75 hours, mostly at night, the differences between 1997 and 2000 were due primarily to using new meteorological fields, indicated by slightly different cloud trajectories. For Day 2 (March 11), the hazard area generally moves in similar directions in both the earlier and present simulations. However, removal mechanisms caused a much smaller hazard area and no FNE area for the 2000 results. The removal mechanisms argument holds true for Day 3 (March 12). The hazard area was much smaller on Day 4 (March 13) because of relatively small emissions. As in 1997, the 2000 results did not show any servicemembers inside the FNE contours.

Figure A-76. GPL possible hazard areas for March 10, 1991

Figure A-76. GPL possible hazard areas for March 10, 1991
2000 modeling upper panel; 1997 modeling lower panel

Figure A-77. GPL possible hazard areas for March 11, 1991

Figure A-77. GPL possible hazard areas for March 11, 1991
2000 modeling upper panel; 1997 modeling lower panel

Figure A-78. GPL possible hazard areas for March 12, 1991

Figure A-78. GPL possible hazard areas for March 12, 1991
2000 modeling upper panel; 1997 modeling lower panel

Figure A-79. GPL possible hazard areas for March 13, 1991

Figure A-79. GPL possible hazard areas for March 13, 1991
2000 modeling upper panel; 1997 modeling lower panel

Figure A-80 shows the enlarged view of the March 10 FNE and M8A1 contours based on the 2000 results. These contours do not exist for the other days because of lower agent concentrations. No troops are within the M8A1 contour, which enclosed the FNE contour. This probably is why there were no reports of the sounding of M8A1 alarms during the time period of the Khamisiyah Pit demolition.

Figure A-80. 2000 First noticeable effects and M8 alarm alarm detection areas

Figure A-80. 2000 First noticeable effects and M8 alarm alarm detection areas

In summary, the 2000 modeling results show much smaller but somewhat different exposure areas due to changes in mesoscale meteorological modeling, the reduced source term, consideration of removal mechanisms, and modified exposure thresholds for GB and GF and the way to account for their combined toxicity. The exposure areas should be interpreted in conjunction with the unit location and personnel data, which also changed significantly since 1997. Thus, the 1997 modeling results and the personnel and location data show that an estimated 99,000 servicemembers are within the GPL contours, with none within the FNE contours; but the 2000 modeling results and the personnel and location data show roughly 102,000 servicemembers within the GPL contours, again with none within the FNE contours. Of these two estimates, about 66,000 servicemembers are within the GPL contours for both the 1997 and 2000 simulations. The 2000 count is slightly higher than 1997's, despite the smaller 2000 hazard areas, because the 2000 unit location and personnel data are more accurate.

Finally, additional sensitivity runs were conducted to study the relative importance of dry deposition versus decay. Although the dry deposition velocity is relatively small at 0.1 cm/s, the ground-based agent cloud is continuously subject to mass removal due to dry deposition. On the other hand, although the peak decay time scales for GB and GF are relatively short (three to four hours at noon), decay occurs only during daytime hours and follows a sinusoidal variation (see Equation A-43). Thus, both the deposition and decay mechanisms contributed significantly to the agents' removal over the time period required for the plume to travel the distance from the source to the farthest extent of the GPL contours.

6. Peer Review Panel

The above refined methodology for modeling the demolition in the Khamisiyah Pit was also described in a separate DHS report (DHS, 2000a) prepared for a peer review panel. The peer review panel for the report consisted of these members:

· Dr. Richard Anthes, President, University Corporation for Atmospheric Research;
· Dr. Steven Hanna, Research Professor, George Mason University; and
· Mr. Bruce Hicks, Director, Air Resources Laboratory, NOAA.

In general, the panel determined that the revised 2000 methodology is satisfactory and an improvement over the methodology used in 1997 (Anthes et al., 2000).


A-I Degradation From Atmospheric Radicals
A-II Recommendations Of Vapor And Inhalation Toxicity Estimates To Be Used In
Khamisiyah Modeling


| First Page | Prev Page | Next Page |