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Response to Comment on “Geographic Sensitivity of Fine Particle Mass to Emissions of SO2 and NOx” We welcome the comments by Boylan and Odman (1) as an important part of the dialogue on the current state of fine particle modeling. First, we reiterate the fact that the published work was aimed at using, in a parallel application, the air quality model developed for the Southern Appalachian Mountains Initiative (SAMI). The problems cited by Boylan and Odman were identified after all the SAMI modeling was completed. The revised version of URM-1ATM was not the SAMI version. It is also important to mention that identifying errors in models is a never-ending process. It is unfortunate that these errors were identified after the SAMI work. However, it is important to keep testing the models under a variety of conditions and configurations to determine just how sensitive and robust they are with various parameterizations and assumptions (errors and all). The real value is in comparing the model outputs based on the SAMI configuration/ assumptions with outputs from the latest version of the model. We are confident that, with time, even more improvements will be identified for URM-1ATM and similar models. As for specific differences in computed sensitivities between the SAMI and newest versions of URM-1ATM, we are frankly surprised that the results are as similar as reported by Boylan and Odman (1). A large part of our analysis focused on the SO2-sulfate link. The sensitivity of sulfate particles to SO2 is apparently little affected by the reported changes to the model. This indicates that the simulated SO2-sulfate relationship is quite consistent in this model when it comes to differences in atmospheric chemistry. Boylan and Odman (1) report that the simulated response of nitrate and organic carbon particles to SO2 appears to be even less sensitive in the revised model than we reported (2). These sensitivities were already found to be quite small. In the case of nitrate, we reported sensitivities of between 0 and 0.4 µg m-3 for a 10% reduction in SO2 emissions (2). Boylan and Odman (1) suggest that the revised model would, under similar conditions, predict sensitivity magnitudes of less than 0.2 µg m-3 per 10% SO2 reduction. For organic carbon, we reported sensitivities of no more than (1.5% per 10% SO2 reduction (2). Again, Boylan and Odman (1) report model differences that could possibly reduce these sensitivities to something like (0.2% per 10% SO2 reduction. These results reinforce our conclusion that particle species other than sulfate were relatively insensitive to SO2 emission changes. Finally, Boylan and Odman (1) report different sensitivities for sulfate and organic carbon particles in response to changes in NOx emissions. Our paper reported sulfate sensitivities of * Corresponding author phone: (256)386-3643; fax: (256)386-2499; e-mail:
[email protected].
10.1021/es0404724 Not subject to U.S. Copyright. Publ. 2004 Am. Chem. Soc. Published on Web 08/03/2004
(0.5% per 10% reduction in NOx emissions (2) (responses differed somewhat for low-level versus elevated sources). On the basis of the revised model, Boylan and Odman (1) suggest these sensitivities, under similar conditions, could be as large as (1%/10% reduction in NOx. In the case of organic carbon, our results found sensitivities were nearly always negative and were computed to be as small (negative) as -1% per 10% NOx reduction (2). Given the revised version of URM-1ATM, these sensitivities could turn out to be smaller (more negative), perhaps down to -1.3% per 10% reduction in NOx. These differences, although in some cases large in terms of the sensitivity metrics, are actually quite small in terms of absolute particle concentrations. None of the differences from the revised URM-1ATM would change the conclusions in our paper regarding the relative importance of SO2 to sulfate and PM2.5, nor do the Boylan and Odman comments (1) change our conclusions regarding the relative geographic relationships for source and impact regions. Boylan and Odman (1) take issue with our statements regarding the applicability of DDM. These statements were included in the SAMI Air Quality Modeling Final Report (3). They are based on the best knowledge available at the time of the report. Boylan and Odman (1) suggest that simulated sensitivities of some particle species may be well represented by DDM for emission changes as large as 50%. This is great if true. However, we preferred to remain on the cautious side when stating the applicable range for the DDM technique because some relationships between emissions and secondary gases/particles are decidedly more nonlinear than others. Also, our statement that DDM cannot provide estimates of changes in emissions to simultaneous changes in multiple emission species is true for DDM as implemented in the SAMI URM-1ATM. It is also true that, as the technique is generalized (to accommodate multiple emission species) within a given computer model, the coding rapidly becomes more complex, the model run times increase, and the results become more difficult to interpret. In the end it comes down to a tradeoff, judged subjectively by the modeler, between his/her ability to simplify modeling using a technique like DDM and his/her ability to understand what a model’s results imply about the nature of the atmosphere.
Literature Cited (1) Boylan, J. W.; Odman, M. T. Environ. Sci. Technol. 2004, 38, xxxx-xxxx. (2) Mueller, S. F.; Bailey, E. M.; Kelsoe, J. J. Environ. Sci. Technol. 2004, 38, 570. (3) Odman, M. T.; Boylan, J. W.; Wilkinson, J. G.; Russell, A. G.; Mueller, S. F.; Imhoff, R. E.; Doty, K. G.; Norris, W. B.; McNider, R. T. SAMI Air Quality Modeling Final Report; Report for the Southern Appalachian Mountains Initiative: Asheville, NC, 2002.
S. F. Mueller,* E. M. Bailey, and J. J. Kelsoe Tennessee Valley Authority P.O. Box 1010 Muscle Shoals, Alabama 35662-1010 ES0404724
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