Grantee Research Project Results
1999 Progress Report: Investigation of Indicators for Ozone-NOx-VOC Sensitivity
EPA Grant Number: R826765Title: Investigation of Indicators for Ozone-NOx-VOC Sensitivity
Investigators: Sillman, Sanford
Institution: University of Michigan
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1998 through September 30,1999
Project Amount: $306,407
RFA: Air Pollution Chemistry and Physics (1998) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
Objective:
The project seeks to investigate the use of observation-based indicators for identifying the sensitivity of ozone to nitrogen oxides (NOx) and volatile organic compounds (VOCs). In the past, O3-NOx-VOC sensitivity has been based on model predictions, which are difficult to test in the real world. Sillman (1995) found that model NOx-VOC predictions are correlated with the values of certain "indicator ratios," involving total reactive nitrogen (NOy) and peroxides. Measured values of these ratios might be used directly to determine whether ozone is primarily sensitive to NOx or to VOC. Measured values of these species also can be used to evaluate whether model predictions for the impact of reduced NOx and VOC emissions on ozone are accurate.
The project also seeks to develop methods for estimating the role of large power plants in ozone formation, based on model-measurement comparisons.
Progress Summary:
During the past year, a major effort was undertaken to investigate rates of ozone formation in large power plants. This effort was based on a previous measurement-based study (Ryerson, et al., Journal of Geophysical Research 1998;103:22569) associated with the Southern Oxidant Study. A model was developed to represent the dynamical and photochemical evolution of a power plant as it moved downwind. This model differed from previous models in that it used much finer horizontal (1 km) and vertical (25 m) grid resolution. Results were used to obtain estimates for the rate of ozone production per emitted NOx in power plants, and to compare these estimates with estimates of ozone production rates derived directly from measurements.
The major finding of this investigation concerned the impact of large power plants on far-downwind locations. It is known that large power plants cause increased ozone at locations 100-200 km downwind of the plume source. Typically, the ozone production efficiency (equal to the amount of excess ozone produced per emitted NOx) is much lower for power plant NOx emissions than for NOx from other sources. However, model results predicted that power plants would generate ozone further downwind (300-400 km) with higher efficiency. At this downwind distance, the ozone generated per NOx from power plants is comparable to the ozone generated per NOx from other emission sources. Summary figures from this study are included at the end of this section (see Figures 1-4).
In addition, a series of investigations were completed during the past year relating to the concept of indicators for NOx-VOC sensitivity. These include: (1) an investigation of the theoretical basis for proposed NOx-VOC indicators, in terms of chemistry; and (2) a case study of the use of indicators, in combination with measured VOC, to evaluate ozone chemistry in Nashville, TN. These investigations formed the basis of a Ph.D. thesis (see Publications/Presentations section). The results include:
1. Ozone-NOx-VOC chemistry is directly linked to the balance of odd hydrogen sources and sinks. This balance forms the theoretical basis for measurable species that might serve as NOx-VOC indicators.
2. Indicators involving the ratio of peroxides to nitric acid or peroxides to total reactive nitrogen (NOy) are directly linked to the balance of odd hydrogen. These remain valid over a wide range of environmental conditions.
3. The ratios O3/NOy and O3/NOz (where NOz represents NOy-NOx) are the most useful of the NOx-VOC indicators, because NOz is more easily measurable than peroxides, and has been proposed for inclusion in the PAMS network. The values of these ratios can be correlated with NOx-VOC sensitivity for a broad range of conditions, but correlation can vary in different environments (e.g., see Lu and Chang, Journal of Geophysical Research 1998;103:3453-3462). This variation in the behavior of O3/NOy and O3/NOz can be explained in terms of the chemistry of odd hydrogen. O3/HNO3 also is useful as a NOx-VOC indicator.
4. Various "smog production algorithms" have been proposed as observation-based methods for evaluating NOx-VOC sensitivity (e.g., Blanchard, et al., NARSTO review, in press). These methods are based on results from smog chambers. They do not appear to work well in photochemical models.
5. Models for ozone formation in urban areas are critically dependent on assumed emission rates for anthropogenic and biogenic VOCs. These emission rates can be evaluated, indirectly, by comparing model and measured VOCs. Such model-measurement comparisons can effectively set limits on the range of uncertainty of emission inventories. This provides an important basis for evaluating model performance, and forms a natural complement to evaluation through measured NOx-VOC indicators.
The following figures are from an article, currently in press in Journal of Geophysical Research. They also were presented at the annual meeting of the American Meteorological Society (see Publications/Presentations section).
Results compare measurements of O3 and NOx in the Cumberland (Tennessee) power plant plume (Ryerson, et al., 1998) with model results. Model results include two scenarios: an initial scenario with plume emission estimates from Ryerson, et al. (1998). This emission estimate is 33 percent higher than the measured NOx in the plume. In the modified scenario, power plant emissions were reduced by 33 percent. This generated model NOx and O3 that agreed well with measured values (Figures 1, 2, and 3).
The critical result is shown in Figure 4. This figure shows the predicted ozone production efficiency (i.e., number of ozones produced per emitted NOx) for ozone in the late afternoon, at various downwind distances from different source types. The source types include: an urban source (the city of Nashville), a relatively small power plant (Gallatin, TN), a large power plant (Cumberland, TN), and a large power plant in an environment with low biogenic emissions.
In this figure, at distances 6-9 hours downwind from the plume source, the ozone produced per NOx is much higher for an urban source or small power plant than it would be for a large power plant. This is the same conclusion reached by Ryerson, et al. (1998). However, at distances 18 hours downwind (representing emissions from an urban or power plant source at midnight, followed by 18 hours transport downwind), the amount of ozone produced per NOx from the large power plant is comparable to the ozone produced per NOx from the urban source or from a small power plant.
Ozone production efficiency is higher at far downwind distances because the plume has had more time to disperse. A dispersed plume produces ozone at a higher rate of ozone per NOx. When the plume is emitted in the morning, it has little time to disperse and has a low rate of ozone production per NOx. When a plume is emitted during the evening, it disperses for several hours before it starts to produce ozone. By morning, the power plant plume has been dispersed over a wider area and produces ozone at a higher rate per NOx.
Figure 1. NOx in the Cumberland (Tennessee) power plant plume, 7 hours downwind from the plume source. Measurements (points), initial model (solid line), and modified model (dashed line).
Figure 2. Excess NOx flux in the Cumberland power plant plume (E25 molecules/sec) versus downwind time (hours). Measurements (points w/error bars), initial model (solid line), and modified model (dashed line).
Figure 3. Excess O3 flux in the Cumberland (Tennessee) power plant plume (E25 molecules/sec) versus downwind time (hours). Measurements (points w/error bars), model base case (solid line), modified model (dashed line).
Figure 4. Model ozone production efficiency in plume versus downwind distance, for Cumberland plume (dashed line), Cumberland with low biogenics (dotted line), the smaller Gallatin power plant (line-circle), and Nashville (solid line).
Future Activities:
The major uncertainty associated with O3-NOx-VOC indicators concerns possible aerosol interactions. Previous investigations have been confined to gas-phase chemistry only. Aqueous chemistry also has been ignored. Over the next year, we expect to prepare an evaluation of NOx-VOC indicators and indicator chemistry using the Community Multiscale Air Quality (CMAQ) modeling system (Models-3), including both gas-phase, aqueous, and aerosol chemistry. A major field measurement campaign currently is planned for Houston, TX, during summer 2000. These field measurements will be used as a basis for evaluating the performance of NOx-VOC indicators in an environment where aerosols (and, possibly, aqueous chemistry) are likely to have a major impact.
Publications are being prepared based on results from the recently completed Ph.D. thesis, listed above.
Over the next year, a Web site will be developed that provides practical guidance for investigators who want to use the method of photochemical indicators and other observation-based methods for evaluating O3-NOx-VOC sensitivity. It is hoped that this may be especially useful to state agencies in developing State Implementation Plans for ozone violations.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 17 publications | 7 publications in selected types | All 6 journal articles |
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Type | Citation | ||
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Sillman S. Ozone production efficiency and loss of NOx in power plant plumes: photochemical model and interpretation of measurements in Tennessee. Journal of Geophysical Research 2000;105(D7):9189-9202. |
R826765 (1999) R826765 (Final) |
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Supplemental Keywords:
tropospheric, oxidants, ambient air, chemical transport, nitrogen oxides, sulfates, organics., RFA, Air, Toxics, VOCs, tropospheric ozone, Engineering, Chemistry, & Physics, air quality standards, risk assessment, nitrous oxide, air modeling, EPA Model 3 Systems, ozone, power plants, ambient emissions, bias, indicator ratios, modeling predictions, Nitric oxide, model measurement comparisonsRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.