(Draft) OTAG Air Quality Analysis Results Relevant to Assessment of Coarse Grid States

 

None of the (roughly 40) OTAG Air Quality Analysis (AQA) reports and summaries (available at http://capita.wustl.edu/otag/Reports/Reports.html) have focused specifically on UAM-V fine grid ozone contributions from locations in the coarse grid. However taken collectively, these AQA studies do suggest that the direction and overall influence of ozone transport can be highly variable from site to site and day to day. The distance of detectable transport influence is also highly variable, ranging from less than 10 miles up to about 400 miles. At this maximum distance, all coarse grid areas have a potential to contribute to high ozone levels at some locations within the fine grid. Transport influences will generally decrease with distance, and are not equally probable for all sections of the coarse grid. Some of the available AQA reports include information which may be more specifically relevant to the coarse grid issue.1-8 Potentially relevant results include:

 

 1. How "ozone problem areas" are defined is a key issue in assessing coarse grid contributions. When only peak 1 hour concentrations in excess of 124 ppb are considered, the "problem areas" (with exceptions of coastal ME and urban centers in TX and LA) tend to be relatively distant from the coarse grid areas. With longer averaging times (seasonal baseline1, mean2, 8-hour max3), the spatial extent of "problem areas" increases, particularly in Central and Southwestern sections of the OTAG region, including areas on both sides of the coarse grid line in AK and MO.3 Aside from implications of future standards, a focus on longer averaging times also increases the probability that future meteorological influences will be similar to those observed in the past.1

 

 2. Many of the analyses suggest relatively minor contributions to high ozone levels in the fine grid from Northern coarse grid areas (i.e. northern: MN, WI, MI, NY, VT, NH and ME). Spatial pattern analyses 2,3 indicate that these Northern areas themselves are typically characterized by relatively low ozone for peak concentrations (1hr, 8hr, and 90th percentile) as well as for moderate to low ends of the distribution (seasonal average, 50th and 10th percentiles). Backward trajectories,5,6 forward trajectories,7 and surface wind analyses 8 all indicate that ozone levels at locations throughout the northern half of the fine grid are lowest with flows from the north. As with the Northwestern coarse grid areas (#4 below), the maximum fine grid impacts from these Northern areas are likely to coincide with low fine grid ozone levels; their contributions to high ozone concentrations are likely to occur to the north of OTAG (i.e. southern Canada).

 

 3. Many of the analyses suggest relatively minor contributions to high ozone levels in the fine grid from Southeastern coarse grid areas (i.e. Fl and Southern GA, AL, MS). Spatial pattern analyses 2,3 indicate that these Southeastern areas are typically characterized by relatively low ozone for peak concentrations (1hr, 8hr, and 90th percentile) as well as for moderate to low ends of the distribution (seasonal average, 50th and 10th percentiles). Chemical tracer analyses4 at rural sites just north and south of the Southeastern coarse grid boundary (in GA, Al, MS) indicate predominantly local "urban" influences and short transport distances in this region. Back trajectories at sites just north and south of the Southeastern coarse grid boundary (in FL, GA, MS, LA) indicate highest concentrations during local stagnation or with northerly flows, and lowest concentrations with flows from the south.5,6 Consequently, maximum fine grid impacts from the Southeastern coarse grid area typically coincide with low ozone levels.

 

 4. Several of the analyses suggest relatively small contributions to high ozone levels in the fine grid from Northwestern coarse grid areas (i.e. MN, ND, SD, NB, IA). Spatial pattern analyses 2,3 indicate that Northwestern areas are typically characterized by low ozone for peak concentrations (1hr, 8hr, and 90th percentile) as well as for moderate to low ends of the distribution (seasonal average, 50th and 10th percentiles). Backward trajectories,5,6 forward trajectories,7 and surface wind analyses 8 all indicate that ozone levels at locations in Northwestern sections of the fine grid are lowest with flows from the northwest. For certain episodes with persistent westerly flows, there is a potential for detectable influence from these Northwestern areas in the Lake Michigan region (such conditions should be well represented by the July 91 and 95 model episode runs).

 

 5. Some of the analyses suggest potentially important contributions to high ozone levels in the fine grid (or coarse grid) from Southwestern coarse grid areas (i.e. TX, OK, LA, AK) - especially if lower ozone thresholds are considered (i.e. 8 hour max). The Southwestern coarse grid region is characterized by complex, variable patterns of emissions, ozone concentration and meteorological flows. Minimum, average and peak ozone levels tend to be higher here than in other coarse grid areas. 2,3 The region includes several current urban non-attainment areas - with those in eastern TX and LA being sufficiently close together that interstate transport influences are likely (in either direction). Back trajectory analyses 5,6 indicates potential for transport into the Southwest from fine grid areas to its northwest (a feature not well represented by the model episodes). Peak Southwestern levels are more typically associated with very light winds from a persistently southerly direction (well-represented in the model episodes).7 Consequently the potential for maximum regional transport occurs to the north just along the western edge of the fine/coarse grid. Current fine grid non-attainment areas of concern include Memphis and St. Louis. Consideration of maximum 8-hour concentrations is especially important here, as the region exhibits relatively poor correlations between occurrences of peak 1 hour and 8-hour maxima 3 (i.e. they tend to occur on different days and/or locations). Maximum 8-hr levels in excess of 80 ppb are relatively common in large sections of TX, OK, AK and MO,2,3 which would be frequently subject to flows from the Southwest.

 

 References

1. Statistical Characteristics of Spectrally-Decomposed Ambient Ozone Time Series Data, 8/26/96 (http://capita.wustl.edu/otag/reports/StatChar/otagrep.htm).

2. Spatial Pattern of Daily Maximum Ozone over the OTAG Region: 9/16/96 (http://capita.wustl.edu/OTAG/Reports/otagspat/otagspat.html).

3. Pattern of 8-Hour Daily Maximum Ozone and Comparison with the 1-Hour Standard: 9/16/96 (http://capita.wustl.edu/otag/Reports/8hdmaxo3/dmax8hr.html).

4. Draft Technical Report Ozone/NOy Tracer Relationships at Three SOS-SCION Sites: 11/4/96

(http://capita.wustl.edu/OTAG/reports/ONTSCION/Ontscion.html).

5. VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to OTAG AQA Workgroup:12/3/96 (http://capita.wustl.edu/otag/reports/Status_Dec96/Status_Dec96.html).

6. VT DEC Air Trajectory Analysis of Long-Term Ozone Climatology: Status Report to OTAG AQA Workgroup: 11/7/96 (http://capita.wustl.edu/otag/Reports/VTTRAJ5/StatusReport1.html).

7. Update on the Characterization of Transport over the Eastern US: 2/1/97 (http://capita.wustl.edu/Otag/Reports/Sricont/Sricont.html).

8. Ozone as a Function of Local Wind Direction and Wind Speed: Evidence of Local and Regional Transport:5/7/97(http://capita.wustl.edu/otag/Reports/OTAGWIND/OTAGWIND.html)