September 12, SW Colorado PSD Increment Study Southwest Colorado Nitrogen Dioxide ( NO 2 ) Prevention of Significant Deterioration (PSD) Increment Study PHASE I Colorado Department of Public Health & Environment Air Pollution Control Division Technical Services Program September 12,

September 12, SW Colorado PSD Increment Study Why was this periodic increment study done? Federal rules require states to periodically review the adequacy of their PSD plans to prevent significant deterioration of air quality (i.e., to prevent increment violations). Federal Land Managers expressed concern about the Class I NO 2 increment at Mesa Verde. Colorado had concerns about growth from minor sources in the study area.

September 12, SW Colorado PSD Increment Study Project Phases For convenience, this study was divided into two phases: –Phase I : quantified increment consumption over a broad area and identified areas with apparent increment violations. –Phase II: studied ‘hot spots’ found in the phase I study A phased approach was helpful because we didn’t have to spend lots of time trying to resolve every apparent violation found in the phase I study. It kept the process moving.

September 12, SW Colorado PSD Increment Study Models Model selection was done in late 1998; modeling was performed in Emissions Models: EPA’s MOBILE5 and Non-Road Emission Factor models Meteorological Models: 1990 MM4, CALMET Air Quality Models: CALPUFF, ISCST3

September 12, SW Colorado PSD Increment Study Modeling Domain

September 12, SW Colorado PSD Increment Study Inventory Area, Mobile, and Point inventories were developed. Point sources were identified as increment consuming, baseline, or retired baseline. For area, mobile, and baseline sources, changes in actual emissions since the minor source baseline date were estimated.

September 12, SW Colorado PSD Increment Study Estimating changes in actual emissions since the minor source baseline date can be problematic (a moving target)

September 12, SW Colorado PSD Increment Study General Model Application Although the domain included New Mexico, we limited receptors to Colorado for several reasons. CALPUFF receptors were used for Class I areas regardless of the distance from source to receptor. ISCST3 was used for Class II areas.

September 12, SW Colorado PSD Increment Study CALPUFF Model Application AREA sources were used in CALPUFF for only those few grid cells with significant increment consuming emissions. This was necessary so the model wouldn’t run forever. We wanted to use CALPUFF for both the Class I and Class II receptors, but the CALPUFF model run time (with only the Class I receptors) was about 15 days of CPU time; thus, we could not finish the project on schedule if we modeled the Class II receptors with CALPUFF.

September 12, SW Colorado PSD Increment Study ISCST3 Model Application The inventory had over 700 increment consuming point sources with numerous meteorological towers. For permitting, we typically select met data for the source under review, but when 700 sources are ‘under review,’ met selection is problematic. Many ISCST3 model runs were done with different met sets. Results were interpreted based on the ‘most representative’ met data for a given ‘hot spot.’

September 12, SW Colorado PSD Increment Study Baseline and Increment Consuming NOx Stationary Sources in Modeling Domain Class I Mesa Verde and Weminuche are federal Class I areas where the annual nitrogen dioxide (NO 2 ) PSD increment is 2.5 micrograms per cubic meter (  g/m 3 ). All other areas are Class II areas where the NO 2 PSD increment is 25  g/m 3.

September 12, SW Colorado PSD Increment Study Change in Mobile/Area Gridded Inventory

September 12, SW Colorado PSD Increment Study Most Increment Consuming Emissions in the Study Area are from Point Sources

September 12, SW Colorado PSD Increment Study

Results No Class I increment violations found. No ‘regional’ Class II violations found. Apparent NO 2 increment violations due to complex terrain and building downwash were found in the vicinity of a large compressor station.

September 12, SW Colorado PSD Increment Study Public and Peer Review We consulted with source operators throughout the project, particularly for sources that appeared to have problems. We requested comments on the draft report from various stakeholders (U.S. EPA, New Mexico, Southern Ute Indian Tribe, Source Operators, Environmental groups).

September 12, SW Colorado PSD Increment Study Limitations and Problems The report had a large section on study limitations. A few of the important limitations included: –NOx chemistry treatment –meteorological models –lack of observations –inventory, inventory, inventory, inventory, inventory …

September 12, SW Colorado PSD Increment Study Closing Thought… If you do a periodic study, consider restricting receptors to a single ‘baseline area’ to avoid confusion. Why? –Sources located outside the ‘baseline area’ can contribute to increment consumption regardless of the ‘minor source baseline date’ in the area where they are located. –Why? According to EPA, only the ‘minor source baseline date’ applicable to the ‘baseline area’ under review is relevant. –For more, see April 5, 1999 memo from Bill Harnett (U.S. EPA)