## Meteorological Determination for the Martin Drake Power Plant

Modeling, Meteorology, and Emission Inventory Unit Air Pollution Control Division / Technical Services Program Colorado Department of Public Health & Environment

September 3 , 201 5

The process completed by the Modeling, Meteorology, and Emission Inventory Unit (MMEIU) to determine if any of the available meteorological datasets are representative of transport and dispersion of the Martin Drake Power Plant plumes for 1-hr SO 2 NAAQS designations are formally documented in this report.

On September 21, 2011, U.S. EPA issued draft guidance on state implementation of the 1-Hour SO 2 NAAQS. This guidance identifies air quality modeling as an alternative to monitoring for demonstrating attainment with the 1-hr SO 2 NAAQS. In October 2011, to prepare for this potential modeling exercise, MMEIU completed an assessment to determine if there are any available meteorological datasets that are representative of transport and dispersion conditions at large stationary SO 2 sources in Colorado

, including the Martin Drake Power Plant. The 2011 assessment was documented in brief and informal notes/figures that would need to be edited to fully convey the elements of the assessment if formal documentation is needed. As a result of comments from the Air Quality Control Commission regarding meteorological data representativeness for the Drake Power Plant during the August 20, 2015 hearing, formal documentation of the meteorological determination completed for this power plant is provided below.

Meteorological data requirements for r egulatory applications of AERMOD are specified in U.S. EPA’s Guideline on Air Quality Models (40 CFR Part 51 Appendix W; November 2005), which is referenced in all of U.S. EPA guidance documents for SO 2 designations modeling. The Guideline describes factors that need to be consider ed when determining representativeness of meteorological data:

“The meteorological data used as input to a dispersion model should be selected on the basis of spatial and climatological (temporal) representativeness as well as the ability of the individual parameters selected to characterize the transport and dispersion conditions in the area of concern. The representativeness of the data is dependent on: (1) The proximity of the meteorological monitoring site to the area under consideration; (2) the complexity of the terrain; (3) the exposure of the meteorological monitoring site; and (4) the period of time during which data are collected. ”

Regarding regulatory applications of AERMOD, the Guideline explains the following:

“ Of paramount importance is the requirement that all meteorological data used as input to AERMOD must be both laterally and vertically representative of the transport and dispersion within the analysis domain. Where surface conditions vary significantly over the analysis domain, the emphasis in assessing representativeness should be given to adequate characterization of transport and dispersion between the source(s) of concern and areas where maximum design concentrations are anticipated to occur. ”To determine if available meteorological data is representative of transport and dispersion of plumes

from the Martin Drake Power Plant, it is necessary to obtain an understanding of the driving forces behind wind conditions at the facility. With the exception of mesoscale or synoptic weather systems moving through the area, local wind flows are driven by differential heating of the surrounding terrain by the sun.

The Martin Drake Power Plant is located adjacent to the foothills of Colorado Springs directly east of Pikes Peak, near the confluence of Monument Creek and Fountain Creek

. Fountain Creek flows southeast down Ute Pass through Manitou Springs along U.S. Route 24 into Colorado Springs, merging with Monument Creek near the Drake Power Plant, and in to the Arkansas River in Pueblo

. Monument Creek flows from the foothills near Palmer Lake (western end of Palmer Divide) to near Monument, then along Interstate 25 to its confluence with Fountain Creek

. (Figure 1)

The terrain features that would affect wind conditions at the power plant are Pikes Peak to the west and Palmer Divide to the north. The sides of Pikes Peak through which Fountain Creek flows are east facing and the uplifted land of the Palmer Divide is south facing. The east facing sides of Pikes Peak will receive more direct sunshine in the morning than the Palmer Divide, which will cause the air to rise sooner over the mountain than the air over the Palmer Divide. Due to differential heating and the proximity of the power plant to the mountain, winds at/near the power plant will be upslope winds that are expected to flow up the valley of Fountain Creek during most daytime hours and up the valley of Monument Creek at a much lower frequency. During nighttime and early morning hours, drainage winds flowing from the valleys of Fountain Creek and Monument Creek towards the power plant are expected. (Figure 2)

The highest impacts from the power plant are expected to occur when the plumes are trapped under an inversion with light surface winds. For the majority of the time when this condi ti on exists, there would be light drainage winds transporting the power plant plumes down along Fountain Creek to the southeast. There will also be a limited number of inversion events when there will be upslope winds that transport the power plant plumes up the valley of Fountain Creek or Monument Creek due to a synoptic weather system. High impacts are also expected when the plume impacts the higher terrain southwest through northwest of the power plant. This will occur under stable to neutral conditions along with upslope winds. The se upslope winds could be driven by local winds or synoptic weather systems.

The only available meteorological data collected in the Colorado Springs area is measured at the airport

, which is just over 11 km east of the power plant. At the airport, the upslope and downslope winds are driven by the higher terrain to the north of the airport, the Palmer Divide

. There are no other significant terrain features that influence the winds at the airport. This makes the wind directions at the airport predominantly northerly (downslope) and southerly (upslope).

Since the wind directions at the power plant are also expected to have a northerly component for some hours due to the Palmer Divide/Monument Creek and some synoptic weather systems would cause wind conditions to be comparable within the region, there will be a portion of the winds at the airport and the power plant that are similar; however, the airport data will not be representative of winds for all other periods when Pikes Pike/Fountain Creek has a greater influence than the Palmer Divide/Monument Creek on plume transport, including the periods when the inversions are expected to cause the highest power plant impacts. It is also important to consider that the power plant will have a lower frequency of sunshine due to a higher occurrence of convective-type clouds and the upslope cloud cover that will last longer in the winter at the power plant than at the airport, both of these weather conditions will affect plume dispersion and are conducive for higher impacts.The wind speeds at the airport are expected to be greater than the wind speeds at the power plant due

to the low roughness length of the land cover around the airport anemometer. Also, wind speed and direction vary with height above terrain; thus, for elevated plumes such as those at the power plant (200+ feet), it is necessary to obtain wind conditions above 10 meters (anemometer height at the Colorado Springs Airport) to construct realistic wind profiles to model plume transport. Modeling with non-representative meteorological data affects the location and magnitude of plume impacts.

There are no available meteorological datasets that are representative of transport and dispersion conditions of the Martin Drake Power Plant plumes due to the differences in meteorological conditions between the Colorado Springs Airport and the Martin Drake Power Plant and no other meteorological dataset is available for the area. The collection of meteorological data at multiple levels up to plume height, or to a minimum height of 60 meters needs to be conducted to obtain representative meteorological data for modeling Martin Drake Power Plant plumes with AERMOD.

## PALMER DIVIDE

PIKES PEAK Fountain CreekMonument Creek

Fountain Creek 

### CO Springs Airport Martin Drake Power Plant I-25

I-25 Hwy 24

Hwy 24



# ¢

0 2.5 5 1.25 Miles 

# Figure 1. Locations of Martin Drake Power Plant and Colorado Springs Airport

Legend

EcoBase.DBO.HIGHWAYS_CDOT
National Elevation Dataset Elevation (meters) High : 4397

Low : 50

## PALMER DIVIDE

PIKES PEAK Fountain CreekMonument Creek

Fountain Creek 

### CO Springs Airport Martin Drake Power Plant upslope

downslope downslope upslope



# ¢

0 2.5 5 1.25 Miles 

# Figure 2. Expected Wind Directions at the Martin Drake Power Plant and Colorado Springs Airport

Legend

National Elevation Dataset Elevation (meters) High : 4397

Low : 50