Environmental Engineering Reference
In-Depth Information
3. Winter Ozone Model Performance Evaluation
As part of the model performance evaluation of the 2005-2006 base year
simulations, the model's ability to simulate observed winter high ozone events in
southwestern Wyoming during February 2006 was assessed. CAMx sensitivity
tests were conducted that were guided by field study data collected by the
Wyoming Department of Environmental Quality (WDEQ) during the winters of
2007 and 2008 and analyzed by WDEQ and their contractors [1]. The field study
data indicate that the following conditions are necessary in order for high
wintertime ozone events to occur in southwestern Wyoming: shallow inversion
(limited mixing), white snow on ground (high albedo), few or no clouds, stagnant
and/or re-circulating slow surface winds, high morning total non-methane hydro-
carbon (TNMHC) concentrations (e.g., ~4,000 ppbC), and high morning TNMHC/
NOx ratios (e.g., 90:1).
The sensitivity of the CAMx modeling system to each of the components of the
above conceptual model of wintertime ozone formation was tested. In these
sensitivity tests, several changes were made to model inputs to adapt the model for
the simulation of conditions typical of high winter ozone days. For snow-covered
surfaces in Southwest Wyoming, the CAMx snow albedo was increased from its
default value of 0.5-0.75 based on UV up/down radiometer measurements at the
Boulder monitoring site in southwestern Wyoming from the WDEQ field study. In
the region where high ozone was observed, a thin cloud deck was removed, PBL
heights were capped at 100 m on high ozone days in accord with observations [2],
and the oil and gas emission inventory TNMHC emissions were adjusted upward
so that the inventory matches the observed TNMHC/NOx ratio. The average
TNMHC/NOx ratio in the Jonah-Boulder area in the oil and gas emission
inventory was approximately 13:1, compared to the observed ratio of 90:1 (note
that this is a comparison of observed ozone precursor levels with the emission
inventory). The adjustment to the oil and gas emission inventory had by far the
Boulder, WY Ozone Time Series for February 18-28, 2006
Jonah, WY Ozone Time Series for February 18-28, 2006
100
140
Observed Ozone
CAMx Default Configuration
CAMx Enhanced Inputs
Observed Ozone
CAMx Default Configuration
CAMx Enhanced Inputs
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0
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Julian Date (local time)
Julian Date (local time)
Fig. 2.
CAMx winter ozone sensitivity test. Ozone time series for Boulder (left) and Jonah (right)
Wyoming for CAMx run in default configuration suitable for modeling summer ozone episodes
and with enhanced inputs indicated by results of WDEQ field study
