High Resolution Ozone Modeling in Portland/Vancouver Airshed

Elevated ozone levels occur periodically in the summer time in Portland/Vancouver airshed with exceedances of the 1-hr National Ambient Air Quality Standard (NAAQS) in the recent past. The region is influenced by complex terrain effects and unique meteorological conditions from the Columbia River Gorge and the Cascades, therefore wind field and air quality modeling is often difficult. In order to better understand the ozone formation and transportation in the region as well as to support the development of the next ozone maintenance plan, a photochemical modeling study is being conducted with MM5/SMOKE/CMAQ modeling system.

Two base case runs with a 4-km resolution (July 1998 and July 1997) are performed due to the high ozone levels and their representative meteorological patterns. With the unique meteorological influence from the Columbia River Gorge and the Cascades, a third episode with a 1-km grid resolution will be conducted to better characterize the flow pattern in the urban area. In addition, one future case run is also planned to investigate the future risk of exceeding the standard under ozone productive meteorological conditions.

With 3 exceedances of 1-hr standard at the Carus monitoring site during July 22-31, 1998 this event is selected as the first episode to be modeled. An emission inventory composed of point, area, mobile, and biogenic emissions are developed specifically for the episode. MM5 is used to generate meteorological field with analysis nudging in the coarse domain and observational nudging in the inner 4km domain. Preliminary CMAQ results show a large under-prediction on peak ozone days which might be related to over-estimation of PBL height by MM5. More extensive model performance evaluation and modeling of other episodes are undergoing.