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Atmospheric Modeling and Analysis Research

Climate Impact on Regional Air Quality (CIRAQ)

Air quality is determined both by emissions of air pollutants, including ozone and particulate matter precursors, and by meteorological conditions, including temperature, wind flow patterns, and the frequency of precipitation and stagnation events. For air quality management applications, regional-scale models are used to assess whether various emission control strategies will meet National Ambient Air Quality Standards (NAAQS).

These modeling applications typically assume present meteorological conditions, which means that potential changes in climate are not included in the assessment. However, with emission controls implemented over several decades, future climate trends could impact the effectiveness of these controls.

EPA's scientists initiated the Climate Impact on Regional Air Quality (CIRAQ) project in 2002 to develop a pilot modeling study to incorporate regional-scale climate effects into air quality modeling. The project involved collaboration across multiple federal agencies and with academic groups that had global-scale modeling expertise and, who were supported through EPA's Science To Achieve Results (STAR) grant program.

The Goddard Institute for Space Studies (GISS) global climate model (GCM) version 2' was used to simulate the period from 1950-2055 at 4° degrees latitude by 5° degrees longitude resolution. Historical values for greenhouse gases (as CO2 equivalents) were used for 1950-2000, with future greenhouse gas forcing following the Intergovernmental Panel on Climate Change's A1B scenario.

Colleagues at the Pacific Northwest National Laboratory downscaled GCM outputs using the Penn State and National Center for AtmosphericResearch (NCAR) mesocale model (MM5) to simulate meteorology over the continental U.S. at 36 kilometers resolution for two 10-year periods centered on 2000 and 2050.

For the first phase of this project, the effect of climate change alone was considered, without accounting for changes in emissions of ozone and PM precursors. Hourly emissions were simulated using the SMOKE model. Anthropogenic emissions were based on the EPA 2001 version ad (2001ad) modeling inventory, projected from the 1999 National Emission Inventory version 3. Biogenic emissions were calculated using the BEIS model and the simulated future meteorology. Air quality was simulated for two 5-year periods (from 1999-2003 and 2048-2052) using CMAQ version 4.5.

Differences in mean and 95th percentile maximum daily 8-h average ozone concentrations

Differences (5-year future − 5-year current) in mean (top) and 95th percentile (bottom) maximum daily 8-hour average (MDA8) ozone concentrations. Results show summertime increases of 2-5 parts per billion in mean MDA8 concentrations in Texas and parts of the eastern U.S.. Greater increases are found in 95th percentile concentrations, suggesting increased severity of ozone episodes. Still larger increases are predicted for the September-October time period, suggesting a lengthening of the ozone season (Nolte et al., J. Geophy. Res., 2008).

As the next step, EPA is investigating the combined effect of climate change together with emission changes on air quality. Emission projections for different scenarios of economic growth and technological utilization have been developed. Air quality simulations using these emissions projections and the climatological meteorology described above has been conducted using CMAQ version 4.7.

Contacts: Chris Nolte

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