The Impact of Climate Change on Air Quality–Related Meteorological Conditions in California. Part II: Present versus Future Time Simulation Analysis

In this study, the Weather Research and Forecasting (WRF) model was applied to dynamically downscale the Parallel Climate Model (PCM) projection for the climate change impact on regional meteorological conditions in California. Comparisons were made for meteorological fields that strongly influence regional air quality between the current (2000-06) and future (2047-53) downscaling results to infer potential air pollution changes in California. Changes in both the meteorological fields and the implied future air quality vary by region and season. Analyses showed that the normalized number of stagnation days (NNSD) integrating all stagnation events, during which most of the air pollution episodes occur, in California's San Joaquin Valley (SJV) will increase and the intensity of stagnation will be stronger in the future for the two main air pollution seasons (i.e., summer and winter). Increases in surface wind and planetary boundary layer height (PBLH) were observed for the coastal part of Los Angeles County (LAC) during summer, suggesting stronger ventilation in this region. Contrary situations were seen in other parts of the South Coast Air Basin (SoCAB) and SJV. Although a surface wind change was not evident in SJV during winter, there was a significant PBLH decrease. Climate-change-induced variations in surface wind and PBLH were only statistically significant in coastal SoCAB and the southern portion of SJV relative to the corresponding interannual variability; changes in temperature are significant throughout the regions studied. The sea breeze along the coast of California plays an important role in the state's climate and air quality, especially during summertime owing to the stronger intensity compared to wintertime. Analysis of the land-sea temperature contrast and the southwesterly wind along the California coastline indicated that the summertime sea breeze will be stronger in the Central Valley (CV) but weaker for the SoCAB region in the future.