Climate impact on airborne particulate matter concentrations in California using seven year analysis periods

摘要

The effect of global climate change on the annual average concentration offine particulate matter (PM) in California was studied using aclimate-air quality modeling system composed of global through regionalmodels. Output from the NCAR/DOE Parallel Climate Model (PCM) generatedunder the "business as usual" global emissions scenario was downscaledusing the Weather Research and Forecasting (WRF) model followed by airquality simulations using the UCD/CIT airshed model. The system representsmajor atmospheric processes acting on gas and particle phase speciesincluding meteorological effects on emissions, advection, dispersion,chemical reaction rates, gas-particle conversion, and dry/wet deposition.The air quality simulations were carried out for the entire state ofCalifornia with a resolution of 8-km for the years 2000–2006 (present climatewith present emissions) and 2047–2053 (future climate with present emissions).Each of these 7-year analysis periods was analyzed using a total of 1008simulated days to span a climatologically relevant time period with apractical computational burden. The 7-year windows were chosen to properlyaccount for annual variability with the added benefit that the air qualitypredictions under the present climate could be compared to actualmeasurements. The climate-air quality modeling system successfullypredicted the spatial pattern of present climate PM concentrationsin California but the absolute magnitude of the annual average PMconcentrations were under-predicted by ~4–39% in the major airbasins. The majority of this under-prediction was caused by excessventilation predicted by PCM-WRF that should be present to the same degreein the current and future time periods so that the net bias introduced intothe comparison is minimized.Surface temperature, relative humidity (RH), rain rate, and wind speed werepredicted to increase in the future climate while the ultra violet (UV)radiation was predicted to decrease in major urban areas in the San JoaquinValley (SJV) and South Coast Air Basin (SoCAB). These changes lead to apredicted decrease in PM mass concentrations of ~0.3–0.7 μg m in the southern portion of the SJV and~0.3–1.1 μg m along coastal regions of California including the heavily populatedSan Francisco Bay Area and the SoCAB surrounding Los Angeles. Annual averagePM concentrations were predicted to increase at certain locationswithin the SJV and the Sacramento Valley (SV) due to the effects of climatechange, but a corresponding analysis of the annual variability showed thatthese predictions are not statistically significant (i.e. the choice of adifferent 7-year period could produce a different outcome for theseregions). Overall, virtually no region in California outside of coastal + central Los Angeles, and a small region around the port of Oakland in theSan Francisco Bay Area experienced a statistically significant change inannual average PM concentrations due to the effects of climatechange in the present~study.The present study employs the highest spatial resolution (8 km) and thelongest analysis windows (7 years) of any climate-air quality analysisconducted for California to date, but the results still have some degree ofuncertainty. Most significantly, GCM calculations have inherent uncertaintythat is not fully represented in the current study since a single GCM wasused as the starting point for all calculations. The PCM results used in thecurrent study predicted greater wintertime increases in air temperature overthe Pacific Ocean than over land, further motivating comparison to other GCMresults. Ensembles of GCM results are usually employed to build confidencein climate calculations. The current results provide a first data-point forthe climate-air quality analysis that simultaneously employ the fine spatial resolution and long time scales needed to capture the behavior ofclimate-PM interactions in California. Future downscaling studiesshould follow up with a full ensemble of GCMs as their starting point, andinclude aerosol feedback effects on local meteorology.