Regional-scale transport of air pollutants: impacts of Southern California emissions on Phoenix ground-level ozone concentrations

摘要

In this study, WRF-Chem is utilized at high resolution (1.333 km gridspacing for the innermost domain) to investigate impacts of southernCalifornia anthropogenic emissions (SoCal) on Phoenix ground-level ozoneconcentrations ([O]) for a pair of recent exceedance episodes. First,WRF-Chem control simulations, based on the US Environmental ProtectionAgency (EPA) 2005 National Emissions Inventories (NEI05), are conducted toevaluate model performance. Compared with surface observations of hourlyozone, CO, NO, and wind fields, the control simulations reproduceobserved variability well. Simulated [O] are comparable with theprevious studies in this region. Next, the relative contribution of SoCaland Arizona local anthropogenic emissions (AZ) to ozone exceedances withinthe Phoenix metropolitan area is investigated via a trio of sensitivitysimulations: (1) SoCal emissions are excluded, with all other emissions asin Control; (2) AZ emissions are excluded with all other emissions as inControl; and (3) SoCal and AZ emissions are excluded (i.e., allanthropogenic emissions are eliminated) to account only for Biogenicemissions and lateral boundary inflow (BILB). Based on the USEPA NEI05,results for the selected events indicate the impacts of AZ emissions aredominant on daily maximum 8 h average (DMA8) [O] in Phoenix. SoCalcontributions to DMA8 [O] for the Phoenix metropolitan area range froma few ppbv to over 30 ppbv (10–30 % relative to Control experiments).[O] from SoCal and AZ emissions exhibit the expected diurnalcharacteristics that are determined by physical and photochemical processes,while BILB contributions to DMA8 [O] in Phoenix also play a key role.Finally, ozone transport processes and pathways within the lower troposphereare investigated. During daytime, pollutants (mainly ozone) near theSouthern California coasts are pumped into the planetary boundary-layer overthe Southern California desert through the mountain chimney and pass channeleffects, aiding eastward transport along the desert air basins in southernCalifornia and finally, northeastward along the lower Gila River basin inArizona, thereby affecting Phoenix air quality during subsequent days. Thisstudy indicates that local emission controls in Phoenix need to be augmentedwith regional emission reductions to attain the federal ozone standard,especially if a more stringent standard is adopted in the future.