Responses of Photochemical Air Pollution in California?s SanJoaquin Valley to Spatially and Temporally Resolved Changes inPrecursor Emissions br

Ground-level ozone adversely affects human health andecosystems. The effectiveness of control programs depends on whichprecursor(s) are controlled, by how much, and where and whenemission reductions occur. We use the adjoint of the CommunityMultiscale Air Quality model to investigate odd oxygen (Ox equivalent to O3+NO2) sensitivities in California's San Joaquin Valley (SJV) to precursoremissions from local and upwind sources. Sensitivities are mapped anddisaggregated by hour and day. Taken together, impacts of precursoremissions in the San Francisco Bay area and Sacramento Valley aresimilar in magnitude to impacts of local SJV emissions. Same-dayemission sensitivities are mostly attributable to local sources, with themost influential anthropogenic emissions of VOCs (volatile organiccompounds) and NOx(nitrogen oxides) occurring in the morning (9-11 am) and early afternoon hours (1-3 pm), respectively. For thenorthernmost SJV receptor, the influence from Sacramento Valley emissions peaks 5-6 h later than Bay area emissions; thisdifference diminishes for SJV receptors located further downwind. Results show a shift toward more NOx-sensitive conditions in theafternoon with all but the southernmost receptor shifting from VOC- to NOx-sensitive conditions. We also evaluate opportunities tocontrol pollution through shifts in precursor emission location and timing