Transport and Mixing Processes Affecting the Evolution of Aerosols in the Sacramento Valley during CARES

During June 2010, the Carbonaceous Aerosols and Radiative Effects Study (CARES) was conducted to obtain measurements on the evolution of carbonaceous aerosols and their optical and hygroscopic properties in the Sacramento urban plume as it is routinely transported to the northeast over the Sierra Nevada Mountains. Carbonaceous aerosols (black carbon and organic matter) have been shown to play a major role in direct and indirect radiative forcing of climate; however, there are still significant knowledge gaps regarding secondary organic aerosol formation, black carbon mixing state, and optical and hygroscopic properties of fresh and aged aerosols. During summer, the Sacramento urban plume transport is controlled by extremely consistent, terrain-driven upslope winds that draw polluted air to the northeast over the oak and pine trees in the Blodgett Forest area by late afternoon. The Sacramento-Blodgett Forest corridor therefore serves as a mesoscale (~100 km) daytime flow reactor in which the urban aerosols undergo significant aging due to coagulation, condensation, and photochemical processes. Downslope winds are expected to transport biogenic aerosols towards urban areas at night. We will present initial findings regarding the characteristics of the diurnally varying flows and boundary layer structure in the region, based on radar wind profiler, sodar, radiosonde, and aircraft measurements. The effect of the thermally-driven flows on the transport and mixing of aerosols will be described using measurements that include the vertical structure of aerosols obtained from NASA’s High Spectral Resolution Lidar (HSRL) deployed on an aircraft. HSRL data will also be used to find evidence of mountain venting process that loft aerosols from the Sacramento Valley into the free atmosphere over the foothills. The Weather Research and Forecasting (WRF) model was used operationally during CARES, and an assessment of the predicted thermally-driven flows will be made as well as comparing simulated tracers with aerosol layers identified by the HSRL.