Articles | Volume 14, issue 18
Atmos. Chem. Phys., 14, 10013–10060, 2014

Special issue: Carbonaceous Aerosols and Radiative Effects Study (CARES)

Atmos. Chem. Phys., 14, 10013–10060, 2014

Research article 22 Sep 2014

Research article | 22 Sep 2014

Modeling regional aerosol and aerosol precursor variability over California and its sensitivity to emissions and long-range transport during the 2010 CalNex and CARES campaigns

J. D. Fast1, J. Allan2, R. Bahreini4,3, J. Craven5, L. Emmons6, R. Ferrare7, P. L. Hayes3,****, A. Hodzic6, J. Holloway4,3, C. Hostetler7, J. L. Jimenez3, H. Jonsson8, S. Liu9, Y. Liu1, A. Metcalf5,**, A. Middlebrook4, J. Nowak10, M. Pekour1, A. Perring4,3, L. Russell11, A. Sedlacek12, J. Seinfeld5, A. Setyan*,14, J. Shilling1, M. Shrivastava1, S. Springston12, C. Song1, R. Subramanian13, J. W. Taylor2, V. Vinoj1,***, Q. Yang1, R. A. Zaveri1, and Q. Zhang14 J. D. Fast et al.
  • 1Pacific Northwest National Laboratory, Richland, Washington, USA
  • 2School of Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 4Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
  • 5California Institute of Technology, Pasadena, California, USA
  • 6National Center for Atmospheric Research, Boulder, Colorado, USA
  • 7NASA Langley Research Center, Hampton, Virginia, USA
  • 8Center for Interdisciplinary Remotely Piloted Aerosol Studies, Marina, California, USA
  • 9Los Alamos National Laboratory, Los Alamos, New Mexico, USA
  • 10Aerodyne, Inc. Billerica, Massachusetts, USA
  • 11Scripps Institute of Oceanography, University of California – San Diego, San Diego, California, USA
  • 12Brookhaven National Laboratory, Upton, New York, USA
  • 13Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
  • 14Department of Environmental Toxicology, University of California – Davis, Davis, California, USA
  • *now at: Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland
  • **now at: Department of Mechanical Engineering, University of Minnesota – Twin Cities, Minneapolis, Minnesota, USA
  • ***now at: Indian Institute of Technology, Bhubaneswar, India
  • ****now at: Department of Chemistry, University of Montreal, Montreal, Quebec, Canada

Abstract. The performance of the Weather Research and Forecasting regional model with chemistry (WRF-Chem) in simulating the spatial and temporal variations in aerosol mass, composition, and size over California is quantified using the extensive meteorological, trace gas, and aerosol measurements collected during the California Nexus of Air Quality and Climate Experiment (CalNex) and the Carbonaceous Aerosol and Radiative Effects Study (CARES) conducted during May and June of 2010. The overall objective of the field campaigns was to obtain data needed to better understand processes that affect both climate and air quality, including emission assessments, transport and chemical aging of aerosols, aerosol radiative effects. Simulations were performed that examined the sensitivity of aerosol concentrations to anthropogenic emissions and to long-range transport of aerosols into the domain obtained from a global model. The configuration of WRF-Chem used in this study is shown to reproduce the overall synoptic conditions, thermally driven circulations, and boundary layer structure observed in region that controls the transport and mixing of trace gases and aerosols. Reducing the default emissions inventory by 50% led to an overall improvement in many simulated trace gases and black carbon aerosol at most sites and along most aircraft flight paths; however, simulated organic aerosol was closer to observed when there were no adjustments to the primary organic aerosol emissions. We found that sulfate was better simulated over northern California whereas nitrate was better simulated over southern California. While the overall spatial and temporal variability of aerosols and their precursors were simulated reasonably well, we show cases where the local transport of some aerosol plumes were either too slow or too fast, which adversely affects the statistics quantifying the differences between observed and simulated quantities. Comparisons with lidar and in situ measurements indicate that long-range transport of aerosols from the global model was likely too high in the free troposphere even though their concentrations were relatively low. This bias led to an over-prediction in aerosol optical depth by as much as a factor of 2 that offset the under-predictions of boundary-layer extinction resulting primarily from local emissions. Lowering the boundary conditions of aerosol concentrations by 50% greatly reduced the bias in simulated aerosol optical depth for all regions of California. This study shows that quantifying regional-scale variations in aerosol radiative forcing and determining the relative role of emissions from local and distant sources is challenging during `clean' conditions and that a wide array of measurements are needed to ensure model predictions are correct for the right reasons. In this regard, the combined CalNex and CARES data sets are an ideal test bed that can be used to evaluate aerosol models in great detail and develop improved treatments for aerosol processes.

Final-revised paper