Intercontinental transport, chemical transformations, and baroclinic systems

The adjoint sensitivity technique is used to diagnose the transport pathway and chemical transformations of a pollutant plume from Asia to Hawaii in a three‐dimensional chemical transport model (HANK) driven by winds from the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Modeling System (MM5). The simulation takes place within the timeframe of the Mauna Loa Observatory (MLO) Photochemistry Experiment (MLOPEX) 2c (15 April to 15 May 1992). Unlike trajectories, the adjoint technique takes into account all model processes including diffusion and convection in accounting for the transport of the pollutant plume and clearly defines the footprint of the plume. The transport of the pollutant plume is primarily analyzed from an isentropic perspective. The plume is lofted upward out of the Asian boundary layer in a very narrow band immediately above the surface cold front in the warm conveyor belt of a developing synoptic storm. Once lofted into the free troposphere, the plume travels almost isentropically to Hawaii, with very little additional mixing. The chemical and physical transformations within the plume are consistent with measurements on Hawaii as part of MLOPEX 2c: in particular, the initial loss of water and HNO 3 , the conversion of NO y from PAN to NO x to HNO 3 , the slow mixing rate in the free troposphere and the small ratio of HNO 3 to NO x are consistent with the measurements. The chemical signature of pollutant plumes at MLO is investigated. The plumes have much higher concentrations of hydrocarbons than average and low concentrations of HNO 3 . The concentrations of PAN, NO x , H 2 O, and H 2 O 2 were not found to distinguish rapid transport from Asia.