Trajectory:
The paths of small air parcels can be traced forward or backward in time as they move through time and space along their trajectories.

Endpoints:
The location of an infinitesimally small air parcel at a particular point in space at a given time.

Air Parcel Trajectory Models:
Mathematical and experimental techniques are used to calculate air parcel (infinitesimally small volumes of air) trajectories to better understand atmospheric transport and dispersion. The most common air parcel trajectory models are HYSPLIT, FLEXTRA and METEX.

HYSPLIT Model:
The HYSPLIT model was developed by National Oceanic and Atmospheric Administration (NOAA) and is one of the most extensively used air parcel trajectory models in the atmospheric sciences community. It provides a complete system for computing simple air parcel trajectories, as well as complex transport, dispersion, chemical transformation, and deposition simulations. TraPSA uses its latest version, HYSPLIY_4, as its trajectory generator.

Receptor Modeling:
Receptor modeling is the application of multivariate statistical methods for the identification and quantitative apportionment of air pollutants to their sources. Chemical transport models can be combined with receptor models to provide improved apportionments. There is now a substantial capability to provide useful insights into the sources of pollutants and their atmospheric processing that can help inform air quality management options.

Trajectory Ensemble Receptor Models:
Trajectory ensemble receptor models are a subset of receptor models that can be used to understand the relationship of the source/receptor combining pollutant concentration measurement data and air parcel back trajectory calculations. A series of different trajectory ensemble receptor models have been developed and applied for air pollution source likely location apportionment. Current popular algorithms for trajectory ensemble receptor models includes Conditional Probability Function (CPF), Concentration Field Analysis (CFA), Concentration Weighted Trajectory (CWT), Potential Source Contribution Function (PSCF), Residence Time Weighted Concentration (RTWC), and Simplified Quantitative Transport Bias Analysis (SQTBA).