HYSPLIT model limitations, product interpretation, and information
The version of HYSPLIT used to produce these results does not incorporate the effects of:
- chemical reactions
- dense gases
- byproducts from fires, explosions, or chemical reactions
- materials released that are not neutrally-bouyant
- deposition - unless the operator enters appropriate wet and dry deposition parameters
- particulate transport - unless the operator enters information about the particle (size,
deposition rates, etc)
- complex terrain - other than what is resolved by the meteorological model's terrain
- varying emission rate, except for a chemical simulation which uses the
ALOHA model to calculate a time-varying emission rate
Other considerations:
- Exposure guidelines (AEGL, ERPG, PAC) are based on a contact duration of 1 hour.
- HYSPLIT's minimum time step is 1 minute, so the model cannot be used for transport less than the distance it
takes for the pollutant to move in 1 minute. However, keep in mind that the meteorology may not
adequately represent the transport/dispersion at the point of release regardless of the model time step.
In general, for transport scales less than 1 km, the separate desktop
ALOHA model is recommended because it uses a short range, short duration dispersion model.
- For ground-based hazardous material releases, near-surface meteorological conditions, including the
10-meter winds, 2-m temperatures, and surface fluxes (which determine the stability and thus the vertical
dispersion of material) are important determinants of the horizontal and vertical spread of the material.
Numerical weather prediction (NWP) forecasts of these meteorological variables are inputs to the HYSPLIT
model, but the NWP models are limited in a number of ways in their representation and forecast of
near-surface conditions. Frequently, the meteorological forecast data available to run HYSPLIT has been
spatially averaged, resulting in representativeness problems for modeling dispersion in, for example,
areas of complex terrain.
- Meteorological data used to force HYSPLIT is available at relatively coarse temporal resolution
(1-3 hours), which can result in errors in rapidly changing conditions, even if the NWP model itself can
faithfully emulate these conditions. NWP models also have limitations in their ability to accurately
forecast the onset and spatial scales of phenomena that may affect dispersion, such as sea-breezes and
mountain-valley circulations.
- Finally, the meteorological data used by HYSPLIT is bi-linearly interpolated in
space and linearly interpolated in time to the location and time of a hazardous material release and
during the forecast of the subsequent diffusion of materials. If important variations
in the meteorological fields occur between grid points or between meteorological data output times,
concentrations errors may result.
Model products:
- Air Concentrations
- Air concentrations produced by HYSPLIT represent the mass of pollutant in a unit of volume (mass/volume), typically
calculated near the surface (0-100 meters above ground level). If the amount of pollutant released to the atmosphere
is known at the time of the model simuation, the operator can enter that value as a rate per hour. If the release
rate us unknown, as is typically the case for emergency response situations, the operator will run the model
using a unit source release per hour. In that case the resulting concentrations are really dispersion factors and
can be multiplied by the release rate once the source information becomes available to produce realistic air
concentations.
- Deposition
- Deposition can be produced by HYSPLIT if the operator provides information on the pollutant removal rates. Deposition
refers to the mass of pollutant removed from the atmosphere either by dry or wet processes. Dry deposition occurs in
the lowest layer of the model where the pollutant is removed due to settling and removal by objects near the ground based
upon the relation that the deposition flux equals the deposition velocity times the ground-level air concentration. Wet
deposition is modeled in two ways; (1) pollutants are scavenged from the atmosphere within a cloud layer by rain or snow
using a washout ratio between the pollutant in air (g/liter of air in the cloud) to that in rain (g/liter) measured at the
ground, and (2) pollutants are removed below the cloud layer using a removal time constant that is not affected by
the rainfall amount or rate.
- Particle Plots
- In some model simulations, plots of the particle positions at specific intervals are displayed to give the end user
a visual representation of the dispersion in the horizontal and vertical domain of the model. When animated, these
plots show the movement of each particle tracked by the model.
For further details on HYSPLIT, see the following sources:
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