Overview

HYSPLIT interface does not incorporate the effects of:

• chemical reactions
• dense gases (heavier than air)
• byproducts from fires, explosions, or chemical reactions
• materials released that are not neutrally-bouyant
• deposition - unless the user enters appropriate wet and dry deposition parameters
• particulate transport - unless the user 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

Other considerations:

• HYSPLIT's source term is in units of mass of a released gas, not a liquid - chemicals must be entered in mass units (lbs, grams, micrograms, etc). HYSPLIT will not determine the quantity of material released based on a scenario (tank, puddle, etc). The user can use source term estimates from models such as the CAMEO/ALOHA model to determine the amount of material released from these types of scenarios.  

• Exposure guidelines (AEGL, ERPG, TEEL) are based on a contact duration of 1 hour

• HYSPLIT's 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. 

Common Cases:

Crude Oil Fire:

• HYSPLIT is not designed to model the by-products of fire, however, it can be run to give a depiction of how the smoke plume may spread

• To model the dispersion of the smoke plume choose to run the HYSPLIT dispersion model with an unknown/generic gas. 

• Define the release duration to be the same as the total duration to simulate the continuous release of smoke from the fire.

• Be sure to set the proper start time of the fire to ensure enough "smoke" is already released into the atmosphere at model startup (i.e fire started an hour ago, make sure to set the start time of the release to this time, which may be different than the start run time of the model).

• Attempt to get an approximate height of the plume

Anhydrous Ammonia

General Description

A clear colorless gas with a strong odor. Shipped as a liquid under its own vapor pressure. Gas generally regarded as nonflammable. Although gas is lighter than air, vapors from a leak initially hug the ground. Prolonged exposure of containers to fire or heat may cause violent rupturing and rocketing. Long-term inhalation of low concentrations of the vapors or short-term inhalation of high concentrations has adverse health effects.

Propane

General Description

**HEAVIER THAN AIR***

A colorless gas with a faint petroleum-like odor. It is shipped as a liquefied gas under its vapor pressure. Easily ignited. The vapors are heavier than air and a flame can flash back to the source of leak very easily. The leak may be either a liquid or vapor leak. Liquid propane will flash to a vapor at atmospheric pressure and appears white due to moisture condensing from the air. Under prolonged exposure to fire or heat the containers may rupture violently and rocket.

Converting concentration to PPM

HYSPLIT concentrations are normally output in units of mass/volume (µg m-3, mg m-3, etc.). Emergency managers generally prefer units of parts per million (by volume) or ppm. One part per million is equal to a volume of a given gas mixed in a million volumes of air. The conversion from concentration to ppm is influenced by the temperature and pressure of the air parcel and the molecular weight of the chemical being converted. One simplified conversion (used in the HYSPLIT calculation) is as follows:

Cppm = pollutant concentration (µg m-3) * RT / PM

Where: R = the universal gas constant [0.08314 hPa m3 K-1 mol-1] T = temperature [K] P = atmospheric pressure [hPa] M = molecular weight of gas [g/mol]

The molecular weights of hazardous materials can be found in the CAMEO/ALOHA database. HYSPLIT assumes a temperature of 293K and a pressure of 1000 hPa. Substituting these values into the above equation results in:

Cppm = pollutant concentration (µg m-3) * (0.08314 hPa m3 K-1 mol-1 * 293 K) / (1000 hPa * M)

or Cppm = pollutant concentration (µg m-3) * 0.02436 m3 mol-1 / M

and for concentration in mg m-3 Cppm = pollutant concentration (mg m-3) * 24.36 / M