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G1.1.2 Evaporative-Related Emissions
In addition to exhaust emissions from GAV operations, there also are evaporative-related
emissions of hydrocarbon. There are six types of evaporative-related emissions calculated for
GAVs: crankcase, refueling losses, running losses, hot soak, diurnal, and resting losses. The six
types of evaporative-related emissions can be divided into two categories: operating or resting.
Operating emissions occur while a vehicle is operating, and are applicable to every trip.
Operating emissions result from both roadway and parking lot vehicle activity. Resting emissions
only occur when the vehicle is at rest, and are applicable to vehicle trips including a vehicle stop.
The six types of evaporative-related emissions are listed in the table below by category. Each
type of evaporative-related emissions is discussed in more detail below.
Table G-1 : Types Of Evaporative-Related Emissions
· Crankcase - Crankcase emissions occur while a vehicle is operating and, therefore, are
dependent on vehicle miles traveled. Crankcase emissions result from fuel displacement.
· Refueling Loss - Refueling loss emissions are the spillage and displacement of fuel vapor
from the vehicle fuel tank to the atmosphere when gasoline-fueled vehicles are refueled.
Stage II and on-board vapor recovery systems are being implemented to control refueling
losses. Refueling loss emissions usually are calculated using the gallons of fuel dispensed.
For ground access vehicles that operate and refuel only on the airport, such as fleet vehicles
(e.g., parking lot shuttle buses, rental car shuttle buses, airport fleet
1 Operating emissions occur for every vehicle trip (i.e., result from both roadway and parking lot vehicle
activity).
2 Resting emissions only occur for vehicle trips including a vehicle stop (i.e., only result from parking lot
vehicle activity including a park).
Operating Emissions1 Resting Emissions2
Crankcase Hot Soak
Refueling Losses Diurnal
Running Losses Resting Losses
G-7
vehicles), all fuel consumed is attributable to the airport and the fleet operator typically knows
the amount of fuel dispensed and consumed. For other ground access vehicles that also are
operated for purposes other than accessing the airport (e.g., passenger vehicles, employee
vehicles), attributing the refueling losses of all gasoline in the vehicle tank would
overestimate those airport-related emissions. To more accurately estimate refueling losses of
those vehicles that also operate for non-airport purposes, emissions are estimated using only
the gallons of fuel used to access the airport (as discussed in section G2. Data Sources,
below). Refueling emissions also can be calculated based on vehicle miles traveled, but will
be less accurate due to inherent assumptions regarding inputs.
· Running Loss - Running loss emissions are similar to hot soak emissions; both are heatrelated,
evaporative emissions that occur while the vehicle is being operated. Running
loss emissions are negligible at first, but increase significantly as trip duration lengthens.
· Hot Soak - Hot soak emissions occur at the end of each vehicle trip as elevated
operation temperatures lead to continued evaporation after a vehicle has been parked
(e.g., in an airport parking lot).
· Diurnal - Diurnal emissions also occur while the vehicle is parked during periods of
rising ambient temperatures. Total diurnal emissions are calculated using two equations:
partial/full-day and multiple-day. The partial/full-day equation calculates diurnal
emissions from vehicles that are at rest for a day or less (e.g., in a short-term parking lot,
employee parking lot, or the first day of a multiple-day parking lot). The multiple-day
equation calculates diurnal emission from vehicles that are at rest for multiple days (e.g.,
in a long-term parking lot). For multiple-day parking lots, emissions from the first day of
rest are calculated using the partial/full-day equation. Emissions from the remaining days
of rest (i.e., the second day of rest and longer) are calculated using the multiple-day
equation.
· Resting Loss - Resting loss emissions occur while the vehicle is at rest (e.g., in a parking
lot) and result from the permeability of fuel system components. Resting emissions are
calculated using the number of hours the vehicles are at rest. Resting evaporative
emissions also can be calculated based on vehicle miles traveled, but will be less
accurate due to inherent assumptions regarding inputs.
Simplified procedures for calculating these evaporative-related emissions from GAV are
presented below. These calculations can be used to determine emissions for any time period of a
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