|Abstract or Summary
- The length of time that atmospheric pollutants released from low-level
sources in the midwestern United States can expect to remain in
the atmosphere is discussed. The pollution is assumed to be removed
from the atmosphere by dry deposition and precipitation scavenging.
Layer-average trajectories originating from Kansas City, Missouri are
used to determine the Lagrangian probability of dry and wet conditions.
The residence time of these pollutants is estimated based on parameterizations
for the effective scavenging rates during wet and dry conditions.
This investigation shows that, in summer, the probability that
precipitation is being experienced by the pollutant is twice as great as
the probability of precipitation at the origin of the pollution; this
same ratio of probabilities is three in winter. Therefore, when precipitation
scavenging is the more important removal mechanism, the statistics
for the length of wet and dry periods at the source region overestimate
the residence time by a factor of about two to three.
By taking into consideration the Lagrangian probability of wet and
dry periods, the relative importance of dry deposition and precipitation
scavenging is discussed as a function of the wet and dry removal rates.
It is seen that for a time- and vertical-average dry deposition velocity
as large as 1 cm/sec, then dry deposition would normally be the bore
important removal process for the meteorological conditions in the midwest
to eastern United States.
Estimates for the expected atmospheric lifetimes of aerosol particles
and trace gases are reported as functions of dry deposition velocities
and collection efficiencies (or washout ratios). For example, lead
particles of mass mean diameter ~0.5 μm, should have a residence time ~8
days in winter, and ~3 days in summer, based on available data for the
dry deposition velocity and washout ratio. In general, the residence time
can be expected to be about twice as long during the summer season than
The winter, monthly average distribution of pollutant mass is shown,
based on the steady-state Gaussian approximation solution of the convective
diffusion equation. The calculations are based on a statistical
analysis of the 12 hourly positions of a series of trajectories. Thus,
monthly average "diffusion" and removal are incorporated into the