PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00001/1408
212 Pb as a Tracer for Urban Aerosols
Paul Bredt, George Klinger, Larry Greenwood, John Smart
Aerosol particles generated in urban areas influence human health, global warming, and regional haze. Methods
involving naturally evolved lead-212 ( 212 Pb) and lead-214 ( 214 Pb) are being developed to track aerosol distribution as a
function of time, location, and altitude. Distribution tracking is fundamental to understanding the life cycle of these
particles.
Project Description
Aerosol samples were collected in the Portland, Oregon,
area and were counted in real-time for 212 Pb and 214 Pb
activity. The goal of this work was to examine the release
of these naturally occurring radioisotopes across a major
metropolitan area. Previous studies have used these
isotopes as tracers to study the horizontal and vertical
mixing of air masses (Gaggeler et al. 1995; Assof and
Biscaye 1972). However, the source strengths for these
isotopes, 212 Pb in particular, are poorly characterized
(Sheets and Lawrence 1999). Both of these isotopes are
released from soil and share radon in their decay chains.
However, 212 Pb has a physical half-life (t1/2) of 10.64
hours and is a daughter of 220 Rn with a t1/2 =55.6 seconds,
while 214 Pb has a t1/2 =26.8 minutes and is a daughter of
222 Rn with a t1/2 =3.8 days. Due to these half-lives, 212 Pb
activity is dominated by local sources while 214 Pb activity,
which generally has local sources, is dominated by
regional and global sources. Data from the Portland
sampling compared well with a simple one-dimensional
model for the transport of marine air over a fixed
222 Rn/ 220 Rn source.
Approach
Aerosol samples were collected during three successive
days in and around the Portland metropolitan area during
the month of June 2000. The samples were collected by
filtering air through a 3M Filtrete GS-100 filter media
mounted on the front side of a high volume air sampler.
Samples were collected at a nominal flow rate of
7.5 m 3 /min over 20 minutes. After collection, the filters
were pressed into a pellet and were counted on a gamma
spectrometer located in a lead cave in the sampling van.
To determine the filter collection efficiency, tests were
run with two filters mounted on the air sampler, then both
filters were counted individually. Approximately 80 to
85% of the 214 Pb and 212 Pb activity was found on the first
filter. Similar efficiencies are expected for these isotopes
given they are both associated with similar particle sizes
(0.07 to 0.25 µm for 212 Pb, and 0.09 to 0.37 µm for 214 Pb)
(Papastefanou and Bondietti 1987).
Different routes through Portland were followed on each
of the sampling days. The routes started in an area of low
to moderate population density, crossed near the urban
center, then generally continuing across the city into a low
density area opposite to the starting location. Samples
were collected over a relatively short time window of
approximately 5 hours to minimize effects of changing
atmospheric boundary layer heights. Changes in the
altitude of the boundary over the course of a day can
significantly affect the effective mixing volume and
therefore significantly affect the concentration of radon
and radon daughters measured at ground level. The
boundary layer is generally observed to drop at night and
rise to higher altitudes by the middle of the afternoon.
This results in higher concentrations at night relative to
later in the day. This cyclic effect in has been referred to
as the “universal radon wave” (Gargon et al. 1986).
Meteorological data including wind speed, wind direction,
humidity, and barometric pressure were collected at each
sampling site. In addition, hourly routine aviation
weather reports from the National Weather Service were
collected at five airports: Portland International, Astoria,
Kelso, Troutdale, and Hillsboro. These airports were near
and/or upwind of Portland during the collection period.
Results and Accomplishments
No correlations were found between 212 Pb/ 214 Pb activity
and proximity to the urban center. This shows that the
fluxes of 220 Rn and 222 Rn under the regional weather
conditions during this work were dominated by local
geologic sources and not by urban construction.
While the measured 212 Pb activity was constant over the
three sampling days within the associated counting error,
the activity of 214 Pb was dependent on wind velocities.
Figure 1 shows the hourly wind speed and direction
Human Health and Safety 273