Project Title: 212Pb as a Tracer for Urban Aerosols

Measurements of ²¹²Pb and ²¹⁴Pb in Urban Aerosols
at Portland Oregon

Paul Bredt, George Klinger, Larry Greenwood, John Smart

Pacific Northwest National Laboratory

Richland, WA 99352

Aerosols particles generated in urban areas influence human health, global warming and regional haze. Methods involving naturally evolved Radon daughters, ²¹²Pb and ²¹⁴Pb, are being developed to track aerosol distribution as a function of time, location, and altitude which is fundamental to understanding the life cycle of these particles.

Aerosol samples were collected in the Portland area and counted in real-time for ²¹²Pb and ²¹⁴Pb activity. The goal of this work was to examine the release of these radioisotopes across a major metropolitan area. Previous studies have used these isotopes as tracers to study the horizontal and vertical mixing of air masses[1]^,[2]. However, the source strengths for these isotopes, ²¹²Pb in particular, are poorly characterized[3]. Both of these isotopes are released from soil and share radon in their decay chains. Lead-212 has a t_½=10.64 hours and is a daughter of ²²⁰Rn which has a t_½=55.6 seconds, while ²¹⁴Pb has a t_½=26.8 minutes and is a daughter of ²²²Rn which has a t_½=3.8 days. Due to these half-lives, ²¹²Pb activity is dominated by local sources while ²¹⁴Pb activity, which also has a local source component, is dominated by regional/global sources.

Aerosol samples were collected during three consecutive days in and around the Portland metropolitan area during the month of June, 2000. Samples were collected over a 20 minute period at various locations on filter media and then counted using a portable gamma spectrometer. Different routes through Portland were followed each of the sampling days. The routes started in areas of low to moderate population density, crossed near the urban center then generally continued across the city into low density areas 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. Meteorological data including wind speed, wind direction, humidity, and barometric pressure were collected for each sampling site. Additionally, hourly METAR data (routine aviation weather reports) was collected at 5 airports: Portland International, Astoria, Kelso, Troutdale, and Hillsboro. These airports were near or upwind of Portland during the collection period.

No correlations were found between ²¹²Pb or ²¹⁴Pb activity and proximity to the urban center. This shows that the fluxes of ²²⁰Rn and ²²²Rn under the regional weather conditions during this time period were dominated by local/regional geologic sources and not by urban influences.

While the measured ²¹²Pb activity was constant over the three sampling days within the associated counting error, the activity of ²¹⁴Pb was found to be dependent on wind velocities. Sample data compared well with a simple 1D model for the transport of marine air over a fixed ²²²Rn/²²⁰Rn source. Variability in the ²¹⁴Pb activity was well correlated with transport time from the Oregon coast. Lead-214 appears to be an good tracer for examining the mixing rates of marine air over land masses on the time scale of 1 to 20 hours. While not directly demonstrated by this work, ²¹²Pb appears to be a good tracer for examining the mixing rates of marine air on the time scale of the first 2 to 5 hours.