Measurements of Pb-212 and Pb-214 in surface air around Lake Michigan and their implications for atmospheric boundary layer mixing.

摘要

The dynamics of the atmospheric boundary layer are a result of turbulence generated at the Earth's surface. The extent of mixing in the boundary layer is studied by using radionuclides as tracers under different conditions such as: wind direction dependent on passing synoptic systems, upwind and downwind lake shore sites, urban versus rural setting, and urban setting at two different altitudes. Data were collected by high volume air samplers at each site, which filtered air for day and night intervals several days in a row. The filters were placed in a GeLe detector and gamma rays of {dollar}sp{lcub}212{rcub}{dollar}Pb and {dollar}sp{lcub}214{rcub}{dollar}Pb decays were counted. From these measurements, the concentrations of {dollar}sp{lcub}212{rcub}{dollar}Pb and {dollar}sp{lcub}214{rcub}{dollar}Pb in the sampled air were determined. Among the types of radionuclide behavior are diurnal variation in activities, synoptic-scale variation, variation due to different surface roughness characteristics and position with respect to the wind at the lake shore, and limited variation in activity at high altitude at the urban site. A series of one dimensional models were developed to interpret the data. The horizontal advection model predicted the effect on {dollar}sp{lcub}212{rcub}{dollar}Pb and {dollar}sp{lcub}214{rcub}{dollar}Pb activities of advecting air over a 100 km zero source region at constant velocity. {dollar}sp{lcub}212{rcub}{dollar}Pb activities decrease substantially for velocities greater than 1 m/s, whereas {dollar}sp{lcub}214{rcub}{dollar}Pb does not decrease much until velocities reach 100 m/s. The vertical diffusion model predicted {dollar}sp{lcub}212{rcub}{dollar}Pb and {dollar}sp{lcub}214{rcub}{dollar}Pb vertical profiles for different vertical diffusivities, {dollar}kappasb{lcub}rm z{rcub}.{dollar} A one order of magnitude change in {dollar}kappasb{lcub}rm z{rcub}{dollar} produces a {dollar}sqrt{lcub}10{rcub}{dollar} change in activity in surface air. The results of both models are equivalent for {dollar}sp{lcub}212{rcub}{dollar}Pb activities. Comparing {dollar}sp{lcub}212{rcub}{dollar}Pb and {dollar}sp{lcub}214{rcub}{dollar}Pb, vertical mixing affects both radionuclides similarly, but horizontal advection fractionates the two radionuclides. The diurnal box models predict activities an order of magnitude lower than the steady state models, in close agreement with observations. Furthermore, an order of magnitude increase can occur between minimum and maximum activities during a single cycle. If applied to the steady state vertical mixing model, this change indicates that at least a one order of magnitude change in the magnitude of {dollar}rmkappasb{lcub}z{rcub}{dollar} is possible on a diurnal time scale.