Beryllium-7 Measurements in the Houston and Phoenix Urban Areas: An Estimation of Upper Atmospheric Ozone Contributions

摘要

Natural radionuclides have been proposed as a means of assessing the transport of ozone (O_3) and aerosols in the troposphere. Beryllium-7 (~7Be) is produced in the upper troposphere and lower stratosphere by the interaction of cosmogenic particles with atmospheric nitrogen and oxygen. ~7Be has a 53.29-day half-life (478 keV γ) and is known to attach to fine particles in the atmosphere once it is formed. It has been suggested that O_3 from aloft can be transported into rural and urban regions during strato-spheric-tropospheric folding events leading to increased background levels of O_3 at the surface. ~7Be can be used as a tracer of upper atmospheric air parcels and the O_3 associated with them. Aerosol samples with a 2.5-μm cutoff were collected during 12-hr cycles (dayight) for a 30-day period at Deer Park, TX, near Houston, in August-September of 2000, and at Waddell, AZ, near Phoenix, in June-July of 2001. A comparison of ~7Be levels with 12-hr O_3 averages and maxima shows little correlation. Comparison of nighttime and daytime O_3 levels indicate that during the day, when mixing is anticipated to be higher, the correlation of ~7Be with O_3 in Houston is approximately twice that observed at night. This is consistent with mixing and with the anticipated loss of O_3 by reaction with nitric oxide (NO) and dry deposition. At best, 30% of the O_3 variance can be explained by the correlation with ~7Be for Houston, less than that for Phoenix where no significant correlation was seen. This result is consistent with the intercept values obtained for ~7Be correlations with either O_3 24-hr averages or O_3 12-hr maxima and is also in the range of the low O_3 levels (25 ppb) observed at Deer Park during a tropical storm event where the O_3 is attributable primarily to background air masses. That is, maximum background O_3 level contributions from stratospheric sources aloft are estimated to be in the range of 15-30 ppb in the Houston, TX, and Phoenix, AZ, area, and levels above these are because of local tropo-spheric photochemical production.