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

Abstract

Natural radionuclides have been proposed as a means of assessing the transport of ozone (O₃) 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 O₃ from aloft can be transported into rural and urban regions during stratospheric–tropospheric folding events leading to increased background levels of O₃ at the surface. ⁷Be can be used as a tracer of upper atmospheric air parcels and the O₃ associated with them. Aerosol samples with a 2.5-µm cutoff were collected during 12-hr cycles (day/night) 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 O₃ averages and maxima shows little correlation. Comparison of nighttime and daytime O₃ levels indicate that during the day, when mixing is anticipated to be higher, the correlation of ⁷Be with O₃ in Houston is approximately twice that observed at night. This is consistent with mixing and with the anticipated loss of O₃ by reaction with nitric oxide (NO) and dry deposition. At best, 30% of the O₃ 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 O₃ 24-hr averages or O₃ 12-hr maxima and is also in the range of the low O₃ levels (25 ppb) observed at Deer Park during a tropical storm event where the O₃ is attributable primarily to background air masses. That is, maximum background O₃ 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 tropospheric photochemical production.