Springtime Surface Measurements of 7Be, 210Pb, and Speciated Hg and Their Relationships with Stratospheric Air in Houston, TX



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The primary goal of this study was to verify the influence of stratospheric air to Houston’s surface ozone. The stratosphere contains approximately 90% of the atmospheric ozone, making it a potent ozone source to the free troposphere. Measurements of various chemical species and meteorological parameters were used to identify stratospherically enhanced airmasses in the Spring 2012. Our results showed higher 7Be and HgII concentrations at the surface were associated with atmospheric layers with high O3, low relative humidity, and enhanced potential vorticity. While irreversible mixing is common between the upper troposphere and lower stratosphere in the springtime, this analysis determined that a source of strong subsidence is needed in order to propagate this mixing down to the surface. For Houston, this means a cold frontal passage or a prolonged period of high pressure over the area. In order to see strong stratospheric signals, the low pressure system must be strong and pass close to Houston. This study shows the stratospherically impacted air is usually not expressed immediately at the time of frontal passage, rather the mechanism of descent is associated with subsidence of the post-frontal high pressure. Consequently, high 7Be concentrations are associated with surface winds from the south. While this study utilizes 24-hour integrated 7Be samples, our results show that a 6-hour bulk aerosol samples would still be well above the 7Be limit of detection. Use of 6-hour sampling would allow for better resolution of the nature and timing of the downward transport UT/LS air to the surface. This study is the first to utilize HgII measurements as a tracer of stratospheric air at the surface. Post-frontal air shows a clear enhancement of HgII for several days suggesting a relationship to ozone levels in the surface layer.



Ozone, Beryllium-7, Lead-210, Speciated mercury, Stratospheric intrusion, UTLS