Influences on Air Quality in Georgetown, Texas

Georgetown, Texas is a city of approximately ten thousand residents situated twenty miles north of Austin, Texas. With few primary industrial sources, the main influence on local air quality in the city is mobile sources on Interstate Highway 35. Regional influences include the City of Austin, 40 km to the south, and the Dallas-Fort Worth metroplex 250 km to the north. Georgetown occupies what is considered the northern end of the Austin metropolitan area, which has grown in population over the past two decades. As a result of this growth, average concentrations of tropospheric ozone have grown as well (REF). In addition to these local sources, long-range transport of ozone and ozone precursors is a contributor. Two data sources were used for this study. A Texas Capital Area Council of Governments (CAPCOG) monitoring station was situated on the west side of Georgetown (EPA site number: 48-491-0690) measured tropospheric ozone concentrations from September 2007 to November 2013. The second data source for this study is four ozonesonde profiles taken with weather balloon launches at Southwestern University. Each vertical profile was taken using an En-SCI Electrochemical Concentration Cell (ECC) ozonesonde connected to an iMet radiosonde. The two instruments were launched together on a 600 g En-SCI weather balloon. Data were transmitted to the operator by the radiosonde for the duration of the flight and recorded in the field on a laptop computer. Two ozonesonde launches were paired in an effort to sample both northerly (post-frontal) and southerly flow. These two approach angles reflect the influence of different air masses, a continental airmass to the north and a maritime airmass over the Gulf of Mexico to the south. Northerly flow often brings additional influences on tropospheric ozone. Post-frontal northerly flow reflects a continental airmass with entrained ozone and ozone precursors from outside the Central Texas region. Additionally, the passage of cold fronts leads to vertical transport of ozone-rich stratospheric air into the troposphere. Days with significant amounts of stratosphere-troposphere exchange often result in high surface ozone concentrations. The other two ozonesonde launches were each conducted in the post-frontal environment. These profiles and surface data will be evaluated along with HYSPLIT trajectories and other resources to characterize the tropospheric ozone environment in this growing Texas community.