Free Tropospheric Ozone Spatial Gradients Between Smith Point and Houston, Texas Observed During DISCOVER-AQ

Ozone is an important atmospheric trace gas that protects the earth from ultraviolet (UV) rays, yet when present near the surface, poses numerous negative risks for plant and human health. In September 2013 as part of DISCOVER-AQ, ozonesondes were launched nearly concurrently from two locations in Texas 57.54 km apart: Houston and Smith Point. Free-tropospheric (3 - 10 km) data from the Houston flights conducted on the same days as Smith Point flights show higher average ozone mixing ratios, with their differences reaching greater than 15% – a surprising result given their spatial proximity. Stratospheric ozone mixing ratio differences for the same flights were consistently less than 5%, enhancing confidence that the tropospheric differences are real. An analysis of concurrent P3-B aircraft and ozonesonde data further indicated a real difference in ozone concentrations between the two sites. This project explores and evaluates a number of possible factors that might result in such a persistent difference and determine its cause. The primary hypothesis explores the possibility of a “chimney effect” due to Houston’s urban heat island (UHI), in which ozone is lofted higher with deep convection. Temperature data was analyzed to determine if UHI effects extend beyond the surface, but showed that Houston’s UHI is confined to the boundary layer. Convective available potential energy (CAPE) was examined to evaluate the relative vertical stability of air masses at the two sites. This analysis showed that Houston had a greater air mass instability and potential for deep convection at the time of the flights. Additionally, days with high CAPE values at Houston but not Smith Point showed average ozone mixing ratios enhancements over Houston reaching over 20% difference, while days with low CAPE values at both sites showed average ozone mixing ratios less than 10% different at most altitudes. This indicates that deep convection plays a role in the persistent ozone offset between Houston and Smith Point. The results from this unique dataset provide insights into how urban areas may influence the free tropospheric ozone budget.