LaRC Modeling of Ozone Formation in San Antonio, Texas

Ozone (O₃) is one of the most important trace species within the troposphere and results from photochemistry involving emissions from a complex array of sources. Ground-level O₃ is detrimental to ecosystems and causes a variety of human health problems including respiratory irritation, asthma and reduction in lung capacity. However, However, the O₃ Design Value in San Antonio, Texas, was in violation of the federal threshold set by the EPA (70 ppb, 8-hr max) based on the average for the most recent three-year period (2014-2016). To understand the sources of high O₃ concentrations in this nonattainment area, we assembled and deployed a mobile air quality laboratory and operated it in two locations in the southeast (Traveler’s World RV Park) and northwest (University of Texas at San Antonio) of downtown San Antonio during summer 2017 to measure O₃ and its precursors, including total nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Additional measurements included temperature, relative humidity, pressure, solar radiation, wind speed, wind direction, total reactive nitrogen (NO_y), carbon monoxide (CO), and aerosol composition and concentration.

We will use the campaign data and the NASA Langley Research Center (LaRC) Zero-Dimensional Box Model (Crawford et al., 1999; Olson et al., 2006) to calculate O₃ production rate, NO_x and hydroxyl radical chain length, and NO_x versus VOCs sensitivity at different times of a day with different photochemical and meteorological conditions. A key to our understanding is to combine model results with measurements of precursor gases, particle chemistry and particle size to support the identification of O₃ sources, its major formation pathways, and how the ozone production efficiency (OPE) depends on various factors. The resulting understanding of the causes of high O₃ concentrations in the San Antonio area will provide insight into future air quality protection.