NOx source apportionment in a coastal urban air shed using stable isotope techniques

NOx (NO + NO2) emission decreases urban air quality and its subsequent deposition can be a significant source of excess nitrogen loading to coastal waters. Photochemical reactions between volatile organic com- pounds, and NOx in the atmosphere creates ozone (O3). Previous studies suggest that the City of Corpus Christi is in a NOx limited zone, so an increase in NOx would lead to an increase in O3. The first step to NOx emission mitigation is to quantify the contributions of NOx sources. This study uses stable isotope techniques to measure point and nonpoint NOx sources in order to quantify three main NOx sources in the Corpus Christi air shed: vehicular, biogenic, and industrial sources. Each of these sources have unique iso- topic compositions or “source signatures”, specifically 15N-NOx values, which are different for each source and allow the use of isotope mixing models to determine source contribution. NOx and NO2 passive air samplers will be deployed at three City of Corpus Christi NOx and ozone monitoring stations each month for one year and the nitrogen and oxygen isotopic composition ( 15N, 18O) of each sample will be measured. The 15N-NOx value in ambient air, and the 15N-NOx values of the NOx sources, will be applied to a Bayesian isotope mixing model to quantify source contributions. Limited preliminary data suggests that vehicular emissions is the main contribution, followed by industrial emissions, then biogenic. Results will help aid in the creation of an ozone action plan for the City of Corpus Christi.