Evaluation of El Niño-Southern oscillation influence on ozone exceedances along the United States Gulf Coast

Abstract

An investigation into climate-scale influences on tropospheric ozone concentrations in cities along the United States Gulf Coast was performed using a thirty-year database of observed surface ozone data. Hourly surface ozone data were obtained from eight surface Environmental Protection Agency monitoring stations from Texas to Florida. Eight-hour peak concentrations were computed for each day in the record and a database created for further study. To account for regulatory changes, each time history was detrended over the thirty-year period. Days with precipitation, which tend to have low surface ozone concentrations due to low solar insolation and wet deposition of ozone precursors on surfaces, were also removed. The resulting database was stratified by Oceanic Niño Index, which is a measure of the state of the El Niño-Southern Oscillation (ENSO) based on water temperatures in the eastern Pacific Ocean. A common seasonal dependency was identified for all eight stations, with peaks in ozone concentrations in spring and autumn regardless of ENSO phase. The summer ozone minimum was more pronounced for the western Gulf of Mexico than the east. This extension of work done for the city of Houston, Texas confirmed that the influence of ENSO on surface air quality applies to the entire coast of the Gulf of Mexico. Statistically significant enhancements of 5–10 ppbv were found during the months of March and April for El Niño years relative to neutral years. Distributions fit to each city's dataset of 8-h peak ozone values for spring and fall confirmed the difference between the behavior of ozone concentrations in spring and fall for different ENSO phases. El Niño spring data possessed a broader distribution more centered at a higher mean value than the neutral or La Niña datasets for each city. Taken together, the higher mean and broader distribution of 8-h ozone concentrations lead to 30% more exceedances of the National Ambient Air Quality Standard of 70 ppbv for El Niño years relative to neutral years for the composite Gulf Coast dataset. These changes in the ozone climate of the Gulf Coast during El Niño years can be attributed in part to an area of anomalously low pressure in the eastern Gulf of Mexico which influences meridional and zonal flow along the coast. This feature leads to stagnation and a persistent continental airmass along the northern Gulf coast. Smaller but still statistically significant differences between El Niño and neutral autumn were also identified for some stations and were attributed to increased frontal activity during El Niño autumn which leads to an earlier, longer ozone season in previous research. This work extends and improves understanding of the influence of the El Niño-Southern Oscillation on air quality in a large and economically important region of the United States.