Temporal variability and case study of high O<SUB align=right>3 episodes in two southeastern US national parks

Abstract

Despite a decreasing trend nationwide, eight-hour O3 concentrations in 25 of US national parks have increased by 8% during last decade. This study presents a 13-year observation of high O3 at the Great Smoky Mountains (GRSM) and Mammoth Cave (MACA) national parks, both among the 25 impacted parks. Although there is no monotonic increase, the later half witnessed three-fold exceedances than the former. O3 exceedances occurred most frequently in June at MACA, and in August or September at GRSM. High O3 episodes at MACA occurred during daytime or early evening, but exceedances at GRSM can be found in any hour. Air masses with high O3 at GRSM came from all directions, whereas those at MACA are predominantly from the southwest. Case studies show that high O3 episodes at MACA are developed under clear sky, high temperature, low humidity, and weak winds traveling in a uniform anti-cyclonal pathway.