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

Despite a decreasing trend nationwide, eight-hour O<SUB align="right"><SMALL>3</SMALL></SUB> concentrations in 25 of US national parks have increased by 8% during last decade. This study presents a 13-year observation of high O<SUB align="right"><SMALL>3</SMALL></SUB> 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. O<SUB align="right"><SMALL>3</SMALL></SUB> exceedances occurred most frequently in June at MACA, and in August or September at GRSM. High O<SUB align="right"><SMALL>3</SMALL></SUB> episodes at MACA occurred during daytime or early evening, but exceedances at GRSM can be found in any hour. Air masses with high O<SUB align="right"><SMALL>3</SMALL></SUB> at GRSM came from all directions, whereas those at MACA are predominantly from the southwest. Case studies show that high O<SUB align="right"><SMALL>3</SMALL></SUB> episodes at MACA are developed under clear sky, high temperature, low humidity, and weak winds traveling in a uniform anti-cyclonal pathway.