High Background Ozone Events in the Houston-Galveston-Brazoria Area and Ozone Variability during Cold Front Events over the Contiguous United States

Meteorology is one of main factors that influences background ozone (O3) which is particularly variable over the Houston-Galveston-Brazoria (HGB) and the whole U.S. Long-term meteorological events such as heat waves, stagnation, thunderstorms, cold front passages, and ozone data were analyzed to identify the days of meteorological events and high ozone episodes during the ozone season (April-October) of 2000-2015. Only thunderstorms showed a decreasing effect on peak and background ozone; other meteorological events are all associated with higher levels of peak and background ozone. Background ozone plays a more important role than local ozone in relative contribution to ozone exceedances in the HGB. Cold front events were studied in detail due to unexpected long-term increasing effects on ozone over the HGB during the ozone season (April-October) 2003-2016. On a long-term and large-scale view, cold fronts over the HGB area could be regarded as an interruption in the cleansing effects of predominantly marine southerly flow from over the Gulf of Mexico. The study area was expanded from the HGB to the Contiguous United States in the next step. I found long-term effects of cold fronts on MDA8 ozone over the Contiguous United States during 2003-2017. During non-winter seasons cold fronts showed decreasing effects in most of regions while showing increasing effects at some areas in the midsouth, along the coast of the Gulf of Mexico, south of the East Coast during all frontal days, and in the northeast during prefrontal days. The effects of cold fronts reversed during winter. Connections between cold fronts and other climate factors (e.g., pre-frontal stagnation, southernly flow from the Gulf of Mexico, and winter stratospheric intrusions) can help explain long-term effects.