The influence of the Chesapeake Bay Breeze on Maryland air quality

On March 12, 2008, the U.S. EPA strengthened the National Ambient Air Quality Standard (NAAQS) for 8-hour average ground-level ozone from 85 ppb to 75 ppb. The new standard reflects over 1,700 scientific studies that linked adverse health effects at the level of the old standard or below. Maryland has greatly improved its air quality over the past several years. However, one region within the Baltimore Nonattainment Area (BNAA) presents many challenges in meeting this new standard. The upper Chesapeake Bay region suffers from the highest ozone concentrations throughout the state, largely due to its location downwind of two major metropolitan areas as well as the development of the bay breeze. The formation of this micro-scale circulation has been known to create a sharp gradient of observed ozone concentrations across its boundary. A climatological study of the Chesapeake Bay breeze will be presented, specifically focusing on the upper bay region of the BNAA. The results of this study will describe bay breeze frequency during the months of June-August in 2004-2008 with respect to high ozone episodes. In addition, a comparison of ozone concentrations between inland air monitoring sites and coastal sites will help quantify the ozone load that can be attributed to the bay breeze.

To further illustrate the Chesapeake Bay breeze trend and its influence on ozone accumulation, two case studies will be shown. In both cases, the bay breeze affect on ozone was underestimated. The air quality episodes were predicted to reach the Unhealthy for Sensitive Groups category of the Air Quality Index (AQI) while ozone actually increased to the Unhealthy AQI category. On June 8, 2007, the bay breeze began to form at approximately 16 UTC. In this case, the bay breeze boundary penetrated further inland than is commonly observed, allowing for the daily maximum 8-hour average ozone concentration to reach 108 ppb, Unhealthy AQI category, at the Aldino air monitoring site. The second air quality episode on June 25, 2009 experienced an earlier bay breeze onset at 14 UTC. Ozone concentrations quickly rose as the bay breeze grew stronger leading to a daily maximum 8-hour ozone value of 109 ppb, Unhealthy AQI category, at the Edgewood air monitoring site. During this episode, the ozone plume was easily tracked by comparing the time in which the maximum 1-hour ozone concentration was observed for each affected site. The plume followed the bay breeze boundary and traveled northeast toward Philadelphia, PA.