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List of State Officials - Martin O'Malley, Governor; Anthony Brown, Lt. Governor; Shari T. Wilson, MDE Secretary 

Volume IV, Number 5

 June 2010

eMDE is a quarterly publication of the Maryland Department of the Environment. It covers articles on current environmental issues and events in the state. 

High-elevation Air Monitoring Stations Reveal the “Ozone Reservoir”

By Michael Woodman, Air and Radiation Management Administration

Click on photo to view larger image

Piney Run Air Monitoring Site Located In Garrett County Western Maryland. 

Typical Day/Night Ozone Cycle (from NHDES) 

The three elevated monitoring sites 

Locations of the three high elevation air monitoring sites. 

Back to this issue's cover page 

A layer of ozone hangs high above Maryland on summer nights but drops to become part of the air we breathe the following day, MDE testing shows.

Data from MDE’s state-of-the-art, high-elevation monitoring site at Piney Run in Garrett County demonstrates the existence of an “Ozone Reservoir” a couple of thousand feet up in the air. Two other high elevation ozone monitoring sites – Shenandoah National Park in Virginia and Methodist Hill in Pennsylvania – and flight data gathered by University of Maryland College Park research aircraft as part of the Regional Atmospheric Measurement Modeling and Prediction Program confirm the existence of this “Ozone Reservoir” and document the presence of pollution transported from the west.

At low elevations during the summer ozone season, ozone concentrations typically start decreasing a few hours after sunset, which results in ozone concentrations being at their lowest in the early morning hours. This is in contrast to what is observed at high elevation sites in and around Maryland, where ozone concentrations tend to stay high during the night hours. This phenomenon, referred to as the Ozone Reservoir, is due to the nocturnal temperature inversion that forms approximately 1,500 – 2,000 feet above sea level. The inversion creates a barrier that prevents ozone, generated earlier in Maryland but primarily in other states, from mixing vertically downward to low elevation surfaces. The high-elevation monitors are at the perfect altitude and location to measure relatively high ozone concentrations, even at night. In the late morning this inversion breaks and vertical mixing allows the aloft ozone to mix down to the low elevations, where it can play a major role in the creation of a high ozone day in Maryland. Breathing ozone can cause respiratory problems.

Research has shown that the Ozone Reservoir stretches across state lines. On some mornings, the ozone over Maryland that fills the Ozone Reservoir is transported into the state from the Ohio River Valley. In the mid 1990s, the high-elevation monitoring sites typically measured ozone concentrations as high as 110 parts per billion (ppb) and the 1-hour ozone air quality standard at the time was 125 ppb. In the 2000s the high ozone concentrations seen at the high-elevation monitors was lower – in the 85 – 90 ppb range, but that is above the 8-hour air quality standard of 75 ppb. This reduction indicates that nitrogen oxide emission controls implemented at power plants and other large emission sources in and around Maryland in the mid-2000s have reduced overall ozone concentrations and lowered the amount of ozone being transported into Maryland from other states. 

To reduce air pollution generated in Maryland, the State enacted the Healthy Air Act, the most sweeping air pollution program ever in Maryland and the toughest power plant emission law on the East Coast.

Still, at certain times, as much as 70 percent of the pollution in our air comes from other states. Maryland is continuing to push for tougher national and regional air quality standards and is part of the multi-state Ozone Transport Commission, which works to implement regional requirements.

For more information on the Air Monitoring Program’s high elevation monitoring site and other measurements click here

Click here to learn more about MDE’s air quality forecasting program


©2010 Copyright MDE

Editorial Board
Maryland Department of the Environment
1800 Washington Boulevard, Baltimore, MD 21230