High O3 Days

Ozone is mostly a summertime pollutant and only reaches unhealthy levels in New Hampshire a few days per year. Read below for an overview of the most common scenarios leading to high ozone in the state.

Some of the ozone on these days is formed from precursor pollutants (NOx and VOCs) generated in New Hampshire, but most of the smog-causing air pollutants on high ozone days are emitted by major cities and power plants in states to the south and southwest of New Hampshire. In fact, most of the ozone problems in New Hampshire occur along the southern and southeastern borders of the state, or at high elevations, where ozone and its precursors enter the state from many miles away.

Pollution entering the state from other places is called air pollution transport. The graphics below illustrate air flow during the highest ozone days in the Northeast compared to the cleanest days. Cleaner air tends to come from the northwest, while polluted air often enters the region from the southwest after passing over the industrial Midwest or urban areas and highways to the south. Please see the report Air Pollution Transport and How It Affects New Hampshire for more information about air pollution transport.

chart of hourly ozone on a high dayExample High Ozone Day in Southern New Hampshire
On August 21, 2013, the maximum eight-hour ozone concentration exceeded the 2008 ozone NAAQS of 0.075 ppm (75 ppb or parts per billion), and the hourly ozone maximum reached nearly 100 ppb in Portsmouth (see chart on left).

Typical of ozone patterns in New Hampshire, ozone levels were low at night, but steadily increased during the day. This pattern occurs because ozone is destroyed when it contacts vegetation and other objects overnight, but it begins to reform in the heat of the day. The late afternoon peak in ozone concentration is common in New Hampshire due with the arrival of ozone and ozone precursors transported long distances with the wind.

The August 2013 event also demonstrates a sea breeze effect. The seacoast experienced the state’s highest ozone that day because a sea breeze drew onto land polluted air that had accumulated offshore.  This pollution originated in urban and industrial areas to the south, drifted offshore, and moved north along the coast.
diagram of the sea breeze effect
High Elevation Ozone
The mountains in the state sometimes experience the highest ozone concentrations. Ozone monitors located at the summits of Pack Monadnock (2288 feet above sea level) in Peterborough and Mount Washington (6989 feet above sea level) are positioned to measure longer-range air pollution transport that may not affect communities at lower elevations. Ozone aloft travels more quickly, is unimpeded by objects and landscape features, and is less exposed to chemicals that can break down the ozone that has accumulated and risen to these altitudes during the day. This is why ozone levels may not only be highest on the mountaintops, but can stay high after sunset. The chart below shows how the average high-day ozone on the Mount Washington summit stays nearly level throughout the day, while at a ground-level site near the base of the mountain, it fluctuates day to night.