High PM Days

Particle pollution can be high during any season, but 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 PM2.5 in the state.

Wood Smoke
In New Hampshire, the highest concentrations of PM2.5 often occur during winter nights when wood smoke, primarily from residential wood burning, accumulates overnight under calm conditions. When calm winds occur on a cold night, a temperature inversion can form, creating a layer of stable air that traps pollution near the ground. Valley areas are most susceptible to pollution buildup because the bowl-like topography further prevents air movement. For more information about wood smoke, please click on Wood Smoke and Wood Stoves/Outdoor Wood Boilers.

New Hampshire commonly measures its highest wintertime PM2.5 concentrations in Keene, a valley city in the southwestern part of the state where residential wood burning is common. For example, the graph below shows how PM2.5 concentrations rose on two consecutive January evenings in 2012, breaking up on the 8th as the warmth of the sun broke the temperature inversion and allowed the pollution to disperse. Read more about winter wood smoke in Keene at Keene, a Case Study.

chart of hourly PM2.5 on a high day

Though New Hampshire sources produce particle pollution, much of it travels here from outside the state. South or southwest winds can transport particles and other types of pollution to New Hampshire from upwind sources any time of year. Wildfire smoke from distant fires occasionally blows over the region, creating high particle pollution concentrations in the state (see chart at left). Even on those winter nights when wood smoke builds up primarily from local wood burning, the worst levels tend to coincide with additional pollution transported here from upwind areas. Please see the report Air Pollution Transport and How It Affects New Hampshire for more information about air pollution transport.

Wildfiressatellite image of wildfire smoke
Although rare, large smoke plumes from wildfires can bring significant levels of particle pollution into New Hampshire. On Memorial Day in 2010, smoke from fires raging in Canada descended over New England, and daily average PM2.5 levels reached the 60s micrograms/m3 in some parts of southeastern New Hampshire. This satellite image shows the widespread smoke plume, visible as a bluish-white haze.