Here is a plot of the levels of small particles (PM2.5, smaller than 2.5 microns), capable of moving deep into the lungs, at Kent, Washington (provided to me by Nick Bond, state climatologist) from 1990 to now during the winter. You will note a big drop during the 1990s, with levels now a quarter of what they had been.
What about larger particles at Kent (PM 10)? Major decreases!
Want to know about Seattle? Huge drops in PM10 (see below). The red lines shows the warming level of the national air quality standards. Seattle exceeded in in the late 80s and early 90s. Now we are well below.
Live in Tacoma? Big improvements there as well.
Or consider the number of days of moderate or worse air quality over King County: a big decline in the 1990s
So why has air quality gotten better? There are many reasons.
Probably the most important is the reduction in wood burning by our local residents and better wood stoves. We are simply burning less wood and fireplaces and wood stoves have been a huge source of particles and toxics.
Back in the 80s, the Seattle Times was FULL of ads for wood stoves. Many new homes had wood fireplaces. The situation is very different today. The ads are gone. Wood burning is less popular, and many homes have gas fireplaces. Just as important, Federal and local regulations have required that new wood stoves are far less polluting.
A lot of credit should be given to local and Federal air quality agencies such as Puget Sound Clean Air Agency (PSCAA), the State Dept. of Ecology, and EPA, who have worked aggressively to reduce emissions. Only low-emission wood stoves are now sold as result of their actions. When the meteorology is poor for dispersion of pollutants (weak winds, strong and low level inversion), agencies such as PSCAA call burn bans that restrict burning wood other than for primary heating.
Local, state, and Federal agencies have also worked to reduce emissions from cars, trucks, and large vessels, in addition to lessening emissions from industrial sources.
Getting back to wood. In general, it is a very dirty and polluting way to heat a home (see graphic from PSCAA). Natural gas is hugely better and inexpensive natural gas has encouraged folks to use this fuel.
Meteorologically, we are now moving into the worst air quality season for home fireplace/stove smoke. Nights are getting longer and cooler, so folks want to get the wood burning. But this is also the season of strongest, low-level inversions, as relatively clear skies allow the surface to radiate heat to space, producing an inversion---warming of air with height. Inversions are very stable and tend to act as atmospheric caps that keep pollutants, like smoke, near the surface. Winds are also relative light this time of year, before the big storms approach.
Want a good example of a low-level inversion? Consider this morning! Here is the temperature plots with height at the Sand Point (Seattle) vertical profiler, run by the Puget Sound Clean Air Agency. Large increase in temperature with height in the lowest several hundred meters of the atmosphere. Pollutants will be trapped near the surface. And air quality has declined at some locations around the Puget Sound region.
The worst wood smoke air quality tends to be in valleys, where the smoke and low-level cool air below the inversion tends to pool. Some of the low areas of Lake Forest Park, north of Seattle, are notorious. What is happening there this morning with the strong inversion? (see graphic of small particle levels). Oh oh....bad news. A spike in particle concentrations.
So the bottom line is that regional air quality has greatly improved, particularly in reducing small particle pollution. But wood smoke is still a serious issue, particularly during inversion conditions and in valley areas.