WHATCOM COUNTY, Wash. — As Whatcom County faces the third day of extremely hot weather today, June 28th, residents are seeing a haze forming in the low altitudes. In confirmation, air quality measurement stations around the county are reporting air quality has worsened and is now in the MODERATE range.
As of 1pm today, all but the northeast corner of the county reported Air Quality Index (AQI) values in the MODERATE range while northeast areas remained in the GOOD range. But this is not the result of typical sources of pollution, including wildfires. The explanation, according to the Washington Department of Ecology is relatively rare occurrence in the Pacific Northwest brought on by the high-pressure system and the heat it brought.
The short explanation is heat-induced biogenic Secondary Organic Aerosols (SOA), naturally occurring particles are occurring far more due to the extended hot weather, are being trapped in the low altitudes by a high-pressure system which as resulted in the visible haze and worsening air quality measurements.
A more detailed explanation appeared today on the Washington Smoke Information blog.
The ridge of high pressure over the state giving us this extreme heat is also responsible for forcing air downward and trapping pollutants within a shallow layer. This is known as a subsidence inversion. Since winds are relatively light, all the pollution generated by human activities tends to accumulate over time. But that’s not all.
Trees also emit organic gases such as terpenes and isoprene, more so on hot days. These pollutants of biogenic origin undergo various complex chemical reactions in the atmosphere, giving rise to small particles known as Secondary Organic Aerosols (SOA). “Secondary” because the aerosols are not released directly from sources but formed as a result of subsequent physical and chemical processes in the atmosphere. The diurnal nature of the elevated pollution levels over the last few days are consistent with how we expect biogenic SOA to behave. A WSU air quality model is showing the same thing.
Biogenic emissions are also one of the ingredients of ground level ozone, aka “smog” formation. They react with oxides of nitrogen (mostly from traffic and industrial sources) under the right meteorological conditions to form ozone. All those factors came together yesterday and will do so again today to cause high ozone levelsRanil Dhammapala- Washington State Department of Ecology
via The Washington State Smoke Information blog (June 28, 2021)
The amount of SOAs is expected to drop as the high-pressure system moves on allowing for cooling and releasing what visible haze has gathered. This will result in air quality improvements and clearer air- assuming there are no fires. The current weather forecast calls for the high-pressure system to move on beginning tomorrow.