Portland, OR Air Quality Index (AQI) Today

Portland occupies the Portland Basin at the northern end of the Willamette Valley, with the Coast Range to the west, the Cascades to the east, and the Tualatin Mountains — the West Hills locally — rising at the city’s own western edge. The basin holds air whenever the wind stops, and that is what the bad days here have in common.

Two mechanisms fill it, at opposite ends of the year. The winter one is routine. On still, cold nights a warmer layer caps the colder air near the ground and stops it mixing out, and Oregon DEQ attributes this area’s cool-season PM2.5 spikes to that inversion combined with residential wood burning, most often in November and December. It accumulates over days and lifts when the weather pattern changes, not before.

The other is wildfire smoke, and the Columbia River Gorge is why it can be extreme. The Gorge is the one near-sea-level gap through the Cascades, so an east wind the range would otherwise block accelerates through the gap instead and carries smoke straight into the basin. Before 2020 the Portland area had never recorded air worse than Unhealthy. In September of that year, during exactly that kind of east-wind event, DEQ logged three Very Unhealthy days and five Hazardous ones. A westerly flushes the basin in an afternoon; an easterly means whatever is in the air is staying.