Have breathing problems? I would suggest limiting your time outside for the next couple of hours. There’s no Air Quality Alert in effect, but there’s something pretty interesting and unfortunate going on with the air quality around #Cincinnati now.

By accident, I happen to look at a map of air quality indices (AQI) this morning, and I notice the air quality was categorized as “unhealthy for sensitive groups” for many communities in and around Cincinnati, especially near I-71 and I-75.

These are government sensors, but citizen sensors – like those from Purple Air (I have one) – show the AQI is actually much higher in spots and even going into the unhealthy for everyone category:

The AQI index is 100+ in Middletown, Blue Ash, Mt. Auburn, and a couple of other local sites. Here’s the trend for Mt. Auburn:

…and Blue Ash:

…and Camp Washington:

Calculating the AQI is a bit weird because it’s not a true observed value like a temperature. The AQI is a forward-leaning 8-hour average, meaning it’s the currently function of the current AQI and forecast AQI values for the next 7-hours. As time goes on, values for previous hours are recalculated based on more observed values and less on forecast values.

So why is the AQI spiking? The AQI is based on a few different pollutants, including ozone and PM2.5. The graphics attached are the AQI based on PM2.5. What’s a common source of elevated PM2.5? Wildfire smoke. An explanation of PM2.5 can be found here, and the caption of an image from this link is important to review:

Emissions of combustion of gasoline, oil, and diesel fuel can drive up the AQI, so seeing “less than ideal” values around interstates is not unusual. But this morning’s spike is significant.

For days I’ve been talking about the wildfire smoke overhead being aloft. For the most part it is, but notice how the RAP model has elevated levels of near-surface smoke around #Cincinnati (black dot in the graphic below) this morning:

So how did this smoke aloft get to the ground? The answer is simple: it has rained. Rain – especially if it’s heavy – is a efficient way of bringing air/wind/energy aloft down to the ground. In this case, it was able to take a lot of PM2.5-laden air down to the ground.

So as rain moves out and the flow at and above the ground turns more northwesterly, smoke and higher PM2.5 concentrations will move south. Here’s an evolution of near-surface smoke from 8am to 5pm today.