I'd be interested to know the decay rate when neither the window is opened or the air purifier is on.

Not the Op but I have PM 2.5 sensors in my house. There is one next to the stove, another in the main bedroom above the kitchen, another in the office above the main bedroom, and another in the garage.

Here's two samples. One was last night when I cooked pancakes, grits, sausages, and hash browns on a gas stove. The fan came on at 6:35:37.

The other is cooking one fried egg in the morning for breakfast.

https://imgur.com/a/zwBU6wA