Not exactly what you're asking but here's an anecdote:

I recently got an air quality monitor because I was worried about CO2 levels in my apartment having the windows closed and AC running all day every day. It turns out the levels were higher than I'd like (nearly 2000 PPM) and just cracking a few windows open very slightly, less than an inch, is enough to reduce the concentration to between 400 (outside air) and 900 depending on how many people are home. The window AC unit was still able to handle cooling the apartment with a relatively small change in usage.

I also run an air filter constantly so my PM2.5 and PM10 levels were already near-zero, but I'd imagine cracking a window helps with that.

I went down a similar journey over the last couple years (CO2, VOC, PM1.0/2.5/10 sensors). In my apartment I sit around zero for particulate with windows closed. I jump up significantly with windows open unfortunately (for my allergies) in the midwest.

I do wish my AC/Heat had more air exchange, similar to your space my CO2 levels rise dramatically after an hour or two of occupation with windows closed.

Any recommendations on a good air quality monitor? It's been in the back of my mind for a while, but I haven't looked into it much. Ideally it'd be something that doesn't use or at the very least require a cell phone app.

I chose the Qingping Air Monitor Lite ($100) specifically because it can connect to HomeKit, but it doesn’t require it and you can use it totally fine without ever connecting it to the network! Though you might need to connect it once if you want to modify default settings like changing it to Fahrenheit. I actually find the device to have a really responsive offline interface that you can use by swiping the top of it.

To give you an idea of why I wanted a “smart” one because I know on HN it’s a polarizing topic: I can include it in my HomeKit dashboard and see it from the other room or when I’m not home, get alerts when it reaches a certain level, record data automatically, and trigger automations like turning on my air filter.

I'm just waiting for a "green building" code to specify "all windows shall open."

Imagine the panic that would cause in the construction industry.

I think generally "green building" (at least the mainstream stuff) focuses on insulation + air tightness, and then mechanical ventilation. In that context, operable windows would be mostly be a "just in case of overheating" or similar, and the expectation would be CO2 et al would be managed by the ventilation.

Building codes already require operable windows, and some local codes now require active ventilation systems.

The stock of office buildings probably turns over fairly slowly. I've actually never worked in an office where the windows opened.