If your air purifier has a screen and shows the air quality, it's most probably using a PM sensor like the one I write about here.

How accurate is PMS7003? If for example, you need to know the precise and exact concentration of particulate matter in the air - then currently available low-cost PM sensors are not to be considered accurate. But, if you need to know if the air quality in your home is good, fair, moderate, poor or very poor, then it's good enough.