What is the difference in an AirVisual Outdoor, Purple Air and Clarity Node-S?

These are three low-cost sensors that are supported on the AirVisual Platform. IQAir also supports regulatory monitors as well.
An AirVisual Outdoor is a weatherized outdoor air quality monitor with modular, field-swappable PM and CO₂ sensors and measures the following: PM2.5, PM1, PM10, CO₂, temperature, and humidity. The AirVisual Outdoor is offered with optional solar/battery powered hardware, global cellular connectivity, and data assurance support.
PurpleAir is an air quality monitoring solution for home enthusiasts and air quality professionals alike. It measures PM2.5.
Clarity Node-S is a sensing-as-a-service solution, inclusive of solar and battery powered hardware, global cellular connectivity, and data assurance support. Not intended for individual consumer purchases. It measures PM2.5 and NO₂.

To find more information on how AirVisual calculates this data, see the link below:

What is the air quality index (AQI)? | IQAir