The categories answer different questions: air quality monitors use one or more sensors to estimate specific pollutants or environmental factors such as particulate matter, carbon dioxide, volatile compounds, temperature, or humidity; nebulizers turn prescribed liquid medicine into an inhalable aerosol for respiratory treatment.
The comparison is governed by different mechanisms. For air quality monitors, examine sensor target and averaging interval; for nebulizers, examine aerosol and nebulizer cup. Extra capabilities come later, after the primary purpose is known.