Dedicated 20 amp outlet tripped twice
Since you said the breaker is AFCI and the breaker is now tripping I would guess the controls in the air purifier or the heater control contacts are getting dirty. When contacts get dirty or pitted they arc more than new and will trip AFCI circuits. I love square d but AFCI and GFCI breaker troubleshooting stinks as they don’t have the indicators that other brands have to show what the trip was caused from. 

I agree that you probably were not overloading the circuit and that it has recently started tripping can show some arcing in the contacts. 

AFCI protection is there to detect wiring problems, being right at the panel and in your garage I would eliminate the AFCI it is not required but the GFCI is required. 
Some might say it is protecting the home but a UL listed air purifier and space heater may keep tripping the breaker (the breaker could also be going bad). I have had many problems with AFCI breakers tripping on circuits that have speed control or ballasts all electronics no contacts so my state allows them to be removed with problem equipment.