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PurpleAir versus the official monitor

Oxford Friends of the Earth is a thriving and effective local group that has been active for over 40 years. Our purpose is to work together to promote a healthy and sustainable environment

  • The City Council’s official monitor and the PurpleAir monitors use different methods to measure particulates pollution. The official monitor uses a regular weighing system to assess small particle pollution. The low-cost PurpleAir monitors use an optical laser counter to measure the extent of small particle pollution. 
  • The relationship between the measures of particulate emissions from the official City Council monitor and from PurpleAir depends on the density of the particles e.g. dust particles are far heavier for a given size than are wood smoke particles. When dust is a big source of air pollution, the official monitor will show worse readings than a PurpleAir monitor. 
  • ‘Co-location studies’, where the two types of monitors located next to each other and are tracked over time, show that the average density of particles is also affected by air temperature and relative humidity. 
  • To convert the PurpleAir monitor measure to units of what the official monitor would show, one needs to apply a time-varying conversion factor – the correlation between the two is not constant. 
  • We are currently conducting an empirical study in Oxford (with the support of the City Council) to calculate more precise conversion factors for Oxford between the two types of monitor.