Improvements to the Passive Ozone Measurement System Used by GLOBE Schools

A study was conducted to improve the accuracy of the passive ozone sensor Eco-Badge test card/Zikua system (Vistanomics, Inc). Specifically we have determined the relationship between ozone concentration and absorbance (color change) of the tin(II)-diphenylcarbazide complex used on the test cards, and the dependence of the color change upon temperature and humidity. Over 700 ambient measurements of hourly test card absorbances with concurrent temperature and humidity, as well as an independent measurement of ambient ozone (2B Technologies, Model 202) collected during the summer 2006 were used to derive a calibration equation for ozone concentration that compensates for temperature and humidity interactions and provides an uncertainty estimate of the measured ozone using rigorous statistical methods. The equation was then applied to previously reported GLOBE surface ozone data to correct for temperature and humidity. Several GLOBE sites were chosen that had consistently reported concomitant measurements of ozone, temperature and humidity and were also in close proximity geographically to an EPA ozone monitoring station. Preliminary results suggest improved agreement between the revised data and EPA values.