Monitoring ambient ozone with a passive measurement technique method, field results and strategy

A low-cost, accurate and sensitive passive measurement method for ozone has been developed and tested. The method is based on the reaction of ozone with indigo carmine which results in colourless reaction products which are detected spectrophotometrically after exposure. Coated glass filters are mounted in a short polystyrene badge-type vessel in which the diffusion path is restricted by a Teflon membrane filter. From wind tunnel experiments good concentration-independent linearity has been found. The detection limit of the method is about 23 ppbv for 1 hour exposure. Tests with interfering oxidants, such as peroxyacetyl nitrate and NO2, indicated no significant effects for common ambient conditions. No humidity effect was found when the relative humidity varied from 20 to 80%. The uptake rate of the sampler is dependent on wind velocity. In general an average air velocity of approximately 0.5 m/s is necessary to ensure a stable collection rate of ozone. This implies the necessity to measure wind speed during exposure or to place the samplers in the wind stream of a small fan. The method is suitable for hourly to daily mean measurements of ambient ozone. Under field conditions an accuracy of 11 +/- 9% (40 experiments) was established in comparison to a continuous UV-photometric monitor.