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Electrochemical and solid state sensors to detect Nitrogen Dioxide at ppb levels have been available for many years, but the problem has always been their near 100% cross response to ozone. This is not helped by the way Ozone and Nitrogen Dioxide mirror each other, with one dropping as the other rises, so their combined value can be meaningless. One way around this is to measure Ozone and subtract its contribution from the combined value. However, most sensors for ozone have a near 100% response to Nitrogen Dioxide. The relative difference in response to the two gases by two sensors has been used to estimate both, but this requires complex web-based algorithms and results can be variable. More recently some sensors have been developed which use an Ozone “filter” above the sensor to remove Ozone and therefore its cross response, but this has been shown to deteriorate with time, so that their correlation to reference analysers deteriorates rapidly as the ozone cross response of the sensor increases. It also reduces the response to Nitrogen Dioxide, especially if a thicker layer is applied to reduce deterioration.

The solution is to use an ozone sensor which measures ozone without NO2 cross-response. This is achieved by Aeroqual’s unique, patented GSS (Gas Sensitive Semiconductor) sensor. Not only is it specific to Ozone, its unique ABC (Automatic Baseline Correction) technology means its calibration stays stable long-term. We can therefore measure the sum of the Nitrogen Dioxide and Ozone concentrations with one sensor and Ozone with another. Accurate Nitrogen Dioxide values can therefore be calculated live, within the instrument and without the need for web-based algorithms. As a result, co-location studies have repeatedly shown correlation between the AQS and bigger and more expensive reference analysers with an R2 better than 0.95.

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