Until now we have referred to Aeroqual’s sensor technology as “Near Reference” but with the development of a new specific NO2 sensor combined with the AQS1’s reference flow technology we are now seeing data, from this tiny box, of the accuracy you would expect from a reference (MCERTS) analyser. This will allow you to get accurate data from confined spaces for NO2, Ozone, PM1, PM2.5, PM10 and TSP. Aeroqual’s unique technology means it can be calibrated directly from calibration gas on site for full QA/QC. LSO (site visits) are only required every 3-6 months so reducing operation costs and saving time. The simplicity of operation of this system further reduces operation costs and improves reliability and data capture rates. This is shown by the study below.

Comparison of AQS1 vs Reference NO2 over 3 Months:

From April to July 2020 an AQS1 was operated close to our reference chemiluminescent NOx analyser (see figure 1). Neither instrument was adjusted during this period, so it has not been possible to apportion drift between the two instruments. However, the data does show just how closely the two analysers tracked each other over a period of more than three months without calibration or adjustment.

The graphs below show how closely the two instruments agreed over three months for when peaks in levels occurred near to the beginning and end of the trial.

Three Month Regression Plots of Hourly Mean Values

The regression plots below show how the AQS1 agreed with the Reference analysers over the three months of the trial. Nitrogen Dioxide levels were unusually low during this period resulting in a lower R2 value for NO2 but correlation at the 40ug/m3 annual limit was excellent. Ozone levels were much higher at times and shows even better correlation, although this is mainly due to Aeroqual’s unique ozone measurement technology.

Time Series Plots and Data Capture

The reference NOx analyser was offline on 17th and 18th June due to a fault. A further failure occurred on 27th and 28th July. The trial was curtailed when both reference analysers were taken away for repair. The AQS1 showed no faults during the three months and is still in operation on 14th September. Data capture for the AQS1 was 100%.

The AQS’s new NO2 analyser module can not only be calibrated directly from gases on site, but it has also shown to have such a stable calibration that this may only be required every 1 or possibly every 2 years. This means that, not only can you be confident that the data is reliable long term. It also means you do not have to factor in the cost of routine “LSO” visits to site.

How is this possible? Unlike other apparently similar systems, the AQS has a flow through system which switches sample air with internally generated zero air so it is, in effect, being zero checked continuously. This together with the stability of the sensor used gives it its unrivalled measurement stability.

To prove this we ran our reference AQS1 (number 847) next to an MCERTS reference chemiluminescent NOx analyser last year as we described in our earlier case study ( https://www.campbell-associates.co.uk/post/comparison-of-aeroquals-aqs-1-with-mcerts-approved-reference-analysers ) carried out for three months between April and July 2020. This shows how the two continued to agree over the whole period to a remarkable degree. More recently (March 2021) we have had the same AQS1 NO2 independently checked against reference gases and this has shown the NO2 values are still correct when operating on the factory calibrations from January 2020. Such calibration stability is unheard of.

Calibration March 2021

Method

Calibration was carried out using GPT (Gas Phase Titration). This is the same equipment as is used to check the calibration and converter efficiency of chemiluminescent NOx analysers and as such this type of equipment is already used on site.

NO2 (Nitrogen Dioxide) was generated by mixing NO (Nitric Oxide) with O3 (Ozone) in a balance of zero air. In the test below calibration gases were fed to both the AQS1 and an MCERTS reference chemiluminescent NOx analyser. Gases were also fed to a reference. To start with NO from a certified high concentration gas standard cylinder was first diluted with zero scrubbed ambient air to create a 250 ppb standard. Neither analyser responded. The ozonater was then switched on and titrated with the NO to create an NO2 250 ppb standard. The same was then used to generate a 90 ppb standard. The NO was then switched off and the analysers tested at 90 and 250 ppb. The results are shown below.

Results

Conclusions

The calibration data proves how stable the AQS1 is over a long period of time (more than 12 months) without any adjustment. It also shows that the NO2 module in the AQS1 is extremely selective and has no response to either NO or O3. Data from the AQS1 was not only close to the traditional “reference” analyser but proved to be far more stable than would be expected from a chemiluminescnet NOx analyser. It is also small enough to be easily moved and installed without an air-conditioner. Indeed it can be carried in one hand and can even be installed on a lamp-post.