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.

You may also be interested in the articles below:

• The challenges with electrochemical NO2 sensors in outdoor air monitoring. https://www.aeroqual.com/challenges-electrochemical-no2-sensors-outdoor-air-monitoring
• Gas Sensitive Semiconductor (GSS) Technology https://www.aeroqual.com/company/our-technology/gss-technology

BACKGROUND

Air pollution is one of the most significant challenges facing London. London is in breach of European legal limits for Nitrogen Dioxide (NO2) and many areas exceed safe limits for Particulate Matter (PM) as set by the World Health Organisation. The Mayor has proposed measures to tackle emissions from road transport in particular diesel vehicles. This is to be extended for NRMM relating to construction sites which is also a significant contributor to adverse Air Quality in London. Current estimates of emissions from NRMM used on construction sites are shown to be responsible for 7% of NOx emissions, 14% for PM2.5 and 8% of PM10 emissions across the Capital.

WHAT DO CONSTRUCTION AND DEMOLITION COMPANIES NEED TO DO?

In order to comply with the GLA’s NRMM LEZ policy there is a requirement for developments to keep an inventory of all NRMM used at the site. 

This inventory of all NRMM will state the emission limits for all equipment. All machinery should be regularly serviced and service logs kept on-site for inspection. This documentation should be made available to local authority officers as required.

The NRMM register is an automated online inventory which is handled at: http://nrmm.london/

Details of all NRMM with a net power between 37kW and 560kW should be recorded as it is delivered to the site with an indication of the proposed duration of use no matter how short or long this may be. If you know what machines will be used they can be entered in advance, this can be particularly useful where an exemption is required.

REGIONS COVERED FOR NRMM STANDARDS

This map shows the area of the Non-Road Mobile Machinery (NRMM) standards will apply. These standards have been active since 1st September 2015. There are two zones – Greater London (shown in blue), and the Central Activity Zone (CAZ) and Canary Wharf (shown in orange). The central London areas have a tighter emissions standard applied to them.

A Useful Practical guide can be seen here:

http://nrmm.london/sites/default/files/NRMM-Practical-Guide.pdf

Campbell Associates offer automated web based monitors for construction and demolition sites.  This includes PM10, PM2.5 particulate/ dust monitors, NOx, NO, NO2 ,  Noise and Vibration for sale and hire.

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.