We hear this question regularly, from dust suppression systems in mines to filtering systems in enclosed railways or ventilation systems in road tunnels. The approach may vary but the fairly obvious answer is to measure the targeted pollutant – whether it’s dust, NO2 or something else – before and after the mitigation process.

Air quality monitoring around mining sites are often driven my compliance requirements and trigger alerts may be required: the pollutant levels monitored can be used to control the systems themselves. The photo shows a current rail station project where two AQMesh pods are being used to continuously monitor the real-time performance of the purification system being tested.

A previous project in Marseille used eight AQMesh pods to understand exactly how in-tunnel ventilation could be controlled to minimise air pollutant exposure by people living near the ring road covered sections. The ‘Boreas Project’ included microsensor monitoring around tunnel entrances and comparison with a nearby reference station informed ventilation management protocols.

The same principle works equally well with heating, ventilation and air conditioning systems (HVAC). AQMesh is used in a high-profile office building, monitoring intake air, with levels scrutinised to manage the ventilation system. Initial readings showed that the HVAC management regime had actually been concentrating pollutants inside the building – a situation that has happily been reversed, while optimising power usage and spend on filter consumables.

AQMesh can also be used to monitor air composition for tougher ventilation applications, such as coal mines. Methane, PM2.5, PM10, CO2, H2S, atmospheric pressure, etc. can be meaningfully measured on a continuous basis to check that systems are performing as expected.

A good quality small sensor air quality monitoring system should last 10 years or more, and of course certified monitoring stations (reference, FRM, FEM) should last much longer.

Looking at the many small sensor systems used for outdoor air pollution monitoring, the question may be ‘how long will my air quality monitor work between services’? The best systems operate for two years between sensor changes, and sensors should be low cost and locally replaceable, with no need to return equipment to the manufacturer for service or calibration.

Whether considering the operational life of the product or time between each service, it is important not to assume that all products are the same. Some systems are not designed to work for more than a year or so, which might meet the needs of a shorter project and budget, but it may cause a series of unscheduled equipment failures in the middle of an important monitoring period. So, it’s worth asking for examples of long-running projects where long-term operation of the system can be proven.

Any intervention during the operation of an air quality system costs money, even if it is ‘only’ the time and effort of local teams, so this must be minimised. Scheduled service work is undesirable, but unscheduled maintenance demands are worse. AQMesh sensor replacements are based on carefully calculated duty cycles – balancing initial costs against expected life and product performance – and the pods have been proven over years of operation around the world.

Of course, any equipment will be affected by operating conditions but small microsensor users should not expect their system to fail prematurely in harsh conditions. ‘Fit for purpose’ systems will be proven in intense heat, heavy rain, strong winds, freezing temperatures and snowfall: an expected 10-year lifetime should not just apply to easy conditions.

Having said all that, product lifetime will always be extended by careful use and following the manufacturer’s advice. The AQMesh user manual is now supplemented by a range of remote diagnostic tools, which detect signs of product ‘distress’, such as lower voltage from a dirty solar panel or potentially blocked air sample path.

Even if few air quality monitoring projects extend to 10 years, a product which is designed to last that long is fit to be used in a series of projects and can be upgraded or traded in, as requirements change.

Here at AQMesh we have a number of long-standing users who re-use their pods for new projects, and we are happy to discuss how we can help you with your own air quality monitoring requirements.

“How many air quality monitors do I need?” is a question we regularly hear, and the easy responses – “it depends”, “how big is your budget?” – are not very helpful.

To give a better idea, it will depend on all these factors:-

Which pollutants you want to measure

Some pollutants are mixed better / are more homogeneous / more background in ambient air, such as PM2.5 and O3. Others are less so, and can be affected by a local source, like NO. Background pollutants can be measured with fewer measurement points than those which will vary greatly over short distances. For example, O3 may range between 50ppb and 60ppb across a city, but NO could vary from 0ppb to 1,000ppb within just 100m.

Your analysis capability

If you have the resources or skills to carry out detailed analysis, you will get more information out of fewer measurement points. For example, if using wind speed and direction data alongside air quality readings, you can look at an area in terms of pollution sources and areas potentially affected by air quality. The resulting plots and mapping allow reading levels to be visualised across space. Taking it further, measurement points can be linked to emissions inventories and modelling can fill the gaps to give an estimated reading for every geographical point. The more measurement points, the more accurate the estimates are likely to be. Additionally, some analysis techniques – such as long distance scaling or network calibration – require a minimum number of measurement points in order to work, which will therefore determine how many pods you might need. As an example, the long distance scaling method offered by AQMesh requires a minimum of 6 different locations.

The area you’re monitoring in

Multiple pollution sources (think busy city vs. a factory in open countryside) create a more complex air quality situation, as do canyons (naturally confined air corridors or streets between high buildings). A single source within an open environment could achieve a lot with just one pod upwind and one downwind, but a city environment means that NO or NO2 readings could be massively different just other sides of a road junction.

Local conditions

If your air quality monitoring location is generally windy you will have to work harder (install more measurement points to pick up plumes) to capture pollutant bursts before they are swept away.

Environmental justice

We have seen customers distribute pods based on one per ZIP code, to achieve fairness to local communities. This is a good idea in itself, but a ZIP code can include a wide range of pollution levels so all the factors about choosing a precise monitoring point still apply.

And, of course, budget!

Seriously, small sensor systems are described as monitoring ‘hyperlocal’ air quality for a reason and even the densest networks will be leaving some gaps where air quality variation is not recorded. So, measurement points can be added infinitely – air quality mapping of an area will improve in accuracy, but there are obviously diminishing returns.

Because, even after all this, “it depends”, just talk to us about your air quality monitoring requirements and we will be more than happy to share our recommendations and give you a more helpful answer to “how many air quality monitors will I need?”

We have a little competition going on between our customers, even if they don’t know it: who can present us with the challenge of the most remote operation for continuously monitoring air quality?

We are used to dealing with people for whom visiting their site is a full day’s driving or even means chartering a plane. So, we get it: air quality monitoring equipment set up on these remote sites needs to work out there – and stay working.

Having had longer to review, test and reject communication technology than other available air quality monitoring systems, we long ago settled on the cellular phone network as the most effective way to achieve reliable data transfer globally in the widest possible range of environments. Whilst we do have one other trick up our sleeve (we’ll come onto that) mobile networks continue to change around us, and we’ve taken advantage of that and invested in this approach over others.

2G is the little star of machine-to-machine communications – we started with that and in many parts of the world it’s very much available, underpinning numerous M2M systems. We’ve already digested withdrawal of 3G provision in most locations, and AQMesh is future-proofed with 5G+, LTE and NB-IoT capability. However, it’s not as simple as that, with band availability varying hugely in different regions. Our global SIM will roam to available networks and bands, but sometimes there’s only one band available in a particular area and it requires a specific local SIM. As well as having the tools to identify this situation, we support users through installation of a local SIM and securing connections.

Even if the network is weak and connection is intermittent, AQMesh is designed to store sensor output on the hardware until a connection is achieved. What this means is that even with an unreliable network signal, the pod will connect when it can and catch-up readings if there is a period where it can’t make a connection. Whilst it’s great to have regular updates and set up near real-time exceedance alerts – which is the norm – it’s reassuring to know that no data will be lost even in areas of marginal network access.

So, having installed an AQMesh pod (very quick process) and ensured connectivity (simply connect power in 99% of cases), how do you keep it running for years? We have developed a range of remote diagnostics tools, allowing us to proactively detect faults and data quality queries, update firmware over the wire and even power cycle if necessary. This is an important tool for the remote support for life we provide as standard with every pod we sell or rent.

A combination of our global SIM and supporting local SIMs has so far achieved communications with our air quality monitors used in mining, oil and gas, construction and various other industries – even on ships – but we are often asked whether we can handle the situation where there really is no cellular network at all. As a result, we offer a PoE to satellite modem option, if you really do need to send air quality data from the furthest corners of the planet. Just get in touch if you think you can present us with our remotest communications challenge yet 😉

Can’t decide which air quality monitoring approach to use? Go for all of them!

We often come across customers who are agonising over which sensor system – or even which technology – to use and we suggest a hybrid network. Whilst the best air quality monitoring networks will always include a reference station, which can provide data traceability back to an approved standard, there are good reasons to broaden the range of measurement approaches.

We are yet to come across a project where budget was not a consideration. We are also very familiar with the questions, “how many measurement points do I need” or “how many can I have with my budget?”

By using each sort of technology to its strengths, hybrid networks simply help your budget go further. The common denominator required is that all measurement devices of a given type read the same; that readings are repeatable and precise. From that point it is possible to cross-relate time- and location-specific measurements, applying correction factors as necessary. This approach can work when choosing between small sensor systems, as long as you are satisfied that similar instruments will produce similar readings: using different brands in your project may take a little longer to manage, but reduces risks and leaves options open.

So, measurements from air samples or passive samplers, analysed in a laboratory, can be compared to reference or equivalence readings, as well as output from small sensor systems, ranging from more expensive (but still much cheaper than reference) near-reference small sensor systems, mid-range and even the cheapest or ‘home-made’ microsensor platforms. Examples we have come across include networks managed by Cheltenham Borough Council, UK, which uses a combination of diffusion tubes, reference equipment and AQMesh pods, and Kitchener, Canada which combines AQMesh with reference. SAMHE also integrates indoor and outdoor air quality measurements, and there is a shale oil producer in the Baltic region which uses reference equipment for H2S and SO2 alongside data from AQMesh. The Breathe London pilot even incorporated data from mobile Google cars.

Increasingly, the final output of such networks drives the crossing of such boundaries. For example, all local air quality measurements across Iceland are published on a single platform, with the network operator ensuring that data accuracy is managed through appropriate data quality assurance measures.

We are happy to discuss all types of hybrid air quality monitoring networks and how AQMesh can play its part in your objectives.

Communities and industry are monitoring air quality around the same areas, so they both want the same thing, right? Er, no, not really ..

Even though both types of AQMesh user may measure the same pollutants using the same instrument, their objectives and needs are often different. The community users we deal with – mostly in the UK and USA, but plenty of other places too – tend to be more interested in identifying pollution events and relating that to what they are experiencing. The first step is a sort of validation of what they believe to be happening all around them. This is not to say that the review of data is selective or unscientific, it’s just experience-focused. For example, school monitoring projects are generally focused on identifying periods of elevated air pollution outside and around the school at different times in the school day and finding the cause / source.

On the other hand, air quality monitoring around communities by the industries that may be the source of the pollution takes a different approach. Our industrial customers – from oil and gas, construction, mining, landfill and other sectors – want accurate air pollution measurements to demonstrate that they are within compliance of local environmental regulations. Another aspect is that there is often more than one potential source of pollution in an area so an industrial AQMesh user may be keen to understand more about what pollution is coming from where (and hopefully proving that a neighbouring facility is causing the issue, not them). Accurate wind data is required to carry out such source apportionment analysis. AQMesh offer a wind speed and direction sensor option and normally only one pod in an area needs to be gathering this information.

Whilst communities and industries may have slightly different air pollution monitoring objectives, they recognise the benefit of using the same instruments, so the data is comparable. A version of this desire to be able to make meaningful comparisons is where government monitoring uses a particular type of equipment and the potential industrial polluters being monitored choose to use the same technology. For example, a community in Texas, USA, installed AQMesh pods outside suspected polluters, so the industrial facility (or rather a consultancy they hired) bought AQMesh systems to monitor themselves. This helps to build trust that data collection and analysis will be done correctly and in an unbiased manner. Another factor bringing all parties together is when there is a natural cause for the pollution affecting people, such as volcanoes (see our news items about airport and community in Iceland) and wildfires.

In the USA, industrial companies are aware that use of uncertified equipment – other than FRM / FEM – means they cannot be obliged to report on data. This creates a ‘safe space’ for potential polluters to understand the air quality around their operations and their impact on it, ahead of compliance demands.

And then there are data centres, where the focus is not on the potential for pollutants to harm people but infrastructure. Hydrogen sulphide monitoring can warn of potential damage to sensitive copper circuits and HVAC maintenance intervals can be managed by monitoring of PM levels, helping to prevent machinery failures.

So, whilst different customers are all using the same air quality monitoring systems and measuring the same pollutants, the reasons driving the project may be entirely different. Either way, our experienced team can support a range of objectives and help interpret your data with meaningful context.

So how do you do it? Many presentations at the recent ASIC conference revolved around calibration of small sensor air quality systems, including that given by AQMesh Technical Business Development Manager, John Downie. Offered the opportunity to poll the ASIC audience, we chose to ask delegates ‘Please indicate which of these calibration methods for small sensors you would consider for your small sensor network’. Whilst numbers responding were not huge, ‘periodic co-location of each instrument with FRM/FEM creating seasonal correction factors’ came out top, indicating a thorough – if labour-intensive – approach is generally most common and taken as the ‘norm’.

So what if you don’t have the ‘boots on the ground’ to carry out the many instrument movements that would be necessary with a network of any size? Is there a short-cut? Can AI help? The poll did show that about a quarter of respondents were considering ‘desk-based calibration, using machine learning or AI’, about twice the proportion thinking about ‘desk-based calibration using network calibration method without machine learning or AI’.

We tried using AI on electrochemical sensor output a few years ago but found that – at the time – the machine learning was very good at latching onto interfering factors, such as temperature, and less good at finding the weak (valid) signal in amongst the noise. We were greatly impressed by the later (non-AI) network calibration work done by University of Cambridge, our partners in the Breathe London pilot, and have developed our own approach. This series of repeatable calculations can be applied to a network of five instruments upwards and we feel there are some clear differences (advantages, of course) comparing the AQMesh approach to local calibration with AI-driven approaches:

The fundamental issue with machine learning and AI being used either for compensation of sensor interferences or for calibration adjustments is that their whole premise is to “evolve”: they spot differences and change to account for those changes. This means that over time the results may be closer to reference, but they will never follow the same process for adjustment as they did previously. This raises significant traceability questions about when processing changes and makes it impossible for these methods to become certified to a standard which require a fixed processing of inputs, managed by a version number.

It’s worth noting that PAS 4023 (Annex D) distinguishes between sensor systems that are ‘self-contained’ – with readings derived by the system alone, using a series of repeatable calculations – and those that require training against a reference station. The PAS also emphasises how important it is that individual sensor systems should perform very similarly to one another, so that remote comparisons across a network can be made with confidence. Another reason why a proven, global correction algorithm used by every instrument has the edge over individual site-based AI. Strong inter-instrument comparability also means that a network can be meaningfully compared against itself, in the absence of a reference station, as offered by AQMesh’s network normalisation option.

Requests to measure hydrogen sulphide (H2S) in ambient air at unthinkably high levels seem to be at odds with our efforts to detect and report single-figure parts per billion H2S emissions. So, why are we asked for such high ranges?

We think this may be explained by operators who are used to measuring high concentration in a gas stream – typically biogas or industrial – and then simply transferring the range across when looking at fenceline monitoring. It’s great to see the growing interest in monitoring fugitive emissions at site boundaries – including H2S – but we need to dial back the gas range expected when looking at ambient pollution.

H2S sensors for gas stream measurement are offered at parts per million ranges from 0-50ppm to 0-10,000ppm. Bear in mind that US Department of Labor guidelines say that H2S odour becomes offensive at only 3-5ppm, with prolonged exposure causing headaches, nausea and insomnia, and causes “nearly instant death” at 1,000-2,000pm. Dilution of any emission in swirling ambient air means that parts per million measurements are inappropriate, and even significant H2S leaks usually register peaks of just a few parts per billion by the time gas has reached the fence line.

That’s why the AQMesh sensor measures from 0-10,000ppb (0-10ppm), with a limit of detection of less than 1ppb. Measuring at such low levels means operators can pick up emissions much earlier and much further away from the source than would be the case with the higher range sensor typically used for measuring the gas stream. Picking up a low level at a suitable point on the industrial boundary should avoid dangerous levels of H2S building up near the source.

Monitoring in ambient air is gentler on sensors, too, so if you are used to sensor poisoning and condensate problems, that benefit does offset the ‘needle in a haystack’ challenge of picking up fugitive H2S emissions. With the potential to move an AQMesh pod from location to location, and add a wind speed and direction sensor to help with source apportionment, it is very satisfying to support our users doing just that.

Did you know that we have always been part of wider group of companies, offering a portfolio of emissions, leak detection and gas stream monitoring products, software and services, specialising in methane?

For decades, the Ecotec group has been designing, selling and supporting equipment for monitoring landfill gas and biogas. Gazomat in France developed laser-based portable and mobile equipment, used extensively to check very low level methane emissions from natural gas pipelines and networks. California-based Oxigraf specialises in oxygen sensor technology.

What does this mean for AQMesh? Our range of ambient air monitoring sensors – including TVOC, H2S and CO2 – matches and complements the group portfolio of industrial emissions monitoring solutions. We now also offer a methane option, delivering readings alongside all the other channels AQMesh provides, including wind speed and direction. This means we have continuous, stationary methane monitoring options at sub-ppm limit of detection with the laser sensor, or a lower sensitivity small sensor CH4 option.

Let us know if you’d like to know more about our industrial fenceline monitoring options or an introduction to the rest of the Ecotec range of equipment.

There’s no doubt that small sensor systems can have an advantage over their cumbersome reference station cousins – in terms of maintenance requirements. We are often asked about ‘service’ requirements for our pods and the honest answer is that the default position, in normal working conditions, is ‘none’. However ..

There are consumables, conditions can have an impact and sometimes ‘stuff happens’. We have seen small sensor systems claiming they require no maintenance at all, so would like to offer some thoughts on these three areas:

Consumables

Some sensors and sensor components need to be replaced at a defined interval (two years for AQMesh). This may be because the product is designed to optimise accuracy and component duty cycle, such as the AQMesh pump sampling particulate matter, which is expected to last two years on standard settings. If the pump is not replaced at two years, in-field failure is the likely outcome at some point. AQMesh uses a pump to provide reliable sampling over a set period. An alternative, such as a fan, may not be offered with a finite duty cycle, or with one that is theoretically longer, but failure modes can be undetectable remotely and readings affected. Is that better?

Many small sensor systems use Alphasense’s electrochemical sensors, which come with a recommended two-year life. Sure, they last longer than two years and keep producing numbers, but who has tested the sensors beyond two years and will be answerable for accuracy after two years? Users of these systems need to look ‘under the hood’ here – is the system claiming to be maintenance-free ignoring sensor manufacture advice? Are they reducing operation or sampling to extend the duty cycle?

Monitoring conditions

In a nice, clean monitoring location it is possible to predict performance of a monitoring system over several years. But, when the system is faced with tough conditions – such as sandstorms or an acidic environment – some intervention may be necessary to keep the system running. Good design and remote diagnostics can help to minimise this, but it is often the pods installed in the most remote locations (hardest to reach if necessary) that are hit by the toughest conditions.

‘Stuff happens’

Most of the stuff that happens to AQMesh pods is hard to predict. One academic study, monitoring air quality around treated road surfaces, mounted pods a few centimetres above the road and they were swamped in a rainstorm. One contractor installed a pod on its side. Solar-powered pods are sometimes installed with great difficulty, only to find the system is constantly in shade. We encourage as much pre-installation planning as possible – and provide a wealth of manuals, guides and videos to avoid these scenarios – but there are sometimes situations which require on-site intervention. We have seen ‘maintenance free’ fulfilled by offering replacement equipment under warranty, instead of maintenance, but customers still have to make a site visit anyway, so we don’t really buy that one.

AQMesh started out offering uninterrupted 2-year gas monitoring with autonomous power but we quickly found that much monitoring was project-based and that users preferred to have the comfort of planned maintenance arrangements that would maximise the chances of achieving high data capture and high data quality throughout the project.