Whilst not the only AQMesh pods still in regular use since the product was commercially launched in 2013, two AQMesh pods are still in use in Spain and demonstrate the long life of this small sensor air quality monitoring system.
Over the last six years, the two pods have been loaned by AQMesh distributor in Spain, Envirodata, to a number of customers for trials and demonstrations, as well as being involved in a range of projects.
In one of these studies the pods were selected to determine how accurately small sensor systems could measure ambient levels of NO2. The collaborative research between the European Life Photoscaling Project, Tecnaire-CM, Greater Madrid Region and Madrid City Council sought to assess the effectiveness of different photocatalytic pavements in reducing levels of NO2 pollution. The AQMesh pods were co-located with two of the region’s larger air quality monitoring stations under traffic and urban background conditions to compare AQMesh readings with reference and calibrate field readings. A paper was published in August 2018 as a result of this study, which determined that with local calibration AQMesh meets the Air Quality Directive’s standards of accuracy at high concentrations of NO2.
These very first AQMesh pods have also been used to determine baseline air quality levels for the La Nucia project in Alicante, in anticipation of the development of a new sports facility. More recently, one pod has been used to evaluate levels of ozone (O3) during the hotter weather in an area within San Agustin, and the other is currently being used by the Madrid Regional Government. Previously, the municipality of Madrid used AQMesh to measure air pollution as part of a traffic reduction scheme, including to reduce speed limits and restrict access to downtown Madrid.
Luis Lombana of Envirodata comments “AQMesh technology is providing us with a unique solution to local air quality monitoring. The combination of compact hardware with advanced data processing algorithms makes AQMesh the best system for mapping city-wide pollution. We have been using these two AQMesh pods for demonstrations and projects since 2013. They have been upgraded by the manufacturer on a few occasions and we are pleased to be able to offer the best AQMesh performance to users with these two old friends. We have no plans to stop using them although we now also sell units with the additional options available, such as PM monitoring.”
The two pods were initially built to measure NO, NO2, O3, CO and SO2, powered by an internal lithium battery, which powers a ‘gas only’ pod for over two years. From 2015 the pod was able to take advantage of developments in the electrochemical sensors, particularly factory sensor characterisation introduced in 2015 (v4.0) and the ozone filter introduced to the NO2 sensor in 2016, which dramatically improved discrimination of O3 and NO2, particularly with the AQMesh proprietary data processing algorithm v4.2. The pods will now be upgraded remotely (a setting change on the AQMesh server), with full traceability, to v5.1 which offers significant improvements in accuracy at higher temperatures.
As well as the original 5-gas suite of sensors, AQMesh can now offer two additional gases – H2S and CO2 – as well as particulate matter (PM1, PM2.5 and PM10), plus noise, wind speed and direction. Other power sources are available with solar being the most popular.
AQMesh pods have been operating in the field since 2013, and so far have been active in over 60 countries in a range of air quality applications. The pods have always been built to a rugged and robust standard, with hundreds of pods installed for years in city centres around the world. The inconspicuous units generally avoid vandalism, damage or malfunction, and in one instance a customer’s pod has still continued to function and transmit data despite being crushed during an accident. They have also proven to withstand extreme weather conditions, continuing to work under several feet of snow during the North American winter and the hot desert climates in the summer months in Australia, South Africa and the Middle East.
The AQMesh small sensor air quality monitoring system already offers flexibility of monitoring location – through independent power and communications – as well as high data quality and traceability. Now the platform is even more flexible offering up to 20 different data channels. The wide range of sensors, including gases such as NO2, particulate matter (PM1, PM2.5 and PM10) wind and noise can be specified, from a single sensor to a ‘fully loaded’ mini air quality monitoring station.
The small, post-mounted monitoring pod sends raw sensor output to the AQMesh server using the mobile phone network and readings can be accessed by a secure login or API connection. Cloud data management allows settings to be changed online and makes a high level of remote support possible – ideal for monitoring in distant or hard-to-reach locations. Recent developments for 2019 mean that AQMesh offers an increased level of modularity unrivalled by any other small-sensor air quality monitoring system.
The full range of sensors includes gases NO, NO2, O3, CO, SO2, H2S and CO2. Recent work has shown the value of measuring a targeted combination of gases for a given application. For example, H2S and SO2 is popular with the oil and gas industry and measurement of CO2 is valuable as an indicator of combustion to calculate an index. The new wind speed and direction module means local pollution source apportionment can be carried out. All AQMesh pods measure environmental conditions – temperature, atmospheric pressure and RH% – in the same compact unit.
New pods can be built to almost any configuration and modifications can be and are carried out to existing pods returned to the factory. Some modifications are also possible in the field, such as adding extra gas sensors.
Most AQMesh pods are now powered using solar power and the new AQMesh solar module allows the pod and solar pack to be mounted on the same post, using the standard clips supplied, with the power cable simply connected together to start monitoring. The updated solar pack comes as a complete package with a 20W panel, more efficient charging, an additional port for powering two pods, and is now a smart system with a Bluetooth mobile app for checking the output of the panel and running basic diagnostics. Battery and direct (9-24V DC) power options are available.
AQMeshData.net is the AQMesh server which performs secure data processing using carefully developed, unique correction algorithms which compensate for cross-gas effects and environmental factors. AQMesh algorithms are fixed by version and completely traceable, with no use of machine learning or artificial intelligence. Continual development of these algorithms, using datasets from around the world which compare AQMesh readings with co-located reference data, allows AQMesh to demonstrate outstanding, accurate and repeatable performance when co-located against certified reference or equivalent methods, in any part of the world.
AQMesh pods require very little ongoing maintenance, meaning the cost of ownership remains low. Sensor replacement is recommended every 2 years and the demonstrated data stability is very high. AQMesh users also benefit from dedicated, global support from its UK base, where it is designed, developed, manufactured and distributed, as well as a network of trained distributors.
North Americans will be well aware of the particularly harsh weather in the early months of 2019, but AQMesh has taken conditions in its stride. The AQMesh stated operating range of -20°C to + 40°C is backed up by long-term operation across a wide range of climates.
AQMesh pods used by Minnesota Pollution Control Agency (MPCA) to measure pollutant gases and particulate matter, such as NO2 and PM2.5 , installed on streetlights around Minneapolis St. Paul have seen temperatures as low as -25.4°C (-13.7°F) and continue to run smoothly.
Monika Vadali, leading the MPCA project, commented “We are quite impressed with the temperatures we have seen this winter”. However, when asked for a photo of the pods, she said “I can’t get to any of the pods as we have had so much snow and cold that there is 5-6ft of snow around some of the poles, making access difficult.” Temperatures recorded elsewhere have not been so low this year but AQMesh pods have been installed over previous winter in Sweden, Finland, Canada and central Europe, with temperatures regularly dropping below -20°C.
Despite such harsh weather conditions, the AQMesh pods have continued to monitor and communicate data to the AQMesh server, where it is securely accessed by users. The hardware design has been refined to ensure the equipment has the resilience to survive, with minimal maintenance, for years. The initial concept was for the pod to measure pollutant gases, particularly NO2, for two years on a single lithium battery. Although many users now run shorter projects or choose solar or DC power sources, the principle – and challenge – is unchanged.
In addition to its physical design, data processing on the AQMesh server includes carefully developed correction algorithms which compensate for extreme conditions. Remote diagnostics also identify unexpected patterns in sensor output, which may affect confidence in the data, which is then flagged.
At the same time as the AQMesh pods were under several feet of snow in Minnesota, pods in the southern hemisphere have been regularly operating at temperatures in excess of 30°C, hitting 44.7°C in South Africa and 46.2°C in Australia.
AQMesh pods have been deployed long-term (many pods are in their sixth year of deployment around the world) and as temperatures rise across some of the hottest locations where pods are deployed – Kurdistan, Pakistan, Myanmar, Arizona (USA), Ghana – the latest generation of sensors and processing algorithm will continue to provide reliable and traceable air quality measurements in locations where other monitoring equipment cannot readily be deployed.
In the southern regions of the USA, with very hot temperatures and varying levels of humidity, AQMesh pods maintained high precision and accuracy against co-located certified reference equipment, with a correlation R2 of 0.92 for ozone, compared to collated FEM. Many parts of the world where AQMesh operates record relative humidity (RH%) over 90%, often on a regular or sustained basis.
AQMesh also stands up to high winds and extreme rainfall and is now available with an optional wind speed and direction sensor to complement its extensive range of measurable parameters. The meteorological data gathered by this sensor can help distinguish between local and regional sources of pollution.
AQMesh has been used in a project at the Port of Kiel, Germany, to measure emissions of nitrous oxides (NOx) and fine particulate matter (PM) around its cruise ship terminal.
This year the port attracted 166 visits by 33 different cruise ships, bringing a record breaking 600,000 visitors into the city. Emissions from the cruise ship terminal and its impact on the local air quality has been in discussion for some time, as the city’s references stations indicate that nitrogen dioxide (NO2) levels regularly exceed the World Health Organisation’s annual mean limit of 40μg/m3.
The joint project with Eurofins and Olfasense, who combined AQMesh air quality monitors with the Ortelium dynamic atlas system, measured and studied levels of NO2 and PM at the cruise ship terminal over several months.
AQMesh pods, supplied by its German distributor Envilyse, measured NO, NO2, O3 alongside PM1, PM2.5 and PM10, as well as relative humidity, temperature and atmospheric pressure. After being co-located with passive samplers at the installation site to provide the greatest degree of accuracy, real time sensor data from the AQMesh pods was fed into Ortelium.
The Ortelium atlas allowed measurements from the AQMesh pod to be visualised in real time and, combined with meteorological data feeds, showed how the emission levels changed during arrival, berthing and departure of the cruise ships.
Data analysis from this study concluded the cruise ships could not be attributed to high levels of NO2. This is similar outcome to a study carried out at a UK airport, which concluded that local traffic was in fact more of an issue than the airport activity.
Plumes from shipping are notoriously difficult to detect and analyse from land, but AQMesh now has a carbon dioxide (CO2) sensor which allows a combustion plume to be detected from elevated CO2 levels. Pollutants can then be evaluated in this context.
AQMesh is in use at a variety of harbours and ports around the world including the UK, Italy, Norway, Netherlands, Germany and Vietnam. The pods can now monitor up to 6 gases using the latest generation of sensors, as well as PM1, PM2.5, PM10 and total particle count (TPC) with a light-scattering optical particle counter.
Recent co-location comparison trials against certified reference equipment continue to prove AQMesh performance and reliability for localised air quality monitoring.
Trials in the USA, UK and Western Europe this year have delivered high correlation coefficients (R2 values) for key pollutants such as nitrogen dioxide (NO2), ozone (O3) and fine particulate matter (PM2.5). An R2 value of 0.92 against reference for O3 was achieved in Southern USA over the Summer, as well as an R2 value of 0.94 against reference for NO2 in Northern USA during the cold season.
Co-location trials for AQMesh and field equivalent methods have been taking place globally for several years, with the results published on the AQMesh website, demonstrating how performance and accuracy continues to improve with each new version of the product. A number of independent studies have also been carried out, verifying the AQMesh system’s capability.
AQMesh is a small sensor air quality monitoring system for measuring pollutant gases and particles in ambient air. It is a flexible, quick to install and easy to use air quality monitor that can deliver localised, real-time readings, aiming to improve the spatial resolution, scope and accuracy of gathering air quality data.
Its range of wireless power options includes a recently improved smart solar panel, which is now larger than the previous and has a more efficient charge, allowing for year-round operation for standard gas and particulate AQMesh pods across Western Europe and regions on a similar latitude.
AQMesh pods can now monitor up to 6 gases out of NO, NO2, NOx, O3, CO, SO2, CO2 and H2S using the latest generation of sensors, as well as PM1, PM2.5, PM10 and total particle count (TPC) with a light-scattering optical particle counter. In addition to pollutants, AQMesh can measure noise, relative humidity, pod temperature and atmospheric pressure, all within a single compact unit. Data is completely secure on the AQMesh cloud server, only accessible by a secure login, which allows the user to manage their pods, view customisable graphical data, and download the data for further analysis.
AQMesh is currently in use throughout the world in a variety of air quality monitoring applications and projects, including smart city networks, indoor-outdoor air quality management, employee health and safety, traffic pollution mitigation studies and air quality modelling. Recent case studies show it forming part of a major ‘hyperlocal’ street-by-street monitoring system throughout London (UK), as well as being used in a similar project across 50 zip code areas in Minnesota (USA).
AQMesh has been measuring ozone (O3) using small sensors since 2011 and the readings from the latest generation electrochemical sensor, using AQMesh v4.2.3 processing, as compared to co-located certified reference readings, consistently show an R2 of over 0.9 with an accuracy ±10ppb (20µg/m3).
AQMesh pods have been measuring ozone levels around the world and co-location comparison studies show very good performance against reference equipment from the latest sensor and processing version. Ozone levels have been particularly high across Western Europe over this summer but are a regular concern in many parts of the world, including the USA. However, there are huge gaps between O3 monitoring points, to different degrees across the world, depending on monitoring equipment budgets. A lower cost small-sensor monitoring solution can provide valuable data within the areas currently lacking in this air quality information. Data validity is typically demonstrated by comparison with a local reference station, although AQMesh is also widely used where no reference data is available.
O3 at ground level is formed by reactions with nitrogen oxides (NOx) and volatile organic compounds (VOCs) from traffic and industrial emissions in the presence of sunlight. As such, hotter, sunnier weather can dramatically increase O3 pollution.
The World Health Organisation (WHO) currently states the daily limit of O3 levels to be 100μg/m3 over an 8-hour mean and advise that prolonged exposure to high levels of O3 can have severe effects on human health, including causing asthma, inflammation of the airways, reduced lung functionality and lung disease. Measuring O3 as a part of an air quality monitoring routine is therefore becoming increasingly important, especially in hotter climates and areas of increased VOC emissions.
O3 can be complicated to measure due to its high sensitivity to environmental conditions and cross-gas effects. Most small sensors for measuring O3 are either electrochemical or metal oxide, but electrochemical sensors (such as those used in AQMesh) have the advantage of low power requirements and can therefore be installed more flexibly. AQMesh pods are compact, wireless units and are available with a variety of power options, including solar panels, which allow them to be installed exactly where monitoring needs to take place.
During summer 2018 AQMesh has been measuring ozone at hundreds of locations across five continents and co-location comparisons show consistently high levels of accuracy. To quote two of many such studies, in an industrial region of the USA, AQMesh O3 measurements compared to FEM gave an R2 of 0.97, and in a similar comparison study in Western Europe the R2 value for O3 was 0.95. AQMesh pods measuring gases can run continuously for over two years using a battery but other power options are available, including solar. Particulate matter (TPC, PM1, PM2.5, PM10) can also monitored with an AQMesh pod, alongside gases including NO, NO2, O3, CO, SO2, CO2 and H2S, as well as pod temperature, RH% and pressure.
The accuracy of AQMesh readings has been proven through an extensive series of global co-location comparison trials and is the proven, commercially available low-cost air quality monitoring system for both pollutant gases and particulate matter, as well as simultaneously monitoring a range of environmental conditions.
AQMesh is now able to offer CO2 and H2S within its range of gas options for local air pollution monitoring.
The NDIR CO2 sensor, which can be offered within a single AQMesh pod alongside five other gases out of NO, NO2, O3, CO, SO2 or H2S, as well as PM1, PM2.5, PM10, temperature, pressure and humidity, has been developed to deliver a higher performance than those typically used for indoor air quality monitoring. It has been rigorously tested against Picarro reference equipment, resulting in an R2 value of 0.93. Pod-to-pod correlation of over 20 AQMesh pods has shown R2 values of 0.98 and 0.99, and the sensor has a MAE (mean absolute error) of less than 20ppm.
In addition to monitoring deviations from ambient levels of CO2, elevated CO2 levels can indicate that monitoring is taking place in a combustion plume and levels of other gases can be interpreted accordingly. For example, the ratio of CO2 to the other pollutant gases present can indicate whether those gases were emitted by a local or distant source.
An additional electrochemical sensor has been introduced to offer H2S measurements. After integrating the sensor, measurements have been compared to readings from a Honeywell SPM Flex installed at a sewage treatment site with an R2 value of 0.87 over a measurement range of 0-150ppb. Particularly of interest to the oil and gas industry, in association with the SO2 monitoring already available on AQMesh, it can be used to measure emissions from sour gas and residual emissions from flaring operations.
AQMesh can measure up to 6 pollutant gases in various combinations, as well as particulate matter, humidity, atmospheric pressure and noise in one small, compact easy-to-install unit. There is a range of wireless power options, including lithium battery packs and solar panels, with information sent in near real-time to a secure server via cellular GPRS. Data can be accessed by a secure login or can be streamed via an API connection.
AQMesh pods are in use across the globe in a variety of indoor and outdoor air pollution monitoring applications, and are becoming increasingly popular in smart cities and networks. Pod performance has been proven through extensive testing in worldwide co-location comparison trials with reference equipment, which have delivered impressive and reliable correlation results.
A new paper published by the American Chemical Society (ACS Sensors) reviews the use of amperometric electrochemical gas sensors for monitoring inorganic gases that affect urban air quality. Written by John Saffell and Ronan Baron of Alphasense, the paper gives a full explanation of how the sensors work, how they have developed and a review of how they have been used. Key relevant studies are summarised as well as major studies using the sensors.
The paper mentions AQMesh as ‘the first provider of integrated air quality networks’ using the Alphasense sensors, and also refers to AQMesh correction algorithms (the authors consider data correction necessary) and the comparison field trials published regularly on the AQMesh website. University of Cambridge, Citi-Sense and IDAD projects, which used AQMesh, are mentioned: of the range of sensor systems submitted for the 2 week field comparisons trial in Aveiro, ‘The AQMesh unit achieved the highest correlation coefficient and the lowest mean absolute errors: R2 was 0.70 for O3, 0.89 for NO2, 0.86 for CO and 0.80 for NO. Other sensor boxes were unable to provide the same degree of correlation, possibly because of the lack of data correction for temperature variations.’
Recent co-location comparison trials using the latest AQMesh processing (v4.2.3) have further proven AQMesh performance with impressive R2 values in excess of O.8 and 0.9 for NO2 in Benelux, Slovakia and Spain.
Particulate matter (PM2.5) was also monitored throughout the Slovakia trial and delivered an R2 value of 0.98.
Correlation values for NO continue to be high throughout all AQMesh co-location trials, with results exceeding 0.9 in nearly all cases.
For more information on co-location trials and performance please contact us.
Leading small sensor air quality monitor, AQMesh, has recently been shown to work alongside passive samplers and air quality models, as well as complementing reference station networks. A recent study shows AQMesh calibration against diffusion tubes and two 2017 conferences have highlighted the potential of such systems used in an air quality network.
At the Dispersion Modellers User Group Meeting (DMUG, London, April 2017), CERC showed that the model optimisation of 7-day average NO2 concentrations using AQMesh readings as well as reference data affects concentration contours, giving a general reduction, but increase in some areas. The study shows the potential to achieve a higher level of local air quality accuracy from this combined approach.
The CERC study uses near real-time NO2 data from 20 AQMesh pods and 5 reference stations across Cambridge, UK over three months. The presentation acknowledged the potential from large networks of low cost sensors installed across a city: accuracy and reliability is generally lower than reference monitors, but larger spatial coverage is possible. The study addressed how these sensor data can best be used in modelling. See the full presentation here.
It is important to distinguish between small air quality sensors and sensor systems. The best sensor systems offer optimisation of sensor output through quality control, platform design and sensor output processing and correction for cross-gas effects and environmental effects. Teams which want to invest in similar air quality studies or city-wide monitoring projects can get a head start by using AQMesh – the most developed such system. See the Cambridge case study of latest AQMesh performance.
International air quality experts meeting at RIVM in the Netherlands in February reviewed a range of studies using small sensor systems to measure air pollution, particularly NO2 and particulate matter. Whilst highlighting the need for good characterisation of sensors and the pace of development of this new technology, discussions focused on how to make the best use of ‘low cost’ sensors.
Two speakers highlighted the ways in which a small sensor system can be characterised. The best method is by regularly co-locating with a reference station and comparing readings; another method is similar but one sensor system is co-located against reference and then moved around the other units to allow comparison. Other options include comparison with passive samplers or by comparing co-located sensor systems against each other.
AQMesh NO2, PM10 and PM2.5 readings from the Citi-Sense project (2015-16, AQMesh v3.5) – a superceded version of AQMesh without standard sensor quality control or characterisation – were compared with maps of reference data using data fusion and showed encouraging results. An even earlier AQMesh project in Asia (2014, AQMesh v3.0) compared source apportionment plots generated using reference data to those from co-located AQMesh pods and the conclusions to be drawn about pollution sources were the same.
AQMesh continues to build on years of global studies and continued development, offering the most ‘project-ready’ small sensor system which can be used in an air quality network – including reference stations, passive samplers and modelling – to proven effect. See more information about AQMesh performance and versions.
The AQMesh team has carried out a test which shows that calibration (scaling) of AQMesh against one, or ideally several, diffusion tubes, is a viable option when no local reference station is available.
When three AQMesh pods and four NO2 diffusion tubes were co-located against a reference station, the diffusion tube readings were so consistent with the reference average reading for the period that the diffusion tube average could be used to apply a slope correction to AQMesh data.
By careful management of the co-location phasing it is possible to record sufficient data points to also correct any offset.
Passive samplers for key pollutants such as NO2 are widely available and are often used to reach areas not covered by more expensive and bulky reference stations. Small sensor systems or so-called ‘low cost’ sensor systems can help to fill the data gap between reference real-time readings, passive sampling single average measurements and model output. Calibration of small sensors systems, such as AQMesh, against certified and validated measurements provides a trail of data validity.
An AQMesh pod used in the recent Citi-Sense project in Norway was returned to us at the end of the project, and we were surprised at the condition. Despite the damage, this robust little pod was still working perfectly, with data still being received.
AQMesh pods are able to measure up to 6 gases, including NO2 and CO2, particulate matter, noise and atmospheric conditions within a variety of environments – and it seems they can also possibly withstand significant damage.
For more reasons to choose AQMesh, click here.
The Assessment of Air Quality Microsensors verses reference methods: The EuNetAir joint exercise has recently been published, and shows an R2 against for reference for AQMesh of >0.8 for NO2.
EuNetAir is the European Network on New Sensing Technologies for Air Pollution Control and Environmental Sustainability, working towards European-wide air quality control standards.
It is generally accepted that whilst measurements from air quality reference stations are highly accurate, they are not sufficiently location-specific. Key pollutants – such as NO2 and PM2.5 – vary dramatically over short distances and time intervals, but the large size, maintenance requirements and relatively high cost of reference equipment limits the places it can be installed. Diffusion tubes can offer a very cheap alternative and are much easier to install in specific locations, however they only offer a single reading over a number of weeks, and air quality professionals therefore rely on modelling techniques to fill the gaps. With research continuing to prove the extent to which air pollution varies significantly over space and time, the answer would be a reliable and accurate tool for taking real-time, localised measurements.
A number of new low-cost air quality monitoring systems are available, each with benefits and shortcomings. It is fair to say that the available sensors, whether electrochemical, optical or metal oxide, are all working at or close to their limit of detection to provide the low ppb or µg/m3 level of sensitivity required for any of the common ambient air quality applications. However, several systems offered for these applications provide readings in ppm or even % level readings – which clearly makes them inappropriate for ambient air monitoring. Some are also not fit for long-term outdoor use, as they are not fully weather proof or cannot cope with the expected temperature ranges. However, at least one system – AQMesh – does operate across a wide range of conditions and territories, so having established that a viable product exists, can it deliver the accuracy required?
Performance is clearly a major consideration for any user and comparing readings from a lower cost system against a reference station is the obvious place to start. One immediate challenge is ensuring meaningful results. Particularly in roadside applications or where there is an immediate source of pollution, all sensors and intakes must be within a metre of each other and at an equal distance from the immediate source. Most sensors, not unreasonably, also require an uninterrupted air flow around them – mounting immediately above hot or wet surfaces will not give accurate readings. On the other hand, some limitations of reference equipment come to the fore when comparing with a different type of measurement. For example, single channel NOx analysers switch between measuring NO and NOx, calculating NO2 as the difference. This switching can have dramatic effects on readings for the two gases (which are measured separately and directly by other sensors) at short reading intervals, such as 1 minute. Similarly, any differences in clock synchronisation or reading averaging protocol (time beginning or time ending) can make the difference between a regression comparison R2 of 0.9 and 0.1, which can render comparisons meaningless.
Comparisons of particulate measurements are also problematic due to the range of reference-equivalent methods available and the limitations, in many ways, of the reference method itself. Since the expanded uncertainty of the reference equivalent measurements for PM10 and PM2.5 allows up to 25%, this should be borne in mind when making comparisons with lower cost particulate sensors. Overall, for both gases and particulate matter, if several identical low cost systems are co-located, the user should expect a high level of repeatability (R2 > 0.9) and should expect to be able to adjust accuracy by ‘calibrating’ – adjusting slope and offset – against a co-located reference/equivalent station. Some systems, such as AQMesh, then allow this scaling adjustment to be applied automatically to all future readings, minimising the need for manual data correction. Access to a calibrated reference station and careful co-location is currently key to getting value out of any of the current generation of emerging sensor systems, although the objective of good accuracy without the need for a reference station is being actively pursued.
First questions about these systems often include ‘How do I run gas through it to calibrate it?’ and ‘Can I calibrate (or test) it in the laboratory?’ In systems such as AQMesh the air sample is not pumped, for good power-saving reasons (low power is essential for battery operation), and so it is not obvious how a conventional gas calibration would work. More importantly, although the sensors generally do give very good results in laboratory tests with known dry, single gases, these bear no relation to real ambient field measurements with a combination of damp, humid gases at potentially varying temperature and pressure. Overall, there is no proven substitute for co-location with a reference station. Even with all of these considerations, some of these small, lower cost air quality systems, such as AQMesh, can deliver very impressive comparison results and provide a new source of air quality data. Those with in-built power and communications offer genuine freedom to gather measurements from any location and research teams worldwide are using such systems to understand pollution around cities, inside and outside buildings, at different heights, in street canyons, around industrial facilities and within neighbourhoods, at different times of day, and so on. This new granularity of measurement and flexibility of location gives air quality management teams a real tool to carry out ‘before and after’ studies and evaluate a range of policy or pollution mitigation activities. Where a number of sensor systems are used, and particularly in combination with wind speed and direction information, the relative measurements and source distribution can provide very powerful insights about where to target pollution mitigation activity.
One such low cost outdoor air quality monitoring system offering this type of flexibility is AQMesh, which has proven its repeatability, accuracy and performance through a series of these careful co-location comparisons with calibrated reference stations in a variety of global locations and applications. The small size, battery power and wireless communications technology mean users can benefit from reliable and accurate real-time, localised air quality measurements in a broad range of studies.
How accurate is ‘accurate’?
One area of discussion is what level of accuracy is ‘good enough’. Although this depends on the application, it is still tempting to look for a very high level of agreement between the low cost sensor system and reference equipment. Whilst this may be the goal, the lower cost systems are considerably cheaper and have the benefit of being correctly located so perhaps it is better to have slightly less accurate readings from the right location than highly accurate readings from the wrong location? For some applications it is really only the relative readings which are required, and systems like AQMesh provide very high levels of precision between identical systems. Or it may only be appropriate to provide a ‘traffic light’ indication for communicating air quality to the public. Until more general guidance is available, users will have to take a view on accuracy relevant to their application.
Publishing air quality data
Another area of confusion is regarding data privacy vs online publication of air quality data. Most of the new air quality systems take advantage of remote data management and online access. This makes sense for a number of reasons. Hard-wired communications infrastructure is a barrier to freedom of location and new systems generally communicate either using the mobile network, radio or wi-fi. Online access to data is also very convenient and less resource hungry. Few of us who readily use mobile phones, online banking and many of the commonplace applications of modern life fully understand security of communications and the reality of data hosting. The bottom line is that air quality data from sensor systems using wireless communications can be as secure as any other online application. Confusion is caused by the systems which are focused on citizen engagement and offer automated sharing and publication of data, but these are the exception and in most cases, such as AQMesh, data is private and secure.
The new generation
Current low cost air quality sensor systems are a very mixed bag. Some products may well appear to offer the same measurements and even claimed accuracy as the more thoroughly developed and tested systems and the user has little choice but to ask searching questions and ask for demonstration of performance and reference projects before purchasing. But the need for such systems is clear and performance is already good enough for many leading institutions and organisations to be actively using the technology. Sensor and sensor system manufacturers are seizing on every new shared comparison dataset and development in technology to make further improvements. The insights that these sensor systems can offer are real and relevant and there is no substitute for trying the technology in any given application to see what it can offer. Many users have found that one insight can lead to another and, working with a clear understanding of the strengths and weaknesses of the systems, the benefits of making a start with this new tool are overwhelming.
The emergence of sensors capable of measuring the gases and particles that make up air pollution, especially in cities and industrial areas, has driven many academic studies which evaluate the sensors and compare performance against reference methodology. Such projects provide valuable and thorough assessment, but they currently work slower than the pace of low cost sensor system development. In addition, EU and US authorities are developing methodology appropriate for certification of such systems. In the meantime, practitioners across a number of fields are adopting AQMesh as the leading commercially available sensor system, and generating valuable information to support policy and commercial decisions.
The obvious and first application for this sort of technology is in identification and management of pollution ‘hotspots’ in cities and collecting background measurements across a much wider range of locations than has previously been possible. UK local authorities and similar bodies in a range of countries have been using AQMesh to monitor specific locations for NO2 and now particulate matter, evaluating mitigation methods, such as barriers between traffic and pedestrians. These studies have often been in conjunction with academic teams who have also investigated issues such as the impact of industrial chimneys close to high rise buildings and pollution at different levels of multi-storey residential buildings. One project uses AQMesh to support development of a walking-to-school campaign, comparing different walking routes and educating parents. AQMesh has also been used to minimise NOx and particulate matter inside buildings through management of building ventilation systems.
Building contractors have been searching for an instrument to measure dust construction sites, which does not require an external power source, and a number of projects are now using AQMesh for this purpose. Civil engineering applications include monitoring pollution in road tunnels and associated ventilation ducts. Studies in several countries, including Scotland, Ireland and Greece focus on understanding the relationship between traffic volumes, mitigation measures and air quality. Local air quality data, used in combination with wind speed and direction information, can provide powerful pollution source attribution, relevant to fence line monitoring of industrial sites, airports, and so on.
These new sensors systems can provide a new stream of information, complementary to and calibrated by reference stations. But being self-sufficient in terms of power and communications, they offer freedom of location and open new opportunities.
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During the months of October 2015 through to February 2016, two AQMesh pods were co-located against reference in Boulder, Colorado (USA), focusing on NO, NO2, O3 and CO measurement.
The trial resulted in impressively high correlation between AQMesh units and reference equipment, in some cases delivering R2 values higher than 0.98 (NO) and 0.87 (NO2).
Through partnership with IDAEA-CSIC two AQMesh pods were co-located with a reference station and scaled locally in Barcelona (ref. Advanced user mode), providing R2 of 0.97 for NO and 0.89 for NO2. One pod was then moved to a second reference point within the city to establish repeatability of results using the scaling factors derived from the first reference. This resulted in a correlation of 0.98 for NO, and 0.86 for NO2. This trial shows that a high degree of air quality monitoring accuracy can be achieved through local scaling and this accuracy is maintained when AQMesh is then used to monitor independently in new locations.
AQMesh performance has been developed and refined through a series of global co-location comparison trials, with the results now available in the AQMesh website’s new performance section.
By co-locating AQMesh pods next to an air quality monitoring reference station and carrying out analysis, R2 values can be identified. R2 values range between 0.0 and 1.0, with the highest value indicating a perfect relationship between the data. Recent co-location trials delivered consistently high R2 values, proving a close match in readings from the pods and reference station.
New trials have been completed in a variety of locations with differing environmental factors, including Sweden, London (UK), Germany and Scotland.
Browse all trials for all parameters here.
Urban background co-location (AQMesh and reference) of a single AQMesh pod gives R2 of 0.95 for NO and 0.81 for NO2 after local scaling (ref. Advanced user mode). Based on 4 weeks of data in the UK in January 2015. The next stage in this University of Leicester project is to co-locate all their 11 AQMesh pods with the reference station to calculate and apply scaling to achieve a high level of accuracy along with the already high performance between pods as well as against the reference station. The individual pods will them be moved out across the city of Leicester for a number of traffic monitoring studies, including a multi-point round-a-bout study looking at the differences in traffic pollution at the various stages of the process as well as other interesting applications for AQMesh…
Heavy traffic site in a North German city. Co-location of two AQMesh pods with reference gives R2 of 0.96 for NO and 0.74 for NO2 after local scaling (ref. Advanced user mode) and pod to pod R2 of >0.97 for both NO & NO2. Based on 10 days of data from Germany in February 2015. Data courtesy of our German distributor Envilyse. The trial is still ongoing and an update will be sent out soon…
Example data can be found here.