Author: Jessica Burniston
- Research shows that increasing the number of measurement points improves the spatial resolution of urban air quality models.
At the RSC AAMG event on ‘Air Quality Monitoring: Evolving Issues and New Technologies’ Professor Rod Jones of the University of Cambridge presented a paper showing very encouraging results. “Because we know that all the pods read the same and because we have a comparison between one pod and a reference instrument we can say that all pods are working equivalently across the city. What we are seeing is correspondences in excess of 0.7, 0.8, against reference and that is very good for straight out of the box”, commented Professor Jones.
These findings are from a project in Cambridge where 20 pods, initially co-located at the AQMesh UK factory, were placed at key points around Cambridge. The objective was to demonstrate what could be shown about ambient air quality at key points in the city, using a larger number of measurement nodes to understand how air quality varies across the city, particularly in relation to key transport corridors and areas of construction activity.
The 20 AQMesh pods measure gases (NO, NO2, O3, CO and SO2, particulate matter (PM1, PM2.5 and PM10) and environmental conditions (atmospheric pressure, relative humidity and pod temperature). 19 of the pods are powered by lithium battery and have been gathering data at 15 minute reading averages since June 2016, without any need to visit the sites to change batteries or carry out maintenance. The twentieth pod is currently running at one minute reading intervals, mounted at a Cambridge City Centre reference station, using a high capacity lithium battery. Each pod is small enough to be easily mounted to a post, wall or enclosure and all pods have been functioning without fault throughout the project.
As part of the project, data has also been pushed directly from the AQMesh server to a University of Cambridge server where it is automatically displayed in near real time. A further aspect of this initial project is to compare collected AQMesh data with ADMS-Urban modelled data for the same area and then use the real-time AQMesh data to improve the airTEXT air quality forecasts for Cambridge.
The next steps are to calibrate the pods and to analyse the data in more detail. Co-locating one of the pods with the reference station has allowed slope and offset values to be calculated, as all pods had already been co-located, allowing the same scaling to be applied to all pods. These values can then be applied to the AQMesh server, improving the accuracy of all data gathered after that point. Initial analysis using wind rose plots has shown the extent to which pollution can be attributed to local road traffic at each point. Further analysis of data will show how pollution varies across the locations and by time of day or week.
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.
January, for many of us, is a surprising time to focus on solar energy but a new solar power unit designed specifically to power the AQMesh air quality monitoring system can power a full-specification pod throughout any western European winter.
Lithium batteries established AQMesh as the first reliable ‘small sensor’ air quality monitoring device that is completely independent of power supply, as well as communications infrastructure – but even the best batteries need to be replaced eventually. The solar power unit includes a back-up battery and requires just a single connection to a matching lead on the pod. The unit is also very simple to mount on a post, wall or fence as required, and includes a compass for alignment and the facility to tilt the panel to achieve maximum solar energy.
“We designed this power unit specifically for AQMesh. AQMesh is low-power so we used the smallest possible solar panel necessary to measure all parameters at all reading and transmission frequencies”, comments AQMesh Technical Director, Steve Earp. “We know that users are concerned about large or heavy objects on posts but the combined package of an AQMesh pod on a post with this solar power unit has a very low impact.”
Stephen Hoskin, Sales Manager at Air Monitors Ltd, has installed and supported hundreds of AQMesh pods in the UK in a variety of environments. He says “For longer monitoring projects involving the measurement of particles, the AQMesh solar power unit provides a convenient, cost efficient and unobtrusive way of keeping the pod running continuously without having to change the battery. The solar power unit is very easy to install, featuring a push fit connector for the cable from the pod and contains a small back up battery to power the pod even on long dark winter days.”
AQMesh can measure NO, NO2, O3, CO, SO2, PM1, PM2.5, PM10, noise, pod temperature, atmospheric pressure and relative humidity from a single pod – and can now be powered by a DC connection to a solar power unit measuring 445 x 250 x 290mm and weighing 5.5kg. AQMesh has been installed in a range of applications including pollution hotspot identification, and performance has been routinely demonstrated and proven in a series of global co-location comparison trials.
For more information visit www.aqmesh.com/results
Last Monday, 17th October, BBC’s Newsnight focussed on outdoor air quality issues, with particular reference to the ongoing Client Earth court case against the government for delay in compliance with air quality regulations.
In the episode, David Grossman cycles along the A3 with an AQMesh pod attached to his bike, capturing air pollution readings in real-time. The AQMesh pod proves there are high levels of pollutants – such as NO2 – in the air as he travels.
If you are in the UK and have a TV licence, you can watch the episode here (the air quality section begins at approximately 35 mins into the episode).
Watch our webinar from today about AQMesh, covering topics including:
- What is AQMesh?
- Which ambient air pollutants does it measure – and how?
- What applications is it used for?
The AQMesh team will be delivering an Introduction to AQMesh webinar on 26th October 2016 at 4.30pm (GMT daylight time).
Topics covered include:
- What is AQMesh?
- Which ambient air pollutants does it measure – and how?
- What applications is it used for?
You can join the webinar from you computer, tablet or smartphone using the link below:
You can also dial in using your phone:
United States +1 (571) 317-3116
Access Code: 755-778-333
More phone numbers:
France +33 (0) 170 950 590
Italy +39 0 699 26 68 65
United Kingdom +44 (0) 20 3713 5011
(First GoToMeeting? Try a test session: http://help.citrix.com/getready )
For more information or help about the upcoming webinar please contact us.
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.
Contact us for more information.
AQMesh has an impressive collection of global users and performance results measuring ambient air quality in applications ranging from traffic planning and urban hotspots to industrial fence line monitoring, building ventilation management, or environmental lobbying. This pole-mounted unit comes with an independent power supply and communications, so offering the possibility of measuring air quality exactly where it is required. Readings are accessed securely online at intervals as short as 5 minutes and can either be downloaded manually or linked to another application.
Each small (2.5kg) pod can measure up to six gases, including NO2 and now CO2, as well as PM2.5, PM10, and environmental conditions. As the units use standard fittings, installation is speedy and a city can be fitted with 20 units in one morning. Accuracy is optimised by using the best available sensors for detecting low levels of ambient pollutants and applying processing which gets the best possible performance from them.
During the five years of the product’s development, the AQMesh team has built on the founding partnership with the University of Cambridge and collaborated with air quality teams worldwide to constantly compare performance against reference stations. These co-location comparison trials have shown how closely AQMesh can track reference readings and also provided continuous insights into how performance can be improved. These impressive comparison trial results are shown in the performance section on the AQMesh website.
AQMesh does not offer an alternative to a full reference station but can help to fill the data gap between sparse reference method monitoring points and diffusion tubes which lack time resolution. In fact, the combination of AQMesh and reference method monitoring stations is particularly powerful, as co-location against the reference station and scaling applied to the AQMesh server provides a reliable calibration method.
Each individual AQMesh pod measures only 20cm and can monitor NO, NO2, O3, CO, SO2, CO2, PM1, PM2.5, PM10, noise, pod temperature, relative humidity and atmospheric pressure, available as a range of options. The power source can also be chosen to suit the application, with independent supply being key. In addition to its range of battery options, AQMesh can now be powered by a small, neat and self-contained solar power unit. This low power platform is key to the versatility of the equipment, meaning that it can be located literally anywhere and operated for years with minimal or even no maintenance.
AQMesh is designed and manufactured in the UK and supported globally, and as worldwide concern for air quality continues to grow, AQMesh is rapidly being proven to be the only reliable solution with the accuracy and precision for local ambient air pollution monitoring. Hundreds of units are in use worldwide by leading academic institutions, environmental monitoring authorities and private companies in a range of pollution monitoring projects, including real-time NO2 monitoring and NOx measurement for traffic pollution analysis. AQMesh is also currently in use across a number of cities as part of the EU-funded Citi-Sense project.