Pollution Guardian: update for UK’s Clean Air Day

Thursday 17th June has been nominated as “clean air day” in the UK so a good time for an update on the Pollution Guardian project.

Three mechanical concepts for the Pollution Guardian in-vehicle sensor

As mentioned in earlier blogs, the Pollution Guardian is our project to develop a low cost solution for monitoring and alerting road drivers to areas of high air pollution and so enable them to take counter measures. We have been working on this project together with the University of Surrey and it is supported by an Innovate UK grant to prove the concept and further develop prototypes.

We have now come to the conclusion of the current phase of work which has involved:

  • Construction and characterisation of 20-30 prototypes with an updated electronic and mechanical architecture
  • MVP (minimum viable product) development of a mobile app geared to consumer test
  • Cloud back-end development to support data capture and visualisation on the mobile app
  • Collaboration with a UK facility management business to run a solution trial in Q1/2021
  • Analysis of pollution data and user experience feedback to guide the next steps
  • Creation of short films to help us explain the project (see here)

A couple of credits here to the people who have been helping us achieve some of the above:

We will be creating a few more blogs and releasing some additional films covering the more recent work in the coming weeks, so watch this space.

Why bother with the Pollution Guardian?

Busy North Circular road

Introduction

As we arrive in the final quarter of our Innovate UK sponsored project and with an ongoing Covid pandemic, it is worth us asking the question why are we bothering with the Pollution Guardian? Let’s start with the facts:

Thus, we have a significant health and air quality issue, made worse by vehicle use, but which is potentially most harmful to the vehicle users themselves. The obvious solution would be to get more people to drive less or drive more zero emissions vehicles but:

  • Changing habits and lifestyle is tricky to pull off.
    • Whilst we have achieved remarkable things during the pandemic, are we motivated enough to address the silent killer of poor air quality?
    • Unfortunately, the evidence is pointing otherwise as ride as a service and online to home delivery services can push up road congestion and air pollution
  • What about electric vehicles?
    • The UK government’s stated aim is to ban sales of new combustion engine vehicles starting from 2030/2032/2035 depending on whom you choose to believe.
    • This is a potentially worthy objective but there are big challenges: charging points, range anxiety and the affordability of electric vehicles.

So, what can we do about it now?

The Idea

Well, whilst in a car as driver or passenger, you can control your own ventilation by rolling up the windows and using the air recirculation button. This has been shown to have a big impact on exposure to poor air quality .  We took the view that it ought to be possible to make an affordable sensor to warn car occupants of poor AQ and so take countermeasures, if we could incorporate existing technology items, smartphones in fact, into the solution. Thus the pollution guardian project was born.

Now we are some way down the line since starting out, how can we assess if what we have been developing could work for people in practice? Our answer: to organise a real life trial incorporating real users, not associated with the project, and let them use the device for a few weeks and listen to their feedback.

  • Are they more conscious of their air quality whilst driving?
  • Has the use of the Pollution Guardian influenced their behaviour at all?
  • What do they like/dislike about the solution?
  • Can we show any differences over time from the data that we gather?

Thus we are engaged now in putting together a real life trial and the next blogs will focus on the various aspects around this.

What happened to the traffic in 2020?

Much has been made over the reduction in air pollution in 2020 during the Coronavirus pandemic. Certainly, the first period of lockdown saw much reduced commuting activity as people were asked to stay at home or work from home and it was certainly my perception that the volume of traffic was less outside of the main commuting times.

Deserted A3 carriageway during lockdown

Deserted A3 carriageway near Guildford

However, this description and picture above are not accurate descriptions of 2020. Driving around West and South West London in August showed that there was indeed plenty of traffic and plenty of air pollution around.

Heavy traffic on Great West Road in London Aug 2020

Busy Great West Road in West London in August 2020

As this blog is motivated by our Pollution Guardian project, you might be interested to see what we observed on the route around West London. Below, some estimates of the Nitrogen Dioxide gas concentration taken en-route from inside the test vehicle over a 10-15 minute period.

NO2 gas concentration vs. time showing rapid change on West London drive

Pollution Guardian prototype NO2 gas level estimates (ppb) against time (s)

Note that whilst the Pollution Guardian prototyping work is still ongoing, the absolute numbers in the graph above were our estimates based on the individual prototype sensor outputs and algorithms at the time. They are however valid to look at as a high quality, lab grade, NO2 gas analyser on board our vehicle, also reported similar or higher gas concentration figures over the same time period.

What does this all tell us?

  • Despite the pandemic, traffic and pollution levels over the summer period in city/urban areas were not so much below the “business as usual” levels.
    • Indeed, the BBC reports that even in the midst of the January 2021 lockdown, motor vehicle traffic is already at 50% of normal levels.
    • This suggests traffic and its associated air pollution will be returning to normal in 2021 when the lockdown eases.
  • Air pollution levels can vary a lot from lower to higher levels of concern over relatively short distances i.e. hotspots are discrete spots.
  • The Pollution Guardian looks capable of following these fast changes in air quality and can be a useful tool to bring awareness and raise alerts on worsening conditions.

Lab testing with the University of Surrey air pollution chamber

As part of the Pollution Guardian project, our collaborators, the University of Surrey Global Centre for Clean Air Research (GCARE) planned to develop an air pollution test chamber for affordable sensor devices to be evaluated against high quality laboratory grade test measuring instruments.

The objective for the chamber was to be able to set up stable, predictable air pollution and environmental conditions to enable the replication of field conditions from ANYWHERE in the world.

In its first iteration, the chamber, subsequently branded “Envilution”, was capable of controlling:

  • Chamber air temperature
  • Humidity, including pre-filtering to create “zero air” as a baseline (dried and filtered air)
  • Particulate Matter (PM) levels, with options to create particulates from a variety of sources
  • Nitrogen Dioxide, NO2, gas concentration

By the end of the second quarter of the project, the chamber was ready to start testing with our Pollution Guardian prototypes.

The Envilution air pollution test chamber created by the University of Surrey

Figure 1: Envilution air pollution test chamber created by the University of Surrey

The chamber conditions:  temperature, relative humidity, particulate matter level and NO2 concentration were sampled from a port in the chamber using the same high quality instrumentation mentioned in the previous blog.

In our testing, we planned on exploring the behaviour of the prototype sensor units against changes in the levels of temperature, humidity, particulate matter and NO2 gas concentrations. Whilst some of these tests were more straightforward to set up, others turned out to be more tricky e.g. humidity testing.

Mostly when weather websites predict humidity levels, they actually refer to the “relative humidity”. This is a measure between 0-100% of how much humidity the air is capable of carrying, relative to the air temperature and pressure. As you get toward 100%, you can get a lot of condensation forming on surfaces and in warm conditions struggle to keep cool by sweating.

In our lab tests, we aimed to see how our sensors responded overall to the relative humidity in the range of 10-85%. The initial approach was to use an aquarium to generate the humidity and mix this with “zero air” (cleaned and dried) to achieve a target relative humidity.

Set up to deliver zero air and combine with water vapour from an aquarium

Figure 2: Zero air combining with water vapour from the aquarium

As we discovered, it was quite hard when starting off with zero air to turn this into high humidity air without also having condensation issues within the ducting leading to the test chamber. The GCARE team were however quite resourceful when coming to practical techniques, and we managed to explore the other parts of the humidity range using a combination of a nebuliser to create small water vapour particles and the higher end range of humidity through rearrangement of the sets of connections into the chamber.

I will tell a bit more about the performance of our sensors in the chamber in subsequent blogs whilst briefly mentioning the results from our temperature sensor during a temperature test.

Set of temperature sensor responses during lab test

Figure 3: CP1 prototype temperature responses during a lab heating/cooling experiment

The CP1 prototype units’ temperature sensor responses were generally well correlated with one another, but slightly higher than the chamber temperature recording. This was attributable to the internal heat dissipation of the prototype, when in operational mode.

 

Pollution Guardian: roadside experimentation together with the University of Surrey

One of the first activities together with the University of Surrey’ Global Centre for Clean Air Research, led by Professor Prashant Kumar, was to join a live field campaign to measure the air pollution next to Stoke Road in Guildford, Surrey. The University of Surrey’s campaign measured roadside (pavement) air pollution using both high-end specialised air pollution instrumentation along with more affordable devices.

Stoke Road can be a busy road at peak times and is closely connected to the A3 slip road for London and the South East as well as the direct connecting route to the nearby town of Woking.

Traffic on Stoke Road Guildford

Figure 1: Stoke Road during a morning rush hour.

The setup involved measuring the air pollution at a height of approximately 1.5meters above the pavement, a comparable height for pedestrians breathing in roadside air. Thus the need to brush off some old woodwork skills and create a container and support platform for our units.

Bird box platform for Pollution Guardian sensor units

Figure 2: “bird table” box for holding pollution guardian units

Why measure pollution on Stoke Road? There is a children’s playground right next to Stoke Road, so monitoring pollution levels there is of real interest and also the University has a co-located air pollution measurement station, using sensors from the iSCAPE project. With the support from the Guildford Borough Council, this site is being continuously used by GCARE team as a part of their iSCAPE’s Guildford Living Lab activities.

Now whilst we are designing the Pollution Guardian for measurements in the car, it was of some interest to us to obtain an early view of how our own “CP1” version prototypes performed compared to the specialised air pollution equipment made by GRIMM and more expensive particulate material testers such as Dylos devices.

stoke road air pollution testing campaign site

Figure 3: Roadside setup next to Stoke Road

What were the learnings?

Good results are possible for fine particulate matter (PM2.5) measurement:

  • After aligning measurements in time, we are able to obtain a correlation of 0.8 between Pollution Guardian units and the high-end particle spectrometer.

However, NO2 pollutant gas measurement is difficult in the field:

  • Pollution Guardian NO2 sensor readings were well correlated with one another but..
  • Even after warm-up, we can only obtain 0.3 correlation with a reference measuring unit
    • Some NO2 spikes detected by Pollution Guardian units were ignored by the reference NO2 detector and vice-versa.
  • One effect we could feel and could see in the “spikiness” of our NO2 readings was the impact of a Northerly wind blowing along the road.
    • We would go on to look at how to better protect our units against wind gusts, whilst not compromising our key use case of the car environment.

So, whilst the reasons for lower correlation on NO2 gas readings were not yet clear, this was something we were obviously keen to explore in the planned lab testing with the University of Surrey’s GCARE Air Quality Lab where we would have better control over the environmental conditions.

Update on our first prototype & air pollution on BBC news again

BBC London ran a recent story on air pollution exposure during commuting:

It now seems a long while since our last blog post, but air pollution is barely out of the news with the latest BBC story about air pollution exposure during commuting. This topic is fairly close to our hearts with the pollution guardian activity inspired by the fact that air pollution exposure is highest within motor vehicles.

Earlier on this year, we put together our first prototype pollution sensor containing a particulate sensor for PM2.5 measurement and a Nitrogen Dioxide (NO2 ) gas sensor. PM2.5 refers to the measurement of very small particles, less than 0.0025mm in diameter which can go deep into the lungs and containing a variety of payloads e.g. road, brake or tyre dust or chemical compounds from combustion. Nitrogen Dioxide gas can exacerbate respiratory conditions and at street level is mainly produced by diesel vehicles.

How does out first prototype look?

Close up photo of our first protoype unit

Why did we make this first prototype?

  • to create a self-contained sensor unit with its own internal power supply to enable easier bench and field (car) testing
  • to shrink the volume of air trapped within the unit in order to keep the unit responsive to the current air conditions
  • to enable experimentation with parameters to find the best balance of sensitivity versus stability

We will write a bit more about the “infrastructure” of the prototype and surrounding app and cloud solution in later blogs, but the main question we had when we put the hardware unit together was does it work?

Well, the good news was that the prototype actually worked as expected across a range of functional areas with only one bug with battery charging which was solved by a simple hardware modification to the units.

How well did the units work compared to our initial cardboard prototypes? Pretty good in practice:

  • having “always on” internal power meant that the sensors could be ready to start work within a couple of seconds; previously the sensors could require several hours from being powered up to being ready to measure
  • fast responsiveness to Nitrogen Dioxide gas e.g. able to pick out smelly vehicles passing by

NO2 measurement against time

And where does that spike of air pollution correspond to? A smelly van on the southbound slip road of the M25/A3 junction…

Map of NO2 air pollution hotspot on M25/A3 southbound sliproad

 

Smart Phone Applications for the Pollution Guardian Project

Most Internet of Things (IOT) products are associated with using very efficient protocols such as MQTT over a special purpose radio link like LoRa. However the IOT solution we are building in the Pollution Guardian project is quite different for the following reasons:

  • As a consumer device we need interaction locally in real time.
  • Our aim was to produce hardware which could be purchased for the minimum cost to the consumer and so offer benefits to as many people as possible. This meant a minimized UI on the device itself.

These drivers brought us to the following decisions: we would build a device which reaches the internet via a smart phone bridge. We would therefore use Bluetooth Low Energy to communicate between the device and the phone.

 

 

We have chosen to build our prototypes using Android as this offers a lower cost to start up than using IOS in terms of devices and computing facilities, we have been careful however throughout the project not to lock ourselves into Android-only infrastructure solutions.

As a roadmap we realized that we needed two versions of the application: first an ‘engineering’ application optimized for our testing in laboratory and in technical field test and second a ‘consumer’ application which would deliver the experience design for a real customer.

The engineering application was the first target since it was the means to learn about our sensor package and how it reacted to changes in pollution conditions. Even for this application we have stuck to the principle of cloud data storage as the most effective means to leverage the value of the data collected – so no memory cards or local only databases were to be implemented. Because our background was in device-side development rather than infrastructure, Firebase Real Time Database emerged immediately as a very good candidate for data storage.

 

 

Further benefits of using Firebase emerged for authentication with code snippets available for sign-on and sign-up. Firebase also handles ‘beneath the hood’ all the buffering, reconnecting and error handling needed for practical operation with a mobile device where connection to the internet may or may not be available at any moment in time.

For the engineering phase, extraction of measurement results files for further offline analysis is essential and Firebase’s admin API library enabled this in a python environment. The data could then be further analyzed with data science tools in Jupyter notebooks.

So far we have extracted data from hundreds of miles of road-test and hours of static testing – the tool set works well and extremely reliably for us. Future posts will share more about the Engineering App and how we further developed our application and cloud designs.

Pollution Guardian: Measuring air pollution with cereal packet prototypes

As mentioned in the previous blog, we put in a lot of preparation work over summer 2018 in order to be able to quickly start work on the Pollution Guardian project and de-risk certain areas. One of the first items to be actioned was the creation of a custom circuit board to explore the performance of our selected, affordable Nitrogen Dioxide (NO2) gas sensors and the type of electronics needed to setup, amplify and filter the output of these sensors. We called it our “bench test” circuit board as we thought it would only get used indoors on the bench..

Bench test circuit board

With the bench test board, we designed a couple of options for the electronics solution:

  • to provide a backup option, in case the one circuit failed to perform
  • provide us alternate options based on relative performance
  • to enable optimisation of the bill of materials cost

Designing & making the circuits went relatively smoothly,  but it is not until you receive circuits that you can really see what is going on. Some mistakes we discovered quickly, whilst others took a period to debug. Mostly our mistakes were due to the misinterpretation of and assumptions on the component specifications; our first solutions to these issues actually went on to compound our problems.

After a period of investigation and debugging, we were finally able to identify and solve the underlying issue. As part of this work, we lashed these prototype circuits together with an off-shelf development board (TI cc2640R2) all contained in a cereal packet wrap for protection. We called our first units  Dougal & Zebedee and took them out of the lab and into the car:

Dougal and zebedee prototypes

What we found when we started testing out the units was that whilst our units appeared quite sensitive to temperature change, they were actually capable of detecting low levels of NO2 inside the car.

The picture below shows a trace of NO2 measurements whilst traversing the one way gyratory system in the centre of Guildford – note that a decreasing level on the graph trace implies a higher NO2 concentration:

Air pollution on Guildford gyratory system

 

So, at the conclusion of our cardboard proto work, we knew we had the right potential sensitivity toward NO2 which we could work with on the next prototype to calibrate and try to get consistent between test units.

 

 

 

Pollution Guardian: From competition win to prototyping

BBC cartoon programme Magic Roundabout characters

Magic Roundabout characters

Feasibility projects like the Pollution Guardian imply a certain level of risk and it was critical for our business to secure some grant support from Innovate UK to help us mitigate those risks.

Looking back, it seems a long time since we made our application for funding, but considering the risks ahead, we prepared ourselves in order to get started quickly in case of a positive outcome of our bid:

  • Working on the system architecture, driving decisions on the hardware and software platforms to use within development
  • Further research on the key components
  • Talking to collaborators and contractors to re-check availability
  • Re-planning the market research

Now, many academic papers point to the difficulty in using affordable sensors e.g. around variability, stability and accuracy, so one of our biggest challenges in the project is to put together an affordable solution based on these sensors. Our approach was to tackle this issue head on, building a very early prototype and using a minimum “data gathering platform” around it to understand the pitfalls & performance issues.

This early work cut across several disciplines:

  1. Electronics, designing a custom circuit to best interface with affordable gas sensors
  2. Firmware, building on a development platform to gather and share sensor data over wireless
  3. Mobile app, customised to gather the local sensor data & upload it to a data store
  4. Mobile backend, a real time database to capture the data streams and tools to help explore the data
  5. Mechanics, how to wrap this early prototype for real world use
  6. Early system testing & validation approach

We will be adding a few blogs to the site to cover progress on the above items; suffice to say the first mechanical housing for the unit followed the “breakfast cereal box” approach. After wrapping up the first sensor unit, its looks gave us the name for our prototype, Dougal, after the dog character from the programme, “The Magic Roundabout”.

Dougal, the Pollution Guardian cereal box prototype

Is it worth entering Innovate UK competitions?

We, at All about the Product, consider ourselves blessed to have won a grant from Innovate UK in the Open March 2018 competition along with our collaborators at the University of Surrey. Thus we are have now embarked on our “Pollution Guardian” Startup project. To help others considering such a bid we thought it might be useful to write this blog.

Why should you do it?

  • Investment: Quite simply the grants from Innovate can be the difference between properly exploring a business idea and letting it wither on the vine. We have found that the grant we received has made it possible not only to buy materials and hire specialist contractors but also to spend the time to unlock the potential of our own competences and experience.
  • Collaboration with academia: This benefit was not immediately obvious to us when we started our journey but has become a key component our activities as we execute our project. The good thing is that universities can attract 100% funding as a collaborator and with that you can get not only world class know-how in a certain field but also a different world view which will enrich your project no end.
  • Support from Innovate: You have a Monitoring Officer appointed who will review your project every quarter. While some may regard this as an examination to pass in order to get your funding released, we have found this discipline useful to keep structure in the project and facilitate periods of reflection on progress. Of course having the backing of ‘Supported by Innovate UK’ also opens doors and helps convey immediately that this project already has gone through a lot of scrutiny.

How do you win?

The simple answer is that we do not know. We had more than one attempt for different applications and through those attempts we did change our proposition significantly. So you could argue immediately that the application process in itself forces you to improve your idea and so is a worthwhile exercise in its own right.

Choosing the right competition to enter is often cited as very important with the idea that you should enter the most specific competition to your idea as possible. Since we won the Open Competition after failing in the Emerging and Enabling competition we cannot perhaps be considered a typical case.

Learning from previous bids and listening to feedback is important – you do get scored feedback on any bid you make. However winning is not simply getting the highest score since Innovate do consider the portfolio of investments they are making. You are only allowed to make the same application twice so do learn from previous feedback but do not give up too easily.

 

 What happens if you do win?

You get the email and read “We are pleased to inform you that your application has been successful” – so what now? In our case this was 3 months after we submitted our application and 5 months after the competition opened. You are not able to start the project immediately and there are quite a few more hoops you need to jump through before you get your all-important offer letter – in our case this took another two and a half months.

So you find yourself starting a project which you planned at least 6 months ago and in this time you may find contractors you had identified are no longer available so you will probably need to do some re-planning – things change after all.

Is it all worth it?

Our experience is that it is for the reasons outlined above. It is not necessarily a fast process for a start-up eager to get moving but if you hang in there it can be very rewarding.

https://www.gov.uk/government/organisations/innovate-uk

https://allabouttheproduct.com/

 

Origins of the Pollution Guardian Startup

We have now completed the first quarter of the Pollution Guardian project part funded by Innovate UK and it seemed a good time to reflect on where this whole thing came from in the first place.

It all started from a growing awareness, through many stories in the press, about the negative health effects of air pollution. Furthermore this pollution, unlike the ‘pea-souper’ of the 1950s and 1960s was largely invisible. However its immediate term effects were not invisible to the large numbers of people suffering from various respiratory illnesses such as asthma. It seemed there should be a demand to ‘shine a light’ on this pollution.

When starting a business idea it is important not only to understand customer needs but also the business’s capability to do something about it. At All about the Product we are veterans from the mobile phone industry and are steeped in both technical and commercial experience to bring highly sophisticated consumer electronic communications products to market. Any solution we were going to build would therefore be an ‘Internet of Things’ solution.

As many people have told us, for a good product idea it is not only important to know whether there are high levels of pollution present but also there needs to be something to do about it. We therefore became interested in the effects of pollution inside vehicles where there was an opportunity to control this enclosed space more than the entire atmosphere of a city. On further investigation we found articles which showed that very often the level of pollutants inside a car exceed those at the road-side [1] – we had our application focus.

We wanted a product within economic reach of the majority of the population and therefore a low cost consumer internet of things product for use in cars and other vehicles was born.

As this idea developed, we spoke to many stakeholder organisations such as health charities, workers unions, academia and local government. What we learnt from these discussions underlined the fact that there was this invisible problem there and many felt powerless to combat it. Our motivation to do something only grew.

We put our engineering hats and did a quick study to show that producing such a product was feasible. However when we looked at the skills needed to pull this product off, we quickly realised we lacked knowledge about urban pollution and the experience of pollution measurement. Fortuitously our local University – the University of Surrey has a very strong department in this field, so we contacted Professor Kumar at the Global Centre for Clean Air Research (GCARE) and set up a meeting. It quickly emerged that our skills complemented those of the university in making this product investigation happen. We agreed to join forces in making a bid for one of Innovate UK’s competitions and after a couple of attempts we won one! Thus the Pollution Guardian Project was born.

[1] Cepeda et al :Lancet Public Health 2017; 2:e23-34

https://allabouttheproduct.com/

https://www.surrey.ac.uk/global-centre-clean-air-research

Sensing the Air Quality

Venue for sensing the air quality and emissions program

In our home page, we mentioned that within product incubation, we were working on our own solution activities. The area of interest is within the internet of things (IoT) and is  targeted at low cost air pollution/air quality (AQ) solutions at this time. Thus we were pleased to participate in the 46th Intelligent sensing program, organised by the UK knowledge transfer network.

What were the main learnings from the event?

  • Big concerns on the level of small particulates, specifically PM2.5 and below.
  • EU AQ measurement standardisation is scoping “informative” sensing solutions.
  • Growing interest in AQ sensor networks.
  • UK government desire to leverage crowdsourced pollution data.
  • Key sensor criteria & findings from low cost sensing components.
  • Urban & journey pollution mapping.

Particulates

Can we meet the EU reduction targets for PM2.5 particulates by 2020? Should we be counting the number of particles rather than particle mass/m^3? Just one PM10 particle weighs as much as 1000 PM1 particles.

AQ sensor networks

Presentations from Alphasense and AirMonitors Ltd. pointed to the additional value coming from “lower than reference” quality sensors connected up as a network e.g. the ability to dis-aggregate pollution sources.

Crowdsourcing, sensor criteria and findings

The UK Environment Agency are looking to identify how they can better leverage crowdsource AQ data as a larger monitoring network than their existing 150 reference stations across the UK. However, the main concern is over the quality of this data. This then leads us to think about the air quality sensor and sensor system criteria:

  • Sensitivity: enough for the purpose e.g. movements within the general background outdoor AQ level
  • Specificity: responding to a specific gas pollutant and not being easily spoofed by the presence of other gases
  • Stability: the sensor performance remains predictable enough, compared to a reference over its intended lifespan

A couple of presentations mentioned using  low cost sensors based upon existing metal oxide technology – the learning here being that the sensor system stability is a challenging issue to manage. Mitigation seems so far to have been to adopt electrochemical type sensors but which can be significantly higher cost.

Hamamatsu was one supplier at the event claiming good individual gas detection capability using light absorption sensing e.g. a sensor set to the band gap of a specific gas. This looks an interesting avenue to explore further.

Overall, the sensor selection is one of the most significant factors for us to consider in our low cost AQ sensing solution.

Urban & air pollution monitoring from vehicles

A couple of interesting talks measuring pollution across journeys.

  • Motorway driving is quite bad for particulates exposure due to vehicle pollution “plumes”.
  • Air re-recirculation in cars provides quite some benefit to exposure levels, as long as the CO2 levels & humidity levels in the cabin don’t build up too much.
  • Urban NO2 hotspots experienced by cyclists on pathways, not only near road junctions but also when passing by industrial areas.
  • AQ pollution hotspots can move position dependent on the wind direction e.g. crossing over to the opposite side of the road. So, dynamic measurements or models are important.