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Measuring an NO2 Reduction on London Car Free Day

EarthSense Data Scientist, Jordan White writes a blog on his day in London for World Car Free Day and filming for BBC One Sunday Morning Live


A call from the BBC

We received a call from the BBC inviting us to their broadcasting house in Westminster, London to feature on a segment of the BBC One programme, Sunday Morning Live on 22nd September 2019 - World Car Free Day with the City of London participating in the event.


We have a good on-going relationship with the BBC following the launch of our pollution postcode checker and were invited to discuss the Zephyr® air quality sensor as a tool for measuring urban air pollution.


World Car Free Day - London

Some of the world’s biggest cities took part in World Car Free Day with London’s Car Free Day mission “committed to championing traffic-free city centres and active travel” brought through the closure of all roads to vehicle traffic in parts of City of London, Tower Hamlets and Southwark.


It was an ideal opportunity to take the Zephyr® sensor and to see whether the reduction in cars on the road would be observable. So, on the morning of Sunday 22nd October I made my way to London with a Zephyr® in tow.


Filming with for BBCOne 'Sunday Morning Live'

I arrived in Westminster at the BBC broadcasting house where we began filming for the Sunday Morning Live segment. This included interviews from a host of others taking part in London Car Free Day covering topics from how the event was organised and some of the benefits of having less cars on the road.


The Zephyr® air quality sensor was highlighted as a method of measuring urban air pollution at high temporal and spatial resolution.


Filming for BBC One Sunday Morning Live on 22.09.2019 at the BBC Broadcasting House in Westminster, London.

You can watch the full programme here or fast forward to the segment at 55 mins - (available until 22.10.2019)


The eerie calm of London

Following the filming, I walked into the from Westminster with a mobile Zephyr® sensor which I had placed into a small backpack. Zephyr’s can be hard wired into a post, solar powered or run on battery power for 24-48 hours for experiments like this one.


It was business as usual in Westminster, however, the City of London was quite surreal. There absolutely no cars on the road and an eerie silence.

I came across a street festival with dedicated chill-out areas, children’s play areas, food stalls and local performers – quite the opposite of seeing people rushing through the streets and trying to cross busy junctions in the masses.


London Car Free Day 22.09.2019

Reimagine Festival in the streets of London on World Car Free Day

Measuring NO2 with the mobile Zephyr® sensor

During my walk into the city, the Zephyr® sensor was sampling the air around me, taking measurements every 10 seconds, then uploading the data to our database at 15-minute intervals.


The increased number of measurements and fast turnaround of data not only provides a good representation of the air around us but also allows us to respond quicker to air quality issues such as redirecting traffic through pollution hotspots or activating clean air zones in cities.


A reduction in NO2 in the City of London

Given previous studies carried out by EarthSense, an improvement to local air quality was envisaged, though the level of improvement was less known.


I had expected to see a reduction in NO2 due to the lack of vehicles on the roads compared to a regular Sunday afternoon.


The images below shows my journey from Westminster with NO2 concentrations decreasing in the City of London where the interventions were taking place.


Jordan's walk into the City of London showing NO2 concentrations measured by the Zephyr® air quality sensor

By accounting for the diurnal trend of NO2 on the data recorded by the Zephyr®, the differential trend can be observed. This highlights the relatively unpolluted and polluted periods of the day in London.


From this, Westminster (business as usual) is often in the relatively polluted parts of the data. The mean concentration of NO2 between Westminster and City of London was 34.1 µg m3, which is factored into the graph for a better view of relative pollution exposure (note in absolute terms the City of London was less polluted partially because of the diurnal cycle of NO2).


A 35% reduction in NO2 can be observed in the data between the heavily congested streets of Westminster and the peaceful City of London during Car Free Day. This large difference shows the immediate impact of reducing emissions can have on improving the air we breathe. I have no doubt that the road closures were disruptive to some, but many people were also seen to be making the most of the open space with sports and leisure.


London is so often a place where ideas start, and I hope to see more cities create positive spaces to improve the environment at a local scale to give the chance of improving the urban environment for generations to come.

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