For over 20 years, environmental scientists have operated a pioneering atmospheric observatory at the top of the BT Tower, measuring pollution in the air we breathe, the greenhouse gases that drive climate change, and the weather conditions that shape London.

At 190 metres tall, the BT Tower Atmospheric Observatory filters and analyses air as it moves upwards from the streets and buildings of central and Greater London. It is one of the very few sites in the world capable of directly measuring emissions from a major city centre.

NCAS researchers have used air pollution monitoring equipment mounted on the BT Tower to collect real-world insight into the quality of the air that the people of London are breathing every day, for the last decade. We asked them about the observatory, what changes there have been in air pollution, and what air quality could be like in the near future.

A gas inlet mast on top of the tower pumps air into the 35th floor laboratory, collecting pollution from nearby roads, commercial residential buildings, and some urban parkland. The observatory can detect air pollution emissions within a 2-4 km radius. Depending on the weather conditions, concentrations of air pollution can be detected from much further afield – Greater London, South West England, and Europe.

“To measure air pollutant concentrations of gases like ozone, we use equipment similar to those found in usual air quality monitoring stations. But for measuring emissions we need to use instruments that can take samples much faster. For example our measurements of nitrogen oxide (NO) and nitrogen dioxide (NO₂), which combined are referred to as NO_x, are taken 5 times per second. These are coupled with fast measurements of the wind speed, direction and turbulence by the University of Reading to tell us how much is being emitted 2-4 km upwind of the tower, every hour. The UK Centre for Ecology & Hydrology also operate fast greenhouse gas analysers, that measure the emissions of carbon dioxide and methane, and the University of Manchester operates instrumentation that makes fast measurements of black carbon,” explains Dr Will Drysdale, research scientist at the National Centre for Atmospheric Science and University of York.

NO_x is produced when fuels are burned, like in cars and boilers, and is a public health concern. Ozone, when found near the Earth’s surface rather than up in the ozone layer, can trigger a variety of health problems – particularly for children, the elderly, and people of all ages who have lung diseases. Black carbon is soot from combustion and is a component of  PM_2.5, which refers to particulate matter with a diameter of 2.5 micrometres or less. These are very fine particles that can enter the lungs and bloodstream, and are known to have both short-term and long-term effects on human health.

“Air pollutant concentration calculations and instrument calibration are completed regularly. Emissions require a more complicated data processing pipeline, bringing together the different data streams from the BT Tower’s measurements e.g pollutant concentration, air turbulence – and relying on the University of York’s supercomputer called Viking to do the data analysis. Then more data is used to add scientific context. For example, we use data collected by automatic traffic counters to explore road emissions, and compare it with other air quality monitoring stations and emissions inventories. These inventories are either national or city wide, and provide “bottom-up” estimates of emission at around 1 km scales, which we then compare with our “top-down” measurements. The data is made available via the Centre for Environmental Data Analysis or the European Fluxes Database,” describes Dr Will Drysdale.

Most air quality stations measure the concentration of pollution that is present in the air. The BT Tower Atmospheric Observatory goes a step further – from its unique position high above London, it measures emissions produced by the city itself. This information helps scientists and policymakers understand whether actions to reduce harmful air pollution and greenhouse gases are working, and where further improvements can be made. 

These measurements support cleaner air, climate action, and evidence-based decisions not just in London, but across the UK. The BT Tower Atmospheric Observatory also provides the only long-term measurement of nitrogen oxide emissions from a megacity in the world.

The site has been used by NCAS and other leading research organisations, such as Imperial College London, UK Centre for Ecology & Hydrology (UKCEH), University of Manchester, University of Reading, and University of York. It has also supported major UK Research and Innovation-funded research programmes.

Research from the BT Tower Atmospheric Observatory has helped reveal how London’s air pollution sources have changed over time.

One of the most significant changes has been the decline in emissions from road traffic. Nitrogen oxides emissions from vehicles in central London are estimated to have fallen by about 73 % between 2016 and 2025. This reduction is partly due to the success of air quality policies in central London, the increasing prevalence of Euro 6 vehicles and electric cars, and changes to congestion that resulted from pandemic-reduced mobility.

Measurements from the BT Tower also showed that carbon dioxide emissions in central London fell by around 60 % during COVID-19 lockdowns, closely reflecting reductions in traffic. These unique observations demonstrated how direct emissions measurements can reveal changes that conventional monitoring may miss.

The impact of cleaner vehicle technologies has also been seen in measurements of black carbon. “We found only minimal emissions from traffic, which is consistent with emissions reductions associated with Euro 6, the standard for diesel engines specified by ULEZ. This is in contrast to previous measurements in London, collected in 2012, which showed a strong association between traffic and black carbon. At the time, diesel particle filters had only been introduced relatively recently. And while domestic wood burning is also known to be a source of black carbon in the UK, this was predictably not found to be the case in central London which is mainly commercial rather than residential,” explains Dr James Allan, senior research scientist at the National Centre for Atmospheric Science and University of Manchester.

As emissions from traffic have declined, other sources have become relatively more important. Measurements showed that the combustion of natural gas in boilers accounted for 72 % of nitrogen oxides emissions within a few kilometres radius of London’s BT Tower between 2021 and 2023. Industrial and non-domestic boilers, which are larger than those in private homes, are the main contributors in central London, which is not a highly residential area. Domestic heating also emerged as a more significant contributor in some areas of Greater London. This evidence informed the 2026 Warm Homes Plan, supporting efforts to reduce fuel poverty, improve health outcomes, and cut emissions.

The research also highlighted the growing importance of other urban pollution sources. Emissions of black carbon from non-road mobile machinery, including generators and heavy machinery used on building sites, have started to surpass the black carbon emissions from vehicles. This is particularly clear in areas with dense construction.

As the benefits of policies targeting vehicle emissions continue to emerge, researchers suggest that future efforts to improve air quality will need to widen their geographic reach and focus beyond emissions from diesel exhausts.

The success of measures such as the Ultra Low Emission Zone, alongside stricter vehicle emission standards, has led to marked improvements in urban air quality. Electrified transport, combined with traffic reduction schemes, could further improve the quality of the air people breathe in London and other cities.

However, as emissions from road traffic continue to fall, other sources of pollution are becoming more prominent. Researchers say further progress will require tackling emissions from heating as well as transport.

“Getting combustion out of cities is central to further improving air quality in the UK. We will either need to electrify the heating of homes and businesses or think much more seriously about how pollution from gas burning can be cleaned up, as we currently do for cars, trucks and buses,” says Professor Alastair Lewis, senior research scientist at the National Centre for Atmospheric Science and University of York.

Researchers also caution that improvements in one pollutant can sometimes have unintended consequences for another. A reduction in nitrogen dioxide levels by cutting diesel and petrol emissions in cities such as London can lead to an increase in ozone because of changes in atmospheric chemistry. This unwanted side-effect can however be overcome by cutting emissions in other places, like solvents from paints and aerosols. 

Research also points to construction as an increasingly important source of urban air pollution. As emissions from vehicles have declined, black carbon emissions from non-road mobile machinery, including generators and heavy machinery used on building sites, have become more significant.

Dr James Allan outlines how important regulatory attention is for the construction sector: “Even if observed emissions from construction equipment comply with emission standards, black carbon output from generators, machinery and construction vehicles remains significant. The Ultra Low Emission Zone in London, and stricter vehicle emission standards, have led to marked improvements in urban air quality. Traditionally non-road mobile machinery has always lagged road vehicles in terms of regulation, but rules for the construction sector are being tightened up, so we could start to observe more positive outcomes for air quality.”

Together, these insights suggest that while London’s air quality is likely to continue improving in the coming years, achieving further gains will depend on addressing a broader range of pollution sources, including building heating systems and construction activities, while carefully monitoring pollutants such as ozone as the city’s emissions profile changes.

As plans for the BT Tower’s future evolve, with its transformation into a hotel, preserving the observatory will protect vital environmental research and ensure monitoring of future changes in air pollution emissions.