BT Tower (London, UK): an urban atmospheric pollution observatory

Biosphere-atmosphere exchange of carbon dioxide (CO₂) has been on the scientific agenda for several decades and new technology now also allows for high-precision, continuous monitoring of fluxes of other trace gases such as methane (CH₄), carbon monoxide (CO) and nitrous oxide (N₂O).

Compared to the natural environment, flux measurements in the urban environment, which is home to over 50% of the population globally, are still rare despite high densities of anthropogenic sources of pollutants. 

Direct measurements can be used to study the temporal dynamics and spatial distributions of pollutant sources and can also provide an independent validation of emission budgets estimated by atmospheric emission inventories. Atmospheric emissions inventories are used to construct budgets based on assumed spatial distributions and source strengths of a range of pollutants. Inventorying is currently the only accepted method for constructing the annual emission budgets the COP21 signatory countries report to the United Nations Framework Convention on Climate Change (UNFCCC).

Despite the widespread use of this approach, there is a need for independent validation of the estimated emissions budgets, which can be done by a "top down" approach such as the one used at the BT tower observatory.

UKCEH began using the BT tower as a tall tower measurement platform as early as 2006 under the NERC-funded project REPARTEE. The 190 m-tall BT tower is located in central London, making it an excellent platform for pollution monitoring at the heart of Europe's largest city. 

Measurements of fluxes of CO, CO₂ and CH₄ by eddy-covariance began in 2011 under the auspices of NERC-funded project ClearfLo. The site has been in continuous use since 2011 and our records of CO₂ and CH₄ fluxes are among the longest in the world. 

In addition to greenhouse gases, UKCEH played a pivotal role in implementing measurements and calculations of fluxes of nitrogen oxides (NO_x) by eddy-covariance at the BT tower under ClearfLo in collaboration with scientists from the National Centre for Atmospheric Sciences (NCAS). 

Between 2013 and 2016, measurement activities at the BT tower were supported by NERC-funded GAUGE project (Greenhouse gAs Uk and Global Emissions) and more recently by National Capability funding.

The eddy-covariance system used at the BT tower consists of a 3-D ultrasonic anemometer (R3-50, Gill Instruments), a Picarro cavity ringdown spectrometer (CRDS) model 1301-f for the measurement of CO₂, CH₄ and H₂O mole fractions (until 2021) and an Aerolaser fast CO monitor model AL5002 (until 2015). A dual laser Aerodyne QCL spectroscopic instrument capable of also measuring CH₄ and H₂O, ethane (C₂H₆), nitrous oxide (N₂O) and nitrogen dioxide (NO₂) replaced the Picarro CRDS, and a Li-Cor 7000 measures CO₂ mole fractions. The anemometer is mounted on top of a lattice tower located on the roof of the BT tower giving an effective measurement height of 190m above street level. The gas analysers are located a few floors below the roof, in an air conditioned room. Air is sampled from ca. 0.3 m below the anemometer head at 20–25 L min^-1 using a 45m long Teflon tube of OD 9.53mm (3/8''). 

Our research into greenhouse gases and air quality conducted at the BT Tower observatory has generated (and continues to generate) a great number of outputs both in the scientific press and mainstream media outlets.

Notable findings include:

• Near two-fold underestimation of central London methane (CH₄) emissions reported by the atmospheric emissions inventories (Helfter et al., 2016).
• Measured and inventoried carbon dioxide (CO₂) emissions are in good agreement, demonstrating that the urban sources of this gas are well understood.
• CO₂ emissions dropped by nearly 60% during the first few weeks of the covid-19 pandemic lockdown, matching the reduction in traffic. Over the two years of the pandemic (2020 & 2021) measured emissions of CO₂ and CH₄ dropped by 20% and 30%, respectively, but rebounded to pre-pandemic levels in 2022.
• NO_x emissions from traffic decreased by ca. 73% between 2017 and 2020 due to more stringent traffic regulations and reduced congestion during the pandemic.

Publications

• Duarte Rocha et al. (2024). "Unprivileged groups are less served by green cooling services in major European urban areas", Nature Cities 1(6). 

• Cliff et al. (2023). "Pandemic restrictions in 2020 highlight the significance of non-road NO_x sources in central London".

• Nicolini et al. (2022). "Direct observations of CO₂ emission reductions due to COVID-19 lockdown across European urban districts",
  STOTEN 830.

• Drysdale et al. (2022), "Eddy covariance measurements highlight sources of nitrogen oxide emissions missing from inventories for central London", Atmos. Chem. Phys., 22, 9413–9433.

• Helfter et al.  (2016). “Spatial and temporal variability of urban fluxes of methane, carbon monoxide and carbon dioxide above London, UK”, Atmospheric Chemistry and Physics, 16(16): 10543-10557.

• Bohnenstengel et al. (2015). "Meteorology, air quality, and health in London: The ClearfLo Project." Bulletin of the American Meteorological Society 96(5): 779-804.

• Lee et al. (2015). "Measurement of NO_x Fluxes from a Tall Tower in Central London, UK, and Comparison with Emissions Inventories." Environmental Science & Technology 49(2): 1025-1034.

• O'Shea et al. (2014). "Area fluxes of carbon dioxide, methane, and carbon monoxide derived from airborne measurements around Greater London: A case study during summer 2012." Journal of Geophysical Research-Atmospheres 119(8): 4940-4952.

• Harrison et al. (2012). "Atmospheric chemistry and physics in the atmosphere of a developed megacity (London): an overview of the REPARTEE experiment and its conclusions." Atmospheric Chemistry and Physics, 12(6): 3065-3114.

• Helfter et al. (2011). "Controls of carbon dioxide concentrations and fluxes above central London." Atmospheric Chemistry and Physics 11(5): 1913-1928.

• Wood et al. (2010). "Turbulent Flow at 190 m Height Above London During 2006-2008: A Climatology and the Applicability of Similarity Theory." Boundary-Layer Meteorology 137(1): 77-96.

Media

Helfter, C. interview for Great British Railway Journeys, Series 14 Episode 8 Paddington to Ongar, first aired on BBC2 28/06/2023.

Coverage of impacts of COVID-19 lockdowns on GHG emissions in central London

• https://www.dailymail.co.uk/sciencetech/article-8332273/Air-pollution-London-dropped-60-cent-coronavirus-lockdown.html
• https://www.ft.com/content/37bde18a-e2bb-4034-b96c-7145f922e1c6
• https://www.timeout.com/london/news/bt-tower-records-58-percent-reduction-in-carbon-emissions-during-london-lockdown-052120
• https://theecologist.org/2020/may/19/lockdown-halves-london-carbon-emissions
• https://www.newscientist.com/article/2243875-coronavirus-set-to-cause-biggest-emissions-fall-since-second-world-war/)
• https://www.energyvoice.com/other-news/240936/londons-carbon-emissions-plummet-by-60-during-lockdown-analysis-finds/
• https://phys.org/news/2020-05-london-carbon-emissions-fall-covid-.html
• https://wattsupwiththat.com/2020/05/19/londons-co2-emissions-cut-by-almost-60-during-lockdown/
• https://news.sky.com/story/coronavirus-lockdown-emissions-drop-dramatically-but-its-having-little-impact-on-climate-change-say-scientists-11991129

• Blog post with Natural History Museum “Nature in Lockdown”: https://www.nhm.ac.uk/discover/nature-liberated-by-lockdown.html

• ICOS blog post (13/05/2020): “Clear evidence of reduction in urban CO2 emissions as a result of COVID-19 lockdown across Europe” https://www.icos-cp.eu/event/933

• UKCEH blog post https://www.ceh.ac.uk/news-and-media/blogs/london-co2-emissions-fallen-60-percent-lockdown