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 and an Aerolaser fast CO monitor model AL5002 (until 2015). 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''). The Picarro CRDS is fitted with an in-house auto-calibration system and were calibrated weekly using two different mixtures of CH₄ and CO₂ in nitrogen (above and below typical ambient concentrations) until September 2014 when the calibration gases were replaced by natural compressed air mixtures traceable to WMO standards.
 

Some notable results obtained from measurements atop the BT tower include:

• Good agreement between annual budgets of CO₂ (39.1 ktons km^-2 year^-1) and CO (89 ktons km^-2 year^-1) with data from the London Atmospheric Emissions Inventory (LAEI).
• The measured fluxes of CH₄ (72 tons km^-2 year^-1) were more than double the LAEI value, which suggests that the inventory does not characterise the urban sources of methane as well as those of carbon monoxide and carbon dioxide.
• Emissions of CO and CO₂ were strongly correlated with air temperature. We attribute the increase in winter time emissions of CO (which represent 45% of the annual budget) to reduced vehicle fuel combustion efficiency and "cold starts".  For CO2, the seasonal differences are thought to be controlled by a rise in natural gas consumption (resulting in more CO2 emissions from combustion) in winter and perhaps also a drawdown from the vegetation in the summer (this was particularly apparent in the seasonal cycle of CO₂ fluxes from the wind sector of Regent's Park, where we recorded negative fluxes - i.e. an uptake - during the summer). 
• The correlation between CH₄ and air temperature was only statistically significant in the East and West wind sectors. We speculate that emissions in these wind sectors are dominated by sources of CH₄ with strong seasonality, such as leaks from the natural gas network and/or emissions from sewage which are temperature-dependent.
• CO₂ and CH₄ fluxes were positively correlated with population density in all wind sectors, except S for CO₂ and S, SE and E for CH₄. This indicates heterogeneous source distributions and/or densities with temporal dynamics which differ from the other wind sectors.

Publications

• Helfter, C., 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, S. I., et al. (2015). "Meteorology, air quality, and health in London: The ClearfLo Project." Bulletin of the American Meteorological Society 96(5): 779-804.
• Lee, J. D., 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, S. J., 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, R.M., 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, C., et al. (2011). "Controls of carbon dioxide concentrations and fluxes above central London." Atmospheric Chemistry and Physics 11(5): 1913-1928.
• Wood, C. R., 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.