Melanie Williams from the National Physical Laboratory (NPL) outlines the results of a London-based pilot study that led to a new emissions measuring offer unveiled by Astrium to complement the current ‘inventory approach’ models and validate national and international energy saving measures.
The National Physical Laboratory (NPL) is partnering with Astrium Services to deliver a new emissions measurement service to enable countries and cities to better quantify their greenhouse gas (GHG) emissions. This service will use actual measurements of carbon dioxide and methane in the atmosphere, together with a bespoke atmospheric model, to provide a much greater level of accuracy than is available at present.
The service is based on a successful pilot across London. This article takes a preliminary look at the measurements which came from this study. A detailed scientific paper will also be published in due course.
The drive to reduce emissions
Countries have committed to reducing their GHG emissions. National plans are in place to decarbonise the power, transport and energy intensive industrial sectors and citizens are being encouraged to save energy in all aspects of their daily lives. The UK and the EU are at the forefront in introducing measures to incentivise the transition to a low carbon economy. Cities as well as nations are starting to look at the GHG emissions and how they can reduce emissions from traffic and local heating systems.
As countries incentivise their industries and citizens to reduce their GHG emissions, and put in place measures like biomass, nuclear energy, carbon capture and storage, they will want to see real local reductions in greenhouse gas concentrations in the atmosphere.
Enhancing current methods
Emissions from a nation or a city are currently calculated using the ‘inventory approach’. Through this activity, data – such as the number of miles driven by an average person – are multiplied by ‘emissions factors’ which turn that activity data into the tonnes of carbon dioxide equivalent.
The new Emissions Measurement Service will complement and improve the reporting using actual concentration measurements. We are confident it will achieve this as parallels with ‘traditional pollutants’ such as acid gases, show that inventories often underestimate emissions.
The measurements will be taken at permanent elevated sites around the country, from aircraft and eventually use satellite measurement of carbon dioxide too. These measurements will then be fed into the sophisticated model, to be merged with inventories of natural and manmade GHG sources and sinks. This combination of actual measurement with traditional modelling will enable better validation of energy saving and decarbonising measures and determine if they are having the intended effect of reducing actual GHG emissions.
NPL will produce the highest quality calibration gas standards to underpin the quality of the data used in the service. This is important as data quality is critical when looking for small changes over a number of years.
Pilot study in London
During the summer of 2012, Astrium Services and partners NPL, Le Laboratoire des Sciences du Climat et l’Environnement (LSCE) and the National Centre for Earth Observation ran a three month pilot study in London, to measure carbon dioxide, methane and carbon monoxide concentrations at four sites in or near the city. The pilot study set out to predict and solve any practical issues before the launch of the full service in 2013. In addition, the consortium hoped to answer some interesting scientific questions such as: • How do concentrations of carbon dioxide and methane vary between rural, Outer London and London Centre sites? • How do meteorological conditions impact on these measured concentrations? • How do expected concentrations from known sources compare with the measured concentrations for this period?
Gases measured by the London pilot study
Carbon Dioxide – The most important greenhouse gas in terms of tonnes emitted annually. Comes from burning fossil fuel in power generation, road and air transport as well as natural sources (e.g. photosynthesis).
Methane – Emitted from waste management, agriculture, gas distribution, mining, aviation and road traffic. Also produced by natural sources. More potent than carbon dioxide as a greenhouse gas by a factor of 25.
Carbon Monoxide – Produced almost exclusively by incomplete combustion of fossil fuels. Will be used in the Emissions Measurement Service as a marker for manmade sources of GHG emissions.
Four state of the art greenhouse gas sensors were installed at sites in and around London. One was sited to the west of London in the suburbs at Teddington. Two were sited in London at Poplar and Hackney and a fourth was sited at Detling, in the countryside to the east of London (see map below). All sites were elevated at 10m above ground level to avoid the measurements being too dominated by local sources. The sites were chosen to be on an approximate west to east transept across London, using existing air quality measurement infrastructure. The predominant wind direction is from the west.
It is important that the measurements are made to the highest accuracy and so the sensors were calibrated throughout the measurement campaign with synthetic gas standards, produced gravimetrically to have the same isotopic composition as that of naturally occurring carbon dioxide in the atmosphere.
The sensors were first characterised in the laboratory for their linear response and stability, so the uncertainty of the final measurement can be quoted. Measurements and instrument diagnostics were transmitted continuously to the control centre, to ensure that the equipment was operating correctly and the immediate availability of carbon dioxide concentrations.
The measurements started in June and continued until the end of September. The results are presented below in the context of the questions posed at the start.
How do concentrations of carbon dioxide and methane vary between rural, Outer London and London Centre sites? An increase in average concentrations over the measurement period was seen for all species between the rural site at Detling and the Outer London site in Teddington.
The average increase was 7.2 ppm for CO2. There was a further increase in average concentrations between Teddington and the average of the Inner London sites of 1.04 ppm. The red line in the graph shows the baseline CO2 concentration for 2011 for the UK of 395 ppm, according to the UK’s Met Office and DECC (Department of Energy and Climate Change) measurements.
It is difficult to make a comment about the magnitude of concentration increases but they are in line with that seen from other pollutants, e.g. Nitric Oxide (NO) from combustion processes. This increase is due to emissions from local traffic and fossil fuel fired heating.
Impact of meteorological conditions
The variation in the concentrations of CO2, with wind speed and direction are best understood from polar plots, where the colour represents the measured concentration at a specific wind direction and speed. The distance from the centre of the plot corresponds to wind speed, i.e. colours close to the origin are concentrations measured at very low wind speeds, while colours further from the origin represent concentrations measured at higher wind speeds.
The angular position represents the wind direction. The white areas represent wind directions and speeds that did not occur due to the short nature of the measurement campaign (three months).
At Detling, high levels (red and yellow) would be expected in the NW if CO2 from London is the main source detected. Raised concentrations from NNE were seen, which are likely to be from Kingsnorth Power Station.
At Teddington, high levels of CO2 (red and yellow) are expected from the NE if CO2 from London is the main source detected. Raised concentrations in the middle suggest CO2 from local sources at low wind speeds dominate. There is poor dispersion of locally generated CO2 under these conditions.
Raised concentrations in the middle of the plot for Poplar and Hackney suggest CO2 from local sources at low wind speeds dominates the measurements. Higher concentrations from the SE at Poplar are probably from traffic associated with the Blackwall Tunnel.
The results show that concentrations were affected by wind conditions. At times of low wind speed, the concentrations are higher as a result of poorer dispersion of local emissions. Concentrations as high as 520 ppm CO2 are seen at Teddington, Poplar and Hackney compared with the average peak in the daily value of 412ppm to 424ppm. The dominance of local sources is expected as the measurements were taken at the relatively low heights of 10-15m.
Expectation versus measurement
The average variation in measured CO2 concentration during a day is shown for each site below. The shaded area on the y-axis represents the uncertainty in the mean y-value due to the standard deviation of the results over that averaging period, so as to cover 95% of the variation. It is not the overall measurement uncertainty.
The variations of carbon dioxide concentrations over the day largely reflect the normal diurnal cycle. The trial was undertaken over the summer, so photosynthesis from vegetation reduces CO2 concentrations during the daytime. Higher peaks in the morning and evening are seen at Teddington, Hackney and Poplar compared with Detling, which is probably due to the effect of local traffic.
These diurnal variations have been compared with modelled data, from LSCE (Laboratoire des Sciences du Climat et l’Environnement), using the most recent greenhouse gas inventory for London (2010). The comparison is shown for each site below. A direct comparison is difficult to make because inventory data for 2012 are not yet available, as they will only be published in 2014. This comparison does, however, indicate that measured CO2 concentrations over the period are mainly higher than predictions based on the inventories available.
Impact of the pilot
Monitoring and controlling greenhouse gas emissions is crucial at international, national and major city level, to both reduce the level of polluting emissions and to promote the uptake of green, low carbon technologies. This will become even more important as growing populations will make our future cities the primary contribution to manmade global climate change, and countries will need to implement robust strategies to report and reduce their carbon footprints.
The London measurement campaign demonstrated the viability of the GHG Emissions Measurement Service being developed by Astrium Services and its partners, who will increase further their capabilities by participating in similar initiatives in Paris and Rotterdam.
A long term site will be installed within the next few months in the UK, sampling the same species at 100m. Boundary layer and other meteorological measurements will be made at the same site.
Sampling at 100m will provide measurements of CO2 concentrations which are not unduly influenced by local sources, a lesson from the London campaign. When these measurements are combined with modelling, then a comparison with concurrent, UK wide, GHG emissions inventories will be carried out.
The site will also be used to test different calibration protocols using NPL gas standards to maximise the accuracy of the measured CO2 concentrations. In addition, sampling will be carried out at 50m and 10m, to gain information on the concentration gradient of CO2.
This will be followed by the launch of the full Emissions Monitoring Service, for cities and countries that wish to track and verify their greenhouse gas emissions’ reductions over the long term.
For further information on the London pilot or the Emissions Measurement Service see http://event.astrium.eads.net/en-london-pilot/ or contact [email protected] or [email protected]
Published: 27th Nov 2013 in AWE International