The NAME model produces a time-integrated particle density (g s m-3) map which represents the surface in uence (bottom 100 m agl) of air experienced at a particular measurement site (as explained in Section 3.2). When this metric is multiplied by the gridded surface area (m2) and divided by the NAME particles' mass (g) from the duration of release, a dilution metric with the units m s-1can be calculated. This can be used with a de ned scenario of emissions, for example the NAEI, to create a modelled, or pseudo-, concentration. NAME air history maps are available for each hour, so an hourly time series of pseudo-observations can be created and directly compared with the measured concentrations. The NAEI (g s-1 m2) is regridded to the NAME dilution map spatial resolution (1.5 x 1.5 km) and the two products multiplied to produce these pseudo-concentration time series. Figure 4.17 shows these time series for all four sites throughout August 2013. A baseline must be added to the pseudo-observations to represent methane sourced from outside the domain of interest. The baseline used was calculated for the inversion system. Its calculation method is described in Section 5.10.
4.4 Pseudo-observations
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Date CH4 (ppb)
Haddenham baseline HAD (pseudo) HAD (measured)
2013−08−01 2013−08−08 2013−08−15 2013−08−22 2013−08−29
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Date CH4 (ppb)
Tacolneston Baseline TAC (pseudo) TAC (measured)
2013−08−01 2013−08−08 2013−08−15 2013−08−22 2013−08−29
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Date CH4 (ppb)
Weybourne Baseline WEY (pseudo) WEY (measured)
2013−08−01 2013−08−08 2013−08−15 2013−08−22 2013−08−29
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Date CH4 (ppb)
Tilney Baseline TIL (pseudo) TIL (measured)
2013−08−01 2013−08−08 2013−08−15 2013−08−22 2013−08−29
Figure 4.17: Time series of measured and pseudo-concentrations of methane at all four sites for August 2013. Pseudo-observations calculated by multiplying the NAME dilution matrix (s m-1) by the NAEI methane emission eld (g s m-3).
Figure 4.17 shows the variations between the modelled and measured observations correlate well. Sharp peaks seen in the measured concentrations are often also seen in the pseudo-observations, however the dierences in these magnitudes can vary signi cantly. Several potential reasons for these dierences exist. The NAEI is available as an annual total for a speci c year (2012). To produce hourly
Chapter 4 Methane measurement analysis
values, these totals are averaged down to the desired hourly period. The NAEI therefore assumes a constant emission from every source throughout the de ned time period.
The NAEI publishes ten source sector emission maps for methane which include agriculture, waste and oshore sources. The oshore source sector, rather confusingly, incorporates emissions from the natural gas pipelines situated in-land. These maps can be used to calculate pseudo-concentrations speci c to the source sector. Figure 4.18 shows a fractional time series of all sectors for the same time period as Figure 4.17. When large peaks of methane are measured the fractional contribution of the waste sector (primarily land ll emissions) also increases. These sources are known to be extremely variable over time (Scheutz and Kjeldsen, 2004). This is exempli ed further at the Haddenham site, which is situated close to several land lls and experiences variable methane concentrations over relatively short time periods. Time varying sources will cause aggregation and systematic errors within the inversion as InTEM solves assuming that emissions are static over the observation time period (1 year is most commonly used within this thesis). These can be reduced by increasing the spatial and temporal resolutions to a ner scale.
Another possible reason for the observed dierences between measured and pseudo-observations could be due to the NAEI being year speci c. Dierent practices adopted by the local land lls could have been implemented since 2012, and hence not accounted for in the NAEI, although this is unlikely given measurements commenced only six months after the NAEI was published. The NAEI methodology for the inventory compilation was renewed for the 2012 inventory, and the emissions dier with the previous inventory assembled for the year 2009. The major dierence between the two inventories was the waste sector, which saw more pinpointed emission estimates in the 2012 inventory. Methane capture techniques across many land lls were also incorporated into the 2012 inventory (European Environment Agency, 2012). Figure 4.19 compares the two NAEI inventories (2009 and 2012) with the measured observations at Haddenham.
The other three sites were analysed but are not shown here due to there being less obvious dierences. The 2009 NAEI is less able to create the sharp peaks of methane seen in the measured time series, and seems to underestimate the emission sources. Additionally, there are instances where peaks are not observed in the 2009 inventory (23 and 29 August 2013). This shows additional sources have been added to the 2012 inventory. Changes to the waste emissions are more prevalent in the Haddenham time series due to its local source in uences. A less
4.4 Pseudo-observations
striking dierence is shown for the other sites' time series as previous analysis showed they were less in uenced by the waste sector (Figure 2.4).
0.00 0.25 0.50 0.75 1.00
Aug 05 Aug 12 Aug 19 Aug 26 Sep 02
Date
Fraction
variable had.agri.p had.domcom.p had.energyprod.p
had.indcom.p had.indproc.p had.nature.p
had.offshore.p had.othertrans.p had.roadtrans.p
had.waste.p
0 25 50 75 100
Aug 05 Aug 12 Aug 19 Aug 26 Sep 02
Date
ppb
Figure 4.18: Time series of the fractional contribution of each NAEI methane source sector as pseudo-observations for Haddenham during August 2013 (top), and the equivalent pseudo-observations (ppb) with no baseline included (bottom).
Source sectors are: agriculture, domestic combustion, energy production, industrial combustion, industrial processes, nature, oshore, other transport, road transport and waste.
There are some periods in Figure 4.17 where peaks are seen in the measurements but not in the pseudo-observations. For example, there is a period from 23 - 26 August 2013 where the Tacolneston and Weybourne sites show elevated methane levels, however these do not appear in either of the pseudo-observations, nor in the other two measurement sites. If all sites have seen this increase then the elevated concentrations could be due to large sources from outside of the spatial domain. Analysis of the NAME air history maps show air to have come from the north but then from the east (Europe) when not over land. This could imply a European source that, for some reason, is not being picked up at Haddenham or Tilney, or it could be a temporary source unaccounted for in the NAEI.
Chapter 4 Methane measurement analysis
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Haddenham
Date CH4(ppb)
baseline
HAD (pseudo 2012) HAD (measured) HAD (pseudo 2009)
2013−08−01 2013−08−15 2013−08−29
Figure 4.19: Time series of measured and pseudo methane concentrations at Haddenham for August 2013 (ppb). Pseudo-observations for the 2009 and 2012 NAEI are both shown. 23 and 29 August 2013 show instances where additional sources have been added to the 2012 inventory as peaks are not observed in the 2009 inventory.
Scatter plots of the pseudo- and measured observations for each individual site can be seen in Figure 4.20. All sites show a shallower gradient between the two variables than compared to the 1:1 line. The pseudo-observations as they never replicate the large peaks seen in the measurements. Tacolneston has the greatest correlation with an R2value of 0.42. This site measures at a higher altitude than the other three sites (50 - 100 m rather than 15 - 25 m) and hence is less in uenced by local point sources. This could mean that Tacolneston is experiencing a less biased `view' of the East Anglian emissions. Further discussions regarding this and the in uence of the dierent sites in the inversion system can be found in Chapter 6.