Source attribution analysis

The SNAQ instrument ran between 09 November  08 December 2012. The methane GC instrument was not running for the rst week of December.

Figure 4.22 shows the methane concentration time series of both Haddenham and Tacolneston for this period. Equivalent time series for CO2, CO, wind speed, wind direction and boundary layer height are also shown for comparison. Some correlations can be seen in this gure, i.e. a low boundary layer height implies a higher methane concentration.

4.5 Haddenham measurements and modelling case study

20002800

CH4(ppb)

HAD TAC

(A)

300600

CO (ppb)

CO

(B)

350500

CO2(ppm)

CO₂

(C)

01020

Wind Speed (m/s)

Modelled WS Measured WS

(D)

0200

Wind Direction ( ° )

Modelled WD Measured WD

(E)

01500

Date

Boundary Layer (m) Modelled BL

2012−11−09 2012−11−16 2012−11−23 2012−11−30 2012−12−07 (F)

Figure 4.22: Time series showing A) methane concentration at Haddenham and Tacolneston (ppb), B) carbon monoxide measured using the SNAQ node (ppb), C) carbon dioxide measured using the SNAQ node (ppm), D) modelled and measured wind speed (m s^-1), E) modelled and measured wind direction (°), F) boundary layer height from the NAME model (m). All data measured at the Haddenham site or modelled using Haddenham location co-ordinates (with the exception of Tacolneston methane concentration in A).

NAME can be used to help attribute dierent source regions to the measured time series. The rst few days show varied concentrations of methane at both Haddenham and Tacolneston which can be attributed to both boundary layer eects and the air mass history coming from over the UK (Figure 4.23.A).

Chapter 4 Methane measurement analysis

The wind direction then changes so that the air comes from over Europe, and wind speed drops (Figure 4.23.B). This accounts for above baseline methane concentrations observed at both sites from 15 - 19 November 2012. The short lived high levels of methane at Haddenham correlate with nighttime periods and most likely come from local sources. The NAME air history map in Figure 4.23.C shows an increased surface inuence south of Haddenham at the time of the large methane spike observed in the early hours of 21 November 2012. The air history map suggests both local and London sources could contribute to this observation.

Methane concentrations remain low due to a prevailing wind direction coming from over the English Channel or Atlantic Ocean (Figure 4.23.D), with the exception of 23 November where reduced wind speed and a low boundary layer height contribute to this high peak. The low methane concentrations after this period are due to a change in wind direction to come from over the North Sea (Figure 4.23.D) where relatively few sources are seen compared to on land. The

nal above baseline observations of methane seen on 30 November 2012 could be attributed to air coming from north west England where large industrial methane sources are located.

4.5 Haddenham measurements and modelling case study

A)

-10 -5 0

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5055 5055

2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 11/11/2012 10:00

B)

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2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 16/11/2012 10:00

C)

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5055 5055

2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 19/11/2012 00:00

D)

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2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 24/11/2012 00:00

E)

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5055 5055

2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 28/11/2012 01:00

F)

-10 -5 0

-10 -5 0

5055 5055

2.0E-12 2.7E-11 3.6E-10 4.8E-09 6.3E-08 8.4E-07 1.1E-05 1.5E-04

0-100m time integrated particle density / g s m^-3 had 30/11/2012 20:00

Figure 4.23: NAME air history maps with source release from Haddenham between 11-30 November 2012. Maps show the time integrated density of the surface inuence (bottom 100 m) for a one hour source release released at A) 11-11-2012 10:00 B) 16-11-2012 10:00 C) 19-11-2012 00:00 D) 24-11-2012 00:00 E) 28-11-2012 01:00 F) 30-11-2012 20:00.

Relationships between methane and carbon dioxide or carbon monoxide can give an indication of the sources of these gases. A positive correlation between methane and carbon monoxide implies a source of incomplete combustion.

Chapter 4 Methane measurement analysis

Anthropogenic sources of incomplete combustion can be found in the UK, for example oil and gas industry plants or domestic gas usage. Methane's relationship with carbon dioxide is much more diverse than with carbon monoxide as it depends on available sources and sinks for both compounds. Various methanogens reduce the amount of carbon dioxide in the surrounding area and produce methane (Chapter 1). Conversely, landlls are known sources of carbon dioxide as well as methane (60 % methane to 40 % carbon dioxide) due to the decomposition of organic matter (Hegde et al., 2003). This suggests that a positive correlation between these two gases could be a landll source signal, as well as the other incomplete combustion sources mentioned.

Figure 4.24 show correlation plots for methane and carbon monoxide split into dierent wind directions (determined by NAME). Modelled boundary layer heights are shown on the z-axis. The prevailing wind for this period (south west) is reected in the correlation plots. Active landlls around Haddenham can be found to the west and (more closely) to the south. These are on the outskirts of Huntingdon and Cambridge, respectively. Methane sources from landlls and incomplete combustion would be expected from both these quadrants.

Figure 4.24 shows that northerly winds seem to be associated with fewer sources of methane for this period, while carbon monoxide concentrations rise to over double their baseline value. Observations of carbon monoxide without methane could imply shipping emissions from the North Sea (Endresen, 2003).

The other wind direction plots show a positive correlation, particularly from the west (C) and the south (D), with the west appearing more linear than the south.

A negative relationship between these concentrations and boundary layer height can be seen, where higher ppb values are associated with lower heights (< 600 m). This implies that this positive correlation can be partially attributed to dispersion eects, although the time series of carbon monoxide (Figure 4.22.B) shows a variation of only ~50 ppb (from boundary layer maximum to minimum).

Since the methane and carbon monoxide ranges are larger than this diurnal

uctuation, this could be showing an incomplete combustion source of methane.

This correlation can be seen in Figure 4.24.D however many methane outliers can be seen which do not have the expected carbon monoxide concentration for an incomplete combustion source. The equivalent correlation plots for carbon dioxide and methane are not shown here but, in general, show similar correlations for the north and east quadrants. Less dened correlations can be seen for the other two wind directions, however the high methane concentrations observed from the south also show above baseline carbon dioxide concentrations (> 500 ppm). This