NMX-J-666-ANCE-2014 (24/11/2014) PLÁSTICOS CELULARES PARA USOS EN APARATOS

Fluxes were calculated using a spreadsheet developed by Prof. Andy Baird and Dr. Sophie Green for the Defra SP1202 project1_{. The flux calculation was based on a} modified version of Equation 3.2:

ࡲ

=

whereFg is the gaseous flux in mg m-2day-1, ݃௠ (mg) is the mass of the chamber gas (calculated byV x ߩ௚ as in Equation 2) and all other components are as in Equation 2. The field and laboratory data required for flux calculations were the concentrations of CH4(ppm) from the gas samples, the measurements of CO2

1_{For more information see}

http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Compl eted=0&ProjectID=16991

(ppm) concentrations, the temperature and barometric pressure readings from inside the chamber at the start and end of the chamber tests, the surface area covered by the collar and the volume of the chamber headspace. The chamber headspace volume should include any area between the peat surface and the top of the collar protruding from the peat surface, as well as the actual chamber volume. Also required for a flux calculations were values of standard temperature (K) and pressure (kPa) (STP), the volume of one mole of the gas of interest under STP and the molecular mass of the gas of interest. All of this subsequent

information was as per the International Union of Pure and Applied Chemistry.

The spreadsheet calculations work as follows. First, for each CH4sample or CO2 measurement taken in the chamber test, the volume (m3_{) of the gas relative to the} chamber volume was calculated. This volume was then converted to the

equivalent volume (m3_{) of the gas at STP, which was then converted into moles of} gas, and finally into a mass (mg). An ordinary least-squares linear regression was applied to the mass data for each chamber test, which gave a rate for the gas: an increase (positive value) if gas was being lost from the peat to the atmosphere, and a decrease (negative value) if there was gas uptake by the peat, or the vegetation. The regression was applied to mass data expressed over the time of the chamber test, which accounts for the ݀݃௠⁄ part of Equation 3. However, there were݀ݐ criteria that had to be met for this resulting rate to be converted to a flux. The gradient of the rate of change had to be significant, and the r2value of the

regression had to be greater than 0.8. If these two criteria were met, then the final part of Equation 3.3 was applied to the data: 1 ܣ⁄ , where 1 is replaced by the slope coefficient from the regression applied to the mass data. The final result was the mass flux density (ܨ௚) in mg m-2day-1.

The spreadsheet also accounted for zero fluxes. If the difference between the maximum and minimum CH4concentrations sampled from the chamber was lower than 0.3 ppm, then a flux of zero was returned. For CO2measurements, this

threshold concentration change was 1 ppm. If the rate of mass change within the chamber could not be fitted with a significant straight line with r2> 0.8 and did not fit the criteria for a zero flux, the chamber test was rejected and no flux recorded

On three occasions, ebullition events were detected within the results of CH4 chamber tests. The chamber tests in question were from collar A1 on 20/06/2012, collar B1 on 18/07/2012 and collar B5 on 14/08/2012, as shown in Figure 3.8. Ebullition events were identified via the CH4concentration detected in the sample when analysed via GC. The change in CH4concentration during a chamber test was expected to be roughly linear, and so the resulting concentrations were plotted to check that this was the case. The concentrations of any samples that were outside of the expected linear trend were scrutinised to deem if they could be the result of an ebullition event. In these cases, depending on the timing of the ebullition event, the samples before or after were excluded from the flux calculations in order to prevent pre- and post-ebullition concentrations being included in the same flux calculation, as shown in Figure 3.8, where the hollow data points are the excluded ones. Five samples were taken in each chamber test. In collar A1 the ebullition event was detected in the first sample, meaning that this first sample was excluded and the remaining four post-ebullition samples were used to calculate the flux. In collar B1, the ebullition event was detected in the fifth sample, so that sample was excluded and the remaining four pre-ebullition samples were used to calculate the flux. In collar B5, the second sample contained the evidence of the ebullition event, and the concentration in the third sample was lower as mixing of the ebullitive release occurred within the chamber. Therefore, the first and second samples were excluded and the remaining three post-ebullition samples were used in the flux calculation. These two flux calculations using ebullition event data at Site B were then the two highest fluxes recorded throughout the entirety of the fieldwork period

Figure 3.8: Chamber test results where ebullition events occurred