Current source verification

An important verification for the GDACCS is to investigate whether the source of current variation is only from the collector, as would be expected if the instrument was operating successfully and without additional variation from leakage currents etc. This was demonstrated simply by comparing one minute mean current from the FP and CP whilst

exposed to the atmosphere as in normal operation, and then removing one of the collecting plates. The results of such an experiment can be seen in Figure 4.28. As expected, a strong linear relationship exists between the FP and CP currents as both experience the same JD and JC (at least proportionally) and are therefore subject to the

same variation inherent in both these components. However, on removal of the FP collecting plate (leaving just the support and electronics box as in Figure 4.27a) it can be seen that the FP current output is nearly uniform, with CP variation unaffected.

0.0 0.5 1.0 1.5 2.0 2.5 -0.50 0.00 0.50 1.00 1.50 2.00 ICP (pA) I F P ( p A ) Normal FP removed

Figure 4.28 Results showing a strong linear relationship between one minute mean currents measured from the FP and CP collectors (green circles). The lack of similar variation once the FP collector is removed from the support is also shown (red crosses).

It is of particular note that although uniform, the FP current output was not zero once the FP collecting plate was removed, neither is the intercept zero when FP and CP currents are plotted in a xy scatter plot like Figure 4.28 (green circles). This implies a constant offset (as close linearity is conserved and no appreciable trend in FP is seen once the collector is removed) is present in the GDACCS. As more data was collected, it was found that this offset remained characteristic but sometimes with strong bimodal tendencies; often remaining constant for several hours before swapping quickly between one value and another, usually around sunset or sunrise. Reasons for this bimodal offset are unclear, but may be due to a form of leakage within the circuit, or a sharp change in JT,

these times (Oluwafemi et. al., 1975). This pattern of GDACCS collecting plate offset is easily recognisable in the data and can be corrected for during data processing by finding the y-intercept of FP vs CP (as seen in Figure 4.28) and subtracting this from the FP values. It is this corrected value of FP that is used to find JC (in pAm-2), such that:

0.136

I

I

y

J

=

−

−

Where y is the y-intercept (in pA) of FP vs CP (i.e. the predicted value of FP when CP=0 using a least squares line of best fit). Equation (4.13) still holds for JD determination as this

calculation is independent of any intercept.

A slight offset of similar magnitude can also be seen in some of the AECP data, but is less noticeable due to the larger current values. Another possible source of near-constant offset are contact potentials5 where two pieces of metal are joined. These have been minimised by using the same type of relatively inert metal (stainless steel) throughout the design, however it is possible that contact potentials exist in the instrument nonetheless.

The removal of this offset is justified by the fundamental assumption that if JC and JD were

zero, it would be expected that current from both FP and CP should also be zero, if equations (4.11) and (4.13) are to be satisfied. As this offset is only a net offset between the FP and CP, the relative contribution from FP and CP is not known during routine operation, so no absolute correction can be applied to individual FP and CP measurements. As it is only the difference in FP and CP current that is required for JC

retrieval, removal of this net offset allows this component to be determined.

The likely uncertainty in JC determination by the GDACCS arises from the uncertainty in

current measurement by the picoammeters and calculation of the y-intercept. The picoammeter uncertainty is considered as ±0.04pA (in accordance with the AECP as both use the same type of picoammeter). The uncertainty in y-intercept determination is dependent on the specific dataset, although using the 0.009pA standard error for the intercept seen in Figure 4.28 (green circles) this component is likely to be small. In determining JC these three uncertainties are combined, and then divided by a factor of

0.136, as seen in equation (4.14). The uncertainty in JC (∆JC) is therefore estimated to be:

5

A contact potential is present when two electrically conductive materials of different electron work functions have been brought into thermal equilibrium with each other, usually through physical contact.

2 2 2 -2 0.04 0.04 0.01 0.42pAm 0.136 C J + + ∆ = = (4.15).

The estimated uncertainty of 0.42pAm-2 is the uncertainty of an individual measurement. This initial estimate of uncertainty can be refined by calibration with a reference system (the AECP), which is discussed later in this section.