Hydrogen sulphide (H2S) analyser calibration issue

We shared the adjusted data with the UK Health Security Agency (UKHSA) so that it could provide you with health advice. UKHSA has produced a report, which you can read here.

The adjustment method we developed involves using the historic sulphur dioxide (SO2) calibration slopes as a surrogate for the hydrogen sulphide (H2S) slopes (which are not available). The SO2 calibration slopes are not available for all our instruments in the MMFs for the same period. We were not recording SO2 calibration slopes routinely in March 2021.

This means that we are unable to apply the adjustment method to specific data points where there is no SO2 calibration slope available, and it is not possible to state what level of uncertainty applies.

Only the H2S data was affected by the calibration issue. It is still possible to infer useful information from this data despite the undefined levels of uncertainty in relation to absolute values. For example, interaction of parameters with meteorological conditions or relative relationships with other parameters. This unadjusted historic H2S data should be identified as containing undefined levels of uncertainty in relation to absolute values.

Good Practice Guide 11 A beginner's guide to uncertainty of measurement (GPG11) written by The National Physical Laboratory is a short introduction to the basic concepts of uncertainty, which apply to all scientific data.

GPG11 states, “It is important not to confuse the terms ‘error’ and ‘uncertainty’. Error is the difference between the measured value and the ‘true value’ of the thing being measured. Uncertainty is a quantification of the doubt about the measurement result.

Whenever possible we try to correct for any known errors: for example, by applying corrections from calibration certificates. But any error whose value we do not know is a source of uncertainty”.

We have not previously referred to the level of uncertainty associated with our monitoring data. However, that does not mean that there was no uncertainty before we identified the hydrogen sulphide (H2S) analyser calibration issue. Uncertainty is not specific to Walleys Quarry, it is in all scientific data.

As part of this data adjustment process, we have, following advice from the peer review group, given specific consideration to better understanding the uncertainties of the adjusted data and created an uncertainty budget which helps to understand the level of uncertainty. You can find the uncertainty budget at annex B of the peer review report, which is available here.

Following a recommendation from the peer review group, we are now considering how we clarify uncertainty in future measurements in investigative atmospheric air monitoring.

The level of uncertainty is comparable to other similar air quality monitoring done in the UK, for example on the Automatic Urban & Rural Network (AURN). You can find more information about the AURN here.

We have not previously shared the level of uncertainty on hydrogen sulphide (H2S) or any other parameters. A degree of uncertainty is associated with all scientific measurements. We explained the factors that affect uncertainty in our uncertainty budget, which is available at annex B of the peer review report, which is available here. The adjusted data includes additional uncertainties because the data was adjusted retrospectively.

Only the H2S data was affected by the calibration issue. Following a recommendation from the peer review group we are now considering how the Environment Agency describes uncertainty in the data for parameters we measure during investigative atmospheric air monitoring.

The independent peer review was led by Defra’s Chief Scientific Adviser. Whilst we originally hoped the group would complete its work by the end of January 2024, there was a delay to publication of the report as the group had supplementary questions which needed to be answered before it could share its findings.

In order to preserve the independence of the peer review group, the Environment Agency was not a member and had no control over the outcome of the review, or the timescales involved.

In our update on 16 November 2023, we told you that we had engaged an independent third party accredited by the United Kingdom Accreditation Service (UKAS), to audit our MMFs in the vicinity of Walleys Quarry. This audit will take place at the end of August 2024.