|Most of the comments from the initial review of this manuscript were related to the need for additional detail regarding the quality control procedures for the MARGA instrument. The authors have revised the manuscript to address some but not all of the comments. In their response to the reviewer comments, the authors indicate that a paper focusing on the QA/QC procedures for the MARGA is forthcoming. Ideally, such a QA/QC assessment and publication would have preceded publication of the long-term monitoring dataset that is this subject of the current manuscript. This reviewer can appreciate that such timing of related manuscripts is not always possible. However, the current manuscript must be able to stand on its own merit from a QA/QC standpoint. In that regard, I have pointed out below specific information contained in the responses to the original review comments that should and can be easily incorporated into the manuscript. These suggestions amount to a very minor revision but will make the paper stronger without detracting from the forthcoming methods paper. I recommend publication subject to treatment of the following specific comments. |
Reviewer 1 comment: Is there any estimation of detection limits (or quantitation limits) or/and measurement uncertainties available?
Author Response: Please refer to QA/QC manuscript text on treatment of detection limits for the presented period.The calculated DLs have been reported to UKAir (http://uk-air.defra.gov.uk/data/) on a monthly basis from 2012 and only 1.4% of potential data were filled with ½ DL. From 2015, the protocol for calculating the DL will using 3σ of the measured concentration of low concentration external standard. It is expected that with the finalised protocol the reported DLs will be smaller and closer to the below). No uncertainty analysis has been carried out yet but it is planned to be addressed in a future manuscript on the QA/QC of the MARGA.
Reviewer response: The author’s statement “The calculated DLs have been reported to UKAir (http://uk-air.defra.gov.uk/data/) on a monthly basis from 2012 and only 1.4% of potential data were filled with ½ DL.” is important and should be incorporated into the new “Quality analysis and quality assurance” section of the manuscript.
Reviewer 2 comment: How often were external liquid standards analyzed?
Author Response: Only on small number of occasions have external solutions been analysed but it was not until 2012, following the instrument upgrade, could external standards be successfully run. Due to other operational issues this is only been implemented as a protocol on a quarterly basis in 2015, using certified standards of the major anions and cations. External standards however have occasionally been used to confirm the peaks on chromatograms.
Reviewer response: The use of external standards to periodically confirm peaks is important and should be incorporated into the new section of the manuscript “Quality analysis and quality assurance”
Reviewer 2 comment: How were the LiBr standards prepared and were they independently checked on another analytical system?
Author Response: The LiBr stock is prepared in house. The stock LiBr is used to produce a working solution for the instrument. Since 2012, where protocols were implemented an accredited laboratory has independently analysed both the stock and the working solutions on a quarterly basis.
Reviewer response: Please include a statement in the new QA/QC section of the manuscript regarding the quality of the in-house LiBr standards based on the independent laboratory analysis.
Reviewer 2 comment: How were potential biases between the two sample boxes evaluated? How often was the sample flow rate measured independently of the mass flow control system (i.e., at the inlet with a calibrated flow meter)?
Author Response: Following the upgrade of the instrument in November 2011, the instrument boxes sample flow rate was measured by a calibrated mass flow controller and monthly 3 point calibrations were carried out on both MFCs. A summary description has been included in the text of the manuscript.
Reviewer response: I cannot find this summary description in the text. The authors should at least note in the new QA/QC section that mass flow controllers were calibrated monthly. How were calibrations conducted? This does not need to be a lengthy description but a couple of sentences would be helpful.
Reviewer 2 comment: To what extent were the independently measured flows consistent with the flow reported by the mass flow controller?
Author Response: Following the upgrade, due to greater control of the MFCs the air flows were found to compare well with independent flow meters. Only on occasions did they differ after denuders or SJACs had been either moved or cleaned. As a result it is now procedure to recalibrate flows following any change in the sample boxes.
Reviewer response: How well did they compare? Within a few %? Some quantitative information would be helpful. The last sentence is important and should be added to the QA/QC section.
Reviewer 2 comment: How often were the inlets cleaned?
Author Response: Following the implementation of the protocols in 2012, inlets are replaced quarterly, the PM10 head and PM2.5 cyclone are cleaned too on a quarterly basis. This frequency increases if there is evidence of pollution events and visual dirt in the denuder or SJAC.
Reviewer response: The last sentence is important and should be incorporated into the new QA/QC section.
Reviewer 2 comment: What was the process for reducing the raw hourly data? That is, how were blanks, external standards, and flow rate audits incorporated into the data reduction process?
Author Response: Blanks at present are only used to remove periods of contamination in the system. Due the frequency of flow rate audits, air concentration data is only corrected where there is evidence of drift in the MFC.
Reviewer comment: The last sentence is important and should be incorporated into the new QA/QC section.