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Prof. Duncan Laxen
Associate
Analysis of Bias in
Diffusion Tubes
Introduction
 Nitrogen dioxide diffusion tubes play an
important role in air quality monitoring and
assessment. It has long been known that bias
adjustment factors need to be applied and that
these factors are laboratory specific
 In 2017 the Scottish Government had noted
concerns that bias adjustment factors seemed
to have been falling year on year
 It wanted to know why and whether this was
real, as it could affect reported trends in
concentrations
 AQC, together with Prof. Mat Heal of Edinburgh
University, were commissioned to investigate
Reports available at: www.aqconsultants.co.uk/Resources/Download-Reports.aspx
The Presentation will:
 Set out some of the interesting analyses from the
study
 Summarise the Conclusions
 Show the ‘real’ trends in concentrations 2008-2017
in Scotland (and for 2005-2016 in the UK)
Background
• Early 2000s
 Knew that the performance of diffusion tubes was very
different from one supplier to another
 Understood many of the factors influencing tube
performance in the field – turbulence across mouth of the
tube – chemistry in the tube
 Recognised need for bias adjusting tubes
• 2008
 Guidance published to improve consistency
• 2017
 Concerns in Scotland that trends in bias adjustment were
causing misleading trends in published diffusion tube results
Original Concern
Declining Bias
Adjustment
Factor for
Edinburgh
Scientific
Services Lab
2008-2016
Use of Diffusion Tubes by Scottish
Local Authorities
Laboratory Tube Type
Number of Local Authorities
using the Lab in 2017
Glasgow Scientific Services 20% TEA in Water 13
Edinburgh Scientific Services 50% TEA in Acetone 8
Tayside Scientific Services
a
20% TEA in Water 4 (reduced by one in 2018)
Aberdeen Scientific Services
a
20% TEA in Water 3
Gradko 50% TEA in Acetone 1
Gradko 20% TEA in Water 1
Socotec
b
50% TEA in Acetone
c
1 (increased by one in 2018)
Diffusion Tubes Not Used - 1
Total - 32
Number of Co-Location Sites for Scottish
Labs to Generate Bias Adjustment Factor
Highly
Variable Year
to Year and
small
numbers
Gradko & Socotec
(ESG) more like
30 sites
Result for Individual Co-Location Sites in
Edinburgh
Sites change year
to year
Individual sites no
overall trend (ignore
dashed line)
Literature Review
• Large number of factors can affect bias
 Lab preparation and analysis, wind speed,
temperature, humidity, chemistry, uncertainty in
automatic analysers
• The key factors affecting bias:
 Chemistry within the tubes - will depend
on concentrations of NO2, NOx and O3
 Wind turbulence – across the mouth of
the tube, related to exposure conditions
Bias Adjustment Trends for Scottish Labs
Variation in Scottish
Labs and apparent
downward trend in
national factors
Also shows
different
labs/tubes have
different bias-
adjustment
factors
Bias Adjustment Trends for Gradko and
Socotec Tubes
Labs with many
co-location
results much
more stable and
no downward
trend
Also shows different
labs/tubes have
different bias-
adjustment factors
Marylebone Road – Trends
in Bias
Note different
Labs/Tubes have
different bias
Upward trend in
bias = downward
trend in bias
adjustment
Marylebone Road – Concentrations
NO2 - down
NOx - steady
NO2:NOx - down
London Bloomsbury – Concentrations
NO2 - down
NOx - down
NO2:NOx - steady
Marylebone Road
 There is an increase in local NOx at Marylebone Road
 This leads to a reduction in NO2:NOx
 Previous empirical and modelling shows bias will increase as
NO2 and NOx decrease, due to chemistry in the tube
 Bias-adjustment results at Marylebone Road consistent with
declining NO2 and/or increasing fresh NOx affecting chemistry
in the tube
 i.e. the increasing bias (decreasing bias-adjustment factors) at
Marylebone Road consistent with effects of in-tube chemistry
‘Real’ Trends in Concentrations in Scotland
2008-2017
43 Automatic
sites (mostly
roadside)
‘Real’ Trends in Concentrations in Scotland
Pollutant Trend (%/yr) and Significance
All sites
(n=43)
Road
(n=31)
Rural
(n=6)
Urban
Background
(n=6)
NO2 Concentration -2.33 *** -2.30 *** -3.76 *** -3.59 ***
NOx Concentration -2.25 * -2.35 ** -2.64 * -3.75 ***
NO2:NOx Ratio -0.35 * -0.19 (n/s) -1.98 *** +0.09 (n/s)
Ozone Concentration +0.17 (n/s) - +0.02 (n/s) +0.36 (n/s)
Significant
downward
trend for NO2
~ 23% decline
over 10 years
‘Real’ Trends in NO2
Concentrations in
UK 2005-2016
156 Automatic sites
Area 1 -1.51% per year
Area 2 - n/s
Area 3 -1.61% per year
Note different trends at different sites
Key findings
 Bias adjustment factors still vary from laboratory to laboratory
despite national guidance
 Bias adjustment factors vary even more from site to site
 The overall bias adjustment factor is much more stable when a
large number of co-location results used. Scottish laboratories
have a limited and highly varying number of co-location results
 Automatic monitors show a significant ‘real’ downward trend at
sites across Scotland
Main Conclusion
 “The apparent evidence for a downward trend in the overall
bias adjustment factors for Scottish Laboratories appears to be
mainly an artefact of the changes in the sites that have been
used to derive the overall factor, together with some chemistry-
driven downward trends due to the declining NO2 and NOx
concentrations”
Uncertainty in Diffusion Tube Results and
Trends in Concentrations Reduced if
 Bias Adjustment factors used are:
 based on consistent set of co-location sites;
 based on as large a number of co-location sites as possible;
 derived from laboratories operating with good precision; and
 not changed from local to national factors from year to year
Final Word
 Diffusion tubes still have an important role to play in monitoring
annual mean NO2 concentrations and trends in concentrations
but care must be taken with their use to ensure the most
reliable information is provided
Head Office
23 Coldharbour Road, Bristol
BS6 7JT
Tel: 0117 974 1086
London Office
119 Marylebone Road, London
NW1 5PU
Tel: 020 3873 4780

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Analysis of bias in diffusion tubes - Professor Duncan Laxen

  • 1. Prof. Duncan Laxen Associate Analysis of Bias in Diffusion Tubes
  • 2. Introduction  Nitrogen dioxide diffusion tubes play an important role in air quality monitoring and assessment. It has long been known that bias adjustment factors need to be applied and that these factors are laboratory specific  In 2017 the Scottish Government had noted concerns that bias adjustment factors seemed to have been falling year on year  It wanted to know why and whether this was real, as it could affect reported trends in concentrations  AQC, together with Prof. Mat Heal of Edinburgh University, were commissioned to investigate Reports available at: www.aqconsultants.co.uk/Resources/Download-Reports.aspx
  • 3. The Presentation will:  Set out some of the interesting analyses from the study  Summarise the Conclusions  Show the ‘real’ trends in concentrations 2008-2017 in Scotland (and for 2005-2016 in the UK)
  • 4. Background • Early 2000s  Knew that the performance of diffusion tubes was very different from one supplier to another  Understood many of the factors influencing tube performance in the field – turbulence across mouth of the tube – chemistry in the tube  Recognised need for bias adjusting tubes • 2008  Guidance published to improve consistency • 2017  Concerns in Scotland that trends in bias adjustment were causing misleading trends in published diffusion tube results
  • 5. Original Concern Declining Bias Adjustment Factor for Edinburgh Scientific Services Lab 2008-2016
  • 6. Use of Diffusion Tubes by Scottish Local Authorities Laboratory Tube Type Number of Local Authorities using the Lab in 2017 Glasgow Scientific Services 20% TEA in Water 13 Edinburgh Scientific Services 50% TEA in Acetone 8 Tayside Scientific Services a 20% TEA in Water 4 (reduced by one in 2018) Aberdeen Scientific Services a 20% TEA in Water 3 Gradko 50% TEA in Acetone 1 Gradko 20% TEA in Water 1 Socotec b 50% TEA in Acetone c 1 (increased by one in 2018) Diffusion Tubes Not Used - 1 Total - 32
  • 7. Number of Co-Location Sites for Scottish Labs to Generate Bias Adjustment Factor Highly Variable Year to Year and small numbers Gradko & Socotec (ESG) more like 30 sites
  • 8. Result for Individual Co-Location Sites in Edinburgh Sites change year to year Individual sites no overall trend (ignore dashed line)
  • 9. Literature Review • Large number of factors can affect bias  Lab preparation and analysis, wind speed, temperature, humidity, chemistry, uncertainty in automatic analysers • The key factors affecting bias:  Chemistry within the tubes - will depend on concentrations of NO2, NOx and O3  Wind turbulence – across the mouth of the tube, related to exposure conditions
  • 10. Bias Adjustment Trends for Scottish Labs Variation in Scottish Labs and apparent downward trend in national factors Also shows different labs/tubes have different bias- adjustment factors
  • 11. Bias Adjustment Trends for Gradko and Socotec Tubes Labs with many co-location results much more stable and no downward trend Also shows different labs/tubes have different bias- adjustment factors
  • 12. Marylebone Road – Trends in Bias Note different Labs/Tubes have different bias Upward trend in bias = downward trend in bias adjustment
  • 13. Marylebone Road – Concentrations NO2 - down NOx - steady NO2:NOx - down
  • 14. London Bloomsbury – Concentrations NO2 - down NOx - down NO2:NOx - steady
  • 15. Marylebone Road  There is an increase in local NOx at Marylebone Road  This leads to a reduction in NO2:NOx  Previous empirical and modelling shows bias will increase as NO2 and NOx decrease, due to chemistry in the tube  Bias-adjustment results at Marylebone Road consistent with declining NO2 and/or increasing fresh NOx affecting chemistry in the tube  i.e. the increasing bias (decreasing bias-adjustment factors) at Marylebone Road consistent with effects of in-tube chemistry
  • 16. ‘Real’ Trends in Concentrations in Scotland 2008-2017 43 Automatic sites (mostly roadside)
  • 17. ‘Real’ Trends in Concentrations in Scotland Pollutant Trend (%/yr) and Significance All sites (n=43) Road (n=31) Rural (n=6) Urban Background (n=6) NO2 Concentration -2.33 *** -2.30 *** -3.76 *** -3.59 *** NOx Concentration -2.25 * -2.35 ** -2.64 * -3.75 *** NO2:NOx Ratio -0.35 * -0.19 (n/s) -1.98 *** +0.09 (n/s) Ozone Concentration +0.17 (n/s) - +0.02 (n/s) +0.36 (n/s) Significant downward trend for NO2 ~ 23% decline over 10 years
  • 18. ‘Real’ Trends in NO2 Concentrations in UK 2005-2016 156 Automatic sites Area 1 -1.51% per year Area 2 - n/s Area 3 -1.61% per year Note different trends at different sites
  • 19. Key findings  Bias adjustment factors still vary from laboratory to laboratory despite national guidance  Bias adjustment factors vary even more from site to site  The overall bias adjustment factor is much more stable when a large number of co-location results used. Scottish laboratories have a limited and highly varying number of co-location results  Automatic monitors show a significant ‘real’ downward trend at sites across Scotland
  • 20. Main Conclusion  “The apparent evidence for a downward trend in the overall bias adjustment factors for Scottish Laboratories appears to be mainly an artefact of the changes in the sites that have been used to derive the overall factor, together with some chemistry- driven downward trends due to the declining NO2 and NOx concentrations”
  • 21. Uncertainty in Diffusion Tube Results and Trends in Concentrations Reduced if  Bias Adjustment factors used are:  based on consistent set of co-location sites;  based on as large a number of co-location sites as possible;  derived from laboratories operating with good precision; and  not changed from local to national factors from year to year
  • 22. Final Word  Diffusion tubes still have an important role to play in monitoring annual mean NO2 concentrations and trends in concentrations but care must be taken with their use to ensure the most reliable information is provided
  • 23. Head Office 23 Coldharbour Road, Bristol BS6 7JT Tel: 0117 974 1086 London Office 119 Marylebone Road, London NW1 5PU Tel: 020 3873 4780