1. Uncertainties in atmospheric observations
Wenche Aas
EMEP/CCC

2. Sources of uncertainties
Sampling and analytical method
Detection limit
Interference
Instrument drift, calibration
Positive or negative artefact
Sampling procedure
Contamination
Temperature and period for storage
Transport
Representativity.
Local farming (NH3)
Nearby roads (NOx; O3)
Dust (PM, Ca..)
Lab- and field intercomparison
Field inter-comparison; model comparison
Repr. studies, i.e passive sampling. Model comparison

3. Monitoring programme:
Level 1
Main ions in precipitation and in air
heavy metals in precipitations
ozone
PM10 and PM2.5 mass
meteorology
at ca 125 sites
Level 2, supersite (joint EMEP/GAW)
POPs
Heavy metals in air and aerosols
VOC
EC/OC, OC speciation
Mineral Dust
PM speciation incl. gas particle ratio
+ all level 1 activities
15-20 sites
Both levels are mandatory by all Parties

4. Data quality objectives
Acidifying and eutrophying compounds
15-25% uncertainty in annual average (10-15% for indiv meas.)
Heavy metals
30% uncertainty in annual average (15-25% for indiv meas.)
40 % uncertainty for As, Cd, Ni in the EU 4th DD (70% in dep)
50% uncertainty for Hg (total gas) in the EU 4th DD
POPs (not defined in EMEP)
50% uncertainty for PAH in the EU 4th DD (70% in dep)
PM (not defined in EMEP)
25% accuracy in continuous measurements EU 1st DD
Photooxidants (not defined in EMEP)
: 15% accuracy in continuous O3, NOx measurements, EU 3rd DD

5. Lab intercomparisons annually, 2005
Bias: RB %
Spread: 2RSD %

6. WMO
ICP

7. SO2 field intercomparison
Preila (LT) using filterpack
Zarra (ES) , abs (H202) and monitor
TCM ain Germany (historic data) at DE09 (left and DE03 (right)

8. SO2. UV fluoresence monitor, interference

9. NO2. Chemiluminisence (Mo converter)
Not selective for NO2

10. QA flag Lab flag Green: Bias < 10% Spread <20% (S,N) Blue:
Field flag
Green:
Bias < 10%
Spread <25%
Blue:
Bias < 50%
Spread <50%

11. QA Flag for main ions in 2005 In precip In air
Analysis in lab are in general better than 20% (both in air and precip)
Total uncertainty (field intercomp):
about half the measurements is better than 25%
rest better than 50%

12. Lab intercomparison of HM annually
Average per cent error, 2005

13. Hg intercomparison at DE02 in 2006
Tot Hg(g) in air
Tot Hg in precip

14. Heavy metal deposition, CEN WG20
Birkenes, comparing wet only (analysed at UBA) and bulk (NILU)

15. Parallell wet only, CEN WG 20
Relative SD in deposition measurements:
From To Cd 9%
18% Pb 7%
15% As 6%
19% Ni
11%
20%

16. POP lab intercomparison,2002

17. Artefact in gas/particle for N
IT01, Jan 2007
Underestimation of N
Artefact free measurements using denuders only done at Montelibretti
IT01, June 2007

18. Estimates of the positive artefact of OC in PM10 and PM2
Estimates of the positive artefact of OC in PM10 and PM2.5/PM1 -June 2006
QBQ-approach

19. Measurement and model intercomparison?? ES NO

20. Uncertainties in trends
SO2
SO4 in air

21. Representativity, NO2
ES07
Comparing EMEP model and obs. in light of population density
IT01
NL91
BE32
AT02

22. Conclusions Many factors influence uncertainty in measurements
Methodology, sampling procedure, Representativity
Need to distinguish between uncertainty in one data point, in averages and trends; and distinguish between bias and spread
In general, the measurements are within DQO if reference methods are used and the site is representative, but there are exceptions
Need to have better control of artefacts, especially nitrogen gas/particle and EC/OC
More intercomparison of other species than main comp and HM are needed

23. Data quality objectives
Acidifying and eutrophying compounds
15-25% uncertainty in annual average (10-15% for indiv meas.)
Heavy metals
30% uncertainty in annual average (15-25% for indiv meas.)
40 % uncertainty for As, Cd, Ni in the EU 4th DD (70% in dep)
50% uncertainty for Hg (total gas) in the EU 4th DD
POPs (not defined in EMEP)
50% uncertainty for PAH in the EU 4th DD (70% in dep)
PM (not defined in EMEP)
25% accuracy in continuous measurements EU 1st DD
Photooxidants (not defined in EMEP)
: 15% accuracy in continuous O3, NOx measurements, EU 3rd DD