1. Inter-Laboratory Comparison Exercise CPC CALIBRATION
CALIBRATION AEROSOL AND LABORATORY SETUP
Key Findings
PMP Meeting
Alexander Terres

2. Round Robin: Status All measurements and evaluation finished
Report circulated to all participants for final comments
Transfer of findings to CPC calibration sub-group
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

3. Key Findings: Aerosol Generator
Soot aerosol provided lower counting efficiencies than emery oil at 23nm (29% vs 49%) and at 41nm (78% vs. 91%)
Soot-like generators offered a low uncertainty, even without standardization  Inter-laboratory standard deviation 2% - 3% for in-house CAST and Palas at 41nm and 70nm
Inherently, soot has more degrees of freedom than electrospray + emery oil  Required: Proven setup, regular checks and standardization (e.g. fuel/air ratio, size distribution)
Thermal treatment necessary for stability and universal application (mandatory for PCRF and PN-PEMS)
Transfer function from soot-like to emery oil is possible (but was not part of the RR)
To be checked: Can the CPC meet D50 and D90 with soot by changing temperatures?
Using emery oil is a trade-off: Low uncertainty from a purpose-specific generator (only for CPC),
but high uncertainty from overall procedure with various generators (Emery oil + soot + NaCl + …)
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

4. Key Findings: CPC/Reference Repeatability
Within 1 Laboratory
Same reference CPC vs. same electrometer
1 year, 40 calibrations
Standard deviation/range of valid measurements:
@41nm: 1.5 / 90%-96%
@70nm: 1.3 / 90%-95%
Without calibration: 0.7 – 1.2
Blue/green: before/after calibration
 variability within 1 laboratory
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

5. Key Findings: Circulated CPC
Notable deterioration between starting measurement and final measurement at manufacturer (w/ emery oil)
At 23nm: 51%  46%
At 41nm: 93%  88%
At 55 nm: 96% 93%
Sample flow was reduced but within tolerance (efficiency is flow corrected)
In addition to laboratory facilities uncertainty,
Circulated CPC seems to be a significant source of uncertainty between labs
Normalization to cancel out reference counter did not improve comparability between labs
CPC stability between labs hard to estimate (see above). Influence of transport, drying and contamination possible
Emery oil calibrated CPCs require recalibration to meet legal targets at 23/41nm for soot-aerosol  validate!
(not part of RR) Comparability between CPC models better when KF is always applied to calibration at 23/41nm  Some CPC models internally apply a KF all the time  Standardization necessary!

6. Key Findings: Procedure
Plateau uncertainty:
At 70 nm the CPC did not reach the plateau with soot (2-3 percentage points less)
At 70 nm the std. deviation was 2-3 percentage points w/ miniCAST, PALAS (range: 90%-96%),
Doubly charged correction at this size was 3% (Lab A) to 7% (Lab B) when using electrometer
Plateau uncertainty is best indicator for improvement in calibration (biggest impact in the field – PMP use!)
Implementation of ISO procedure + reference aerosol
Open questions at 23nm where uncertainty is largest (not investigated in this RR):
DMA calibration
Thermal stability, soot morphology (miniCAST) (measurements from one lab showed an influence)
CPC drift, contamination

7. Key Findings: Procedure/Documentation
Calibration results should be shown in the form: ηCPC = 95% ±3%
documentation of the uncertainty budget
Example in ISO for calibration labs
CPCs can deteriorate during one calibration interval (1 year)
Important: quality check of calibration setup with shorter frequency (1-2 months)
WLTP regulation includes these checks for test CPCs
No calibration for the aerosol generator available
Regular documentation of performance + operation points necessary
Needed for complete traceability of PN calibration
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

8. Key Findings: 10nm
Only few measurements with 10nm Round Robin CPC (due to lack of suitable reference and/or lack of time)
Impression: uncertainty at 10nm/15nm similar or better than engine exhaust CPC at 23nm
Material influence at 10nm/15nm: seems smaller than for 23nm; few data
 General points, calibration:
Sub-10nm reference device needed: electrometer, ultrafine CPC (full flow!)
Huge impact of diffusion losses:
Residence times must be the same, mathematical correction significantly increases the uncertainty and can manipulate the results
Generator:
miniCAST able to produce 10nm particles, composition not investigated  concentration /cm³
Thermal stability at 10/15nm needs to be demonstrated
DMA:
Long-DMA suitable for 10/15nm ?
Traceable DMA calibration unclear (typically extrapolation from 70nm/100nm PSL)

9. Key Findings: Conclusions
Generator: soot-like provides low level of uncertainty; still more degrees of freedom than emery oil Reference Counter: influence relatively small, traceability needed, flow correction a must CPC: deterioration during 1 year Procedure: Implement common procedure following ISO generator for PMP Documentation: for calibration lab – provide uncertainty budget + regular quality checks (e.g. monthly) 10nm: Soot generator needs further investigation, e.g. properties of CAST aerosol, concentration with PALAS DMA calibration needs further investigation Aerosol influence might be different than for 23nm CPCs
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

10. Key Findings: Outlook
Overall PN measurements uncertainty will be improved significantly if:
A single standard aerosol is defined
It is used for both stationary PMP-compliant devices and mobile PN-PEMS
An agreement is made on K-factor application
Traceability and ISO are implemented by all users
These topics should be covered by calibration procedures for future post Euro 6 EU legislation.
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |

11. Thank you for your attention
Terres: CPC Calibration Round Robin | Key Findings, 47th PMP Meeting |