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Leibniz Institute for

Tropospheric ResearchProblems and Solutions Specific to Ambient Aerosol

Measurements

Problems and Solutions Specific to Ambient Aerosol

Measurements

Alfred Wiedensohler

Leibniz Institute for Tropospheric Research

WMO-GAW World Calibration Centre for Aerosol Physics

Metrology of Airborne Nanoparticles, Standardisation and Applications (MANSA)

NPL, June 8-9, 2010

Background



Measurements

- Long-term observations of physical aerosol properties became common

due to an increase interest in regional climate and air quality

- Especially, measurements of number size distributions, light scattering

and light absorption (black carbon) are of great interest

- An important goal of the community is to obtain comparable data sets

- Measurements have to be thus performed under the comparable

conditions

- The data have to be traceable and its treatment transparent and

reproducible

- The goal is now to standardize aerosol sampling, measurement

guidelines and data handling

EUSAAR



Measurements

- The EU-infrastructure project EUSSAR is funded since 2006

- The aim of EUSSAR was to establish 20 aerosol supersites for continuous observations of key aerosol properties in Europe

- These properties are i) number size distributions, ii) light scattering, iii) light absorption, and iv) organic/elemental carbon

- One main focus is to standardize these measurements and to ensure

quality assurance and control

- The quality assurance includes comparable sampling protocols and data

analysis

- The quality control includes comparisons against “reference instruments”and stations audits

- The data delivered to the data base EBAS should be transparent and traceable

General Sampling Considerations



Measurements

- Sample air should be brought into the laboratory through a vertical stack

with an aerosol inlet that is well above ground level

- Because gas analysers have other requirements, a dedicated inlet stack is

required for the aerosol samples

- The size of the entrance configuration should be well designed to provide a

high inlet sampling efficiency for aerosol particles over a wide range of

wind speeds

- The sample flow should be laminar in the sample tube to avoid additional

losses of small particles due to diffusion and turbulent inertial deposition.

The ideal flow should have a Reynolds number of about 2000

- The inlet material should be weather- and sunlight-resistant, conductive,

and non-corrosive such as stainless-steel

Water Take-Up



Measurements

- Atmospheric aerosol particles can take up a significant amount of water at

relative humidities below saturation called hygroscopic growth

- This hygroscopic growth influences physical aerosol measurements

- Increase of particles size – factor ~1.5 at 80% RH

- Increase in light scattering – factor ~2 at 80% RH

- Increase of the noise level of filter-based light absorption measurements

- The sampling philosophy is thus to determine the physical aerosol properties

at a relative humidity lower than 40-50% - dry state

Humidity Control



Measurements

How to reach the RH requirement?

- In case the room temperature is higher than 20°C, no dryer is needed if the

ambient dew point temperature never exceeds 10°C

- Aerosol dryers are needed for each instrument if the dew point temperature

is always below the room temperature

- The whole inlet flow have to be dried before entering the room in case that

the dew point temperature is occasionally above the room temperature

- There are basically FOUR possibilities to dry the aerosol

- Aerosol diffusion dryer

- Membrane dryer

- Drying by dilution with dry particle-free air

- Drying by heating (careful!)



Measurements

Aerosol diffusion dryer

- A diffusion dryer works on the base of silica

- Advantage: no dry air is needed

- Disadvantage: the silica has to be frequently changed

Membrane dryer

- A membrane dryer (e.g. Nafion) is based on the principal that water vapor is

transported through a membrane surrounded by a counter flow with low

humidity

- Advantage: no frequent maintenance

- Disadvantage: dry air is needed

Dilution

- The aerosol is dried by dilution with dry particle-free air

- The dilution ratio has to be exactly known

Heating

- Heating is not recommended to avoid the evaporation of semi-volatile material



Measurements

Sketch of an automated

aerosol diffusion dryer for long-

term operation

Diffusion Dryer

Nano-Particle Measurements



Measurements

- In the following, the focus will be on the characterization of Nanometer

particles (number size distribution measurements)

- The knowledge of particle number size distribution is essential to better

understand the aerosol in terms of climate effects and air quality

- Mobility size spectrometers are widely used to determine the number size

distribution of Nanometer particles

- To use mobility size spectrometers following calibrations are mandatory

- Sizing calibration (latex calibration)

- Calibration of the Condensation Particle Counter

- Flow calibrations by an independent volume flow meter

Standardization for Long-Term Observations (EUSAAR)



Measurements

Additional requirements for long term measurements

- The hard- and software set-up has to be modified according a

standardized set-up for mobility size spectrometers (next slide)

- Aerosol and sheath air volume flow rates have to be measured and logged

- Dryers should be mounted in the aerosol and sheath air flow

- Relative humidity (below 40 %) and temperature in the aerosol and sheath

air flows have to be measured and logged

- Absolute pressure in the system has to be measured and logged

- The inversion routines and correction functions for particle losses should

be transparent and traceable

- Recommendation: These standardizations should be also adopted for the

commercial mobility size spectrometers

Standard SMPS Set-Up (EUSAAR)



Measurements

The Main Question



Measurements

How to reach high quality measurements and final data?

- Follow the recommendations for sampling and humidity control

- Modify the existing mobility size spectrometers

- Test your inversion software (in case you use a custom-made)

- Perform regular calibrations

- Perform regular comparisons against reference instruments

- Allow independent audits of your station

- Educate operators and people producing the final data

EUSAAR Networking



Measurements

What did EUSAAR to reach the aim of a network?

- Modification of custom-built and commercial mobility size spectrometers

according to the standard set-up

- Comparisons of inversion routines

- Standardization of calibrations

- Intercomparison workshops

- Mobile reference instruments for on-site intercomparisons

- Training courses

- Stations audits

- Standardization for a traceable three-level data format for EBAS

Comparison of Inversion Routines



Measurements

1 10 100 1000

2000

4000

6000

8000

10000

12000

14000

dN

/ d

log

Dp [cm

-3]

Dp [nm]

IFT

LUND

BOL

NILU

UHEL

JRC

PSI

LAMP



Measurements

1 10 100 1000

2000

4000

6000

8000

10000

12000

14000

dN

/ d

log

Dp [

cm

-3]

Dp [nm]

IFT

McMurry

Brechtel

GRIMM ISO15900

GRIMM Reichelt

TSI ISO15900

Latex Calibration



Measurements

0.10 0.15 0.20 0.25 0.30

0

5000

10000

15000

20000

25000

30000

NILU

FMI

UHEL

NUIG

UBIR

IFT REF

IFT TDMPS

TNO

PSI

LAMP

JRC

± 3.5%

dN

/ d

log

Dp [cm

-3]

Dp [µm]

CPC Calibration



Measurements

0 5 10 15 20 25 30 35 40 45

0

20

40

60

80

100

Co

un

tin

g E

ffic

ien

cy o

f b

uta

no

l C

PC

s

for

silv

er

pa

rtic

les

[%]

Particle Diameter Dp [nm]

IFT TDMPS, 3010, ∆T=17

NILU DMPS, 3010, ∆T=25

UHEL DMPS, 3010, ∆T>25

ISPRA DMPS, 3772, ∆T=17

IFT SMPS, 3010, ∆T=17

FMI TDMPS, 3772, ∆T=25

FMI TDMPS, 3772, ∆T=25

LAMP SMPS, 3010, ∆T=17

NUI SMPS, 3010, ∆T=17

Intercomparison of Mobility Size Spectrometers



Measurements

0.01 0.1 0.5

0

1000

2000

3000

4000

5000 NILU

FMI

UHEL

IFT REF

IFT TDMPS

TNO

PSI

dN

/ d

log

Dp [

cm

-3]

Dp [µm]

Comparison Number Concentration - reference CPC



Measurements

174.00 174.02 174.04 174.06 174.08 174.10

0

1000

2000

3000

4000

5000 NILU

FMI

UHEL

IFT REF

IFT TDMPS

TNO

PSI

IFT CPC

int.

co

nc.

[cm

-3]

time [DOY]

Comparison Number Concentration > 100 nm



Measurements

174.00 174.02 174.04 174.06 174.08 174.10

0

200

400

600

800

1000

1200

1400 NILU

FMI

UHEL

IFT REF

IFT TDMPS

TNO

PSI

int.

co

nc. [c

m-3]

time [DOY]

Mobile Reference Mobility Size Spectrometer



Measurements

On-Site Latex Size Calibration



Measurements

100 200 300 400

0.1

1

10

p1

p5

p25

Median

p75

p95

p99

raw

co

nce

ntr

atio

n [#

/cm

³]

Mobility diameter [nm]

ref. SMPS

SMPS_HP

Hohenpeissenberg

On-Site Size Distribution Intercomparisons



Measurements

Hyytiälä, Finland, one month average

Data Structure



Measurements

- Traditionally, highly time-resolved data is stored as one-hour averages in

the EMEP data base (EBAS)

- For aerosol properties, this data is converted to standard temperature and

pressure

- This traditional data shows no transparency how the data were treated or

corrected

- Traceability back to raw data is not possible

- We developed a three-level data structure for the aerosol properties

mentioned above within EUSAAR

EUSAAR Three-Level Data Structure



Measurements

The data treatment for each level is described in a separate document

The user obtains a optimum transparency and traceability of the data

Level-0

- Standardized raw and system data in original time resolution

Level-1

- Corrected data in original time resolution

Level-2

- Final one-hour averaged data are normalized to standard temperature and

pressure

Summary



Measurements

Recommendations

- Follow the guidelines for sampling and humidity control

- Modify your instrument (e.g. mobility size spectrometer) in hard- and

software

- Perform frequent calibrations

- Compare your instrument against a reference instrument (workshop or on-

site)

- Allow site audits

- Send your operators/young scientists to comparison workshops and/or

training courses for capacity building



Measurements

Thank you very much for your attention

Melpitz: PM10 inlet and dryer

Documents

05 Alfred Wiedensohler - National Physical Laboratory · Alfred Wiedensohler ... - In case the room temperature is higher than 20 °C, ... training courses for capacity building