1

Rosaria Erika Pileci1, Michele Bertò1, Robin Modini1, Stefania Gilardoni2,

Matteo Rinaldi2, Douglas Orsini2, Angela Marinoni2, Martin Gysel1

1 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland

2Institute of Atmospheric Science and Climate, Italian National Research Council (CNR-ISAC) via Gobetti 101, 40129 Bologna, Italy

PM2018 – Matera – 24/05/2018

Physical and chemical properties of freshly emitted and aged particles determined from mobile measurements in the Po Valley

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Introduction:

Mass Absorption Coefficient (MAC) of Black Carbon

MAC is a key parameter when estimating black carbon radiative forcing. Uncertainties in MAC

Uncertainties in radiative forcing Uncertainties in climate predictions

𝐌𝐀𝐂𝐁𝐂 [𝐦𝟐𝐠−𝟏] = 𝛃𝐚𝐛𝐬,𝐁𝐂 [𝐌𝐦−𝟏]

𝐦𝐁𝐂 [𝛍𝐠

𝐦𝟑]

Introduction:

MAC, experimental uncertainty and variability

(Zanatta et al., Atmos. Env., 2016)

MAC at European

background sites:

~10.0 𝐦𝟐𝐠−𝟏 (@ 637 nm)

𝐌𝐀𝐂 for freshly BC:

𝟕. 𝟓 ± 𝟏. 𝟐 𝐦𝟐𝐠−𝟏 (@ 550 nm)

6.5 𝐦𝟐𝐠−𝟏 (@ 637 nm, AAE = 1) (Bond & Bergstrom, Aerosol Science

and Technology, , 2006)

Part of MAC variability is due to

measurements uncertanties!

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MAC of internally mixed “coated

BC” increases by up to a factor

of ~2 compared to bare BC core.

Introduction:

MAC, link to BC physical quantities

• Mixing state

• Core sizes

• Morphology

• Refractive index

Aim of the work

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• Studying the difference in properties between

freshly emitted and aged particles

• Explain the MAC variability with variations in BC physical properties:

- BC mass size distribution; - Mixing state

aging

• Atmospheric aging processes lead to internally mixed BC

• With mobile and stationary measurements in different locations!

Mobile measurements with MOSQUITA (to investigate spatial variability)

Licor (CO2) ; O3

CAPS PMssa (780 nm) (Extinction and

scattering) --------------------------

MAAP (637 nm) --------------------------

PAX (870 nm) (Absorption and

scattering) -------------------------- AETHALOMETER 7𝛌

HR ToF

AMS

SP2 (rBC mass and

mass size distribution,

coating thickness)

CPC

UHSAS (Size

distribution)

Drives repeated

San Pietro Capofiume Rural background

Remote background Monte Cimone

Urban background

Highway

Freshly emitted particles

Rural background road

Aged particles

July 2017

12 trips

4.000 km

Results: Black carbon optical properties (Drives between Bologna and Milano)

8 8

fresh traffic BC

aged BC Evidence that atmospheric

aging increases the MAC of BC.

Results: The Challenge of mobile absorption measurements

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MAAP signals very noisy when doing mobile measurements.

Does it introduce a systematic bias? Can we recover the data?

Mobile Stationary

Results: CAPS – a good promise

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Extremely good correlation between CAPS PMssa absorption and rBC mass from

the SP2 at a time resolution of 5 secs

But how accurate are the absolute values?

Results: PAX, a challenge lost

50

40

30

20

10

0

-10

-20

Ab

sorp

tion

co

eff

icie

nt

(1/M

m)

13:3013.07.2017

14:00 14:30 15:00 15:30 16:00 16:30 17:00 17:30 18:00 18:30 19:00

Date & Time

Bologna - Monte Cimone Absorption coefficient time serie - PAX drift problem

Unsuccessful measurements both mobile and stationary!

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Results: MAC spatio variability

𝑯𝒊𝒈𝒉𝒘𝒂𝒚 𝑩𝒐𝒍𝒐𝒈𝒏𝒂 𝑪𝒊𝒕𝒚 𝑺𝒂𝒏 𝑷𝒊𝒆𝒕𝒓𝒐

𝑪𝒂𝒑𝒐𝒇𝒊𝒖𝒎𝒆

(14.4 ± 0.5 − 14.6 ± 0.6)

(9.8 ± 0.1 − 16.2 ± 1.1)

(18.4 ± 0.4 − 26.5 ± 0.7)

(Bond & Bergstrom, Aerosol Science and Technology, 2006)

(Zanatta et al., Atmospheric Environment, 2016)

5.3 @ 780 nm, AAE = 1

8.2 @ 780 nm, AAE = 1

Mixing state: delay time method

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BC mass

Op

tica

l dia

me

ter

Thinly-to-moderately

coated Thickly coated

Results: MAC vs Fraction of thickly coated particles

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MAC [18.1 – 26.2] background sites 14% - 18 % thickly coated particles

MAC [9.8 – 16.2] traffic dominated sites 1% - 6% thickly coated particles

Results:

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𝑩𝒐𝒍𝒐𝒈𝒏𝒂 𝑪𝒊𝒕𝒚

120 𝑛𝑚 < max (𝐷𝐵𝐶 ) < 155 𝑛𝑚

𝑺𝒂𝒏 𝑷𝒊𝒆𝒕𝒓𝒐 𝑪𝒂𝒑𝒐𝒇𝒊𝒖𝒎𝒆

max (𝐷𝐵𝐶) ≃ 150 𝑛𝑚

BC mass size distributions

𝑯𝒊𝒈𝒉𝒘𝒂𝒚 max (𝐷𝐵𝐶) ≃ 110 𝑛𝑚

Conclusions

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Mobile absorption measurements are tricky. There are uncertainties and this

complicate the absolute value of MAC

Nevertheless there is clear evidence of MAC variation:

explained with difference in BC coating thickness

Future analysis

- Quantitave mixing state;

- Improve data analysis: determine best available absorption measurements!

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Acknowledgments