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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO2901

NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1

Supplementary Information for

Black carbon absorption enhancement in the atmosphere

determined by particle mixing state

Dantong Liu1, James Whitehead1, M. Rami Alfarra1,2, Ernesto Reyes-Villegas1, Dominick V.

Spracklen3, Carly L. Reddington3, Shaofei Kong1,4,5, Paul I. Williams1,2, Yu-Chieh Ting1,

Sophie Haslett1, Jonathan W. Taylor1, Michael J. Flynn1, William T. Morgan1, Gordon

McFiggans1, Hugh Coe1 and James D. Allan1,2

1School of Earth, Atmospheric & Environmental Sciences, University of Manchester, UK

2National Centre for Atmospheric Science, UK 3Institute for Climate and Atmospheric Science, School of Earth and Environment, University of

Leeds, Leeds, UK 4School of Atmospheric Physics, Nanjing University of Information Science & Technology, China

5now at School of Environmental Studies, China University of Geosciences (Wuhan), China

Black-carbon absorption enhancement in theatmosphere determined by particle mixing state

S1. Summary of conducted experiments

Fig. 1. A schematic diagram of the set up for the diesel engine emissions experiments.

Table 1. A summary of black carbon particles sampled in this study.

Soot Sources Sampling

conditions Details Abbreviation

Light duty diesel engine - EURO 4 Laboratory Chamber Cold idle condition without

warm up or engine load CI

Light duty diesel engine - EURO 4 Laboratory Chamber

Normal running condition

with 2000rpm, 30% load and

10 minutes warm up

NR

Light duty diesel engine - EURO 4 Laboratory Chamber Thermodenuded normal

running condition NR-TD

Morning Rush hour traffic Ambient 29/10-10/11, 2014 TR

Evening traffic, wood burning Ambient 29/10-10/11, 2014 SF

Evening traffic, wood burning and bonfire Ambient 18:30, 05/11- 03:30, 06/11 BN

S2. Source attribution of particles during the ambient experiment

Fig. 2. Time series of AMS and SP2 data and source attribution from the ambient measurements. A)

the aerosol loadings during the entire ambient experiment; B) The mass spectra of the derived ME-2

factors; C) The diurnal variation of aerosol loadings and ME-2 factors excluding the Bonfire event; D)

the aerosol loadings during Bonfire night; the whiskers, boxes, the line and cross in represent the 10%

(90%), 25% (75%) percentiles, median and mean values for each component.

0.80

0.75

0.70

0.65

0.60

0.55

0.50

29/10/2014 31/10/2014 02/11/2014 04/11/2014 06/11/2014 08/11/2014 10/11/2014

date and time

70

60

50

40

30

20

10

0

Aero

sol m

ass loadin

gs (

µg m

-3)

Bonfire Event OM BC SO4

NO3

A

0.12

0.08

0.04

0.00

1009080706050403020m/z

0.12

0.08

0.04

0.000.100.080.060.040.020.000.100.080.060.040.020.000.250.200.150.100.050.00

BBOA

Re

lative

In

ten

sity

HOA

COA

SVOOA

LVOOA

B

0.70

0.65

0.60

0.55

0.50

20151050Hours of day

1.0

0.8

0.6

0.4

0.2

0.0

4

3

2

1

0

Aero

sol m

ass load

ings (

µg

m-3

) 50

40

30

20

10

0Aero

sol m

ass loa

din

gs (

µg m

-3)

3210BC OM SO4 NO3

non-Bonfire Bonfire Event BC OM NO3

SO4

HOA SFOA COA SVOOA LVOOA

TR SF BN

C D

S3. The calibration and detection efficiency of the SP2 instrument

Fig. 3. The SP2 incandescence response to the particle mass for both thermodenuded NR and Cold

Idle BC.

Fig. 4. The SP2 detection efficiency as a function of rBC mass and Dve for the laboratory diesel in

normal running condition. The arrows show the change when increasing SP2 laser power.

800

600

400

200

0

N (

cm

-3)

4 5 6 7 8 9

0.12 3 4 5 6 7 8 9

12

CPMA mass (fg)

1.0

0.8

0.6

0.4

0.2

0.0

SP

2/C

PC

35 40 50 60 75 100 130

rBC Dve (nm)

CPC SP2

increase SP2 laser current from 2600 to 3000mA

S4. The density and shape of diesel soot particles

Fig. 5. The effective density as a function of particle mass for laboratory diesel soot particles

generated under different conditions.

1.4

1.2

1.0

0.8

0.6

0.4

Eff

ective

de

nsity (

g c

m-3

)

9

0.12 3 4 5 6 7 8 9

12 3 4 5 6 7 8 9

10Particle mass (fg)

Normal Running (NR) Cold Idle NR with TD Cold Idle with TD

S5. The optical models used to estimate the scattering properties of black carbon

particles

Fig. 6. A schematic of the models used to estimate the optical properties of the black carbon particles.

For internally mixed black carbon particles, two different approaches for computing the effective

refractive index (m) of the mixture are considered.

𝑚 = √𝜀 , (S1)

where ε denotes the electric permittivity. The Maxwell-Garnett mixing rule1 assumes that an inclusion

with a permittivity ε1 and volume fraction f1 is embedded in a host matrix of permittivity ε2. The

resulting effective permittivity εeff is obtained from

f1ε1−ε2

ε1+2ε2=

εeff−ε2

εeff+2ε2 , (S2)

The Bruggemann mixing rule 2 assumes the two materials are embedded in a host medium with an εeff

given by

f1ε1−εeff

ε1+2εeff+ (1 − f1)

ε2−εeff

ε2+2εeff= 0 , (S3)

The Bruggemann rule treats the two materials more symmetrically.

The RDG approximation used in this study assumes a monomer diameter Ds=50nm and assumes the

coating volume is evenly distributed on each monomer with the same coating content 3. The number

of monomers Ns is given by

Ns=(Dc/Ds)3 , (S4)

Where Dc is the diameter of the BC core.

The resulting integrated scattering cross section is given by

SSca=Ns2×Ss,Sca , (S5)

Where Ss,Sca is the scattering cross section of the monomer and Ssca is that of the entire particle.

S6. The transition in optical behaviour resulting from changes in mixing ratio, MR, as a

function of particle mass

Fig. 7. The measured and modelled optical properties as a function of mass ratio as presented in Fig. 1

but for different particle masses. The grey line on each plot shows the ratio between the externally

mixed model prediction of the scattering cross section compared to that from the core-shell model

(Sext/Sc-s). The values in brackets denote the range of non-BC densities used to calculate the

uncertainty in each data set represented by the bars in each figure. The scattering signal measured at

1fg may be biased towards smaller MrBC due to the reduced instrument S/N ratio for small particles.

0.70

0.65

0.60

0.55

0.50

0.1 1 10MR

2.0

1.5

1.0

0.5

0.0

S/S

c-s

Diesel NR(1.04) NR TD(1.04) Cold Idle(1.04)

Ambient TR (1.1) SF (1-1.2) BN (1-1.2)

Model only

Sext/Sc-s

Particle mass 1fg

Externally mixed

Transition

Internally mixed

Unknown

0.70

0.65

0.60

0.55

0.50

0.1 1 10MR

2.0

1.5

1.0

0.5

0.0

S/S

s-c

Diesel NR (1.04) NR TD (1.04) Cold Idle (0.9-1.1)

Model only Sext/Sc-s

Particle mass 3fg

Internally mixedExternally mixed

Transition Unknown

0.70

0.65

0.60

0.55

0.50

0.1 1 10MR

2.0

1.5

1.0

0.5

0.0

S/S

c-s

Diesel NR (1.04) NR TD (1.04) Cold Idle (0.9-1.1)

Ambient TR (1) SF (1-1.2) BN (1-1.2)

Model only

Sext/Sc-s

Particle mass 5fg

Internally mixedExternally mixed

Transition Unknown

0.70

0.65

0.60

0.55

0.50

0.1 1 10MR

2.0

1.5

1.0

0.5

0.0

S/S

c-s

Particle mass 10fg

Ambient TR(1.1) SF(1-1.2) BN(1-1.2)

Model Only

Sext/Sc-s

Internally mixedExternally mixed

Transition Unknown

MR

Normal

Running

Cold Idle TR SF BN Average

MP=1fg

Transition Start 0.28 0.26 0.31 0.30 0.29 0.3

Transition End Transition* 2.82 transition 2.51 2.63 2.7

2fg

Transition Start external** transition external 1.48 1.44 1.5

Transition End external 4.5 external 2.86 2.90 3.3

3fg

Transition Start external 1.36

Transition End external 4.15

5fg

Transition Start transition transition 0.56 0.48 0.35 0.5

Transition End transition 3.23 transition 2.24 2.55 2.7

10fg

Transition Start 0.46 0.42 internal† 0.4

Transition End transition 0.88 internal 0.9

*means there is no data beyond the transition region

**means there is no data beyond region characterised by the external mixing model best representing the data

†means all of the data is within internal mixing model regime

Table 2. Threshold values of MR characterising the transition in the optical properties of black carbon

at various particle masses. The BC calculated using the hybrid model is considered to be effectively

externally or internally mixed when either the external mixing or core-shell model can represent the

measured scattering to within 20%.

S7. SP2 measured scattering enhancement (Esca) as a function of MR

Fig. 8. The influence of MR on the scattering enhancement (Esca) using the Mie-core-shell and external

mixing approaches for a range of different particle masses and experimental conditions. A) Esca at

1064 nm at different particle masses derived from Mie-core-shell and external mixing model. B) SP2

measured Esca under a variety of environments mapped in the model frame at particle mass 2fg; the

grey bar shows the transition regime. The top panel shows the internal mixing fraction, with the dash

line showing the lineal fitting against MR.

S8. Direct measurements of absorption enhancement

Laboratory-generated diesel

Fig. 9. Results from cold idle laboratory diesel. A) BC properties since engine emission injection into

the aerosol chamber, from top to bottom: rBC mass, rBC core mass median diameter, MR,bulk and

absorption coefficient (Babs) measured by PASS-3, the right axis is the internal mixing faction

assuming threshold MR at 1.5 and 3; B) measured MAC and Mie calculated uncoated rBC MAC;

bottom panel is the average Eabs at three wavelengths calculated as the ratio between measured MAC

and Mie calculated MAC of uncoated rBC core, with the error bar denoting the standard deviation.

Ambient experiment

Fig. 10. Absorption measurements in the ambient atmosphere. A) An example of measured absorption

coefficient (Babs) switching between direct and heated line in 30mins during bonfire night. The

markers show the average within 30mins. B) Babs from direct line correlated with the heated line. Eabs

is obtained by ODR.

500

400

300

200

100

0

Ba

bs (

Mm

-1)

12:0005/11/2014

18:00 00:0006/11/2014

06:00

UTC time

PASS measured Babs Heated line Direct line

A

30

25

20

15

10

5

0

Ba

bs D

ire

ct

(Mm

-1)

3020100

Babs Heated (Mm-1

)

Slope=1.02±0.03TR

B 40

30

20

10

0

Ba

bs D

ire

ct

(Mm

-1)

403020100

Babs Heated (Mm-1

)

Slope=1.16±0.03

SF

300

250

200

150

100

50

0

Ba

bs D

ire

ct

(Mm

-1)

250200150100500

Babs Heated (Mm-1

)

Slope=1.23±0.07

BN

Table 3. A summary of measured and calculated Eabs at 532nm for all of the sources in this study. The

error in the hybrid model is the uncertainty when varying the threshold MR spanning over the

transition regime. The error for ambient measurement is the error in the slope derived from the ODR

fitting. The other errors denote standard deviations with respect to the average. The values in brackets

are the calculated Eabs when applying different transition regimes at different particle masses.

Sources Eabs

Measurement

Eabs by

Hybrid model

Eabs by

Mie-core-shell

MR,bulk Internal mixing

fraction (MR>2)

Laboratory diesel

NR 1.01±0.02 1.14±0.11 1.31±0.04 1.13±0.03 0.25±0.02

CI start 1.02±0.03 1.03±0.02 1.23±0.03 1.10±0.06 0.14±0.03

CI middle 1.10±0.08 1.16±0.04 1.40±0.02 2.70±0.21 0.42±0.08

CI end 1.30±0.06 1.27±0.07 1.54±0.03 4.38±0.07 0.58±0.02

Ambient

TR 1.02±0.03 1.03±0.02 1.22±0.06 0.38±0.13 0.07±0.05

SF 1.16±0.03 1.19±0.04(1.21) 1.36±0.05 0.59±0.11 0.18±0.08

BN 1.23±0.07 1.25±0.05(1.32) 1.46±0.03 1.08±0.17 0.38±0.11

S9. Summary of previous field experiments used as data sources in this work

The CalNEx-LA experiment took place from 15 May–15 June 2010. The site was located on the

Caltech campus in Pasadena, CA, USA. The site was influenced by advected plumes from the source-

rich western LA basin as well as fresher more local emissions 4. The main source of black carbon in

the region is thought to be diesel emissions. Biomass burning did not strongly influence the site

during the measurement period.

The Weybourne experiment was conducted at the Weybourne Atmospheric Observatory (WAO)

(52.9504◦ N, 1.1219◦ E) from June–July, 20115. The WAO is located near the North Norfolk coast in

the UK and represents a rural environment remote from major populated or industrial areas. This site

routinely experiences polluted plumes from the UK and mainland Europe.

The Clean Air for London (ClearfLo) winter campaign was conducted from 11 January to 8 February,

2012 6. The experimental site was located in the grounds of a school in North Kensington, London,

which is representative of a typical urban background environment. The London urban environment

in winter is significantly influenced by solid fuel burning sources, such as wood burning for

residential heating as well as from traffic.

The South American Biomass Burning Analysis (SAMBBA) campaign was carried out in Brazil in

September and October 20127. The aircraft based study focussed on characterising smoke from

biomass burning in regions remote from other black carbon emission sources.

S10. Black Carbon and Organic Carbon inventories used in GLOMAP

We use the MACCity emissions inventory to represent the anthropogenic emissions of black carbon

and organic carbon 8, which provides annually-varying emissions for the period 1979–2010. Fire

emissions of black carbon and organic carbon were taken from the Global Fire Emissions Database

(GFED) version 3 emission inventory (GFED3 9). GFED3 provides yearly-varying, monthly-mean

fire emissions of aerosol and gas-phase species from 1997 to 2011 at 0.5º×0.5º resolution. The global

emissions are derived using estimates of burnt area, active fire detections, and plant productivity from

the Moderate resolution Imaging Spectroradiometer (MODIS) satellite (post year 2000) combined

with estimates of fuel loads and combustion completeness for each monthly time step from the

Carnegie-Ames-Stanford-Approach (CASA) biogeochemical model.

Fig. 11. The global BC and OC emissions for fossil fuel, biofuel and biomass burning.

S11. The MR for different sources used in GLOMAP

The MR used in GLOMAP for fossil fuel, biofuel and open biomass burning, as derived from BC and

OC inventories.

Fig. 12. The global annually averaged MR for fossil fuel, biofuel and open biomass burning.

-50

0

50

La

titu

de

(De

gº)

-100 0 100Longitude(Degº)

10

8

6

4

2

MR

bio

fue

l

-50

0

50

La

titu

de

(De

gº)

-100 0 100

Longitude(Degº)

10

8

6

4

2

MR

fossil fu

el

-50

0

50

Latitu

de (

Deg º

)

-150 -100 -50 0 50 100 150Longitude (Deg º)

10

8

6

4

2

MR

open b

iom

ass b

urn

ing

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