1. 25 March 2015
Robert Healy
Chemical and optical properties of
black carbon particles in Toronto
(CHEMBC)
2. 2
Background- Black carbon
- “Black carbon” particles strongly absorb visible light across all wavelengths
- Sources include incomplete combustion of fossil fuels (vehicles, industry) and biomass
(domestic burning, forest fires)
- Exert a positive direct radiative forcing (warming effect on climate)
- High uncertainty associated with climate impacts
http://www.catf.us/climate/http://www.geog.ucsb.edu/
5. 5
- Black carbon (BC) particles are small, typically composed of multiple spherules,
each with a diameter of about 20 nm
100 nm
Black carbon absorption
7. 7
- BC particles accumulate organic and inorganic secondary coatings during
atmospheric transport
organic/inorganic coating
300 nm
Black carbon absorption
8. 8
- Coated black carbon can still absorb incoming radiation directly
Black carbon absorption
9. 9
- Enhanced absorption of solar radiation has been proposed to occur through a
“lensing effect” (refraction) for coated BC
- This enhancement effect has been demonstrated in laboratory studies
- But estimates of the enhancement for ambient BC vary greatly (6-50%), and thus
measurements in different environments are needed
BC absorption enhancement (Eabs)
Cappa et al., Science 2012
10. 10
Motivation
- Does BC coating composition and thickness affect absorption efficiency?
- If so, what is the impact in Toronto?
???
13. 13
Campaign and instrumentation
Soot particle aerosol mass
spectrometer
(SP-AMS)
Proton transfer
reaction mass
spectrometer
(PTR-MS)
Photoacoustic soot spectrometer (PASS)
Thermodenuder
Aerosol time of flight
mass spectrometer
(ATOFMS)
14. 14
Campaign and instrumentation
Instrument Function
Photoacoustic soot spectrometer
(PASS-3)
Measures aerosol absorption/scattering at
405 nm and 781 nm
Soot particle aerosol mass spectrometer
(SP-AMS)
Measures BC and coating material
concentrations quantitatively
Aerosol time-of-flight mass spectrometer
(ATOFMS)
Measures single particle composition and
mixing state qualitatively
15. 15
250 °C
Remove coatings using
thermal denuder
Experimental configuration
- Measure absorption for coated and uncoated (denuded) particles
- Is there a difference?
Measure absorption (babs)
using PASS
Measure absorption (babs)
again using PASS
21. 21
Absorption Ångström exponent (AAE)
Absorption(babs)
800700600500400
Wavelength (nm)
Lensing-related absorption
BC absorption
AAE = 1
AAE = 1
- Impact of optical lensing absorption enhancement on AAE
- If lensing is occurring, we should observe an enhancement at 405 nm AND 781 nm
22. 22
Absorption Ångström exponent (AAE)
Absorption(babs)
800700600500400
Wavelength (nm)
Brown carbon absorption
BC absorption
AAE = 1
AAE = 2
- AAE of “brown carbon”, associated with biomass burning, is typically ~2 or greater
- Could explain the absorption enhancement being observed only at 405 nm
29. 29
Mie theory predictions of lensing Eabs at 781 nm
1.1 2.0 1.1 1.9 1.2 2.0
By comparison we observe 1.0 for our campaign
30. 30
Conclusions
- Optical lensing is not relevant for BC in Toronto, even when the site is influenced by
BC particles with large coatings
- Direct absorption by brown carbon is relevant when the site is influenced by wildfires,
however, and is responsible for over 50% of direct absorption at 405 nm at times
- If radiative forcing is estimated using composition data and Mie theory, regional
warming is substantially overpredicted
- Care must be taken when accounting for BC internal mixing in climate models, and
further studies of absorption enhancement in other environments are necessary
34. 34
Single particle sampling- online:
ATOFMS
- Single particle information retained
- Enables source identification and investigation of chemical processing
- Data typically qualitative only
Data output:
Single particle
mixing state
(qualitative)
35. 35
Quantitative approach
- Derived ATOFMS mass spectral relative sensitivity factors (RSF) for OA, BC,
NH4, NO3 and SO4
- Calculated quantitative chemical composition estimates for each single particle
Healy et al. Atmos. Chem. Phys. 2013
RSF
quantitative
36. 36
Quantitative approach
- Chemical composition of each particle in the population can also be summed to
produce size-resolved bulk composition information
30
25
20
15
10
5
0
dM/dlogDp(µgm
-3
)
900
800
700
600
500
400
300
200
Aerodynamic diameter (nm)
NH4
NO3
SO4
OA
BC
quantitative
38. 38
Parameterisation of particle mixing state
• To what extent is each chemical species represented in a particle?
• Shannon entropy is used to determine this representation, similar to
applications in biodiversity and ecology
• For a single particle (i)…
𝐻𝑖 =
𝑎=1
𝐴
−𝑝𝑖
𝑎
ln 𝑝𝑖
𝑎
Mass fraction of species a (for example SO4) in particle i
Shannon entropy
Riemer & West, Atmos. Chem. Phys. 2013
39. 39
Single particle diversity (Di) examples
• Example 1: 2 species, equal proportions
Hi = (-0.5ln(0.5))+(-0.5ln(0.5)) = 0.69
The single particle diversity (Di), defined as e(Hi) = 2.0
OA BC
40. 40
Single particle diversity (Di) examples
• Example 2: 2 species, unequal proportions
Single particle diversity (Di) = 1.8
OA
BC
Typical fresh combustion particle
41. 41
Single particle diversity (Di) examples
• Example 4: 5 species, unequal proportions
Single particle diversity (Di) = 4.2
OA
BC
NH4
NO3
SO4
Typical aged particle
42. 42
The MEGAPOLI campaign Paris 2010
PARIS
SIRTA
LHVP
20km
GOLF
Livry
- ‘MEGAPOLI’ winter campaign site locations
ATOFMS
Sampling took place over 4 weeks in Jan/Feb 2010
43. 43
Paris single particle example 1
Relativepeakarea(arbitraryunits)
200180160140120100806040200
m/z
+
-
C
+
C3
+
C4
+
C2
-
C5
+
C3
-
C5
-
C2
+
Ca
+
C4
-
HSO4
-
SO4
OA
BC
Di = 2.0
Healy et al. Atmos. Chem. Phys. 2014
46. 46
Whole dataset: Paris
• We can average Di across all particles for every hour of the
measurement period
• This average value is termed Dα, and represents how well mixed
chemical species are at the single particle level only
• What about the bulk aerosol?
47. 47
Bulk aerosol diversity (Dγ)
• Bulk diversity is easier to calculate for each hour
• Shannon entropy calculated from the mass fractions of each species
present in the bulk aerosol
NO3
SO4
OA
BCNH4
Bulk aerosol diversity Dγ = 4.5
48. 48
Quantifying aerosol mixing state (χ) in Paris
• Relating Dα and Dγ gives the quantitative mixing state index (χ)
• Expressed as a percentage
• Average value for Paris χ = 59%
49. 49
Mixing state (χ) as a function of time
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/20104/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/2010
4/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
50. 50
Dependence of χ on air mass origin
26. Jan 2010 18:00 to 21:00
-5 0 5 10 15 20 25 30
42
44
46
48
50
52
54
56
[ns/kg]
0.005 0.01 0.02 0.04 0.08 0.16 0.32 0.64 1.28 2.56
Continental
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/2010
4/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/2010
4/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
51. 51
Dependence of χ on air mass origin
03. Feb 2010 18:00 to 21:00
-20 -15 -10 -5 0 5
40
45
50
55
[ns/kg]
0.005 0.01 0.02 0.04 0.08 0.16 0.32 0.64 1.28 2.56
26. Jan 2010 18:00 to 21:00
-5 0 5 10 15 20 25 30
42
44
46
48
50
52
54
56
[ns/kg]
0.005 0.01 0.02 0.04 0.08 0.16 0.32 0.64 1.28 2.56
Marine
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/2010
4/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
1.0
0.8
0.6
0.4
0.2
0.0
Bulkpopulation
massfraction26/1/201027/1/201028/1/201029/1/201030/1/201031/1/20101/2/20102/2/2010
3/2/2010
4/2/20105/2/2010
6/2/2010
7/2/20108/2/2010
9/2/2010
10/2/201011/2/2010
Date
70
60
50
40
4.0
3.5
3.0
2.5
2.0
D
4.5
4.0
3.5
3.0
D
C M C
D
D
NH4
NO3
SO4
OA
BC
53. 53
Conclusions Part 2
• ATOFMS data can be used to estimate single particle composition
• Calculating single particle diversity and bulk aerosol diversity enables an
assessment of aerosol mixing state (χ)
• Aerosol mixing state depends on local emissions, chemical processing and
regional transport
• Aerosol diversity measurements can be used to evaluate the error
introduced in climate models when internal mixing is assumed
54. 54
Thank you
Questions?
University College Cork
J.C. Wenger, J.R. Sodeau, I.P. O’Connor,
E. McGillicuddy, J. Arndt
University of Toronto
J.M. Wang, C.-H. Jeong, A.K.Y. Lee, M.D. Willis,
N. Hilker, J.P.D Abbatt, G.J. Evans
University of Illinois
N. Riemer, M. West
Norwegian Institute for Air Research
S. Eckhardt, A. Stohl
57. 57
Aerosol absorption and the Beer-Lambert Law
I = I0e-αlc
I = outgoing light
I0 = incident light
α = absorption cross section (m2 g-1) (α is often termed the “MAC” value)
l = path length (m)
c = concentration (g m-3)
babs is the product of α and c (m-1)
and is measured directly by the PASS
Scattering (and extinction) calculations are analogous to absorption
58. 58
Terminology
BC (black carbon, measured quantitatively by SP-AMS)
babs (absorption coefficient measured by PASS)
Eabs (absorption enhancement)
MAC (mass absorption cross-section)
NR-PMBC (non-refractory particulate matter on BC, measured by SP-AMS)
NR-PMBC/BC (coating-to-core mass ratio for BC-containing particles)
69. 69
Assumptions
• All particles are spherical with a density of 1.5 g cm-3 for aerodynamic
diameter to mobility diameter conversions prior to scaling
• All particles are composed exclusively of NH4, SO4, NO3, OA and BC
• ATOFMS ‘sees’ all particle types with equal efficiency at a given size
70. 70
Why do we need Dα and Dγ? Example 1
NO3NH4
OA BC
NO3NH4
NO3NH4
NO3NH4
NO3NH4
NO3NH4
OA BC
OA BC
OA BC
OA BC
OA BC
Dα = 2.0 Dγ = 4.0
NO3 OA
BCNH4
SINGLE PARTICLE BULK AEROSOL
71. 71
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4 NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
Dγ = 4.0Dα = 4.0
Why do we need Dα and Dγ? Example 2
SINGLE PARTICLE BULK AEROSOL
73. 73
Diurnal dependence of Dα Dγ and χ
5.0
4.5
4.0
3.5
3.0
D
3.23.13.02.92.82.72.62.5
D
20
15
10
5
0
Hourofday
OA BC
OA BC
OA BC OA BC
OA BC
OA BC
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
NO3 OA
BCNH4
75. 75
Relative sensitivity factors by species
3
4
5
6
1
2
3
4
5
6
10
2
3
4Relativesensitivityfactor(arbitraryunits)
SO4
OA
NH4
NO3
BC
Box-plot of hourly mass spectral relative sensitivity factors (n = 610). Median,
75th percentile and 90th percentile are denoted by the solid line, box and
whisker respectively.
