“Unde Fuligo?”
Aerosol Black Carbon
Analysis of Multi-Wavelength Aethalometer Data for the apportionment of ‘Black’ versus ‘Brown’ Carbon
Tony Hansen Magee Scientific, Berkeley, CA
Presented to USEPA ORD Research Triangle Park, NC 23rd February, 2017 The ‘Model AE33’ Aethalometer
2 Aethalometer – Continuous optical analysis
Air stream containing BC particles
Reference I0
BC Sensing I
Light Source Light Detectors
Filter collects particles continuously: Gradually becomes darker and darker
3 The Aethalometer™ - 1986
• Draw air sample continuously through filter, collect aerosol. • Measure rate of decrease of optical transmission at 880 nm. • Calculate rate of increase of ‘Attenuation’ (‘ATN’) • ATN is proportional to loading of Black Carbon • Calculate concentration of BC in air stream.
4 Measurement Principle – Optical Absorption
–( ba ∙ D ) Beer’s Law : I = I0 e
I0 is incident intensity; I is transmitted intensity D is total density of material traversed;
ba is absorption coefficient.
Measure I and I0 ; use known value of ba ; determine D → measure amount of absorbing material
5 Measurement Principle – Optical Absorption
–( ba ∙ D ) Beer’s Law : I = I0 e
I0 is incident intensity; I is transmitted intensity D is total density of material traversed;
ba is absorption coefficient.
Measure I and I0 ; use known value of ba ; determine D → measure amount of absorbing material
Note ! – ba is function of wavelength λ ; D may be mixture of materials with varying absorption spectra
6 From Black& to Color
7 7 The Aethalometer™ - 1996
• Draw air sample continuously through filter, collect aerosol. • Measure rate of decrease of optical transmission continuously at multiple wavelengths. • Calculate rate of increase of ‘Attenuation’ (‘ATN’) as function of λ. • ATN is proportional to loading of Light Absorbing Carbon , sometimes represented by a combination of ‘Black’ and ‘Brown’ Carbon • Calculate concentration in air stream as function of λ.
8 Measurement Principle – Optical Absorption
–( Σj ba(j, λ) ∙ D(j) ) Beer’s Law : I = I0 e
I0 : incident intensity; I is transmitted intensity D(j) : density of material of species j
ba (j, λ) : absorption coefficient of species j, function of wavelength λ
Measure I and I0 as function of wavelength λ, deconvolute
9 An early observation (1984)
10 Absorption Analysis across the spectrum
• Aethalometers provide analysis at 7 wavelengths: 370, 470, 520, 590, 660, 880 and 950 nm
• “Black” materials absorb uniformly across the spectrum: the 880 nm analysis is quantitative for ‘Black Carbon’.
• Other species – aromatic organic compounds – sometimes called “Brown Carbon” - can show increased absorption at shorter wavelengths.
11 Spectral Optical Analysis – real-time data
7-wavelength Aethalometer data - biomass burning plume impact at remote site. Data as recorded
370 430 470 520 590 700 950 Best-fit Baseline 25000
20000
15000
10000 Equivalent ng/m3
5000
0 0:00 6:00 12:00 18:00 0:00
27 June 2008 Data courtesy of Bryan Fabbri, NASA ‘COVE’ program, 2008 12 Absorption spectrum of biomass smoke - USA
'Added material' - absorbance vs. wavelength
120 Data Best-fit model 100
80
60
40 Relativeabsorbance 20
0 300 400 500 600 700 800 900 1000 Wavelength, nm
13 Absorption spectrum of biomass smoke - Europe
Haze of biomass smoke over Paris
Ambient measurements of light absorption by agricultural waste burning organic aerosols
O. Favez, S. C. Alfaro, et al., J. Aerosol Science 40, 613 (2009)
14 Europe : Renewable fuels : wood smoke
• Wood/biomass is a ‘sustainable fuel’ (from fossil CO2 perspective) and is encouraged for political/economic reasons. • Biomass burning is a major energy source in rural areas • Combustors range from high-efficiency ‘district’ central heating to individual wood-stoves
15 PM Emissions from ‘outdoor’ fires in US
16 How much smoke from RWC in USA?
For each 1°F colder, 1% increase in woodsmoke fraction of urban BC
17 Analytical Method: Separate the 2 species
Multi-wavelength optical measurement to enhance thermal/optical analysis for carbonaceous aerosols L.-W. A. Chen, J. C. Chow, et al., Atmos. Meas. Tech., 8, 451–461, 2015
18 Results from DRI 7-λ thermal-optical analysis
Multi-wavelength optical measurement to enhance thermal/optical analysis for carbonaceous aerosols L.-W. A. Chen, J. C. Chow, et al., Atmos. Meas. Tech., 8, 451–461, 2015
19 7-wave data : “Black” Carbon ~ λ-1
Angstrom Exponent (curvature) of BC absorption is 1
20 7-wave data : “Brown” Carbon ~ λ-α
Angstrom Exponent (curvature) of BrC absorption is α (~ 2 to 5 )
21 Combined measured data is a complex curve
22 Mathematical de-convolution needs 6+ data points
(expanded Actual data is vertical scale) combination of two curves
BC magnitude Angstrom Exponent BrC magnitude (curvature) of BrC absorption
23 Observations in China
Black carbon and wavelength-dependent aerosol absorption in the North China Plain based on two-year Aethalometer measurements 24 L. Ran, Z.Z. Deng, et al., Atmos. Environ. 142, 132-144 (2016) AAE of BrC component – Forest Fire Smoke
• Wildfire east of Fresno, CA: summer, hot, dry
Data courtesy of Prof. K. Hammond, UC Berkeley 25 AAE of BrC component – Woodstove Smoke
• Residential Wood Combustion in Felton, CA: winter, cool, damp
26 Data courtesy of M. Gilroy, MBUAPCD Identification of components
• Black Carbon has Mass Absorption Coefficient of 7.8 m2/g at 880 nm which defines this material. It has an identifiable physical structure. • We can interpret the optical absorption at 880 nm; associated with the “Alpha = 1” component; as a mass of BC .
• However, Brown Carbon is not a unique material.
27 Mass Absorption Coefficient of Brown Carbon
• Brown Carbon is probably a mixture of compounds whose combined optical absorption increases more steeply at shorter wavelengths. • Literature results suggest an optical absorption in the range of 0.4 ~ 0.6 m2/g at 880 nm • This allows us to interpret the optical absorption measured at 880 nm; associated with the “Alpha > 1” component; as an estimate of BrC mass.
28 Empirical relationship to PM2.5 mass
Correlation between Aethalometer ‘BrC’ optical absorption (calculated as the equivalent mass of BC); versus measured PM2.5 concentrations; in situations of overwhelming impact from biomass smoke.
Mass proportionality factor = 25 Mass proportionality factor = 14 BrC MAC = 7.8 / 25 = 0.31 m²/g BrC MAC = 7.8 / 14 = 0.56 m²/g
Data courtesy of Prof. K. Hammond, UC Berkeley; and M. Gilroy, MBUAPCD 29 Relationship between BC and BrC components
• Biomass burning will create both BC and BrC aerosols. • Their relative proportions will depend on combustion conditions. • In scenarios completely dominated by wood smoke, comparison of the relative magnitudes of the BC and BrC components provides an empirical characterization of the sources.
30 BrC/BC ratios in wood smoke
Fresno: Felton: Summer wildfire. Winter RWC. Hot, dry. Cool, damp. Possibility of low-oxygen smoldering.
Data courtesy of M. Gilroy, MBUAPCD; and Prof. K. Hammond, UC Berkeley 31 BrC/BC ratios in wood smoke
• Residential Wood Combustion in Felton, CA: possibility of both flaming and smoldering
Data courtesy of M. Gilroy, MBUAPCD 32 Summary of “Angstrom Analysis”
• The optical absorption at wavelength λ of aerosol containing both BC and BrC may be described as
-1 -α [mass BC] * σBC * λ + [mass BrC] * σBrC * λ
• With sufficient data points across the spectrum, we can separate these components and identify the parameters.
33 Summary of “Angstrom Analysis”
• The optical absorption at wavelength λ of aerosol containing both BC and BrC may be described as
-1 -α [mass BC] * σBC * λ + [mass BrC] * σBrC * λ ? ? • With sufficient data points across the spectrum, we can separate these components and identify the parameters. • CAUTION ! BrC is not a unique substance!
34 Implementation to Aethalometer data
• All 7-wavelength Aethalometer data can be processed by this analysis ….. • …. including the large archive of current and historical Aethalometer data.
• This analysis can be applied in real time for identification of BrC and wood-smoke impacts.
35 Summary
• Both Black and Brown Carbon aerosols have serious effects on Health, Visibility and Climate.
• BC and BrC are highly variable and must be measured.
• By means of a single instrument it is possible to separate and identify BC and BrC in real time.
• The relationship between the analyses may indicate attributes of the combustion.
• There is an empirical relationship between the BrC analysis and PM2.5 mass.
36 End of presentation
• If you are interested in any of the material shown in the slides, please write to me: • Tony.Hansen @ MageeScientific.com
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