Transcript Visible light absorption by organic carbon during FLAME

Preliminary results from spectral
characterization of aerosol absorption
during FLAME
Colorado State University
Gavin McMeeking
Sonia Kreidenweis
Jeffrey Collett, Jr.
Lynn Rinehart
Guenter Engling
Rich Cullin
Kip Carrico
University of Nevada/DRI
Pat Arnott
Kristin Lewis
Lawrence Berkeley NL
Melissa Lunden
Thomas Kirchstetter
October 30, 2006 – Group meeting
US Forest Service
Cyle Wold
Patrick Freeborn
Biomass burning climate effects
Focus of this work
Biomass burning visibility effects
Focus of this work
adapted from Malm et al. (2004)
Classifying carbon
Terms describing carbonaceous aerosol are defined from how
each is measured and used
Light Absorbing Carbon
Chemical structure controls light absorption (electrons are highly mobile in
EC/BC)
Evidence of visible light
absorption by organic carbon
Andreae and Gelencser, 2006 (AG06)
Brown carbon: Light-absorbing organic matter in atmospheric aerosols of various
origins – soil humics, HULIS, tarry materials from combustion, bioaerosols
Patterson and McMahon, 1984 and Bond, 2001
Observed smoldering material and residential coal combustion can contain
large amounts of Cbrown
Kirchstetter et al, 2004
Demonstrated an OC contribution to spectral light absorption for several biomass
from SAFARI – same technique used in this study
Hoffer et al., 2005, Havers et al., 1998, Gelencser et al. 2000 and others
Fine continental aerosol contains organic carbon with properties similar to natural
humic/fulvic substances.
Andreae and Crutzen, 1997
Biogenic materials and their oxidation and polymerization products can absorb light
Impacts of Cbrown (AG06)
Light absorption measurements
The presence of Cbrown will lead to uncertainty in the conversion of measured
attenuation to a BC concentration if the conversion factor differs from that assumed
for soot.
Tropospheric photochemistry
Care must be taken when extrapolating absorption measurements at mid-visible
wavelengths over the solar spectrum. Downward UV irradiance can be
underestimated if the light absorbing carbon has a stronger wavelength dependency
than that typically assumed for soot.
Cloud chemistry and cloud light absorption
If a significant fraction of Cbrown is soluble in water it can alter cloud droplet light
absorption, particularly in the UV. Could be an important process in clouds formed
on/near smoke plumes.
Impacts of Cbrown (AG06)
Thermochemical analysis
Significant contributions of Cbrown to tropospheric fine aerosol could bias traditional
measurement techniques that are carried on to calculations of light absorption.
Cbrown is volatilized over a wide range of temperatures and may be classified partly as
OC and partly as EC (Mayol-Bracero et al., 2002).
Biomass smoke contains inorganic components that catalyze oxidation of soot and
Cbrown, resulting in lower evolution temperatures (Novakov and Corrigan, 1995).
Not known if Cbrown is prone to charring and if so, to what extent the TOT and TOR
OC/EC correction methods are applicable. Larger differences between measurement
techniques are seen for non-urban and biomass burning samples than for urban and
laboratory-generated soot samples (Chow et al., 2001).
Experimental setup for FLAME
May/June 2006
Filter samplers for chamber burns
Hi Vol
IMPROVE
Biomass types burned during
FLAME (chamber)
Lawrence Berkeley Lab visit
September/October 2006
Visible light attenuation measurements
Light box
Ten LED light source
Spectrometer
(Ocean Optics S2000)
400 – 1100 nm range with sub
nanometer resolution
Attenuation calculation
ATN
Attenuation
1
 100 ln
T
Transmission
Sample intensity
Reference filter
intensities
I s I r ,o
T 
I s ,o I r
Sample filter intensity
Attenuation spectral dependence (Bohren and Huffman, small particles) :
constant
ATN  K λ
β
attenuation exponent
assumed = 1 for EC
wavelength
Untreated ATN measurements
Base case measurement of
filter attenuation as a
function of wavelength
Measured for all burns A-S
on at least one 1.14 cm2
punch from “B” HiVol quartz
filter sample (PM2.5)
Ceanothus HiVol sample
Normalized light ATN for selected burns
Absorption exponent ranged from 0.8 (chamise, lignin) to ~ 3.5
(Alaskan duff, rice straw)
ATN base case results
Filter solvent treatments
Selected filter samples were extracted with hexane, acetone
and water to determine role of organic carbon and water
soluble carbon on filter attenuation
Each organic solvent extraction was performed for ~ one
hour and water extraction for 12 hours
Filter solvent treatments
Puerto Rican mixed woods
No treatment
Acetone extraction
Filter treatments: Lodgepole Pine
(normalized)
water extraction results in no change
~ 1.5
1.0
0.9
acetone extraction reduces attenuation coefficient
Filter treatments: Alaskan duff
hexane extraction results in increase (normalized only)
3.5
water extraction results in largest reduction
3.1
2.3
1.0
Filter treatments: Alaskan duff
very weak attenuation by back half of filter
Total carbon measurements:
Evolved Gas Analysis (EGA)
Total carbon measurements:
Evolved Gas Analysis (EGA)
to CO2 analyzer
light source
fan
fiber optic to
spectrometer
sample oven catalyst oven
O2 carrier gas
light
collector
EGA light source upgrade
solid quartz tube
brighter light source
filter holder
Example EGA (no treatment)
Burn B, chamise
- flaming combustion
- Low OC/EC ratio
EC
attenuation at 544 nm
OC
Example EGA (no treatment)
Burn P, southern pine
- mixed combustion
OC
EC
slight increase
due to oven heat
pyrolized carbon ATN
Example EGA (no treatment)
Burn G, Alaskan duff
- Smoldering combustion
- High OC/EC ratio
OC
no EC?
pyrolized carbon ATN
Attenuation coefficients
Attenuation divided by total carbon mass
How will results change when ATN divided by EGA-determined OC and EC?
Effect of solvent treatments on EGA
Front/back half filter EGA results
Burn L, lodgepole pine
- mixed combustion
whole filter (26 ug cm-2)
front half (20.5 ug cm-2)
back half (5.1 ug cm-2)
EGA spectrometer measurements
Burn M, PR fern
- mixed combustion
oven signal
No attenuation
Large change in ATN
(EC evolving here)
‘charring’
EGA spectrometer measurements
Burn G, Alaskan duff
- smoldering combustion
- very little EC
oven signal
No attenuation
No large ATN change
as seen in PR fern
‘charring’
Summary of work done so far
Attenuation
- Strong relationship between combustion phase and attenuation/absorption
exponent
- Exponent decreases following acetone treatment and in one case water treatment
- Attenuation coefficients (by TC) higher for flaming fuels versus smoldering fuels
- Smoldering biomass fuel emissions that contain very little-to-no EC still attenuate
light at low wavelengths.
Evolved gas analysis
- Measurements across entire visible range may provide additional insight on OC/EC
charring issues and light attenuation as a function of carbon evolution temperature
- Smoldering fuels charred significantly during EGA analysis
- All treatments led to a reduction in TC, most likely due to mechanical separation of
particles from the filter matrix
- Water treatment not only reduced amount of EC and increased the EC evolution
temperature, most likely due to the removal of inorganic ions acting as catalysts
Attenuation
Work to be done
- Quantify ATN and ATN coefficient changes under different treatments for TC and
OC/EC
- Compare filter-based ATN measurements to photoacoustic absorption
measurements, nephelometer scattering measurements and extinction cell data
- Compare ATN measurement results to chemical composition data from a variety of
sources
- Explore the significance of ATN results to visibility, radiative forcing and UV
photochemistry for areas influenced by biomass burning emissions
- Additional measurements of ATN for different polarity solvent treatments
- Compare smoldering and flaming phases of combustion for the same fuel (FLAME2)
- Analyze IMPROVE backup filters for gas-phase adsorption effects on ATN
- Compare FLAME results to other studies (fresh vs. aged smoke?)
- Develop method to deconvolute attenuation into brown carbon and black carbon
components
Evolved gas analysis
- Compare EGA results for OC/EC determination to Sunset analyzer
- Analyze spectrometer measurements taken during EGA runs and try to determine
optical properties of pyrolized carbon
Acknowledgements
Joint Fire Science Program
National Park Service
USFS – Missoula Fire Science Laboratory
US DOE Global Change Education Program
Jeff Gaffney
Milton Constantin
LBNL staff
John McLaughlin and Jeff Aguiar
Front half vs back half ATN
Punches from selected burn samples were sliced into front and
back halves and analyzed in an attempt to characterize gasses
adsorbed onto the filter
EGA measurement details
Sample heated in pure oxygen atmosphere
Only temperature and light transmission can be used to make OC/EC split.
Constant heating rate of 40 C / minute
No temperature steps as seen in the IMPROVE and NIOSH methods.
Light transmission measurement over entire visible range
Use of white light and spectrometer gives light transmission as a function of
wavelength. Method may aid OC/EC split determination if OC and pyrolized OC has a
different absorption wavelength than EC.
Evolved C converted to CO2
Measured with LiCor CO2/H2O IR gas analyzer
Magnesium dioxide catalyst at 800 C
Attenuation for selected burns (log)
flaming
smoldering