Upload
isabel-moore
View
217
Download
0
Tags:
Embed Size (px)
Citation preview
Preliminary results from spectral characterization of aerosol absorption during FLAME
Gavin McMeekingSonia KreidenweisJeffrey Collett, Jr.
Lynn RinehartGuenter Engling
Rich CullinKip Carrico
Melissa LundenThomas Kirchstetter
Pat ArnottKristin Lewis
Cyle WoldPatrick Freeborn
Colorado State University
Lawrence Berkeley NL
University of Nevada/DRI
US Forest Service
October 30, 2006 – Group meeting
Classifying carbonTerms describing carbonaceous aerosol are defined from how each is measured and used
Light
Abso
rbin
g C
arb
on
Chemical structure controls light absorption (electrons are highly mobile in EC/BC)
Evidence of visible light absorption by organic carbon
Brown carbon: Light-absorbing organic matter in atmospheric aerosols of variousorigins – soil humics, HULIS, tarry materials from combustion, bioaerosols
Andreae and Gelencser, 2006 (AG06)
Kirchstetter et al, 2004
Hoffer et al., 2005, Havers et al., 1998, Gelencser et al. 2000 and others
Patterson and McMahon, 1984 and Bond, 2001Observed smoldering material and residential coal combustion can contain large amounts of Cbrown
Demonstrated an OC contribution to spectral light absorption for several biomass from SAFARI – same technique used in this study
Andreae and Crutzen, 1997Biogenic materials and their oxidation and polymerization products can absorb light
Fine continental aerosol contains organic carbon with properties similar to naturalhumic/fulvic substances.
Impacts of Cbrown (AG06)
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 assumedfor soot.
Light absorption measurements
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.
Tropospheric photochemistry
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.
Cloud chemistry and cloud light absorption
Impacts of Cbrown (AG06)
Significant contributions of Cbrown to tropospheric fine aerosol could bias traditionalmeasurement techniques that are carried on to calculations of light absorption.
Cbrown is volatilized over a wide range of temperatures and may be classified partly asOC and partly as EC (Mayol-Bracero et al., 2002).
Biomass smoke contains inorganic components that catalyze oxidation of soot andCbrown, 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 andlaboratory-generated soot samples (Chow et al., 2001).
Thermochemical analysis
Visible light attenuation measurements
Light box
400 – 1100 nm range with sub nanometer resolution
Spectrometer(Ocean Optics S2000)
Ten LED light source
Attenuation calculation
TATN
1ln100
r
or
os
s
I
I
II
T ,
,
Attenuation Transmission
Sample intensity
Sample filter intensity
Reference filter intensities
βλKATN wavelength
attenuation exponentassumed = 1 for EC
constant
Attenuation spectral dependence (Bohren and Huffman, small particles) :
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)
Filter solvent treatmentsSelected 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 treatments: Lodgepole Pine
water extraction results in no change
0.9
~ 1.5
acetone extraction reduces attenuation coefficient
1.0
(norm
aliz
ed)
Filter treatments: Alaskan duff
1.0
2.3
3.5
3.1 water extraction results in largest reduction
hexane extraction results in increase (normalized only)
Total carbon measurements:Evolved Gas Analysis (EGA)
to CO2 analyzer
O2 carrier gas
light source
sample oven catalyst oven
light collector
fiber optic to spectromete
r
fan
Example EGA (no treatment)
Burn B, chamise
EC
OC
attenuation at 544 nm
- flaming combustion- Low OC/EC ratio
Example EGA (no treatment)
Burn P, southern pine
ECOC
pyrolized carbon ATN
- mixed combustion
slight increasedue to oven heat
Example EGA (no treatment)
Burn G, Alaskan duff
no EC?
OC
pyrolized carbon ATN
- Smoldering combustion- High OC/EC ratio
Attenuation coefficientsAttenuation divided by total carbon mass
How will results change when ATN divided by EGA-determined OC and EC?
Front/back half filter EGA results
whole filter (26 ug cm-2)
front half (20.5 ug cm-2)
back half (5.1 ug cm-2)
Burn L, lodgepole pine- mixed combustion
EGA spectrometer measurements
No attenuation
‘charring’
Burn M, PR fern- mixed combustion
Large change in ATN(EC evolving here)
oven signal
EGA spectrometer measurements
No attenuation
‘charring’
Burn G, Alaskan duff- smoldering combustion- very little EC
No large ATN changeas seen in PR fern
oven signal
Summary of work done so far
- 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.
Attenuation
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
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
Attenuation
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 ProgramNational Park Service
USFS – Missoula Fire Science LaboratoryUS DOE Global Change Education Program
Jeff GaffneyMilton Constantin
LBNL staffJohn 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
Only temperature and light transmission can be used to make OC/EC split.
Sample heated in pure oxygen atmosphere
No temperature steps as seen in the IMPROVE and NIOSH methods.Constant heating rate of 40 C / minute
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.
Light transmission measurement over entire visible range
Measured with LiCor CO2/H2O IR gas analyzerMagnesium dioxide catalyst at 800 C
Evolved C converted to CO2