Light Absorbing Carbonaceous Aerosols (BC and BrnC) and their Climate Impacts

Transcription

1 Light Absorbing Carbonaceous Aerosols (BC and BrnC) and their Climate Impacts J I U M E N G L I U E A S : C L O U D S, A E R O S O L S A N D C L I M AT E M A R. 2 5,

2 Terminology Light absorbing carbonaceous aerosols are divided into two categories: black carbon (BC) and brown carbon (BrnC). BC: a strong absorber of visible and near-ir light; generally implied to have optical properties and composition similar to soot carbon. BrnC: Light-absorbing organic matter in atmospheric aerosols of various origins, e.g., soil humics, humic-like substances (HULIS), tarry materials from combustion, bioaerosols, etc.

3 Current progress on BC Microphysical properties of black carbon Constraints on BC atmospheric abundance: acceptable and explainable bias among models Climate impacts: large uncertainty, BrnC involved (Hansen et al., JGR, 2005)

4 Scientific questions for BrnC What is the current measurement capability of BrnC? What do we know about the optical properties of BrnC? How optically important is BrnC comparing to BC? What do we know about the climate impacts of BrnC?

5 Some background of BrnC 350 nm 800 nm o Heterogeneous (aqueous) reactions of carbonyls (eg. glyoxal), BSOA with amine, NH 3, NH 4 + o Aromatic SOA formed under high Nox o Jaoui et al., JGR, 2008 Abs, Mm appears brown wavelength, nm

6 Sources of Brown carbon Sources of Brown carbon Primary Sources Secondary Sources Biomass Burning Coal Combustion Vehicles Other Primary sources Formed in heterogeneous reactions from dienes.(limbeck et al., 2003). Formed in multiphase reactions with OH radicals in cloud water.(gelencser et al., 2003). Produced by Anthropogenic/biogenic VOC under high-no x condition (Zhang et al., 2011 ) Other Mechanisms

7 Scientific questions for BrnC What is the current measurement capability of BrnC? What do we know about the optical properties of BrnC? How optically important is BrnC comparing to BC? What do we know about the climate impacts of BrnC?

8 Remote sensing o Single-angle techniques: cannot distinguish the scattering component of extinction from the absorption component o Eg., mono-static lidaror simple sun photometry o Multi-angle and multi-wavelength observations o Eg., AERONET and ARM o Successful in characterizing aerosol light absorption for specific aerosol types during specific events(such as biomass burning) o Pros: atmospheric distribution o Apply radiative inversion methods to infer optical properties o Yield column-integrated, effective values, potentially including different aerosols and mixing states o Eg. Ångstrom exponent ~1.2 from AERONET

9 In-situ, no filter based o Convert optical signal into other signals o Photoacoustic technique o eg., PAS, creating a pressure increase o Refractive index-based techniques o refractive index change due to the absorption of a laser beam o Thermal in-situ techniques o particle incandescence, rely upon the heating and expansion of the air surrounding absorbing particles o Extinction-minus-scattering techniques o measure the sum of the absorption coefficients of the particle ensemble and the surrounding air o Interferences from gaseous absorption o Temperature/pressure sensitive

10 In-situ, filter-based o Direct measurement of aerosol light absorption concentrate and deposit aerosols on particle filters o Pro: highly time-resolved (seconds) o Usually with several fixed wavelengths o Potential systematic errors due to multiple scattering by filter medium and deposits of aerosols, angular distribution of scattered light, etc. o Measure BrnC+BC absorption o effective BC [Moosmuller et al., 2009]

11 [Moosmuller et al., 2009]

12 Filter-based, in Lab o Highly wavelength resolved o Chemically-resolved o Heterogeneous (aqueous) reactions of carbonyls (eg. glyoxal), BSOA with amine, NH3, NH4+ o Aromatic SOA formed under high NOx Abs, Mm wavelength, nm Data courtesy: Xiaolu Zhang

13 Filter-based, in Lab o Comprehensive Molecule in solution Absorption of solution m=n+ik, Ångstrom exponent, etc. Particles in air BrnC b ap (λ) Absorption of particles in the atmosphere o Insight into chemical properties of BrnC o No black carbon interferences o Whole spectra (vs. several fixed wavelengths) o Limited time resolution o Only applies for dissolved components

14 Absorption Scientific questions for BrnC What is the current measurement capability of BrnC? What do we know about the optical properties of BrnC? How optically important is BrnC comparing to BC? What do we know about the climate impacts of BrnC? Absorption Ångstrom exponent (AAE) Mass Absorption Coefficients Refractive index MAC=0.12 g/m 2 m 2 /g WSOC

15 AAE Remote sensing: (AERONET, mix of BC, BrnC, dust, etc.) 4.55±2.01 (OC) In situ measurements Ambient AAE: (mix of BC, BrnC, dust, etc.) Treated BrnC AAE: 3-6 (a suite of assumptions, eg., BC AAE =1) In lab measurements from solution spectra From single compounds: 4-6 for organic solvents, 6-8 for watersolubles Semi-controlled: up to 15 reported (pine burning) Ambient bulk organics: 1-7 [Bahadur et al., PNAS, 2012]

16 Mass Absorption Coefficients Wavelength dependent Large variation Aerosol properties Variety of locations Analytical methods Sources Single compound vs. bulk Measurement vs. model Abs, Mm wavelength, nm Sample λ (nm) MAC (m2/g) Reference Brown carbon produced by aging SOA with 100 ppb NH3 (lab) Updyke et al., 2012 Tar balls from smoldering combustions of wood (lab) 532 Humic-LIke Substances (HULIS) extracted from filter samples from various sites in Europe (calculated from k= ) (calculated from k = ) Chakrabarty et al.,2010 Dinar et al., 2008 Methanol extracts from wood combustion particles (lab) Chen and Bond, 2010 Refractory organic carbon from biomass burning in North America (INTEX/ICARTT) Clarke et al., 2007 Brown carbon in particles collected in Asia (EAST-AIRE) Yang et al., 2009 Acetone extracts from biomass burning aerosols in Africa (SAFARI 2010) Kirchstetter et al., 2004

17 Refractive Index Refractive indices are ALWAYS inferred by assuming a theory and applying it to optical measurements. i.e., for imaginary part, Wavelength dependent Large variation Reported values: at nm Assumptions applied Aerosol mixing state Aerosol density Alexandre et al., Science, 2008

18 Scientific questions for BrnC What is the current measurement capability of BrnC? What do we know about the optical properties of BrnC? How optically important is BrnC comparing to BC? What do we know about the climate impacts of BrnC? Seasonally Regionally Globally Vertically

19 Seasonally Undefined Major differences: Seasonal difference of organics abundance Different sources result in difference in optical properties Ammonium Nitrate Sulfate EC Organic carbon Summer Undefined Ammonium Organic carbon EC Nitrate [Bahadur et al., PNAS, 2012] Sulfate Winter Figure courtesy: Zhenyu Du

20 Regionally Limited measurements/data available Values vary among different locations Sources, eg., anthropogenic vs. biomass burning, etc. Wavelength (nm) brnc fraction Location Reference 300 Up to 50% Rondˆonia, Brazil Hoffer et al., ~40% Mexico City Barnard et al., ~30% Xianghe, China Yang et al., ~10% Xianghe, China Yang et al., ~20% CA Bahadur et al., % Los Angeles region, CA Cappa et al., % Los Angeles region, CA Cappa et al., % Chung et al., 2012

21 Vertical profile Interpolation from models Run model with different mixratios of BrnC and BC to get vertical profiles Constrain with observations Why? No direct measurement of pure BrnC Better agreement, BrnC to BC ratio increases with altitude Increasing importance of BrnC with higher altitude Absorption (x10-5 m -1 ) Park et al., 2010

22 Globally Two ways Remote sensing measurement Global models Constrained with satellite data Chung et al. 2012: 20% contribution from BrnC to carbonaceous aerosols at 550nm Constrained with AERONET data Partition BC and BrnC assuming no BrnC absorption at 675nm Global models Feng et al., 2013: 8 26% % at 450nm and up to 56% at 350 nm Park et al., 2010: 27% contribution at 550nm Problems: Very limited studies: needs further comparison between measurements and model results Large uncertainty from both measurement and models eg. Optical properties of BrnC unclear

23 Scientific questions for BrnC What is the current measurement capability of BrnC? What do we know about the optical properties of BrnC? How optically important is BrnC comparing to BC? What do we know about the climate impacts of BrnC? Park et al., 2010

24 Climate impacts Very limited studies Currently only focusing on direct forcing indirect forcing involves more complex BrnC properties such as CCN activity, hygroscopicity, aerosol mixing state, etc. Estimations varied a lot Probably because the optical properties applied into models are largely uncertain BrnC forcing at TOA, Wm -2 BrnC forcing at surface, Wm -2 Reference Park et al., ~ ~-0.75 Chung et al., Feng et al., 2013

25 Conclusions and the way forward Measurement capabilities: further validation, including inter-comparison among instruments needed Optical properties: more information needed regarding to chemical/microphysical properties of organics Optical importance and climate impacts: models On the particle level a. Mixing state of the aerosols (internal/external) b. Water-uptake properties (hygroscopicity) c. CCN/IN activity of organics (wettability, surface tension) d. Size distribution of BrnC e. Optical properties On the model scale a. Three-dimensional mass abundance/mixing of carbonaceous aerosols b. Cloud processing with carbonaceous aerosols

26 Any Questions?

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