2010 March 17: Black Carbon Detection Systems: FAQ on the SP2 and PASS

FAQ on the SP2 and PASS
Frequently Asked Questions for the SP2 and the PASS:

1. Why is it important to measure black carbon aerosols?
2. What are the differences between the SP2 and the PASS?
3. What are the advantages of the PASS against filter-based methods like the Aethalometer?
4. Why does the SP2 use a 1064 nm laser, while the PASS-3 uses different wavelengths like 405/532/781/870 nm, and the Aethalometer uses 880 nm?
5. Can the SP2 detect brown (light-absorbing organic) carbon?
6. How does BC mixing state affect BC optical properties? What if BC is coated with brown carbon?
7. Can the SP2 detect the type of material (sulfates, organic matter) coating BC?
8. What does “the SP2 measures BC very well” mean?
9. How many intercomparisons have been conducted between the SP2, PASS, and other instruments like the Aethalometer, MAAP, PSAP, OC/EC etc.?

1. Why is it important to measure black carbon aerosols?

Black carbon (BC) particles play a major role in climate change (Jacobson, 2002; Bond, 2007; Levy et al., 2008) by absorbing solar radiation and re-radiating the energy as heat, warming the local atmosphere. BC is emitted by combustion processes, particularly in diesel engine exhaust and biomass smoke. Hence, BC is used as a marker of these anthropogenic processes for air quality monitoring (Subramanian et al., 2006). The mixing state of BC is also important from a climate change perspective, as BC coated with non-absorbing matter such as ammonium sulfate or organic compounds can absorb up to 50% more light than uncoated BC ( Bond et al., 2006).

2. What are the differences between the SP2 and the PASS?

Data from both the SP2 and the PASS are related to the black carbon content in the aerosol, and both instruments detect light scattered by the aerosol. However, there are differences between the two techniques, as described below.

The PASS measures ensemble particulate light absorption (and scattering) at different wavelengths. The light absorption in the near-infrared (781 nm or 870 nm) can be correlated against BC mass concentration, but this calibration can be spatio-temporally variable depending on the BC mixing state. The scattering detectors of the PASS can be used as a nephelometer, and the combination of absorption and scattering can produce climate-relevant properties like the single scattering albedo (SSA) from a single instrument.

The SP2 measures the incandescence from (typically) single BC-containing particles, which can be correlated to the BC mass with the help of BC proxies like Aquadag or fullerene soot. This calibration is independent of BC mixing state, and hence the SP2 is a much better measure of the BC mass concentration than the PASS (or other optical/thermal measurement techniques). Since the SP2 detects single particles, the SP2 can also measure the BC number concentration. A scattering detector is included in the SP2, which detects single particle scattering at 1064 nm, and the scattering signal can be used to indicate the BC mixing state at the single-particle level. The scattering detector can also be used to detect non-BC-containing aerosol number and mass concentrations.

PASS   SP2
Measures ensemble absorption Measures single-particle incandescence, correlated to BC mass/particle
Measures ensemble scattering (at 405 nm, 532 nm, 781 or 870 nm) Measures single-particle scattering (at 1064 nm), used to optically size aerosol particles
Absorption dependent on BC mixing state and light-absorbing organic matter Incandescence independent of BC mixing state
Data can be used to determine ensemble SSA and (with PASS-3) Angstrom Absorption Exponent Data can be used to detect single-particle BC mixing state
Absorption at 532 nm calibrated using NO2 gas, or by extinction techniques Incandescence calibrated to BC mass using DMA-sized Aquadag, fullerene soot or glassy carbon. (DMA refers to differential mobility analyzer, used to extract a single-mobility sized particle (closely approximating a mono-size) from a polydisperse aerosol.)
Scattering calibrated using PSL (monodisperse polystyrene latex spheres used for particle-size calibration) or ammonium sulfate Scattering calibrated using PSL or DMA-sized ammonium sulfate
Absorption at 781 or 870 nm can be correlated to BC mass concentration, but calibration can be spatio-temporally variable depending on BC mixing state Can detect BC mass and number concentrations, independent of BC mixing state

 

3. What are the advantages of the PASS against filter-based methods like the Aethalometer?

Filter-based methods like the Aethalometer and Particle Soot Absorption Photometer (PSAP) detect light transmission through a fibrous filter sample. Due to the filter matrix, the light transmission is subject to multiple scattering effects, and various corrections (e.g. Bond et al. 1999; Virkkula et al. 2005) have to be made for this scattering artifact, in order to obtain the particulate light absorption. Further, non-absorbing aerosol can affect the measured light absorption over and above the filter matrix effects (e.g. Lack et al. 2008; Cappa et al. 2008).

The PASS measures in-situ particulate light absorption, and hence avoids these filter matrix and non-absorbing aerosol effects altogether. The PASS can also measure particulate light scattering, which neither the Aethalometer nor the PSAP does.

4. Why does the SP2 use a 1064 nm laser, while the PASS-3 uses different wavelengths like 405/532/781/870 nm?

The PASS-3 measures particulate light absorption and scattering, and the regions typically of interest for climate research are the wavelengths in the visible sub-spectrum. The selection of the infrared wavelengths (781 or 870 nm) is based on a desire to isolate absorption by BC, as non-BC particulate absorption becomes more prominent at sub-600 nm wavelengths (Chen and Bond, 2010; Sun et al. 2007), and there is very little gaseous absorption around 781 and 870 nm.

The SP2 uses a 1064 nm laser to generate high energy densities inside the cavity (~1 MW/cm2) to ensure incandescence of the BC particle, and the Nd-YAG pumped laser system is well-understood, reliable and long-lived for these power levels.

5. Can the SP2 detect brown (light-absorbing organic) carbon?

This is a question that needs further study, but given that light-absorbing organic carbon does not appear to absorb significantly at 1064 nm, the answer would appear to be “no.”

6. How does BC mixing state affect BC optical properties? What if BC is coated with brown carbon?

Coating of BC with non-BC material typically enhances the absorption cross-section, with the enhancement dependent on the coating thickness. For more information on BC optical properties and brown (non-BC, light-absorbing organic) carbon, the following publications are recommended as a good start:

– KA Fuller, WC Malm, and SM Kreidenweis (1999). “Effects of mixing on extinction by carbonaceous particles.” J. Geophys. Res., 104(D13): 15,941-15,954.

– TC Bond, G Habib, and RW Bergstrom (2006). “Limitations in the enhancement of visible light absorption due to mixing state.” J. Geophys. Res., 111(D20211), doi:10.1029/2006JD007315.

– H Sun, L Biedermann, and TC Bond (2007). “Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol.” Geophys. Res. Lett., 34, L17813, doi:10.1029/2007GL029797.

– Y Chen and TC Bond (2010). “Light absorption by organic carbon from wood combustion.”Atmos. Chem. Phys., 10, 1773-1787.

7. Can the SP2 detect the type of material (sulfates, organic matter) coating BC?

No. However, the SP2-AMS (also called SPAMS), a collaboration between DMT and Aerodyne Research, Inc., can be used to determine the BC mass/particle as well as the composition of the material coating the BC particle.

8. What does “the SP2 measures BC very well” mean?

– The SP2 measurement of BC mass is independent of the BC mixing state and the calibration does not vary based on the ensemble aerosol composition, unlike other BC measurements techniques, including filter-based optical methods like the Aethalometer and thermal-optical OC/EC instruments.

– Since the SP2 is a single-particle instrument, it can detect BC mass concentrations below 10 ng/m3.

– The SP2 can be used to determine BC mass on a 1-second or faster time-resolution, which is useful for aircraft-based atmospheric characterization, as well as for combustion source tests such as the transient periods in dynamometer-based vehicle driving cycle tests.

9. How many intercomparisons have been conducted between the SP2, PASS, and other instruments like the Aethalometer, MAAP, PSAP, OC/EC etc.?

Intercomparisons between the SP2 and other BC-measuring techniques have been conducted at Boston College (in 2005 and 2008) and at DMT (in 2009). The results from the first Boston College study have been published:

JG Slowik, ES Cross, J-H Han, P Davidovits, TB Onasch, JT Jayne, LR Williams, MR Canagaratna, DR Worsnop, RK Chakrabarty, H Moosmüller, WP Arnott, JP Schwarz, R-S Gao, DW Fahey, GL Kok, and A Petzold (2007). “An Inter-Comparison of Instruments Measuring Black Carbon Content of Soot Particles.” Aerosol Science and Technology, 41(3):295–314.

Manuscripts from the other two studies are expected to be published in late 2010.

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