Emerging Technologies for Measurements of Black Carbon Gregory L. Kok Droplet Measurement Technologies Boulder, Colorado Darrel Baumgardner Centro de Ciencias de la Atmosfera Universidad Nacional Autónoma de México Mexico City, Mexico A&WMA International Specialty Conference: Leapfrogging Opportunities for Air Quality Improvement May 10-14, 2010 Xi an, Shaanxi Province, China
Need for More Comprehensive Black Carbon Measurements Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. Ramanathan, V. and G. Carmichael, 200: Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221-22. Problem: More high quality measurements of aerosol optical properties are needed but most of the current techniques require extensive corrections.
Aerosol light-absorption measurements have typically been accomplished using filter-based samplers. The attenuation of light across the filter is measured as a proxy for light absorption. This approach inherently overestimates the in situ aerosol light because these filters are multiple scattering substrates that amplify absorption. There have been dozens of studies and papers published that attempt to correct the measurements from filter-based absorption measurements.
The inter-comparison shows a large variation between the responses to absorbing aerosol particles for different types of 10 instruments. The unit to unit variability between instruments can be up to 30% for Particle Soot Absorption Photometers (PSAPs) and Aethalometers. The large variation between the response to absorbing aerosol particles for different types of instruments indicates that current correction functions for absorption photometers are not adequate
Will correction algorithms ever be adequate to compensate for multiple, complex and still unknown factors that limit filter-based techniques? Solution: Remove the filters.
The Photoacoustic Technique: A Leapfrogging Opportunity for Air Quality Improvement
Photoacoustic Technology: No filters, no interference from light scattering and can be calibrated. Aerosols absorb laser light and heat the surrounding air (1-2º C) causing pressure wave.
The technique is insensitive to the composition of black carbon: Laboratory inter-comparisons with 9 types of natural and synthetic BC, using two Photoacoustic Spectrometers, show very little variation over a wide range of BC mass concentrations. 100 9 r 2 =0.99 1 6 2 DMT PASS (Mm -1 ) 5 4 3 2 10 9 6 9 6 9 3 4 15 1=printex-5 2=Aquadag 3=Fullerene 4=nigrosine 5=GC spheres 6=Aqua Black =DFGS =DFGS+AS 9=DFGS+HM 5 5 6 9 10 2 3 4 5 6 9 100 DRI PASS (Mm -1 )
Comparison between two filter-based techniques indicates larger variability and sensitivity to composition of BC 2 r 2 =0.3 1 Aethalometer 660nm (ng m -3 ) 10 4 9 6 5 4 6 5 3 1 2 4 3 2 9 1=printex-5 2=Aquadag 3=Fullerene 4=nigrosine 5=GC spheres 6=Aqua Black =DFGS =DFGS+AS 9=DFGS+HM 10 2 3 4 5 6 9 100 Particle Soot Absorption Photometer 660 nm (Mm -1 )
The next-generation Photoacoustic Extinctiometer (PAX) is much smaller, lighter and requires less power than the existing photoacoustic instrument for aerosol measurements.
Like its larger predecessor (PASS), the PAX is the ONLY instrument that simultaneously measures light absorption and
scattering in order to derive extinction, the parameter needed for visibility and climate change studies.
PAX Specifications: -AC or DC operation -Self contained data system and pump -Factory and in the field calibrated. -Insensitive to flow rate Parameters Directly Measured: -Absorption coefficient (0 nm): <0.25 Mm -1 -Scattering coefficient (0 nm): <0.25 Mm -1 @ 60 sec averaging @ 60 sec averaging Parameters Derived: -Extinction coefficient (0 nm): <0.25 Mm -1 -Single scattering albedo -Effective black carbon mass @ 60 sec averaging
Outlook: The PAX will be deployed at several urban sites this summer for intercomparison with other instruments that measure aerosol optical properties. See a unit at the AWMA meeting in Calgary Available in late 2010 to the general community. More information: http://www.dropletmeasurement.com/products