AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Isolation and Quantification of Black Carbon to Study Climate Impacts
ALLISON AIKEN, Manvendra Dubey, Rahul Zaveri, John Shilling, Alla Zelenyuk, Claudio Mazzoleni, Gavin McMeeking, Ezra Levin, Paul DeMott, Sonia Kreidenweis, Los Alamos National Lab
Abstract Number: 595 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Absorbing aerosols (AA) represent a large uncertainty in climate models today. While refractory black carbon (rBC) is historically the most studied AA, the absorption enhancements (Eabs) assumed in climate models remain un-validated and depend strongly on morphologies and composition. When using core-shell Mie Theory, Eabs can be as large as a factor of 2 for non-absorbing coatings on a rBC core. Ambient measurements from field campaigns indicate that rBC Eabs depend strongly on source types (Cappa et al., 2012, Liu et al., 2015). rBC absorption is difficult to constrain in ambient data due to the presence of other absorbing species, e.g. brown carbon, absorbing dusts. For this reason, we present two measurement techniques to isolate physical and optical properties specific to rBC. The first well-established method using a thermal denuder for the removal of non-refractory species employs laser-induced incandescence (LII) in the SP2 to detect rBC and photoacoustic spectroscopy to directly measure aerosol absorption. rBC mass and size distributions are coupled with absorption measurements to directly quantify rBC optical properties from both lab and ambient studies. The second technique is a new method that uses the laser-induced vaporization (LIV) that occurs within the SP2 to remove the incandescent particles, dominated by rBC for ambient aerosols. For the first method (rBC Isolation) rBC-specific mass absorption coefficients (MAC’s), single scatter albedo (SSA) and absorption enhancements (Eabs) are measured directly for SOA coating experiments on mobility-diameter selected diesel soot cores. Measurements are also compared with Mie theory calculations. Results from both ambient and laboratory data, including urban and biomass burning sources, will be presented indicating the importance of regional and source-specific studies. The second method (rBC Removal) was recently developed to isolate the effects of rBC by removing rBC from mixed aerosol samples (Levin et al., 2014). Here the SP2 is repurposed as a pre-treatment, similar to how the thermal denuder is used in the first method. The LII and the subsequent LIV that occur within the SP2 are used to remove the particles that incandesce. Removal rates and the efficacy of the method are presented based on laboratory quantification using the rBC surrogate Aquadag and a second SP2 for detection of the remaining incandescent particles (Aiken et al., 2016). Removal of Aquadag is efficient for particles >100 nm mass-equivalent diameter (dme), enabling application for microphysical studies. However, the removal of particles ≤100 nm dme is less efficient due to the formation of small particles by fragmentation and recondensation of the volatilized material. In addition, incomplete vaporization that can also occur for smaller rBC particles in the SP2 can also contribute to the particles that remain after LII. While small in mass, the particles remaining after undergoing LII and LIV in the SP2 have a significant effect on the residual absorption and incandescent particle number remaining. Therefore and in summary, future work is required before the rBC removal technique can be used as a pre-treatment for wide-ranging applications similar to how the thermal denuder is employed.