Laboratory Experiments and Modeling to Study the Aging of Black Carbon during Atmospheric Transport

 

Authors

R. Subramanian — Carnegie Mellon University
Georges Saliba —
Janarjan Bhandari — MTU

Sharma Noopur — Michigan Technological University
Barbara Scarnato — Naval Postgraduate School
John E Shilling — Pacific Northwest National Laboratory
Shang Liu — University of Colorado, Boulder
Manvendra K. Dubey — Los Alamos National Laboratory
Gourihar Kulkarni — Pacific Northwest National Laboratory
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Alexander Laskin — Purdue University
Rahul Zaveri — Pacific Northwest National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
Claudio Mazzoleni — Michigan Technological University
Allison C Aiken — Los Alamos National Laboratory
Duli Chand — Pacific Northwest National Laboratory

Category

Absorbing aerosol

Description

Direct radiative forcing by black carbon (BC)-containing aerosols depends on the relative fraction of non-BC material (mixing state) and the morphology of individual particles both of which are highly uncertain. We investigated the optical properties, chemical composition, and morphology of combustion soot fresh and coated with secondary organic aerosol (SOA) with targeted lab experiments. In the first set of experiments (SAAS-PNNL), diesel soot was size-selected with a DMA, introduced into dual smog chambers, and progressively coated with α-pinene SOA. For some of these experiments, water was nucleated around freshly emitted BC, which was then brought to supercooled conditions or homogeneously frozen. The supercooled water droplet and the ice crystal residuals were collected on substrates and analyzed with an electron microscope, and changes in optical properties induced by the water effect on the BC structure were calculated using the discrete dipole approximation. The microscopy analysis of the BC residuals from the compact ice chamber SAAS-PNNL experiments showed that water and even more, ice processing, results in remarkable changes in the BC structure. These structural changes affect the BC optical properties as estimated from the discrete dipole approximation simulations, in particular enhancing the scattering cross section and therefore significantly affecting the BC radiative forcing. In a second set of experiments (SAAS-CMU), soot from European White Birch bark burning in a rocket stove was size-selected with a DMA, introduced into a single smog chamber, and then progressively coated with α-pinene SOA. Both fresh and coated soot were sampled using a variety of instruments and filter samplers. The mass absorption cross section (MAC) and mass scattering cross section (MSC) of fresh and coated BC aggregates were compared with predictions from Mie and Rayleigh-Debye-Gans (RDG) theories using an ensemble parameter, the total average organic to BC ratio (OA:BC.) Soot parameters for RDG modeling were obtained from SEM images. Simple forcing efficiency (SFE) of the aggregates were also modeled and compared with measurements. Mie theory reproduced the MAC and SFE of the aggregates across the whole OA:BC spectrum. The good agreement in SFE between measurements and Mie predictions across the full OA:BC spectrum, indicates little error in calculations when assuming a core and shell morphology for aged BC aggregates emitted from cook stove operation.