Black Carbon Absorption Enhancements (Eabs) from SOA Coatings

 
Poster PDF

Authors

Allison C Aiken — Los Alamos National Laboratory
Manvendra K. Dubey — Los Alamos National Laboratory
Rahul Zaveri — Pacific Northwest National Laboratory
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory

Category

Absorbing aerosol

Description

Absorbing aerosols (AA) represent a large uncertainty in climate models today. While black carbon (BC) is historically the most studied AA, the absorption enhancements (Eabs) assumed in climate models remain un-validated and depend strongly on their 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 BC core. Ambient measurements from field campaigns indicate that BC Eabs depend strongly on source types (Cappa et al., 2012, Liu et al., 2015). However, BC absorption is difficult to constrain in ambient data due to the presence of other absorbing species, e.g. brown carbon, absorbing dusts. For these reasons, the Soot Aerosol Aging Study (SAAS) was designed. SAAS was conducted at the PNNL’s Environmental Chamber to investigate the relationship between internally-mixed BC with different morphologies and measured BC light absorption enhancements. By using the same BC core (120 nm size selected diesel) and non-absorbing aerosol components SAAS isolates the effect of morphology on diesel BC absorption enhancements. Three different types of experiments were conducted using size-selected diesel BC and α–pinene secondary organic aerosol (SOA) formed in the chamber: (1) SOA coating BC, (2) BC coagulated with SOA, (3) BC coagulated then coated with SOA. Direct on-line measurements were made with the single particle soot photometer (SP2) from fresh and aged BC. The BC measurements are coupled with photoactoustic measurements spanning the visible region to probe changes in BC light absorption when mixed with SOA. BC measurements from SPLAT-II confirm the presence of collapsed BC cores with SOA coatings. Here we focus on the enhancements at 781nm for the SOA coating experiments that are tracked through SOA growth and quantified with SP2 coating thickness. Thermal denuder (TD) experiments are used to determine Eabs, calculated using 2 different methods that agree well. TD loss rates are taken into account and determined by two methods, quantified at 14%-30% for 100-300 degC. Eabs are reported from 1.08–1.48 at varying temperatures: 100, 150, 250, and 300 degC for coatings of 50+ nm, which increase with temperature due to more complete removal of the coatings. Our observations of Eabs, BC core size, coating thickness and morphology are used to perform closure studies and evaluate Mie radiative modules used in climate models.