Black Carbon Aging using SOA Coatings and Coagulation to Probe Morphology-dependent Absorption Enhancements
Authors
Manvendra K. Dubey — Los Alamos National Laboratory
Claudio Mazzoleni — Michigan Technological University
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Rahul Zaveri — Pacific Northwest National Laboratory
Allison C Aiken — Los Alamos National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Shang Liu — University of Colorado, Boulder
Category
Absorbing Aerosol
Description
Figure 1. Measured absorption enhancement (E_abs) of diesel BC and SOA at 781 nm from three different experiment types during SAAS.
Black carbon (BC) has been named as potentially being the 2nd most important global warming factor behind CO2 (Bond et al., 2013). However, large uncertainties exist on the extent of the warming that BC causes, specifically due to morphology and mixing state, e.g. Cappa et al., 2012. Core-shell BC is expected to enhance absorption by up to a factor of 2, but has not been well documented in ambient data due to the difficulty of isolating BC absorption in the presence of other absorbing species. On the other hand, non-core shell BC morphologies, which have been observed in wildfire emissions (Sedlacek et al., 2012), are predicted to have little to no absorption enhancement. For these reasons, the Soot Aerosol Aging Study (SAAS at Pacific Northwest National Laboratory’s Environmental Chamber) was designed to investigate the relationship between internally-mixed BC with different morphologies and measured BC light absorption enhancements. By using the same BC (120 nm size selected diesel emissions) and non-absorbing aerosol components we can isolate the effect of morphology on BC absorption enhancements.
Three different types of experiments were conducted using size-selected diesel BC and 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. Here we focus on the enhancements at 781 nm that are tracked throughout SOA growth on BC and quantified with SP2 coating thickness. Thermal denuder (TD) experiments are conducted and absorption enhancements are calculated from two different methods that agree well with each other, confirming the observed results. BC measurements are also compared with co-located measurements from SPLAT-II and filter analysis using SEM and TEM.
BC coagulated with SOA produces minimal absorption enhancement values (<1.1), whereas coatings have significant enhancement values at 300 degrees C, up to 1.7 +/- 0.4 for thickly coated BC. Since we did not observe appreciable enhancements for the coagulated BC, we expect that ambient emissions dominated by this particle type to have enhancements due to other sources, such as brown carbon (BrC) that is often co-emitted (Saleh et al., 2014).
