Transmission Electron Microscopy Analysis of Tarball Formation and Volatility from Biomass-burning Aerosol Particles during the 2013 BBOP Campaign

Abstract:

Tar balls, a subset of organic aerosol that is characterized by spherical shape and amorphous composition, are presumed to be brown-carbon (BrC) particles that contribute to light absorption in the atmosphere. Because of their distinctive shapes, compositions, and lack of crystallinity, tar balls are uniquely identifiable using transmission electron microscopy (TEM). Using samples collected during BBOP, we performed a detailed investigation of their compositions, formation processes, and occurrence. They primarily originated from wildfires and are most abundant in relatively aged plumes (>several hours from emission). Their features are similar to those from Mexico measured during the 2006 MILAGRO campaign, but their number fractions are more abundant in the Idaho, Oregon, and Washington samples of BBOP. In order to determine the TB robustness with respect to heating, an experiment was carried out on a TEM grid containing a mixture of organic particles, nanosphere soot, and TBs during which the sample temperature was ramped from ~30 to 650 ºC. The TEM results collected at 30 and 600 ºC, the latter representing the filament temperature used by the AMS, indicate that material of lower volatility is lost by 600 ºC. Although there was some loss of material during the 15-minute temperature ramp, TBs were still present at the higher temperature. These preliminary findings suggest that the efficiency of AMS detection of TBs may be significantly less than unity and potentially translate to an underreporting of the aerosol mass for wildfire plumes. If tar balls have low AMS detection efficiencies, an underestimate in organic aerosol mass will result.