Detection of tar brown carbon with a single particle soot photometer (SP2)

Abstract

Abstract. We investigate the possibility that the refractory, infrared-light-absorbing carbon particulate material known as “tarballs” or tar brown carbon (tar brC) generates a unique signal in the scattering and incandescent detectors of a single particle soot photometer (SP2). As recent studies have defined tar brC in different ways, we begin by reviewing the literature and proposing a material-based definition of tar. We then show that tar brC results in unique SP2 signals due to a combination of complete or partial evaporation, with no or very little incandescence. Only a subset of tar brC particles exhibited detectable incandescence (70 % by number); for these particles the ratio of incandescence to light scattering was much lower than that of soot black carbon (BC). At the time of incandescence the ratio of light scattering to incandescence from these particles was up to 2-fold greater than from soot (BC). In our sample, where the mass of tar was 3-fold greater than the mass of soot, this led to a bias of <5 % in SP2-measured soot mass, which is negligible relative to calibration uncertainties. The enhanced light scattering of tar is interpreted as being caused by tar being more amorphous and less graphitic than soot BC. The fraction of the tar particle which does incandesce was likely formed by thermal annealing during laser heating. These results indicate that laser-induced incandescence, as implemented in the SP2, is the only BC measurement technique which can quantify soot BC concentrations separately from tar while also potentially providing real-time evidence for the presence of tar. In contrast, BC measurement techniques based on thermal–optical (EC: elemental carbon) and absorption (eBC: equivalent BC) measurements cannot provide such distinctions. The optical properties of our tar particles indicate a material similarity to the tar particles previously reported in the literature. However, more- and less-graphitized tar samples have also been reported, which may show stronger and weaker SP2 responses, respectively.

Rapid evolution of aerosol particles and their optical properties downwind of wildfires in the western US

“…Quantities related to aerosol oxidation state, f 44, O : C, and OSc monotonically change with age for all flights, albeit with some scatter and flightto-flight differences. f 60, which is a surrogate for primary levoglucosan, and related compounds (Cubison et al, 2011) decrease with age for all flights (Fig. S10c).…”

“…Evidence to the contrary, namely, that TBs are detected with the same efficiency as other organic aerosol by an AMS, is that TBs have the correct magnitude and volatility (up to 200 • C) to coincide with BBOA-3, a low-volatility PMF factor of BB smoke sampled at MBO by Zhou et al (2017). Sedlacek et al (2018b) and Corbin and Gysel-Beer (2019) showed that, depending on laser power, TBs can be charred by an SP2 YAG laser identical to that used in the SP-AMS. The charred residue has the potential to be detected as BC in a SP-AMS.…”

An overview of optical and thermal methods for the characterization of carbonaceous aerosol

“…Main advantages of SP2 are the possibility to have online measurements of BC containing particles, over a quite large range of mass concentrations, with information on size too. Hints on coating materials and/or on internal mixing of BC particles can be achieved, especially in the case of volatile materials [193][194][195]. Smaller rBC particles can also be detected, although with reduced counting efficiency [192].…”

Comparison of co-located refractory black carbon (rBC) and elemental carbon (EC) mass concentration measurements during field campaigns at several European sites

“…The OC-EC split can be also biased by EC precombustion: EC can thermally evolve in the presence of oxidizing species (Watson et al, 2005;Corbin et al, 2014Corbin et al, , 2015, and soluble inorganic compounds (Chow et al, 2001;Yu et al, 2002) and metal salts (Aakko-Saksa et al, 2018) can catalyze EC pre-combustion. If the amount of EC undergoing pre-combustion is significant relative to the amount of PC formed during the analysis, the optical correction (transmittance or reflectance) is not able to account for it, and this may cause an underestimation of the EC concentration.…”