A study of the morphology and effective density of externally mixed black carbon aerosols in ambient air using a size-resolved single-particle soot photometer (SP2)

Abstract

Abstract. The morphology and effective density of externally mixed black carbon (extBC) aerosols, important factors affecting the radiative forcing of black carbon, were studied using a tandem technique coupling a differential mobility analyzer (DMA) with a single-particle soot photometer (SP2). The study extended the mass–mobility relationship to large extBC particles with a mobility diameter (dmob) larger than 350 nm, a size range seldom included in previous tandem measurements of BC aggregates in the atmosphere. The experiment was conducted at an urban site in Beijing during a 19 d winter period from 23 January to 10 February 2018. Ambient dry particles were selected by the DMA, and the size-resolved extBC particles were distinguished from particles with a thick coating (internally mixed) according to the time delay between the incandescence signal peak and the scattering peak detected by the SP2. The masses of the extBC particles were then quantified. The time differences between the DMA size selection and the SP2 measurement were processed previously. The normalized number size distributions were investigated at the prescribed dmob sizes in the range of 140–750 nm to provide the typical mass of extBC at each dmob. On this basis, the mass–mobility relationship of the ambient extBC was established, inferring a mass–mobility scaling exponent (Dfm) (an important quantity for characterizing the morphology of fractal-like BC aggregates) with a value of 2.34±0.03 in the mobility range investigated in this study. This value is comparable with those of diesel exhaust particles, implying a predominant contribution of vehicle emissions to the ambient extBC in urban Beijing. Compared to the clean period, a higher Dfm value was observed in the polluted episode, indicating a more compact BC aggregate structure than that in the clean period. The effective densities (ρeff) of the extBC in the same dmob range were also derived, with values gradually decreasing from 0.46 g cm−3 at 140 nm mobility to 0.14 g cm−3 at 750 nm mobility. The ρeff values were slightly lower than those measured using the DMA–aerosol particle mass analyzer (APM) system. The difference in ρeff values was likely due to the lower BC masses determined by the SP2 compared to those measured by the APM at the same mobility, since the SP2 measured the refractory BC (rBC) mass instead of the total mass of the BC aggregate, which consists of both rBC and a possible fraction of nonrefractory components measured by the APM. The ρeff values in the 280–350 nm dmob range were much closer to the values for soot aggregates reported in the literature. It might be related to the more compact structure of BC aggregates in this range, resulting from the reconstruction effect by volatile and/or semivolatile components in the atmosphere. The reconstruction effect might also result in a hiatus in the increased dynamic shape factor in the range of 200–350 nm, which presented an overall increase from 2.16 to 2.93 in the 140–750 nm dmob range.

Impact of combustion conditions on physical and morphological properties of biomass burning aerosol

“…The maximum asymptote of (2.97 ± 0.05) indicates that the particles emitted during burns with MCE 0.92 are homogeneous spheres having mass mobility exponents of $3, which is consistent with the brown carbon emitted during smoldering peat burning in a previous study (Sumlin et al 2018). Furthermore, the fitting coefficient base represents the minimum asymptote with a value of 2.26 ± 0.05, indicating that particles emitted during burns with MCE > 0.95 are non-spherical with mass mobility exponents similar to the previously studied externally mixed BC and diesel exhaust particles, with D fm ranging from 2.3 to 2.41 (Wu et al 2019;Rissler et al 2014;Maricq and Xu 2004;Park et al 2003a, Park, Kittelson, andMcMurry 2004). However, the minimum asymptote value found in this study is lower than the value (D fm ¼ 2.48) proposed by Olfert and Rogak (2019) in their recent review article, but the variation in the literature is wide.…”

“…where C is a pre-factor and D fm is the mass mobility exponent (Sumlin et al 2018;Radney and Zangmeister 2016;McMurry 2004, Park et al 2003a). D fm is commonly used to describe the particle morphology, with a value $3 for spherical particles and <3 for non-spherical particles (Wu et al 2019;McMurry 2004, Park et al 2003a;McMurry et al 2002). Furthermore, D fm can be converted to the fractal dimension, which is estimated from transmission electron microscopy image analysis (Park, Kittelson, and McMurry 2004).…”

“…Even though the DMA-particle mass analyzer system is very popular in determining the q eff of engine exhaust particles (Olfert et al 2017;Park, Kittelson, and McMurry 2004;Maricq and Xu 2004;Sakurai et al 2003;Park et al 2003a) and ambient aerosols (Wu et al 2019;Qiao et al 2018;Rissler et al 2014), the q eff measurement of primary aerosol emitted from BB are still limited (Sumlin et al 2018;Zhai et al 2017;Li et al 2015). Most of the studies that determined aerosol mass from the particle size distribution applied an assumed density ranging from 1.0 g cm À3 to 1.7 g cm À3 regardless of the diameter, aging conditions, and morphology of the particles (Corbin et al 2019;Kumar et al 2018;Gkatzelis et al 2016;Saliba et al 2016;Vakkari et al 2014;Flowers et al 2010).…”

Technical note: Measurement of chemically resolved volume equivalent diameter and effective density of particles by AAC-SPAMS

“…Combining a single-particle soot photometer (SP2) with a (volatility) tandem differential mobility analyzer ((VT)DMA) can measure the ρ I e of particles mixed with soot (Y. Wu et al, 2019;Han et al, 2019). The measurement of chemically resolved ρ III e can be achieved by coupling a DMA with an online aerosol mass spectrometer such as the Single-Particle Laser Ablation Time-of-Flight Mass Spectrometer (SPLAT) (Zelenyuk et al, 2005(Zelenyuk et al, , 2006Alexander et al, 2016), aerosol mass spectrometer (AMS) (Dinar et al, 2006;Schneider et al, 2006;Kiselev et al, 2010), aerosol time-of-flight mass spectrometer (ATOFMS) (Spencer and Prather, 2006;Spencer et al, 2007), and single-particle aerosol mass spectrometry (SPAMS) (G. Zhai et al, 2017).…”

“…Size and ρ e govern the transport properties of a particle both in the atmosphere and in the human respiratory system (Seinfeld and Pandis, 1998;Liu and Daum, 2008) and directly and/or indirectly influence the potential of the particle to absorb or reflect solar radiation (Tang, 1997;Zhao et al, 2019;Liu and Daum, 2008). ρ e can also provide information concerning particle morphology (Yon et al, 2015) and serve as a tracer for atmospheric processing (Guo et al, 2014;Yin et al, 2015;Liu et al, 2015). However, the quantitative relationship between aerosol properties, namely size and ρ e , and their effects on air quality, human health, and global climate change are not yet well understood, which is partly because important aerosol properties cannot be measured by current techniques.…”