Radiative Absorption Enhancements Due to the Mixing State of Atmospheric Black Carbon

Abstract

Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.

Formation of secondary organic aerosol coating on black carbon particles near vehicular emissions

“…were about 5-7 h in the afternoon given that the rate constant between OH radicals and NO x for HNO 3 formation (k rxn ) is equal to 7.9 × 10 −12 cm 3 molecules −1 s −1 (Brown et al, 1999;Cappa et al, 2012). Such estimation further supports our hypothesis that OOA-1 and OOA-2 were fresh SOA produced in the local atmosphere.…”

“…A complex mixture of gas-phase organic compounds with a wide range of volatilities and molecular structures are co-emitted with BC from vehicles, contributing prominently to the urban SOA burden (Gentner et al, 2017, and references therein). Although it is not straightforward to identify the role of individual SOA precursors, previous studies have shown that organic coating thickness of BC particles and their degree of oxygenation increased with photochemical age or oxidant levels in the atmosphere (Cappa et al, 2012;Liu et al, 2015;Wang et al, 2017). Of particular concern is the timescale that is required for sufficient SOA condensation to modify the physical, chemical, and optical properties of BC near emission sources.…”

“…Real-time and mass-based chemical compositions of organic coatings on ambient BC particles were seldom reported until the recent development of an Aerodyne soot-particle aerosol mass spectrometer (SP-AMS; Cappa et al, 2012;Massoli et al, 2012Massoli et al, , 2015Onasch et al, 2012;Liu et al, 2015;Lee et al, 2016;Willis et al, 2016). In this study, we investigate formation of organic coatings on BC particles by deploying a SP-AMS in Fontana, California, which is located in the broader South Coast Air Basin and includes the greater Los Angeles area.…”

“…The hydrophilic nature of SOA coating has been shown to modify hygroscopicity of ambient BC for cloud droplet activation (Kuwata et al, 2009;McMeeking et al, 2011;Laborde et al, 2013;Liu et al, 2013). Increasing coating thickness may enhance light absorption of BC due to a "lensing effect" depending on the degree of particle aging (Jacobson, 2001;Cappa et al, 2012;Peng et al, 2016;Liu et al, 2017) and alter BC morphology from highly fractal to compact structures and thus their aerodynamic properties (Moffet and Prather, 2009;Schnitzler et al, 2014;Guo et al, 2016;Peng et al, 2016). Understanding the mixing state of ambient BC and the chemical char-acteristics of its associated coatings is therefore particularly important to evaluate their fate and environmental impacts.…”

Variation of secondary coatings associated with elemental carbon by single particle analysis

“…While freshly emitted EC may contain limited coating, atmospheric aging such as coagulation with other particles, condensation of vapors, and in-cloud processing could cause EC to internally mix with other chemical species (e.g., Moffet and Prather, 2009). Cappa et al (2012) and Lan et al (2013) had observed a limited enhancement due to the mixing state of ambient EC, however, other studies confirmed the enhancement effect of internally mixing under various conditions (Wei et al, 2013;Zhang et al, 2008). The internally mixing can result in enhanced absorption by nearly 2-fold and scattering capacity by approximately 10-fold at 80% relative humidity (RH) relative to fresh particles (Zhang et al, 2008).…”