Markedly enhanced absorption and direct radiative forcing of black carbon under polluted urban environments

Abstract

Black carbon (BC) exerts profound impacts on air quality and climate because of its high absorption cross-section over a broad range of electromagnetic spectra, but the current results on absorption enhancement of BC particles during atmospheric aging remain conflicting. Here, we quantified the aging and variation in the optical properties of BC particles under ambient conditions in Beijing, China, and Houston, United States, using a novel environmental chamber approach. BC aging exhibits two distinct stages, i.e., initial transformation from a fractal to spherical morphology with little absorption variation and subsequent growth of fully compact particles with a large absorption enhancement. The timescales to achieve complete morphology modification and an absorption amplification factor of 2.4 for BC particles are estimated to be 2.3 h and 4.6 h, respectively, in Beijing, compared with 9 h and 18 h, respectively, in Houston. Our findings indicate that BC under polluted urban environments could play an essential role in pollution development and contribute importantly to large positive radiative forcing. The variation in direct radiative forcing is dependent on the rate and timescale of BC aging, with a clear distinction between urban cities in developed and developing countries, i.e., a higher climatic impact in more polluted environments. We suggest that mediation in BC emissions achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.black carbon | absorption | air quality | radiative forcing | climate B lack carbon (BC) particles, produced from incomplete fossil fuel combustion and biomass burning, are ubiquitous in the atmosphere and have profound impacts on air quality and climate (1-4). As a key short-lived climate forcer, the magnitude of BC direct radiative forcing (DRF) is dependent on the mixing state, i.e., whether particles are externally or internally mixed with other aerosol types (5, 6), and atmospheric aging by coating with secondary aerosol constituents (such as organics and sulfate) enhances the mass absorption cross-section (MAC) (5-9). Previous laboratory studies conducted under controlled experimental conditions yielded a broad range of MAC enhancements from 1.05 to 3.50, varying with the diameter, morphology, and coating of BC particles (7-15). On the other hand, a field measurement indicated a negligible absorption enhancement of ambient BC particles under a variable mixing state (16). In addition, BC aging and absorption enhancement also strongly impact visibility and atmospheric stability.Few direct measurements have been conducted to capture aging and quantify the related absorption variation of BC particles under ambient conditions. In particular, atmospheric measurements at fixed sites are affected by transport, local emissions, and chemistry, and quantification of the evolution in the BC properties (such as morphology, mixing state, and absorption and scattering coefficients) during aging involves complex decoupli...

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

“…During the peak of SOA production, the average mass loadings of rBC and VOOA were about 0.2 and 4 µg m −3 , respectively, which were a few factors to orders of magnitude lower than those generated in some recent aging experiments of soot particles (Metcalf et al, 2013;Li et al, 2017). Peng et al (2016) recently showed that the timescale for producing sufficient fresh SOA to completely modify rBC properties strongly de-pends on pollution levels. Our observations may provide insight into the design of soot aging experiments for investigating the formation rate of fresh SOA coatings (e.g., growth rate of coating thickness) as well as their environmental impacts under a more atmospherically relevant condition.…”

“…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.…”

“…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. Peng et al (2016) recently performed on-site chamber experiments to examine coating formation on sizeselected BC seeds using SOA precursors from particle-free ambient air, demonstrating that only a few hours of photochemical aging can lead to complete particle morphology modification and light absorption enhancement of BC in polluted urban regions. Moffet and Prather (2009) also provided field evidence that fresh BC can quickly evolve in terms of particle morphology in a photochemically active urban environment by developing coatings of secondary species over a timescale of several hours, highlighting the importance of local SOA chemistry in BC aging mechanisms.…”

Climate Feedback on Aerosol Emission and Atmospheric Concentrations

“…Cloud processing and photochemical aging of hydrophobic aerosols like soot or mineral dust particles may change their hygroscopic properties, in turn modifying growth and atmospheric lifetimes of the particles [78]. A multi-model study of climate impacts on aerosol finds that a global increase in most aerosol species is explained by a decrease of large-scale precipitation over land in a warmed climate.…”

The Absorption Ångström Exponent of black carbon: from numerical aspects

“…These three BC geometries are considered in the present study: lacy aggregates, compact aggregates and aggregates with coating, and are referred as the Fresh BC, Compact BC and Coated BC. The optical properties of the Coated BC were investigated by a core-shell model with a Mie theory in the past, which assumes a spherical BC core in the center of coating sphere (Chung et al, 2012, Lack et al, 2012Peng et al, 2016;Moffet and Prather, 2009), and this may introduce significant differences on BC optical properties. In this study, a compact aggregate is used for BC, and a spherical coating is added as the coating material following the ideal model developed by Liu et al (2017).…”