Roles of water-soluble aerosol coatings for the enhanced radiative absorption of black carbon over south asia and the northern indian ocean

Black Carbon (BC), formed by incomplete combustion, absorbs solar radiation and heats the atmosphere. We investigated the enhancement in optical absorption of BC due to coatings of water-soluble (WS) species in the polluted South Asian atmosphere. The BC Mass Absorption Cross-section (MAC; 678 nm) was estimated before and after removal of the WS components. Wintertime samples were collected from three South Asian receptor observatories intercepting large-footprint outflow: Bangladesh Climate Observatory Bhola (BCOB; integrating outflow of the Indo-Gangetic Plain), Maldives Climate Observatories at Hanimaadhoo (MCOH) and at Gan (MCOG), both reflecting outflow from the South Asian region. The ambient MAC observed at BCOB, MCOH and MCOG were 4.2 ± 1.4, 7.9 ± 1.9 and 7.1 ± 1.5 m2 g-1, respectively. The average enhancement of the BC MAC due to WS coatings (i.e., ws-EMAC) was identical at all three sites (1.6 ± 0.5) indicating that the anthropogenic aerosols had already evolved to a fully coated morphology at BCOB and/or that subsequent aging involved two compensating evolution processes of the coating. Inspecting the key coating component sulfate; the sulfate-to-BC ratio increased threefold when transitioning from BCOB to MCOH and by about 1.5 times from BCOB to MCOG. Conversely, both WS organic carbon (WSOC)/BC and water-insoluble OC (WIOC)/BC ratios declined with distance: WSOC/BC diminished by 84 % from BCOB to MCOH and by 80 % from BCOB to MCOG, while WIOC/BC dropped by about 63 % and 59 %, respectively. Such declines in WSOC and WIOC reflect a combination of photochemical oxidation and more efficient washout of OC compared to BC. The observed changes in the SO42-/BC and WSOC/BC ratios across South Asia highlight the significant impact of aerosol composition on the optical properties of Black Carbon (BC). These findings emphasize the need for detailed studies on aerosol composition to improve climate models and develop effective strategies for reducing the impact of anthropogenic aerosols on the climate.

A review of quantification methods for light absorption enhancement of black carbon aerosol

Black carbon (BC) is a distinct type of carbonaceous aerosol that has a significant impact on the environment, human health, and climate. A non-BC material coating on BC can alter the mixing state of the BC particles, which considerably enhances the mass absorption efficiency of BC by directing more energy toward the BC cores (lensing effect). A lot of methods have been reported for quantifying the enhancement factor (Eabs), with diverse results. However, to the best of our knowledge, a comprehensive review specific to the quantification methods for Eabs has not been systematically performed, which is unfavorable for the evaluation of obtained results and subsequent radiative forcing. In this review, quantification methods are divided into two broad categories, direct and indirect, depending on whether experimental removal of the coating layer from an aged carbonaceous particle is required. The direct methods described include thermal peeling, solvent dissolution, and optical virtual exfoliation, while the indirect methods include intercept-linear regression fitting, minimum R squared, numerical simulation, and empirical value. We summarized the principles, procedures, virtues, and limitations of the major Eabs quantification methods and analyzed the current problems in the determination of Eabs. We pointed out what breakthroughs are needed to improve or innovate Eabs quantification methods, particularly regarding the need to avoid the influence of brown carbon, develop a broadband Eabs quantification scheme, quantify the Eabs values for the emissions of low-efficiency combustions, measure the Eabs of particles in a high-humidity environment, design a real-time monitor of Eabs by a proper combination of mature techniques, and make more use of artificial intelligence for better Eabs quantification. This review deepens the understanding of Eabs quantification methods and benefits the estimation of the contribution of BC to radiative forcing using climate models.

The Measurement of Atmospheric Black Carbon: A Review

Black Carbon (BC), the second-largest contributor to global warming, has detrimental effects on human health and the environment. However, the accurate quantification of BC poses a significant challenge, impeding the comprehensive assessment of its impacts. Therefore, this paper aims to critically review three quantitative methods for measuring BC: Thermal Optical Analysis (TOA), the Optical Method, and Laser-Induced Incandescence (LII). The determination principles, available commercial instruments, sources of deviation, and correction approaches associated with these techniques are systematically discussed. By synthesizing and comparing the quantitative results reported in previous studies, this paper aims to elucidate the underlying relationships and fundamental disparities among Elemental Carbon (EC), Equivalent Black Carbon (eBC), and Refractory Black Carbon (rBC). Finally, based on the current advancements in BC quantification, recommendations are proposed to guide future research directions.

Size-resolved mixing state and optical properties of black carbon at an urban site in Beijing

The size-resolved (200-700 nm) mixing state and optical properties of black carbon (BC) in Beijing in the spring of 2019 were investigated using a tandem system consisting of an aerodynamic aerosol classifier, a nephelometer, and a single particle soot photometer. The results showed that the coating thickness distribution exhibited a clear bimodal pattern for BC-containing particles with a fixed aerodynamic diameter (D_ae). Based on the coating thickness, BC-containing particles can be classified as having external and internal mixing states. The number fraction of internal BC-containing particles increases with increasing D_ae and reaches 95% when D_ae = 700 nm. Both the BC core diameter and coating thickness simultaneously increased with an increasing D_ae of BC-containing particles. The dynamic shape factor (χ) of BC-containing particles decreased from 1.43 to 1.0 as D_ae increased from 200 nm to 400 nm and varied around 1.0 when D_ae = 500-700 nm. This demonstrated that thickly coated BC-containing particles were more likely to have regular shapes. An observation-constrained simulation on the basis of Mie theory showed that the coating plays an important role in light absorption. The amplification of BC absorption by the coating increased from 1.21 to 1.75 with increasing D_ae due to the thicker coating of BC-containing particles with a larger D_ae. The single-scattering albedo was dependent on size, increasing from 0.83 to 0.98 with increasing D_ae. The size-dependent characteristics of BC-containing particles were similar under different pollution conditions, but BC-containing particles tended to be larger with a thicker coating and have a larger absorption enhancement under polluted conditions (PM_2.5 > 75 μg/m³) than under clean conditions (PM_2.5 < 35 μg/m³). This study highlights the strong dependence of the microphysical and optical properties of BC on size.

Degradation of PAHs during long range transport based on simultaneous measurements at Tuoji Island, China, and at Fukue Island and Cape Hedo, Japan

We investigated the degradation of polycyclic aromatic hydrocarbons (PAHs) during long-range transport. Aerosols were collected simultaneously at remote sites on Tuoji Island, China; Fukue Island, Japan; and the Cape Hedo Atmosphere and Aerosol Measurement Station (CHAAMS), Okinawa, Japan in April, October, and December from 2012 to 2013. These remote sites were convenient for investigating the degradation of PAHs during long-range transport. PAHs were analyzed via gas chromatography/mass spectrometry. We identified air masses that passed over all sites and combined our measurements with a chemical transport model. We estimated the relative contributions of the PAHs at the three sites by normalizing the PAH concentrations to elemental carbon. Benzo[a]pyrene persisted in 5-16% of samples. The results of this study are consistent with laboratory studies in which secondary organic aerosol (SOA) coatings protected PAHs from degradation by ozone. We detected an inhibition of the degradation PAHs by SOA coatings by collecting PAHs simultaneously at the three sites. To elucidate the major sources of the SOAs, we carried out a positive matrix factorization analysis to identify the major sources of SOA coating, which controls the lifetime of PAHs. In spring and winter, the contribution of vehicle emissions was higher (46%) at Tuoji Island than at CHAAMS (13%). In contrast, the contribution of coal combustion was higher at CHAAMS (59%) than at Tuoji Island (28%). This result implies that during long-range transport, PAHs derived from coal combustion are more slowly degraded than PAHs derived from vehicle emissions. We found that the viscosity of SOA coatings derived from vehicle emissions in China was low, and the corresponding PAHs were rapidly degraded. In contrast, the viscosity of SOA coatings derived from coal combustion was high, and degradation of the corresponding PAHs was relatively slow. These results imply that PAHs derived from coal combustion have long lifetime.

Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition

Absorption by black carbon strongly affects regional and global climate. Yet, large discrepancies between standard model predictions and regionally specific observations—often with observed absorption lower than expected—raise questions about current understanding of black carbon absorption and its atmospheric impacts. Through a combination of measurement and modeling, our analysis resolves the discrepancy by showing that particular laboratory designs or atmospheric conditions engender distinct compositional heterogeneity among particles containing black carbon. Lower-than-expected absorption results largely from increased heterogeneity, although slightly lowered absorption occurs even in a purely homogeneous system. This work provides a framework that explains globally disparate observations and that can be used to improve estimates of black carbon’s global impact.

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC’s radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model–measurement differences. We show that accounting for these two effects—variability in per-particle composition and deviations from the core-shell approximation—reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC’s radiative effect on climate.

Changes in black carbon and PM2.5 in Tokyo in 2003-2017

Black carbon (BC) particles cause adverse health effects and contribute to the heating of the atmosphere by absorbing visible solar radiation. Efforts have been made to reduce BC emissions, especially in urban areas; however, long-term measurements of BC mass concentration (M_BC) are very limited in Japan. We report M_BC measurements conducted in Tokyo from 2003 to 2017, showing that M_BC decreased by a factor of 3 from 2003 to 2010 and was stable from 2010 to 2017. Fine particulate concentrations (PM_2.5) decreased by a much smaller factor during 2003-2010. The diurnal variations of BC size distributions suggest that the BC in Tokyo originates mainly from local sources, even after 2010. Our three-dimensional model calculations show that BC from the Asian continent contributes a small portion (about 20%) of the annual average M_BC in the Kanto region of Japan, which includes Tokyo. This indicates that continued reduction of BC emissions inside Japan should be effective in further decreasing M_BC.

Microscopic comparison of aerosol particles collected at an urban site in North China and a coastal site in Japan

In order to understand the physicochemical properties of aerosol particles in Japan and China, transmission electron microscopy was used to analyze individual aerosol particles collected at two very different environmental sites, i.e., a continental site (T1) in North China and a coastal site (T2) in Japan in springtime. The average PM_2.5 concentration (52μg/m³) at T1 was much higher than T2 (20μg/m³) from 20 to 23 March 2014. Our study shows that sulfur-organic matter (S-OM) particles were the most abundant at both T1 and T2, and individual spherical primary organic matter (POM) particles were only observed at T1. More anthropogenic fly ash and metal particles were observed at T1, consistent with the heavier air pollution at T1 than T2, and the overall complexity of aerosol composition at T1 exceeded that at T2, due to the influence of regional industrial emissions. Further examination of S-OM particles suggest that S-OM coated particles accounted for 29.6% of total observed particles at T2 but only 8.6% at T1. However, the average thickness of OM coating was larger at T2 than at T1, indicating that the particles at T2 had aged longer than those at T1. These comparisons suggest that the OM coating thickness on sulfate depends on the transport distance of the air mass and on the concentration of atmospheric oxidants but should not be used to represent pollution levels. Compared with the coastal air in Japan, we know that urban aerosol particles in North China not only attain high concentrations but also have more complex aerosol components.

Black carbon radiative effects highly sensitive to emitted particle size when resolving mixing-state diversity

Post-industrial increases in atmospheric black carbon (BC) have a large but uncertain warming contribution to Earth's climate. Particle size and mixing state determine the solar absorption efficiency of BC and also strongly influence how effectively BC is removed, but they have large uncertainties. Here we use a multiple-mixing-state global aerosol microphysics model and show that the sensitivity (range) of present-day BC direct radiative effect, due to current uncertainties in emission size distributions, is amplified 5-7 times (0.18-0.42 W m-2) when the diversity in BC mixing state is sufficiently resolved. This amplification is caused by the lifetime, core absorption, and absorption enhancement effects of BC, whose variability is underestimated by 45-70% in a single-mixing-state model representation. We demonstrate that reducing uncertainties in emission size distributions and how they change in the future, while also resolving modeled BC mixing state diversity, is now essential when evaluating BC radiative effects and the effectiveness of BC mitigation on future temperature changes.

Black Carbon Sources Constrained by Observations in the Russian High Arctic

Understanding the role of short-lived climate forcers such as black carbon (BC) at high northern latitudes in climate change is hampered by the scarcity of surface observations in the Russian Arctic. In this study, highly time-resolved Equivalent BC (EBC) measurements during a ship campaign in the White, Barents, and Kara Seas in October 2015 are presented. The measured EBC concentrations are compared with BC concentrations simulated with a Lagrangian particle dispersion model coupled with a recently completed global emission inventory to quantify the origin of the Arctic BC. EBC showed increased values (100-400 ng m^-3) in the Kara Strait, Kara Sea, and Kola Peninsula and an extremely high concentration (1000 ng m^-3) in the White Sea. Assessment of BC origin throughout the expedition showed that gas-flaring emissions from the Yamal-Khanty-Mansiysk and Nenets-Komi regions contributed the most when the ship was close to the Kara Strait, north of 70° N. Near Arkhangelsk (White Sea), biomass burning in mid-latitudes, surface transportation, and residential and commercial combustion from Central and Eastern Europe were found to be important BC sources. The model reproduced observed EBC concentrations efficiently, building credibility in the emission inventory for BC emissions at high northern latitudes.

Tethered balloon-born and ground-based measurements of black carbon and particulate profiles within the lower troposphere during the foggy period in Delhi, India

The ground and vertical profiles of particulate matter (PM) were mapped as part of a pilot study using a Tethered balloon within the lower troposphere (1000m) during the foggy episodes in the winter season of 2015-16 in New Delhi, India. Measurements of black carbon (BC) aerosol and PM <2.5 and 10μm (PM_2.5 & PM₁₀ respectively) concentrations and their associated particulate optical properties along with meteorological parameters were made. The mean concentrations of PM_2.5, PM₁₀, BC_370nm, and BC_880nm were observed to be 146.8±42.1, 245.4±65.4, 30.3±12.2, and 24.1±10.3μgm^-3, respectively. The mean value of PM_2.5 was ~12 times higher than the annual US-EPA air quality standard. The fraction of BC in PM_2.5 that contributed to absorption in the shorter visible wavelengths (BC_370nm) was ~21%. Compared to clear days, the ground level mass concentrations of PM_2.5 and BC_370nm particles were substantially increased (59% and 24%, respectively) during the foggy episode. The aerosol light extinction coefficient (σ_ext) value was much higher (mean: 610Mm^-1) during the lower visibility (foggy) condition. Higher concentrations of PM_2.5 (89μgm^-3) and longer visible wavelength absorbing BC_880nm (25.7μgm^-3) particles were observed up to 200m. The BC_880nm and PM_2.5 aerosol concentrations near boundary layer (1km) were significantly higher (~1.9 and 12μgm^-3), respectively. The BC (i.e BC_tot) aerosol direct radiative forcing (DRF) values were estimated at the top of the atmosphere (TOA), surface (SFC), and atmosphere (ATM) and its resultant forcing were - 75.5Wm^-2 at SFC indicating the cooling effect at the surface. A positive value (20.9Wm^-2) of BC aerosol DRF at TOA indicated the warming effect at the top of the atmosphere over the study region. The net DRF value due to BC aerosol was positive (96.4Wm^-2) indicating a net warming effect in the atmosphere. The contribution of fossil and biomass fuels to the observed BC aerosol DRF values was ~78% and ~22%, respectively. The higher mean atmospheric heating rate (2.71Kday^-1) by BC aerosol in the winter season would probably strengthen the temperature inversion leading to poor dispersion and affecting the formation of clouds. Serious detrimental impacts on regional climate due to the high concentrations of BC and PM (especially PM_2.5) aerosol are likely based on this study and suggest the need for immediate, stringent measures to improve the regional air quality in the northern India.

Effects of mixing states on the multiple-scattering properties of soot aerosols

The radiative properties of soot aerosols are highly sensitive to the mixing states of black carbon particles and other aerosol components. Light absorption properties are enhanced by the mixing state of soot aerosols. Quantification of the effects of mixing states on the scattering properties of soot aerosol are still not completely resolved, especially for multiple-scattering properties. This study focuses on the effects of the mixing state on the multiple scattering of soot aerosols using the vector radiative transfer model. Two types of soot aerosols with different mixing states such as external mixture soot aerosols and internal mixture soot aerosols are studied. Upward radiance/polarization and hemispheric flux are studied with variable soot aerosol loadings for clear and haze scenarios. Our study showed dramatic changes in upward radiance/polarization due to the effects of the mixing state on the multiple scattering of soot aerosols. The relative difference in upward radiance due to the different mixing states can reach 16%, whereas the relative difference of upward polarization can reach 200%. The effects of the mixing state on the multiple-scattering properties of soot aerosols increase with increasing soot aerosol loading. The effects of the soot aerosol mixing state on upwelling hemispheric flux are much smaller than in upward radiance/polarization, which increase with increasing solar zenith angle. The relative difference in upwelling hemispheric flux due to the different soot aerosol mixing states can reach 18% when the solar zenith angle is 75°. The findings should improve our understanding of the effects of mixing states on the optical properties of soot aerosols and their effects on climate. The mixing mechanism of soot aerosols is of critical importance in evaluating the climate effects of soot aerosols, which should be explicitly included in radiative forcing models and aerosol remote sensing.

Gas uptake and chemical aging of semisolid organic aerosol particles

Organic substances can adopt an amorphous solid or semisolid state, influencing the rate of heterogeneous reactions and multiphase processes in atmospheric aerosols. Here we demonstrate how molecular diffusion in the condensed phase affects the gas uptake and chemical transformation of semisolid organic particles. Flow tube experiments show that the ozone uptake and oxidative aging of amorphous protein is kinetically limited by bulk diffusion. The reactive gas uptake exhibits a pronounced increase with relative humidity, which can be explained by a decrease of viscosity and increase of diffusivity due to hygroscopic water uptake transforming the amorphous organic matrix from a glassy to a semisolid state (moisture-induced phase transition). The reaction rate depends on the condensed phase diffusion coefficients of both the oxidant and the organic reactant molecules, which can be described by a kinetic multilayer flux model but not by the traditional resistor model approach of multiphase chemistry. The chemical lifetime of reactive compounds in atmospheric particles can increase from seconds to days as the rate of diffusion in semisolid phases can decrease by multiple orders of magnitude in response to low temperature or low relative humidity. The findings demonstrate that the occurrence and properties of amorphous semisolid phases challenge traditional views and require advanced formalisms for the description of organic particle formation and transformation in atmospheric models of aerosol effects on air quality, public health, and climate.

Effect of intrinsic organic carbon on the optical properties of fresh diesel soot

This study focuses on the retrieval of the normalized mass absorption cross section (MAC) of soot using theoretical calculations that incorporate new measurements of the optical properties of organic carbon (OC) intrinsic to fresh diesel soot. Intrinsic OC was extracted by water and an organic solvent, and the complex refractive index of the extracted OC was derived at 532 and 355-nm wavelengths using cavity ring-down aerosol spectrometry. The extracted OC was found to absorb weakly in the visible wavelengths and moderately at blue wavelengths. The mass ratio of OC and elemental carbon (EC) in the collected particles was evaluated using a thermo-optical method. The measured EC/OC ratio in the soot exhibited substantial variability from measurement to measurement, ranging between 2 and 5. To test the sensitivity of the MAC to this variability, three different EC/OC ratios (21, 11, and 12) were chosen as representative. Particle size and spherule morphology were estimated using scanning electron microscopy, and the soot was found to be primarily in the form of aggregates with a dominant aggregate diameter mode in the range 200-250 nm. The measured refractive index of the extracted OC was used with a variety of theoretical models to calculate the MAC of internally mixed diesel soot at 532 and 355 nm. We conclude that Rayleigh-Debye-Gans theory on clusters of coated spherules and T-matrix of a solid EC spheroid coated by intrinsic OC are both consistent with previous measurements; however, Rayleigh-Debye-Gans theory provides a more realistic physical model for the calculation.

Effects of dicarboxylic acid coating on the optical properties of soot

Soot is a major component of atmospheric aerosols responsible for absorption of visible solar radiation. Internal mixing of soot with transparent materials can enhance its ability to absorb and scatter light, resulting in a larger role of soot in climate forcing. We have investigated the absorption and scattering of visible light (532 nm) by soot aerosol internally mixed with succinic and glutaric acids using a combination of a cavity ring-down spectrometer and an integrating nephelometer. The measurements were performed for flame-generated soot aerosol with well-characterized morphology and mixing state in the particle size range from 155 to 320 nm. Thin coatings of dicarboxylic acids on soot aggregates (with a mass fraction of 0.1-0.4) enhance significantly light scattering (up to 3.8 fold) and slightly light absorption (less than 1.2 fold). Cycling the coated soot aerosol through high relative humidity (humidified to 90% RH and then dried to 5% RH) promotes further increase in light absorption and scattering for soot internally mixed with glutaric acid, but not for soot mixed with succinic acid. The larger effect of glutaric acid on light absorption and scattering is caused by the irreversible restructuring of soot aggregates induced by the coating material. Our results indicate that the enhancement in the optical properties of soot by transparent coatings is strongly related to the ability of the coating materials to change the morphology of soot aggregates.