Particle Size as a Key Driver of Black Carbon Wet Removal: Advances and Insights

Abstract

Black carbon (BC), as a potent light-absorbing aerosol, is mainly removed from the atmosphere through wet deposition. The efficiency of this process depends on the capacity of BC particles to serve as cloud condensation nuclei (CCN) or ice nuclei (IN). Newly emitted BC particles are typically small in size and highly hydrophobic, which limits their activation potential. However, atmospheric aging processes involving interactions with sulfates, nitrates, or organic matter enhance their hydrophilicity and nucleation capacity. Particle size serves as the critical link between aging and removal processes. Larger or coated BC particles are more readily activated and removed, while smaller particles require higher supersaturation levels. Both observations and models indicate that uncertainties in BC particle size distribution and aging processes lead to significant discrepancies in lifetime and transport estimates. This paper reviews recent research on the size dependence of wet removal of BC, evaluates current observational and modeling results, and proposes key research priorities to more accurately constrain its role in the climate system.

1. Introduction

Black carbon (BC) is an important light-absorbing aerosol originating from the incomplete combustion of biomass and fossil fuels [1,2,3], and it typically exhibits a particle size ranging from several tens to several hundred nanometers [4,5,6,7]. Black carbon enters the atmosphere and absorbs solar radiation directly. It enhances positive radiative forcing at the top of the atmosphere and modifies cloud radiative and microphysical processes. These effects disturb the energy balance of the cloud–precipitation system and influence regional and global climate [8,9,10,11,12,13,14,15,16,17]. BC deposited on snow, glaciers, and sea ice through wet and dry deposition reduces surface albedo and accelerates melting [8,16,17]. This decrease in reflectivity increases the absorption of solar radiation and contributes to regional and global warming. These processes show that black carbon acts not only as an atmospheric aerosol but also as a surface-altering agent that amplifies climate [4,16]. BC deposition occurs via both dry and wet processes. Observational and modeling studies indicate that under typical conditions, dry deposition contributes roughly 15–40% of total BC deposition, while wet deposition accounts for approximately 60–85%, depending on regional precipitation, BC aging state, and surface type [16,18]. In regions with low precipitation or at high altitudes, the relative importance of dry deposition increases, but wet deposition remains the dominant removal pathway in most areas [16,18]. The primary removal pathway of BC from the atmosphere is wet deposition, which is closely linked to the spatial and temporal distribution of precipitation. Wet deposition consists of two components, as shown in Figure 1. One mechanism is in-cloud scavenging. Aerosol particles become cloud droplets or ice crystals and are removed by precipitation. The other mechanism is below-cloud scavenging. Falling raindrops or snow efficiently capture aerosol particles during their descent, influencing aerosol removal and cloud–precipitation interactions [18,19,20]. The efficiency of wet deposition depends strongly on particle nucleation activity and hygroscopic growth properties. Wet deposition is more effective than dry deposition. It removes larger amounts of BC in a short time. It also determines the atmospheric lifetime of BC [20,21,22,23]. Nevertheless, the microphysical properties of BC are shaped by complex interactions with cloud and fog processes, which remain highly variable, introducing significant uncertainties in the quantification of wet deposition efficiency.

Whether BC is effectively removed by wet deposition largely depends on its ability to act as a cloud condensation nucleus (CCN) and ice nucleus (IN). Fresh BC is generally hydrophobic, irregular in shape, and too small to efficiently serve as CCN [25,26]. However, BC undergoes physicochemical aging during atmospheric transport, primarily through interactions with sulfate, nitrate, organics, and natural atmospheric oxidants such as ozone. These reactions enhance the hydrophilicity of BC and generate hydroxyl and nitrate radicals, thereby increasing its CCN activation potential, i.e., its ability to act as cloud condensation nuclei under supersaturated conditions [19,26,27]. In addition to oxidation by atmospheric oxidants, the physicochemical aging of BC also occurs through repeated evaporation–condensation and freezing–melting cycles within cloud systems. These in-cloud processes promote the coating of BC with soluble materials such as sulfates, nitrates, and organics, thereby increasing its hygroscopicity and enhancing wet deposition efficiency [16,19,26,27]. Thus, wet removal is influenced not only by precipitation patterns but also by the evolving physicochemical properties of BC [28,29]. A key factor linking aging to wet scavenging is particle size, which governs cloud activation and collision–coalescence efficiency [16,30,31,32,33]. Small BC requires a higher supersaturation, which refers to the condition in which the water vapor pressure exceeds the saturation vapor pressure with allowing cloud droplets to form for activation, whereas larger or coated BC is more readily removed [34]. Observations and models alike indicate that uncertainties in BC size distribution and aging rates can result in large differences in its simulated lifetime and transport [18,35,36]. At the same time, variations in particle size distribution affect collision and turbulent diffusion processes, which in turn modify the removal efficiency across precipitation systems of various scales [36,37,38]. Recent aircraft and high-resolution measurement studies suggest that larger black carbon particles have greater activation and collision–capture efficiencies and therefore act more effectively as cloud condensation nuclei during droplet formation, leading to enhanced wet removal [39,40,41,42,43]. Research on numerical models continues to advance the parameterization of BC size spectra and mixing state evolution. These efforts aim to improve simulations of BC vertical distribution and lifetime [41,44,45,46,47]. However, current research still lacks systematic quantification of the size-dependent wet removal mechanisms of BC particles in multi-cloud systems. Significant gaps remain in both observational coverage and model parameterization. Considering the critical influence of particle size on BC wet removal, this study systematically summarizes the mechanisms governing size distribution in wet deposition. It also reviews recent progress in observations and modeling, identifies existing research gaps, and outlines future research directions. These efforts provide a scientific basis for understanding the atmospheric behavior of BC and its climatic impacts.

2. Theoretical Mechanisms: Particle Size-Dependent Activation and Wet Removal of Black Carbon

2.1. Key Role of Particle Size in Cloud Droplet Activation

Freshly emitted BC particles primarily originate from the incomplete combustion of biomass and fossil fuels. Their initial sizes are highly dispersed, but they are mainly distributed within the nucleation and Aitken modes, ranging from tens to several hundred nanometers, and exhibit fractal chain-like or aggregate morphologies with extremely high porosity and relatively large dynamic shape factors [48,49,50]. These BC particles lack hydrophilic functional groups and soluble coatings formed under high-temperature and oxygen-deficient conditions, which makes them hydrophobic. As a result, they require higher critical supersaturation (Sc) to be activated as CCN [51,52,53,54,55,56,57]. In the typical atmospheric boundary layer, Sc generally remains below 0.4% and rarely exceeds 0.5% [56,58]. Therefore, freshly emitted BC particles within this size range are rarely able to participate effectively in cloud droplet formation. Due to their low activation frequency, fresh BC mainly exists as dry aerosol in the atmosphere, absorbing solar radiation strongly and generating direct positive radiative forcing in the upper atmosphere [32,59,60,61,62,63,64]. During atmospheric transport, low-volatility products formed from the oxidation of volatile organic compounds (VOCs), i.e., secondary organic aerosols (SOAs), effectively irreversible on atmospheric timescales, condense onto BC surfaces. Similarly, gaseous sulfuric acid, nitric acid, and ammonia can form sulfate, nitrate, and ammonium coatings on BC surfaces through heterogeneous reactions or condensation [65,66,67]. These coatings not only increase the particle’s total mass but also enhance overall hygroscopicity via volume effects [5,66]. Atmospheric oxidants include ozone, hydroxyl radical, and nitrate radical. They react on BC surfaces through heterogeneous processes [32,67]. These reactions generate oxygen-containing functional groups. They increase the hydrophilicity of BC and alter its surface chemistry, affecting its atmospheric behavior [16,68]. The collision and coagulation of BC with hygroscopic particles, such as sulfates, nitrates, or sea salt aerosols, leading to internally mixed particles, represents one of the most effective pathways for BC aging [5,32,69].

BC particles undergo oxidation, condensation of SOA, and coating processes with sulfates, nitrates, and other species, acquiring hydrophilic surface layers gradually [70,71,72,73,74]. Condensation generally increases BC particle size, while coagulation may decrease particle number concentration and simultaneously increase the mean diameter. Observations indicate that the geometric mean diameter (GMD) of BC can grow from ~80 nm to over 200 nm during atmospheric transport [75,76,77,78,79]. Both field measurements and model simulations demonstrate that even extremely thin soluble coatings (<2 nm) can substantially enhance BC hygroscopicity, lowering the Sc significantly and enabling activation as CCN under typical atmospheric Sc conditions [30,39,53,80,81,82,83,84,85]. The formation of coatings also leads to a marked increase in particle size. The Köhler equation [51] shows that particles with similar hygroscopicity and larger diameters have lower Sc values [20]. Even coatings with moderate hygroscopicity can strongly decrease the Sc of BC. This effect results from simple volumetric growth of the coating [63]. Furthermore, the chemical composition of the coating plays a decisive role in determining hydrophilicity. Due to their high solubility and strong hygroscopicity, inorganic salts such as sulfates and nitrate are far more effective than most organics at promoting water uptake and droplet nucleation, shifting particle size distributions toward larger values and further lowering activation Sc [86,87,88,89,90,91,92,93]. Aged BC is more efficient in acting as CCN than freshly emitted BC. This property increases its influence on cloud microphysical processes.

BC exhibits a pronounced environmental influence as IN. In mixed-phase and cirrus clouds, BC functions as a substrate for heterogeneous nucleation. This mechanism controls the formation of cirrus clouds [94,95]. Laboratory and field studies under immersion and contact freezing conditions within the temperature range of −38 °C to −10 °C show that both fresh and atmospherically aged BC have limited ice-nucleating activity. Their efficiency is comparable to that of aqueous droplets of the same size. Therefore, BC can be considered nearly negligible as IN [96,97,98]. At lower cirrus temperatures (T ≤ −38 °C), BC containing particle morphologies and high surface area may induce freezing via pore condensation or deposition nucleation. However, these pathways typically require elevated ice Sc, and the proportion of available active sites remains low [99,100]. The ice-nucleating efficiency of BC reflects its chemical composition and structure. Atmospheric processes strongly shape this efficiency. These processes include photochemical oxidation, coating by secondary species, mixing-state transitions, and variations in Sc. This sensitivity to both intrinsic and extrinsic factors contributes to the large variability and contradictions reported across experimental and modeling studies [16,95,101,102]. Aging and mixing-state evolution further amplify the size effect. Growth in BC particle size is generally associated with condensation of sulfates, nitrates, and organics, as well as coagulation with other aerosols [5,32,103]. As a result, larger BC particles often represent a state of advanced aging characterized by enhanced hygroscopicity and greater activation potential. Hence, BC particle size not only directly governs the Sc for activation but also encodes information on atmospheric processing and mixing-state evolution, thereby exerting a dual control on both CCN and IN activity.

2.2. The Predominant Influence of Particle Size Distribution on Wet Removal Efficiency

Wet deposition is the primary sink for BC, and its efficiency exhibits a pronounced dependence on particle size distribution. This dependence is expressed not only in the contrasting efficiencies across size ranges but also in the coevolution of size and surface properties during aging [18,19,20,21,22,23]. Mechanistically, wet deposition can be classified into in-cloud and below-cloud processes, each characterized by distinct and size-dependent removal pathways [104,105]. In-cloud scavenging removes aged BC with coating layers and diameters preferentially in the accumulation mode (100–300 nm), driven by CCN and IN activation. In contrast, freshly emitted hydrophobic BC particles are typically smaller in size, which requires higher Sc for activation and is inefficiently removed through cloud processes [22,106,107]. Such small particles are instead more efficiently removed below clouds by precipitation collection. Removal efficiencies are further shaped by microphysical processes: ultrafine particles (<50–100 nm) are efficiently scavenged by Brownian diffusion, and coarse particles (>1000 nm) by interception and inertial impaction, while mid-accumulation mode particles (~100–300 nm) exhibit a well-known “valley of minimum efficiency” and are removed by either mechanism poorly [104,108,109]. This size-selective behavior exerts strong control on BC lifetime and transport. Observational and modeling studies further show that transport efficiency is highly sensitive to precipitation occurrence, phase, and intensity, as well as to convective depth and air-mass history. Precipitating convection and cold-cloud processes strongly enhance in-cloud removal and suppress long-range transport, whereas non-precipitating or weak precipitation conditions favor below-cloud scavenging and dry deposition [38,110,111,112]. Overall, the strong size dependence of wet deposition represents a central factor and a major source of uncertainty in quantifying the atmospheric lifetime and radiative impacts of BC.

Sricharoenvech et al. demonstrated that wet deposition, through its efficient removal of BC, strongly influences the particle’s lifetime and transport range, which in turn directly controls BC’s atmospheric residence time [43]. Moteki et al. emphasized the need for explicit size representation in models examining size-dependent wet removal from the boundary layer to the free troposphere [106]. Croft et al. showed that alternative in-cloud scavenging schemes in ECHAM5-HAM substantially affect simulations [113], while Liu and Matsui achieved major improvements in global BC modeling by refining wet removal parameterizations similarly [114]. Complementary work by Yang et al. highlighted the importance of condition-dependent, multi-component scavenging efficiencies, underscoring the value of size-resolved approaches in regional contexts [24]. Field observations and model simulations demonstrate that microscale particle properties such as size, mixing state, and coating determine the efficiency of BC removal through in-cloud condensation and below-cloud collision. These properties further control the vertical distribution and radiative effect of BC [43,106,113,114,115,116]. Recent progress in cloud microphysics modeling has incorporated explicit size-resolved wet removal and demonstrated that empirical bulk scavenging rates fail to reproduce the spatial and temporal variability or radiative influence of BC in convective and mixed-phase clouds. Model-observation discrepancies can be reduced by incorporating particle-size-dependent activation thresholds, coating evolution, and raindrop-size-selective collection [112,116,117,118,119,120]. Consequently, incorporating size-resolved schemes together with representations of mixing state and precipitation conditions enhances the performance of BC simulations in vertical distribution, lifetime, and radiative effects. The improvement is consistent with evidence from multi-site observations and laboratory studies. This establishes a stronger foundation for advancing the understanding of BC-climate feedbacks and for supporting reliable mitigation assessments. Overall, particle size is the key factor that determines both the dominant wet removal pathways and their efficiency in cleansing BC from the atmosphere.

3. Observational Evidence: Empirical Support for Size-Dependent Effects

Recent studies using precipitation samples and field observations have highlighted the pivotal role of BC particle size in governing wet deposition. Micron-sized and larger particles are efficiently removed through dry deposition processes such as inertial impaction, interception, and gravitational settling, whereas submicron particles are primarily influenced by Brownian diffusion and by cloud droplet activation, after which they can be scavenged by cloud droplets or by the diffusion layer surrounding falling raindrops [22,117,121,122,123]. The relative importance of these removal pathways is strongly dependent on precipitation type and raindrop size distribution, contributing to significant variability in wet deposition efficiency [18,41]. Atmospheric aging processes such as secondary organic aerosol condensation and coating by sulfate and nitrate further modulate BC particle size, surface chemistry, and mixing state to enhance hygroscopicity and cloud activation potential [49,124,125,126,127,128]. Sensitivity studies indicate that wet deposition dominates the atmospheric lifetime of fine BC, while uncertainties in particle size distribution and mixing state parameterizations can amplify spatiotemporal variability in BC fluxes and concentrations. This underscores the strong size-dependent nature of BC wet removal and the inherent uncertainties in modeling its atmospheric fate [18].

3.1. Lab and Cloud Chamber Evidence of Particle Size Effects on BC

Laboratory and cloud chambers provide versatile platforms to investigate Brownian diffusion of particles. In addition, flow reactors allow researchers to study inertial collisions of coarse-mode aerosols. Using aerosol wind tunnels, researchers can investigate how secondary species coat particles and assess how particle size affects the activation of CCN and IN. Together, these experimental platforms provide a comprehensive approach to studying nanoscale processes. By precisely controlling air temperature and relative humidity and by adjusting gas composition, concentration, and illumination conditions, researchers can both isolate confounding factors and realistically reproduce key atmospheric aerosol aging processes, which in turn facilitates a detailed understanding of how specific physical and chemical mechanisms govern BC removal [129]. Since the 1980s, cloud-based BC removal has been extensively studied, with most research focusing on the distribution and variability of BC mass or volume concentrations in cloud interstitials and droplets, as well as the factors that influence these distributions. Aerosol nucleation mechanisms, particularly those involving sulfuric acid, play a critical role in cloud condensation nuclei formation. The CLOUD experiment demonstrated that even at extremely low ammonia concentrations (≤100 ppt), sulfuric acid nucleation rates can increase by over two orders of magnitude due to the stabilizing effect of ammonia on sulfuric acid clusters [130]. Experimental studies manipulating relative humidity, precursor gas concentrations, and light intensity have directly observed that, as aerosols age, BC particle surfaces progressively acquire coatings of SOA and inorganic species such as sulfates and nitrates. This coating decreases the Sc threshold required for cloud activation and shifts the particle size distribution toward larger diameters [131,132,133,134,135]. Additionally, BC removal efficiency generally increases with distance from the emission source, reflecting the cumulative effects of aging and wet scavenging [136,137].

At central Vienna and the North Sea coast, two representative monitoring sites, aerosol samples collected with cascade impactors showed that BC exhibits a multimodal size distribution including a dominant accumulation mode, a finer Aitken mode linked to fresh combustion, and a coarse mode. Correlations within the accumulation mode indicated active physical and chemical evolution during transport [138]. Laboratory chamber studies further demonstrated the role of particle size in BC activation and uptake processes. Simulations in the “QUALITY” chamber revealed rapid external-to-internal mixing of fresh BC, with limited short-term changes in hygroscopicity. Cloud chamber experiments showed that BC incorporation into droplets depends strongly on particle size and concentration: under low BC concentration (<5 μg/m³), nearly 90% was activated, while efficiency dropped to ~20% at high BC concentration (>20 μg/m³) due to Sc limitations and competition effects [138,139]. Dew point conditions and particle size jointly regulated droplet partitioning and effective Sc. Size-resolved experiments at Lawrence Berkeley National Laboratory confirmed that up to ~80% of BC mass can be incorporated into fog droplets with efficiency varying by particle size and surface properties. Field campaigns, such as the Southern California Air Quality Study, further revealed strong diurnal variability in BC size distributions linked to atmospheric aging [140]. Cloud cycling experiments also demonstrated that BC-containing aerosols act as CCN, which drives SO₂ oxidation to sulfate within droplets. Repeated cycles altered size distributions and acidity, modulating conversion efficiency [141]. Collectively, these studies highlight that BC hygroscopicity, activation potential, and climate relevance are strongly size-dependent and mediated by physicochemical evolution.

Cloud chamber experiments have played an important role in clarifying the microphysical mechanisms of aerosol–cloud interactions, but they also face several inherent limitations. These experiments are performed on restricted spatial and temporal scales. Such scales differ greatly from the turbulent dynamics, convective updrafts, and precipitation processes in the real atmosphere, which limits their representativeness [142,143]. Many studies rely on idealized conditions. They often use highly purified aerosol species or simplified mixtures and therefore overlook the widespread organic coatings, inorganic salts, and chemical aging present in the atmosphere [101,106,144]. In addition, cloud chambers usually trigger nucleation or ice formation through rapid cooling, controlled supersaturation, or artificially high particle concentrations. These conditions seldom occur in nature and may bias the results [94,145,146,147]. Instrumental sensitivity and resolution also impose strong constraints. They make it difficult to follow the full evolution of aerosols from emission to aging and to final removal by cloud droplets or ice crystals [148]. Another critical issue is the inconsistency between chamber-derived activation thresholds or ice-nucleating particle concentrations and values reported by field or aircraft measurements. The differences often reach several orders of magnitude and add uncertainty to the direct use of chamber results in climate model parameterizations [149,150,151]. For these reasons, chamber studies must be combined with long-term field observations, aircraft campaigns, and numerical modeling to achieve reliable assessments of aerosol impacts on clouds and climate at broader spatial and temporal scales [152,153,154,155].

3.2. Field Evidence on BC Size and Mixing State from SP2–CCN Measurements

The single-particle soot photometer (SP2) operates on the principle of laser-induced incandescence (LII) and can directly measure the mass of refractory black carbon (rBC) which represents the refractory, highly graphitized fraction of black carbon that vaporizes at temperatures above ~4000 K when exposed to the SP2’s laser beam, producing an incandescence signal on individual aerosol particles in real time. The instrument excites rBC particles to induce incandescence and infers their mass. At the same time, it records the scattering signal, which provides information on the optical size or coating thickness contributed by secondary components such as sulfates, nitrates, and organics surrounding the rBC core. This capability allows the SP2 to measure mass-equivalent diameters, number size distributions, and mixing-state parameters of rBC during field and airborne measurements. It serves as a core tool for characterizing the physicochemical properties of atmospheric rBC. Since its introduction in the mid-2000s [156], the SP2 has been widely applied in diverse atmospheric environments worldwide. Measurement methods, calibration procedures, and data processing protocols have gradually become standardized. To investigate the activation potential of BC as CCN, researchers often couple the SP2 with CCN counters (CCNc) or instruments for measuring particle hygroscopicity, such as the humidified tandem differential mobility analyzer (HTDMA) and aerosol or centrifugal particle mass analyzers (APM/CPMA) [74,115]. Techniques such as DMA–SP2 and CPMA–SP2 allow coordinated measurements that resolve particle mass and size [37]. These measurements systematically link BC particle size, coating state, and coating thickness to CCN activation efficiency under different Sc conditions. Such studies have been applied in CCN closure experiments and have substantially improved understanding of BC’s climatic effects and the microphysical mechanisms of aerosol–cloud interactions [30].

Field and aircraft observations, as illustrated in Figure 2, in different regions consistently demonstrated the strong influence of particle size on the wet removal efficiency of BC [87,157,158]. The transformation of the mixed state greatly strengthens BC’s capacity to serve as both CCN and IN. The coating enlarges the wet particle size of BC and lowers its Sc so that the particles are more likely to activate into cloud droplets [158,159]. On the other hand, coating creates conditions that allow soluble substances to form and surface tension to change, which promotes the activation of BC particles [160]. The relative contribution of larger BC particles within the CCN population is significantly higher than that of particles with no coating or thin coatings [41,161]. Studies using cloud-entry instruments or cloud-drop separators for direct sampling show that medium to large rBC nuclei with thick coatings are more easily captured within cloud droplets or ice crystals. These particles actively participate in cloud-drop formation [157,162]. The critical activation diameter derived from Köhler theory decreases accordingly. This change causes a strong increase in the activation fraction of rBC under common Sc conditions. Airborne observations show that the activation rate of BC increases with coating thickness and nucleation size in air masses affected by pollution or long-range transport [50,163,164]. These conclusions were repeatedly validated across multiple field activities [165].

In the Tokyo metropolitan area, ground-based measurements before and after precipitation showed a clear size-dependent scavenging efficiency that aligned with variations in nucleation properties, and the data also enabled the first indirect estimate of in-cloud effective Sc [166]. At the Jungfraujoch station in the Swiss Alps, CCN measurements under controlled Sc conditions revealed that the critical activation diameter centered around 87 nm, and local topography exerted a dominant control on effective peak Sc [133]. Observations in southern China during a cloud event indicated that removal efficiency ranged from 2.7 percent to more than 50 percent. Particle size had a stronger influence during the initial formation stage. The mixing state became more important at later stages. Internally mixed BC combined with organic material exhibited lower removal efficiency [167]. Aircraft campaigns over East Asia revealed that BC mass per particle decreased when vertical transport efficiency declined. This observation shows that larger BC-containing particles were removed first [106]. Complementary ground measurements in Beijing confirmed that larger BC particles with thick coatings contributed more effectively as CCN [134]. Single-particle observations during cold cloud and snowfall events further showed preferential incorporation of large and dense BC into droplets and graupel, followed by release during ice-phase processes, which provided direct mechanistic evidence for size-dependent BC removal [168].

Significant differences exist in the aging pathways and final mixing states of BC from various emission sources. These differences strongly influence the hydrophilization rate and the capacity of BC to act as CCN [169,170]. BC released from biomass burning is often accompanied by large amounts of organic matter. Growth of coated particles decreases the critical activation diameter. The combined effect substantially increases the probability of CCN activation under atmospherically relevant Sc levels [139,171]. BC from traffic emissions interacts more frequently with inorganic salts such as sulfates and nitrates. The hydrophilization in this case proceeds at a relatively fast pace. Local variations in precursor concentrations and nucleation conditions often lead to coatings of irregular thickness [157,172]. The source dependence produces large variability in activation potential. BC particles that share the same mass-equivalent diameter can display order-of-magnitude differences in CCN activation fractions [157,173]. Direct and precise characterization of coating thickness and mixing state through SP2 is therefore essential. Comparative studies across different emission types provide valuable constraints on the role of BC in droplet activation. Such efforts are necessary to improve the parameterization of CCN activity in regional and global climate models [82,166]. Freshly emitted BC close to sources usually has small core sizes and thin coatings. The CCN activity under such conditions remains extremely limited. Transport over longer distances results in the condensation of photochemically oxidized compounds (such as sulfuric acid and sulfate particles) and SOA. This process drives aging and produces internally mixed particles. Aging increases particle size and bulk hygroscopicity. As a result, the critical Sc decreases and the CCN activity of BC becomes strongly enhanced. The transformation rate depends on the surrounding chemical environment. Aircraft campaigns and field observations repeatedly confirm this relationship [162,174,175].

SP2–CCN joint measurements represent a central approach in recent studies. This technique provides single-particle information on BC core size and coating while also linking these properties to activation behavior. Such measurements reveal direct evidence for the coupling between aging mechanisms and CCN activity [157]. Despite these advantages, several methodological challenges remain. The lower detection threshold of SP2 generally corresponds to core diameters between 70 nm and 150 nm. Sub-100 nm BC near sources may therefore be underestimated in terms of CCN contribution [169,170]. Retrieval of coating thickness from scattering signals depends on assumptions regarding refractive index, particle morphology, and calibration procedures. These assumptions can produce systematic uncertainties [140,171]. Deriving CCN activation from SP2 observations also requires assumptions on Sc conditions and solute properties. Sensitivity tests demonstrate that such assumptions may lead to significant variations in closure analysis [176]. A combination of SP2 with CPMA reduces these uncertainties. Coordinated multi-instrument observations and closure experiments remain crucial for a robust assessment of BC–CCN interactions and for improved representation in models.

4. Modeling Studies: Sensitivity of BC Scavenging to Particle Size Distributions

In the wet removal of BC particles, it remains difficult to distinguish in-cloud scavenging from below-cloud deposition. Multiple factors also exert substantial influence on the overall wet deposition process. Accurate simulation requires coupling collision–kinetics theory to precipitation microphysics parameterizations and explicitly representing the spatiotemporal evolution of BC mixing states in climate models [176,177]. Many studies have conducted systematic simulations of BC distribution and deposition patterns on global and regional scales based on observational datasets and previous research findings, as described in

Table 1. Climate and aerosol models for black carbon and cloud processes.

Model/Tool	Type	Main Function	Features & Applications	References
GEOS-Chem	Global 3D chemical transport model	Simulates in-cloud scavenging in large-scale mixed-phase clouds	Supports multiple representations of scavenging efficiency, reflecting different research perspectives and mathematical descriptions	[187]
ECHAM-HAM	Numerical climate model system	High-resolution simulation of aerosol–cloud interactions	Couples aerosols as cloud condensation nuclei/ice nuclei affecting cloud microphysics (e.g., droplet spectra, cloud phase, albedo, and lifetime), and quantifies anthropogenic and natural aerosol impacts on radiation and hydrological cycles	[98,104,142,186,188,189,190,191]
GISS ModelE	Coupled climate system model	Simulates the radiative forcing of aerosols	Integrates atmospheric circulation, ocean circulation, and land surface modules; quantifies effects of sulfate, black carbon, etc.; ModelE2.1/2.2 extends carbon cycle processes, applicable to paleoclimate reconstruction and future climate projections	[6,14,178,192,193]
WRF-Chem	Regional-scale online coupled meteorology–chemistry model	Captures local circulation features and simulates black carbon transport and deposition	Kilometre-scale nested grids suitable for complex highland terrains; studies black carbon effects on glacier albedo and water resources; ongoing improvements in wet deposition parameterization	[112,194]

. Variations in particle size representation within regional air quality and global climate models can strongly affect predicted CCN concentrations, removal efficiencies, vertical distributions, and radiative forcing. These variations have important implications for climate impact assessments and policy evaluations [18,178,179,180]. Chemical transport models employ several approaches to parameterize particle size distributions [181,182,183]. Modal methods assume that aerosol size distributions follow predefined analytical functions. Aerosols are represented as a sum of overlapping modes, each corresponding to a subgroup of particles with similar sources or formation mechanisms. Typical modes include nucleation, Aitken, accumulation, and coarse modes. Modal approaches have been implemented in the GLOMAP-mode of UKCA and the Modal Aerosol Module of CESM/CAM. These approaches provide a practical framework to describe aerosol size spectra and mixing states while maintaining computational efficiency for removal calculations [181,184,185,186].

Sectional methods divide the particle size spectrum into multiple bins and track mass, number, and other properties for each bin. Models such as TOMAS (TwO-Moment Aerosol Sectional) and GLOMAP-bin, which simulate aerosol number, mass, and chemical composition in multiple size classes, use this approach to simulate condensation, coagulation, condensational growth, and deposition with high fidelity [195,196,197,198]. Moment-based methods occupy an intermediate position between modal and sectional approaches. These methods forecast selected statistical moments of the particle distribution instead of the full spectrum [199]. Cloud microphysics schemes often apply moment-based methods to predict cloud droplet and ice crystal size distributions [200,201,202]. Assumptions regarding BC particle size distributions and aging rates produce substantial differences in simulated transport, deposition, and radiative forcing. Such differences represent a major source of uncertainty in quantifying BC climatic effects. Accurate representation of particle size sensitivity and aging dynamics is essential for reducing uncertainty in aerosol–cloud interactions and improving climate model predictions.

The simulation of BC particle emissions and their aging represents a major source of uncertainty in climate models. Modal approaches often inject BC directly into the accumulation mode or partially into the Aitken mode and then simulate the conversion toward the accumulation mode using parameterized aging processes. This approximation introduces a limitation because the rate of BC aging depends strongly on emission strength, atmospheric composition, diurnal cycles, and the stability of the boundary layer and free troposphere. Urban environments promote rapid aging within a few hours due to high concentrations of condensable gases and enhanced photochemical activity, associated with elevated levels of atmospheric oxidants such as ozone (O₃), nitrogen oxides (NO_x), hydroxyl (OH) radicals, and nitrate (NO₃) radicals. Remote regions require several days for fresh BC to acquire a significant internal mixing state. Such spatial and temporal variations increase the uncertainty in model predictions [103]. Different models apply distinct parameterizations for aging. Some use fixed timescales to describe the conversion of freshly emitted BC into an internally mixed state. Other models simulate condensation of sulfate and secondary organic aerosols, coagulation, and chemical transformation. Wet deposition efficiency depends strongly on particle size distribution. Mass median diameter and the proportion of particles in the accumulation mode play dominant roles. Observations indicate that larger BC particles activate more efficiently as cloud condensation nuclei while smaller particles travel longer distances [203,204]. Placing all BC in the accumulation mode overestimates wet deposition and underestimates atmospheric lifetime, which typically ranges from a few days for coarse particles to 5–10 days for accumulation-mode particles in the troposphere [4,5,6,7,16]. Emission of BC in the Aitken mode and gradual aging reduce deposition efficiency and prolong particle residence time. As a result, BC can be transported to remote regions [205]. The internal mixing state and surface coatings of BC determine CCN activity. Coatings formed through condensation of soluble species and chemical reactions increase hydrophilicity and the probability of cloud-mediated removal [199]. Different models represent aging rates and mixing mechanisms differently. Variability in the description of coating formation and hygroscopic growth generates further uncertainty in predicting BC lifetime, vertical distribution, and radiative forcing [200]. Fixed aging timescales used in many models cannot capture changes caused by photochemistry, humidity, or temperature. This simplification leads to systematic bias in predicted mixing states and CCN activity [206,207,208,209].

Recent studies that compare sectional and modal modeling approaches demonstrate the importance of mechanistic detail. Sectional schemes, including the Sectional Aerosol module for Large Scale Applications (SALSA2.0), reproduce observed BC optical properties and mass concentrations and size distributions with higher accuracy than modal approaches [209]. Sectional models capture the gradual growth of BC particles from nucleation sizes to accumulation sizes. Modal models tend to introduce significant deviations when aging is rapid or local number concentrations are high. Aircraft observations indicate that freshly emitted BC requires several hours to days to reach full internal mixing. The aging time in the atmosphere is longer than assumed in most modal models [210].

In the aerosol module, the M7 module in ECHAM-HAM/Hamburg/Messina MOZART (HAMMOZ) applies a modal approximation method. The model assumes that a fraction of emitted BC enters the accumulation mode immediately after emission or transitions from the Aitken mode to the accumulation mode within a very short period. Changing the emission assumption from direct injection into the accumulation mode to emission in the Aitken mode followed by slower aging significantly alters BC column burden, long-range transport efficiency, and deposition at high latitudes at the regional scale [188]. Increasing the initial geometric mean diameter of emitted BC from 50 nm to 150–200 nm enhances wet removal and shortens the overall atmospheric lifetime by 20% to 40%. This adjustment reduces the long-range transport load [193]. Kokkola et al. [209] compared the SALSA2.0 sectional scheme and the M7 modal scheme. The sectional scheme reproduces BC optical properties, mass concentration, and size distribution more accurately [211]. It captures the transition of BC from small diameters near emission sources to larger diameters in remote regions during the evolution of coal and biomass burning emissions. The modal scheme shows significant bias when aging rates are fast or particle number concentrations are high. Dahlkötter et al. [210] reported that freshly emitted BC requires several hours to days of physical and chemical aging to reach a well-mixed state. This timescale exceeds the default assumptions of most modal models. Reddington et al. demonstrated that differences in initial size distribution and aging rate produce up to 30 percent variation in predicted BC atmospheric lifetime, which strongly affects climate impact estimates [211]. Models of different complexity provide multiple perspectives on BC aging. Sectional and two-moment schemes allow precise simulation of aerosol number size distribution and CCN activity. Particle-resolved models track individual particle evolution and explicitly represent the distribution of aging timescales under idealized conditions. These models also simulate particle responses to Sc and diurnal cycles [212]. They quantify the relative importance of physical processes under various atmospheric environments and help reduce uncertainty in climate modeling of BC effects.

Numerical modeling studies have applied sensitivity experiments to assess uncertainties in model predictions that arise from aerosol particle size representation and related parameterization schemes [182,198]. These studies use a consistent model framework and change one or several critical parameters to determine their individual impact on simulation results [196]. Global aerosol microphysical simulations indicate that CCN (0.2%) concentrations depend strongly on particle size and uncertainties in their representation. Variations in primary emissions and nucleation rates alter particle size distributions and significantly influence CCN concentrations. Under conditions of abundant condensable material, primary emissions control particle size and CCN concentrations more than nucleation rates. Differences in CCN between pre-industrial and industrial periods show limited sensitivity to nucleation parameterization but remain affected by uncertainties in emissions and secondary organic aerosol particle sizes. Even when particle size representation is accurate, nucleation parameters still dominate CCN uncertainties [213]. Refining the modal partitioning scheme improves model performance. In the Modal Aerosol Module version 4, the accumulation mode is separated into soluble and insoluble sub-modes, which enhances the representation of BC transformation from hydrophobic to hydrophilic and its mixing state. This improvement increases the accuracy of simulated BC column burdens [185,214]. Multi-model comparisons conducted by Aerosol Comparisons between Observations and Models, which provides a coordinated intercomparison of global aerosol models, demonstrate that differences in wet deposition efficiency and aerosol mixing state parameterization generate substantial uncertainty in BC atmospheric lifetime and vertical distribution [193]. Variations in these parameters also produce a wide range of estimates for BC radiative forcing, especially for direct radiative effects [18]. Therefore, although refining modal schemes enhances model fidelity, the overall uncertainty in BC climate effect assessment depends on a precise and consistent representation of key physicochemical processes throughout the particle lifecycle.

5. Discussion

5.1. Aerosol Size Dynamics Driven by Aging: From Formation to Atmospheric Removal

The climatic effects of BC particles exhibit strong nonlinearity and complexity. This behavior results from the evolving particle size distribution and chemical composition during atmospheric aging, as shown in Figure 3. The evolution begins with the nucleation of new particles. These particles grow from molecular clusters to detectable nanoscales through aggregation and condensation [215,216]. Water vapor plays a central role in nucleation [176]. At this stage, the particle population is dominated by an ultrafine mode [50,217]. Subsequent growth occurs through condensation, coagulation, chemical oxidation, and cloud microphysical processes. BC particle chemical composition changes dynamically, and the number of surface-active nucleation sites increases. Condensation deposits gas-phase material onto particle surfaces to drive continuous growth. Coagulation reduces particle number concentration and shifts the distribution toward larger sizes under high-number-density conditions. Condensation predominantly controls growth for particles at molecular to tens-of-nanometer scales, whereas coagulation and hygroscopic growth dominate mass increase for particles larger than 100 nanometers [24]. Coagulation reshapes the aerosol size distribution by reducing the number of Aitken-mode particles and transferring them to the accumulation mode between 100 and 1000 nanometers [20,218,219]. Losses during coagulation are significant and compete with condensation, jointly determining the final number of particles [20,220]. Aging drives BC particles from the nucleation mode below 20 nanometers to the Aitken mode between 20 and 100 nanometers. A portion of the particles grows further into the accumulation mode above 100 nanometers [217,218,219]. This process alters particle size, composition, and mixing state, enhancing hygroscopicity. Many particles that were initially too small or hydrophobic to activate become effective cloud condensation nuclei, increasing cloud droplet number concentrations and complicating cloud microphysical processes [192,221,222,223].

Particle size and surface chemical composition jointly control the activation of aerosol particles as CCN. Critical Sc increases for smaller particles [50]. Surface composition alters particle hygroscopicity and modifies activation efficiency [224]. These factors determine cloud droplet number concentration and size distribution [225]. Higher CCN activity generates more numerous and smaller droplets. This formation pattern slows droplet coalescence and delays raindrop formation [142]. The effect influences precipitation efficiency and cloud lifetime [226]. Wet removal shows strong size selectivity. Large particles readily activate as CCN and are scavenged by raindrops. Small particles may fail to grow or return to the atmosphere after cloud droplet evaporation [227]. This selective process reshapes aerosol size distribution and chemical composition below and after clouds. Dry deposition rates rise with particle size and are governed by gravitational settling, inertial impaction, and deposition resistance [228]. Wet deposition introduces greater uncertainty. It strongly affects the removal of hygroscopic particles in the 0.1–10 µm range [18]. Differences in model treatment of below-cloud scavenging and in-cloud removal contribute to discrepancies between simulated and observed aerosol concentrations and lifetimes [229]. Regional simulations indicate that below-cloud scavenging can account for a substantial fraction of total wet deposition under certain meteorological conditions [230]. Accurate representation of size-selective and mixing-state-dependent wet removal is critical to understand aerosol spatiotemporal evolution and climate effects. BC particles act as both outcomes and drivers of size distribution during their lifecycle. The distribution reflects multiple physical and chemical processes and modulates CCN activity, hygroscopic growth, and removal efficiency, thereby influencing aerosol environmental and climatic impacts [20,231].

5.2. Regional Differences

The physicochemical properties of BC particles and their environmental and climatic impacts differ significantly across regions. These differences strongly influence regional climate, air quality, and ecological health. BC particles vary in emission sources, transport processes, removal pathways, and final deposition. The timing of high-intensity industrial and traffic emissions differs among Europe, East Asia, and South Asia. Ice core records on the Tibetan Plateau capture these variations. Xu et al. analyzed BC and organic carbon records from five ice cores across the Tibetan Plateau over the past 50 years [232]. The study examined the spatial differences of source regions. Four ice cores recorded the highest BC concentrations in the 1950s and 1960s, which may reflect industrial emissions from Europe. The Zuoqiupu ice core in southeastern Tibet showed no clear trend. This observation indicates that European BC particles were largely removed during transport and reached southeastern Tibet in very low amounts. BC concentrations in the Zuoqiupu ice core rose sharply after the 1980s. Southeastern Tibet is a typical monsoon region, and the increase reflects the rapid industrial development in South Asia. East Asia and South Asia rank among the regions with the most complex and highest aerosol concentrations in the atmosphere. High-intensity human emissions and natural sources drive these concentrations [233,234]. Rapid industrialization and urbanization produced large amounts of emissions from coal combustion, vehicle exhaust, and metallurgical and chemical industries [235]. Agricultural residue burning and biomass use in households further increased BC emissions [16,236]. In northern Indian plains, crop residue burning and winter boundary layer inversions elevated PM_2.5 concentrations above 300 μg m⁻³, making the region one of the most polluted in the world [237]. Europe and North America gradually implemented air pollution control policies after the late 20th century. Aerosol levels in these regions remain influenced by urban emissions and wildfire events. Recent years have seen a marked increase in wildfire frequency and intensity in California and Canada. Between 2017 and 2020, wildfire plumes in western North America caused PM_2.5 concentrations to rise by three to ten times over short periods. These events changed the size distribution and optical properties of carbonaceous aerosols [214,238].

Regional background conditions strongly influence aerosol size distributions and new particle nucleation. High background aerosol loads in East Asia and South Asia suppress nucleation events. An enhanced condensation sink reduces the survival of newly formed molecular clusters and prevents their growth into detectable particles [215,231]. Observations in Beijing and surrounding areas show that nucleation events occur less frequently under high-pollution conditions than at clean background sites in Europe [239]. Newly formed nucleation-mode particles often fail to grow to sizes relevant for cloud condensation nuclei due to rapid condensation losses [240]. Background air in Europe and North America is cleaner and exhibits lower levels of condensation sinks. Nucleation events occur more frequently and contribute more significantly to CCN populations [217,241]. Long-term observations at Hyytiälä, Finland, indicate that nucleation events account for more than half of all days in spring and summer. Nucleation-mode particles grow to 50–100 nm within one to two days and become effective CCN [242]. The APPalAIR project in the eastern United States shows that under clean conditions, new particle nucleation directly affects cloud droplet number concentrations and can modify regional climate feedbacks [213]. Regional differences also shape the overall aerosol size distribution. Accumulation-mode particles dominate number concentrations in East Asia and South Asia, whereas the nucleation-mode contribution remains low [243]. In Europe and North America, nucleation-mode and Aitken-mode particles are more abundant under cleaner conditions, producing size distributions with pronounced bimodal or trimodal characteristics [25]. These differences directly determine the influence of aerosols on radiative forcing and cloud droplet activation.

The evolution of aerosols in the atmosphere depends on the initial particle size distribution, aging rates, and removal mechanisms. In East Asia and South Asia, high concentrations of precursor gases such as SO₂ and VOCs cause nucleation-mode and accumulation-mode particles to acquire coatings of organics, sulfates, and nitrates within hours or days. This process accelerates the formation of hygroscopic coatings [32,244]. Rapid aerosol aging converts hydrophobic BC into hydrophilic particles. This conversion increases the efficiency of BC removal through wet deposition [245]. Wintertime temperature inversions and dry conditions favor dry deposition. Dry deposition dominates under these conditions and prolongs aerosol lifetime [246]. Conditions in Europe and North America differ. Long-term emission reductions have lowered SO₂ and VOC concentrations. Coating formation and oxidation processes occur slowly. Aerosol aging depends primarily on photochemical reactions [247]. Under clean background conditions, BC and organic aerosols can remain hydrophobic for several days or longer. Dry deposition becomes an important removal pathway under these conditions [170]. Wildfire events alter these processes. BC from wildfires is often coated with thick organic layers. Its aging and removal efficiency differ significantly from fossil fuel-derived BC [238]. Smoke plumes frequently reach the upper troposphere. Particles in these plumes have extended lifetimes and can be transported across continents to Europe or East Asia [248]. Seasonal variations in removal processes are also significant. Summer monsoon systems in East Asia and South Asia produce high wet deposition efficiency. Winter aerosols remain largely within the boundary layer. Dry deposition is enhanced during this season [244]. Precipitation in Europe and North America is relatively evenly distributed. Wet removal contributes significantly to BC removal throughout the year [18,22]. These regional and seasonal differences indicate that aerosols with identical size and composition can have vastly different atmospheric lifetimes and climatic effects. Differences in lifetime and removal shape the spatiotemporal distribution of aerosols. They also influence the role of aerosols in radiative forcing, CCN formation, and climate feedbacks. Accurate representation of regional emission inventories, size distribution parameterizations, and removal processes is essential in global climate models and regional air quality simulations. Such representation improves the reliability of simulation results and the scientific basis for policy assessment [20,246].

6. Conclusions and Outlook

In global studies of BC distribution, the coupling among particle size distribution, aging processes, nucleation ability, and removal mechanisms forms a crucial link between cloud and fog microphysical processes and global climate effects. Particle size distribution is not only a primary physical parameter controlling BC direct radiative effects and indirect effects on clouds and climate, but also strongly influences atmospheric lifetime, transport distance, and ultimate removal pathways [16,18,20,189]. In the WRF-Chem model, the dynamic coupling of size and chemical composition determines optical properties and deposition rates, thereby affecting the balance of shortwave and longwave radiation [194]. Aging processes drive changes in particle size distribution and mixing state. During long-range transport of BC from biomass burning, the formation of SOA in the first few days increases particle mass and size. Subsequent photochemical degradation and cloud processing may reduce particle mass or weaken light absorption. Particle size and aging state directly control activation as CCN or IN. Larger particles with high hygroscopicity activate at lower Sc. In contrast, unaged, hydrophobic, or externally mixed particles require a higher Sc for activation [249]. Chang et al. compared different activation schemes in model simulations and examined their sensitivity on CCN and cloud droplet number concentrations [250]. The study indicated that the representation of size distribution and mixing state contributes substantially to uncertainties in indirect climate effects.

6.1. Summary of Main Conclusions

Particle size plays a critical role in determining whether aerosols can activate as CCN or IN, thereby influencing cloud droplet and ice crystal formation processes. Newly emitted submicron aerosols smaller than 100 nm often fail to activate due to their small size and hydrophobicity. During aging, heterogeneous oxidation occurs. Condensation of semi-volatile substances such as secondary organic aerosols, sulfates, and nitrates increases particle size. Aging also alters particle hygroscopicity and mixing state. These changes allow particles to reach the critical diameter required for CCN activation [247,251,252]. Particle growth increases removal efficiency by cloud droplet collision. Large particles are more likely to be captured by cloud droplets. They are also removed faster through inertial impaction during wet deposition. This removal mechanism shortens the atmospheric lifetime of aged, large particles despite their higher nucleation potential [18,62,253]. Particle size evolution, therefore, exerts a dual effect on aerosol–cloud interactions and atmospheric residence time. Observational studies show strong regional differences. East Asia and South Asia exhibit complex aging processes and evolving size distributions due to dense populations, strong combustion sources, and humid climates [239]. Europe and North America display clearer seasonal cycles. Wintertime emissions increase the fraction of small particles. Summer precipitation enhances wet removal [16,157]. Background measurements at high-altitude and polar sites indicate gradual particle growth during long-range transport. These observations demonstrate the universal role of aging at the global scale [107]. Modeling studies identify size distribution and aging rate parameterizations as major sources of uncertainty. Most global models adopt modal schemes that approximate aerosol size distributions using lognormal functions. These schemes produce deviations when simulating size-dependent removal and mixing state transitions [123,190]. Size-resolved schemes better capture nucleation and removal processes. However, they require high computational resources and are difficult to apply in global climate simulations [180,191,254]. Discrepancies between simulations and observations limit the precise assessment of BC climate effects.

6.2. Sources of Uncertainty and Future Research Directions

Despite extensive experimental, observational, and modeling studies, substantial uncertainties remain in the coupling of aerosol size, aging, and removal processes. Aging rate assumptions differ significantly among models. Some models assume that BC particles convert from hydrophobic to hydrophilic within a single day. Other models suggest that the conversion requires several days [255]. These differences directly lead to inconsistent estimates of aerosol lifetime and long-range transport potential. Most models assume either complete internal mixing or complete external mixing. In reality, aerosols exhibit partial mixing across multiple scales. Simplifying assumptions cause models to underestimate or overestimate the effects of particle growth on nucleation and removal [212,247]. Observational techniques have limitations in size resolution and spatial coverage. SP2 instruments detect BC only above 70–150 nm equivalent diameter. This restriction reduces the ability to identify small-particle nucleation modes [107]. Long-term observations are sparse in key emission regions such as Southeast Asia and Africa. The lack of data increases uncertainty in regional climate assessments [16]. Regional climate factors also influence size-dependent nucleation and removal efficiency. Factors such as humidity, precipitation type, and cloud droplet size distribution alter aerosol behavior. Differences in observations among regions indicate that these background conditions constitute an additional major source of uncertainty. These factors significantly affect aerosol lifetime and climatic impact [134].

To address these challenges, future research should be deepened in several areas. Globally, particularly in South Asia, Africa, and South America, long-term station development should be strengthened to obtain systematic particle size distribution and mixed-state observation data [221]. Concurrently, new technologies such as high-resolution mass spectrometry, simultaneous CCN and IN measurements, and online particle size distribution monitoring should be developed to enhance the ability to capture aging processes. Cloud chamber experiments can reveal the physical relationships between particle size and nucleation/clearing under controlled conditions, while field observations reflect the true atmospheric background. Future research should strengthen the coupling between these two processes, develop standardized experimental protocols, and quantify size-dependent aging rates and removal efficiencies [101]. Global climate models require further incorporation of size-resolving capabilities or improved modal parameterization schemes to enhance the accuracy of describing particle size evolution. Multi-model comparisons and observational constraints can effectively narrow model differences [190,212]. The particle size distribution-aging-removal chain is not only an atmospheric science issue but also closely related to air quality and emission reduction policies. Future efforts should establish an interdisciplinary research framework directly linking aerosol microphysical processes with regional air quality management and global climate policies, providing a more scientific basis for decision-making [206]. Additionally, the BC deposition record in ice cores serves as the final outcome and evidence of the entire transport, aging, and deposition process of BC particles. The particle size distribution recorded in BC can directly reflect the influence of transport processes and emission sources, making it a crucial medium for studying BC particle size and evolution. Combined with air mass transport patterns, this record can be used to reconstruct the size-differentiated evolution of BC and its influencing factors.

In summary, the particle size distribution strongly influences aerosol activation, aging, and removal processes, thereby linking microphysical evolution with atmospheric lifetime and climate effects. Future research should integrate observational, experimental, and modeling efforts to reduce uncertainties, thereby providing robust scientific support for global climate assessments and regional emission reduction policies. Only by establishing a closed-loop system encompassing “micro-mechanisms—observational evidence—model simulations” can we achieve a systematic understanding of the aerosol evolution chain and effectively bridge scientific insights with policy implementation.