Table 1 Summary of the physiochemical properties of LACs.
From: Lifecycle of light-absorbing carbonaceous aerosols in the atmosphere
LAC type | Black carbon (BC) | Brown carbon (BrC) | ||
|---|---|---|---|---|
Highly absorbing BrC | Moderately absorbing BrC | Weakly absorbing BrC | ||
Common present forms or other names | Soot, elemental carbon (EC), involatile carbon | Tar ball (in continuous state) | Atmospheric humic-like substances (HULIS, in continuous state) | Water-soluble BrC |
Sources (formation mechanism) | High-temperature combustion (flame synthesis of PAHs) | More absorbing BrC from flaming biomass or solid fuel burning than lower-temperature smoldering combustion (pyrolysis and near-source condensation, oxidation, functionization after emission) | Some primary OA after photobleaching; some secondary OA from VOC condensation | |
Carbon bonding | sp2 dominated | A range of sp2/sp3 | Moderate sp2/sp3 | Low sp2 |
Composition | Graphite layers | Chromophores, e.g., aromatics associated with nitrated, carbonyl, and unsaturated function groups | Chromophores being decomposed or newly formed | |
Morphology | ||||
 Shape and particle size | Aggregates of monomers in diameter 10–50 nm, overall diameter 80–220 nm | Sphere mainly at accumulation mode in diameters of hundreds of nanometers; or as coatings | Secondary species, size depending on the condensed substrate | |
 Volatility  (vaporization temperature) | Refractory (~4000 K) | Extremely low volatility (~1000 K) | Low to medium volatile | Semi-volatile |
 Viscosity | Solid | Amorphous solid | Amorphous, semi-solid or liquid depending on temperate and RH | Mostly liquid |
Optical properties | ||||
 Imaginary refractive indexa | Weak spectral dependence ~0.7–0.8 | ~0.3–0.5, slightly enhanced near-UV, also near-IR | In the order of 0.01–0.1, mainly near-UV | In the order of 0.001–0.01, only near-UV |
 Absorption Ångström exponent (AAE) | AAE ~1 | AAE slightly >1 | Moderate AAE 2–4 | High AAE ~3–8 |
Hygroscopic properties | ||||
 Solubility | Insoluble to any solvents | Mostly insoluble | Partly soluble (usually more in organic solvents) | Soluble |
 Cloud condensation nuclei | Hard CCN; enhanced CCN after aging | Barely CCN but may enhance through oxidation | More CCN | |
 Ice nuclei | Maybe deposition IN | Maybe deposition/immersion IN when glassy-solid/semi-solid | Hard IN | |
 Atmospheric evolution | Chemically inert; more compact shape after aging, possible absorption enhancement | Chemically unstable; subject to photolysis/photooxidation degradation; may enhance absorption by NO3 oxidation or ammonia uptake | ||
