Variability in morphology, hygroscopicity, and optical properties of soot aerosols during atmospheric processing

Abstract

The atmospheric effects of soot aerosols include interference with radiative transfer, visibility impairment, and alteration of cloud formation and are highly sensitive to the manner by which soot is internally mixed with other aerosol constituents. We present experimental studies to show that soot particles acquire a large mass fraction of sulfuric acid during atmospheric aging, considerably altering their properties. Soot particles exposed to subsaturated sulfuric acid vapor exhibit a marked change in morphology, characterized by a decreased mobility-based diameter but an increased fractal dimension and effective density. These particles experience large hygroscopic size and mass growth at subsaturated conditions (<90% relative humidity) and act efficiently as cloud-condensation nuclei. Coating with sulfuric acid and subsequent hygroscopic growth enhance the optical properties of soot aerosols, increasing scattering by Ϸ10-fold and absorption by nearly 2-fold at 80% relative humidity relative to fresh particles. In addition, condensation of sulfuric acid is shown to occur at a similar rate on ambient aerosols of various types of a given mobility size, regardless of their chemical compositions and microphysical structures. Representing an important mechanism of atmospheric aging, internal mixing of soot with sulfuric acid has profound implications on visibility, human health, and direct and indirect climate forcing.climate ͉ clouds ͉ radiative properties ͉ human health ͉ anthropogenic pollution S oot aerosols produced from fossil-fuel combustion, automobile and aircraft emissions, and biomass burning are ubiquitous in the atmosphere, comprising Ϸ10-50% of the total tropospheric particulate matter (1-6). Once emitted into the atmosphere, soot particles are subjected to several aging processes, including adsorption or condensation of gaseous species (7-9), coagulation with other preexisting aerosols, and oxidation (10-12). Model calculations have shown that, when associated with other nonabsorbing aerosol constituents (e.g., sulfate), soot seems more absorptive and exerts a higher positive direct radiative forcing, and the warming effect by soot nearly balances the net cooling effect of other anthropogenic aerosols (5, 13). Also, on the basis of mesoscale model simulations, absorption of solar radiation by internally mixed soot aerosols causes warming in the middle atmosphere and reduction in cloudiness over the tropics (4). The mixing state and associated physical, optical, and geometrical properties of soot particles are of critical importance in evaluating the effects of light-absorbing aerosols and improving climate predictions by using global climate models (GCMs). Current knowledge on such an issue is very limited for developing an accurate representation of soot particles in GCMs, leading to underestimation of climatic forcing (14).Hygroscopic aerosols also act as cloud-condensation nuclei (CCN) that impact cloud formation and the lifetime and albedo of clouds (4, 6). Freshly generated soot particles exist...

Amplification of light absorption of black carbon associated with air pollution

“…S1), in which considerable industrial manufacturing has led to 23 heavy emissions of air pollution, especially in southern Hebei. 24 Ambient aerosol particles were collected by a PM1 cyclone and a diffusion silica gel dryer, and they were then analyzed 25 by an aethalometer (A E33, Magee Scientific Corp.), an Aerosol Chemical Speciat ion Monitor (ACSM, Aerodyne Research 26 Inc.) and a single particle soot photometer (SP2, Droplet Measurement Technologies Inc.). The AE33 can measure the 27 absorption coefficient (σab) of sampled aerosols in seven spectral regions (370, 470, 520, 590, 660, 880 and 950 n m).…”

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

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

Elucidating severe urban haze formation in China

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