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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...

Rapid depletions of atmospheric mercury (Hg 0 ) and ozone coinciding with high halogen concentrations have recently been observed in several high-Arctic and sub-Arctic regions. The lack of kinetic data on the halogen-initiated reactions of elemental mercury precluded the drawing of any conclusions on the chemistry of these mercury depletion events. We carried out extensive kinetic and product studies on the reactions of gaseous Hg 0 with molecular and atomic halogens (X/X2 where X = Cl, Br) at atmospheric pressure (750 ± 1 Torr) and room temperature (298 ± 1 K) in air and N2. Kinetics of the reactions with X/X2 was studied using relative and absolute techniques by cold vapor atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS). The measured rate constants for reactions of Hg 0 with Cl2, Cl, Br2, and Br were (2.6 ± 0.2) × 10 -18 , (1.0 ± 0.2) × 10 -11 , <(0.9 ± 0.2) × 10 -16 , and (3.2 ± 0.3) × 10 -12 cm 3 molecule -1 s -1 , respectively. The reaction products were analyzed in the gas phase, from the suspended aerosols, and from the wall of the reactor using MS, GC-MS, and inductively coupled plasma mass spectrometry (ICP-MS). The major products identified were HgCl2 and HgBr2 adsorbed on the wall. Suspended aerosols, collected on the micron filters, contributed to less than 0.5% of the reaction products under our experimental conditions. The importance of halogen reactions of mercury in the formation of particulate matter, which tends to be bioaccumulative, is discussed herein.

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. To simulate atmospheric processing, combustion soot agglomerates were altered by sulphuric acid vapor condensation, relative humidity (RH) cycling, and evaporation of the sulphuric acid and water by heating. Primary investigated properties were particle mobility size and mass. Secondary properties, derived from these, include effective density, fractal dimension, dynamic shape factor, and the mass fraction of condensed material. A transformation of the soot particles to more compact forms occurs as sulphuric acid and water condense onto fresh soot. The particle mass increases and initially the mobility diameter decreases, indicating restructuring of the soot core, likely due to surface tension forces. For a given soot source and condensing liquid, the degree of compaction depends strongly on the mass (or volume) fraction of condensed material. For water and sulphuric acid condensing on combustion soot, a mass increase of 2-3 times is needed for a transformation to spherical particles. In the limit of spherical particles without voids, the effective density then approaches the inherent material density, the fractal dimension approaches 3 and the dynamic shape factor approaches 1. Our results indicate that under typical atmospheric conditions, soot particles will be fully transformed to spherical droplets on a time scale of several hours. It is expected that the morphology changes and addition of soluble material to soot strongly affect the optical and hygroscopic properties of soot.