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.

Effects of coating of dicarboxylic acids on the mass-mobility relationship of soot particles.

Atandem differential mobility analyzer (TDMA) and a differential mobility analyzer-aerosol particle mass analyzer (DMA-APM) have been employed to study morphology and hygroscopicity of soot aerosol internally mixed with dicarboxylic acids. The effective densities, fractal dimensions, and dynamic shape factors of soot particles before and after coating with succinic and glutaric acids are determined. Coating of soot with succinic acid results in a significant increase in the particle mobility diameter, mass, and effective density, but these properties recover to their initial values once succinic acid is removed by heating, suggesting that no restructuring of the soot core occurs. This conclusion is also supported from the observation of similar fractal dimensions and dynamic shape factors for fresh and coated/heated soot aggregates. Also, no change is observed when succinic acid-coated aggregates are cycled through elevated relative humidity (5% to 90% to 5% RH) below the succinic acid deliquescence point. When soot is coated with glutaric acid, the particle mass increases, but the mobility diameter shrinks by 10-40%. Cycling the soot aerosol coated with glutaric acid through elevated relative humidity leads to an additional mass increase, indicating that condensed water remains within the coating even at low RH. The fractal dimension of soot particles increases after coating and remains high when glutaric acid is removed by heating. The dynamic shape factor of glutaric acid-coated and heated soot is significantly lower than that of fresh soot, suggesting a significant restructuring of the soot agglomerates by glutaric acid. The results imply that internal mixing of soot aerosol during atmospheric aging leads to changes in hygroscopicity, morphology, and effective density, which likely modify their effects on direct and indirect climate forcing and deposition in the human respiratory system.

Enhanced light absorption and scattering by carbon soot aerosol internally mixed with sulfuric acid.

Light absorption by carbon soot increases when the particles are internally mixed with nonabsorbing materials, leading to increased radiative forcing, but the magnitude of this enhancement is a subject of great uncertainty. We have performed laboratory experiments of the optical properties of fresh and internally mixed carbon soot aerosols with a known particle size, morphology, and the mixing state. Flame-generated soot aerosol is size-selected with a double-differential mobility analyzer (DMA) setup to eliminate multiply charged particle modes and then exposed to gaseous sulfuric acid (10(9)-10(10) molecule cm(-3)) and water vapor (5-80% relative humidity, RH). Light extinction and scattering by fresh and internally mixed soot aerosol are measured at 532 nm wavelength using a cavity ring-down spectrometer and an integrating nephelometer, respectively, and the absorption is derived as the difference between extinction and scattering. The optical properties of fresh soot are independent of RH, whereas soot internally mixed with sulfuric acid exhibits significant enhancement in light absorption and scattering, increasing with the mass fraction of sulfuric acid coating and relative humidity. For soot particles with an initial mobility diameter of 320 nm and a 40% H(2)SO(4) mass coating fraction, absorption and scattering are increased by 1.4- and 13-fold at 80% RH, respectively. Also, the single scattering albedo of soot aerosol increases from 0.1 to 0.5 after coating and humidification. Additional measurements with soot particles that are first coated with sulfuric acid and then heated to remove the coating show that both scattering and absorption are enhanced by irreversible restructuring of soot aggregates to more compact globules. Depending on the initial size and density of soot aggregates, restructuring acts to increase or decrease the absorption cross-section, but the combination of restructuring and encapsulation always results in an increased absorption for internally mixed soot. Mass absorption cross-sections (MAC) for fresh soot aggregates are size dependent, increasing from 6.7 +/- 0.7 m(2) g(-1) for 155 nm particles to 8.7 +/- 0.1 m(2) g(-1) for 320 nm particles. After exposure of soot to sulfuric acid, MAC is as high as 12.6 m(2) g(-1) for 320 nm particles at 80% RH. Our results imply that optical properties of soot are significantly altered within its atmospheric lifetime, leading to greater impact on visibility, local air quality, and radiative climate forcing.

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

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 approximately 10-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.

Heterogeneous chemistry of organic acids on soot surfaces.

We have investigated the heterogeneous interaction between a number of carboxylic acids and soot generated from different fuel sources and formation mechanisms. A low-pressure fast flow reactor in conjunction with ion drift-chemical ionization mass spectrometry detection was employed to study uptake of monocarboxylic (benzoic, oleic, and steric) and dicarboxylic (glutaric, maleic, oxalic, and phthalic) acids on deposited soot surfaces formed by combustion of methane, propane, and kerosene. Most acids exhibited irreversible uptake on the soot surfaces and the uptake coefficient was measured in the range of 9 x 10(-4) to 1 x 10(-1) estimated based on the geometric surface areas. Brunauer, Emmett, and Teller surface areas of the deposited soot surfaces were measured and the soot bulk and surface chemical compositions were evaluated with Fourier transform infrared spectroscopy and attenuated total reflection spectroscopy. Plausible uptake mechanisms were discussed on the basis of the measured soot physiochemical properties by comparing the mono and dicarboxylic acids.

Determination of PAHs in dust from Shanghai by optimized SFE and GC/MS.

EPA 16 priority polycyclic aromatic hydrocarbons (PAHs) were extracted from dust by supercritical fluid extraction (SFE). Parameters (pressure, temperature and time) affecting the collection efficiencies of PAHs were assessed according to the extraction efficiency, and 30 MPa, 80 degrees C and 30 min dynamic extraction time were confirmed to be the best and simplest conditions of SFE to extract 16 priority PAHs from dust. The characterization of the extracts was carried out by gas chromatography with mass detector in selective ion mode (GC/MS/SIM). Ultrasonic extraction (USE) was used as a comparison with SFE. The results showed that the SFE method has a better efficiency than USE for the extraction of PAHs in dust. With the optimized conditions of SFE, the distribution of PAHs in dust samples in urban areas of Shanghai from Mar 10, 2005 to May 28, 2005 was investigated. The results demonstrated that traffic emission, especially from gasoline engines, was the main source of PAHs in dust of Shanghai.