Alexei F. Khalizov

Nucleation and growth of nanoparticles in the atmosphere.

Nucleation and Growth of Nanoparticles in the Atmosphere Renyi Zhang,* Alexei Khalizov, Lin Wang, Min Hu, and Wen Xu Department of Atmospheric Sciences andDepartment of Chemistry, Center for Atmospheric Chemistry and Environment, Texas A&M University, College Station, Texas 77843, United States Department of Environmental Science & Engineering and Institute of Global Environment Change Research, Fudan University, Shanghai 200433, China State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

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 ≈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.

Formation of nanoparticles of blue haze enhanced by anthropogenic pollution

The molecular processes leading to formation of nanoparticles of blue haze over forested areas are highly complex and not fully understood. We show that the interaction between biogenic organic acids and sulfuric acid enhances nucleation and initial growth of those nanoparticles. With one cis-pinonic acid and three to five sulfuric acid molecules in the critical nucleus, the hydrophobic organic acid part enhances the stability and growth on the hydrophilic sulfuric acid counterpart. Dimers or heterodimers of biogenic organic acids alone are unfavorable for new particle formation and growth because of their hydrophobicity. Condensation of low-volatility organic acids is hindered on nano-sized particles, whereas ammonia contributes negligibly to particle growth in the size range of 3–30 nm. The results suggest that initial growth from the critical nucleus to the detectable size of 2–3 nm most likely occurs by condensation of sulfuric acid and water, implying that anthropogenic sulfur emissions (mainly from power plants) strongly influence formation of terrestrial biogenic particles and exert larger direct and indirect climate forcing than previously recognized.

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.

Processing of Soot by Controlled Sulphuric Acid and Water Condensation—Mass and Mobility Relationship

The effects of atmospheric processing on soot particle morphology were studied in the laboratory using the Differential Mobility Analyzer-Aerosol Particle Mass Analyzer (DMA-APM) and the DMA-DMA (Tandem DMA) techniques. 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.

Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor

The hygroscopic properties of submicron soot particles during internal mixing with gaseous sulfuric acid have been investigated using a combined tandem differential mobility analyzer (TDMA) and differential mobility analyzer-aerosol particle mass analyzer (DMA-APM) technique. Fresh particles exhibit no change in mobility size and mass at subsaturated conditions, whereas particles exposed to gaseous sulfuric acid (10(9)-10(10) molecule cm(-3), 12 s contact time) experience significant mobility size and mass changes with increasing relative humidity (RH). The DMA-APM measurements reveal that particles of all sizes exposed to H2SO4 vapor gain mass with increasing RH because of absorption of water by sulfuric acid coating. However, on the basis of mobility size measurements using TDMA, upon humidification H2SO4-coated soot agglomerates display distinct hygroscopic growth patterns depending on their initial size and the mass fraction of condensed sulfuric acid. While small particles experience an increase in their mobility sizes, larger particles exhibit a marked shrinkage due to compaction. We suggest that determination of the hygroscopic properties of soot particles using a TDMA alone can be inconclusive. Restructuring of the soot agglomerates and filling of the voids that accompany the condensation of water-soluble materials and subsequent water absorption lead to little or no observable changes in particle mobility size at subsaturated RH even for particles that contain aqueous coatings. Extrapolation of our experimental results to the urban atmosphere indicates that initially hydrophobic soot particles acquire sufficient sulfate coating to become efficient CCN (cloud condensation nuclei) within a time period ranging from a few hours to a few days, dependent on the ambient H2SO4 level. The results imply that internal mixing with sulfuric acid through H2SO4 vapor condensation likely represents a common aging process for a variety of atmospheric aerosols. The variations in the size and hygroscopicity of soot particles during atmospheric processing influence their optical properties, cloud-forming potential, and human health effects. (Less)

Hydrogen-bonding interaction in molecular complexes and clusters of aerosol nucleation precursors.

Complexes and clusters bridge the gap between molecular and macroscopic levels by linking individual gaseous molecules to newly formed nanoparticles but the driving forces and mechanism for the formation of complexes and clusters in the atmosphere are not well understood. We have performed ab initio and density functional quantum chemical calculations to elucidate the role of organic acids in the formation of complexes with common atmospheric nucleating precursors such as sulfuric acid, water, and ammonia. A central feature of the complexes is the presence of two hydrogen bonds. Organic acid-sulfuric acid complexes show one strong and one medium-strength hydrogen bond whereas the corresponding hydrogen bonds in organic acid-ammonia complexes are characterized as medium-strength and weak. The formation of strong hydrogen bonds in organic acid-sulfuric acid complexes is explained by the well-established resonance-assisted hydrogen bonding theory. Organic acid-sulfuric acid and organic acid-organic acid complexes possess the largest binding energies among the homomolecular and heteromolecular dimers, about 18 kcal mol(-1) from the composite theoretical methods. Topological analysis employing quantum theory of atoms in molecules (QTAIM) shows that the charge density and the Laplacian at bond critical points (BCPs) of the hydrogen bonds of the organic acid-sulfuric acid complex (e.g., benzoic acid-sulfuric acid and cis-pinonic acid-sulfuric acid) are 0.07 and 0.16 au, respectively, which falls in or exceeds the range of one strong and one medium-strength hydrogen bonding criteria.

Heterogeneous chemistry of alkylamines with sulfuric acid: implications for atmospheric formation of alkylaminium sulfates.

The heterogeneous interaction of alkylamines with sulfuric acid has been investigated to assess the role of amines in aerosol growth through the formation of alkylaminium sulfates. The kinetic experiments were conducted in a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The measurements of heterogeneous uptake of methylamine, dimethylamine, and trimethylamine were performed in the acidity range of 59-82 wt % H(2)SO(4) and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (gamma) in the range of 2.0 x 10(-2) to 4.4 x 10(-2). The measured gamma value was slightly higher in more concentrated sulfuric acid and at lower temperatures. The results imply that the heterogeneous reactions of alkylamines contribute effectively to the growth of atmospheric acidic particles and, hence, secondary organic aerosol formation.

Heterogeneous Reactions of Alkylamines with Ammonium Sulfate and Ammonium Bisulfate

The heterogeneous reactions between alkylamines and ammonium salts (ammonium sulfate and ammonium bisulfate) have been studied using a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) at 293 ± 2 K. The uptake of three alkylamines, i.e., monomethylamine, dimethylamine, and trimethylamine, on ammonium sulfate shows a displacement reaction of ammonium by aminium, evidenced by the release of ammonia monitored using protonated acetone dimer as the reagent ion. For the three alkylamines, the initial uptake coefficients (γ(0)) range from 2.6 × 10(-2) to 3.4 × 10(-2) and the steady-state uptake coefficients (γ(ss)) range from 6.0 × 10(-3) to 2.3 × 10(-4) and decrease as the number of methyl groups on the alkylamine increases. A different reaction mechanism is observed for the uptake of the three alkylamines on ammonium bisulfate, which is featured by an acid-base reaction (neutralization) with irreversible alkylamine loss and no ammonia generation and occurs at a rate limited by diffusion of gaseous alkylamines to the ammonium bisulfate surface. Our results reveal that the reactions between alkylamines and ammonium salts contribute to particle growth and alter the composition of ammonium sulfate and bisulfate aerosols in the atmosphere.

Role of OH-Initiated Oxidation of Isoprene in Aging of Combustion Soot

We have investigated the contribution of OH-initiated oxidation of isoprene to the atmospheric aging of combustion soot. The experiments were conducted in a fluoropolymer chamber on size-classified soot aerosols in the presence of isoprene, photolytically generated OH, and nitrogen oxides. The evolution in the mixing state of soot was monitored from simultaneous measurements of the particle size and mass, which were used to calculate the particle effective density, dynamic shape factor, mass fractal dimension, and coating thickness. When soot particles age, the increase in mass is accompanied by a decrease in particle mobility diameter and an increase in effective density. Coating material not only fills in void spaces, but also causes partial restructuring of fractal soot aggregates. For thinly coated aggregates, the single scattering albedo increases weakly because of the decreased light absorption and practically unchanged scattering. Upon humidification, coated particles absorb water, leading to an additional compaction. Aging transforms initially hydrophobic soot particles into efficient cloud condensation nuclei at a rate that increases in the presence of nitrogen oxides. Our results suggest that ubiquitous biogenic isoprene plays an important role in aging of anthropogenic soot, shortening its atmospheric lifetime and considerably altering its impacts on air quality and climate.