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 % H2SO4 and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (γ) in the range of 2.0 × 10−2 to 4.4 × 10−2. The measured γ 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.

The heterogeneous reaction of nitrogen dioxide (NO2) on fresh and coated soot surfaces has been investigated to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere. Soot surfaces were prepared by incomplete combustion of propane and kerosene fuels under lean and rich flame conditions and then processed by heating to evaporate semivolatile species or by coating with pyrene, sulfuric acid, or glutaric acid. Uptake kinetics and HONO yield measurements were performed in a low-pressure fast-flow reactor coupled to a chemical ionization mass spectrometer (CIMS), using atmospheric-level NO2 concentrations. The uptake coefficient and the HONO yield upon interaction of NO2 with nascent soot depend on the type of fuel and combustion regime and are the highest for samples prepared using fuel rich flame. Heating the nascent soot samples before exposure to NO2 removes the organic material from the soot backbone, leading to a significant increase in NO2 uptake coefficient and HONO yield. Continuous exposure to NO2 reduces the reactivity of soot because of irreversible deactivation of the surface sites. Our results support the oxidation−reduction mechanism involving adsorptive and reactive centers on soot surface where NO2 is converted to HONO and other products. Coating of the soot surface by different materials to simulate atmospheric aging has a strong impact on its reactivity toward NO2 and the resulting HONO production. Coating of pyrene has little effect on either reaction rate or HONO yield. Sulfuric acid coating does not alter the uptake coefficient, but significantly reduces the amount of HONO formed. Coating of glutaric acid significantly increases NO2 uptake coefficient and HONO yield. The results of our study indicate that the reactivity and HONO generating capacity of internally mixed soot aerosol will depend on the chemical composition of the coating material and hence will vary considerably in different polluted environments.

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.

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.

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 (109−1010 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% H2SO4 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 m2 g−1 for 155 nm particles to 8.7 ± 0.1 m2 g−1 for 320 nm particles. After exposure of soot to sulfuric acid, MAC is as high as 12.6 m2 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.

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.

Indirect radiative forcing of atmospheric aerosols by modification of cloud processes poses the largest uncertainty in climate prediction. We show here a trend of increasing deep convective clouds over the Pacific Ocean in winter from long-term satellite cloud measurements (1984–2005). Simulations with a cloud-resolving weather research and forecast model reveal that the increased deep convective clouds are reproduced when accounting for the aerosol effect from the Asian pollution outflow, which leads to large-scale enhanced convection and precipitation and hence an intensifed storm track over the Pacific. We suggest that the wintertime Pacific is highly vulnerable to the aerosol–cloud interaction because of favorable cloud dynamical and microphysical conditions from the coupling between the Pacific storm track and Asian pollution outflow. The intensified Pacific storm track is climatically significant and represents possibly the first detected climate signal of the aerosol–cloud interaction associated with anthropogenic pollution. In addition to radiative forcing on climate, intensification of the Pacific storm track likely impacts the global general circulation due to its fundamental role in meridional heat transport and forcing of stationary waves.

We performed laboratory studies to investigate the OH-initiated oxidation of m-xylene using a fast flow reactor coupled to ion drift-chemical ionization mass spectrometry (ID-CIMS). Seven products consisting of ring-retaining and ring-opening components have been identified and quantified. Three products, methylglyoxal, 4-oxo-2-pentenal and 2-methyl-4-oxo-2-butenal, and dimethylphenols exhibited a dependence of the formation yields on the O2 and NO concentration. We reported for the first time the yields (%) of three other products, 2-methyl-4-oxo-2-penetal (4.89 ± 0.52), diunsaturated dicarbonyls (2-methyl-6-oxo-2,4-heptadienal and 4-methyl-6-oxo-2,4-heptadienal, 7.82 ± 1.0), and epoxy carbonyls (2,4-dimethyl-2,3-epoxy-6-oxo-4-hexenal, 2,6-dimethyl-2,3-epoxy-6-oxo-4-hexenal, and 3,5-dimethyl-2-hydroxyl-3,4-epoxy-5-hexenal, 2.18 ± 0.20). m-Tolualdehyde, with a yield of 6.42 ± 0.89, is consistent with previous studies. The yield of methylglyoxal (15.1 ± 4.2) is consistent with its co-products, but lower than the literature values. The dependence of product distributions on O2 and NO concentration was discussed. The mechanism leading to the formation of the products was proposed.

Abstract

Atmospheric aerosols often contain a substantial fraction of organic matter, but the role of organic compounds in new nanometer-sized particle formation is highly uncertain. Laboratory experiments show that nucleation of sulfuric acid is considerably enhanced in the presence of aromatic acids. Theoretical calculations identify the formation of an unusually stable aromatic acid–sulfuric acid complex, which likely leads to a reduced nucleation barrier. The results imply that the interaction between organic and sulfuric acids promotes efficient formation of organic and sulfate aerosols in the polluted atmosphere because of emissions from burning of fossil fuels, which strongly affect human health and global climate.

Simulations with a regional chemical transport model show that anthropogenic emissions of volatile organic compounds and nitrogen oxides (NOx = NO + NO2) lead to a dramatic diurnal variation of surface ozone (O3) in Houston, Texas. During the daytime, photochemical oxidation of volatile organic compounds catalyzed by NOx results in episodes of elevated ambient O3 levels significantly exceeding the National Ambient Air Quality Standard. The O3 production rate in Houston is significantly higher than those found in other cities over the United States. At night, a surface NOx maximum occurs because of continuous NO emission from industrial sources, and, consequently, an extensive urban-scale “hole” of surface ozone (<10 parts per billion by volume in the entire Houston area) is formed as a result of O3 removal by NO. The results suggest that consideration of regulatory control of O3 precursor emissions from the industrial sources is essential to formulate ozone abatement strategies in this region.

The competing pathways of H-abstraction by oxygen molecules and 1,5 H-shift of the δ-alkoxy radicals with the Z-configuration arising from OH-initiated reactions of isoprene have been investigated using density functional theory (DFT) and ab initio molecular orbital calculations. The activation and reaction energies of the alkoxy radical reactions were obtained with B3LYP, CCSD(T), and MPW1K and various basis sets. Kinetic calculations employing variational RRKM/ME formalism and separate statistical ensemble (SSE) theory show that a significant fraction of the chemically excited alkoxy radicals undergo prompt 1,5 H-shift. The results also reveal that 1,5 H-shift of thermalized δ-alkoxy radicals dominates over H-abstraction by O2.

We evaluate the impact of anthropogenic and natural NOx sources over the contiguous United States on tropospheric NOx and O3 levels by using a global 3D chemical transport model. The effects of major U.S. surface NOx emission sources (including anthropogenic, biomass burning, and soil emissions) are compared with that of lightning-produced NOx. Summer lightning is shown to play a dominant role in controlling NOx and O3 concentrations in the middle and upper troposphere, despite the fact that fossil-fuel burning represents the largest source of NOx over the U.S. Furthermore, the effect of regional U.S. lightning is propagated through large areas of the Northern Hemisphere by atmospheric circulation. The results reveal that a thorough assessment of atmospheric NOx emission sources and their impact is required to devise control strategies for regional and global air pollution.

Density functional theory (DFT) and ab initio multiconfigurational calculations have been performed to investigate the OH–toluene reaction. The applicability of DFT and ab initio theories to the OH–toluene reaction system is evaluated. The results reveal that the DFT method produces activation and reaction energies and rate constants of the OH–toluene reaction in good agreement with the experimental values. We predict the branching ratios of OH addition to ortho, para, meta, and ipso positions to be 0.52, 0.34, 0.11, and 0.03, respectively, significantly different from a recent theoretical study of the same reaction system.

The formation rates and isomeric branching ratios of the four possible adducts arising from the Cl–isoprene reaction have been calculated using canonical variational transition state theory (CVTST). In addition, RRKM/master equation (ME) formalism is employed to investigate isomerization of the Cl–isoprene adducts. We found that there is no evidence for an energetic barrier for Cl addition to isoprene and the Morse potential well represents the energetics along the reaction coordinate. The results reveal the importance of terminal Cl addition to isoprene in partitioning of the final reaction products and in determining the reaction pathways.