Formation of secondary organic aerosol by reactive condensation of furandiones, aldehydes, and water vapor onto inorganic aerosol seed particles

Suppression of NaNO3 crystal nucleation by glycerol: micro-Raman observation on the efflorescence process of mixed glycerol/NaNO3/water droplets

Although the hygroscopicity of a NaNO(3)/water microdroplet and a polyalcohol/water microdroplet, two of the most important aerosols in atmosphere, has been widely studied, little is known about the relationship between the hygroscopic behavior of mixed NaNO(3)/polyalcohol/water droplets and their structures on the molecular level. In this study, the hygroscopicity of mixed glycerol/NaNO(3)/water droplets deposited on a hydrophobic substrate was studied by micro-Raman spectroscopy with organic-to-inorganic molar ratios (OIRs) of 0.5, 1, and 2. In the mixed glycerol/NaNO(3)/water droplets, glycerol molecules tended to combine with Na(+) and NO(3)(-) ions by electrostatic interaction and hydrogen bonding, respectively. On the basis of the analyses of the changes of symmetric stretching (v(s)-CH(2)), asymmetric stretching (v(a)-CH(2)), their area ratio (Av(a)-CH(2)/Av(s)-CH(2)) of glycerol, and symmetric stretching band of NO(3)(-) (ν(1)-NO(3)(-)) with relative humidity (RH), it was found that the conformation of glycerol was transformed from αα mainly to γγ and partly to αγ with a decreasing RH in the mixed droplets, contrary to the case in the glycerol/water droplet. In addition, the glycerol with γγ and αγ conformation had strong interaction with Na(+) and NO(3)(-) respectively, which suppressed the formation of contact of ions and delayed the efflorescence relative humidity (ERH) for the mixed droplets compared to the NaNO(3)/water droplet.

Understanding the fast pyrolysis of lignin

In the present study, pyrolysis of corn stover lignin was investigated by using a micro-pyrolyzer coupled with a GC-MS/FID (FID=flame ionization detector). The system has pyrolysis-vapor residence times of 15-20 ms, thus providing a regime of minimal secondary reactions. The primary pyrolysis product distribution obtained from lignin is reported. Over 84 % mass balance and almost complete closure on carbon balance is achieved. In another set of experiments, the pyrolysis vapors emerging from the micro-pyrolyzer are condensed to obtain lignin-derived bio-oil. The chemical composition of the bio-oil is analyzed by using GC-MS and gel permeation chromatography techniques. The comparison between results of two sets of experiments indicates that monomeric compounds are the primary pyrolysis products of lignin, which recombine after primary pyrolysis to produce oligomeric compounds. Further, the effect of minerals (NaCl, KCl, MgCl(2), and CaCl(2)) and temperature on the primary pyrolysis product distribution is investigated. The study provides insights into the fundamental mechanisms of lignin pyrolysis and a basis for developing more descriptive models of biomass pyrolysis.

Uptake of glyoxal by organic and Inorganic aerosol

The uptake of glyoxal by a variety of organic and inorganic aerosol types was examined in a Teflon chamber. Rapid glyoxal uptake was observed for all liquid-phase aerosols at all relative humidity levels tested (< 5 to 50% RH). Even for aerosol with known water content, Henry's Law cannot predict glyoxal uptake: H* > (3 +/- 1.5) x 10(8) mol kg(-1) atm(-1) for l-tartaric acid, H* > (1 +/- 0.5) x 10(8) for dl-malic acid and H* = (2 +/- 1) x 10(7) for malonic acid aerosol. Other liquid-phase aerosol particles containing amine functional groups (arginine, aspartic acid, and glutamic acid) took up even more glyoxal (H* > 3 x 10(8)). The trend of higher glyoxal uptake onto aerosol containing more nucleophilic organic compounds suggests that glyoxal is reacting with organic compounds in the aerosol phase. Solid-phase aerosol showed RH-dependent glyoxal uptake, likely due to the existence of surface water layers. However, particle growth rates were the highestfor sodium sulfate aerosol. For organic aerosol, growth rates correlated with the acidity of the carboxylic acid groups of the aerosol material, suggesting that glyoxal uptake is enhanced by mildly acidic conditions.

Secondary Organic Aerosol Formation during the Photooxidation of Toluene: NOx Dependence of Chemical Composition

The photooxidation of toluene is a potential source of secondary organic aerosol (SOA) in urban air, but only a small portion of the compounds present in SOA have been identified. In this study, we analyzed the chemical compositions of SOA produced by photoirradiation of the toluene/NOx/air system in laboratory chamber experiments by a combination of liquid chromatography-mass spectrometry, hybrid high-performance liquid chromatography-mass spectrometry, and iodometry-spectrophotometry. The dependence of the chemical composition on the initial NOx concentration was examined at initial NO concentrations ([NO]0) of 0.2 and 1 ppmv. Fifteen semivolatile products, including aromatic and ring-cleavage compounds, were quantified. However, the quantified products comprised only a small portion ( approximately 1 wt %) of the total aerosol mass. The total SOA yield ( approximately 13 wt %), the ratio of organic peroxides to total SOA mass ( approximately 17 wt %), and the density of SOA ( approximately 1.4 g cm-3) were independent of the NOx level, suggesting that the reaction mechanisms of the formation of major SOA products at [NO]0 = 0.2 and 1 ppmv are essentially the same. The negative-ion mass spectra of SOA samples showed that ion signals attributed to hemiacetal oligomers and/or decomposition products of peroxy hemiacetal oligomers were detected in the range of mass-to-charge ratios (m/z) between 200 and 500. The highest signals were detected at m/z = 155 and 177, and these were tentatively assigned to C7 unsaturated oxacyclic oxocarboxylic acids and C7 unsaturated oxacyclic dicarboxylic acids, respectively. We conclude that the major chemical components of the aerosol are hemiacetal and peroxy hemiacetal oligomers and low-molecular-weight dicarboxylic acids.

Effects of seed aerosols on the growth of secondary organic aerosols from the photooxidation of toluene

Hydroxyl radical (.OH)-initiated photooxidation reaction of toluene was carried out in a self-made smog chamber. Four individual seed aerosols such as ammonium sulfate, ammonium nitrate, sodium silicate and calcium chloride, were introduced into the chamber to assess their influence on the growth of secondary organic aerosols (SOA). It was found that the low concentration of seed aerosols might lead to high concentration of SOA particles. Seed aerosols would promote rates of SOA formation at the start of the reaction and inhibit its formation rate with prolonging the reaction time. In the case of ca. 9000 pt/cm3 seed aerosol load, the addition of sodium silicate induced a same effect on the SOA formation as ammonium nitrate. The influence of the four individual seed aerosols on the generation of SOA decreased in the order of calcium chloride>sodium silicate and ammonium nitrate>ammonium sulfate.

Identification of Fulvic Acids and Sulfated and Nitrated Analogues in Atmospheric Aerosol by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

The water-soluble organic fractions of aerosol samples collected in Riverside, CA, in summer 2005 were analyzed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Elemental compositions of about 1000 molecular species were determined in the range m/z 220-420, and four series of organic compounds were identified, fulvic acids, and S-containing, N-containing, and S- and N-containing molecules. Low-resolution product ion spectra proved the presence of organosulfates, organonitrates, and mixed organosulfates and -nitrates that appear to be structurally closely related to each other and to the fulvic acids. This is the first unambiguous detection of fulvic acid molecules and sulfated components in atmospheric aerosol and the first detection even of nitrated analogues. These species provide new clues to the nature of particulate organic matter in atmospheric aerosol.

Secondary organic aerosol formation by heterogeneous reactions of aldehydes and ketones: a quantum mechanical study

Experimental studies have provided convincing evidence that aerosol-phase heterogeneous chemical reactions (possibly acid-catalyzed) are involved to some extent in the formation of secondary organic aerosol (SOA). We present a stepwise procedure to determine physical properties such as heats of formation, standard entropies, Gibbs free energies of formation, and solvation energies from quantum mechanics (QM), for various short-chain aldehydes and ketones. We show that quantum mechanical gas-phase Gibbs free energies of formation compare reasonably well with the literature values with a root-mean-square (RMS) value of 1.83 kcal/mol for the selected compounds. These QM results are then used to determine the equilibrium constants (reported as log K) of aerosol-phase chemical reactions, including hydration reactions and aldol condensation for formaldehyde, acetaldehyde, acetone, butanal, hexanal, and glyoxal. Results are in qualitatively agreement with previous studies. In addition, the QM results for glyoxal reactions are consistent with experimental observations. To our knowledge, this is the first QM study that supports observations of atmospheric particle-phase reactions. Despite the significant uncertainties in the absolute values from the QM calculations, the results are potentially useful in determining the relative thermodynamic tendency for atmospheric aerosol-phase reactions.

Secondary organic aerosol formation by glyoxal hydration and oligomer formation: humidity effects and equilibrium shifts during analysis

Glyoxal is a significant atmospheric aldehyde formed from both anthropogenic aromatic compounds and biogenic isoprene emissions. The chemical behavior of glyoxal relevant to secondary organic aerosol (SOA) formation and analysis is examined in GC-MS, electrospray ionization (ESI)-MS, and particle chamber experiments. Glyoxal oligomers are shown to rapidly decompose to glyoxal in GC injection ports at temperatures > or = 120 degrees C. Glyoxal dihydrate monomer is dehydrated at temperatures > or = 140 degrees C during GC analysis but shows only oligomers (n < or = 7) upon ESI-MS analysis. Thus both of these analytical techniques will cause artifacts in speciation of glyoxal in SOA. In particle chamber experiments, glyoxal (at -0.1 Torr) condensed via particle-phase reactions when relative humidity levels exceeded a threshold of -26%. Both the threshold humidity and particle growth rates (-0.1 nm/min) are consistent with a recent study performed at glyoxal concentrations 4 orders of magnitude below those used here. This consistency suggests a mechanism where the surface water layer of solid-phase aerosol becomes saturated with glyoxal dihydrate monomer, triggering polymerization and the establishment of an organic phase.

Measurements of secondary organic aerosol from oxidation of cycloalkenes, terpenes, and m-xylene using an Aerodyne aerosol mass spectrometer

The Aerodyne aerosol mass spectrometer (AMS) was used to characterize physical and chemical properties of secondary organic aerosol (SOA) formed during ozonolysis of cycloalkenes and biogenic hydrocarbons and photo-oxidation of m-xylene. Comparison of mass and volume distributions from the AMS and differential mobility analyzers yielded estimates of "effective" density of the SOA in the range of 0.64-1.45 g/cm3, depending on the particular system. Increased contribution of the fragment at m/z 44, C02+ ion fragment of oxygenated organics, and higher "delta" values, based on ion series analysis of the mass spectra, in nucleation experiments of cycloalkenes suggest greater contribution of more oxygenated molecules to the SOA as compared to those formed under seeded experiments. Dominant negative "delta" values of SOA formed during ozonolysis of biogenics indicates the presence of terpene derivative structures or cyclic or unsaturated oxygenated compounds in the SOA. Evidence of acid-catalyzed heterogeneous chemistry, characterized by greater contribution of higher molecular weight fragments to the SOA and corresponding changes in "delta" patterns, is observed in the ozonolysis of alpha-pinene. Mass spectra of SOA formed during photooxidation of m-xylene exhibit features consistent with the presence of furandione compounds and nitro organics. This study demonstrates that mixtures of SOA compounds produced from similar precursors result in broadly similar AMS mass spectra. Thus, fragmentation patterns observed for biogenic versus anthropogenic SOA may be useful in determining the sources of ambient SOA.

Secondary organic aerosols from anthropogenic and biogenic precursors

Secondary organic aerosol (SOA) formation from the photooxidation of an anthropogenic (1,3,5-trimethylbenzene) and a biogenic (alpha-pinene) precursor was investigated at the new PSI smog chamber. The chemistry of the gas phase was followed by proton transfer reaction mass spectrometry, while the aerosol chemistry was investigated with aerosol mass spectrometry, ion chromatography, laser desorption ionization mass spectrometry, and infrared spectroscopy, along with volatility and hygroscopicity studies. Evidence for oligomer formation for SOA from both precursors was given by an increasing abundance of compounds with a high molecular weight (up to 1000 Da) and by an increasing thermal stability with increasing aging time. The results were compared to data obtained from ambient aerosol samples, revealing a number of similar features.