Photoionization of Atmospheric Aerosol Constituents and Precursors in the 7−15 eV Energy Region: Experimental and Theoretical Study

Determination of absolute photoionization cross-sections of some aromatic hydrocarbons

RATIONALE

Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however.

METHODS

PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements.

RESULTS

The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy.

CONCLUSIONS

Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.

New particle formation from the reactions of ozone with indene and styrene

This work reports the experimental study of the ozonolysis of indene in the presence of SO₂ and the reaction conditions leading to the formation of secondary aerosols. The reactions have been carried out in a Teflon chamber filled with synthetic air mixtures at atmospheric pressure and room temperature. As in the case of styrene, SO₂ plays a key role in the oxidation of the Criegee intermediates and enhances the formation of particulate matter. Thus, for the ozonolysis of indene, nucleation was observed for reacted indene concentrations above (4.5 ± 0.8) × 10¹¹ molecule cm^-3 in the absence of SO₂ while new particle formation was observed for concentrations one order of magnitude lower, (3 ± 1) × 10¹⁰ molecule cm^-3, in the presence of SO₂. Within the detection limit of the system, SO₂ concentrations remained constant during the experiments. The formation of secondary aerosols in the smog chamber was inhibited by H₂O and so the potential formation of secondary aerosols under atmospheric conditions depends on the concentration of SO₂ and relative humidity. Computational calculations have been performed for the ozonolysis of both indene and styrene in the presence of SO₂ and water to identify the reaction channels and species responsible for new particle formation. The release of SO₃ and its subsequent conversion into H₂SO₄ from the reaction of the Criegee intermediate H₂COO in the ozonolysis of styrene makes this aromatic have a high potential of aerosol formation in the atmosphere. On the other hand, quantitative conversion of SO₂ into SO₃ does not occur following the ozonolysis of indene.

Rotational spectroscopy of the atmospheric photo-oxidation product o-toluic acid and its monohydrate

o-Toluic acid, a photo-oxidation product in the atmosphere, and its monohydrate were characterized in the gas phase by pure rotational spectroscopy. High-resolution spectra were measured in the range of 5-14 Hz using a cavity-based molecular beam Fourier-transform microwave spectrometer. Possible conformers were identified computationally, at the MP2/6-311++G(2df,2pd) level of theory. For both species, one conformer was identified experimentally, and no methyl internal rotation splittings were observed, indicative of relatively high barriers to rotation. In the monomer, rocking of the carboxylic acid group is a large amplitude motion, characterized by a symmetrical double-well potential. This and other low-lying out-of-plane vibrations contribute to a significant (methyl top-corrected) inertial defect (-1.09 amu Å(2)). In the monohydrate, wagging of the free hydrogen atom of water is a second large amplitude motion, so the average structure is planar. As a result, no c-type transitions were observed. Water tunneling splittings were not observed, because the water rotation coordinate is characterized by an asymmetrical double-well potential. Since the minima are not degenerate, tunneling is precluded. Furthermore, a concerted tunneling path involving simultaneous rotation of the water moiety and rocking of the carboxylic acid group is precluded, because the hilltop along this coordinate is a virtual, rather than a real, saddle-point. Inter- and intramolecular non-covalent bonding is discussed in terms of the quantum theory of atoms in molecules. The percentage of o-toluic acid hydrated in the atmosphere is estimated to be about 0.1% using statistical thermodynamics.

Ionization photophysics and spectroscopy of dicyanoacetylene

Photoionization of dicyanoacetylene was studied using synchrotron radiation over the excitation range 8-25 eV, with photoelectron-photoion coincidence techniques. The absolute ionization cross-section and detailed spectroscopic aspects of the parent ion were recorded. The adiabatic ionization energy of dicyanoacetylene was measured as 11.80 ± 0.01 eV. A detailed analysis of the cation spectroscopy involves new aspects and new assignments of the vibrational components to excitation of the quasi-degenerate A(2)Πg, B(2)Σg(+) states as well as the C(2)Σu(+) and D(2)Πu states of the cation. Some of the structured autoionization features observed in the 12.4-15 eV region of the total ion yield spectrum were assigned to vibrational components of valence shell transitions and to two previously unknown Rydberg series converging to the D(2)Πu state of C4N2(+). The appearance energies of the fragment ions C4N(+), C3N(+), C4(+), C2N(+), and C2(+) were measured and their heats of formation were determined and compared with existing literature values. Thermochemical calculations of the appearance potentials of these and other weaker ions were used to infer aspects of dissociative ionization pathways.

Determination of absolute photoionization cross-sections of aromatics and aromatic derivatives

The absolute photoionization cross-sections of aromatics and aromatic derivatives including toluene, ethylbenzene, n-propylbenzene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, styrene, phenylacetylene, indene, indane, 1-methylnaphthalene, benzyl alcohol and benzaldehyde were measured at the photon energy range from ionization thresholds to 11.7 eV. The experiments were performed by tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Benzene was chosen as a calibration standard, since its photoionization cross-section is well known. Binary liquid mixtures of the investigated molecules and benzene were used in the measurements. Photo-induced fragments from the molecules were also observed, and their photoionization cross-sections are also presented.