Qiao Ma

Experimental and Theoretical Study of Reactions of OH Radicals with Hexenols: An Evaluation of the Relative Importance of the H-Abstraction Reaction Channel.

C6 hexenols are one of the most significant groups of volatile organic compounds with biogenic emissions. The lack of corresponding kinetic parameters and product information on their oxidation reactions will result in incomplete atmospheric chemical mechanisms and models. In this paper, experimental and theoretical studies are reported for the reactions of OH radicals with a series of C6 hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-4-hexen-1-ol, at 298 K and 1.01 × 10(5) Pa. The corresponding rate constants were 8.53 ± 1.36, 10.1 ± 1.6, 7.86 ± 1.30, 8.08 ± 1.33, 9.10 ± 1.50, and 7.14 ± 1.20 (in units of 10(-11) cm(3) molecule(-1) s(-1)), respectively, measured by gas chromatography with a flame ionization detector (GC-FID), using a relative technique. Theoretical calculations concerning the OH-addition and H-abstraction reaction channels were also performed for these reactions to further understand the reaction mechanism and the relative importance of the H-abstraction reaction. By contrast to previously reported results, the H-abstraction channel is a non-negligible reaction channel for reactions of OH radicals with these hexenols. The rate constants of the H-abstraction channel are comparable with those for the OH-addition channel and contribute >20% for most of the studied alcohols, even >50% for (E)-3-hexen-1-ol. Thus, H-abstraction channels may have an important role in the reactions of these alcohols with OH radicals and must be considered in certain atmospheric chemical mechanisms and models.

Kinetic and mechanistic study on gas phase reactions of ozone with a series of cis-3-hexenyl esters

As an important group of green leaf volatiles (GLVs), C6 hexenyl esters, are found to be widely emitted into the atmosphere by plants and vegetation, especially when they suffer mechanical damage. It is indispensable to understand their atmospheric fate for environmental assessment and model simulation. In this paper, the rate constants for reactions of O3 with four cis-3-hexenyl esters have been measured using an absolute method in a flow tube reactor at 298 K and atmospheric pressure. The measured rate constants (in 10−17 cm3 per molecule per s) were 4.06 ± 0.66 for cis-3-hexenyl formate, 5.77 ± 0.70 for cis-3-hexenyl acetate, 7.62 ± 0.88 for cis-3-hexenyl propionate, and 12.34 ± 1.59 for cis-3-hexenyl butyrate, respectively. Theoretical calculations were also carried out for the title reactions to better understand their kinetics and mechanism using density functional theory (DFT) and transition state theory (TST). Geometry optimizations, energy and harmonic vibrational frequency calculations were performed for all of the stationary points at the BHandHLYP/6-311+G(d,p) level of theory. The calculated rate constants were in good agreement with the experimental values. The results showed that the reactivity of the studied compounds towards O3 was obviously dependent on their chemical structure, such as the nature of the substituent, and the relative positions of the double bond and the substituent. The results were also discussed in terms of their atmospheric importance in the degradation of these unsaturated esters by comparing their lifetimes with respect to their reactions with O3 and other main atmospheric oxidants.

VUV photoionization aerosol mass spectrometric study on the iodine oxide particles formed from O3-initiated photooxidation of diiodomethane (CH2I2)

Iodine oxide particles (IOPs) formed from O3-initiated photooxidation of diiodomethane have been investigated based on the combination of a thermal desorption/tunable vacuum ultraviolet time-of-flight photoionization aerosol mass spectrometer (TD-VUV-TOF-PIAMS) with a flow reactor for the first time. Characterization of the home-made flow reactor was performed, which indicates the applicability of its combination with TD-VUV-TOF-PIAMS. Based on that, aerosol mass spectra of IOP formation from photooxidation of CH2I2/O3 were studied on-line taking full advantage of both the virtues of the flow reactor and TD-VUV-TOF-PIAMS. The main chemical components of IOPs, including atomic and molecular iodine (I, I2), iodine oxides (IO, OIO, I2O and I2O3) and hydrogen-containing iodine species (HI, HIO and HIO3), were observed and identified based on the corresponding photoionization energy (PIE) curves, and the probable chemical composition and formation mechanism of IOPs were proposed. The work has not only improved the understanding of the formation mechanism of IOPs, but also demonstrated the capability of TD-VUV-TOF-PIAMS for direct molecular characterization of aerosols in flow reactor experiments, whose potential application in mass spectrometric studies of atmospheric aerosols is anticipated.

Kinetics and mechanisms of gas phase reactions of hexenols with ozone

An absolute kinetic study is reported for the reactions of O3 with a series of C6 hexenols, (Z)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (Z)-4-hexen-1-ol, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-4-hexen-1-ol. At 298 K and atmospheric pressure, the rate constants (in units of 10−17 cm3 molecule−1 s−1) were measured to be 7.44 ± 1.03, 5.47 ± 0.71, 7.09 ± 0.91, 16.6 ± 2.2, 6.19 ± 0.72 and 10.5 ± 1.4, respectively. To gain a deeper insight into the reactivity and mechanism, theoretical calculations were also performed for the title reactions with the methods of density functional theory (DFT) and transition-state theory (TST). The geometries, energies, and harmonic vibrational frequencies of each stationary point were obtained at the BH&HLYP/6-31+G(d,p) level of theory. The calculated rate constants are in good agreement with the experimental data, and the reactivity of hexenols with O3 shows a strong dependence on their chemical structure based on the theoretical results. Finally, lifetimes of the C6 hexenols, with respect to their reactions with some important atmospheric oxidants such as O3, OH and NO3 radicals, have also been discussed in the article.

The influences of ammonia on aerosol formation in the ozonolysis of styrene: roles of Criegee intermediate reactions

The influences of ammonia (NH3) on secondary organic aerosol (SOA) formation from ozonolysis of styrene have been investigated using chamber experiments and quantum chemical calculations. With the value of [O3]0/[styrene]0 ratios between 2 and 4, chamber experiments were carried out without NH3 or under different [NH3]/[styrene]0 ratios. The chamber experiments reveal that the addition of NH3 led to significant decrease of SOA yield. The overall SOA yield decreased with the [NH3]0/[styrene]0 increasing. In addition, the addition of NH3 at the beginning of the reaction or several hours after the reaction occurs had obviously different influence on the yield of SOA. Gas phase reactions of Criegee intermediates (CIs) with aldehydes and NH3 were studied in detail by theoretical methods to probe into the mechanisms behind these phenomena. The calculated results showed that 3,5-diphenyl-1,2,4-trioxolane, a secondary ozonide formed through the reactions of C6H5ĊHOO· with C6H5CHO, could make important contribution to the aerosol composition. The addition of excess NH3 may compete with aldehydes, decreasing the secondary ozonide yield to some extent and thus affect the SOA formation.