K. Wirtz

Simulating the Formation of Secondary Organic Aerosol from the Photooxidation of Aromatic Hydrocarbons

Environmental Context. Atmospheric particulate material can affect the radiative balance of the atmosphere and is believed to be detrimental to human health. Secondary organic aerosols (SOA), which make a significant contribution to the total atmospheric burden of fine particulate material, are formed in situ following the photochemical transformation of organic pollutants into relatively less-volatile, oxygenated compounds which can subsequently transfer from the gas phase to a particle phase. SOA formation from the atmospheric photooxidation of aromatic hydrocarbons—present, for example, as a result of automobile use—is believed to be important in the urban environment and yet the mechanisms are not well understood. For example, even the reasons for observed variations in the relative propensity for SOA formation, from the photooxidation of various simple aromatic hydrocarbons, are not clear. Abstract. The formation and composition of secondary organic aerosol (SOA) from the photooxidation of benzene, p-xylene, and 1,3,5-trimethylbenzene has been simulated using the Master Chemical Mechanism version 3.1 (MCM v3.1) coupled to a representation of the transfer of organic material from the gas to particle phase. The combined mechanism was tested against data obtained from a series of experiments conducted at the European Photoreactor (EUPHORE) outdoor smog chamber in Valencia, Spain. Simulated aerosol mass concentrations compared reasonably well with the measured SOA data only after absorptive partitioning coefficients were increased by a factor of between 5 and 30. The requirement of such scaling was interpreted in terms of the occurrence of unaccounted-for association reactions in the condensed organic phase leading to the production of relatively more nonvolatile species. Comparisons were made between the relative aerosol forming efficiencies of benzene, toluene, p-xylene, and 1,3,5-trimethylbenzene, and differences in the OH-initiated degradation mechanisms of these aromatic hydrocarbons. A strong, nonlinear relationship was observed between measured (reference) yields of SOA and (proportional) yields of unsaturated dicarbonyl aldehyde species resulting from ring-fragmenting pathways. This observation, and the results of the simulations, is strongly suggestive of the involvement of reactive aldehyde species in association reactions occurring in the aerosol phase, thus promoting SOA formation and growth. The effect of NOx concentrations on SOA formation efficiencies (and formation mechanisms) is discussed.

OH-initiated oxidation of benzene

The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Simulating the Formation of Secondary Organic Aerosol from the Photooxidation of Toluene

Environmental Context. Atmospheric particulate material can affect climate by absorbing and scattering solar radiation and by altering the properties of clouds. They are also implicated as a health risk. Secondary organic aerosol (SOA) material makes an important contribution to this particulate burden. SOA material results from the transfer of gas-phase species into a particle state after the formation of products from the reaction of atmospheric volatile organic compounds (VOCs) with oxygen. SOA from the oxidation of aromatic hydrocarbons, such as toluene, a gasoline fuel component, is important in the polluted urban environment and yet formation mechanisms are not well understood. Abstract. The formation and composition of secondary organic aerosol (SOA) from the gas-phase photooxidation of toluene has been simulated using the Master Chemical Mechanism version 3.1 (MCM v3.1) coupled to a representation of the transfer of organic material from the gas phase to a particle phase. The mechanism was tested against data from a series of toluene photooxidation experiments performed at the European Photoreactor (EUPHORE) outdoor smog chamber in Valencia, Spain. Simulated aerosol mass concentrations compared reasonably well with the measured SOA data after absorptive partitioning coefficients were increased by a factor of between 20 and 80, although the simulated onset of SOA growth was delayed with respect to the experiments. A simplified representation of peroxyhemiacetal adduct formation, from the reaction of organic hydroperoxides with aldehydes in the condensed organic phase, was included in the mechanism and this reduced the required scaling of partitioning coefficients and reduced the time-lag in simulated SOA growth. These observations, and the dependence of SOA formation efficiency upon the initial NO concentration, strongly imply the significant occurrence of association reactions in the condensed organic phase and the important role of organic hydroperoxides in SOA formation. Aerosol data from photooxidation experiments of intermediate degradation products (butenedial, 4-oxo-pentenal, and ortho-cresol) were also simulated using the developed mechanism.

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. E-2-Hexenal and E,E-2,4-hexadienal were found to undergo rapid isomerization to produce Z-2-hexenal and a ketene-type compound (probably E-hexa-1,3-dien-1-one), respectively. Both isomerization processes were reversible with formation of the reactant slightly favoured. Values of j(E-2-hexenal)/j(NO(2)) = (1.80 +/- 0.18) x 10(-2) and j(E,E-2,4-hexadienal)/j(NO(2)) = (2.60 +/- 0.26) x 10(-2) were determined. The gas phase UV absorption cross-sections of E-2-hexenal and E,E-2,4-hexadienal were measured and used to derive effective quantum yields for photoisomerization of 0.36 +/- 0.04 for E-2-hexenal and 0.23 +/- 0.03 for E,E-2,4-hexadienal. Although photolysis appears to be an important atmospheric degradation pathway for E-2-hexenal and E,E-2,4-hexadienal, the reversible nature of the photolytic process means that gas phase reactions with OH and NO(3) radicals are ultimately responsible for the atmospheric removal of these compounds. Atmospheric photolysis of Z-3-hexenal produced CO, with a molar yield of 0.34 +/- 0.03, and 2-pentenal via a Norrish type I process. A value of j(Z-3-hexenal)/j(NO(2)) = (0.4 +/- 0.04) x 10(-2) was determined. The results suggest that photolysis is likely to be a minor atmospheric removal process for Z-3-hexenal.

Kinetics of the reactions of pinonaldehyde with OH radicals and with Cl atoms

The rate constant for the reaction of OH radicals with pinonaldehyde has been measured at 293 6 6 K using the relative rate method in the laboratory in Wuppertal (Germany) using photolytic sources for the production of OH radicals and in the EUPHORE smog chamber facility in Valencia (Spain) using the in situ ozonolysis of 2,3-dimethyl-2-butene as a dark source of OH radicals. In all the experiments pinonaldehyde and the reference compounds were monitored by FTIR spectroscopy, and in addition in the EUPHORE smog chamber the decay of pinonaldehyde was monitored by the HPLC/DNPH method and the reference com- pound by GC/FID. The results from all the different series of experiments were in good agree- ment and lead to an average value of k(pinonaldehyde 1 OH) 5 (4.0 6 1.0) 3 10 211 cm 3 molecule 21 s 21 . This result lead to steady-state estimates of atmospheric pinonaldehyde con- centrations in the ppbV range (1 ppbV < 2.5 3 10 10 molecule cm 23 at 298 K and 1 atm) in regions with substantial a-pinene emission. It also implies that atmospheric sinks of pinon- aldehyde by reaction with OH radicals could be half as important as its photolysis. The rate constant of the reaction of pinonaldehyde with Cl atoms has been measured for the first time. Relative rate measurements lead to a value of k(pinonaldehyde 1 Cl) 5 (2.4 6 1.4) 3 10210 cm3 molecule21 s21. In contrast to previous studies which suggested enhanced kinetic reactivity for pinonaldehyde compared to other aldehydes, the results from both the OH- and Cl-initiated oxidation of pinonaldehyde in the present work are in line with predic- tions using structure-activity relationships. q 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 291- 301, 1999

The atmospheric photolysis of o-tolualdehyde.

The photolysis of o-tolualdehyde by natural sunlight has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photolysis rate coefficient was measured directly under different solar flux levels, with values in the range j(o-tolualdehyde) = (1.62-2.15) × 10(-4) s(-1) observed, yielding an average value of j(o-tolualdehyde)/j(NO(2)) = (2.53 ± 0.25) × 10(-2). The estimated photolysis lifetime is 1-2 h, confirming that direct photolysis by sunlight is the major atmospheric degradation pathway for o-tolualdehyde. Published UV absorption cross-section data were used to derive an effective quantum yield (290-400 nm) close to unity, within experimental error. Possible reaction pathways for the formation of the major photolysis products, benzocyclobutenol (tentatively identified) and o-phthalaldehyde, are proposed. Appreciable yields (5-13%) of secondary organic aerosol (SOA) were observed at EUPHORE and also during supplementary experiments performed in an indoor chamber using an artificial light source. Off-line analysis by gas chromatography-mass spectrometry allowed identification of o-phthalaldehyde, phthalide, phthalic anhydride, o-toluic acid, and phthalaldehydic acid in the particle phase.

Studies of photochemical ozone formation in toluene/NOx/air systems by empirical and numerical simulations

Benzene and the alkyl-substituted benzenes, toluene and xylenes, are the most abundant aromatics observed in urban atmospheres (see Table 1). The aromatic fraction reaches 20 to 40% of the total non-methane hydrocarbons with the traffic emission being the major source. Their contribution to the photochemical smog and the formation of photooxidants, especially ozone, have been studied in smog chamber experiments and by simulations using atmospheric chemical models (Atkinson et al., 1980; Killus and Whitten, 1982; Leone et al., 1985). As a result of these studies, the aromatic hydrocarbons are considered to contribute significantly to the tropospheric ozone formation (Carter and Atkinson, 1987). However, the oxidation mechanisms of aromatic hydrocarbons in the atmosphere are up to now not well understood. Recent product studies from the OH-radical initiated oxidation of toluene under NOx-free conditions and in the presence of NOx (Atkinson and Aschmann, 1994; Bierbach et al., 1994) gave a poor carbon balance. Therefore, a detailed oxidation mechanism for the aromatic hydrocarbons is at present not possible.