Sara M. Aschmann

Formation of OH radicals in the gas phase reactions of O3 with a series of terpenes

The gas phase reactions of O3 with ethene, isoprene, and a series of monoterpenes have been investigated at 296 ± 2 K and atmospheric pressure of air in the presence of cyclohexane at concentrations sufficient to essentially totally scavenge any OH radicals formed. The expected products of the OH radical-initiated reaction of cyclohexane, cyclohexanone and cyclohexanol, were observed in all cases. From a knowledge of the chemistry of cyclohexane in these reaction systems and as a result of cyclohexanone and cyclohexanol formation yield data obtained in subsidiary experiments, the formation yields of OH radicals in these O3-alkene reactions were derived. The OH radical formation yields obtained from the gas phase reactions of O3 with alkenes were ethene, 0.12; isoprene, 0.27; camphene, ≤0.18; 3-carene, 1.06; limonene, 0.86; myrcene, 1.15; ocimene (a cis-, trans- mixture), 0.63; β-phellandrene, 0.14; α-pinene, 0.85; β-pinene, 0.35; sabinene, 0.26; and terpinolene, 1.03, all with estimated overall uncertainties of a factor of ∼ 1.5. For a-pinene the effect of varying the water vapor concentration was investigated and no change in the OH radical formation yield was observed over the range of (2.5–24) × 1016 molecules cm−3 of water vapor. The experimental conditions were such that formation of OH radicals from HO2 radicals was of minor importance, and the OH radical formation yields given above refer to direct formation of OH radicals and not HO2 radicals.

Product formation from the gas-phase reactions of OH radicals and O3 with a series of monoterpenes

The formation yields of nine carbonyl products are reported from the gas-phase OH radical-initiated reactions (in the presence of NOx) and the O3 reactions with seven monoterpenes. The products were identified using GC/MS and GC-FTIR and quantified by GC-FID analyses of samples collected on Tenax solid adsorbent cartridges. The identities of products from camphene, limonene and β-pinene were confirmed by comparison with authentic standards. Sufficient quantities of products from the 3-carene, limonene, α-pinene, sabinene and terpinolene reactions were isolated to allow structural confirmation by proton NMR spectroscopy. The measured total carbonyl formation yields ranged from non-detectable for the OH radical reaction with camphene and the O3 reactions with 3-carene and limonene to ∼0.5 for the OH radical reaction with limonene and the O3 reaction with sabinene.

Emission rates of organics from vegetation in California's Central Valley

Rates of emission of speciated hydrocarbons have been determined for more than 30 of the most dominant (based on acreage) agricultural and natural plant types found in Californias Central Valley. These measurements employed flow-through Teflon chambers, sample collection on solid adsorbent and thermal desorption gas chromatography (GC) and GC-mass spectrometry analysis to identify more than 40 individual organic compounds. In addition to isoprene and the monoterpenes, we observed sesquiterpenes, alcohols, acetates, aldehydes, ketones, ethers, esters, alkanes, alkenes and aromatics as emissions from these plant species. Mean emission rates for total monoterpenes ranged from none detected in the case of beans, grapes, rice and wheat, to as high as 12–30 μg h−1 g−1 for pistachio and tomato (normalized to dry leaf and total biomass, respectively). Other agricultural species exhibiting substantial rates of emission of monoterpenes included carrot, cotton, lemon, orange and walnut. All of the plant species studied showed total assigned compound emission rates in the range between 0.1 and 36 νg h−1 g−1.

The emission of (Z)-3-hexen-1-ol, (Z)-3-hexenylacetate and other oxygenated hydrocarbons from agricultural plant species

Abstract The oxygenated hydrocarbon species identified as emissions from 29 agricultural and natural plant species found in Californias Central Valley are reported. Prevalent as emissions from these agricultural crops were (Z)-3-hexen-1-ol and (Z)-3-hexenylacetate, with the latter compound often the dominant hydrocarbon emission. Quantitative emission rates for (Z)-3-hexen-1-ol and (Z)-3-hexenylacetate are reported for 18 agricultural and two natural plant species, and a brief discussion of the expected atmospheric chemistry of these two compounds is given.

Terpenes emitted from agricultural species found in California's Central Valley

More than a dozen monoterpenes have been identified as emissions from agricultural and natural plant species occupying large acreages in the Central Valley of California, including as dominant emissions camphene, 2-carene, Δ3-carene, limonene, myrcene, trans-ocimene, β-phellandrene, α-pinene, β-pinene, sabinene, γ-terpinene, and terpinolene. Isoprene was not a significant emission from any of the crop species examined but was emitted by a Valley Oak. In addition to the monoterpenes, sesquiterpenes were emitted from approximately one third of the species investigated, in some cases at higher levels than the monoterpene emissions from the same plant. The possible contributions of these biogenic emissions to the ozone exceedances in the Central Valley should be considered in planning future emission control strategies.

Formation of acetone from the OH radical‐ and O3‐initiated reactions of a series of monoterpenes

The formation yields of acetone from the gas-phase reactions of the OH radical (in the presence of NO) and O3 with a series of monoterpenes have been measured at room temperature and atmospheric pressure of air. The acetone formation yields ranged from <2–3% for the OH radical reaction with limonene and the O3 reactions with limonene and α-phellandrene to 50% for the O3 reaction with terpinolene. Combining these acetone formation yields with literature estimates of emission rates of monoterpenes from vegetation leads to an estimate of acetone formation from the atmospheric photooxidation of monoterpenes of ∼10–11 Tg yr−1 globally, a significant fraction of the global acetone source strength of 40–60 Tg yr−1. Reaction mechanisms leading to acetone formation from these monoterpene reactions are discussed.

Products of the gas phase reactions of the OH radical with α‐ and β‐pinene in the presence of NO

Products of the gas phase reactions of the OH radical with α- and β-pinene in the presence of NO have been investigated using gas chromatography and in situ atmospheric pressure ionization mass spectrometry (API-MS). Acetone was identified and quantified by gas chromatography with flame ionization detection and combined gas chromatography-mass spectrometry, with formation yields of 0.110±0.027 from the α-pinene reaction and 0.085±0.018 from the β-pinene reaction. Acetone is a first-generation product and may arise after initial H atom abstraction from the tertiary allylic C-H bond at the 1 position and/or after OH radical addition at the 2 and 3 positions. Using API-MS and API-MS/MS analyses, in addition to the formation of pinonaldehyde from α-pinene and nopinone from β-pinene, we have observed the formation of hydroxynitrates, dihydroxynitrates and dihydroxycarbonyl products of molecular weights 215, 231 and 184, respectively, from both α- and β-pinene. Reaction schemes leading to the formation of these multifunctional product species, consistent with our understanding of the atmospheric chemistry of organic peroxy and alkoxy radicals, are presented.

Formation yields of methyl vinyl ketone and methacrolein from the gas-phase reaction of o3 with isoprene.

The formation yields of methyl vinyl ketone and methacrolein from the gas-phase reaction of O 3 with isoprene were determined at 296±2 K and 740 Torr total pressure of air. Sufficient cycloherane was added to the reactant mixture to scavenge >97% of the OH radicals formed in the O 3 -isoprene reaction, and secondary reactions of O 3 with methyl vinyl ketone and methacrolein were taken into account. The formation yields obtained were 0.159±0.013 for methyl vinyl ketone and 0.387±0.030 for methacrolein. Experiments were also carried out in the absence of added cycloherane, and the methyl vinyl ketone and methacrolein data were consistent with formation from both the OH radical and O 3 reactions with isoprene

Products of the gas-phase reactions of O(3 P) atoms and O3 with α-pinene and 1,2-dimethyl-1-cyclohexene

Products and mechanisms of the gas-phase reactions of O(3P) atoms and O3 with the cycloalkenes α-pinene and 1,2-dimethyl-1-cyclohexene have been investigated at 296±2 K and 740 torr total pressure, using gas chromatography and in situ atmospheric pressure ionization tandem mass spectrometry. The O(3P) atom reaction with α-pinene led to the formation of α-pinene oxide and two isomeric C10H16O ketones with yields of 0.77±0.06, 0.18±0.03, and 0.06±0.02, respectively. The O(3P) atom reaction with 1,2-dimethyl-1-cyclohexene produced 1,2-dimethyl-1-cyclohexene oxide with a yield of 0.51±0.16. Product studies of the O3 reactions in the presence of cyclohexane as an OH radical scavenger showed the formation of α-pinene oxide and pinonaldehyde from α-pinene with yields of 0.021±0.007 and 0.143±0.024, respectively, and the formation of 1,2-dimethyl-1-cyclohexene oxide, 5-oxohexanal, and 2,7-octanedione from 1,2-dimethyl-1-cyclohexene with yields of 0.020±0.006, 0.19±0.05, and 0.07±0.02, respectively. No evidence for the formation of O(3P) atoms from either the reactions of α-pinene or 1,2-dimethyl-1-cyclohexene with O3 was obtained (with estimated O(3P) atom yields of <0.03 and <0.04, respectively), and the formation of the epoxides is attributed to a direct reaction. An OH radical formation yield from the 1,2-dimethyl-1-cyclohexene reaction of 1.02±0.16 was measured using 2-butanol to scavenge the OH radicals and form 2-butanone. Experiments to elucidate the formation route(s) to 2,7-octanedione and pinonaldehyde were carried out, and it is postulated that these carbonyl products are formed from reactions of the thermalized biradicals with water vapor.

Rate constants for the gas-phase reactions of OH and NO3 radicals and O3 with sabinene and camphene at 296±2 K

Abstract The rate constants for the gas-phase reactions of sabinene and camphene, two monoterpenes emitted from vegetation, with OH and NO 3 radicals and O 3 have been determined at 296±2 K and one atmosphere total pressure of air. The OH and NO 3 radical reaction rate constants were determined using relative rate techniques. Using rate constants of k (OH + isoprene) = 1.01 × 10 −10 cm 3 molecule −1 s −1 , k (NO 3 + trans -2-butene) = 3.87 × 10 −13 cm 3 molecule −1 s −1 and k (NO 3 + 2-methyl-2-butene) = 9.33 × 10 −12 cm 3 molecule −1 s −1 , the following OH and NO 3 radical reaction rate constants (in cm 3 molecule −1 s −1 were obtained: OH radical reaction; sabinene, 1.17 × 10 −10 and camphene, 5.33 × 10 −11 ; NO 3 radical reaction; sabinene, 1.01 × 10 −11 , and camphene, 6.54 × 10 −13 . The absolute O 3 reaction rate constants determined were (in cm 3 molecule −1 s −1 units): sabinene, 8.07 × 10 −17 , and camphene, 9.0 × 10 −19 . These rate constants are compared to literature data for other structural-related alkenes and monoterpenes.