No NO Isoprene, a five-carbon diene produced by plants, is the most abundant nonmethane hydrocarbon released into the atmosphere and plays an important role in tropospheric chemistry. Isoprene is also thought to affect climate by acting as a source of atmospheric aerosols. Paulot et al. (p. 730 ; see the Perspective by Kleindienst ) now describe how isoprene may lead to the formation of secondary organic aerosols. In laboratory experiments, the photooxidation of isoprene in low-NO conditions, such as those which occur in vegetated regions far from anthropogenic influence, produced high yields of dihydroxy epoxides, a suspected precursor of the aerosols. This discovery could help to explain some of the more puzzling aspects of isoprene chemistry in remote regions.
Abstract. We describe a nearly explicit chemical mechanism for isoprene photooxidation guided by chamber studies that include time-resolved observation of an extensive suite of volatile compounds. We provide new constraints on the chemistry of the poorly-understood isoprene δ-hydroxy channels, which account for more than one third of the total isoprene carbon flux and a larger fraction of the nitrate yields. We show that the cis branch dominates the chemistry of the δ-hydroxy channel with less than 5% of the carbon following the trans branch. The modelled yield of isoprene nitrates is 12±3% with a large difference between the δ and β branches. The oxidation of these nitrates releases about 50% of the NO x . Methacrolein nitrates (modelled yield 15±3% from methacrolein) and methylvinylketone nitrates (modelled yield 11±3% yield from methylvinylketone) are also observed. Propanone nitrate, produced with a yield of 1% from isoprene, appears to be the longest-lived nitrate formed in the total oxidation of isoprene. We find a large molar yield of formic acid and suggest a novel mechanism leading to its formation from the organic nitrates. Finally, the most important features of this mechanism are summarized in a condensed scheme appropriate for use in global chemical transport models.
We report experimental evidence for the formation of C 5 -hydroperoxyaldehydes (HPALDs) from 1,6-H-shift isomerizations in peroxy radicals formed from the hydroxyl radical (OH) oxidation of 2-methyl-1,3-butadiene (isoprene). At 295 K, the isomerization rate of isoprene peroxy radicals ðISO 2 Þ relative to the rate of reaction of ISO 2 þ HO 2 is k 295 isom k 295 ISO 2 þHO 2 ¼ ð1:2 AE 0:6Þ Â 10 8 mol cm À3 , or k 295 isom C 0.002 s À1 . The temperature dependence of this rate was determined through experiments conducted at 295, 310 and 318 K and is well described by k isom ðTÞ k ISO 2 þHO 2 ðTÞ ¼ 2:0  10 21 expðÀ9000=TÞ mol cm À3 . The overall uncertainty in the isomerization rate (relative to k ISO 2 þHO 2 ) is estimated to be 50%. Peroxy radicals from the oxidation of the fully deuterated isoprene analog isomerize at a rate B15 times slower than non-deuterated isoprene. The fraction of isoprene peroxy radicals reacting by 1,6-H-shift isomerization is estimated to be 8-11% globally, with values up to 20% in tropical regions.Approximately 500 Tg of isoprene (C 5 H 8 ) originating primarily from plants is released to Earth's atmosphere each year. 1 The oxidation of isoprene in the atmosphere is primarily initiated by reaction with hydroxyl radicals (OH). The bulk of this reaction proceeds through addition of the OH to one of the two external olefinic carbon atoms. In the presence of oxygen, six different peroxy radicals (collectively ISO 2 ) are formed from reaction with O 2 (R1).In nearly all chemical mechanisms used to describe atmospheric photochemistry, the subsequent fate of ISO 2 is determined by reaction with either NO (R2a and R2b) or HO 2 (R3).where NY = nitrate yield.This representation is consistent with a wealth of laboratory studies of the reactivity of peroxy radicals. Such studies have typically been performed with sufficient concentrations of NO or HO 2 that the lifetimes of the peroxy radicals are very short-often less than 0.1 s. For vast regions of the atmosphere (including most of the tropics), however, the peroxy radicals are estimated to live for 10's of seconds before finding a reactive partner in either NO or HO 2 (e.g., for observed NO and HO 2 levels over Amazonia, 2 ISO 2 lifetime with respect to R2a-R3 is calculated to be 30-60 s). A number of recent analyses have cast doubt on the traditional representation of peroxy radical chemistry (R2a-R3). In particular, motivated by the inability to explain measured concentrations of OH in regions with elevated levels of biogenically-derived hydrocarbons, 2-6 investigators have hypothesized that the peroxy radicals of the dominant VOC, isoprene, may not follow the traditional reactive pathways. Most recently, Peeters et al. 7 and da Silva et al. 8 have estimated from quantum mechanical calculations that intramolecular hydrogen transfer reactions of the isoprene peroxy radicals (R4) may diminish the impact of isoprene oxidation on HO x concentrations.Here, we report product yields in the oxidation of isoprene and its fully deuterated analog by OH ...
We report fluxes and dry deposition velocities for 16 atmospheric compounds above a southeastern United States forest, including: hydrogen peroxide (H 2 O 2 ), nitric acid (HNO 3 ), hydrogen cyanide (HCN), hydroxymethyl hydroperoxide, peroxyacetic acid, organic hydroxy nitrates, and other multifunctional species derived from the oxidation of isoprene and monoterpenes. The data suggest that dry deposition is the dominant daytime sink for small, saturated oxygenates. Greater than 6 wt %C emitted as isoprene by the forest was returned by dry deposition of its oxidized products. Peroxides account for a large fraction of the oxidant flux, possibly eclipsing ozone in more pristine regions. The measured organic nitrates comprise a sizable portion (15%) of the oxidized nitrogen input into the canopy, with HNO 3 making up the balance. We observe that water-soluble compounds (e.g., strong acids and hydroperoxides) deposit with low surface resistance whereas compounds with moderate solubility (e.g., organic nitrates and hydroxycarbonyls) or poor solubility (e.g., HCN) exhibited reduced uptake at the surface of plants. To first order, the relative deposition velocities of water-soluble compounds are constrained by their molecular diffusivity. From resistance modeling, we infer a substantial emission flux of formic acid at the canopy level (∼1 nmol m −2 ·s −1 ). GEOS−Chem, a widely used atmospheric chemical transport model, currently underestimates dry deposition for most molecules studied in this work. Reconciling GEOS−Chem deposition velocities with observations resulted in up to a 45% decrease in the simulated surface concentration of trace gases.biosphere−atmosphere exchange | isoprene | dry deposition | OVOCs | fluxes
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