Olaf Böge

Methyl-Nitrocatechols: Atmospheric Tracer Compounds for Biomass Burning Secondary Organic Aerosols

Detailed chemical analysis of wintertime PM₁₀ collected at a rural village site in Germany showed the presence of a series of compounds that correlated very well with levoglucosan, a known biomass burning tracer compound. Nitrated aromatic compounds with molecular formula C₇H₇NO₄ (M(w) 169) correlated particularly well with levoglucosan, indicating that they originated from biomass burning as well. These compounds were identified as a series of methyl-nitrocatechol isomers (4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol, and 3-methyl-6-nitrocatechol) based on the comparison of their chromatographic and mass spectrometric behaviors to those from reference compounds.Aerosol chamber experiments suggest that m-cresol, which is emitted from biomass burning at significant levels, is a precursor for the detected methyl-nitrocatechols. The total concentrations of these compounds in the wintertime PM₁₀were as high as 29 ng m⁻³, indicating the secondary organic aerosol (SOA) originating from the oxidation of biomass burning VOCs contributed non-negligible amounts to the regional organic aerosol loading.

Gas-phase ozonolysis of α-pinene: gaseous products and particle formation

Abstract The gas-phase ozonolysis of α -pinene has been studied in a stopped-flow system at 295±2 K and 950 mbar of synthetic air as well as under flow conditions at 295±0.5 K and 1000 mbar of synthetic air. Gaseous products were analyzed using on-line GC-MS/FID and FT-IR measurements. The formation of new particles ( d p⩾3 nm ) was followed by means of a differential mobility particle sizer system and an ultra-fine condensation particle counter. First, the reaction of OH radicals with c -hexane was reinvestigated. Products were c -hexanol with a yield of 0.35±0.06 and c -hexanone with a yield of 0.53±0.06. The rate coefficients of the consecutive reaction of OH radicals with c -hexanol and c -hexanone of (2.7±0.4)×10 −11 and (6.1±0.9)×10 −12 cm 3 molecule −1 s −1 , respectively, were obtained. From the reaction of OH radicals with c -hexanol, a c -hexanone yield of 0.58±0.07 was observed. Using the c -hexane scavenger technique, an OH radical yield of 0.68±0.10 was measured for the reaction of O 3 with α -pinene applicable for H 2 O concentrations of ∼1.5×10 15 and 2.6×10 17 molecule cm −3 . The formation yield of pinonaldehyde was found to be strongly dependent on the experimental conditions. Generally, the pinonaldehyde yield decreased for increasing α -pinene conversion. In the presence of an OH radical scavenger, maximum pinonaldehyde yields were 0.42±0.05 and 0.32±0.04 for [H 2 O]∼1.5×10 15 and 2.6×10 17 molecule cm −3 , respectively. Under the present conditions the pinonaldehyde formation cannot be described via the reaction of a Criegee intermediate with H 2 O. This finding is corroborated by measurements of the co-product H 2 O 2 . The formation yield of α -pinene oxide of 0.03±0.015 was unaffected by experimental conditions. Newly formed particles were measured for a relatively wide range of experimental conditions. Particle formation was only detectable for an α -pinene conversion above 3×10 11 molecule cm −3 . The results of the present study suggest that the formation of new particles from the pure O 3 + α -pinene reaction is unlikely under atmospheric conditions.

Formation of phenol and carbonyls from the atmospheric reaction of OH radicals with benzene.

The gas-phase reaction of OH radicals with benzene has been studied in a flow tube operated at 295 +/- 2 K and 950 mbar of synthetic air or O2. Ozonolysis of tetramethylethylene (dark reaction) with a measured OH radical yield of 0.92 +/- 0.08 or photolysis of methyl nitrite in the presence of NO served as the OH sources. For investigations in the presence of NOx, the conditions were chosen so that more than 95% of the OH/benzene adduct reacted with O2 even for the highest NO2 concentration occurring in the experiment. In the absence of NOx, a phenol yield from the reaction of OH radicals with benzene of 0.61 +/- 0.07 was measured by means of long-path FT-IR and UV spectroscopy over a wide range of experimental conditions. This yield was confirmed by measurements performed in the presence of NOx. Detected carbonyls were glyoxal, cis-butenedial and trans-butenedial with formation yields of 0.29 +/- 0.10, 0.08 +/- 0.03 and 0.023 +/- 0.007, respectively, measured in synthetic air and in the presence of NOx. There was no significant difference in the product yields applying both experimental approaches for OH generation (dark reaction or photolysis). Nitrobenzene and o-nitrophenol were detected in traces. The yield of nitrobenzene increased with increasing NOx resulting in a maximum formation yield of 0.007. The detected products in the presence of NOx account for approximately 78% of the reacted carbon. Butenedial yields from benzene degradation are reported for the first time. In the absence of NOx, glyoxal, cis-butenedial and trans-butenedial were also detected, but with distinctly lower yields compared to the experiments with NOx.

Highly Oxidized Multifunctional Organic Compounds Observed in Tropospheric Particles: A Field and Laboratory Study.

Very recent studies have reported the existence of highly oxidized multifunctional organic compounds (HOMs) with O/C ratios greater than 0.7. Because of their low vapor pressure, these compounds are often referred as extremely low-volatile organic compounds (ELVOCs), and thus, they are able to contribute significantly to organic mass in tropospheric particles. While HOMs have been successfully detected in the gas phase, their fate after uptake into particles remains unclear to date. Hence, the present study was designed to detect HOMs and related oxidation products in the particle phase and, thus, to shed light on their fate after phase transfer. To this end, aerosol chamber investigations of α-pinene ozonolysis were conducted under near environmental precursor concentrations (2.4 ppb) in a continuous flow reactor. The chemical characterization shows three classes of particle constituents: (1) intact HOMs that contain a carbonyl group, (2) particle-phase decomposition products, and (3) highly oxidized organosulfates (suggested to be addressed as HOOS). Besides chamber studies, HOM formation was also investigated during a measurement campaign conducted in summer 2013 at the TROPOS research station Melpitz. During this field campaign, gas-phase HOM formation was found to be correlated with an increase in the oxidation state of the organic aerosol.

Laboratory studies on secondary organic aerosol formation from terpenes

The formation of secondary organic aerosol (SOA) following the ozonolysis of terpene has been investigated intensively in recent years. The enhancement of SOA yields from the acid catalysed reactions of organics on aerosol surfaces or in the bulk particle phase has been receiving great attention. Recent studies show that the presence of acidic seed particles increases the SOA yield significantly (M. S. Jang and R. M. Kamens, Environ. Sci. Technol., 2001, 35, 4758, ref. 1; M. S. Jang, N. M. Czoschke, S. Lee and R. M. Kamens, Science, 2002, 298, 814, ref. 2; N. M. Czoschke, M. Jang and R. M. Kamens, Atmos. Environ., 2003, 37, 4287, ref. 3; M. S. Jang, B. Carroll, B. Chandramouli and R. M. Kamens, Environ. Sci. Technol., 2003, 37, 3828, ref. 4; Y. Iinuma, O. Böge, T. Gnauk and H. Herrmann, Atmos. Environ., 2004, 38, 761, ref. 5; S. Gao, M. Keywood, N. L. Ng, J. Surratt, V. Varutbangkul, R. Bahreini, R. C. Flagan and J. H. Seinfeld, J. Phys. Chem. A, 2004, 108, 10147, ref. 6). More detailed studies report the formation of higher molecular weight products in SOA (refs. 5 and 6; M. P. Tolocka, M. Jang, J. M. Ginter, F. J. Cox, R. M. Kamens and M. V. Johnston, Environ. Sci. Technol., 2004, 38, 1428, ref. 7; S. Gao, N. L. Ng, M. Keywood, V. Varutbangkul, R. Bahreini, A. Nenes, J. He, K. Y. Yoo, J. L. Beauchamp, R. P. Hodyss, R. C. Flagan and J. H. Seinfeld, Environ. Sci. Technol., 2004, 38, 6582, ref. 8) which could result in a non-reversible uptake of organics into the particle phase. Most of the past studies concentrated on the characterisation of the yields of enhanced SOA and its composition from ozonolysis of terpenes in the presence or absence of acidic and neutral seed particles. Recent findings from cyclohexene ozonolysis show that the presence of OH scavengers can also significantly influence the SOA yield. Our new results from the IfT chemistry department aerosol chamber on terpene ozonolysis in the presence of OH scavengers show that the presence of hydroxyl radical scavengers clearly reduces the amount of formed SOA. The OH scavenger strongly depletes the formation of oligomeric compounds in the particle phase in contrast to previous findings (M. D. Keywood, J. H. Kroll, V. Varatbangkul, R. Bahreini, R. C. Flagan and J. H. Seinfeld, Environ. Sci. Technol., 2004, 38, 3343, ref. 9). This result indicates that hydroxyl radicals play an important role in the formation of precursor compounds (e.g., hydroxy pinonaldehyde) for the particle phase heterogeneous acid catalysed reactions leading to the higher molecular weight compounds and thus the enhancement of SOA yields. Better understanding of the role of hydroxyl radicals in the formation of SOA is necessary to distinguish between the contribution of ozonolysis and hydroxyl radicals to the SOA yield. If the recent findings are a ubiquitous phenomenon in the atmosphere, current atmospheric and climate models might underestimate SOA formation yields, particle phase OC contents and its impact on the atmospheric radiation budget.

Gas-phase reaction of OH radicals with phenol

The gas-phase reaction of OH radicals with phenol was investigated in a flow tube in the temperature range of 266–364 K and a pressure of 100 mbar. The product formation was followed by on-line FT-IR spectroscopy and GC-MS measurements. Newly formed particles were detected by means of a low-pressure CPC (condensation particle counter). In the presence of O2, OH radicals were generated via the reaction sequence H + O2 + M → HO2 + M, HO2 + NO → OH + NO2 and in the absence of O2via H + NO2 → OH + NO. For evaluation of a possible competing process, the rate constant for H + phenol was measured, k(H + phenol) = (2.5 ± 1.5) × 10−13 cm3 molecule−1 s−1 (295 ± 2 K, 25 mbar He). Under the experimental conditions used the H-atom reaction does not compete with the reaction of OH radicals with phenol. At 295 K, the product distribution was studied for different O2, NO and NO2 concentrations. Identified products were catechol, o-nitrophenol and p-benzoquinone. Under all experimental conditions catechol represented the main product. The measured dependence of the catechol yield on NO and NO2 for constant O2 concentrations allowed an estimate of the reactivity of the OH/phenol adduct towards O2, NO and NO2, k(adduct + O2)/k(adduct + NO) > 10−3 and k(adduct + O2)/k(adduct + NO2) = (1.4 ± 0.5) × 10−4. For constant gas composition, in the absence of additional NO2, the product distribution was measured for different temperatures. With increasing temperature the catechol yield increased from 0.37 ± 0.06 (266 K) to 0.87 ± 0.04 (364 K). The yields of o-nitrophenol and p-benzoquinone were nearly constant. Below 295 K, with decreasing temperature enhanced formation of newly formed particles was observed. For realistic atmospheric conditions, a catechol yield of 0.73–0.78 (295 K) can be recommended from this study.

Gas-phase reaction of OH radicals with benzene: products and mechanism

The gas-phase reaction of OH radicals with benzene was studied in O2/He mixtures under flow conditions in the temperature range 276–353 K and at pressures of 100 and 500 mbar using on-line FT-IR spectroscopy and GC-MS measurements. The reaction conditions were chosen so that the initially formed OH/benzene adduct predominantly reacted either with O2 or O3. Under conditions of a predominant reaction of the OH/benzene adduct with O2 the product formation was studied for variable NO concentrations. Identified products were the isomers of hexa-2,4-dienedial, phenol, nitrobenzene, p-benzoquinone and glyoxal. Furan was found in small amounts. For increasing NO concentrations there was a decrease of the phenol yield and the yields of trans,trans-hexa-2,4-dienedial and nitrobenzene increased, resulting in maximum values of 0.36 ± 0.02 and 0.11 ± 0.02, respectively (100 mbar, 295 K). The p-benzoquinone yield of 0.08 ± 0.02 was found to be independent of the NO concentration. The temperature dependence of the phenol yield was measured in the range of 276–353 K for initial ratios of [NO]/[O2] = 1–20 × 10−6 at 500 mbar. For a fixed [NO]/[O2] ratio, a distinct increase of the phenol yield with increasing temperature was observed; initial [NO]/[O2] = 1–1.2 × 10−6, phenol yield: 0.18 ± 0.04 (276 K) and 0.68 ± 0.05 (353 K). Generally, the total yield of carbonylic substances was found to be anti-correlated to the phenol yield. When the OH/benzene adduct reacted with O3, trans,trans-hexa-2,4-dienedial, phenol and formic acid were identified as main products with formation yields of 0.28 ± 0.02, 0.20 ± 0.05 and 0.12 ± 0.02, respectively (100 mbar, 295 K). Further products were p-benzoquinone, CO and unidentified carbonylic substances. For the different experimental conditions, reaction mechanisms are proposed explaining the formation of the observed products. A simple model describing the temperature and NOx-dependence of the phenol yield is presented.

GAS-PHASE REACTION OF NO3 RADICALS WITH ISOPRENE : A KINETIC AND MECHANISTIC STUDY

The gas-phase reaction of NO3 radicals with isoprene was investigated under flow conditions at 298 K in the pressure range 6.8<P(mbar)<100 using GC-MS/FID, direct MS, and long-path FT–IR spectroscopy as detection techniques. By means of a relative rate method, the rate constant for the primary attack of NO3 radicals toward isoprene was determined to be (6.86±2.60)×10−13 cm3 molecule−1 s−1. The formation of the possible oxiranes, 2-methyl-2-vinyl-oxirane and 2-(1-methyl-vinyl)-oxirane, was observed in dependence on total pressure. In the presence of O2 in the carrier gas, the product distribution was found to be strongly dependent on the reaction pathways of formed peroxy radicals. If the peroxy radicals mainly reacted in a self-reaction, the formation of organic nitrates was detected and 4-nitroxy-3-methyl-but-2-enal was identified as a main product. On the other hand, when NO was added to the gas mixture and the peroxy radicals were converted via RO2+NO→RO+NO2, the formation of methyl vinyl ketone as the main product as well as 3-methylfuran and meth-acrolein was observed. From the ratio of the product yields if NO was added to the gas mixture it was concluded that the attack of NO3 radicals predominantly takes place in the 1-position. A reaction mechanism is proposed and the application of these results to the troposphere are discussed.

PRODUCTS AND MECHANISM OF THE GAS-PHASE REACTION OF NO3 RADICALS WITH ALPHA -PINENE

The gas-phase reaction of NO 3 radicals with α-pinene has been studied under flow conditions in the pressure range 20<p/mbar <200 at 298 K using gas chromatography–mass spectrometry/flame ionisation detection (GC–MS/FID), MS and long-path FTIR spectroscopy as detection techniques. NO 3 radicals were generated by thermal decomposition of N 2 O 5 . He, N 2 or O 2 –N 2 mixtures served as carrier gas. In the absence of O 2 in the carrier gas, α-pinene oxide (ca. 60%) and an organic nitrate (ca. 40%) were found to be the main products with minor amounts of α-campholene aldehyde. The yields were slightly pressure dependent. In the presence of O 2 , pinonaldehyde was the predominant product. When NO was added for conversion of the formed peroxyl radicals via RO 2 +NO→RO→NO 2 , the following product yields were obtained at p=200 mbar and [O 2 ]>10 17 molecule cm -3 , pinonaldehyde 75±6%, α-pinene oxide 15±3%, organic nitrates (total) 14±3%, α-campholene aldehyde 3±1%. In the absence of NO, nitroperoxy-group-containing substances were observed, arising from the reaction RO 2 +NO 2 →RO 2 NO 2 . A reaction mechanism is proposed and a tropospheric application of the results is discussed.

Ozone-driven secondary organic aerosol production chain.

Acidic sulfate particles are known to enhance secondary organic aerosol (SOA) mass in the oxidation of biogenic volatile organic compounds (BVOCs) through accretion reactions and organosulfate formation. Enhanced phase transfer of epoxides, which form during the BVOC oxidation, into the acidified sulfate particles is shown to explain the latter process. We report here a newly identified ozone-driven SOA production chain that increases SOA formation dramatically. In this process, the epoxides interact with acidic sulfate particles, forming a new generation of highly reactive VOCs through isomerization. These VOCs partition back into the gas phase and undergo a new round of SOA forming oxidation reactions. Depending on the nature of the isomerized VOCs, their next generation oxidation forms highly oxygenated terpenoic acids or organosulfates. Atmospheric evidence is presented for the existence of marker compounds originating from this chain. The identified process partly explains the enhanced SOA formation in the presence of acidic particles on a molecular basis and could be an important source of missing SOA precursor VOCs that are currently not included in atmospheric models.