@article{FuchsAcirBohnetal.2014, author = {Fuchs, H. and Acir, Ismail-Hakki and Bohn, Birger and Brauers, Theo and Dorn, Hans-Peter and H{\"a}seler, R. and Hofzumahaus, A. and Holland, F. and Kaminski, M. and Li, X. and Lu, K. and Lutz, A. and Nehr, Sascha and Rohrer, F. and Tillmann, R. and Wegener, R. and Wahner, A.}, title = {OH regeneration from methacrolein oxidation investigated in the atmosphere simulation chamber SAPHIR}, series = {Atmospheric Chemistry and Physics}, volume = {14}, journal = {Atmospheric Chemistry and Physics}, number = {15}, publisher = {Copernicus}, issn = {1680-7324}, doi = {10.5194/acp-14-7895-2014}, pages = {7895 -- 7908}, year = {2014}, abstract = {Abstract. Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene-rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50\% larger than calculated by the Master Chemical Mechanism (MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals an OH source that is not accounted for in MCM, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, 0.77 OH radicals (1σ error: ± 0.31) need to be additionally reformed from each reaction of OH with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s-1 for the reaction rate constant if the OH source is attributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OO formed in the MACR + OH reaction as suggested in the literature (Crounse et al., 2012). This fast isomerization reaction would be a competitor to the reaction of this RO2 species with a minimum of 150 pptv NO. The isomerization reaction would be the dominant reaction pathway for this specific RO2 radical in forested regions, where NO mixing ratios are typically much smaller.}, language = {en} } @article{NehrBohnDornetal.2014, author = {Nehr, Sascha and Bohn, Birger and Dorn, Hans-Peter and Fuchs, H. and H{\"a}seler, R. and Hofzumahaus, A. and Li, X. and Rohrer, F. and Tillmann, R. and Wahner, A.}, title = {Atmospheric photochemistry of aromatic hydrocarbons: OH budgets during SAPHIR chamber experiments}, series = {Atmospheric Chemistry and Physics}, volume = {14}, journal = {Atmospheric Chemistry and Physics}, number = {13}, publisher = {Copernicus}, issn = {1680-7324}, doi = {10.5194/acp-14-6941-2014}, pages = {6941 -- 6952}, year = {2014}, abstract = {Abstract. Current photochemical models developed to simulate the atmospheric degradation of aromatic hydrocarbons tend to underestimate OH radical concentrations. In order to analyse OH budgets, we performed experiments with benzene, toluene, p-xylene and 1,3,5-trimethylbenzene in the atmosphere simulation chamber SAPHIR. Experiments were conducted under low-NO conditions (typically 0.1-0.2 ppb) and high-NO conditions (typically 7-8 ppb), and starting concentrations of 6-250 ppb of aromatics, dependent on OH rate constants. For the OH budget analysis a steady-state approach was applied in which OH production and destruction rates (POH and DOH) have to be equal. The POH were determined from measurements of HO2, NO, HONO, and O3 concentrations, considering OH formation by photolysis and recycling from HO2. The DOH were calculated from measurements of the OH concentrations and total OH reactivities. The OH budgets were determined from DOH/POH ratios. The accuracy and reproducibility of the approach were assessed in several experiments using CO as a reference compound where an average ratio DOH/POH = 1.13 ± 0.19 was obtained. In experiments with aromatics, these ratios ranged within 1.1-1.6 under low-NO conditions and 0.9-1.2 under high-NO conditions. The results indicate that OH budgets during photo-oxidation experiments with aromatics are balanced within experimental accuracies. Inclusion of a further, recently proposed OH production via HO2 + RO2 reactions led to improvements under low-NO conditions but the differences were small and insignificant within the experimental errors.}, language = {en} } @article{FuchsHofzumahausRohreretal.2013, author = {Fuchs, H. and Hofzumahaus, A. and Rohrer, F. and Bohn, Birger and Brauers, Theo and Dorn, Hans-Peter and H{\"a}seler, R. and Holland, F. and Kaminski, M. and Li, X. and Lu, K. and Nehr, Sascha and Tillmann, R. and Wegener, R. and Wahner, A.}, title = {Experimental evidence for efficient hydroxyl radical regeneration in isoprene oxidation}, series = {Nature Geoscience}, volume = {6}, journal = {Nature Geoscience}, number = {12}, publisher = {Springer Nature}, issn = {1752-0894}, doi = {10.1038/NGEO1964}, pages = {1023 -- 1026}, year = {2013}, language = {en} } @article{LuRohrerHollandetal.2012, author = {Lu, K. D. and Rohrer, F. and Holland, F. and Fuchs, H. and Bohn, Birger and Brauers, Theo and Chang, C. C. and H{\"a}seler, R. and Hu, M. and Kita, K. and Kondo, Y. and Li, X. and Lou, S. R. and Nehr, Sascha and Shao, M. and Zeng, L. M. and Wahner, A. and Zhang, Y. H. and Hofzumahaus, A.}, title = {Observation and modelling of OH and HO2 concentrations in the Pearl River Delta 2006: a missing OH source in a VOC rich atmosphere}, series = {Atmospheric Chemistry and Physics}, volume = {12}, journal = {Atmospheric Chemistry and Physics}, number = {3}, publisher = {Copernicus}, issn = {1680-7324}, doi = {10.5194/acp-12-1541-2012}, pages = {1541 -- 1569}, year = {2012}, abstract = {Abstract. Ambient OH and HO2 concentrations were measured by laser induced fluorescence (LIF) during the PRIDE-PRD2006 (Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta, 2006) campaign at a rural site downwind of the megacity of Guangzhou in Southern China. The observed OH concentrations reached daily peak values of (15-26) × 106 cm-3 which are among the highest values so far reported for urban and suburban areas. The observed OH shows a consistent high correlation with j(O1D) over a broad range of NOx conditions. The correlation cannot be reproduced by model simulations, indicating that OH stabilizing processes are missing in current models. The observed OH exhibited a weak dependence on NOx in contrast to model predictions. While modelled and measured OH agree well at NO mixing ratios above 1 ppb, a continuously increasing underprediction of the observed OH is found towards lower NO concentrations, reaching a factor of 8 at 0.02 ppb NO. A dependence of the modelled-to-measured OH ratio on isoprene cannot be concluded from the PRD data. However, the magnitude of the ratio fits into the isoprene dependent trend that was reported from other campaigns in forested regions. Hofzumahaus et al. (2009) proposed an unknown OH recycling process without NO, in order to explain the high OH levels at PRD in the presence of high VOC reactivity and low NO. Taking a recently discovered interference in the LIF measurement of HO2 into account, the need for an additional HO2 → OH recycling process persists, but the required source strength may be up to 20\% larger than previously determined. Recently postulated isoprene mechanisms by Lelieveld et al. (2008) and Peeters and M{\"u}ller (2010) lead to significant enhancements of OH expected for PRD, but an underprediction of the observed OH by a factor of two remains at low NO (0.1-0.2 ppb). If the photolysis of hydroperoxy aldehydes from isoprene is as efficient as proposed by Peeters and M{\"u}ller (2010), the corresponding OH formation at PRD would be more important than the primary OH production from ozone and HONO. While the new isoprene mechanisms need to be confirmed by laboratory experiments, there is probably need for other, so far unidentified chemical processes to explain entirely the high OH levels observed in Southern China.}, language = {en} }