7 research outputs found

    Peroxy radicals in the summer free troposphere: seasonality and potential for heterogeneous loss

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    The sum of peroxy radicals (HO2+ΣiRiO2) and supporting trace gases were measured on the Jungfraujoch (3580 m a.s.l.) during the late summer of 2005. The period was marked by extended times of heavy snow which led to reduction in the observed peroxy radicals during the snowy periods that was greater than the concomitant reduction in j(O1D). In the limit a first order loss rate of 0.0063 s−1 can be derived for the peroxy radical loss in the snowy conditions that could be potentially ascribed to a heterogenous loss process. On snow free days photolysis of HCHO is shown to be a significant peroxy radical source. The seasonal trends of the peroxy radical concentrations have been mapped from the winter to summer transition in line with previous experiments. Net ozone production in late summer at the Jungfraujoch was net neutral to marginally ozone destructive. A value of 28±4 pptv is calculated for the ozone compensation point for the snow free days.ISSN:1680-7375ISSN:1680-736

    Distribution of gaseous and particulate organic composition during dark α-pinene ozonolysis

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    Secondary Organic Aerosol (SOA) affects atmospheric composition, air quality and radiative transfer, however major difficulties are encountered in the development of reliable models for SOA formation. Constraints on processes involved in SOA formation can be obtained by interpreting the speciation and evolution of organics in the gaseous and condensed phase simultaneously. In this study we investigate SOA formation from dark α-pinene ozonolysis with particular emphasis upon the mass distribution of gaseous and particulate organic species. A detailed model for SOA formation is compared with the results from experiments performed in the EUropean PHOtoREactor (EUPHORE) simulation chamber, including on-line gas-phase composition obtained from Chemical-Ionization-Reaction Time-Of-Flight Mass-Spectrometry measurements, and off-line analysis of SOA samples performed by Ion Trap Mass Spectrometry and Liquid Chromatography. The temporal profile of SOA mass concentration is relatively well reproduced by the model. Sensitivity analysis highlights the importance of the choice of vapour pressure estimation method, and the potential influence of condensed phase chemistry. Comparisons of the simulated gaseous-and condensed-phase mass distributions with those observed show a generally good agreement. The simulated speciation has been used to (i) propose a chemical structure for the principal gaseous semi-volatile organic compounds and condensed monomer organic species, (ii) provide evidence for the occurrence of recently suggested radical isomerisation channels not included in the basic model, and (iii) explore the possible contribution of a range of accretion reactions occurring in the condensed phase. We find that oligomer formation through esterification reactions gives the best agreement between the observed and simulated mass spectr

    Insights into the Formation and Evolution of Individual Compounds in the Particulate Phase during Aromatic Photo-Oxidation

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    Secondary organic aerosol (SOA) is well-known to have adverse effects on air quality and human health. However, the dynamic mechanisms occurring during SOA formation and evolution are poorly understood. The time-resolved SOA composition formed during the photo-oxidation of three aromatic compounds, methyl chavicol, toluene and 4-methyl catechol, were investigated at the European Photoreactor. SOA was collected using a particle into liquid sampler and analyzed offline using state-of-the-art mass spectrometry to produce temporal profiles of individual photo-oxidation products. In the photo-oxidation of methyl chavicol, 70 individual compounds were characterized and three distinctive temporal profile shapes were observed. The calculated mass fraction (<i>C</i><sub><i>i</i>,aer</sub>/<i>C</i><sub>OA</sub>) of the individual SOA compounds showed either a linear trend (increasing/decreasing) or exponential decay with time. Substituted nitrophenols showed an exponential decay, with the nitro-group on the aromatic ring found to control the formation and loss of these species in the aerosol phase. Nitrophenols from both methyl chavicol and toluene photo-oxidation experiments showed a strong relationship with the NO<sub>2</sub>/NO (ppbv/ppbv) ratio and were observed during initial SOA growth. The location of the nitrophenol aromatic substitutions was found to be critically important, with the nitrophenol in the photo-oxidation of 4-methyl catechol not partitioning into the aerosol phase until irradiation had stopped; highlighting the importance of studying SOA formation and evolution at a molecular level

    Mapping gas-phase organic reactivity and concomitant secondary organic aerosol formation: chemometric dimension reduction techniques for the deconvolution of complex atmospheric data sets

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    Highly non-linear dynamical systems, such as those found in atmospheric chemistry, necessitate hierarchical approaches to both experiment and modelling in order to ultimately identify and achieve fundamental process-understanding in the full open system. Atmospheric simulation chambers comprise an intermediate in complexity, between a classical laboratory experiment and the full, ambient system. As such, they can generate large volumes of difficult-to-interpret data. Here we describe and implement a chemometric dimension reduction methodology for the deconvolution and interpretation of complex gas- and particle-phase composition spectra. The methodology comprises principal component analysis (PCA), hierarchical cluster analysis (HCA) and positive least-squares discriminant analysis (PLS-DA). These methods are, for the first time, applied to simultaneous gas- and particle-phase composition data obtained from a comprehensive series of environmental simulation chamber experiments focused on biogenic volatile organic compound (BVOC) photooxidation and associated secondary organic aerosol (SOA) formation. We primarily investigated the biogenic SOA precursors isoprene, α-pinene, limonene, myrcene, linalool and β-caryophyllene. The chemometric analysis is used to classify the oxidation systems and resultant SOA according to the controlling chemistry and the products formed. Results show that "model" biogenic oxidative systems can be successfully separated and classified according to their oxidation products. Furthermore, a holistic view of results obtained across both the gas- and particle-phases shows the different SOA formation chemistry, initiating in the gas-phase, proceeding to govern the differences between the various BVOC SOA compositions. The results obtained are used to describe the particle composition in the context of the oxidised gas-phase matrix. An extension of the technique, which incorporates into the statistical models data from anthropogenic (i.e. toluene) oxidation and "more realistic" plant mesocosm systems, demonstrates that such an ensemble of chemometric mapping has the potential to be used for the classification of more complex spectra of unknown origin. More specifically, the addition of mesocosm data from fig and birch tree experiments shows that isoprene and monoterpene emitting sources, respectively, can be mapped onto the statistical model structure and their positional vectors can provide insight into their biological sources and controlling oxidative chemistry. The potential to extend the methodology to the analysis of ambient air is discussed using results obtained from a zero-dimensional box model incorporating mechanistic data obtained from the Master Chemical Mechanism (MCMv3.2). Such an extension to analysing ambient air would prove a powerful asset in assisting with the identification of SOA sources and the elucidation of the underlying chemical mechanisms involved