Molecular size evolution of oligomers in organic aerosols collected in urban atmospheres and generated in a smog chamber.

Only a minor fraction of the total organic aerosol mass can be resolved on a molecular level. High molecular weight compounds in organic aerosols have recently gained much attention because this class of compound potentially explains a major fraction of the unexplained organic aerosol mass. These compounds have been identified with different mass spectrometric methods, and compounds with molecular masses up to 1000 Da are found in secondary organic aerosols (SOA) generated from aromatic and terpene precursors in smog chamber experiments. Here, we apply matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to SOA particles from two biogenic precursors, alpha-pinene and isoprene. Similar oligomer patterns are found in these two SOA systems, but also in SOA from trimethylbenzene, an anthropogenic SOA precursor. However, different maxima molecular sizes were measured for these three SOA systems. While oligomers in alpha-pinene and isoprene have sizes mostly below 600-700 Da, they grow up to about 1000 Da in trimethylbenzene-SOA. The final molecular size of the oligomers is reached early during the particle aging process, whereas other particle properties related to aging, such as the overall acid concentration or the oligomer concentration, increase continuously over a much longer time scale. This kinetic behavior of the oligomer molecular size growth can be explained by a chain growth kinetic regime. Similar oligomer mass patterns were measured in aqueous extracts of ambient aerosol samples (measured with the same technique). Distinct differences between summer and winter were observed. In summer a few single mass peaks were measured with much higher intensity than in winter, pointing to a possible difference in the formation processes of these compounds in winter and summer.

Secondary organic aerosol formation by irradiation of 1,3,5-trimethylbenzene-NOx-H2O in a new reaction chamber for atmospheric chemistry and physics.

A new environmental reaction smog chamber was built to simulate particle formation and growth similar to that expected in the atmosphere. The organic material is formed from nucleation of photooxidized organic compounds. The chamber is a 27 m3 fluorinated ethylene propylene (FEP) bag suspended in a temperature-controlled enclosure. Four xenon arc lamps (16 kW total) are used to irradiate primary gas components for experiments lasting up to 24 h. Experiments using irradiations of 1,3,5-trimethylbenzene-NOx-H2O at similar input concentrations without seed particles were used to determine particle number and volume concentration wall loss rates of 0.209+/-0.018 and 0.139+/-0.070 h(-1), respectively. The particle formation was compared with and without propene.

Design, synthesis, and evaluation of a biomimetic artificial photolyase model.

Two new artificial photolyase models that recognize pyrimidine dimers in protic and aprotic organic solvents as well as in water through a combination of charge and hydrogen-bonding interactions and use a mimic of the flavine to achieve repair through reductive photoinduced electron transfer are presented. Fluorescence and NMR titration studies show that it forms a 1:1 complex with pyrimidine dimers with binding constants of approximately 10(3) M(-1) in acetonitrile or methanol, while binding constants in water at pH 7.2 are slightly lower. Excitation of the complex with visible light leads to clean and rapid cycloreversion of the pyrimidine dimer through photoinduced electron transfer catalysis. The reaction in water is significantly faster than in organic solvents. The reaction slows down at higher conversions due to product inhibition.

Sampling gaseous oxidation products of aromatic compounds in gas/particle separation systems.

In this study we performed a direct comparison between two different ambient air samplers to characterize their performance in sampling oxidized gaseous organic compounds, known as oxidation products of aromatics. We investigated compounds with a variety of functional groups and vapor pressures. A polyurethane foam (PUF) adsorbent and an annular diffusion denuder sampler were operated along with particle filters. In both systems the sampling devices were liquid-extracted, followed by derivatization and analysis by GC-MS. The PUF system works very well for aromatic as well as non-aromatic compounds, whereas the denuder shows smaller collection efficiencies for highly volatile non-aromatic compounds. In addition, the sampling efficiencies in the PUF set-up are in good agreement with the calculated vapor pressures of the compounds and also the particle phase is not affected by most compounds.