[1] Despite growing interest in the visible light-absorbing organic component of atmospheric aerosols, referred to as "brown" carbon, our knowledge of its chemical composition remains limited. It is well accepted that biomass burning is one important source of "brown" carbon in the atmosphere. In this study, cloud water samples heavily affected by biomass burning were collected at Mount Tai (1534 m, ASL), located in Shandong province in the North China Plain in summer 2008. The samples were analyzed with high performance liquid chromatography equipped with a UV/Vis absorbance detector immediately followed by electrospray ionization and analysis using a time-of-flight (ToF) mass spectrometer. The high mass resolution and accuracy provided by the ToF mass spectrometer allow determination of the elemental composition of detected ions. Using this approach, the elemental compositions of 16 major light-absorbing compounds, which together accounted for approximately half of measured sample absorption between 300 and 400 nm, were determined. The most important classes of light-absorbing compounds were found to be nitrophenols and aromatic carbonyls. Light absorption over this wavelength range by reduced nitrogen compounds was insignificant in these samples.

The detailed molecular composition of water-soluble atmospheric organic matter (AOM) contained in fog water was studied by use of electrospray ionization ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. We identified 1368 individual molecular masses in the range of 100-400 Da from negative-ion spectra obtained after reversed-phase extraction with a hydrophilic solid phase sorbent. The detected organic anions are multifunctional with a variety of oxygenated functional groups. We observe organic nitrogen, organic sulfur, and organic nitrogen-sulfur compounds as well as many species with only C, H, and O elemental composition. Analysis of the double bond equivalents (DBE = the number of rings plus the number of double bonds to carbon) suggests that these compound structures range from highly aliphatic to aromatic with DBE values of 1-11. The compounds range in their extent of oxidation with oxygen to carbon ratios from 0.2 to 2 with an average value of 0.43. Several homologous series of compounds and multifunctional oligomers were identified in this AOM. The high extent of homologous series of compounds likely originates from primary components that have become oxidized. The multifunctional oligomers appear to represent atmospheric processing of primary and secondary compounds. The isolated water-soluble components identified here are amphiphilic, meaning that they contain both hydrophilic oxygenated functional groups and hydrophobic aliphatic and aromatic structural moieties. Despite the high number of compounds with very high oxygen content, 60% of assigned chemical formulas have measured organic mass-to-organic carbon ratios

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and starting material. This reaction can be expressed as (olefin-OH)* *product (4)The activation energy of this reaction should be less than or equal to that of the back-reaction of (1). The reaction of the energy-rich adduct would then not reform OH, and thus we would not see a decrease in the rate. We would then get the kinetic expression for the apparent rate constant from reactions 1-4.k,Data in the literature strongly suggest the existence of such a reaction for aliene.13 By use of high-intensity, crossed molecular beams and detecting fragments with a photoionization mass spectrometer, C3H3, CH3, and H2C20 were measured. C3H3 is evidence for reaction 3, the H-atom abstraction. It was proposed that CH3 and H2C20 are products of the reaction OH + C3H4 ^(C3H4OH)* -» CH3 + H2C20Since the work was carried out in a molecular beam, product formation requires an efficient unimolecular process.Since there is little falloff at high temperature and the rate of OH addition to aliene is large, the H-atom abstraction reaction could not be seen. However, a careful analysis of the Arrhenius plot for both the experimental and computer-simulated data shows evidence that the H-atom abstraction is of probably of the same order of magnitude in aliene as it is in ethylene. The curvature of this plot above 800 K could be due to the effect of this reaction.