Brown Carbon in Primary and Aged Coal Combustion Emission.

Smog chamber experiments were conducted to characterize the light absorption of brown carbon (BrC) from primary and photochemically aged coal combustion emissions. Light absorption was measured by the UV-visible spectrophotometric analysis of water and methanol extracts of filter samples. The single-scattering albedo at 450 nm was 0.73 ± 0.10 for primary emissions and 0.75 ± 0.13 for aged emissions. The light absorption coefficient at 365 nm of methanol extracts was higher than that of water extracts by a factor of 10 for primary emissions and a factor of 7 for aged emissions. This suggests that the majority of BrC is water-insoluble even after aging. The mass absorption efficiency of this BrC (MAE365) for primary OA (POA) was dependent on combustion conditions, with an average of 0.84 ± 0.54 m2 g-1, which was significantly higher than that for aged OA (0.24 ± 0.18 m2 g-1). Secondary OA (SOA) dominated aged OA and the decreased MAE365 after aging indicates that SOA is less light absorbing than POA and/or that BrC is bleached (oxidized) with aging. The estimated MAE365 of SOA (0.14 ± 0.08 m2 g-1) was much lower than that of POA. A comparison of MAE365 of residential coal combustion with other anthropogenic sources suggests that residential coal combustion emissions are among the strongest absorbing BrC organics.

A Comprehensive Nontarget Analysis for the Molecular Reconstruction of Organic Aerosol Composition from Glacier Ice Cores.

Ice cores are climate archives suitable for the reconstruction of past atmospheric composition changes. Ice core analysis provides valuable insight into the chemical nature of aerosols and enables constraining emission inventories of primary emissions and of gas-phase precursors. Changes in the emissions of volatile organic compounds (VOCs) can affect formation rates and mechanisms as well as chemical composition of aerosols during the preindustrial era, key information for understanding aerosol climate effects. Here, we present an analytical method for the reconstruction of organic aerosol composition preserved in glacier ice cores. A solid-phase-extraction method, optimized toward oxidation products of biogenic VOCs, provides an enrichment factor of ∼200 and quantitative recovery for compounds of interest. We applied the preconcentration method on ice core samples from the high-alpine Fiescherhorn glacier (Swiss Alps), and used high-performance liquid chromatography coupled to high-resolution mass spectrometry as a sensitive detection method. We describe a nontarget analysis that screens for organic molecules in the ice core samples. We evaluate the atmospheric origin of the detected compounds in the ice by molecular-resolved comparison with airborne particulate matter samples from the nearby high-alpine research station Jungfraujoch. The presented method is able to shed light upon the history of the evolution of organic aerosol composition in the anthropocene, a research field in paleoclimatology with considerable potential.

Oxidation of atmospheric humic like substances by ozone: a kinetic and structural analysis approach.

This work explores the heterogeneous reaction between HUmic-LIke Substances (so-called HULIS) and ozone. Genuine atmospheric HULIS were extracted from aerosol samples collected in Chamonix (France) in winter and used in coated flow tube experiments to evaluate heterogeneous uptake of O3 on such mixtures. The uptake coefficient (γ) was investigated as a function of pH (from 2.5 to 10), O3 concentration (from 8 to 33 × 10¹¹ molecules cm⁻³), relative humidity (20 to 65%) and photon flux (from 0 to 1.66 × 10¹5 photons cm⁻² s⁻¹). Reactive uptake was found to increase in the irradiated experiment with pH, humidity and photon flux. The extract was characterized before and after exposure to O3 and/or UV light in the attempt to elucidate the effect of the photochemical aging. Carbon content measurements, UV-vis spectroscopy and functional groups analysis revealed a decrease of the UV absorbance as well as of the carbon mass content, while the functionalization rate (COOH and C═O) and therefore the polarity increased during the simulated photochemical exposure.

Precursor ion scanning-mass spectrometry for the determination of nitro functional groups in atmospheric particulate organic matter.

An analytical method for the quantitative determination of the total nitro functional group (R-NO2) content in atmospheric particulate organic matter is developed. The method is based on the selectivity of NO2(-) (m/z 46) precursor ion scanning (PAR 46) by atmospheric pressure chemical ionization-tandem mass spectrometry (APCI-MS/MS). PAR 46 was experimented on 16 nitro compounds of different molecular structures and was compared with a neutral loss of NO (30 amu) technique in terms of sensitivity and efficiency to characterize the nitro functional groups. Covering a wider range of compounds, PAR 46 was preferred and applied to reference mixtures containing all the 16 compounds under study. Repeatability carried out using an original statistical approach, and calibration experiments were performed on the reference mixtures proven the suitability of the technique for quantitative measurements of nitro functional groups in samples of environmental interest with good accuracy. A linear range was obtained for concentrations ranging between 0.005 and 0.25 mM with a detection limit of 0.001 mM of nitro functional groups. Finally, the analytical error based on an original statistical approach applied to numerous reference mixtures was below 20%. Despite of potential artifacts related to nitro-alkanes and organonitrates, this new methodology offers a promising alternative to FT-IR measurements. The relevance of the method and its potentialities are demonstrated through its application to aerosols collected in the EUPHORE simulation chamber during o-xylene photooxidation experiments and in a suburban area of a French alpine valley during summer.