Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere.

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.

Secondary organic aerosols from anthropogenic and biogenic precursors.

Secondary organic aerosol (SOA) formation from the photooxidation of an anthropogenic (1,3,5-trimethylbenzene) and a biogenic (alpha-pinene) precursor was investigated at the new PSI smog chamber. The chemistry of the gas phase was followed by proton transfer reaction mass spectrometry, while the aerosol chemistry was investigated with aerosol mass spectrometry, ion chromatography, laser desorption ionization mass spectrometry, and infrared spectroscopy, along with volatility and hygroscopicity studies. Evidence for oligomer formation for SOA from both precursors was given by an increasing abundance of compounds with a high molecular weight (up to 1000 Da) and by an increasing thermal stability with increasing aging time. The results were compared to data obtained from ambient aerosol samples, revealing a number of similar features.

Identification of organic acids in secondary organic aerosol and the corresponding gas phase from chamber experiments.

Organic acids in the gas and aerosol phase from photooxidation of 1,3,5-trimethylbenzene in the presence of 300 ppb propene and 300 ppb NOx in smog chamber experiments were determined using a wet effluent diffusion denuder/aerosol collector coupled to ion chromatography (IC) with conductivity detection. Behind the IC, the samples were collected using a fraction collector, for identification of unresolved/unidentified organic acids with IC-mass spectrometry (MS). In total, 20 organic acids were found with MS of which 10 were identified. The organic acids identified offline by IC-MS were then further quantified based on the online IC data. The identification was additionally confirmed with gas chromatography-mass spectrometry. At the maximum aerosol concentration, organic acids comprised 20-45% of the total aerosol mass. The method has a detection limit of 10-100 ng/m3 for the identified carboxylic acids.

Identification of polymers as major components of atmospheric organic aerosols.

Results from photooxidation of aromatic compounds in a reaction chamber show that a substantial fraction of the organic aerosol mass is composed of polymers. This polymerization results from reactions of carbonyls and their hydrates. After aging for more than 20 hours, about 50% of the particle mass consists of polymers with a molecular mass up to 1000 daltons. This results in a lower volatility of this secondary organic aerosol and a higher aerosol yield than a model using vapor pressures of individual organic species would predict.