| Abstract |
Through complex reactions with OH radicals, NO3 radicals and ozone, monoterpenes turn into species that contain carbonyl, carboxyl, and hydroxyl functional groups. These oxidation products have substantially lower vapour pressure than their parent species, and thus they play an important role in the formation of secondary organic aerosols (SOA). A deeper insight concerning the oxidation products of monoterpenes is crucial for better understanding of SOA formation. Although many studies have been conducted in simulation chambers to research the secondary organic compounds generation by oxidation of terpenes, field measurements of oxidation products in the atmospheric particle phase are still sparse.
Compound speciation in SOA is a demanding task because of the wide range of polarities and the unavailability of authentic standards for most of the chemical species. In the present study a simple and highly sensitive analytical technique was developed to determine the most important polar and acidic products arising from terpene oxidation in the atmosphere. This method incorporates (1) gradient solvent extraction with an Accelerated Solvent Extraction system, followed by rotatory evaporation; (2) a clean-up procedure for carbonyl compounds using silica-gel chromatography, (3) derivatization of acidic products and (4) analysis by gas chromatography coupled to mass-spectrometry. Recovery rates of the target compounds were evaluated from spiked filters and polyurethane foam (sampling media) in the extraction/evaporation steps and were quantified at different solvent mixtures and blowdown conditions.
The recoveries for the selected terpene compounds were 52-90% for the combined steps of extraction, evaporation, silica gel clean-up and derivatization. Furthermore two GC-MS systems were optimized in order to achieve low instrumental detection limits; i.e a quadrupole filter mass selective detector (MSD) and a quadrupole ion trap (ITMS). The instrumental detection limits for the selected compounds were <12 pg for MSD and <78 pg for ITMS. The MSD was finally selected as detector and the overall analytical method was applied to ambient aerosols for the quantitative analysis of monoterpene oxidation products in a semi-urban site in Heraklion, Greece.
Furthermore, a method coupling gas chromatography with positive chemical ionization mass spectrometry (using ammonia as reagent gas) was developed for a better structural identification of monoterpene photo-oxidation carbonyl- and carboxylic-type products. This method was used in conjunction with MSD-based electron impact method for the identification of various monoterpene photo-oxidation products in samples collected from chamber experiments. In addition to the well known products reported in previous studies, a highly abundant component having a molecular weight of 172 was observed. The structure of this compound was tentatively elucidated using mass spectrometry data and was consistent with 3-(2-hydroxyethyl)-2,2-dimethylcyclobutanecarboxylic acid. It is particularly interesting that the specific compound was consistently detected in field samples collected from a rural area in estearn, Germany (Melpitz).
In order to study the importance of forested areas in SOA production and to understand the local atmospheric chemistry, gas and particle-phase concentrations of primary and secondary
organic compounds were measured in a Boreal forest in Finland (Hyytiälä) and a rural site in eastern Germany (Melpitz). A series of carbonyl and carboxylic compounds formed through the photo oxidation of monoterpenes and high molecular weight n-alkan-1-ols, n-alkanoic, n-alkenoic and α,ω-dicarboxylic acids directly emitted from conifer leaf epicuticular wax were detected and quantified conjointly in both gas and particles.
Our results suggested that carbonyl and carboxylic products arising from the photo-oxidation of monoterpenes and the primarily emitted semi-volatile organic compounds were important constituents of organic aerosol in both sites. Pinonaldehyde was the most abundant compound among terpene oxidation products (up to 39 ng m-3 in particulate phase and 29 ng m-3 in gas phase), while carboxylic products were detected at lower concentrations, mostly between 0.6 and 10 ng m-3. Pinonic acid was always more abundant than pinic acid. In most samples, the concentrations of the latter were below the detection limit. The low levels of the dicarboxylic acids, (i.e. pinic acid) could be attributed to the low ozone levels experienced in the investigated forest area and the low emissions of monoterpenes.
Pinonaldehyde, limonaldehyde and nopinone have shown higher gas than particle phase concentrations. On the contrary, the low-volatility products, such as pinonic, norpinonic and pinic acids, were mostly detected in the particle phase. It is interesting to note that particle formation events in forest area, were accompanied by higher concentrations of monoterpene photo-oxidation carbonyl- and carboxylic-type products. Among them, pinonaldehyde exhibited its highest concentration especially during night time. It is possible that temperature decrease during the night favoured the absorption of the more volatile compounds (e.g. pinonaldehyde, nopinone) on the surface of newly formed organic particles and caused their growth in size. In contrary, the highest concentrations of monoterpene photo-oxidation derived carboxylic compounds were measured before the onset of a less intensive new particle formation event.
Despite the fact that a clear link between the levels of acids (i.e. pinic, pinonic, norpinonic acid) and the formation of new particles was not observed, it is possible that the products arising from the photo-oxidation of monoterpenes had a large impact on the growth and the composition of sub micrometer particles, due to their gas-to particle conversion.
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Πρωτογενή και δευτερογενή οργανικά αερολύματα : μελέτη της παρουσίας και του σχηματισμού τους σε δασικές και αγροτικές περιοχές της βόρειας και κεντρικής Ευρώπης
