| Abstract |
Large amounts of biogenic, compared to anthropic, non-methane hydrocarbons (NMHCs) are emitted into the atmosphere from a variety of sources. Estimates of biogenic NMHCs emission have been of interest the last two decades because of their potential role in shaping regional photochemical oxidant and aerosol formation and thus balancing global tropospheric chemistry. The atmospheric chemistry of biogenic NMHCs is initiated by the highly reactive O3 and the ubiquitous, transient OH and NO3 radicals. There are few data available concerning the products of these reactions and the reaction mechanisms under simulated atmospheric conditions and even fewer under real atmospheric conditions. In order to study the importance of forested areas and to understand the local chemistry we investigated: a) the composition of the size distributed organic compounds in an urban, non-urban and a forested area b) the secondary organic aerosols formed in the atmosphere by the condensation of low vapor pressure products of isoprene and terpenes photo-oxidation such as α- and β- pinene c) the chemical structure of these products and their ability in new particle formation and d) the gas-to-particle partitioning of secondary organic compounds.
The composition of particle-associated organic compounds in urban, non-urban and forest ares was studied. Significant differences were observed among these areas. Anthropogenic activities as well as environmental tobacco smoke are the major sources of finer aerosol in urban areas while direct emissions of leaves epicuticular waxes enrich coarser particles. On the other hand, non-urban otganic mass is concentrated in particles having diameter from 1.5 to 7.2 μm as a consequence of condensation processes. Air masses origin was determined as the most important factor controlling the composition of non-urban areas. In forested areas, aerosol chemical composition and chemical properties are extensively controlled by the photo-oxidation of α- and β- pinene.
In order to study the mechanism of aerosol formation in-situ, intensive sampling for both gas and particulate phases were performed inside the forest canopy at two different forests. The identification of the two isomers of 2,2-dimethyl-3-acetyl-cyclobutane-ethanoic acid (pinonic acid), cis-2,2-dimethyl-3-carboxyl-cyclobutane-ethanoic acid (cis-pinic acid) and 2,2-dimethyl-3-acetyl-cyclobutane-acetaldehyde (pinonaldehyde) (photo-oxidation products of a α-pinene) and 6,6-dimethyl-bicyclo[3,1,1]-heptan-2-one (nopinone;photo-oxidation product of β-pinene) has been done by means of their mass spectra.
The diurnal distribution pattern of particulate cis- and trans-pinonic acid follows exactly the same pattern as the corresponding variation of Aitken nuclei concentration, supporting that hese rwo acids are chemically coupled with new particle formes over the forests. Although, pinic acid is a dicarboxylic acid, its diurnal pattern follows an unclear pattern. Since, the mechanism is not clear, we can assume that other oxidants such as HO2 and NOx as well as temperature and humidity could influence at pinic acid formation. In addition, the presence of pinonaldehyde and nopinone, despite their high vapor pressure, in the particulate phase can be only explained by the assumption that vapor-phase products condense onto existing particles due to the formation of an intermediate external organic layer of Aitken nuclei. Gas phase concentrations of pinonic and pinic acid, pinonaldehyde and nopinone increased during daytime due to rapid photo-oxidation of α- and β-pinene.
Finally, partitioning ratio (Kom) between gas and particulate phases of above compounds has been calculated (cis-pinonic acid: 0.188; trans-pinonic acid: 0.118; cis-pinic acid: 0.170; pinonaldehyde: 0.075; nonpinone: 0.032). These values verify the importance of carboxylic compounds contribution to secondary organic aerosol formation.
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