| Abstract |
The role of carboxylic acids in the chemistry of the troposphere has become in the past two decades an issue of growing interest. Low molecular weight carboxylic acids, e.g formic and acetic acids, have been shown to be ubiquitous components in the tropospheric aqueous, gaseous and particulate phases. These acids may have a dominant contribution (as high as 60%) to the free acidity of precipitation in remote areas. Studies for these acids in forested areas are very limited and most of them have been conducted in tropical forests. Most of the published studies concerning the presence of carboxylic acids in forested areas did not take into consideration important factors such as precursor and oxidant concentration and as well as meteorological parameters. In order to enhance our knowledge on the atmospheric role of low molecular weight carboxylic acids, we studied their concentration variation simultaneously with the concentration of their precursors, of the atmospheric oxidants and meteorological parameters (temperature, solar radiation, air masses back trajectories) influencing their occurrence. Our study took place in two forest sites at Tabua in Portugal (Eucalyptus forest) and at Pertuli in Creece (conifer forest). The mean atmospheric mixing ratio of gaseous formic acid was at the same level in the two forests. The mean mixing ratio of formic acid was 1.64 ppbv in the Eucalyptus forest and 1.46 ppbv in the conifer forest respectively. Acetic acid mixing ratio in the two forests was quite different. The mean mixing ratio of this acid was 0.96 ppbv in the Eucalyptus forest and 1.95 ppbv in the conifer forest respectively. This difference between these areas was probably due to the difference of the emission sources for these carboxylic acids. The dominant sources in these areas were found to be direct emissions from vegetation and photochemical production from biogenic organic compounds. Nevertheless the type and the age of the trees in these forests are different. Therefore these two factors may affect the direct emissions from trees. Furthermore as different biogenic organic compounds are emitted from these forests at different intensities the photochemical production of these acids is expected to differ between the two sampling sites. Acetic and formic acid in these forest areas present similar diurnal variation. The concentration of the gaseous acids increased throughout the morning, reached a maximum in the afternoon, and declined again towards evening. In presence of rain the concentration of formic and acetic acid decreased indicating that wet deposition was the major sink of these acids. The concentration of acetic and formic acids in the gas phase exceeded the gaseous concentration of the main inorganic acids (HNO3 and SO2). This difference was due both to the intensive production of carboxylic acids in forests areas and to the lack of sources of these main inorganic acids. The presence of these carboxylic acids in fine aerosols was very limited in contrast to their abundance in gas phase. The low quantities, which were detected in particulate phase (fine aerosols), were probably due to their adsorption on existing particles.
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Μελέτη της σύστασης της Ατμόσφαιρας Δασικών περιοχών σε οργανικά και ανόργανα ιόντα
