| Abstract |
The current thesis focuses on the study of the nitrate radicals at the eastern Mediterranean atmosphere, a cross point of cultures and civilizations as well as of air masses of different origin and chemical composition. The nitrate radicals are key species of the atmosphere and one of the three major agents linked to its cleansing capacity from the various anthropogenic and biogenic pollutants. Their production is enhanced during the last years as the anthropogenic activities multiplied by five the nitrogen emissions to the atmosphere since the pre-industrial era. Despite their importance, the nitrate radicals are not well studied mainly due to the measuring difficulties arising from their low mixing ratios in the atmosphere, in addition to the observed high temporal and spatial variation.
These problems were overcame with the help of the long path (10.4km) differential optical absorption spectroscopy (DOAS). The measurements were conducted at the environmental observatory of the University of Crete (Finokalia) during the period 2001-2003. This was the first time ever that these radicals were monitored for such a long period. As a result major conclusions were drawn concerning the seasonal variation of the nitrate radicals and their importance to the tropospheric chemistry. More specifically the thesis achieved to answer the following issues-goals: W Determination of the levels of the nitrate radicals at the eastern Mediterranean.
W To observe their diel and seasonal variation.
W To investigate the role of other chemical species and metrological factors in connection with the observed variations of the nitrate radicals. W To evaluate the chemical lifetime of NO and the mechanisms controlling it on an annual and seasonal base.
W To clarify the role, and the relative contribution, of the nitrate radicals to the oxidizing capacity ofthe atmosphere.
W To comprehend their role in removing nitrogen oxides from the troposphere, on a diel and seasonal base, through the formation of nitric acid and nitrate anions followed by wet and dry deposition.
W To evaluate their involvement on other radicals production such as the peroxy-radicals and hydroxyl radicals which are created through the oxidation of volatile organic compounds by the NO3 radicals during night.
The above mentioned goals were achieved through measurements, statistical analysis (1st phase) and arithmetic OD modeling studies (2nd phase).
During the first phase of the thesis, field measurements were conducted for a period of more than two years. Except nitrate radicals, other species were also measured. Some of them were: Nitrogen monoxide (NO) and dioxide (NO), total nitrogen oxides (NOy), ozone (O3), radon (222Rn), volatile organic compounds (VOC), carbon monoxide (CO), peroxy radicals (ROx), nitric acid (HNO3), nitrate-anions (NO), photo-dissociation constants of ozone (JOD) and nitrogen dioxide (JNO2), aerosol surface (S), temperature (T), relative humidity (RH), wind speed and direction (Ws, Wd).
The second phase of the thesis was the modeling stage. The box model used for in this phase has been evaluated by comparing its results with the experimental ones driven on the first phase. After the evaluation, this model was used to answer questions which are difficult to be answered via the experiment.
For instance the model has been used for the forecast of the diel and seasonal variation of the hydroxyl radicals, a key species in order to quantify the relative contribution of NO radicals to the oxidizing capacity of the atmosphere. Further studies were focused on the estimation of the nitrate radicals and their contribution to nitric acid and nitrates production as well as to peroxy radicals production in the atmosphere.
The following text highlights some of the results of the study.
The NO radicals followed a distinct seasonal dependency with maximum mixing ratios in summer (5.6±1.2 pptv) and minimum in winter (1.2±1.2 pptv).
Episodes with high NO mixing ratios have been encountered mainly in polluted air masses originating from mainland Greece, Central and East Europe, and Turkey.
The data showed that the NO nighttime mixing ratios are primarily dependent on NO (positive correlation) and relative humidity (negative correlation) and to a lesser extend on temperature (positive correlation). As inferred from these observations, on average the major sink of NO radicals in the area is the heterogeneous reaction of dinitrogen pentoxide (N2O5) on aqueous particles whereas the homogeneous gas phase reactions of NO are most important during spring and summer. NO chemistry in the area significantly contributes to VOC oxidation and to the nighttime formation of peroxy radicals, nitric acid and particulate nitrate.
Nighttime production of HNO plus NO~, as initiated by NO radicals, accounts for about 50-65% of the total production rate depending on season. The remainder is produced by the hydroxyl radical (OH) reaction with nitrogen dioxide during daytime. On a yearly mean basis, about 17% of the HNO plus NO- formation results from the reaction of NO3 radicals with dimethylsulfide of marine origin. This shows important interactions between biogenic and anthropogenic compounds contributing to nutrient deposition to the sea.
OD modeling calculations revealed that the nitrate radicals are the dominant producers of peroxy radicals during night and only a small fraction can be attributed to ozone reactions. Moreover the most abundant ROx species of the area was the methyl-peroxy radical which is mainly produced via the reaction of NO radicals and the dimethylsulfide molecule.
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