|Transformation, concentrations and deposition of North Sea aerosols|
Otten, P. (1991). Transformation, concentrations and deposition of North Sea aerosols. PhD Thesis. Universiteit Instelling Antwerpen: Antwerpen. 293 pp.
Universiteit Antwerpen; Centrum voor Micro- en Sporenanalyse (MITAC), more
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|Document type: Dissertation|
In this work, attention was focused on the lower troposphere above the North Sea. It is well known that the marine atmosphere acts as a source, a sink and a reaction reservoir for various chemical constituents. In the past 15 years it has become clear that the atmosphere above the North Sea can be an important source of pollutants for the sea. In chapter 1, the sampling of marine aerosols with a cascade impactor was studied with regard to the possible interaction between particles collected on an impaction surface and reactive gaseous atmospheric components that are drawn through the impactor . NaCI particles, generated by pneumatic nebulisation of a NaCI solution, were collected on an impaction plate of a cascade impactor . After passing through the impactor an air stream contaminated with nitric acid, the resulting particles were analyzed with laser microprobe mass spectrometry (LMMS). This technique allowed to confirm the importance of the reaction between NaCI aerosol particles and HNO3 vapour, resulting in a central NaCI cube covered with a NaNO3 layer. Both dry and wet NaCI particles were shown to react with nitric acid vapour during impactor sampling. In order to avoid possible HNO3-NaCI interaction, a NaF diffusion denuder can be placed in front of the cascade impactor to remove the gaseous HNO3 quantitatively. An alternative procedure involves the use of extra-stages with high cut-off diameter. When loaded with at least a ten-fold excess of NaCI particles, relative to the amount of nitric acid expected to pass through the impactor during sampling, all gaseous HNO3 will be removed. This setup has the obvious advantage of being more compact and less fragile and thus has some possibilities for use on the field, i.e. on the North Sea. Analogous sampling artifacts between sulphuric acid aerosols and ammonia were simulated in laboratory tests. The LMMS technique was shown to be useful for the detection of inorganic ammonium salts in single aerosol particles. It is one of the few techniques that can analyze thermally unstable compounds directly, without prior sample treatment, on the scale of one single particle. The high sensitivity of the LAMMA-500 instrument for Na+, K+ and some other cations, relative to the ammonium ion, limits the possibilities for detecting ammonium salts in ambient aerosol particles. Nevertheless, analysis of cascade-impactor collected aerosols from the North Sea atmosphere, sampled during easterly wind and under a low inversion height, showed the presence of high amounts of ammonium compounds in single particles. LMMS results of a few aerosol samples collected at the university campus during the winter of 1986-1987 indicated the presence of mixtures of ammonium chloride, ammonium nitrate and ammonium sulphate in single particles. In chapter 2, the results of energy dispersive X-ray fluorescence (EDXRF) measurements on 71 aerosol samples, collected between 1984 and 1988 on the North Sea and the Channel, were presented. To our knowledge, this data set is the first one that covers not only an extensive period of time (5 years), but also the complete North Sea area. Atmospheric concentrations of different elements in the lower troposphere above the North are highly variable as a function of place and time. Changes in wind direction and air mass history are reflected without delay in the composition of the North Sea aerosol. Except for CI, all elements show a clear gradient from the southern to the northern part of the North Sea. Fe, Mn, Zn and Pb concentrations are 10 times lower on the average in the northern North Sea atmosphere, compared to the atmosphere above the Southern Bight of the North Sea. Atmospheric Si, S, K, Ca, Ti, Cr, V and Ni concentrations are 3 to 6 times higher above the Southern Bight compared to the northern North Sea atmosphere. Factor analysis performed on all samples collected above the Southern Bight yielded the identificat