DISTRIBUTION OF NITRATE OVER SEA-SALT AND SOIL DERIVED PARTICLES - IMPLICATIONS FROM A FIELD-STUDY

A method for determining the distribution of supermicrometer nitrate b etween size-segregated sea-salt and soil derived particles is presente d. The analysis is based on field data from six measurements at a coas tal site in southern Finland, and on a theoretical treatment taking in to account the transfer of gaseous species onto particle surfaces and their subsequent reaction. Significant amounts of nitrate were found i n both the particle types, with the fraction of nitrate associated wit h soil particles varying from 20-50% in the 1-2 mu m size to near 90% in particles larger than 10 mu m. Overall, the nitrate accumulation fo llowed closely the relative abundances of these two particle types. Tw o overlapping modes in supermicron nitrate mass size distributions cou ld be identified. The lower mode, associated with sea-salt, was locate d between the surface-area and volume distribution of sodium peaking a t about 2-3 mu m of EAD. The upper mode peaked at 3-5 mu m and followe d more closely the surface-area distribution of calcium in all samples . At our site, the accumulation of nitrate into both particle types wa s shown to be limited by an effective 'surface reaction' rate rather t han by gas-phase diffusion. This rate was estimated to be considerably larger for sea-salt particles. Strong evidence in support of the satu ration of nitrate in sea-salt particles were obtained.