NITRATE FORMATION ON SEA-SALT AND MINERAL PARTICLES - A SINGLE-PARTICLE APPROACH

Heterogeneous reactions of NO2 and HNO3 at sub-ppm levels with individual sea-salt and mineral particles were investigated. Particles deposited on filters and on electron microscope grids placed in a Teflon reaction chamber, were exposed to NO2 or HNO3 under controlled conditions. Experiments were carried out under dark conditions and were repeated under u.v. radiation (solar simulation). Nitrates formed on the particles were determined by bulk and individual particle analyses. Individual sea-salt and mineral particles were observed in a transmission electron microscope for the presence of nitrate on the particle surface. The formation of nitrates on sea-salt particles under dark conditions, was in the range of 0.1-3.3 mg NO3- g-1 NaCl. Higher values were obtained for mineral particles: 0.2-8.2 mg NO3- g-1 aerosol ([NO2] = 0.18 and 0.54 ppm; [HNO3] = 0.04 ppm; exposure time 1-7 days; relative humidity = 70%). The formation of nitrates on sea-salt particles increased from 3.0 to 16.1 mg NO3- g-1 NaCl when u.v. radiation was added. Mineral particles did not show a significant increase in nitrate formation under u.v. radiation. Microscopy showed that about 50% of the soil particles reacted with NO2 and HNO3 to form mixed nitrate particles. Almost all sea-salt particles (above 95%) reacted with both gases, although the reaction was not complete and took place only on the particle surface. Application of electron microscopy and a specific microspot technique provided direct evidence for the formation of nitrate on sea-salt and mineral particles exposed to NO2 and HNO3.