HETEROGENEOUS REACTION OF NITROGEN-OXIDES ON SEA SALT AND MINERAL PARTICLES - A SINGLE-PARTICLE APPROACH

Heterogeneous reactions of NO2 and HNO3 at sub ppm levels with sea salt and mineral particles were investigated. Experiments were conducted in a static reaction chamber made of Teflon, where particles deposited on filters and on electron microscope grids were exposed to NO2 or HNO3 under controlled conditions. Nitrates formed on the particle loaded filters were determined by bulk analysis. In parallel, sea salt and mineral particles were observed in an electron microscope to detect the presence of nitrate on the surfaces of each individual particle. Microspot techniques were applied for the later. Under the present experimental conditions the formation of nitrates on sea salt particles was in the range of 0.1 to 3.3 mg NO3/g NaCl. Slightly higher values were obtained for mineral particles: 0.2 to 4.7 mg NO3/g aerosol ([NO2] = 0.18 ppm and 0.54 ppm; [HNO3] = 0.04 ppm; exposure time 1 to 7 days; relative humidity = 70%). Exposure of these particles to UV radiation resulted in a significant increase in nitrate formation: 3.0 to 16.1 for NaCl and 1.4 to 27.9 mg NO3/g aerosol for minerals. Application of electron microscopy and specific microspot techniques provided direct evidence for the heterogeneous reaction of sea salt and mineral particles with NO2 and HNO3 to form a layer of nitrate on the particle surfaces. Forty to 50% of the soil and almost all sea salt particles that were exposed to NO2 and HNO3 form mixed nitrate particles.