Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 1 2004 10.5194/acpd-4-121-2004 http://www.atmos-chem-phys-discuss.net/4/121/2004/ http://www.atmos-chem-phys-discuss.net/4/121/2004/acpd-4-121-2004.html http://www.atmos-chem-phys-discuss.net/4/121/2004/acpd-4-121-2004.pdf 121 140 2004-01-07 Growth of upper tropospheric aerosols due to uptake of HNO₃ S. Romakkaniemi sami.romakkaniemi@uku.fi H. Kokkola A. Petzold A. Laaksonen University of Kuopio, Department of Applied Physics, Kuopio, Finland Deutsches Zentrum für Luft und Raumfahrt Oberpfaffenhofen, Institut für Physik der Atmosphäre, Wessling, Germany The effect of nitric acid on the equilibrium size distributions of upper tropospheric aerosols is calculated as a function of relative humidity. It is shown that HNO₃ concentrations above a few tenths of a ppb can cause substantial increases in haze mode particle concentrations at relative humidities at about 60% and above. The effect can be strongly magnified when letovicite particles are present in addition to sulfuric acid aerosols. This is mainly due to the lowering of the deliquescence RH of letovicite in the presence of gaseous nitric acid at low temperatures. We have also compared equilibrium calculations of the HNO₃ effect with observations of increased haze mode concentrations at relative humidities above 50% (Petzold et al., 2000). Nitric acid mixing ratios on the order of 0.5–2 ppb may explain the observed increase of haze mode particles at least partially.