ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-549-2004 Growth of upper tropospheric aerosols due to uptake of HNO<sub>3</sub> Romakkaniemi S. 1 Kokkola H. 1 Petzold A. 2 Laaksonen A. 1 University of Kuopio, Department of Applied Physics, Kuopio, Finland Deutsches Zentrum für Luft und Raumfahrt Oberpfaffenhofen, Institut für Physik der Atmosphäre, Wessling, Germany 30 03 2004 4 2 549 556 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/4/549/2004/acp-4-549-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/549/2004/acp-4-549-2004.pdf

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 HNO<sub>3</sub> 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. Letovicite particles are less acidic than the sulfuric acid particles and so more nitric acid can be absorbed. This effect can be seen even at RH below 50% 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 HNO<sub>3</sub> 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 0.5-2ppb may explain the observed increase of haze mode particles at least partially.