Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 5 1 2005 10.5194/acp-5-77-2005 http://www.atmos-chem-phys.net/5/77/2005/ http://www.atmos-chem-phys.net/5/77/2005/acp-5-77-2005.html http://www.atmos-chem-phys.net/5/77/2005/acp-5-77-2005.pdf 77 83 2005-01-20 Heterogeneous conversion of NO₂ and NO on HNO₃ treated soot surfaces: atmospheric implications J. Kleffmann P. Wiesen Physikalische Chemie/FB C, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany In the present study, the heterogeneous conversion of nitrogen oxide (NO) and nitrogen dioxide (NO₂) was studied at atmospheric humidity levels on flame soot surfaces treated with gaseous nitric acid (HNO₃). In addition, the heterogeneous reaction of HNO₃ on soot was investigated at atmospheric humidity. <P style="line-height: 20px;"> For the treatment of soot by pure HNO₃ only reversible uptake with a surface coverage of ~1-2x10¹⁴ HNO₃ cm^-2 was observed for HNO₃ mixing ratios in the range 250-800ppbv. Only for higher HNO₃ mixing ratios of &gt;800ppbv the formation of NO and NO₂ was observed. The results were not affected by the addition of NO. In none of the experiments with HNO₃ the formation of nitrous acid (HONO) was observed. For HNO₃ mixing ratios &lt;600ppbv the upper limit yields for HONO, NO₂ and NO were found to be &lt;0.2%, &lt;0.5% and &lt;1%, respectively. Compared to untreated soot, the product formation of the reaction of NO₂ with soot was not significantly affected when the soot surface was treated with gaseous HNO₃ prior to the experiment. Only for high surface coverage of HNO₃ the formation of HONO was suppressed in the initial phase of the reaction, probably caused by the blocking of active sites by adsorbed HNO₃. <P style="line-height: 20px;"> Under the assumption that the experimental findings for the used model flame soot can be extrapolated to atmospheric soot particles, the results show that the reactions of HNO₃ and HNO₃+NO on soot surfaces are unimportant for a "renoxification" of the atmosphere and do not represent an atmospheric HONO source. In addition, the integrated HONO yield of ca. 10¹⁴cm^-2 in the reaction of NO₂ with soot is not significantly influenced by simulated atmospheric processing of the soot surface by HNO₃, and is still too small to explain HONO formation in the atmosphere.