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Heterogeneous conversion of NO2 and NO on HNO3 treated soot surfaces: atmospheric implications

by J Kleffmann, P Wiesen
Atmos. Chem. Phys. ()
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In the present study, the heterogeneous conversion of nitrogen oxide (NO) and nitrogen dioxide (NO2) was studied at atm. humidity levels on flame soot surfaces treated with gaseous nitric acid (HNO3). In addn., the heterogeneous reaction of HNO3 on soot was investigated at atm. humidity. For the treatment of soot by pure HNO3 only reversible uptake with a surface coverage of ∼1-2 × 1014 HNO3 cm-2 was obsd. for HNO3 mixing ratios in the range 250-800 ppbv. Only for higher HNO3 mixing ratios of {>}800 ppbv the formation of NO and NO2 was obsd. The results were not affected by the addn. of NO. In none of the expts. with HNO3 the formation of nitrous acid (HONO) was obsd. For HNO3 mixing ratios {<}600 ppbv the upper limit yields for HONO, NO2 and NO were found to be {<}0.2{%}, {<}0.5{%} and {<}1{%}, resp. 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 expt. 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. Under the assumption that the exptl. findings for the used model flame soot can be extrapolated to atm. soot particles, the results show that the reactions of HNO3 and HNO3+NO on soot surfaces are unimportant for a "renoxification" of the atm. and do not represent an atm. HONO source. In addn., the integrated HONO yield of ca. 1014 cm-2 in the reaction of NO2 with soot is not significantly influenced by simulated atm. processing of the soot surface by HNO3, and is still too small to explain HONO formation in the atm. [on SciFinder(R)]

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