Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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5
2004
10.5194/acpd-4-6747-2004
http://www.atmos-chem-phys-discuss.net/4/6747/2004/
http://www.atmos-chem-phys-discuss.net/4/6747/2004/acpd-4-6747-2004.html
http://www.atmos-chem-phys-discuss.net/4/6747/2004/acpd-4-6747-2004.pdf
6747
6767
2004-10-21
Heterogeneous conversion of NO<sub>2</sub> and NO on HNO<sub>3</sub> treated soot surfaces
J. Kleffmann
kleffman@uni-wuppertal.de
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 (NO<sub>2</sub>) was studied at atmospheric humidity levels
on flame soot surfaces, which were treated with gaseous nitric acid
(HNO<sub>3</sub>). In addition, the heterogeneous reaction of HNO<sub>3</sub> on soot
was investigated at atmospheric humidity.
<br><br>
For the treatment of soot by pure HNO<sub>3</sub> only reversible uptake with a
surface coverage of ~1–2×10<sup>14</sup> HNO<sub>3</sub> cm<sup>−2</sup> was
observed for HNO<sub>3</sub> mixing ratios in the range 250–800 ppbv. Only
for higher HNO<sub>3</sub> mixing ratios of >800 ppbv the formation of NO and
NO<sub>2</sub> was observed. The results were not affected by the addition of NO.
In none of the experiments with HNO<sub>3</sub> the formation of nitrous acid
(HONO) was observed. For HNO<sub>3</sub> mixing ratios <600 ppbv the upper limit
yields for HONO, NO<sub>2</sub> and NO were found to be <0.2%, <0.5%
and <1%, respectively. Compared to untreated soot, the product
formation of the reaction of NO<sub>2</sub> with soot was not significantly
affected when the soot surface was treated with gaseous HNO<sub>3</sub> prior to
the experiment. Only for high surface coverage of HNO<sub>3</sub> the formation of
HONO was suppressed in the initial phase of the reaction, probably caused by
the blocking of active sites by adsorbed HNO<sub>3</sub>.
<br><br>
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 HNO<sub>3</sub> and HNO<sub>3</sub>+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.
10<sup>14</sup> cm<sup>−2</sup> in the reaction of NO<sub>2</sub> with soot is not
significantly influenced by simulated atmospheric processing of the soot
surface by HNO<sub>3</sub>, and is still too small to explain HONO formation in
the atmosphere.