| 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.
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 >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 <600ppbv the upper limit
yields for HONO, NO2 and NO were found to be <0.2%, <0.5%
and <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.
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. |