Heterogeneous conversion of NO2 and NO on HNO3 treated soot surfaces

Introduction Conclusions References Tables Figures Back Close Abstract Introduction Conclusions References Tables Figures Back Close Abstract In the present study, the heterogeneous conversion of nitrogen oxide (NO) and nitrogen dioxide (NO 2) was studied at atmospheric humidity levels on flame soot surfaces, which were treated with gaseous nitric acid (HNO 3). In addition, the heterogeneous reaction of HNO 3 on soot was investigated at atmospheric humidity. 5 For the treatment of soot by pure HNO 3 only reversible uptake with a surface coverage of ∼1–2×10 14 HNO 3 cm −2 was observed for HNO 3 mixing ratios in the range 250–800 ppbv. Only for higher HNO 3 mixing ratios of >800 ppbv the formation of NO and NO 2 was observed. The results were not affected by the addition of NO. In none of the experiments with HNO 3 the formation of nitrous acid (HONO) was observed. 10 For HNO 3 mixing ratios <600 ppbv the upper limit yields for HONO, NO 2 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 2 with soot was not significantly affected when the soot surface was treated with gaseous HNO 3 prior to the experiment. Only for high surface coverage of HNO 3 the formation of HONO was suppressed in the initial phase 15 of the reaction, probably caused by the blocking of active sites by adsorbed HNO 3. 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 3 and HNO 3 +NO on soot surfaces are unimportant for a " renoxification " of the atmosphere and do not represent an atmospheric HONO source. In addition, the inte-20 grated HONO yield of ca. 10 14 cm −2 in the reaction of NO 2 with soot is not significantly influenced by simulated atmospheric processing of the soot surface by HNO 3 , and is still too small to explain HONO formation in the atmosphere.