FORMONA: Formation and occurence of nitrous acid in the atmosphere

• Research unit: EU RFP
• Project number: 95.0550

Key words (English)

Photochemical smog formation; heterogeneous reactions; nitrous acid; soot; amorphous carbon

Alternative project number (English)

EU project number: ENV4-CT95-0055

Research programs (English)

EU-programme: 4. Frame Research Programme - 3.1 Environment

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise:
EPF Lausanne
Département de Génie Rural (DRG)
Laboratoire de pollution atmosphérique et sol (LPAS)

Partners and International Organizations (English)

CNR Roma (I), Universität Wuppertal (D), Aristotle University Saloniki (GR), University of Cambridge (UK), Universität Heidelberg (D), LCSR/CNRS Orléans (F), Institut für Troposphärenforschung, Leipzig (D), Risoe National Laboratory, Roskilde (DK), Max-Planck Institut für Meteorologie, Hamburg (D)

Abstract (English)

Nitrous acid (HONO) is suspected to be one of the precursors of OH initiating the formation of tropospheric ozone early in the morning during photosmog episodes. Homogeneous gas phase reactions such as OH + NO + M ? HONO + M cannot explain the source strength of HONO so that heterogeneous modes of HONO formation are suspected to occur under typical conditions of heavy urban air pollution. We are therefore investigating the formation of HONO resulting from the heterogeneous interaction of NOx with amorphous carbon (Cam) and soot from a diffusion flame from a fuˆndamental point of view. We use the bulk phase as a surrogate for tropospheric combustion aerosols. The main results are the following:
oThe heterogeneous interaction of NO2 with soot generated in a diffusion flame leads to nitrous acid (HONO) and NO formation. HONO formation has not been observed on commercial amorphous carbon samples under our experimental conditions (total pressure in the mTorr range, very low relative humidity). Nitrous acid does not react heterogeneously with soot and does not have a measurable surface residence time.
oThe rate of HONO formation is at least a factor of 10 3 larger than the HONO formation in the 'classical' reaction 2NO2 + H2O -> HONO + HNO3. Nitrogen monoxide (NO) does not appear in the rate law of HONO formation resulting from the interaction of NO2 with soot.
oHONO is generated from the heterogeneous interaction of NO2 with soot in a complex reaction mechanism including saturation, inhibition and the possibility of reactivation under the influence of light and/or heat. The main features are:
+ The source of hydrogen seems to be some hydrogen-containing species or hydrogen itself which is adsorbed onto soot. This conclusion is supported by the experimental result that the absolute amount of HONO generated is independent of the H2O content of the soot sample. Overall, the reaction appears to be a surface oxidation process accompanied by the formation of CO and CO2.
+ The initial uptake coefficient of NO2 on soot increases from 2 to 12% with decreasing NO2 concentration. Steady state values are roughly a factor of 100 lower.
+ On the order of 10 16 molecules of HONO per mg of soot are generated. After a regeneration cycle (light and/or heat) during which CO and CO2 have been observed 75±10% of the original reactivity is attained implying a comparable yield of HONO.
+ The branching ratio between HONO and NO critically depends upon the sampling location of the soot. Sampling close to the flame base favors HONO formation.
+ The absolute amount of HONO seems to be independent of the nature of the fuel used to generate the soot whereas the uptake coefficient for NO2 is highest for ethylene and lowest for toluene soot.
oAging phenomena of the soot samples have been observed at ambient atmospheric pressure leading to a decrease of soot reactivity and of the potential for regeneration.
oResults from molecular diffusion tube experiments involving soot generated from different fuels yield similar values for the NO2 uptake and HONO yields obtained in Knudsen flow reactor experiments. The biexponential arrival time curves of the surviving fraction of NO2 reveal a weak and strong binding site for every type of soot investigated. A correlation between y0, the initial uptake coefficient and the surface residence time has been observed with the yield of HONO varying significantly between the different types of soot.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0550