NITROCAT: Nitrous acid and its influence on the oxidation capacity of the atmosphere

• Forschungsstelle: EU FRP
• Projektnummer: 99.0042-1
• Projekttitel Englisch: NITROCAT: Nitrous acid and its influence on the oxidation capacity of the atmosphere

Schlüsselwörter (Englisch)

Heterogeneous chemistry; flame soot; HONO formation; secondary aerosol; NO2 uptake coefficient; reaction mechanism

Alternative Projektnummern (Englisch)

EU project number: EVK2-1999-00025

Forschungsprogramme (Englisch)

EU-programme: 5. Frame Research Programme - 1.4a.2 Global change, climate and biodiversity

Kurzbeschreibung (Englisch)

See abstract

Weitere Hinweise und Angaben (Englisch)

Full name of research-institution/enterprise:
EPF Lausanne
Département de Génie Rural
Laboratoire de Pollution Air-Sol / LPAS

Partner und Internationale Organisationen (Englisch)

Coordinator: Bergische Universität, Wuppertal (D)

Abstract (Englisch)

HONO is the 'starter' for photochemical smog formation in an urban polluted atmosphere. Its mode of formation is still not known with certainty. The working hypothesis for HONO generation is that it is heterogeneously formed on the surface of enviornmental particulate such as combustion aerosol or soot, but also urban and rural interfaces such as ground cover. Previous results indicate a strong correlation between the type of combustion (lean vs. rich) and the HONO forming potential of soot rather than a significant influence of the type of fuel. Therefore, we have controlled the combustion process that also improved the reproducibility of the heterogeneous kinetics. At EPFL we have extensively used two burners:
(A) Co-flow device or 'lamp' burner leading to a diffusion flame which affords resettability and reproducibility of the combustion conditions, but which does not allow for the determination of the absolute fuel/oxygen mixing ratio (l). Hexane, decane, toluene, Diesel were used as fuels.
(B) CAST (Combustion Aerosol Standard) Device invented by Dr. Lianpeng Jing of METAS and optimised for generating soot under stable conditions of known l. Hexane was the only fuel used.
NO2 interaction with flame soot generated in the laboratory.
The following general conclusions may be drawn from our studies as they result from the heterogeneous interaction of oxidized nitrogen with a reducing substrate (soot):
(A) HONO is formed in a reduction-oxidation reaction on ALL types of soot we have investigated resulting from the heterogeneous reaction of NO2 with soot. In the case where we do not observe HONO it decomposes on flame soot obtained under lean combustion conditions presumably at active functional groups of 'black' soot generated in the co-flow device or soot at l = 0.09 or 0.16 produced in the CAST. On soot from a rich flame HONO does not decompose and is observed in high yields approaching 100% of the reacted NO2.
(B) HONO also results from the interaction of HNO3 on every type of flame soot investigated at reaction probabilities that are up to an order of magnitude larger than for NO2. The reaction HNO3 + NO è NO2 + HONO is slow and necessitates HNO3-coverages of the order of 30% on flame soot. It most likely is unimportant in the troposphere.
(C) The initial uptake coefficient g0 depends on the reactant concentration in a process that is sensitive to molecular saturation but insensitive to sample mass, hence dependent upon the geometric surface area. Pore diffusion theory describing the diffusion of gas in the interstices of the sample is inappropriate for soot as it leads to unreasonable results.
(D) The HONO-forming principle may be dissociated from the black carbon matrix using extraction of soot in THF, but not in benzene as a solvent. It may be transferred to an appropriate support (reverse phase silica-gel) where it continues to reduce NO2 to HONO.
(E) Owing to the fact that HONO has been observed in the field at noontime when photolysis rates are highest, the working hypothesis that HONO may be formed on Secondary Organic Aerosol (SOA) has been tested. The following SOA particles have been generated under controlled conditions:
(a) Toluene/ozone/air(humid) + UV photons leading to concentrations of 106 cm-3. Typical ozone and VOC concentrations were 50-100 to 1000 ppm, respectively. Relative humidity is required in this case because OH is the active oxidizer; (b) Limonene /ozone/air (dry, humid). The presence of photons does not affect the metrology nor the rate of SOA formation, increasing humidity seems to inhibit the SOA formation process somewhat. No HONO has been observed using both types of SOA as substrate for NO2 heterogeneous interaction under a variety of conditions despite the fact that the liquid SOA represented a reducing substrate for NO2 reduction.

Datenbankreferenzen (Englisch)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 99.0042-1