From: Subject: NITROCAT : Nitrous acid and its influence on the oxidation capacity of the atmosphere Date: Fri, 1 Sep 2006 16:51:34 +0200 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.sbf.admin.ch/htm/services/publikationen/international/frp/eu-abstracts/html/fp/fp5/5en99.0042-1.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2962 NITROCAT : Nitrous acid and its influence on the = oxidation capacity of the atmosphere

Swiss Participation in European Research Programmes

EU-Programme :	Environment, Global change

Project-Title :	NITROCAT : Nitrous acid and its influence on the = oxidation=20 capacity of the atmosphere
Keywords :	Heterogeneous chemistry; flame soot; HONO formation; secondary = aerosol;=20 NO2 uptake coefficient; reaction mechanism

Beneficiary :	Hubert Van den Bergh (hubert.vandenbergh@epfl.ch)
Organisation / Company :	EPF Lausanne D=E9partement de G=E9nie = Rural Laboratoire de=20 Pollution Air-Sol / LPAS DGR Ecublens 1015 Lausanne,=20 Switzerland
Contact Person :	Dr. Frank Kirchner (frank.kirchner@epfl.ch), Tel. +41 21 693 61 = 38
Project Duration :	01.03.2000 - 31.07.2002
Contribution BBW/OFES :	CHF 327'936.--
Project-Number :	OFES-No: 99.0042-1 ; EC-No: EVK2-1999-00025

Project Partners :	Coordinator: Bergische Universit=E4t, Wuppertal=20 (D)

Abstract:

HONO is the =93starter=94 for photochemical smog formation in an = urban polluted=20 atmosphere. Its mode of formation is still not known with = certainty. =20 The working hypothesis for HONO generation is that it is heterogeneously = formed=20 on the surface of enviornmental particulate such as combustion aerosol = or soot,=20 but also urban and rural interfaces such as ground cover. Previous = results=20 indicate a strong correlation between the type of combustion (lean vs. = rich) and=20 the HONO forming potential of soot rather than a significant influence = of the=20 type of fuel. Therefore, we have controlled the combustion = process that also improved the reproducibility of the = heterogeneous=20 kinetics. At EPFL we have extensively used two burners:

=B7 Co-flow device or = =93lamp=94 burner=20 leading to a diffusion flame which affords resettability and = reproducibility of=20 the combustion conditions, but which does not allow for the = determination of the=20 absolute fuel/oxygen mixing ratio (λ ). Hexane, decane, = toluene, Diesel=20 were used as fuels.

=B7 CAST (Combustion Aerosol = Standard)=20 Device invented by Dr. Lianpeng Jing of METAS and optimised for = generating soot=20 under stable conditions of known λ . Hexane was the only fuel = used.

NO₂ interaction = with=20 flame soot generated in the laboratory.

The following general conclusions may be drawn from our studies as = they=20 result from the heterogeneous interaction of oxidized nitrogen with a = reducing=20 substrate (soot):

(A) = HONO=20 is formed in a reduction-oxidation reaction on ALL types of soot we have = investigated resulting from the heterogeneous reaction of=20 NO₂ with soot. In the case where we do = not=20 observe HONO it decomposes on flame soot obtained under lean combustion=20 conditions presumably at active functional groups of =93black=94 soot = generated in=20 the co-flow device or soot at λ =3D 0.09 or 0.16 produced in the = CAST. On=20 soot from a rich flame HONO does not decompose and is observed in high = yields=20 approaching 100% of the reacted NO₂.

(B) = HONO=20 also results from the interaction of HNO₃ on = every=20 type of flame soot investigated at reaction probabilities that are up to = an=20 order of magnitude larger than for = NO₂. The=20 reaction HNO₃ + NO →=20 NO₂ + HONO is slow and necessitates = HNO₃-coverages of the order of 30% on flame=20 soot. It most likely is unimportant in the troposphere.

(C) = The=20 initial uptake coefficient λ ₀ depends = on the=20 reactant concentration in a process that is sensitive to molecular = saturation=20 but insensitive to sample mass, hence dependent upon the geometric = surface=20 area. Pore diffusion theory describing the diffusion of gas in the = interstices of the sample is inappropriate for soot as it leads to = unreasonable=20 results.

(D) = The=20 HONO-forming principle may be dissociated from the black carbon matrix = using=20 extraction of soot in THF, but not in benzene as a solvent. It may = be=20 transferred to an appropriate support (reverse phase silica-gel) where = it=20 continues to reduce NO₂ to HONO.

(E) = =20 Owing to the fact that HONO has been observed in the field at noontime = when=20 photolysis rates are highest, the working hypothesis that HONO may be = formed on=20 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=20 10⁶ cm^-3. Typical ozone and VOC = concentrations were=20 50-100 to 1000 ppm, respectively. Relative humidity is required in = this=20 case because OH is the active oxidizer; (b) Limonene /ozone/air (dry,=20 humid). The presence of photons does not affect the metrology nor = the rate=20 of SOA formation, increasing humidity seems to inhibit the SOA formation = process=20 somewhat. No HONO has been observed using both types of SOA as = substrate=20 for NO₂ heterogeneous interaction under a = variety of=20 conditions despite the fact that the liquid SOA represented a reducing = substrate=20 for NO₂ reduction.

Reporting Date :=20 31.07.2002