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Abstract

Nitrous acid formation in the troposphere is described in the light of the most recent advances
of knowledge on NOx chemistry on ground and aerosol surfaces. It is suggested to follow the
stoichiometry of the reaction 2 NO; + H20 -> HNO2 + HNO3 on all ambient surfaces except
those of carbonaceous particles. The influences of ambient humidity on aerosol (water uptake)
and ground surface area (surface to area indices) and on water availability for surface reactions
are considered. It is concluded that the state of the reacting H20 molecule, chemisorbed to the
surface or other, is determining: The ldnetics of the process obeys first order in N02
concentration and eidier pseudo-zeroth order (for low surface coverage with I-12_, using to: -
0.1 - 1 * 10-6 for the various surface types) or first order in water vapour concentration (for
water molecules available on the surface in multilayer adsorption, using km =- 1.36 * 10-20 cm4
moles1 s as determined in a laboratory experiment on a humid vessel wall). A reaction
mechanism is proposed. The scheme differentiates between 4 ground surface and 4 aerosol
surfaces types.
Predicted and observed nitrous acid formation rates from two urban sites are compared. For
this purpose, observed rates and their' error bars were derived with particular care. Despite
large uncertainties for both predicted and observed values, the agreement is good for both low
and high humidities (mean predicted/observed = 138 % for 18 nights in Mainz, 1986/87, and
132 % for 9 nights in Milano, 1994).