Formation of Nitrous Acid: Parameterisation and comparison with observations

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)