Hydrolysis of N2O5 on sub-micron sulfate aerosols

Abstract

The kinetics of reactive uptake of gaseous N₂O₅ on sub-micron aerosol particles composed of aqueous ammonium sulfate, ammonium hydrogensulfate and sodium nitrate has been investigated. Uptake was measured in a laminar flow reactor, coupled with a differential mobility analyser (DMA) to obtain the aerosol size distribution, with N₂O₅ detection using NO chemiluminescence. FTIR spectroscopy was used to obtain information about the composition and water content of the aerosol particles under the conditions used in the kinetic measurements. The aerosols were generated by the nebulisation of aqueous salt solutions. The uptake coefficient on the sulfate salts was in the range γ = 0.0015 to 0.033 depending on temperature, humidity and phase of the aerosol. On sodium nitrate aerosols the values were much lower, γ < 0.001, confirming the inhibition of N₂O₅ hydrolysis by nitrate ions. At high humidity (>50% r.h.) the uptake coefficient on liquid sulfate aerosols is independent of water content, but at lower humidity, especially below the efflorescence point, the reactivity of the aerosol declines, correlating with the lower water content. The lower uptake rate on solid aerosols may be due to limitations imposed by the liquid volume in the particles. Uptake on sulfate aerosols showed a negative temperature dependence at T > 290 K but no significant temperature dependence at lower temperatures. The results are generally consistent with previous models of N₂O₅ hydrolysis where the reactive intermediate is NO₂⁺ produced by autoionisation of nitrogen pentoxide in the condensed phase.