The heterogeneous chemical kinetics of NO3 on atmospheric mineral dust surrogates

Abstract

Uptake experiments of NO₃ on mineral dust powder were carried out under continuous molecular flow conditions at 298 ± 2 K using the thermal decomposition of N₂O₅ as NO₃ source. In situ laser detection using resonance enhanced multiphoton ionization (REMPI) to specifically detect NO₂ and NO in the presence of N₂O₅, NO₃ and HNO₃ was employed in addition to beam-sampling mass spectrometry. At [NO₃] = (7.0 ± 1.0) × 10¹¹ cm⁻³ we found a steady state uptake coefficient γ_ss ranging from (3.4 ± 1.6) × 10⁻² for natural limestone to (0.12 ± 0.08) for Saharan Dust with γ_ss decreasing as [NO₃] increased. NO₃ adsorbed on mineral dust leads to uptake of NO₂ in an Eley–Rideal mechanism that usually is not taken up in the absence of NO₃. The disappearance of NO₃ was in part accompanied by the formation of N₂O₅ and HNO₃ in the presence of NO₂. NO₃ uptake performed on small amounts of Kaolinite and CaCO₃ leads to formation of some N₂O₅ according to NO_3(ads) + NO_2(g) → N₂O_5(ads) → N₂O_5(g). Slow formation of gas phase HNO₃ on Kaolinite, CaCO₃, Arizona Test Dust and natural limestone has also been observed and is clearly related to the presence of adsorbed water involved in the heterogeneous hydrolysis of N₂O_5(ads).