The interaction of N<sub>2</sub>O<sub>5</sub> with mineral dust: aerosol flow tube and Knudsen reactor studies

Abstract

Abstract. The interaction of mineral dust with N 2 O 5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO 3 ) were used to derive reactive uptake coefficients (γ ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10 −2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ (SDCV)=(3.7±1.2)×10 −2 , γ (ATD)=(2.2±0.8)×10 −2 and γ (CaCO 3 )=(5±2)×10 −2 . The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N 2 O 5 .

Heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with airborne TiO<sub>2</sub> particles and its implication for stratospheric particle injection

“…The kinetics of the uptake of N 2 O 5 onto desert dust particles and their surrogates (e.g., Arizona Test Dust, illite) has been reported by several previous studies (Karagulian et al, 2006;Mogili et al, 2006;Seisel et al, 2005;Tang et al, 2010Tang et al, , 2012Tang et al, , 2014Wagner et al, 2008Wagner et al, , 2009). In addition, Seisel et al (2005) observed the formation of nitrate on mineral dust particles due to the uptake of N 2 O 5 using diffuse reflectance FTIR (Fourier transform infrared spectroscopy), and Tang et al (2012) further confirmed that the yield of nitrate is ∼ 2 (as expected from Reaction R1) within the experimental uncertainty, and that the formation of NO 2 is negligible.…”

“…The glass reactor used in this work has a volume of ∼ 30 cm 3 (with a length of 10 cm and an inner diameter of 2 cm), leading to a residence time of ∼ 3 s at a flow rate of ∼ 500 mL min −1 . At atmospheric pressure and 90 • C, the lifetime of N 2 O 5 with respect to thermal decomposition (Reaction R4b) is around ∼ 0.05 s and the lifetime of NO 3 radicals due to the titration by 1 ppmv NO (Reaction R5) is ∼ 0.002 s, as detailed by Wagner et al (2008). The residence time in the reactor is much longer than the lifetime of N 2 O 5 and NO 3 under our experimental condition, ensuring that all the N 2 O 5 should be decomposed and then titrated by NO.…”

“…the flow was laminar. The entrance length required to develop the laminar flow and the mixing length were calculated to be ∼ 8 and ∼ 13 cm, respectively (Keyser, 1984), using a diffusion coefficient of 0.085 cm 2 s −1 for N 2 O 5 (Wagner et al, 2008). For all the experiments, only the middle part of the flow tube (30-80 cm) where the gases have been well mixed and the laminar flow has been fully developed was used to measure the uptake kinetics.…”

“…The residence time in the reactor is much longer than the lifetime of N 2 O 5 and NO 3 under our experimental condition, ensuring that all the N 2 O 5 should be decomposed and then titrated by NO. This scheme has been suggested as an absolute method to calibrate other N 2 O 5 detection methods (e.g., CIMS) (Fahey et al, 1985), and is also widely used to study the heterogeneous reactions of N 2 O 5 with aerosol particles (e.g., Wagner et al, 2008). The NO concentration was measured by a chemiluminescence-based nitrogen oxides analyser (Model 200E, Teledyne Instruments, USA), which has a sampling flow rate of 500 mL min −1 (±10 %) and a detection limit of ∼ 0.5 ppbv with a time resolution of 1 min.…”

“…where D(N 2 O 5 ) is the diffusion coefficient of N 2 O 5 (0.085 cm 2 s −1 at 1 atm and 296 K; Wagner et al, 2008) and d is the diameter of the particle (cm), which is assumed to be the mobility diameter measured by the SMPS. For polydispersed aerosol particles, e.g., TiO 2 particles used in this work, Kn can be calculated by…”