Aerosol Chamber Study of Optical Constants and N₂O₅ Uptake on Supercooled H₂SO₄/H₂O/HNO₃ Solution Droplets at Polar Stratospheric Cloud Temperatures

Abstract

The mechanism of the formation of supercooled ternary H(2)SO(4)/H(2)O/HNO(3) solution (STS) droplets in the polar winter stratosphere, i.e., the uptake of nitric acid and water onto background sulfate aerosols at T < 195 K, was successfully mimicked during a simulation experiment at the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. Supercooled sulfuric acid droplets, acting as background aerosol, were added to the cooled AIDA vessel at T = 193.6 K, followed by the addition of ozone and nitrogen dioxide. N(2)O(5), the product of the gas phase reaction between O(3) and NO(2), was then hydrolyzed in the liquid phase with an uptake coefficient gamma(N(2)O(5)). From this experiment, a series of FTIR extinction spectra of STS droplets was obtained, covering a broad range of different STS compositions. This infrared spectra sequence was used for a quantitative test of the accuracy of published infrared optical constants for STS aerosols, needed, for example, as input in remote sensing applications. The present findings indicate that the implementation of a mixing rule approach, i.e., calculating the refractive indices of ternary H(2)SO(4)/H(2)O/HNO(3) solution droplets based on accurate reference data sets for the two binary H(2)SO(4)/H(2)O and HNO(3)/H(2)O systems, is justified. Additional model calculations revealed that the uptake coefficient gamma(N(2)O(5)) on STS aerosols strongly decreases with increasing nitrate concentration in the particles, demonstrating that this so-called nitrate effect, already well-established from uptake experiments conducted at room temperature, is also dominant at stratospheric temperatures.

Mass Accommodation and Chemical Reactions at Gas−Liquid Interfaces

“…2,146 Most recently, Wagner et al presented results from an aerosol chamber study designed to determine the uptake coefficient of N 2 O 5 on supercooled ternary H 2 SO 4 /H 2 O/HNO 3 solutions at 193.6 K, characteristic of polar winter stratospheric conditions. 148 They deduced uptake coefficients of ∼0.04 for nitric acid/sulfuric acid concentrations of 14:28, 18:25, and 31:17 wt %, respectively, with measured values falling off to 0.016 at 45:6 wt %, as expected from saturation due to the common (nitrate) ion effect. 148 Hanson and co-workers have studied the very important hydrolysis reaction of the reservoir species BrONO 2 on sulfuric acid surfaces using both wetted wall and aerosol flow reactor techniques, 23,149,150 demonstrating that unlike ClONO 2 reactive uptake, γ for BrONO 2 remains very large (∼0.8) up to acid concentrations of over 70 wt % before falling off at higher acid concentrations.…”

“…148 They deduced uptake coefficients of ∼0.04 for nitric acid/sulfuric acid concentrations of 14:28, 18:25, and 31:17 wt %, respectively, with measured values falling off to 0.016 at 45:6 wt %, as expected from saturation due to the common (nitrate) ion effect. 148 Hanson and co-workers have studied the very important hydrolysis reaction of the reservoir species BrONO 2 on sulfuric acid surfaces using both wetted wall and aerosol flow reactor techniques, 23,149,150 demonstrating that unlike ClONO 2 reactive uptake, γ for BrONO 2 remains very large (∼0.8) up to acid concentrations of over 70 wt % before falling off at higher acid concentrations. Hanson has fit the data from these studies to a model based on eq 16 in section 2, obtaining a value of R ) 0.80.…”

“…The N 2 O 5 hydrolysis reactive uptake coefficient remains relatively large (∼0.05−0.15) over the full range of temperatures and acid concentrations studied, while γ for ClONO 2 reactive uptake starts at ∼0.2 for 40 wt % acid at lower temperatures and decreases to as low as ∼0.0001 for 75 wt % acid. These combined data sets were used to produce a phenomenological model of these key hydrolysis reactions that can be used to predict their rates for the full range of stratospheric conditions. , Most recently, Wagner et al presented results from an aerosol chamber study designed to determine the uptake coefficient of N 2 O 5 on supercooled ternary H 2 SO 4 /H 2 O/HNO 3 solutions at 193.6 K, characteristic of polar winter stratospheric conditions . They deduced uptake coefficients of ∼0.04 for nitric acid/sulfuric acid concentrations of 14:28, 18:25, and 31:17 wt %, respectively, with measured values falling off to 0.016 at 45:6 wt %, as expected from saturation due to the common (nitrate) ion effect …”

Production and use of 13N labeled N2O5 to determine gas–aerosol interaction kinetics

“…It is characterized by the uptake coefficient , defined as the probability that a gas kinetic collision of a molecule leads to its uptake at the interface. Studies have been conducted on various types of aerosol particles such as NaCl/sea salt aerosols [6], ammonium sulfate or nitrate particles [7][8][9], organic aerosols [10], mineral dust [2,11] and soot [12]. Various detection methods have been used; Cavity RingDown Spectroscopy (CRDS) [13] and Chemical Ionization Mass Spectrometry (CIMS) [14] have been particularly popular.…”

Infrared Spectrum of Nitric Acid Dihydrate: Influence of Particle Shape

“…In addition to the different spectral signatures of α- and β-NAD, we want to emphasize the effect of particle shape on band positions and intensities in infrared spectra of NAD. In our present study, as part of ongoing laboratory measurements of PSC particles in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe, − we have generated α-NAD particles in two different ways: (1) by shock freezing a HNO 3 /H 2 O gas mixture and (2) by homogeneous nucleation of slowly evaporating supercooled HNO 3 /H 2 O solution droplets at constant temperature on a time scale of several hours. The recorded FTIR extinction spectra of the NAD particles were analyzed by Mie theory as well as T-matrix calculations assuming spheroidal particle shapes with the aspect ratio as adjustable parameter.…”