Salt Pan Preparation
To prepare a salt pan, the amount of the desired salt mixture was first completely dissolved in bidistilled water and afterwards dried in an oven (flushed with zero air) on a Teflon sheet at 50°C for at least 70 h. Depending on the stickiness, the resulting salt crust
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was milled in a ball mill (Retsch MM 2, Haan, Germany), ground in a (household salt mill) or spread untreated on a circular 0.3 m2 Teflon sheet mounted in the middle of the chamber. In this way 13 samples were prepared with a sodium chloride (NaCl) bulk doped with various constituents, such as FeIII chloride hexahydrate (FeCl3·6H2O), magnesium chloride (MgCl2), sodium bromide (NaBr), sodium sulfate (Na2SO4), catechol (C6H6O2), oxalic acid (H2C2O4), or sodium oxalate (Na2C2O4) and irradiated. After every experiment, the pH was estimated by adding 3 mL bidistilled water to 2 g of the salt crust to obtain a saturated solution from which the pH was determined by pH indicator strips (Merck). For more details on the investigated salt mixtures see section 4.2.
Aerosol Preparation and Production al. 1967). In case of Fe2O3, 17 mg were stirred into 100 mL of artificial seawater, forming a suspension. For some samples, the pH was adjusted by adding HCl (Sigma-Aldrich ACS, 37 %) in order to promote the iron dissolution and the associated FeIII-Cl complex formation (see section 1.4.1). The prepared solutions were nebulized with an ultrasonic nebulizer (Quick Ohm QUV-HEV FT25/16-A, 35 W, 1.63 MHz) generating droplets in the µm range, that quickly come into equilibrium with the surrounding and evaporate to a saturated sea-salt solution (containing Fe2O3 agglomerates when applying Sicotrans Orange). The resulting particle number size distributions showed maxima between 290–
480 nm (Figure 1.5), depending on the composition of the nebulized solution. A starting RH of 30–40 % was adjusted in the chamber before injecting the aerosol to avoid a crystallization of the saline aerosol (Siekmann 2008). The injection took typically 30–60 minutes and (in order to avoid a dripping of the condensed droplets into the chamber) a heated transfer tube (made of copper) was applied. For the FeCl3 and the corresponding blank samples, the impact of gaseous pollutants O3, NO2 (Rießner Gase, 104 vpm NO2
with a purity of 98 % in synthetic air) and SO2 was investigated (Rießner Gase, 0.99 % SO2 with a purity of 99.98 % in N2 with a purity of 99.999 %). In a further experimental series, suspensions of Fe2O3 and Aerosil 200 (Evonik Industries, specific surface = 200 m2 g–1) in deionized water were nebulized and exposed to various amounts of evaporated HCl (Sigma-Aldrich, ACS, 37 %).
1.3.3 Instrumentation
Gas Analyzers
The NOX and O3 concentrations in the chamber air were continuously monitored by chemiluminescence gas analyzers (EcoPhysics, CLD 88p, coupled with a photolytic converter, PLC 860, for NO and NOX, and UPK 8001 for O3). The UPK 8001 measures O3 based on its reaction with ethene resulting in exited formaldehyde and the emitted photons are detected. The O3 analyzer was calibrated in parallel by an absorption measurement at 254 nm in a 10 cm cuvette with zero air in the reference channel in an Uvikon XL. The calibration of the EcoPhysics analyzer and its converter efficiency was
13 performed by gas-phase titration of NO with O3. The instruments are described in detail in Bleicher (2012).
Aerosol Measurement
During the experiments, the aerosol number size distributions were monitored by an electrostatic classifier (TSI, 3071) in combination with a bipolar neutralizer (85Kr) and a condensation nucleus counter (TSI, 3020). Scanning and data evaluation was performed by a custom written software from Heinz-Ulrich Krüger (Balzer 2012).
Having passed the neutralizer, the particles exhibit a known bipolar charge distribution.
With increasing particle size, it becomes more probable that the particles carry multiple charges (2e, 3e, etc.). Assuming the charge equilibrium according to Boltzmann, the fraction of particles carrying up to two elementary charges can be estimated by approximating the charge distributions with a logarithmic distribution of particle sizes from 1 to 1000 nm (Wiedensohler 1988):
(1.2)
Here, ai(N) are approximation coefficients listed in Wiedensohler 1988, N is the number of elementary charge units on a particle and DP is the particle mobility diameter. Equation 1.2 is valid for the size ranges from 1 nm to 1000 nm for N = –1, 0, 1 and for the size ranges from 20 nm to 1000 nm for nP = –2, 2. Particles smaller than 20 nm carry mostly one elementary charge, whereas for particles larger than 70 nm a triple charge becomes probable. The fraction of triply and higher charged particles can be calculated after Gunn and Woessner 1956:
(1.3)
where e = elementary charge, = dielectric constant, k = Boltzmann’s constant, T = temperature, cI± = ion concentration, and ZI± = ion mobility. Equations 1.2 and 1.3 are used for the multiple charge correction in our software.
Within the electrostatic classifier, a quasi-monodisperse particle size distribution is obtained based on the different electrical mobilities of the charged and former polydisperse aerosol. The classifier consists of two concentric electrodes whose voltage can be adjusted in order to scan through various electrical mobilities. In dependence of the diameter and the charge, the electrical mobility is defined as
(1.4)
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where ZP is the electrical mobility of the particle, nP is the particle charge in elementary units, is the viscosity of air and C is the slip correction factor (Liu and Pui 1975). The slip correction considers the mean free path of air molecules, which is not negligible for particles smaller than 10 µm. In practice, the electrical mobility selection includes all aerosol particles whose mobility lies within a certain narrow range Zp ± , typically described by a transfer function which ideally has a triangular shape (maximum at Zp).
The half-width of the transfer function is given by
(1.5)
where qa is the aerosol flow rate, qs the sampling flow rate, r1 the outer radius of the classifier center rod, r2 the inner radius of the classifier housing, L the distance between the mid-planes of the classifier entrance slit and sampling slit and V the classifier center rod voltage (Knutson and Whitby 1975). Figure 1.5 illustrate typical, multiple charge corrected number size distributions of several aerosol types applied. The origin of these more or less pronounced bi- or trimodal distributions is probably the laminar and turbulent coagulation of droplets, especially at high precursor temperatures, droplet number concentrations and carrier gas flow rates (Wang et al. 2008) that apply to the nebulizer used.
Additionally, the generated aerosol particles were sampled by a Sioutas cascade impactor (SKC, aerodynamic diameter ranges: >2.5 µm, 1–2.5 µm, 0.5–1 µm, 0.25–0.5 µm and
<0.25 µm; Misra et al. 2002) and subsequently analyzed by SEM-EDX (Scanning-Electron-Microscope with an Energy-Dispersive X-ray detector; Lohninger and Ofner 2014). The resulting images demonstrate the small particle size of the iron oxide powder, forming agglomerates during nebulization of the suspension in water (Figure 1.6a) and the composition of sea-salt particles including dissolved iron species (Figure 1.6b).
15 Figure 1.5: Typical, multiple-charge corrected number size distributions for aerosol particles obtained by the nebulization of artificial seawater (art.sea.), iron-containing (FeCl3 or Fe2O3) art.
sea. mixtures and pure suspensions of Fe2O3 in water. Adopted and merged from Wittmer et al.
(2015b) and Wittmer and Zetzsch (2015).
Figure 1.6: SEM (-EDX) images of the pure iron oxide sample (a) and the FeCl3 doped artificial sea-salt sample (b). Color coding: NaCl – blue, CaSO4 – green, MgCl2 – yellow, KCl – red, FeCl3
– turquoise. Image adopted and modified from Wittmer et al. (2015b).
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