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Abstract. In this laboratory study a multidiagnostic experimental approach including Fourier transform infrared (FTIR) absorption of 1 to 2 μm thick polycrystalline ice films, residual gas mass spectrometry (MS) and total pressure measurement were employed. Both amorphous HCl–H2O and crystalline HCl hexahydrate (HCl · 6H2O) have been investigated. After controlled doping with HCl and evaporation of excess H2O from the ice film, transmission FTIR of pure HCl · 6H2O films and use of calibrated mass spectrometry enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies, for example σ = (6.5 ± 1.9) × 10−19 cm2 molec−1 at 1635 cm−1. Two types of kinetic experiments on pure HCl · 6H2O have been performed under SFR conditions: (a) evaporation of pure HCl · 6H2O over a narrow T range after evaporation of excess H2O, and (b) observation of the phase transition from crystalline HCl · 6H2O to amorphous HCl–H2O under H2O-rich conditions at increasing T. The temperature dependence of the zero-order evaporation flux of HCl in pure HCl · 6H2O led to logJev molec cm−2 s−1 = (36.34 ± 3.20) – (80 810 ± 5800)/2.303 RT with R = 8.314 JK−1 mol−1, which turned out to be rate-limiting for evaporation. HCl · 6H2O has a significant intrinsic kinetic barrier to HCl evaporation of 15.1 kJ mol−1 in excess of the HCl sublimation enthalpy of 65.8 kJ mol−1 at 200 K but is kinetically unstable (metastable) at T ≥ 173 K. The atmospheric importance of HCl · 6H2O is questioned in view of its large nucleation barrier and its dependence on T and P(HCl) compared to the amorphous HCl–H2O phase at upper tropospheric–lower stratospheric (UT/LS) conditions.

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In this laboratory study, 1 to 2 μm thick polycrystalline ice films have been grown under stirred flow reactor (SFR) conditions and subsequently doped with metered amounts of HCl under static conditions. A multidiagnostic approach including FTIR absorption spectroscopy in transmission, residual gas mass spectrometry (MS) and total pressure measurement was employed. Depending on the growth protocol controlling both temperature and partial pressure of HCl (P_HCl), either amorphous HCl/H_2O or crystalline HCl hexahydrate (HCl · 6H_2O) have been obtained. After controlled doping with HCl and evaporation of excess H_2O from the ice film, transmission FTIR of pure HCl · 6H_2O films and use of calibrated residual gas MS enabled the measurement of differential (peak) IR cross sections at several mid-IR frequencies (σ = (6.5 ± 1.9) × 10^-19 cm² molec^-1 at 1635 cm^-1 as an example). Two types of kinetic experiments on pure HCl · 6H_2O have been performed under SFR conditions: (a) evaporation of HCl · 6H_2O under H_2O-poor conditions over a narrow T range, and (b) observation of the phase transition from crystalline HCl · 6H₂O to amorphous HCl/H_2O under H_2O-rich conditions at increasing T. The temperature dependence of the zero-order evaporation flux of HCl in pure HCl · 6H_2O monitored at 3426 cm^-1 led to log(J_ev) molec cm^-2s^-1= (36.34 ± 3.20) - (80 810 ± 5800)/2.303RT with R=8.312 JK⁻¹ mol^-1. HCl · 6H₂O has a significant intrinsic kinetic barrier to HCl evaporation of 15.1 kJ mol^-1 in excess of the HCl sublimation enthalpy of 65.8 kJ mol^-1 at 200 K but is kinetically unstable (metastable) at typical UT/LS conditions of HCl partial pressure (P(HCl)) and temperature. Water-rich HCl · 6H_2O undergoes a facile phase transition from crystalline to the amorphous/ supercooled/disordered state easily observable at T≥ 195 K under both static and SFR conditions. This corresponds to low P(HCl) in the neighborhood of 10^-7 Torr that also prevails at the Upper Troposphere/Lower Stratosphere (UT/LS). The atmospheric importance of HCl · 6H_2O is questioned in view of its nucleation barrier and its dependence on temperature and P(HCl) compared to the amorphous HCl/H_2O phase

The main aim of this study was to analyze dilute acid pretreatment for the Douglas fir wood in order to improve the efficiency of hydrolysis with an ultimate aim to produce bioethanol. Compositional analysis of the untreated Douglas fir biomass revealed the presence of 63.3 % carbohydrate of which 57.2 % was C6 sugars. The total lignin content was approximately 30 %. A partial fractional factorial design was opted for performing the pretreatment experiments employing sulfuric acid (H2SO4). Acid concentration, solids loading, residence time, reaction temperature, and particle size of feedstock were evaluated simultaneously for improving the enzymatic digestibility of Douglas fir biomass. Enzymatic saccharification of the pretreated biomass was done using a commercial cellulase preparation and the total reducing sugars liberated was monitored. Saccharification efficiency was correlated with the parameters and the best combination of parameters for obtaining feedstock suited for enzymatic saccharification was determined.