Abstract
Abstract. In this laboratory study, H2O ice films of 1 to 2 μm thickness have been used as surrogates for ice particles at atmospherically relevant conditions in a stirred flow reactor (SFR) to measure the kinetics of evaporation and condensation of HCl and H2O on crystalline and amorphous HCl hydrates. A multidiagnostic approach has been employed using Fourier transform infrared spectroscopy (FTIR) absorption in transmission to monitor the condensed phase and residual gas mass spectrometry (MS) for the gas phase. An average stoichiometric ratio of H2O : HCl = 5.8 ± 0.7 has been measured for HCl · 6H2O, and a mass balance ratio between HCl adsorbed onto ice and the quantity of HCl measured using FTIR absorption (Nin – Nesc – Nads) / NFTIR = 1.18 ± 0.12 has been obtained. The rate of evaporation Rev(HCl) for crystalline HCl hexahydrate (HCl · 6H2O) films and amorphous HCl / H2O mixtures has been found to be lower by a factor of 10 to 250 compared to Rev(H2O) in the overlapping temperature range 175 to 190 K. Variations of the accommodation coefficient α(HCl) on pure HCl · 6H2O up to a factor of 10 at nominally identical conditions have been observed. The kinetics (α, Rev) are thermochemically consistent with the corresponding equilibrium vapour pressure. In addition, we propose an extension of the HCl / H2O phase diagram of crystalline HCl · 6H2O based on the analysis of deconvoluted FTIR spectra of samples outside its known existence area. A brief evaluation of the atmospheric importance of both condensed phases – amorphous HCl / H2O and crystalline HCl · 6H2O – is performed in favour of the amorphous phase.
Highlights
After the discovery of the ozone hole over Antarctica, the importance of heterogeneous chemistry on frozen surfaces had started to be recognized (Solomon et al, 1986) as well as the role of polar stratospheric clouds (PSCs) as substrates on whose surfaces heterogeneous reactions may take place.PSCs are formed in the stratosphere during polar night when the temperatures drop to as low as 183 K to allow for cloud formation even in the dry stratosphere (Peter, 1997)
Once the selected substrate, amHCl or hexahydrate phase (HH), has been grown according to the corresponding protocol and characterized using its Fourier transform infrared spectroscopy (FTIR) absorption spectrum, the film is set to a chosen temperature
The cumulative dose on the ice sample has been measured as the difference between the admitted dose and the number of molecules effused from the reactor in the aftermath of each pulse
Summary
After the discovery of the ozone hole over Antarctica, the importance of heterogeneous chemistry on frozen surfaces had started to be recognized (Solomon et al, 1986) as well as the role of polar stratospheric clouds (PSCs) as substrates on whose surfaces heterogeneous reactions may take place.PSCs are formed in the stratosphere during polar night when the temperatures drop to as low as 183 K to allow for cloud formation even in the dry stratosphere (Peter, 1997). After the discovery of the ozone hole over Antarctica, the importance of heterogeneous chemistry on frozen surfaces had started to be recognized (Solomon et al, 1986) as well as the role of polar stratospheric clouds (PSCs) as substrates on whose surfaces heterogeneous reactions may take place. Heterogeneous reactions occurring on PSCs convert the major unreactive chlorine reservoir compounds, ClONO2 and HCl, into molecular chlorine, which is rapidly photolysed into atomic chlorine. Reaction (R1) is the most important chlorine-activating reaction in the polar stratosphere because it converts two moles of unreactive chlorine compounds into two moles of atomic chlorine after photolysis in an efficient heterogeneous reaction (Seinfeld and Pandis, 2006): ClONO2(g) + HCl(s) → Cl2(g) + HNO3(s). The Cl2 released into the gas phase from Reaction (R1) rapidly photolyses into free Cl atoms, which establish a rapid cycle of O3 destruction.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
