1. Determination of uptake coefficients ClO radicals with surfaces of sea
salts depend on the ozone concentration
I. I. Morozov, E.S. Vasiliev, and G.V. Karpov,
Semenov Institute of Chemical Physics, Russian Academy of Sciences,
119991, Moscow, Russia,
Experimental Setup
Sea salt aerosols represent an unlimited halogen reservoir. Together with salt deposits on shelf ice they are considered the most likely source of reactive halogens in the Arctic. Heterogeneous reactions on sea salt aerosols or deposits are therefore suspected to be a potential source of gas phase halogen compounds. The possible source mechanism for reactive halogen compound such as ClO in the polar boundary layer or by salt flats is the so called “Bromine explosion”. ClO has been detected at a concentration of up to 1,5 pptv simultaneously with BrO in the area of Great Salt Lake in Utah (Stutz J. et al., 2002). On the other hand, heterogeneous reactivity of HOCl on solid KBr at ambient temperature reveal the formation of Br- and Cl- containing reaction products such as BrCl and Cl2. The presence of adsorbed species such as Br2 are crucial for the kinetics of the reaction of HOCl with solid KBr substrates (Santschi Ch. Et al., 2005). Photolysis of the reaction products (BrCl and Cl2) with subsequent reaction with ozone lead to the chlorine oxide formation.
In this work the interaction of ClO with solid NaCl, NaBr and KBr salt has been investigated. It has been studied on substrates such as thin films sprayed on Pyrex high temperature substrate. The γ values from experiment were correspond to first order uptake kinetics throughout the concentration range.
Introduction
Experimental Setup
Results
Experimental determination of uptake coefficient
Uptake coefficients were calculated from the first-order decay of the radical signal as a function of the injector
position over the salt crystallites. The first-order loss-rate of a gaseous species may be expressed in the form:
-d [ClO]/dt = /4**A/V*[ClO] (1)
ClO + wall -> products
and k = /4**A/V;
where the uptake coefficient  is the parameter to be determined, N is the concentration of a gaseous species,  is the average molecular velocity, A is the reaction surface area and V is the volume of reactor.
The loss-rate parameter k is measured in experiment.
The average velocity may be expressed in the usual form:
 = sqrt(8/*R*T/M) (2)
For the used cylindrical reactor (R=1.2 cm) with central tube
(r=0.55 cm) which surface coated salt crystallites: A = 2**r*L,
V =  *(R2-r2)*L,
A/V = cm2/ cm3.
Table 1. Dependence of ClO uptake coefficient ku on KBr surface
on ozone concentration at T = 292 K.
ku / [O3]/1013 molecule cm-3
7, ,45
9, ,92
7, ,64
6, ,18
6, ,66
5, ,7
7, ,74
2, ,73
1, ,54
2, ,1
0, ,16
0, ,22
It can be assumed that a strong ku dependence on ozone concentration exists also at lower O3 concentrations typical
for real atmospheric altitudes.
Summary
Mass Spectrum of the ClO Flow in the Absence and Presence of KBr Crystallites
The main reaction product is HCl. Small quantities of HOCl and CO2 have been detected
ClO uptake coefficients on the solid NaCl, NaBr and KBr salts were investigated. There is no big difference in uptake coefficients for salts surfaces at room temperature.
Using mass-spectrometry method for investigation of ClO and KBr reaction there was not observed gas spices containing Br atoms. Main product of heterogeneous reactions is HCl. Small amount of HOCl was observed also.
References
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Stutz J., Ackermann R., Fast J.D. and Barrie L., Geophys. Res. Lett. 29, 1380 (2002).
Acknowledgment
The work was supported by RFBR