P. Beichert et Bj. Finlaysonpitts, KNUDSEN CELL STUDIES OF THE UPTAKE OF GASEOUS HNO3 AND OTHER OXIDES OF NITROGEN ON SOLID NACL - THE ROLE OF SURFACE-ADSORBED WATER, Journal of physical chemistry, 100(37), 1996, pp. 15218-15228
A newly designed and constructed Knudsen cell has been tested by measu
ring the reaction probability for gaseous N2O5 on 65% H2SO4/H2O at 210
-230 K to be 0.075 +/- 0.047 (2 sigma), which is in excellent agreemen
t with the literature value. This cell has been applied to the study o
f gaseous HNO3 reactions with NaCl small crystals and ground powders a
t 298 K. A rapid initial uptake of HNO3 and production of gaseous HCl
are observed when the crystals and powders are pumped but not heated p
rior to reaction. After this rapid initial reaction, a constant uptake
of HNO3 and formation of HCl is observed from which a reaction probab
ility of (1.4 +/- 0.6) x 10(-2) (2 sigma) is calculated. When possible
systematic errors (including uncertainties in the effective surface a
rea available for reaction) are taken into account, the overall uncert
ainty is estimated to be about a factor of 2. The measured reaction pr
obability is independent of the size or preparation of the salt crysta
ls as well as the number of layers of salt in the sample holder. This
reaction probability is in excellent agreement with results from the p
revious work of Rossi and co-workers(21,23) and Leu and co-workers(15)
using powders but significantly larger than that measured by Laux et
al.(1)2 using single crystals and an ultrahigh vacuum system. Prior he
ating of the salts while pumping decreased the extent of the initial r
apid reaction but did not affect the subsequent constant reaction upta
ke probability. Experiments on the reaction of HNO3 with NaCl crystals
that had been previously exposed to D2O to replace any surface-adsorb
ed H2O and on the reactions of DNO3 with NaCl show that under all expe
rimental conditions studied here, some water remains on the surface an
d plays a key role in the uptake of HNO3. We propose a new model for t
he reaction of HNO3 with NaCl powders in which HNO3 is taken up into s
trongly adsorbed water (SAW) on the salt. This SAW, for which there is
prior evidence in the literature,(34) appears likely to be held at de
fect sites on the powders. Acidification of this SAW leads to degassin
g of HCl due to dissolution of NaCl into the SAW from the underlying s
alt. As gaseous HNO3 continues to be taken up, HCl degasses and nitrat
e precipitates out as NaNO3. This model represents a fundamental chang
e in the description of the heterogeneous reactions of salt powders. T
he lower reaction probability for single crystals observed by Laux et
al.(12) is consistent with the lack of surface-adsorbed water on relat
ively defect-free single crystals. No uptake of the gases NO2, NO, HCl
, ClNO, ClNO2, or H2O was observed on the finely ground NaCl powder fr
om which an upper limit to the reaction probabilities for these gases
with NaCl of similar to 10(-5) was derived. The atmospheric implicatio
ns of this model are discussed.