Experimental Study for the Identification of the Nascent Product of OH Heterogeneous Reaction with NaCl using Chemical Ionization Mass Spectrometry

An experimental study on the nascent product of the OH heterogeneous reaction with NaCl was performed under dry and wet conditions using a bead-filled flow tube system coupled to a high-pressure chemical ionization mass spectrometer. The ozone concentration in the flow tube for the atomic hydrogen removal was varied in order to control the conversion reaction of molecular chlorine into HCl for the identification of the nascent product. The mass spectrometric observation was that the O3 introduction reduced the concentration HCl, while it increased the concentration of Cl2 and ClO. Based on the experimental results, we suggest that the nascent product of the titled reaction is gaseous Cl2, which is followed by fast conversion into HCl in presence of H. No significant difference in the concentration profile between under dry and wet (RH = 2%) conditions was observed.


Introduction
As a key oxidant, the hydroxyl radical (OH) is involved in a variety of reactions in the atmosphere.Although the main sinks of OH in the atmosphere are gas-phase reactions, such as the reactions with carbon monoxide, methane, or biogenic hydrocarbons, heterogeneous reactions have been suggested as additional sinks in order to explain the overestimation of OH concentration in the atmospheric models.][3][4] However, recently it is reported that the heterogeneous reaction of OH with sea-salt is enhanced under wet conditions, 5,6 which is a probable case in the coastal area.
The OH reaction on sea-salt aerosols plays an important role in the production of chlorine, which is responsible for further reactions in the troposphere and the stratosphere, for example, ozone destruction.Since atomic chlorine (Cl) is an extremely powerful oxidant with the reactivity one or two order-of-magnitude higher than that of OH, 7 considerable attention has been paid to the heterogeneous reaction of OH on the sea-salt, such as NaCl, MgCl 2 , and CaCl 2 .Recently, we observed chlorine-containing ions, which are HClF -(55 amu) and SF 5 Cl − (162 amu), in the flow tube study on the OH heterogeneous reaction with NaCl using chemical ionization mass spectrometry (CIMS).Those ions are likely the products of the ion transfer reaction from to HCl and atom exchange reaction between and HCl, respctivley (see Reaction 1 and 2). 8eaction 1) (Reaction 2) However, the origin of HCl is not yet certain.It is also possible that the OH heterogeneous reaction with NaCl induces the release of HCl directly from the OH uptake surface.In addition, molecular chlorine (Cl 2 ) should not be excluded as the product of the reaction since HCl can be formed from the irreversible conversion of Cl 2 right after release from the OH uptake surface in the presence of atomic hydrogen (H), with the extremely high reaction constant (2.0 × 10 −11 cm 3 molecule −1 s −1 ). 9 Cl 2 + H → HCl + Cl (Reaction 3) A considerable amount of atomic hydrogen may be present in the flow tube as a residue, even after the gas phase reaction of H and O 2 that were supplied as the OH production sources.Therefore, identification of the nascent product of the OH heterogeneous reaction with NaCl by On the other hand, the removal of HCl and Cl 2 by O 3 is negligible; the kinetic constants for O 3 + HCl and O 3 + Cl 2 are 4.7 × 10 −24 and 3.0 × 10 −28 cm 3 molecule −1 s −1 , respectively. 11,12n this study, we monitored HCl, Cl 2 , and ClO as introducing O 3 into the flow tube system in which the OH heterogeneous reaction with NaCl was taking place.Hydrogen atoms remaining even after the OH production was scavenged through O 3 purging, which removed the possibility of the HCl production through the reaction with Cl 2 .The mass spectrometric observation of the ion species provides us information as to what the nascent product of the heterogeneous reaction of OH with NaCl is, which is important in the atmosphere in the coastal area.

Experimental
Experimental studies were performed under flow conditions at 100 Torr and room temperature using an experimental setup described elsewhere. 5,6There were two modifications made in order to enhance the sensitivity: (1)  an increased number of beads (80 instead of 60) that was used to pack the reactor tubes, (2) no pre-pumping before the chemical ionization (CI) region, which allows all species in the flow tube to proceed to the CI region without any loss.The flow-tube was coupled to a high-pressure chemical ionization mass spectrometer equipped with a quadrupole analyzer and an electron multiplier.
The reaction H + O 2 + M was used to produce OH radical, which induced heterogeneous reactions on the NaCl surface at 100 Torr.A molecular hydrogen flow was diluted by helium (Airgas, UHP), and was then discharged inside a Beenaker microwave discharge cavity operating at 30 W to produce hydrogen atoms which then react with O 2 (Matheson Tri Gas, 99.5%).Ozone was generated from O 2 with an ozonizer (ORTEC), trapped in silica gel at −70 o C, and then was evaporized at room temperature into a glass bulb to prepare a O 3 /He mixture.The mixture of Cl 2 /He was prepared by dilution of He and gaseous Cl 2 (Matheson Tri Gas, 99 %).
No reference tube was used as the uptake coefficient was not calculated, which is based on the additivity of kinetic resistances. 13The experimental setup with the gas flow configuration used in this work is schematically described in Figure 1.

Results and Discussion
To verify the reproducibility of Reaction 3 and Reaction 4 in our system, a gaseous flow of Cl 2 was introduced into the empty flow tube as well as with H 2 , both of which were diluted in a carrier gas of He/N 2 .The concentration of H was assumed to be proportional to the concentration of H 2 that passed through the microwave discharge.The mass peaks derived from HCl, i.e., HClF − (m/z 55 amu) and SF 5 Cl − (m/z 162 amu), were not observed until H was produced upon ignition of the microwave discharge.
As shown in Figure 2(a), the concentrations of HCl and Cl 2 are inversely proportional to each other.The inhibition of Reaction 3 by Reaction 4 was also confirmed by introducing O 3 .As the amount of O 3 introduced increased, HCl produced from the reaction with Cl 2 decreased, as can be seen in Figure 2(b).This is because O 3 depleted H that could react with Cl 2 .However, when H atom production is inhibited by turning off the microwave discharger, the concentration of Cl 2 did not change even when O 3 was introduced, which confirmed negligible interference of Cl 2 by O 3 itself.Gaseous HCl, Cl 2 , and ClO were observed under dry and wet conditions, when OH was present at the initial concentration of approximately 10 12 molecule cm −3 .As discussed previously in detail, 5,6 OH radical in the gas phase reacts with NaCl on the surface of glass beads.Although the mechanism for this reaction is still under discussion, Cl 2 is considered as the most probable product of the reaction. 14,15owever, the observations made in the current study revealed that the dominant species of chlorine compounds was HCl, while only a trace amount of Cl 2 was detected.More care must be taken to conclude that HCl was the dominant reaction product.A nascent chlorine molecule that was generated from the above-mentioned reaction might have been transformed immediately into HCl, HOCl, and ClO in the presence of H, OH, and O through some unidentified heterogeneous reactions.
It should also be noted that considerable amounts of H, OH, and O present in the system originated from the OH production.
Chlorine monoxide (ClO) was also produced from Reaction 5 (k = 4.3 × 10 −14 cm 3 molecule −1 s −1 ). 15 2 + O → ClO + Cl (Reaction 5) Atomic chlorine was not observed, due to its reaction with H 2 , OH, and HOCl (k = 1.8 × 10 −14 , 7.1 × 10 −16 , and 1.2 × 10 −12 cm 3 molecule −1 s −1 , respectively).16,17 Figure 3 shows that the HCl concentration monotonically decreased as Reaction 3 was inhibited by the quenching reaction with O 3 under the dry condition.The HCl reduction leveled off at the O 3 concentration of approximately 1.0 × 10 13 molecule cm −3 .On the other hand, the Cl 2 concentration increased at higher [O 3 ] although the change was not as clear as the HCl concentration.As mentioned in the introduction section, this implies that the observed HCl is not the nascent product of the titled reaction, but the conversion product of Cl 2 in the presence of H. Otherwise, [HCl] with respect to [O 3 ] would have given no change as the reaction of O 3 + HCl is extremely slow, which is contrary to our observation.
The assumption that Cl 2 is the nascent product is additionally supported by the observation that the production of ClO increased by one order-of-magnitude at [O 3 ] = 1.0 × 10 13 molecule cm −3 (see Figure 3).Due to the considerable difference in the rates between Reaction 3 and 5, Cl 2 has little chance to react with O in the presence of H.However, Reaction 5 becomes more probable when the removal of H by O 3 occurs, resulting in enhancement of the ClO production.The build-up of ClO leveled off when the reduction of HCl began to slow down.
The concentration changes of HCl, Cl 2 , and ClO in the flow tube as the added O 3 varied were also monitored under a slightly wet condition (RH = 2 %).The relative humidity (RH) was kept to only 2% since no pre-pumping was given in front of the CI region in order to prevent the loss of detection sensitivity.The concentration changes were basically the same as those at dry condition although all the signal intensities of the species were reduced by the factor of ~3 due to the loss of the parent ion ( ) by complex formation with H 2 O.This implies that Cl 2 is more likely to be the nascent product of the titled reaction than HCl at 2 % SF 6 −  of RH or less.
It must be noted that further studies are required to conclude the identification of the nascent product of the titled reaction since the reaction system of the flow tube in this study was too complicated.One of the possible studies will be the computational modeling which can enable us to cover a wide variety of reactions that possibly occur in the flow tube system but evade experimental investigation.A comparison of the modeling results with those from experimental studies is expected to provide valuable information on the nascent product of the titled reaction.

Summary
An experimental study to identify the nascent product of the heterogeneous reaction of OH with NaCl surface was performed under dry and wet conditions using a bead-filled flow tube system coupled to a high-pressure chemical ionization mass spectrometer.The gaseous OH radical generated in situ through H + O 2 + M reaction reacts heterogeneously with NaCl coated on the glass beads in the flow tube, resulting in the ion detection corresponding to HCl.Ozone was introduced into the system to inhibit the conversion reaction of Cl 2 into HCl by removing H through the fast reaction of O 3 + H.The mass spectrometric observation was that the O 3 introduction reduced the concentration of HCl, while it increased the concentrations of Cl 2 and ClO.The experimental results suggest that the nascent product of the titled reaction is gaseous Cl 2 , which is followed by fast conversion into HCl in presence of H.No significant difference in the concentration profile between under dry and wet (RH = 2%) conditions was observed.It is suggested that a further study including computational modeling will strengthen the evidence of the product identification.
of the two pathways is required in order to understand the mechanism of the reaction and to improve the atmospheric model.In this regard, ozone (O 3 ) is the best inhibitor of Reaction 3, as it scavenges H very well with a high reaction constant, 3.0 × 10 −11 cm 3 molecule −1 s −1 .10

SF 6 −Figure 1 .
Figure 1.The schematic diagram of the flow tube system coupled to CIMS.