The heterogeneous reactions of hydroxyl radicals (OH) occur on the surface of various atmospheric aerosols and have been studied to explain discrepancies between field measurements and theoretical values based on atmospheric modeling. Knowledge of such reactions also increases our understanding of the chemical reactions occurring in Earth’s atmosphere.1-4 However, little is known of the OH heterogeneous reactions that occur under real tropospheric conditions when water vapor may be present at up to 4% by volume. Due to its relatively large dipole moment (1.83×10^-18 esu·cm) and its ability to form strong hydrogen bonds, water vapor is easily adsorbed onto atmospheric aerosol surfaces, where it can facilitate various heterogeneous reactions. For example, the heterogeneous reactions of OH with certain inorganic aerosols, such as MgCl₂, sea salt, and SiO₂, are enhanced under wet conditions. In contrast, the reactivity of OH with adsorbed water remains unaffected or slightly decreases on other organic or inorganic aerosols.5,6

Chemical ionization mass spectrometry (CIMS) is widely used in experimental atmospheric studies because it is less destructive than other ionization methods and results in a lesser degree of fragmentation and relatively simple mass spectra.7-9 In CIMS detection, a reagent ion, or “parent ion,” such as SF₆^-, F^-, O₂^-, and NO₂^-, collides with a gas-phase molecule of interest, whereby electron transfer occurs via the following reaction:

where X^- is the reagent ion and A is the gas-phase molecule of interest.

However, technical problems arise in CI experiments at high water-vapor pressures. Providing a sufficient amount of water vapor for CI requires a considerable flow of carrier gas that builds up relatively high pressures of 100-200 Torr inside a flow tube reactor. Such high-pressure conditions slow the diffusion of radicals into the reactor surface significantly.11,12 Increasing the surface area of the reactor by employing a miniature tube array or glass bead packing inside the flow tube simultaneously maximizes the rate of heterogeneous reactions and minimizes diffusion limits by shortening the distance between reactants and the surface.

The sensitivity of CIMS is also affected by high watervapor pressure. Water vapor readily interacts with SF₆^-, the reagent ion, to form SF₄O^- and F^- (HF)_n.13,14 The resulting loss of the reagent ion significantly lowers CIMS sensitivity for many analytes, including OH. Water vapor also forms clusters such as [OH(H₂O)_n]^- with OH^- in the CI region, resulting in a further reduction in OH sensitivity. Although humid conditions significantly reduce CIMS sensitivity, few studies have addressed the dependence of water-parent ions or water-OH complexes on the relative humidity (RH).

This paper describes the use of a flow tube system coupled to a CIMS to explore the interactions among water molecules, reagent ions (SF₆^-), and/or OH radicals that reduce CIMS sensitivity. Preliminary ab initio calculations supported experimental observations and identified the specific locations where various complex-forming reactions occur. In addition, the recovery of experimental sensitivity by introducing a buffer flow of helium (He) is briefly discussed.

Experiments were performed under flow conditions using a modified version of a previously developed apparatus.5,6,15 The modified flow tube system is shown in Figure 1. In the modified system, the reactor tubes in which OH heterogeneous reactions occur were removed because the OH reactions were not of interest. Instead, a mixing tube was connected directly to the CIMS system. Another modification is the use of a switchable water-vapor introduction line.

Hydroxyl radicals were generated by two sequential reactions.

Hydrogen radicals (H) were produced by a 30-W microwave discharge and subsequent reaction with O₂ (99.5%; Matheson Tri-Gas) to generate hydroperoxyl radicals (HO₂), which were then stabilized by He gas (M). HO₂ reacted promptly with excess hydrogen radicals to generate OH. Since Rxn. (3) is relatively fast, the production of HO₂ was negligible (HO₂/OH:~1 × 10^-2). To prevent OH loss via self-recombination, the flow containing OH was expanded in a mixing tube and diluted with N₂ (99.5%; Praxair, Jefferson City, MO, USA), the primary carrier gas. The detection sensitivity of OH was 4.2 × 10⁷ molecules cm^-3 cps^-1.

Conditions of up to 50% RH were established using a water bubbler. Humid gas was selectively injected into the flow tube system or into the CI region using the switchable water-vapor introduction line shown in Figure 1.

A small flow of sulfur hexafluoride (SF₆, 99.99%; Matheson Tri-Gas) was initially diluted with N₂ to 0.1% by volume and exposed to a radioactive polonium source (²¹⁰Po; NRD) to produce SF₆^- reagent ions in the CI region. A mass spectrometer equipped with a quadrupole mass analyzer (Extrel) and a continuous-dynode electron multiplier (Channeltron 4870E; Burle Electro-Optics) were used for quantitative measurements.

Preliminary ab initio calculations were performed to characterize the complex-forming reactions. Hybrid density functional theory at the B3LYP level with a 6-31(d,p) basis set was used for geometry optimization and energy calculations. All calculations were carried out in Gaussian 03.16

Changes in the peak intensities of SF₆^- and its products from reactions with water are shown in Figure 2 (a). The SF₆^- peak in the CI region was drastically diminished under wet conditions. The peak intensities of SF₄O^-, F^- (HF)₂, and F^- (HF)₃ reached the same level as that of SF₆^- at 16% RH. At 50% RH, F^- (HF)₃ was the major fluorine compound, while peaks associated with SF₆^- were negligible. Although SF₄O^- can act as a reagent ion, the degree of CI was lowered through significant losses of both SF₆^- and SF₄O^-.

Figure 2(b) shows that the intensity of the OH peak decreased with the addition of water into the flow tube system, which was due to not only the loss of CIMS sensitivity as described above, but also to the conversion of OH into OH-water complexes.17-20 In particular, the peak intensity of [OH(H₂O)₃]^- exceeded that of OH^- at greater than 10% RH and reached a maximum at 18% RH. The observation that [OH(H₂O)₃]^- becomes the major OHcontaining species under high RH is supported by an infrared spectrometric study with quantum chemical calculation21 and our preliminary ab initio calculation, which predicted the cyclic complex of [OH(H₂O)₃] to be the most stable.

The observed OH-water complexes ([OH(H₂O)_n]) originate from the complexation of OH radicals and water molecules in the flow tube and subsequent ionization in the CI region (Rxn. (4) to (5)) followed by the formation of clusters of OH^- ions with water molecules (Rxn. (6) to (7)).

or,

According to preliminary ab initio calculations on Rxn (4), OH(H₂O) dissociates into an OH radical and a water molecule immediately after production, resulting in an extremely small equilibrium constant of 2.1 × 10^-26 cm³ molecule^-1. This implies that the amount of the OH-water complex is negligible in the CI region. Conversely, Rxn. (7) and further clusterization with water were favorable in terms of enthalpy. Thus, [OH(H₂O)_n]^- in CIMS likely results from Rxns. (6) and (7), with subsequent clusterization in the CI region. High humidity conditions were created in the CI region using the water-vapor introduction line to examine the origin of [OH(H₂O)_n]^-. The resulting distribution of [OH(H₂O)_n]^- under humid conditions was similar to that shown in Figure 2(b), thereby confirming the origin of [OH(H₂O)_n]^- complexes.

An additional buffer flow of He into the CI region caused two opposing effects on CIMS sensitivity. The buffer flow increased the total pressure in the CI region, resulting in a dilution of OH and SF₆^-, which led to a decrease in CIMS sensitivity because lesser amounts of both species are introduced into the mass spectrometer. In contrast, the same effect dilutes water vapor in the CI region and reduces loss of the reagent ion, which increases sensitivity.

The intensity of the OH peak as a function of RH was monitored with an additional He flow of 2 SLPM (standard liters minute^-1) into the CI region, increasing the pressure from 1.9 Torr to 10 Torr.14 As shown in Figure 3(a), the sensitivity for OH decreased as the RH increased up to 38%. However, the sensitivity recovered significantly with additional He buffer flow, which diluted water vapor resulting in reduction of the reagent ion loss and OH-water clusterization. For example, the OH sensitivity increased by a factor of 17 with an additional He buffer flow at 38% RH. Without the additional flow, the OH peak was barely observable.

Figure 3(b) shows intensity changes in the OH^- peak and its associated water complexes at 18% RH as a function of pressure in the CI region. The complexation of OH ions with water occurred even with the additional He flow, and [OH(H₂O)₃]^- was the major OH-containing species. However, the intensity of the OH peak increased slightly as the CI pressure increased, and eventually exceeded the peak intensities of [OH(H₂O)]^-, [OH(H₂O)₂]^-, and [OH(H₂O)₄]^- at over 8 Torr. The recovery of OH intensity through the introduction of an additional He buffer flow provides sufficient sensitivity for experimental investigations on heterogeneous reactions of OH under humid conditions.

A flow tube system was coupled to a CIMS instrument to investigate the interference of water vapor on OH radical detection. The sensitivity of CIMS decreased under humid conditions due to loss of the reagent ion (SF₆^-) via reactions with water. In addition, complexes formed between OH ions and water mostly produced [OH(H₂O)₃]^- ions, which resulted in additional losses in OH sensitivity. The observed OH(H₂O)_n peaks in the CI region under humid conditions and preliminary ab initio calculations on associated reactions suggest that [OH(H₂O)_n]^- ions were produced from OH ions and water molecules in the CI region. OH peak intensity could be partially recovered by introducing an additional He buffer flow into the CI region. This strategy provided sufficient sensitivity for the study of OH heterogeneous reactions under humid conditions.