The uptake of gas-phase SO 2 , H 2 S, and CO 2 by aqueous solutions was studied as a function of pH and temperature using droplet train flow reactor and bubble train reactor apparatuses. All three atmospheric species react with H 2 O and OH -, acidifying aqueous solutions. Studies yielded the reaction rates of SO 2 , H 2 S, and CO 2 with OH -and the mass accommodation coefficient (R) of SO 2 on water as a function of temperature. The second-order aqueous-phase rate coefficients at 291 K for SO 2 , H 2 S and CO 2 reaction with OH -are (1.1 ( 0.2) × 10 10 , (1.7 ( 0.2) × 10 9 , and (4.0 ( 0.7) × 10 3 M -1 s -1 , respectively. As far as we can determine, the rate coefficients for SO 2 and H 2 S have been determined here for the first time. At 291 K, for SO 2 , R ) 0.175 ( 0.015, and as in previous studies, the temperature dependence of R is well represented by the relationship R /(1 -R) ) exp(-∆G obs /RT) with ∆H obs ) -7.6 ( 0.6 kcal mol -1 and ∆S obs ) -29.2 ( 2.1 cal mol -1 K -1 . IntroductionIn this work, we describe uptake studies of gas-phase SO 2 , H 2 S, and CO 2 by aqueous solutions as a function of pH and temperature. These three atmospheric species exhibit similar chemical behaviors that contribute to the acidification of aqueous media in the atmosphere. Sulfur dioxide oxidation is responsible for about 90% of atmospheric H 2 SO 4 , and an estimated 50% of this oxidation occurs via heterogeneous reactions. 1 Hydrogen sulfide is a major biogenic reduced sulfur species, emitted into the atmosphere by vegetation, soils, and marine waters, 2 that is oxidized by the OH radical in the gas phase to produce SO 2 . 3 H 2 S may also be important to O 3 chemistry, as in the aqueous phase, H 2 S dissociates readily to HS -, which, in turn, reacts rapidly with O 3 . 4 Of the three gas-phase species, CO 2 is the least reactive but perhaps the most atmospherically important. Because of its high atmospheric abundance (∼370 ppm) and its infrared absorption properties, CO 2 plays an important role in the global warming process.The pH-dependent reactions of SO 2 and CO 2 in aqueous solution are (R ) SO 2 or CO 2 )The pH-dependent reactions of H 2 S are Here, k 1 is the pseudo-first-order reaction-rate coefficient of the species with H 2 O, and k 2 is the second-order reaction-rate coefficient of the species with OH -. As the current study makes evident, the forward reaction rates for SO 2 , H 2 S, and CO 2 with OH -vary over nearly 6 orders of magnitude. The uptake coefficients vary over a similar range. Two complementary apparatuses were used to span this range of uptake coefficients: a droplet train flow reactor apparatus and a horizontal bubble train reactor apparatus. The uptake measurements yielded rate coefficients for the reaction of SO 2 , H 2 S, and CO 2 with OH -, and the mass accommodation coefficient for SO 2 on water as a function of temperature.In several previous uptake studies, the measured uptake was larger than predicted by bulk-phase parameters. Such enhanced uptake was observed among others in the follow...

Uptake of gas-phase NO 2 by water was studied in both the droplet train and bubble train flow reactors. Aqueous surface reaction of NO 2 (g), reported previously in the literature, is not substantiated by these studies. The uptake of NO 2 (g) is a function of Henry's law coefficient (H NO 2 ) and the second-order NO 2 (aq)-NO 2 -(aq) hydrolysis reaction rate coefficient (k 2 ), in the form H NO 2 (k 2 ) 1/2 . The NO 2 (aq)-NO 2 (aq) hydrolysis rate coefficient is defined by: -d[NO 2 (aq)]/dt ) 2k 2 [NO 2 (aq)] 2 . The coupled nature of the uptake makes it difficult to obtain reliable separate values for the two parameters H and k 2 . Literature values for these parameters vary by as much as a factor of 5. With the bubble train apparatus it was possible to separate clearly the effects of solubility and reaction on NO 2 gas uptake. From these measurements and analysis of literature values, recommended values of these parameters at 293 K are H NO 2 ) (1.4 ( 0.2) × 10 -2 M atm -1 and k 2 ) (3.0 ( 0.9) × 10 7 M -1 s -1 . At 276 K, our best estimates are H NO 2 ) 2.3 (+0.3-0.9) × 10 -2 M atm -1 and k 2 ) (2.2 ( 0.6) × 10 7 M -1 s -1 .

A new horizontal bubble train apparatus has been built to conduct time-resolved gas−liquid interaction studies of interest in atmospheric chemistry. In the apparatus, liquid is pumped horizontally through a 0.4 cm i.d. quartz tube at a controlled speed of 15−35 cm/s. A low-pressure (about 50 Torr) gas flow, carrying the trace gas of interest diluted in helium carrier gas, is injected through a moveable injector into the liquid flow forming well-defined bubbles that fill the diameter of the tube. In a controlled way, 10−25 bubbles are formed per second. The outlet gas flow is then sampled by a differentially pumped mass spectrometer. The position of the bubble injector, which is computer controlled by a stepping motor, determines the bubble travel distance and therefore the gas-liquid interaction time (typically 0.1−10 s). Modeling of the gas uptake and the validation of the apparatus performance are described. The apparatus can measure Henry's law coefficients (H) for non-reactive species in the range 10-3 to 3.0 M atm-1. For reactive species (first-order reaction rate k), the apparatus measures Hk 1/2 values in the range 0.04−150 (M atm-1 s-1/2). If the Henry's law coefficient of a species is known, then first order reaction rates in the range 1−108 s-1 can be measured. Using this bubble train apparatus, the uptake of gas phase formaldehyde as a function of pH (0−11) and NaCl concentration (0−4 M) has been measured. The Setchenow coefficient for formaldehyde in NaCl solution at 293 K has been determined to be K s = 0.072 ± 0.004 M-1.

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Abstract. Uptake of SO2(g) was measured as a function of sulfuric acid concentration in the range pH = 1 to 70 wt% H2SO 4 at 293 K. Uptake studies were carried out in the presence of solvated H202, 03, HONO, NO•, and HNO3. The studies yielded Henry's law coefficients for SO: which were in accord with previous measurements. The results provided first time determinations at high acidities of rate coefficients for the reaction of SO: with H20:, 03, and HONO in the solvated state. The observed SO:(g) uptake was consistent with bulk phase oxidation processes. The measured rate coefficients were used to evaluate the role of heterogeneous interactions with the selected species for aerosol formation in aircraft plumes. Calculations show that these interactions are insignificant compared to gas phase oxidation by the OH radical.