Pergamon J. Aemsol Sci. Vol. 29, Suppl. I. pp. S987-S988, 1998

Q 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Gnat Brimin

0021~8502/98 $19.00+0.00

UPTAKE OF GAS-PHASE S02, H2S AND CO2 INTO DROPLETS AND BUBBLES IN AQUEOUS ACID SOLUTIONS

D. R. WORSNOP, J. T. JAYNE and C. E. KOLB Center Aerosol and Cloud Chemistry

Aerodyne Research, Inc., Billerica, MA 01821

Q. SHI, J. BONIFACE, Y.Q. LI, O.V. RATTIGAN, E. SWARTZ and P. DAVIDOVITS Department of Chemistry, Merkert Chemistry Center

Boston College, Chestnut Hill, MA 02167

KEYWORDS

Mass accommodation; Effective solubility; Liquid-phase reaction

Gas uptake of SOZ, H2S and CO2 into aqueous solutions vary over a wide range resulting from changes in the effective solubility of the species and liquid-phase reaction with OH-. Two separate techniques were used to conduct time-resolved uptake measurements for S02, H2S and CO;?. One experimental approach utilizes a stream of fast moving, monodisperse droplets (50 to 350 pm in diameter) that are passed through a low pressure flow reactor. The other utilizes well defined bubbles (S-1.0 cm in length) containing the trace gas at low pressure entrained in a liquid flow reactor. Short gas-liquid interaction times (2-15 ms) in the droplet apparatus minimize reevaporation of the dissolved species due to saturation of liquid-phase solubility; while the other long interaction time in the bubble train apparatus (0. l- 10 s) increases the number of gas-liquid collisions, and thus the sensitivity to small gas uptake rates. The uptake coefficients for this droplet technique are in the range of y = 10m3 to 1. With the train bubble train technique, rates of heterogeneous processes corresponding to small uptake coefficients in the range 10“ > y > low7 can be measured. Precise control of factors affecting the rate of gas uptake enables the deconvolution of the uptake into its component processes, such as gas-phase diffusion, mass accommodation, surface chemistry, Henry’s law solubility and liquid phase reaction (Worsnop et al., 1989). Results for S02, H2S, and CO2 are shown in Figure 1.

Using the droplet train apparatus S02(g) uptake studies were measured as function of acid concentration (pH 1 - pH 14) and temperature (263-293 K). As was found in earlier work, the uptake of SOz increases with pH due to increasing effective solubility of SO;?. In the newly studied higher pH region, further increase in uptake is observed due to reaction of S02(aq) with OK. The uptake is consistent with bulk phase solubility and pseudo-first order bulk phase reaction of SO? with OR. From the uptake results, rates of both direct and OH- catalyzed SO2 hydrolysis to form HS03- were determined. The SOz(aq) + OK reaction rate is essentially constant (k2 = 3~10~ Mm’ s) in the temperature range studied. The mass accommodation coefficient, a, which also determined as a function of temperature, can be expressed as all-a = exp (-AG/RT). Where AH = -7.78 kcaYmo1 and AS = -29.9 caY(mo1 K). At 283 K a = 0.44.

S987

S988 Abstracts of the 5th International Aerosol Conference 1998

For CO* the Henry’s law solubility was measured as a function of acid concentration (pH 11 - pH 13) with the horizontal bubble train apparatus. From the uptake data the rate constant kz for the reaction of C02(aq) with OA was determined to be k2 = 1.3x lo3 Me1 s at 293 K.

For H2S the Henry’s law solubility was also measured as a function of acid concentration (pH 1 - pH 12) with the horizontal bubble tram apparatus. From the uptake data (< pH 9) the rate constant for the reaction of H#(aq) with OH- is negligible and uptake is controlled by the effective solubility. However in acid concentrations (> pH 9) the reaction is very rapid and kz for the reaction between H#(aq) and OR is determined to be 1.8~10’ Me1 s. Under high pH conditions (pH 12-14) where the effective solubility and liquid-phase reaction lead to large uptake rates, the droplet train apparatus was used to measure the uptake. These results confirm the liquid-phase chemistry measured using the horizontal bubble train apparatus.

We have also used the horizontal bubble train apparatus to investigate the uptake of gaseous SO* by sulfuric acid solution over a wide range of acid concentration (pH = 2 to 70% wt) at 293 K. Approximately a 50% decrease in the effective Henry’s law is observed over the acidity range from pH 1 to 60% wt H2S04 SO* uptake studies were also carried out as a function of oxidant (H202, 03, HONO) concentration over the acidity range pH 1 to 70% wt H2S04. These results are in good agreement to the predicted oxidation kinetics from aqueous (> pH 1) measurements.

10”

lo‘*

r-" loJ

lo4

lo*

1oat 1 I I I I I I

0 2 4 6 6 10 12 14

PH

Figure 1. Uptake of SOz, HzS, and COz as a function of pH at 291 K for gas-liquid contact time of 5 ms.

The implications for atmospheric heterogeneous chemistry, including the air-ocean interface and aerosol formation in aircraft plumes, will be discussed.

ACKNOWLEDGEMENTS

This work has received financial support from NASA, EPA, DOE, and NSF.

REFERENCES

Worsnop, D.R., M.S. Zahniser, C.E. Kolb, J.A. Gardner, L.R. Watson, J.M. Van Doren, J.T. Jayne, P. Davidovits, Temperature dependence of mass accommodation of SO;? and H202 on aqueous surfaces, .I. Phys. Chem., 93, 1159-l 172, 1989.