Sulfur dioxide oxidation in winter orographic clouds

Abstract

The production of sulfate in cloud droplets, and the attendant depletion of sulfur dioxide (SO2) oxidants were studied in wintertime orographic clouds over southeastern Wyoming. By periodic (5-20 min) releases of SO 2 into the cloud the mixing ratio of SO 2 was raised to about double its background value of 0.7 parts per billion by volume (ppbv). Average values of the in-cloud reaction time, pseudo-firstorder SO 2 reaction rate, and hydrogen peroxide (H202)depletion were 400 s, 2.3 x 10 '4 s 'l, and 0.04 ppbv, respectively. The measured sulfate yields and H20 2 depletions show that H202 is the dominant oxidant for SO 2 in this situation; this finding is consistent with model results. Model simulations reveal that the O2/SO 2 reaction pathway (catalyzed by Mn(II) and Fe(III)) was competitive with the H202/802 reaction pathway on one out of the nine observation days. Organohydroperoxides were not depleted and other modeled reaction pathways (O3/SO 2 and HCHO/SO2) were inferred to be inhibited due to either the chemical (pH < 4.7) or physical (temperature below -5 øC, cloud water content less than 0.3 cm 3 m '3) properties of the cloud. The agreement found in this work between observed and predicted properties of the H202/SO2/H20 system contrasts with the work of Chandler et al. (1988a, b, 1989) and Gallagher et al. (1990) who observe a factor of 3 or larger discrepancy between laboratory and field measurements of the H202/802 reaction rate. 10-100 times shorter. Therefore in-cloud reactions constitute a major factor for regional and for global removal of SO 2. The quoted range of in-cloud lifetimes is based on laboratory kinetic investigations of the hydrogen peroxide (H202) and ozone (03) reactions with sulfur in the +4 oxidation state (S(IV)) [Hoffmann and Calvert, 1985; Hoffmann, 1986] and on data obtained in field studies [Hegg and Hobbs, 1982; Gervat et al., 1988]. However, there are important uncertainties in these values. Model calculations based on field observations conducted in northem England [Chandler et al., 1988a, b, 1989; Gallagher et al., 1990] suggest that the H202-S(IV ) reaction rate constants determined either using purified water (Hoffmann and Calvert, 1985] or using authentic precipitation samples [Lee et al., 1986] are a factor of 3-10 too low. The field studies themselves involve important uncertainties, mostly in regard to measurements of the depletion of H202. A small number of simultaneous measurements of sulfate production and of H202 depletion were obtained in experiments conducted at Great Dun Fell in the United Kingdom [Chandler et al., 1989; Clark et al., 1990; Gallagher et al., 1990]. Such data are essential if the field experiments are to be compared reliably with laboratory data and if they are to lead to identification of the dominant reaction pathways for SO 2. This information is necessary for reliable assessments of how changes in SO 2 emissions will affect sulfuric acid deposition. To help clarify the questions just discussed, studies of SO 2 oxidation to sulfate have been co...

Hydrogen peroxide retention in rime ice

“…This correction is obtained by modifying (1) in the following manner: Figure 3a. This reflects both a seasonal trend in H202 abundance at Elk Mountain [Snider, 1990], and the inverse relationship between SO2 solubility and sulfurous acid strength with -0.50, ._P<0.01).…”

“…Cloud water samples were collected as rime using the rotating dual-arm wire grid, referred to as the CWS [Snider, 1989]. The stainless steel grids (width 0.5 mm) were exposed for 5 to 60 min.…”

“…The technique of Kok et al [1986], in which aqueous hydroperoxides rapidly convert p-hydroxyphenylacetic acid to its highly fluorescent dimer in the presence of peroxidase, was employed in this work. The organohydroperoxide concentrations, inferred from catalase addition to a subset of cloud water samples collected at EMO, are always less than 10% of the H202 concentration [Snider, 1989], and therefore differences between the sum of all hydroperoxide concentrations and the H202 concentration ([H202] , M) were ignored. Over half of the 1989 samples were divided into two aliquots: one was mixed with the hydroperoxide reagent within 20 min of collection, the other 2 -5 hours later.…”

“…In addition to these measurements, all recorded at 1 Hz by the data acquisition system, event marks signifying the start and stop of cloud water collection intervals were also recorded. Rime samples were collected on the instrument platform.Cloud water samples were collected as rime using the rotating dual-arm wire grid, referred to as the CWS [Snider, 1989]. The stainless steel grids (width 0.5 mm) were exposed for 5 to 60 min.…”

Modeling and analysis of aerosol processes in an interactive chemistry general circulation model

“…Their associated rates J i (M −1 s −1 ) are listed in Table 3. Conversion of [S(IV)] to [S(VI)] is updated in time, with chemical equilibrium being reestablished at the end of each incremental oxidation step, using The aqueous phase reaction proceeds on a much smaller timescale (2 min) than the model time step of 30 min [ Boucher et al , 2002; Snider and Vali , 1994; Hegg and Hobbs , 1981].…”