[03 H0x0jAm

so that

Combining the kinetic and solubility data permits the rate of reaction to be evaluated for known or assumed conditions of cloud liquid water content and partial pressures of reagent gases (Fig. 3). Here the left-hand ordinate gives the rate of aqueous-phase reaction. The right-hand ordinate gives the effective first-order rate coefficient of aqueous-phase reaction, referred to the gas-phase mixing ratio; for indicated conditions, expressed in units of percent per hour. Note the strong pH dependence, rate increasing with pH, resulting from the pH dependences of sulfur-IV solubility (Fig. 1) and kinetic rate constant (Fig. 2). The ozone reaction is quite rapid at high pH. However, because of production of sulfuric acid as the reaction

Figure 3 Instantaneous rate of aqueous-phase oxidation of S(IV) by H202 and 03, evaluated as a function of pH for representative nonurban reagent concentrations. The rates scale approximately linearly with reagent concentrations. The right-hand ordinate gives the oxidation rate of S02 referred to the gas-phase partial pressure and expressed as percent per hour for a liquid water content L = 1 x 10~6 (1 cm3/m~3), the rate scales approximately linearly with L. For the H202 reaction the indicated aqueous-phase concentration of H202 corresponds to total mixing ratio of this species (gas plus aqueous phase; the two are comparable) of ~0.6 x 10-9. See ftp site for color image.

Figure 3 Instantaneous rate of aqueous-phase oxidation of S(IV) by H202 and 03, evaluated as a function of pH for representative nonurban reagent concentrations. The rates scale approximately linearly with reagent concentrations. The right-hand ordinate gives the oxidation rate of S02 referred to the gas-phase partial pressure and expressed as percent per hour for a liquid water content L = 1 x 10~6 (1 cm3/m~3), the rate scales approximately linearly with L. For the H202 reaction the indicated aqueous-phase concentration of H202 corresponds to total mixing ratio of this species (gas plus aqueous phase; the two are comparable) of ~0.6 x 10-9. See ftp site for color image.

proceeds, the pH rapidly becomes lower, decreasing the rate. Although a strong acid concentration of 10 |iM (pH 5) is quickly reached, in perhaps 10 min, a much greater time, MO h, is required to reach an acid concentration of 50 pM. For this reason the ozone reaction is unlikely to account for cloudwater acidities of 10-4 to 10-3 M commonly observed in regions influenced by industrial emissions of S02.

Now consider the hydrogen peroxide reaction, whose aqueous-phase rate is given by:

This reaction is acid catalyzed; that is, the second-order aqueous-phase rate constant increases with decreasing pH (Fig. 2b). The pH dependences of solubility and reaction kinetics now cancel to yield a reaction rate that is roughly independent of pH throughout the pH range pertinent to cloudwater acidification (Fig. 3). For the conditions given in Figure 3 the rates of the 03 and H202 reactions are equal roughly at pH 5. An effective first-order reaction rate of 100%/h corresponds to a \/e lifetime of S02 of 1 h; the actual lifetime would depend on actual conditions. The H202 reaction is the only identified atmospheric reaction capable of maintaining the S02 oxidation rate sufficiently rapid to produce observed cloudwater H+ and S042-concentrations on time scales pertinent to cloud acidification.

In the case of the ozone reaction, ambient mixing ratios of 03 are generally sufficiently in excess of those of S02 that depletion of 03 need not be considered. However, ambient concentrations of H202, which are typically below 3nmol/mol are often much less than ambient S02 mixing ratios. This leads to a situation where the reaction proceeds rapidly to completion by exhausting the H202 reagent. If on the other hand S02 mixing ratios are the lesser, then the reaction can rapidly and completely exhaust ambient S02. The time scale of this process, a few tens of minutes for representative mixing ratios in the nmol/mol region, leads to a situation where the extent of reaction is controlled by the limiting reagent. Such appears to be the case as indicated by field measurements simultaneously examining H202 and S02 mixing ratios in clouds. A survey of nonprecipitating stratiform clouds indicated that although either species is frequently present at nmol/mol mixing ratios, appreciable mixing ratios of the two species are virtually never simultaneously present, (Daum, 1990). The contribution of this reaction to cloudwater acidification has been directly confirmed by field measurements under well-defined flow conditions, including experiments with artificially introduced S02 and inert tracers, showing concomitant decreases in S02, and H202 and increases in H+ and S042-consistent with this reaction. This reaction is now thought to be the major contributor to atmospheric oxidation of S02, contributing both to acid precipitation and, in the likely event of cloud evaporation to sulfate aerosol, a principal component of atmospheric aerosols.

Based on laboratory and industrial experience nitrogen dioxide (N02) is known to be highly reactive with liquid water forming nitric and/or nitrous acids, the initial reaction being

for which there is a strong thermochemical driving force; the N204 in parentheses indicates the possible participation of the N02 dimer, dinitrogen tetroxide. (This reaction is the basis for industrial1 manufacture of nitric acid.) It was therefore assumed by many atmospheric chemists that N02 would be rapidly taken up in cloudwater in the ambient atmosphere. Consideration of the mechanism of this reaction gives the aqueous-phase rate expression,

Determination of the Henry's law coefficient for N02 and the second-order reaction rate constant permitted evaluation of this rate for atmospheric conditions. Such evaluations have indicated that this rate is much too slow to contribute appreciably to N02 uptake by cloudwater at ambient concentrations. The reason for this, and for the great difference with experience at high N02 concentrations, is that the reaction is second-order in the concentration of a very weakly soluble gas. Comparisons of N02 mixing ratios in clouds with those in clear air in the vicinity of clouds indicates that the fractional uptake of N02 into cloudwater is quite small, lending confirmation to the above picture. Alternative possible mechanisms for N02 uptake include reaction with reducing species dissolved in cloudwater, as N02 is a fairly strong oxidant.

An alternative mechanism that may be important, especially at night, is initiated by the gas-phase reactions

followed by

N03 + N02 N205

with N205 being taken up by cloudwater by reaction to form nitric acid:

The rate of reaction is controlled by the rate of the initiation reaction of N02 with 03, which is several percent per hour at typical ozone mixing ratios of a few tens ofnmol/mol. The reason that this appears to be important at night but not during the day is that photolysis of N03 by visible radiation

is the major sink of N03 during the day, thereby cutting off the overall reaction.

In addition to these acidification reactions several other in-cloud reactions have been identified as of importance or potential importance in atmospheric chemistry. The hydroperoxy radical, H02, which plays an important role in gas-phase photochemistry as part of the chain of reactions leading to ozone formation by oxidation of NO and hydrocarbons, is thought to be rather soluble in water because of its weak-acid dissociation:

The dissolved material undergoes rapid self-reaction to form hydrogen peroxide. It has been suggested that the occurrence of this process can substantially influence the ozone budget in the remote troposphere. However, the process remains somewhat speculative in view of the lack of firm information on the solubility of the H02 radical.

Several studies have demonstrated substantial aqueous-phase formation of H202 by photochemical reactions in collected cloudwater. The exact processes are not yet elucidated but evidently involve trace organic species, which are difficult to characterize. Such reactions may contribute substantially to S02 oxidation in situations where this oxidation is limited by the amount of H202 initially present. More generally, it may be noted that photochemical reactions, in both gas and solution phases, may be enhanced in the tops of clouds because of enhanced photolysis fluxes, by a factor of 5 or more, that result from multiple scattering of solar radiation within clouds.

0 0

Post a comment