25 According to a recent study of Haulet el m. (1977) the S02 production rate during the eruption of Etna in June, 1973 was found to be 3.7 x 103 t day1.

according to Granat et al. (1976) this proportion is even greater (approximately 50 %) we conclude that man contributes substantially to the atmospheric inventory of sulfur compounds.

3L6.3 Transformation of sulfur compounds in the atmosphere

The sulfur compounds, except the sulfate of sea salt particles, are released into the atmosphere in gaseous form. In spite of this fact the majority of atmospheric sulfur in clean air is found in particulate matter as sulfate (see later). Thus, the aim of studies on transformations of sulfur compounds is to determine the nature and rate of oxidation processes leading to sulfate formation.

Unfortunately, our knowledge about the H2S-+S02 transformation is very scanty, although it is speculated that this reaction is the only important sink for hydrogen sulfide of biological origin. According to Kellogg ei al. (1972) S02 forms by the interaction of H2S and atomic oxygen or ozone. These authors believed that the oxidation takes place on the surface of aerosol particles as well as in cloud and precipitation water since the rate of gas phase reactions was found to be insignificant. The reaction with ozone is as follows:

However, it was demonstrated by Penkett (1972) that the oxidation of H2S in solutions is a very slow process. H2S can also react with atomic oxygen:

which is followed by a reaction chain resulting in the formation of S02, S03 and sulfuric acid vapour. It is very doubtful, however, whether reaction [3.48] is important in the troposphere where the level of atomic oxygen is low.

According to the laboratory experiments of Cox and Sandalls (1974) OH radicals play an important role in the oxidation of H2S. They stated that the oxidation of H2S is initiated by the gas phase reaction

The HS radical formed in this way reacts with molecular oxygen:

The rate constant of [3.49] is reported as 3.1 x 10"12 cm3 molecule"1 s"1.

Jaeschke et al. (1978) have shown the importance of this process in the following indirect way. They measured the vertical concentration gradient of H2S and calculated the vertical flux by using the gradient method (see Section 5.1). They have found, on the basis of the decrease of H2S flux with increasing height, that the mean removal coefficient of hydrogen sulfide is 2.27 x 10"5 s "1. This coefficient is equal to the product of the reaction rate and OH concentration (k [OH]), since

This permits calculation ot the steady-state OH concentration in the air layer studied (150-1000 m). Since Jaeschke el ai. (1978) have calculated values comparable to measured OH levels (Perner et ai, 1976), it can be concluded that reaction [3.49] accounts for most of the observed H2S transformation. From the mean removal coefficient, Jaeschke et ui (1978) were also able to determine the mean lifetime of H2S as about 12 hr.

Cox and Sandalls (1974) also studied during their laboratory work the oxidation of DMS by using ppm concentrations (which are much higher than possible atmospheric levels). They found that photochemically generated free radicals such as O and OH reacted rapidly with DMS. An important finding was the absence of S02 during DMS oxidation. However, the appearance of aerosol particles was evident in the system. Cox and Sandalls stated that, due to this process, the residence time of DMS in the atmosphere is only a few hours.

Recently, Paugam (1978) published an interesting atmospheric observation which may be related to the oxidation of DMS. He observed a substantial photolytic aerosol formation on the coast of Brittany (France) during low tide. Paugam believes that this phenomenon is due to the photooxidation of DMS emitted by algae. He also mentioned, on the basis of S02 measurement, that sulfur dioxide does not play an important part in the gas-to-particle conversion.

Our knowledge of the oxidation of sulfur dioxide is more complete because this transformation was studied in more detail, owing to its importance for local air pollution. It is generally accepted that sulfate can be formed from S02 in the following ways:

(a) Sulfuric acid vapour is an end product of homogenous gas phase reactions, including photochemical and thermal steps. Sulfuric acid droplets are formed by vapour condensation.

(b) S02 is oxidized in fog- and cloud drops after its absorption. Sulfate particles are formed by the evaporation of drops.26

(c) S02 is oxidized on the surface of existing aerosol particles.

The study of homogenous gaseous reactions leading to sulfate formation started at the beginning of the fifties. The results of early laboratory works are reviewed by

26 If the cloud evaporates. In case of precipitation formation theSOj" formed is removed from the air by wet deposition (see Chapter 5).

Leighton (1961). A general feature of these studies was the fact that authors assumed direct photooxidation of S02 gas. Thus. Gerhard and Johnstone (1955) speculated that the processes could be summarized by the following three reactions:

S03 + H20-H2S04 [3.53] Their experiments showed that the S02 loss is a first-order process:

Gerhard and Johnstone (1955) demonstrated that k was 10" 3 hr"1 when they used solar radiation to illuminate the reaction chamber containing only purified air, S02 and H20. In a more recent paper, Cox (1972) published O values (<D is the quantum yield, see Subsection 3.4.2) between 1 x 10" 3 and 5 x '10"3, which implies that ku in reaction [3.54] should be about 1 hr"1.

Further studies pointed out that the homogeneous oxidation of S02 becomes faster when the air also contains nitrogen dioxide and ozone. Thus, N02 absorbs solar radiation in the visible spectrum and dissociates (see [3.32]) to give atomic oxygen. Oxygen atoms produce ozone or react with S02 to form sulfur trioxide (Cadle and Powers, 1966):

This oxidation is of third order and its reaction rate is independent of the temperature. Using reaction rate values measured under laboratory conditions and the concentrations of M and O for different levels of the atmosphere, Cadle and Powers calculated that this process can be significant only above 10 km if the S02 concentration is 1 /ig m"3 STP. The residence time of sulfur dioxide molecules is estimated to be 103 hr at an altitude of 10 km, while at 30 km the corresponding figure ranges from 5 hr to 10 hr. Hence it seems probable that this reaction is not important in the troposphere. However, it may play an important role in the formation of the stratospheric sulfate layer (Subsection 4.4.3).

In the S02->H2S04 transformation organic materials play an important part. Thus, if organic substances are added to air containing S02, H20, NOz and 03 the S02 oxidation rate is observed to accelerate. According to the laboratory experiments of Cox and Penkett (1972) the photooxidation rate of S02 is as large as 1-10 % per hr if the air contains appropriate olefins and ozone. Olefins can be detected, however, only in air polluted by exhaust gases. However, Cadle (1972) has speculated that, under clean tropospheric conditions. S02 oxidation is stimulated by the presence of terpenoid hydrocarbons (see Subsection 3.3.3).

It is believed at present (Calvert et al.. 1978) that "the direct photooxidation of S02 by way of the electronically excited states of S02 is relatively unimportant for most conditions which occur within the troposphere". Among others, Calvert et al, (1978) argue that, in the air containing the species noted above, many radicals are formed by photochemical processes. These radicals (e.g. OH, H02 CH2, CH302) oxidize S02 by the following processes:

where reaction [3.56] is the dominant path. In the case of [OH], [H02] and [CH30] concentrations typical for theclean troposphere, S02 transformation rates with an order of 0.1 % hr "1 can be calculated, which is equivalent to a residence time of 5 days. However, the rate is calculated to be greater than 1 % hr "1 in the more polluted lower part of the troposphere, because of the higher concentrations of free radicals.

By homogeneous reactions, vapour phase sulfuric acid is formed. This vapour condenses in the air by bimolecular condensation (H20 molecules also take part in the phase transition). The rate of this process depends, among other things, on the concentration of H 20 and H 2S04 molecules as well as on the temperature (Kiang et al., 1973). In the presence of suitable aerosol particles the condensation is heterogeneous, that is vapour molecules condense on the surface of aerosol particles (Cox, 1974).

Relatively few direct observations supports the photochemical sulfate formation concept; it derives primarily from laboratory experiments.27 Thus, Atkins et al. (1972) demonstrated in Southern England that airborne sulfuric acid concentrations correlate generally with the intensity ol solar radiation and the 03 leve . Similarly, near Budapest during summer daylight, E. Meszaros (1973) found a good positive correlation between the sulfate content of particulate matter and solar radiation intensity, as weil as between radiation and the logarithm of [S04"]/[S02] molar ratios. The chemical parameters mentioned also correlated well with the temperature. The results obtained are plotted in Fig. 16. It can be seen that the linear correlation coefficient between log([S04~]/[S02]) and radiation intensity is + 0.63. In the case of temperature the corresponding coefficient is + 0.69. These results strongly suggest that sulfate ions in particulate matter are formed by

27 .The expression "direct" means that we speak about such measurements when it was chemically proved that particles were composed of sulfate.

photochemical reactions followed by thermal processes. It is emphasized, however, that the above effect cannot be identilied in the winter months (E. Meszaros, 1974b), which means that the sulfate formation in these months is due to other transformation paths.

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