OH NH3
SO2-----→ H2SO4--→ (NH4)2SO4 aerosol
Close to the source of emission dry deposition is formed. The longer the gas is in the atmosphere for the more likely it is to from wet deposition.
Wet deposition occurs in or on hydrometeor, rain or snow. As water moves through the atmosphere it picks up the SO2 particles. Dry deposition is either the absorption of SO2 onto moist surfaces, sedimentation of SO2 particles due to gravity, or impaction of SO2 as the result of air movement over surfaces. Occult deposition is modified wet deposition. SO2 is deposited by wind driven mist or fog particles.
Schematic diagram of the major emission-deposition pathways
In 1950 Meetham suggested that dry deposition of SO2 must be a major removal process to the environment. A deposition velocity of 1.5cms-1 was deduced. Direct measurements of SO2 deposition to grassland (Garland et al, 1973), cereal crops (Fowler and Unsworth, 1979) and forest (Ensman et al, 1994), have shown the major characteristics of the deposition process. Rates of SO2 deposition to canopies of vegetation vary from 1mms-1 to 20mms-1, and comprise of two major sinks:
- Stomata absorb SO2, and the rate of uptake can be calculated from stomatal conductance and ambient SO2 concentrations.
- The external surface of the vegetation consisting of epicuticular wax surface debris and a water layer. Variability in this water layer is sufficient enough to regulate SO2 uptake. The fraction of time that vegetation surfaces are wet in NW Europe varies from 60-80%, depending on the vegetation type. Depth and leaf area index then influence the wetness of the vegetation. The ionic composition of the surface water then determines the solubility and fate of the dissolved SO2.
Flechard et al (1999) stimulated an SO42- deposition model and showed that its deposition was regulate din part by the presence of ambient NH3 and NH4 in solution and by solution pH. The dynamic model stimulates continuous fluxes and SO2 concentrations and related species for several days. A comparison between measured and modelled fluxes at Auchencarth Moss in the Scottish borders from Flechard et al (1999) shows good agreement in both the mean rates of exchange and its temporal variability. This recent work confirms data from previous studies, which suggested the role of NH3 in regulating SO2 deposition (Van Hove, 1989). The concentration ratio of NH3/SO2 exhibits large spatial variability as SO2 is from combustion processes and NH3 largely from livestock emissions. There are however, clear spatial patterns in the average deposition velocity for SO2 in vegetation as a consequence of different relative concentrations of NH3 and SO2 present.
Tingely et al (1971) demonstrated using the tabacco plant that there is potential for interaction between SO2 and NO2. The effects of the separate gases are increased greatly when combined.
Ambient SO2 has declined significantly over the past three decades. E.g., UK, Central England:
1950/1960 50ug m-3 SO2
Present 5ug m-3 SO2
According to Finlayson-pitts and Pitts (1986), typical peak ambient SO2 concentrations vary from <1ppb in remote areas, to 1-30ppb in rural areas, to 30-200ppb in moderately polluted areas, to 200-2000ppb in heavily polluted areas. The adverse effects of SO2 have been noted over the past 300 years. Many experiments have shown SO2 has the potential to affect plant yields. Cowling et al (1973) grew the S23 plant in controlled clean air and in air of varying SO2 concentrations in a green house. The grass was grown for 11 weeks.
The experiments demonstrated that relatively low levels of SO2 inhibited shoot yield, and that increasing SO2 conc seemed to have no more of an adverse effect on the plant.
Plant follicular symptoms to injury include necrotic injury, chlorosis and decreased pss. The extent of damage is related to; the genetic make up of the plant, developmental stage of growth, plant nutrient status, humidity and precipitation.
SO2’s first access to the plant is through stomatal openings. Once inside the leaf it is rapidly dissolved into the aqueous phase of the apoplast to form bisulphate and sulphite. Conversion of toxic sulphite to non-toxic sulphate may occur. The presence of sulphite is inhibitory to peroxidase activity and sulphite oxidation is competitive with the oxidation of phenolic compounds in lignin formation. Necrosis is governed by the accumulation of the oxidation products of phenolic compounds. Part of the SO2 absorbed is re-emitted as H2S, and can be viewed as homeostatic regulation. A certain resistance to SO2 can be built up. Fulber et al (1984) noted that young cucumber leaves absorbing high levels of SO2 were more resistant than mature leaves absorbing smaller amounts. About 60% of the SO2 absorbed were converted to SO42- by oxidation, but the young leaves emitted H2S over 100 times faster.
Short term exposure to SO2 (<50ppb) causes stomata opening; long-term exposure causes stomatal closing. This either enhances or depresses CO2 uptake and water loss. In general acute or chronic exposures to SO2 can result in a reduction in pss rate. Pratt et al (1983) showed an accumulation of total sulphur in SO2 fumigated soybean plants and a reduction in total chloroplast content. Harvey and Legge (1979) found a reduction in total ATP content in pine needles subject to chronic SO2 exposure suggesting a reduction in oxidative phosphorylation or increased energy consumption in stress repair.
Plant communities are hierarchies and SO2 pollution affects are related to connectedness of hierarchy. Guderian (1979) showed that under SO2 exposure interspecies competition is altered. The primary effect of SO2 on more susceptible species was magnified to such a degree that they could no longer compete effectively for vital growth determining factors. Leading to a decrease in sensitive species and an increase in resistant species.
Resistance will evolve when a population contains individuals with heritable differences in characters that effect fitness, e.g. reproduction of offspring. Murdy (1979) showed that animal weed Peppergrass from a SO2 polluted copper basin in Tennessee, USA, showed significantly less flower sterility after exposure of inflorescence to 0.8ppm SO2 for 9hours, than populations outside the basin. The population did not differ in sterility in the absence of SO2. However, resistance has energy costs that may reduce yields, or alter the species niche, therefore it would be an error to assume that breeding or evolution of resistance will always compensate for stress from SO2 and other pollutants.
There are a number of natural and anthropogenic sources that release SO2 into the atmosphere. Fuel combustion, metal smelting and oil and natural gas processing produce vast amounts of SO2. Of the 194 tonnes of SO2 emitted annually 83% is duel to fossil fuel combustion. The heat generated by the combustion process carries the SO2 convectively into the atmosphere. SO2 reacts with NO2 and OH in the atmosphere to produce acid rain. SO2 and acid rain have detrimental effects on vegetation such as chlorosis and reduction of photosynthesis.
