Sulfate, particulate deposition rate

The magnitude of the various processes varies with locality. Over vegetated continental areas typically a fifth of the sulfur is dry-deposited as SO2. In areas of high SO2 concentrations much higher amounts can be deposited. Where sulfate concentrations in particulate material are high, dry deposition rates can be greater. The balance of wet and dry deposition of sulfate particles over the ocean is uncertain, while some authors suggest dry deposition dominates others favor wet deposition (Wameck, 1999). 

In estimating the rate of gas-to-particle conversion involving S02 from anthropogenic sources, Peterson and Junge (1971) assumed that 66% is converted to sulfate and the rest is removed by dry deposition. In addition, it was assumed that sulfate is completely neutralized to ammonium sulfate. An emission rate of 160 Tg S02/yr from the combustion of fossil fuels then gives a production rate for particulate sulfate of 220 Tg/yr. In 1971 the rate of sulfur compound emissions from natural sources was less well known... 

The disk surface is thus exposed only to gaseous SO, not particulates. The American Society for Testing of Materials (ASTM) procedure suggests a 30-day exposure, followed by a standard sulfate analysis [16]. The other method sometimes used is the peroxide candle, similar in its function to the chloride candle, but again using lead peroxide to capture SOy In this procedure, a lead peroxide paste is applied to a paper thimble in the laboratory, and allowed to dry thoroughly before exposure. The thimble is then exposed in an instrument shelter to the test yard environment. In both cases, the SOj deposited results are appropriately reported in terms of deposition rate on the surface in units of mg/m /day. 

Estimating the deposition velocities of gaseous species is considerably more complex than estimating those for substances in particles, in part due to the uncertainties in the sticking and reaction probabilities. Such estimates have not been made but the potential effects of some of the typical gases can be surmised from available data on surface accumulation rates, e.g. sulfate accumulation on indoor zinc and aluminum surfaces is predominantly a result of particulate sulfate deposition rather than a corrosion reaction involving sulfur dioxide (0. 

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