Modeling Acid Rain Formation and Atmospheric Deposition (1)

Outline Acid Rain Formation – History – Precursors emissions – Gas phase chemistry – Oxidation of S(IV) to S(VI) in the aqueous phase

Brief History of Acid Rain First noted by John Evelyn in 17 th century – acidic city air on limestone and marble Farther of acid rain: Robert A. Smith (Scottish chemist). First to study the chemistry in and made the phase ‘acid rain’ in Public awareness of acid rain increases in the US in the 1970s Clean Air Act amendments in 1989: total reduction of 10 million tons of SO2 emissions from power plants. Clean Air Interstate Rule Cap and trade program of SO2 and NOx emissions. Permanently cap emissions in the eastern United States

Effects On ecosystems On buildings Bugini et al., Journal of Culture Heritage

Since 1998, Harvard University wraps some of the bronze and marble statues on its campus, such as this "Chinese stele", with waterproof covers every winter, in order to protect them from erosion caused by acid rain and acid snowHarvard UniversityChinese stele

Acid Rain Precursors

SO2 emission sources

NOx emission sources

NOx emissions

NOx emission sources

Acid Rain Formation Chemistry Gas phase processes Lifetime of NO2 due to OH oxidation is approximately 1 day. Lifetime of SO2 due to OH oxidation is approximately 1-2 weeks. If gas phase oxidation is the only pathway, one would expect acid rain occurs in areas much further downwind of the source region. However, from the rain acidity and SO2 emission map, we notice that both peak in the eastern US. (slow)

Location of coal power plants in US

Rain acidity trend

Acid Rain Formation Chemistry Partitioning of SO2 into cloud droplets Aqueous phase H2SO3 is a weak acid, and could not fully explain the acidity in the rain droplet under reasonable SO2 concentrations. Also, observed SO 4 2- distribution cannot be explained.

Acid Rain Formation Chemistry Aqueous oxidation of S(IV) into S(VI) – due to O3,, Why convert S(IV) to S(VI) can lead to higher H+ concentration?

Acid rain formation chemistry Aqueous oxidation of S(IV) into S(VI) – due to O3 Self-inhibition of this pathway under low pH!

Acid rain formation chemistry Aqueous oxidation of S(IV) into S(VI) – due to H2O2 H2O2 is approximately 7 orders of magnitude more soluble than O3. Thus under typical atmospheric conditions, this reaction pathway is usually more important than the O3 pathway.

Acid rain formation chemistry Aqueous oxidation of S(IV) into S(VI) – due to catalyzed O2 oxidation by Fe and Mn Usually trace amounts of catalyst metals always exists in real cloud or fog droplets, especially in urban areas. Fe(II) appears to not directly catalyze S(IV) oxidation and so it is the quantity of Fe(III) that is important.