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Generally, the components in the roof runoff are derived from wet deposition or dust buildup on the rooftop. However, the chemical reactions between the rainwater and dust buildup (e.g. neutralization reaction between acid and alkali) may change the component mass in the roof runoff. Assume that the proportions of the ith component in the wet deposition and the dust buildup on the rooftop are Pw,i and Pb,i, respectively. The contributions of wet deposition to the component mass in the roof runoff may be possibly investigated by comparing Pw,i with Pb,i and considering the possible chemical reactions in the roof runoff. The following three cases may exist (Table 2)

  1. Pw,iPb,i

Table 2

Impacts of dust buildup and chemical reaction on contribution of wet deposition

Pw,i and Pb,iPossible effects of chemical reactions on the component mass in roof runoffPossible contribution
Pw,iPb,i no or insignificant change Ri ≈ 1 
significant increase Ri < 1 
significant decrease Ri > 1 
Pw,iPb,i no or insignificant change Ri < 1 
significant increase Ri < 1 
significant decrease Ri ≧ 1 
Pw,iPb,i no or insignificant change Ri < 1 
significant increase Ri < 1 
significant decrease Ri < 1 or Ri ≧ 1 
Pw,i and Pb,iPossible effects of chemical reactions on the component mass in roof runoffPossible contribution
Pw,iPb,i no or insignificant change Ri ≈ 1 
significant increase Ri < 1 
significant decrease Ri > 1 
Pw,iPb,i no or insignificant change Ri < 1 
significant increase Ri < 1 
significant decrease Ri ≧ 1 
Pw,iPb,i no or insignificant change Ri < 1 
significant increase Ri < 1 
significant decrease Ri < 1 or Ri ≧ 1 

Pw,i the proportion of the ith component in the wet deposition; Pb,i the proportion of the ith component in the dust buildup on the rooftop; Ri the ratio of mass of the ith component in the wet deposition to that in the roof runoff.

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