Many studies and applications of indicator species approaches have addressed ozonerelated

problems. Sillman (1995), Sillman (1998) and Lu and Chang (1998) provide good

descriptions of the method, identify key ratios and illustrate application of the approach. Even

though the preceding ratios are oriented toward ozone, they can provide insight into why modeled

concentrations of secondary particulate matter are sensitive to changes in VOC, or NOx emissions

(Pun and Seigneur, 1999). Ansari and Pandis (1998) have developed an indicator ratio of species


and applied it to several combinations of secondary particulate matter present under different

environmental conditions. They use this ratio to predict how mass of particulate matter will

respond to reductions in sulfate, nitrate and ammonia. Blanchard et al., (2000) have also

examined how indicator species might be used to assess whether particulate nitrate concentrations

are limited by NOx or by ammonia emissions using mechanisms which incorporate reactions

dealing with secondary particulate matter. These authors identify two ratios of indicator species

which appear potentially useful for identifying limiting precursors for secondary nitrate

particulate matter: (1) the ratio of particulate ammonium plus gas-phase ammonia over the sum of

nitric acid plus particulate nitrate plus particulate sulfate, and (2) the ratio of particulate to total

nitrate. It is likely that additional indicator species approaches will be identified as the user

community continues to gain more experience with chemical mechanisms incorporating

secondary particulate formation and more speciated particulate and gas phase ambient

measurements become available.

Other observational methodo


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