Collecting Measurements for Indicator Species Measurement of “indicator species” is

a potentially useful means for assessing which precursor category limits further production of

ozone or secondary PM2.5 at a monitor’s location at various times of day and under various sets

of meteorological conditions. Many of the gas phase species needed to calculate indicator ratios

are not routinely measured. Some of the species needed to calculate ozone indicator ratios are

NOy and HNO3. Some of the species needed to calculate the gas ratio and excess ammonia (for

nitrate analyses) include NH3 and HNO3. Thus, realizing the potential of the “indicator species

method” as a tool for model performance evaluation and for diagnosing why observed

concentrations do or do not meet previous expectations may depend on deploying additional

monitors and/or measurements.

Making measurements aloft. Almost all measured ambient air quality and

meteorological data are collected within 20 meters of the earth’s surface. However, the

modeling domain extends many kilometers above the surface. Further, during certain times of

day (e.g., at night) surface measurements are not always representative of air quality or

meteorological conditions aloft. Measurements aloft can be made by aircraft (usually during

special studies) or on tall buildings or towers. Concentrations aloft can have marked effects

when they are mixed with ground-level emissions during daytime. Thus, the weight given to


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