88This is particularly true for PM2.5 in areas with little or no ambient speciation data.

assessed by normalizing to account for year-to-year meteorological variations. Second, one must

have an accurate accounting of the year-to-year changes in actual emissions (NOx, VOC, and/or

SO2 and NH3) for the given area and any surrounding areas whose emissions may impact local

concentrations. Third, one must have a solid conceptual model of how ozone or PM2.5 is formed

in the local area (e.g., NOx-limited, ammonia limited, transport-influenced, etc.). Assuming all

of these prerequisites can be met, then it may be possible to develop a curve that relates past

emissions changes to historical and current air quality. Once the relationship between

past/present emissions and air quality is established, this “response factor” can be applied to the

expected emissions reductions from a particular control strategy.


If available, meteorologically adjusted ozone and PM2.5 concentrations can be used to

establish air quality trends. There are several techniques that have been used to examine the

influence of meteorology on air quality. Among them are (a) statistical modeling (U.S. EPA,

2005c ); (b) filtering techniques (Rao and Zurbenko, 1995, Flaum, et al., 1996, Milanchus, et al.,

1998, Hogrefe, et al., 2000), (c) using a probability distribution of meteorological severity based

on climatological data (Cox and Chu, 1993, 1996), (d) using CART analysis to identify

meteorological classes and selecting days from each year so that the underlying frequency of the

identified meteorological classes remains the same (Stoeckenius, 1990, Deuel and Douglas,

1996). Most of this work has examined the relationship between ozone and meteorology. Only

recently have analyses examined the relationship between meteorology and PM2.5. Additionally,

compared to PM2.5, the established relationship between ozone and meteorological variables is

stronger (higher r-square values). In the case of PM2.5, the relationship between concentration

and meteorology is complicated by the fact that PM2.5 components experience high

concentrations at different times of the year and for different reasons. This makes it more

difficult to meteorologically adjust PM2.5 concentrations.


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