Measuring the Effects of the Clean Air Act Amendments on Ambient PM10 Concentrations: The critical importance of a spatially disaggregated analysis
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We examine the effects of the 1990 Clean Air Act Amendments (CAAAs) on ambient concentrations
of PM10 in the United States between 1990 and 2005. We find that non-attainment
designation has no effect on the ‘average monitor’ in non-attainment counties, after controlling
for weather and socioeconomic characteristics at the county level. In sharp contrast, if we allow
for heterogeneous treatment by type of monitor and county, we do find that the 1990 CAAAs
produced substantial effects. Our best estimate suggests that PM10 concentrations at monitors
with concentrations above the national annual standard dropped by between 7μg/m3 and
9μg/m3, which is roughly equivalent to a 11-14% drop. We also show that monitors which were
in violation of the daily standard experience two fewer days in violation of the daily standard the
following year. Empirical results suggest that this treatment effect is independent of whether
the EPA has finalized the non-attainment designation.