northeast and for BCt in the midwest, representing a
collection of 79 days for each 8-year period. The change
is also significant above the 97.5th percentile for COt in the
midwest (12 days for each 8-year period). Our results
indicate an increase in the severity of summertime pollution
episodes in these two regions by 2050. Elsewhere in the
United States in summer and for most regions during other
seasons, we do not detect significant differences in the
frequency distributions of concentrations between presentday
and 2050 climates.