Environmental Engineering Reference
In-Depth Information
modeling and the model evaluation against observed data can be seen elsewhere
[3].
CMAQ outputs are largely characterized in our analysis using intake fractions
(iFs), to elucidate the emissions-to-exposure relationship in a form relevant for
population health risk estimation. The quantitative definition of iF is
iF
j
= Σ
i
(P
i
ΔC
ij
)*BR/Q
j
where iF
j
is the intake fraction for exposure to pollutant j, P
i
refers to the
population in geographic area i, ΔC
ij
(in μg/m
3
) is the change in ambient
concentration at geographic area i related to emissions Q
j
, and BR is a nominal
population breathing rate (assumed to be 20 m
3
/day). Of note, BR is divided back
out in a risk calculation, so this assumption is only included to ensure that iFs are
unitless measures. We estimate iFs for all modeled air toxics and calculate total
population exposure for primary and secondary PM
2.5
, given the challenge of
extracting iF values for secondary PM
2.5
constituents based on CMAQ outputs.
3. Results
In
Fig. 1,
we present iFs for air toxics and cumulative population exposure for
PM
2.5
. We present values at increasing distance from each airport using 36- and
12-km resolution CMAQ results, and we include secondary formation in the
estimated concentrations. Findings demonstrate systematically higher iF values
using 12-km resolution and for less reactive pollutants, with clear influences of
population centers. Most national-scale population exposure is captured within
612 × 612 km domains surrounding each airport (95-100% across all air toxics
and model resolutions), although less so for secondary byproducts and where large
populations are found just outside of these domains (e.g., for PVD). In contrast,
for PM
2.5
, while most of the total population exposure for primary constituents is
found within this domain (96% with 36-km and 98% with 12-km resolution), the
corresponding values are 60% and 64% for the secondary constituents that
dominate public health impacts. More than half of the population exposure to air
toxics is observed within a 108 × 108 km domain surrounding each airport, but for
PM
2.5
, half of the total population exposure occurs outside of 324 × 324 km
domains surrounding each airport. In part because of the larger domain necessary
to capture PM
2.5
population exposure, model resolution has less of an influence on
PM
2.5
population exposure estimates than for air toxics.
