Environmental Engineering Reference
In-Depth Information
from the STN network range from about 1-5 ng/m
3
for 8 of the 10 target metals; Zn
(14 ng/m
3
) and Fe (94 ng/m
3
) are the exceptions (Table
1
).
The STN network has confi rmed that the highest metal concentrations are found
in heavily industrialized cities such as Birmingham, AL, Pittsburgh, PA, and East
Saint Louis, IL. Since these high-concentration areas are the most likely to show
health effects, if such exist, it is important to have confi rmation that measured con-
centrations are a reliable representation of potential exposures. Hyslop and White
(
2008
;
2009
) compared the STN and IMPROVE network detection limits for six
elements, fi ve of which are included in our ten targeted metals: As, Cu, Fe, Ti, Mn,
and Se; they found that the detection limits for all metals were approximately 10
times lower for the IMPROVE network than for the STN network. Detection limits
in ng/m
3
for the STN 2008 data and the IMPROVE 2006 data, for the major metals
of interest in this review were, respectively: Cu (1.4 and 0.043); Fe (2.0 and 0.039);
Mn (2.3 and 0.026); Ni (1.3 and 0.059); V (1.5 and 0.034); and Zn (1.5 and 0.039).
Reff et al. (
2009
) provides a complete discussion of major sources of the target ele-
ments. The laboratories contributing to the two networks have recently agreed to
“harmonize” their measures of uncertainty and detection limits by adopting similar
or identical methods (Gutknecht et al.
2010
). It is anticipated that recent progress in
determining empirical method detection limits (MDLs) for duplicate monitors in
the STN and IMPROVE systems will provide more reliable detections and mea-
surements for PM metals across geographic locations, and ultimately benefi t the
conduct of epidemiological studies.
The Canadian National Air Pollution Surveillance Network (NAPS) monitors the
largest cities in Canada and other sites that are chosen mainly for their intensity of
industrial activity. The European Environment Agency also monitors several air
quality parameters, including PM
10
, PM
2.5
, As, Cd, Ni and Pb. Maps of monitored
sites can be found on-line at
http://www.eea.europa.eu/themes/air/airbase/
.
Useful
data on a large number of samples in eight Canadian cities (Burnett et al.
2000
)
were collected in two additional studies, and in one U.S. city (Steubenville, OH),
long studied closely for its relatively high levels of ambient PM (Connell et al.
2006
). Table
2
summarizes the average ambient concentrations of PM
2.5
and metals
of interest and the percent of PM
2.5
mass from several studies.
Recent progress has been made in estimating emissions of metals on a nation-
wide geographic grid. A special analysis in this report suggested that, at least for the
highest observed ambient concentrations, the emissions estimates are often consis-
tent with those measurements. A far more complete study is needed on the entire
STN data (not just the top 2% as in our analysis) for the years surrounding the latest
emission inventory.
Although much more information is now available on metal concentrations in
ambient air due to the STN network, additional work needs to be done in fi rst
collecting data from duplicate monitors over a period of years on empirical preci-
sion, and then using that information to detect areas of sampling and analysis that
can be improved. When completed, such work will improve detection limits and
will produce more accurate results for the less-common elements.
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