economic analyses used to estimate the benefits
associated with achieving a revised standard compared to maintaining an existing standard. The
regulatory impact analysis prepared for ozone
and particulate matter [67] and separately for particulate matter [76] and ozone [78] are examples.
Substantial controversy developed in the final
stages of revision of the PM2.5 standard
announced in 2006. A majority of the EPA’s
Clean Air Scientific Advisory Committee’s
(CASAC) PM Panel recommended that the EPA
Administrator reduce the PM 2.5 24-h averaging
time standard from the previous level of 65 mg/
m
3 to a level in the range of 25–35 mg/m
3 and that
for the PM 2.5 annual averaging time the standard
be reduced from the previous level of 15 mg/m
3 to
a level in the range of 12–14 mg/m
3 . I was in the
minority and argued that identifying a specific
bright line, upper bound such as 14 mg/m
3 was
an issue that went beyond scientific interpretation
of data and was a policy judgment as to acceptable
risk. The Administrator, ultimately, revised the
PM 2.5 NAAQS, 24-h averaging time standard, to
35 mg/m
3 and retained the annual standard at
15 mg/m
3 [74, 75]. Obviously, the majority of
CASAC was not happy with the Administrator’s
policy judgment call.
Ozone
Ozone is an interesting air pollutant because it is
not directly emitted from industrial sources except
in rare circumstances. The majority of Ozone
found in the ambient environment is formed
from reactions of Volatile Organic Compounds
(VOC) and Nitrogen Oxides (NO x ) in the presence of sunlight. The VOCs originate both from
natural and industrial sources and the NO x is
largely from combustion of hydrocarbon fuels.
Ozone concentrations typically have distinct diurnal cycles with the highest levels observed at
midday and the lowest concentrations at night in
the summer. Lower concentrations are typically
observed in the winter than in the summer. This is
the case because of the manner in which ozone is
formed with sunlight driving the reactions
between NO x and VOCs.
To provide context for the discussion of the
Ozone NAAQS it is important to understand the
background levels of ambient ozone. A team of
investigators at Harvard University led by Daniel
Jacob have made major contributions to understanding tropospheric ambient ozone levels
around the world using a global chemical transport model (GEOS-Chem). The Harvard team
[90] reported the use of the GEOS-Chem model
with 1
latitude by 1
longitude spatial resolution
to quantify the effects of anthropogenic emissions
from Canada, Mexico, and outside North America
to estimate daily maximum 8-h average ozone
concentrations in US surface air. Their simulations for summer 2001 estimated mean North
American and US background contributions of
26 Æ 8 ppb and 30 Æ 8 ppb, as obtained by
eliminating anthropogenic emissions in North
America versus in the United States only. The
background 8-h highest Ozone concentrations
approached 60 ppb. The frequency distribution
of the simulated daily 8-h-maximum surface
Ozone concentrations are shown in Fig. 9. It is
important to recognize that the statistical form of
the Ozone NAAQS is directed at minimizing the
occurrence of the highest ozone events, the right
side tail of the distributions in the figure. The high
levels of background Ozone will make it challenging to achieve any Ozone NAAQS set in the range
of 60–80 ppb (8-h daily maximum).
The initial Ozone NAAQS in the United States
was set in 1971 using photochemical oxidants as
an indicator and with a 1-h averaging time [7,
64]. The standard was later revised with ozone
as the indicator. Later the averaging time was
changed from a 1-h maximum during a 24-h
period to an 8-h rolling average maximum.
There is an abundant database on the health
effects of ozone from epidemiological investigations and toxicity studies conducted in laboratory
animals [38, 71, 80, 81]. In addition, there is a
substantial amount of data acquired from studies
conducted with human volunteers exposed to controlled levels of ozone for short periods of time. In
the setting of the Ozone NAAQS, quantitative
exposure-response data from human studies have
been given the greatest weight. This is especially
appropriate since there are well-established
32
Air Quality Guidelines and Standards
associated with achieving a revised standard compared to maintaining an existing standard. The
regulatory impact analysis prepared for ozone
and particulate matter [67] and separately for particulate matter [76] and ozone [78] are examples.
Substantial controversy developed in the final
stages of revision of the PM2.5 standard
announced in 2006. A majority of the EPA’s
Clean Air Scientific Advisory Committee’s
(CASAC) PM Panel recommended that the EPA
Administrator reduce the PM 2.5 24-h averaging
time standard from the previous level of 65 mg/
m
3 to a level in the range of 25–35 mg/m
3 and that
for the PM 2.5 annual averaging time the standard
be reduced from the previous level of 15 mg/m
3 to
a level in the range of 12–14 mg/m
3 . I was in the
minority and argued that identifying a specific
bright line, upper bound such as 14 mg/m
3 was
an issue that went beyond scientific interpretation
of data and was a policy judgment as to acceptable
risk. The Administrator, ultimately, revised the
PM 2.5 NAAQS, 24-h averaging time standard, to
35 mg/m
3 and retained the annual standard at
15 mg/m
3 [74, 75]. Obviously, the majority of
CASAC was not happy with the Administrator’s
policy judgment call.
Ozone
Ozone is an interesting air pollutant because it is
not directly emitted from industrial sources except
in rare circumstances. The majority of Ozone
found in the ambient environment is formed
from reactions of Volatile Organic Compounds
(VOC) and Nitrogen Oxides (NO x ) in the presence of sunlight. The VOCs originate both from
natural and industrial sources and the NO x is
largely from combustion of hydrocarbon fuels.
Ozone concentrations typically have distinct diurnal cycles with the highest levels observed at
midday and the lowest concentrations at night in
the summer. Lower concentrations are typically
observed in the winter than in the summer. This is
the case because of the manner in which ozone is
formed with sunlight driving the reactions
between NO x and VOCs.
To provide context for the discussion of the
Ozone NAAQS it is important to understand the
background levels of ambient ozone. A team of
investigators at Harvard University led by Daniel
Jacob have made major contributions to understanding tropospheric ambient ozone levels
around the world using a global chemical transport model (GEOS-Chem). The Harvard team
[90] reported the use of the GEOS-Chem model
with 1
latitude by 1
longitude spatial resolution
to quantify the effects of anthropogenic emissions
from Canada, Mexico, and outside North America
to estimate daily maximum 8-h average ozone
concentrations in US surface air. Their simulations for summer 2001 estimated mean North
American and US background contributions of
26 Æ 8 ppb and 30 Æ 8 ppb, as obtained by
eliminating anthropogenic emissions in North
America versus in the United States only. The
background 8-h highest Ozone concentrations
approached 60 ppb. The frequency distribution
of the simulated daily 8-h-maximum surface
Ozone concentrations are shown in Fig. 9. It is
important to recognize that the statistical form of
the Ozone NAAQS is directed at minimizing the
occurrence of the highest ozone events, the right
side tail of the distributions in the figure. The high
levels of background Ozone will make it challenging to achieve any Ozone NAAQS set in the range
of 60–80 ppb (8-h daily maximum).
The initial Ozone NAAQS in the United States
was set in 1971 using photochemical oxidants as
an indicator and with a 1-h averaging time [7,
64]. The standard was later revised with ozone
as the indicator. Later the averaging time was
changed from a 1-h maximum during a 24-h
period to an 8-h rolling average maximum.
There is an abundant database on the health
effects of ozone from epidemiological investigations and toxicity studies conducted in laboratory
animals [38, 71, 80, 81]. In addition, there is a
substantial amount of data acquired from studies
conducted with human volunteers exposed to controlled levels of ozone for short periods of time. In
the setting of the Ozone NAAQS, quantitative
exposure-response data from human studies have
been given the greatest weight. This is especially
appropriate since there are well-established
32
Air Quality Guidelines and Standards