MEMORANDUM

SUBJECT:	Ozone Monitoring Issues related to the Revised NAAQS: Network
Design Options Under Consideration

FROM:	Lewis Weinstock, OAQPS/AQAD/AAMG

TO:	Ambient Ozone Monitoring Regulations Review Docket
(EPA-HQ-OAR-2008-0338)

This memorandum summarizes network design options being considered as
part of the potential revisions to the ozone monitoring network
requirements.  The purpose of this memorandum is to provide a basis for
consultation with the Clean Air Scientific Advisory Committee (CASAC)
Ambient Air Monitoring & Methods (AAMM) Subcommittee on February 10,
2009.  The following paragraphs summarize the current ozone surveillance
network, existing design requirements, and network options under
consideration to improve network coverage.

BACKGROUND	

In the ozone NAAQS final rule (see 73 FR 16436) that revised the level
of the primary and secondary ozone NAAQS to an 8-hour average of 0.075
ppm, EPA also committed to a separate rulemaking that would support the
changes necessary in monitoring network requirements to ensure that the
ozone network was adequate in light of the changes.  EPA has completed
the final draft of this proposed rule and it is currently undergoing
review by the Office of Management and Budget (OMB).  The following is a
summary of the technical issues of the ozone monitoring rule as
transmitted to OMB.

CHANGES TO URBAN NETWORK DESIGN REQUIREMENTS

Presently, States (including the District of Columbia, Puerto Rico, and
the Virgin Islands) and local air quality management agencies when so
delegated by the State are required to operate minimum numbers of
EPA-approved ozone monitors based on the population of each of their
Metropolitan Statistical Areas (MSA) and the most recently measured
ozone levels for each area.  These requirements are contained in 40 CFR
part 58 Appendix D, SLAMS Minimum Ozone Monitoring Requirements, Table
D-2.  These requirements were last revised on October 17, 2006, as part
of a comprehensive review of ambient monitoring requirements for all
criteria pollutants. (See table 1).

	

Currently, the minimum number of ozone monitors required in an MSA
ranges from zero (for an area with a population of at least 50,000 and
under 350,000 and no recent history of an ozone design value greater
than 85 percent of the level of the NAAQS) to four (for an area with a
population greater than 10 million and an ozone design value greater
than 85 percent of the level of the NAAQS).  Because these requirements
apply at the MSA level, large urban areas consisting of multiple MSAs
can be required to have more than four monitors.  

There are 105 MSAs with populations between 50,000 and less than 350,000
that are presently without any ozome monitors supporting design value
calculations for either 2004 to 2006 or 2005 to 2007 (see figure 1).  
These unmonitored MSAs have a total population of approximately 18
million people and include areas in 37 States and Puerto Rico.  The
existing regulations do not require these MSAs to begin monitoring for
ozone.  Comments that were received from State air monitoring agencies
and from multi-state air planning organizations in response to the ozone
NAAQS proposal expressed concern that these requirements ignore the
needs that States and localities have for additional monitors to measure
ozone levels in a variety of locations, particularly in  areas with
populations under 350,000.  The commenters stated that unless this
deficiency is corrected, the health benefits of EPA’s ozone NAAQS
revision would likely be limited to those living in MSAs having
populations of more than 350,000.  Other commenters noted the difficulty
in defining the boundaries of new attainment/non-attainment areas
without additional monitoring in the MSA’s below 350,000 population.

EPA is considering the modification of the minimum ozone monitoring
requirements to require one monitor to be placed in MSAs of populations
of between 50,000 and less than 350,000 in situations when there is an
absence of a design value.  States would likely have to install some new
monitors and/or have the option to relocate existing monitors under
certain conditions.

CHANGES TO NON-URBAN NETWORK DESIGN REQUIREMENTS

The revised secondary standard put into place during the recent NAAQS
revision was intended to provide protection to sensitive vegetation in
less urbanized areas, in particular those Class I and Wilderness areas
set aside by Congress to be protected so as to conserve the scenic value
and the natural vegetation and wildlife within such areas, and to leave
them unimpaired for the enjoyment of future generations.  The secondary
ozone NAAQS also considered the benefits that would be provided to the
public welfare from increased protection of sensitive vegetation in
other Federal, State, Tribal and/or public interest lands that have been
set aside for a similar purpose.  These areas are characterized by the
presence of ozone-sensitive species of native vegetation that have been
shown to be subject to ozone-induced visible foliar injury, impaired
growth, and/or other adverse impacts to a degree that could be
considered adverse.

Currently, existing ozone monitoring requirements and current State
monitoring practices are primarily oriented towards protecting against
human health effects and therefore towards reporting compliance with the
primary NAAQS.  This accounts for the current focus of the monitoring
requirements on urban areas, where large populations reside, in which
significant emissions of ozone-forming precursors are found, and where
ozone concentrations of concern have been historically measured.  EPA
believes that the previously described potential changes to urban
monitoring requirements will be adequate for determining compliance with
the secondary NAAQS in MSAs, noting that the assessment of welfare
effects has not been a traditional objective of urban-based ozone
monitoring networks.

There are no EPA requirements for ozone monitoring in less populated
areas outside of MSA boundaries or in rural areas.  However, at present
there are about 200 State operated ozone monitors in counties that are
not part of MSAs.  EPA operates a network of about 58 ozone monitors as
part of its Clean Air Status and Trends Network (CASTNET). The National
Park Service (NPS) operates about 23 monitors at other CASTNET sites. 
CASTNET ozone monitors are primarily located in rural areas; siting
criteria require distances of at least 40 kilometers from cities of
greater than 50,000 population as well as other separation requirements
from air pollution sources.  Taking into account both State and EPA/NPS
operated non-urban ozone monitors, an analysis of the distribution of
these monitors indicates a relatively high spatial density in the
eastern one-third of the U.S. and in California, with significant gaps
in coverage elsewhere across the country.  Virtually all States east of
the Mississippi River have at least several non-urban ozone monitors,
while many large mid-western and western States have one or fewer
monitors (see figure 2).

EPA believes that the public input received on this issue during the
ozone NAAQS process together with internal technical reviews support the
consideration of additional non-urban monitoring requirements.  These
requirements would support the following objectives: 

(1) Providing better characterization of ozone exposures to
ozone-sensitive vegetation and ecosystems in rural/remote areas to
ensure that potential secondary NAAQS violations are measured.  This
objective would also serve the purpose of providing more consistent
support for studies examining the impact of elevated ozone levels in
wilderness areas, locations with ozone-sensitive natural vegetation, and
in areas such as National Parks.

(2) Assessment of population exposure due to elevated ambient ozone
levels in smaller communities located outside of the larger urban MSAs
covered by urban monitoring requirements.

(3) Assessment of the location and severity of maximum ozone
concentrations that occur in non-urban areas. 

Given these objectives, EPA is considering the addition of a requirement
that each State operate a minimum of three non-urban ozone monitors in
addition to the current and potential urban ozone monitoring
requirements described earlier.  The first required non-urban monitor
could be located in areas such as some Federal, State, or Tribal lands,
including wilderness areas that have ozone-sensitive natural vegetation
and/or ecosystems; lands with other ownership may also be appropriate. 
The second required non-urban monitor could be placed in a Micropolitan
Statistical Area expected to have ozone design value concentrations of
at least 85 percent of the NAAQS.   The third required non-urban monitor
could be placed in the area of expected maximum ozone concentration
outside of any MSA, potentially including the far-downwind transport
zones of currently well-monitored urban areas. 

CHANGES TO REQUIRED OZONE MONITORING SEASON

EPA has done an analysis to address the issue of whether extensions of
currently required monitoring seasons are appropriate in light of the
revised NAAQS.  The analysis has investigated the occurrence of
exceedances of the revised NAAQS (8-hour ozone averages above 0.075 ppm)
in unmonitored months using monitors collecting ozone data year-round in
2004-2006.   Additionally, the analysis examined occurrences of 8-hour
ozone averages of at least 0.060 ppm, the concentration that serves as
the revised AQI breakpoint between the Good and Moderate indicator
level.  A statistical model was used to predict ozone concentrations in
areas where no year-round ozone monitors were operated.  The model is
based on the relationship between maximum 8-hour ozone concentrations
and meteorological variables including temperature and relative
humidity.

Exceedances of the revised ozone NAAQS during months outside of the
required monitoring season occurred in eight States during the 2004-2006
study period (Maine, Massachusetts, New Hampshire, New Jersey, New York,
South Carolina, Vermont, Wyoming).  Except for Wyoming, these
exceedances occurred in a very limited manner, occurring just before the
beginning of the required ozone monitoring season which began on April 1
for those States (i.e., on March 30 or March 31).  In Wyoming, the
frequency of ozone exceedances before the beginning of the required
ozone season was higher, with multiple occurrences noted at several
sites up to two months prior to the April 1 startup of required ozone
monitoring.  

	

As would be expected, the frequency of occurrences of the revised
Moderate AQI level during months outside of the required monitoring
season revealed many examples of such readings.  A total of 32 States
experienced such occurrences; 22 States with Moderate level AQI readings
only before the required season, 9 States with such levels noted before
and after the required season, and 1 State with such levels only after
the season.  In a number of cases, the frequency of such Moderate AQI
levels was quite high, with some States experiencing over a dozen days
during 2004 to 2006 at a high percentage of their operating year-round
ozone monitors.

The specific changes to the required State ozone monitoring seasons that
are under consideration are as follows: Minnesota, an increase of one
month (19 states), two months (6 states), four months (3 states), and
five months (Wyoming).  Ozone season requirements are currently split by
Air Quality Control Region in Louisiana and Texas.  Included in the
above State-by-State accounting is the proposal to lengthen the required
season in the northern part of Louisiana by one month (southern
Louisiana ozone monitors would remain on a required year-round schedule)
and the proposal for the required season in Texas to become year-round
for the entire State. 

NCore stations are required to operate a full suite of gaseous and
particulate matter monitors as well as basic meteorology to support
these objectives.  Given the potential value of NCore data to support
year-round scientific studies, EPA believes that it is appropriate to
require that ozone monitors at NCore stations be operated on a
year-round basis.  Accordingly, EPA is considering a change to the
required monitoring season for NCore stations to a year-round schedule
regardless of the length of the required ozone monitoring season for the
remainder of the SLAMS monitors within a State. 

≥85% of any O3 NAAQS3	Most recent 3-year design value concentrations
<85% of any O3 NAAQS3,4

>10 million	4	2

4 - 10 million	3	1

350,000 - <4 million	2	1

50,000 - <350,0005	1	0

1 Minimum monitoring requirements apply to the Metropolitan statistical
area (MSA).

2 Population based on latest available census figures.

3 The ozone (O3) National Ambient Air Quality Standards (NAAQS) levels
and forms are defined in 40 CFR part 50.

4 These minimum monitoring requirements apply in the absence of a design
value.

5 Metropolitan statistical areas (MSA) must contain an urbanized area of
50,000 or more population.

Figure 1

 

Figure 2

 

 

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