   EPA New England

MEMORANDUM

DATE:     July 27, 2007

SUBJ: 	Technical Support Document for Revision to the Carbon Monoxide
Maintenance Plan for Nashua, New Hampshire

FROM: 	Bob Judge, Environmental Engineer

Air Quality Planning Unit, Office of Ecosystem Protection

  TO: 	Anne Arnold, Manager

Air Quality Planning Unit, Office of Ecosystem Protection 

On May 30, 2007, the State of New Hampshire submitted a formal revision
to its State Implementation Plan (SIP).  The SIP revision consists of a
modification to New Hampshire’s carbon monoxide (CO) maintenance plan
for Nashua.

Background

The City of Nashua, New Hampshire was designated nonattainment for the
CO National Ambient Air Quality Standard (NAAQS) on April 11, 1980 (45
FR 24869).  In 1991, following passage of the 1990 Clean Air Act
Amendments, the city retained this designation by operation of law (see
56 FR 56694), although ambient monitoring showed attainment at that
time. On February 2, 1999, the State of New Hampshire submitted a formal
CO redesignation request for Nashua.  On November 29, 2000, the U.S. EPA
redesignated the city of Nashua nonattainment area to attainment for the
CO standard and approved New Hampshire’s maintenance plan for this
area (65 FR 71060).  Since that time, there have been no violations of
the CO NAAQS of 9 ppm (8 hour average) or 35 ppm (1 hour average) in
Nashua, and CO is trending downward to a level less than half the NAAQS.


Evaluation of New Hampshire’s Submittal

New Hampshire would like to discontinue monitoring for CO in Nashua. 
Nashua is a “sole pollutant” site, monitoring only CO.  The New
Hampshire Department of Environmental Services (DES) allocates
considerable resources each year to cover the lease agreement with the
property owner, maintenance of the monitoring and support equipment,
phone and electricity, travel to and from the site from the main office
in Concord, and staff time to conduct routine and emergency maintenance,
all of which are resources that could better be used elsewhere in the
state’s monitoring program. 

As previously stated, CO levels have been falling in Nashua since the
early 1980s.  (See attached Graph #1 of Nashua CO levels.)  The NAAQS of
9 ppm (8 hour average) has not been violated since 1986, and current
readings of the 8 hr 2nd high design value are in the 2 to 3 ppm range,
and have been for the past three (3) years. Most of the CO reduction has
been due to “fleet turnover,” with newer, cleaner motor vehicles
making up an increasing share of the fleet, and this trend is unlikely
to change. 

In addition to the downward trend shown by the monitoring data, the
state has performed mobile source modeling and conformity analyses which
indicate that winter time CO levels in Nashua will not reach even half
the CO Conformity Budget as far out into the future as 2025, beyond the
end of the maintenance plan.  

 

In this SIP revision, NH is modifying Section 6.5.3 “Selection of a
Nonattainment Indicator” (page 27) portion of the federally approved
CO maintenance plan.  That approved plan states: “The 9/4/92
EPA/Calcagni memorandum (Section 5e) obligates the State to select a
contingency threshold or ‘trigger,’ such as a monitored CO
exceedance or violation, and commit to implementing the appropriate
contingency measure once the threshold is exceeded. For the purposes of
this plan, New Hampshire is proposing to implement its contingency
program in the event that a CO violation (the ‘contingency trigger’)
is monitored in Nashua in any year during the maintenance period and to
consider one or more of the other EPA-approved measures… if
necessary.”     (italics added.)

Based on the downward CO trend, New Hampshire is modifying the
contingency threshold or “trigger” in Nashua.  The state will cease
monitoring for CO in Nashua, and instead, will continue monitoring for
CO in nearby Manchester, and to use these data as a surrogate for Nashua
CO levels. (See attached Graph #2 comparing CO levels in the two
cities.)  Nashua and Manchester are less then 20 miles apart, have
similar populations (Manchester is slightly larger), and both have had
CO concentrations similar to each other for years.  Both were designated
nonattainment in 1990 for CO “by operation of law” though both had
design values below the standard at that time.  In both cases, only the
city itself was designated nonattainment since data did not support an
expansion of the nonattainment area.  Both were redesignated to
attainment in 2000, and both measure CO concentrations well below the
standard at this time.  Both are in Hillsborough County, New Hampshire. 
 In light of this, the italicized portion of Section 6.5.3, above, is
being modified to read as follows:

               

            “For the purposes of this plan, New Hampshire will be
discontinuing CO monitoring in    Nashua upon EPA approval of this
revised plan.  New Hampshire DES will continue to collect and review CO
monitoring data from nearby Manchester, NH on an on-going basis. In the
event the second-highest CO concentration in any calendar year monitored
in Manchester reaches 75 percent of the federal 1-hour or 8-hour
national ambient air quality standard (NAAQS) for CO, New Hampshire
will, within 9 months of recording such concentrations, re-establish a
CO monitoring site in Nashua consistent with EPA siting criteria, and
resume analyzing and reporting those data.  New Hampshire will continue
to commit to implement its contingency program in Nashua in the event
that a CO violation (the ‘contingency trigger’) is monitored at the
re-established Nashua monitoring site at any time during the maintenance
period, and to consider one or more of the other EPA-approved measures
listed in Section 6.5.2 if necessary to reduce CO levels.

           If the Manchester CO monitor measures a violation of either
the federal 1-hour or 8-hour NAAQS for CO, the contingency measures in
Section 6.5.2 will be implemented in Nashua as well, until a
re-established Nashua CO monitor shows that the area is in attainment of
the CO standard.

         When implementing contingency measures, New Hampshire will
review and implement the measures necessary to remedy the violation,
including transportation control measures (TCM) or other additional
vehicle or fuel controls.”

Recommended Action

This SIP revision should be approved.

For the reasons outlined above, EPA agrees that the criteria New
Hampshire DES will utilize as quoted above represents a reasonable plan
modification and approach for shutting down a CO monitor that is
currently measuring concentrations well below the existing 1- hour and
8- hour CO national ambient air quality standards.  The revisions
represent an acceptable alternative contingency triggering mechanism as
part of a CO maintenance plan.  Under this plan, we believe air quality
goals can be maintained, and State monitoring resources, conserved.



ATTACHMENT

Graph #1

Graph #2

 

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