UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

REGION III

1650 Arch Street

Philadelphia, Pennsylvania 19103

DATE: May 12, 2015

SUBJECT: Supplemental Technical Support Document – Determination of Attainment for 

the Balitmore, MD 2008 8-Hour Ozone Nonattainment Area

RE: Response to Comments Concerning Baltimore Area Invalidated and Missing 

Monitoring Data

FROM: Irene Shandruk, Physical Scientist /s/

Office of Air Program Planning 

TO: EPA-R03-OAR-2014-0884

THRU: Cristina Fernandez, Associate Director /s/

Office of Air Program Planning

I. Background

On March 18, 2015, EPA proposed to determine that the Baltimore, Maryland Moderate 

Nonattainment Area (Baltimore Area) has attained the 2008 8-hour ozone National Ambient Air 

Quality Standard (NAAQS) (hereafter referred to as “NPR”). 80 FR 14041. EPA received 

adverse comments on the proposed determination of attainment. The commenter stated that 

Baltimore’s ozone monitors do not fully and accurately capture all maximum ozone exposures 

and asserted there were many instances where ozone monitors “shut off” for at least a day during 

a heat wave when ozone levels were rising. The commenter claimed that had the monitors 

collected data during those periods, ozone exceedances would likely have occurred which may 

have altered the Baltimore Area’s ozone design value and adversely affected any “clean” data for 

the Baltimore Area which might delay EPA’s determination of attainment for the Baltimore Area 

with the 2008 ozone NAAQS.

EPA consulted with the Maryland Department of the Environment (MDE) concerning the 

monitors questioned by the commenter, including the monitors at Davidsonville, Padonia, and 

Aldino. MDE reviewed the monitoring data at these monitors for the dates alleged by the 

commenter as “shutting off.” MDE provided information to EPA supporting the conclusion that 

these monitors did not “shut off” as the commenter alleges but that MDE invalidated data from 

these monitors for brief periods of time due to extenuating circumstances such as condensation 

build up and power failures. 

II. Responsive Information for Comments Concerning Monitor Operation

1. Comment regarding Davidsonville data from 7/17/12-7/19/12 (page 27 of comments on 

EPA’s NPR)
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a. In a Number of Instances, Ozone Monitors Shut Off as Ozone Concentrations Increased, 

Nearing an Exceedance 

When examining just the past five years of hourly data from these ozone monitors, it is clear that 

there were many instances of ozone concentrations increasing and nearing an exceedance, and 

then shutting off and not reporting any data. These instances often happened during peak ozone 

season and/or a summer heat wave. 

The monitor at the Davidsonville Recreation Center (#240030014), which had a 2012-2014 

design value of 74 ppb, displayed this phenomenon numerous times. For example, July 17, 2012 

was a very hot day, with a maximum temperature of 100 degrees, and very high hourly ozone 

values, starting at 10 am, of 83 ppb, 93 ppb, 95 ppb, 91 ppb, 91 ppb, 95 ppb, and 102 ppb, but, at

5 pm, the monitor shut off.

The monitor did not record any data for July 18, 2012, an even hotter day up to 104 degrees, and 

did not come back online until noon on July 19, 2012, which was a relatively cooler day with a 

maximum of 95 degrees. The monitor’s first recorded values when it came back online were, 

chronologically, 74 ppb, 82 ppb, 90 ppb, 90 ppb, 93 ppb, 92 ppb and 93 ppb.

It is likely that the monitor missed a tremendously high ozone exceedance when it was shut off

for the entirety of July 18, 2012, as the preceding day’s 8-hour maximum average ozone level 

was 94 ppb (the monitor’s 3rd highest value that year) and the following day’s was 87 ppb (the 

monitor’s 4th highest value.) Given July 18 was an even hotter day, it is reasonable to infer that 

the ozone maximum would have been in between 87 and 94 ppb, and most likely at the upper end 

of that range. If the monitor had recorded a level even just in the middle of this range at 91 ppb 

or above, it would have been the monitor’s 4th highest value, changed the 2012-2014 design 

value to 76 ppb, and continuously kept the Baltimore area in nonattainment, thus precluding 

EPA from even proposing this CDD.

Response:

MDE disputes that this monitor was “shut off.” The data was simply invalidated. From the 

station log notes: During the 4pm hour on 7-17-12, the window AC unit in the shelter failed. By 

5pm the shelter temperature was 94.8°F, the next hour it was 100°F. The analyzer really 

shouldn't run over 86°F, per the manual. By the afternoon of 7-18-12, we had replaced the AC 

unit and started to cool down the shelter. Even though a note was left about a PC passing on the 

afternoon of 7-18-12 (it did pass), the values were invalidated in Envista through 11:30am on 7-

19-12 because of instability on the minute readings. 

Second, upon MDE review, MDE asserted it is highly unlikely that the monitor missed a 

tremendously high ozone exceedance and it is not reasonable to infer that the max would have 

been between 87 and 94 parts per billion (ppb) unless one assumes that ozone concentrations are 

solely dependent on temperature and behave somewhat linearly from day to day to day at each 

monitor. MDE also asserted this ignores the meteorology and the behavior of the other ozone 

monitors in Maryland on 7/18/12. On 7/17/12, there were 13 monitors registering exceedances 

in Maryland, 5 in the Baltimore area, 5 in the Washington area, 1 in the Philadelphia area and 2 

on the eastern shore. On 7/18/12, even though the max temperature was higher, there were only 
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5 monitors registering exceedances, 2 in the Baltimore area, 2 in the Washington area and 1 on 

the eastern shore. On 7/19/12, there were 7 monitors registering exceedances, 1 in the Baltimore 

area (Davidsonville, tied for daily max in Maryland with PG Equestrian Center), 5 in the 

Washington area and 1 on the eastern shore. On 7/18/2, thunderstorms were initiating around 3 

PM between Beltsville and Davidsonville and they swept through the area between 5 and 6 PM. 

There also was appreciable cloud cover by 3 PM. Ozone concentrations at the PG Equestrian 

Center (the closest monitor to Davidsonville and the site that most closely correlates with 

Davidsonville) had early morning (10 AM) concentrations in 90 ppb range but decreased after 

that due to boundary layer mixing, as evidenced by the preconvective cumulus field which 

developed between noon to 1 PM (per analyses by MDE meteorologist). 

The highest 8-hour ozone max in Maryland on 7/18/12 was 84 ppb, measured at Edgewood, well 

north of the isolated thunderstorms that occurred at Davidsonville. Even if one were to substitute 

the Edgewood max at Davidsonville (which has no sound technical basis) it would have no 

impact on the 2012-2014 design value at Davidsonville. The max at the PG Equestrian Center 

(again, the closest monitor to Davidsonville and most closely correlated) on 7/18/12 was 76 ppb, 

a more likely candidate for substitution (and technically defensible).

2. Commenter regarding Davidsonville 6/29/12-7/1/12 (page 27 of comments on EPA’s 

NPR)

Another example, from the same monitor, begins on June 29, 2012. The maximum 8-hour 

average for that day was 102 ppb but the monitor shut off at 10 pm and does not report data 

again until July 1, 2012 at noon, at which point the first values are 78 ppb, 78 ppb, 73 ppb, 73 

ppb, and 77 ppb. The monitor did not record any data for the entirety of June 30, 2012, which 

was during peak ozone season and a heat wave, with maximum temperatures of 103, 95 and 97 

respectively for the three days.

Response:

According to MDE, there was a power failure at the Davidsonville monitoring station which 

began 6/29/12 at 10:33 PM. Power was not restored until 7/1/12 at 11:37 AM (from the site’s 

datalogger power failure log). This was a direct result of the passage of a derecho which caused 

considerable damage throughout Maryland with many downed trees and power lines and 

extensive power outages that lasted for extended periods throughout Maryland and many other 

states. 

Excerpted from the preface of the January 2013 NOAA Service Assessment “The Historic 

Derecho of June 29, 2012” (http://www.nws.noaa.gov/os/assessments/pdfs/derecho12.pdf):

On June 29, 2012, a derecho of historic proportions struck the Ohio Valley and Mid-Atlantic 

states. The derecho traveled for 700 miles, impacting 10 states and Washington, D.C. The 

hardest hit states were Ohio, West Virginia, Virginia, and Maryland, as well as Washington, 

D.C. The winds generated by this system were intense, with several measured gusts exceeding 80 

mph. Unfortunately, 13 people were killed by the extreme winds, mainly by falling trees. An 
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estimated 4 million customers lost power for up to a week. The region impacted by the derecho 

was also in the midst of a heat wave. The heat, coupled with the loss of power, led to a lifethreatening situation. Heat claimed 34 lives in areas without power following the derecho.

Excerpted from the Executive Summary of the January 2013 NOAA Service Assessment “The 

Historic Derecho of June 29, 2012”:

On June 29, 2012, a devastating line of thunderstorms known as a derecho (deh REY cho) moved 

east-southeast at 60 miles per hour (mph) from Indiana in the early afternoon to the MidAtlantic region around midnight. The states most significantly impacted were Indiana, Ohio, 

Kentucky, Pennsylvania, West Virginia, Maryland, Virginia, Delaware, New Jersey, and North 

Carolina, as well as Washington, D.C. Nearly every county impacted by this convective system 

suffered damages and power outages. Winds were commonly above 60 mph with numerous 

reports of winds exceeding 80 mph. Some areas reported isolated pockets of winds greater than 

100 mph. The storm resulted in 13 deaths, mainly a result of falling trees. One major impact 

from the derecho was widespread power outages. More than 4 million customers were without 

power, some for more than a week after the storms moved through. To make matters worse, the 

area affected was in the midst of a prolonged heat wave. There were 34 heat-related fatalities in 

areas without power because of the derecho.

In addition to the Davidsonville monitoring station, the ozone monitoring stations at Edgewood, 

Rockville, Southern Maryland, PG Equestrian Center and Calvert all lost power. Edgewood’s 

power was restored at 4:00 AM on 6/30/12 and all the other Baltimore area monitors were fully 

operational during the 6/29/12-7/1/12 period while Davidsonville was without power. Despite 

the heat, none of the other Baltimore area monitors registered exceedances during that period, 

again demonstrating that temperature alone is a poor indication of expected ozone levels. The 

derecho effectively swept out much of the ozone in the upwind area to the west all the way to 

Chicago.

3. Comment regarding Padonia monitor 7/22/11 (page 28 of comments to EPA’s NPR)

This phenomenon is prevalent in not just this most recent design value time period, but before 

then as well. For example, the monitor at Padonia Elementary School (#240051007) recorded a 

maximum 8-hour average of 86 ppb on July 20, 2011, which included hourly values as high as 

95 ppb, 98 ppb, 103 ppb, 96 ppb, and 81 ppb. The next day on July 21, 2011, the monitor records 

consistently high hourly values starting at 2 pm of 81 ppb, 86 ppb, 83 ppb and then shuts off at 5 

pm. The monitor did not start reporting again until July 23, 2011 at 11 am, missing the entirety 

of July 22, 2011, which was an extremely hot day conducive to ozone formation.

Response

First, MDE and EPA find this issue irrelevant to the NPR which proposed to determine the 

Baltimore Area attained the 2008 8-hour ozone standard based on 2012-2014 design values. This 

is only presented to bolster the commenter’s claims that these types of “phenomena” are 

“prevalent” and that they discovered “many instances” of these occurring over a five year period. 

If that were the case, the completeness criteria would not have been met.
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Second, it is also irrelevant because the 2011-2013 design value for Padonia was 78 ppb without 

collecting any data on 7/22/11. The site was already out of attainment and any data collected on

7/22/11 would not have changed that fact.

Third, according to MDE, the monitor did not shut off. The data was invalidated due to 

condensation in the sample line which actually got into the monitor resulting in erroneous 

readings. There were temperature control issues in the shelter, with the shelter eventually 

reaching 50oF before being noticed early 7/22/11. It took until noon on 7/23/11 to dry out the 

monitor and bring it back on line.

4. Comment regarding Davidsonville monitor 5/23/13-5/31/13 (page 28 of comments on 

EPA’s NPR)

There were also several instances in which ozone monitors shut off and did not report data for 

several days, sometimes for even over a week, during ozone season. These are times when many 

exceedances may have occurred, and thus could have significantly increased design values, but 

were not recorded because the monitors were not recording data. 

For example, the Davidsonville Recreation Center’s monitor, which is on the cusp of 

nonattainment at 74 ppb, turned off and did not report data for over one week during ozone 

season, from noon on May 23, 2013 through 8 am on May 31, 2013. During this time period 

when the monitor was off, daily maximum temperatures reached 80, 89, 92 and 90 degrees,

which are at and well above the threshold for ozone exceedances in Baltimore. Thus, the monitor 

may well have missed exceedances that would have kept the monitor in nonattainment for 2012-

2014.

Response

This is another instance of temperature control issues in the shelter. According to MDE, the 

temperature in the shelter was turned down for operator comfort on the morning of 5/23/13 and 

was inadvertently not readjusted when the operator left the shelter. The temperature reached 60oF 

and stayed there until 5/29/13 at 11:00 AM. Condensation had formed in the sample line and got 

into the monitor. The monitor was dried out and put back online on 5/31/13.

During the period from 5/23/13 through 5/28/13, the highest 8-hour max in the Baltimore Area 

was 59 ppb and there were no exceedances registered anywhere in Maryland. On 5/29/13 an 8-

hour max of 80 ppb was measured at Fair Hill in the Philadelphia nonattainment area, but no 

exceedances were registered in the Baltimore Area (max 68 ppb), Washington Area, or the 

eastern shore. No exceedances were measured anywhere in Maryland on 5/30/13 (Baltimore 

Area max 71 ppb). On 5/31/13, the Padonia monitor, in the Baltimore Area, was the only 

monitor in Maryland to register an exceedance at 76 ppb. Given the overall low regional ozone 

concentrations, this definitely was not a time when many exceedances may have occurred or 

when the monitor may well have missed exceedances. Even if you substitute the max exceedance 

in Maryland during that entire time period (the 80 ppb at Fair Hill), the 2012-2014 design value 

at Davidsonville would remain unchanged. The likelihood of an exceedance of 88 ppb or greater 

(the value needed to change the Davidsonville design value to 76 ppb) occurring at 

Davidsonville during the period in question is extremely rare.
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5. Comment regarding Aldino monitor 8/17/12-8/20/12 (page 28 of comments on EPA’s 

NPR)

Similarly the Aldino monitor (#240259001), which has a 2012-2014 design value of 73 ppb, is 

missing data from 8 pm on August 17, 2012 through 1 pm on August 20, 2012.50 The daily 

maximum temperatures during the time period included 92, 94 and 80 degrees.51

Response

There was a power failure at the Aldino monitor beginning at 7:46 PM on 8/17/12, a Friday. The 

power came back on at 7:47 PM but went back out at 8:54 PM. When the operator returned to 

the site Monday 8/20/15 at 12:25 PM, it was noticed that 2 circuit breakers had tripped. They 

were reset at that time and the site came back online.

During this entire period, there were no exceedances registered anywhere in Maryland. A cold 

front came through the evening of 8/17/12 (probably triggering the power outage). There were 

clouds and rain throughout the area on 8/19/12 and 8/20/12. It is highly unlikely that any 

exceedances were missed at Aldino during the time the power was out.

III. EPA Analysis

EPA finds MDE’s explanations for inoperable monitors during the periods in question by the 

commenter to be reasonable and supported by the technical evidence as discussed above. A 

variety of technical issues, such as power failures or condensation build-up in the monitors, led 

to missing or invalidated monitoring data and are supported by monitoring station log notes. 

Ozone monitors are sophisticated analytical instruments which mostly operate quite reliably. 

However, there are many possible ways for the monitors to malfunction or to produce erroneous 

or compromised data. It is unreasonable to expect any ozone monitor to operate continuously 24 

hours a day for seven days a week over the seven month ozone season without experiencing any 

operational issues. For instance, ultraviolet lamps can burn out, vacuum pumps can fail, leaks 

can develop, particulate filters can clog and water vapor can condense in the sample manifold 

and be drawn into the monitor, among other possibilities. 

The monitors also must be operated in environmentally controlled buildings or instrument 

shelters. If the air conditioning fails and the monitors overheat, unstable readings may occur. If 

the temperature gets too cold in a shelter on a hot and humid day, condensation can occur and

affect the ozone readings. Monitoring stations frequently house additional monitoring equipment 

and require a high electrical current draw. Ozone exceedance days are often observed on very 

hot and humid days. These conditions contribute greatly to condensation issues and high demand 

creates stress on the electrical grid which can lead to power failures. Thunderstorms also 

frequently occur under these conditions, which can also cause power failures.

A strict schedule of preventative maintenance, operational checks, daily zero and span 

challenges, periodic audits and a minimum of bi-weekly precision checks are in place to insure 
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that any problems are addressed in a timely manner and that the highest possible quality data is 

being produced. 

Since MDE produces daily ozone forecasts, monitoring site operators are alerted ahead of time 

when they can expect ozone exceedance days and extra efforts are taken to insure, to the best of 

their ability, that the monitors are operating properly and to avoid any instrument downtime or 

loss of data due to preventative maintenance, audits or other quality control checks during those 

periods when high ozone levels are forecast.

In conclusion, Appendix P of 40 CFR part 50 formally recognizes the potential issues that can be 

encountered in monitoring ozone concentrations by allowing for some missing data. Each ozone 

monitor must produce daily maximum 8-hour average concentrations for at least 90% of the days 

within the ozone monitoring season, on average for the 3-year design value period, with a 

minimum data completeness requirement in any one year of at least 75% of the days within the 

ozone monitoring season for a design value to be considered valid. All of the Baltimore Area 

ozone monitors meet these requirements for the period in question.