Field evaluations of newly available “interference-free” monitors for nitrogen dioxide and ozone at near-road and conventional National Ambient Air Quality Standards compliance sites

ABSTRACT

Long-standing measurement techniques for determining ground-level ozone (O₃) and nitrogen dioxide (NO₂) are known to be biased by interfering compounds that result in overestimates of high O₃ and NO₂ ambient concentrations under conducive conditions. An increasing near-ground O₃ gradient (NGOG) with increasing height above ground level is also known to exist. Both the interference bias and NGOG were investigated by comparing data from a conventional Federal Equivalent Method (FEM) O₃ photometer and an identical monitor upgraded with an “interference-free” nitric oxide O₃ scrubber that alternatively sampled at 2 m and 6.2 m inlet heights above ground level (AGL). Intercomparison was also made between a conventional nitrogen oxide (NO_x) chemiluminescence Federal Reference Method (FRM) monitor and a new “direct-measure” NO₂ NO_x 405 nm photometer at a near-road air quality measurement site. Results indicate that the O₃ monitor with the upgraded scrubber recorded lower regulatory-oriented concentrations than the deployed conventional metal oxide–scrubbed monitor and that O₃ concentrations 6.2 m AGL were higher than concentrations 2.0 m AGL, the nominal nose height of outdoor populations. Also, a new direct-measure NO₂ photometer recorded generally lower NO₂ regulatory-oriented concentrations than the conventional FRM chemiluminescence monitor, reporting lower daily maximum hourly average concentrations than the conventional monitor about 3 of every 5 days.

Implications: Employing bias-prone instruments for measurement of ambient ozone or nitrogen dioxide from inlets at inappropriate heights above ground level may result in collection of positively biased data. This paper discusses tests of new regulatory instruments, recent developments in bias-free ozone and nitrogen dioxide measurement technology, and the presence/extent of a near-ground O₃ gradient (NGOG). Collection of unbiased monitor inlet height–appropriate data is crucial for determining accurate design values and meeting National Ambient Air Quality Standards.

Introduction

The recent tightening (U.S. Environmental Protection Agency [EPA], 2015a) of the ozone National Ambient Air Quality Standards (NAAQS) does not require upgrading compliance monitoring networks with recently approved “interference-free” Federal Reference Method (FRM) ozone (O₃) instruments such as the Teledyne Air Pollution Instrumentation (TAPI, San Diego, CA) T265 nitric oxide (NO) chemiluminescence (CL) O₃ monitor (EPA, 2016) or the Federal Equivalent Method (FEM) 2B Technologies, Inc. (2B, Boulder, CO), nitric oxide (NO)-scrubbed 211 ultraviolet (UV; 254 nm) photometer (EPA, 2016) that would minimize the interference bias present in the conventional network metal oxide–scrubbed (e.g., copper oxide [CuO], manganese dioxide [MnO₂]) UV photometers presently deployed (Kleindienst et al., 1997; Spicer et al., 2010; Johnson et al., 2014; Zhao and Stephens, 2016). Although current EPA-allowed inlet heights above ground level (AGL) may range within 2–15 m, they cluster in the 3–6 m range, averaging 5.4 m for urban sites (EPA, 2015b) and 10 m AGL for rural EPA Clean Air Status and Trends Network (CASTNET) O₃ compliance sites. However, inlets above 2 m may increase reported concentrations due to positive near-ground O₃ gradients (NGOG) as noted by Horvath et al. (1998), Lee et al. (2002), EPA (2006a), and Velasco et al. (2008). In spite of concerns that “the stability regime during the day in urban areas tends more toward instability because of the urban heat island effect” (EPA, 2006b), there is evidence (Wisbeth et al., 1996; Johnson et al., 1997) that significant daytime NGOGs exist during episodes of elevated O₃ when stagnant conditions exist.

Although data from regulatory state, local, and tribal (SLT) organization monitoring networks can satisfy multiple objectives, their primary goal is determination of a design value (DV), which varies by pollutant, and its compliance with the NAAQS. In some cases, the procedure for calculating compliance area DVs relies on an extremely small fraction of the area’s annual data set.

For example, DVs for O₃ sites are calculated as the fourth highest annual daily maximum 8-hr average O₃ value averaged over three consecutive years and compliance is determined from the highest monitoring site DV within the compliance area. Under current EPA requirements (EPA, 2015c), there are 17 8-hr O₃ averages available for each calendar day and for sites operating year round, there will be up to 6205 8-hr averages per year and 18,615 8-hr values over three consecutive calendar years. Since only the four highest 8-hr averages for each year are considered (12 such averages over the 3-yr period), that site’s DV is derived from the highest 0.06% of the total site’s data collected. Since the data are taken from days with the highest O₃ concentrations, it is probable that DVs will occur on days most conducive to O₃ formation when the atmosphere is commonly hot, humid, and stagnant. Such O₃-conducive days are also likely to be atmospherically stable and occur when the atmosphere contains high concentrations of O₃ precursors, volatile organic compounds (VOCs), and other chemical species known to interfere with the presently deployed conventional UV photometric O₃ analyzers.

Nitrogen dioxide (NO₂) is not thought to possess the same type of vertical near-ground gradient as O₃ due to its many emission sources at or very near ground level as well as photochemical processing between NO and NO₂ (Villena et al., 2011; EPA, 2012; Kenagy et al., 2016). However, the NO₂ FRM CL monitors are also prone to a positive bias due to interfering species such as ammonia, nitrous/nitric acid, and alkyl amines/nitrites/nitrates (EPA, 2005, 2012; ASTM International, 2016). In contrast to O₃ monitoring requirements, the revised O₃ NAAQS do require replacement of conventional NO₂ FRMs with a photolytic-converter upgrade of its conventional molybdenum catalyst converter or by substitution of a cavity-attenuated phase shift (CAPS) FEM photometer at National Core (NCore) multipollutant monitoring sites in compliance areas with populations >1 million (EPA, 2005, 2015a).

The studies at Westport and Hartford were undertaken to compare new less interference-prone instruments and components thought to reduce known interferences in the O₃ and NO₂ regulatory monitoring networks. A secondary interest of the Westport study was to determine the extent to which inlet heights and the NGOG might influence O₃ DVs because the Westport site typically exhibits high O₃ DVs at the current site 6.2 m inlet height.

Approach and methodology

Our upgraded O₃ monitor deployment and inlet height study occurred at the Sherwood Island State Park (EPA 09-001-9003) Westport, Connecticut (CT), site (41.11822, −73.33675) from late June to early October 2015. The upgraded NO₂ monitor study deployed a new 2B model 405 nm NO₂ monitor at the Huntley Place (EPA 09-003-0025) Hartford, CT, near-road site (41.77144, −72.67992) from October 24, 2015, to March 13, 2016.

Westport, CT

The Westport site borders a cove off the north shore of Long Island Sound in a designated flood plain that requires structures such as the instrument shelter to be constructed at least 2 m AGL. To comply with that regulation, the Westport shelter rests on a stilted 2 m platform that raises the existing compliance monitor inlet to 6.2 m AGL.

Ambient air samples at Westport were drawn through Teflon lines (3.175 mm inner diameter [ID]) within opaque protective plastic conduits, two of which terminated in a “candy cane” configuration 2 m above the flat shelter roof at 6.2 m AGL, with the third positioned predominantly upwind at 2 m AGL.

The Connecticut Department of Energy and Environmental Protection (DEEP) provided a TAPI model T400 O₃ monitor that was upgraded by 2B with a NO gas-phase titration (GPT) O₃ scrubber (2B Technologies, Inc., 2014). That monitor (GPTO₃) was collocated with the DEEP’s T400 at the Westport site and fitted with a solenoid valve that allowed alternate sampling from two Teflon lines of equal length. One line was collocated so that its inlet tip at 6.2 m AGL was within 0.5 m horizontally of the DEEP’s conventional TAPI T400 O₃ monitor’s inlet tip. The second GPTO₃ sample line was run through an opaque conduit so that its inlet tip extended 2 m horizontally in the predominant upwind southwesterly direction from the shelter’s exterior wall, complying with EPA guidance that “through-wall” probes be located in the direction of prevailing winds with the highest ambient pollutant concentration of interest at least 1 m horizontally from the prevailing upwind edge of shelter structures or surfaces. The stilted shelter presents little restriction of wind flow, and the 2 m horizontal separation from the shelter wall positioned the inlet tip in free-flowing air, allowing for collection of a representative sample. Both T400 monitors were calibrated over the 0–425 ppb range.

The DEEP’s conventional T400 O₃ monitor sampled continuously at 6.2 m AGL, whereas the modified collocated (GPTO₃) T400 monitor alternated sampling at 6-min intervals between its 6.2 m and 2.0 m inlets. Sample switching by a three-way Teflon valve was triggered by the site’s data acquisition system (DAS), providing data flags indicating which inlet was actively sampling. The 1-min-averaged data were inspected, and values flagged as “incomplete,” “invalid,” or occurring during quality assurance (QA) zero/span or calibration procedures were removed. Valid 1-min data were aggregated into hourly averages, which were truncated to whole ppb, and hours were considered valid if they contained at least 75% of the potential 1-min averages. For the DEEP O₃ monitor, 45 or more valid 1-min averages constituted a valid hour. The upgraded monitor’s sample was split between two inlet heights, so a different data validation protocol was used in which the first minute of each 6-min sample was invalidated due to sample stagnation within the sample lines. Since only 25 min of data were available from each of these alternating inlet lines, if 20 of the 25 1-min samples (80%) were valid, the hour was considered valid.

On four occasions, a significant reduction in response of the DEEP O₃ monitor was noted after replacement of particulate matter filters (Savillex, Eden Prairie, MN; 47 mm diameter, polytetrafluoroethylene [PTFE], 5–6 micron pore size). On those occasions, the 1-hr DEEP average O₃ data were lower than the GPTO₃ monitor by 2–8 ppb for an average of 11 hr, whereas prior to filter replacement the DEEP monitor displayed a value equal to or slightly higher than the GPTO₃ monitor. Since new instrument filters often have an “ozone demand” (EPA, 1997), it is thought plausible that filter scavenging occurred during these replacements, and hourly DEEP data for periods following these four filter replacements were invalidated in our analyses.

Eight-hour averages were valid if they contained six or more valid 1-hr averages. Data completeness for 1-hr and 8-hr averages exceeded 93% from all three O₃ inlets. Per EPA convention, all hourly and 8-hr averages are identified by the starting time of the period in terms of local standard time.

Automated zero and span checks were performed simultaneously on both T400 monitors each day (DEEP span target = 144 ppb, GPTO₃ monitor span target = 170 ppb), whereas a “precision” check was performed every sixth day on both monitors (DEEP precision target = 40 ppb, GPTO₃ monitor precision target = 50 ppb). The test atmospheres for these QA checks were generated by the “internal zero/span” (IZS) module on board each T400 and were synchronized by the DEEP’s DAS. The IZS module contains a desiccant cartridge in series with an activated charcoal cartridge to provide ozone-free, dry “zero” air and a mercury vapor lamp for generation of O₃. With the exception of the precision check results, data from both monitors were adjusted for relative instrument drift using routine daily zero/span information.

Comparison of the upgraded GPTO₃ and conventional 6.2 m TAPI T400 O₃ time series provided a site- and year-related measure of conventional monitor interference bias, whereas comparison of the GPTO₃ monitor’s 6.2 m and 2 m time series provided a measure of the O₃ gradient over the site’s 2–6.2 m inlet height range. Figure 1 notes the current O₃ NAAQS compliance network inlet height distribution, including the rural CASTNET sites, which draw O₃ samples from inlets positioned 10 m AGL.

Figure 1. Distribution of U.S. O₃ compliance monitor inlet heights (meters).

Hartford, CT

A 2B model 405 nm photometer directly measuring NO₂, and nitrogen oxides (NO_x) and NO indirectly by difference, was collocated with a conventional TAPI T200U NO_x CL FRM instrument (calibrated over the 0–425 ppb range for NO and 0–340 ppb for NO₂), directly measuring NO, and NO_x and NO₂ indirectly by difference. The comparison was made at the Huntley Place near-road site in Hartford, CT, during the November 24, 2015, to March 13, 2016, period. The conventional NO_x CL FRM reacts sample NO in ambient air with O₃ to produce electronically excited NO₂*, which luminesces, providing a signal proportional to the NO level; after switching to an alternate flow path, sample NO₂ is converted to NO over a heated molybdenum catalyst and reacted with O₃ to obtain a NO_x measure and the ambient NO₂ level by difference. The 2B 405 nm photometer measures ambient NO₂ directly in a 2 m multiple-reflection white cell at 405 nm; in a second ambient sample, sample NO is ozonized to NO₂ to obtain both a measure of NO_x and ambient NO by difference. Development of direct-measure NO₂ instruments has been prompted by the long-recognized positive bias in the measurement of NO₂ via the CL FRM (EPA, 2012; ASTM International, 2016). As with the DEEP’s T200u monitor, the 405 nm was calibrated in the 0–425 ppb range for NO and 0–340 ppb for NO₂. A recently available folded-cell 2B model 405 nm design upgrades monitor precision to <0.5 ppb (±0.5% of reading with adaptive filter) and response time to 10 sec; this improved analyzer has met FEM certification testing requirements and has received FEM certification (Federal Register, May 11, 2017).

The Hartford, CT, Huntley Place monitoring site is 57 m above sea level and situated on the north side of IH-84, about 0.25 km west of the Connecticut River. The instrument shelter is 15 m from the edge of the IH-84 right-hand westbound travel lane. The annual average daily traffic count at the site is 161,350 (Connecticut Department of Transportation [DOT], 2014).

A 3.175 mm (ID) Teflon sample line for the 2B 405 nm photometer was run through the shelter’s roof inside a plastic conduit that terminated in a down-facing candy cane—a configuration identical to the collocated DEEP’s NO_x monitor and approximately horizontally 0.5 m distant with both inlets at 4.6 m AGL. One-minute-averaged data were logged from both monitors via a DEEP DAS, and any values flagged as “incomplete,” “invalid,” or occurring during QA zero/span or calibration procedures were removed. Valid 1-min data were aggregated into hourly averages, which were truncated to whole ppb, and hours were considered valid if they contained at least 75% of the potential 1-min averages. Daily zero/span checks for NO and NO_x were performed on both monitors by the on-site calibration system.

In keeping with EPA guidance, the on-site calibration system also generated a NO₂ “precision point” every sixth day for both monitors. Challenge gases from the on-site calibrator were introduced to both instruments near the 4.6 m inlet line openings 15 cm downstream behind the inlet line filter (Savillex; 47mm diameter, PTFE, 5–6 micron pore size). The calibration system consisted of a zero air supply (TAPI, model 701), a dilution system calibrator (TAPI model 700 EU), and a cylinder of compressed NO/N₂ (oxygen-free, 37 ppm NO, 0.3 ppm NO₂; Scott Specialty Gas, Compliance Class).

Results and discussion

Westport O₃ data

The final data set from Westport contained 2188 triplets of hourly observations (June 30, 2015, to October 5, 2015), with the only significant data loss occurring August 7 to August 10, 2015, when the DEEP monitor malfunctioned. Regressed GPTO₃ versus DEEP O₃ data from inlets 6.2 m AGL were highly correlated (r² = 0.99, slope = 0.97, intercept = −0.39), indicating that, on average, the conventionally scrubbed DEEP O₃ monitor reported values 3% higher than the alternatively scrubbed GPTO₃ monitor.

Automated span data responses from Westport included 98 checks on the GPTO₃ instrument, which ranged in percent difference ([Target − Response]/Target) × 100) from −3.2% to +6.2%. Span data from the DEEP monitor comprised 94 checks, with a percent difference of −3.6% to +3.2%. There were 14 precision check data points from the DEEP monitor ranging from −4.0% to +3.0% and 13 precision point checks on the GPTO₃ monitor ranging from −3.6% to +8.4%.

On July 17, 2015, and July 23, 2015, the GPTO₃ monitor’s precision point response exceeded the suggested EPA individual precision point limit of ±7%. Subsequent to those precision checks on both July 18, 2015, and July 24, 2015, the instrument’s daily span responses were well within the “7%” limit and no adjustments were deemed necessary to the GPTO₃ monitor’s zero or span response at that time.

The only adjustment to either monitor at Westport occurred on August 3, 2015, when automated zero/span data showed that the GPTO₃ monitor’s zero response had been inexplicably low by ~7 ppb since July 28, 2015. On August 3, 2015, QA personnel from the DEEP, using a Level 3 transfer standard, found the zero response of the GPTO₃ instrument to be −7 ppb and the span response (at 426 ppb) to be 436 ppb or ~2.4% high. The instrument’s zero response was adjusted to “0 ppb,” and the span response at 426 ppb was set to 423 ppb; no further adjustments to zero or span were made throughout the study.

Figure 2a plots superimposed Westport, CT, rolling 8-hr average O₃ values from the conventional (DEEP) T400 6.2 m inlet, O₃ values from the GPTO₃ inlet at 6.2 m, and the difference between those monitors. Figure 2b shows rolling 8-hr averages from the two inlets of the GPTO₃ monitor as well as their O₃ differences at 6.2 m and 2 m. The black tips of the daily O₃ peaks in Figure 2a are a measure of conventional monitor O₃ interference bias also noted in the trace at the bottom of Figure 2a. The black tips of the daily O₃ peaks in Figure 2b are a measure of the degree to which O₃ concentrations at 6.2 m and 2.0 m AGL are impacted by the NGOG. These numerical differences vary as a function of time over an interference range of −7 to +10 ppb in Figure 2a and a NGOG range of −1 to +9 ppb in Figure 2b.

Figure 2. (a) Time series of collocated monitor adjusted 8-hr O₃ values (ppb) and O₃-GPTO₃ (ppb) differences found at 6.2 m AGL at Westport, CT. (b) Adjusted collocated monitor 8-hr O₃ values (ppb) and GPTO₃ (6.2 m)–GPTO₃ (2.0 m) differences (ppb) at Westport, CT.

To gain perspective on the nature of the 6.2 m to 2 m O₃ gradient, the available meteorological parameters (wind speed, wind direction, and temperature) were analyzed with respect to the NGOG. Analysis of hourly data showed that wind speed (WS) was the only parameter demonstrating significant ties to the NGOG and a speed of 1.2 m/sec appeared to be critical. When hourly NGOG values were averaged into WS bins of 0.1 m/sec from 0 to 6.1 m/sec, it was seen that virtually all NGOG average values of 2 ppb or greater occurred at WS values of 1.2 m/sec or less. Only one WS bin (4.3–4.4 m/sec) was found with an NGOG greater than 2 ppb (i.e., 3 ppb) when the WS was greater than 1.2 m/sec, demonstrating that at the Westport site low wind speeds, especially those less than or equal to 1.2 m/sec, are crucial in creating significant NGOGs.

Figure 3 plots the observed overall daily average hourly O₃ values and their differences averaged over the late June to early October 2015 collocated monitoring period at the Westport site. The daily average O₃ conventional-to-upgraded monitor differences at 6.2 m range between 0 and 3 ppb, whereas the upgraded monitor differences between 6.2 m and 2 m range between 1 and 3 ppb. At this site during the study period, the combined average diurnal effect of both the O₃ monitor interference and the 2 m to 6.2 m O₃ gradient (triangles) range between 1 and 4 ppb. Note that interferences appear to increase during the rising portion of the photochemical O₃ production process where oxygenated aromatic species are initially generated (e.g., phenols, aldehydes, etc.); during the latter part of the day, with increased air mass aging, it seems plausible that these initial interferences react further and are likely partially converted into less 254 nm–absorbing ring-opened species and less volatile aromatic species (e.g., carboxylic acids, particles).

Figure 3. Hourly diel plot of Westport, CT, O₃ values (ppb) and differences averaged over the June to October 2015 monitoring period.

Twelve daily maximum 8-hr averages ≥70 ppb are shown as peak values in Figure 2a. The monitor’s fourth highest daily maximum 8-hr average values observed during the June 30 to October 5, 2015, period are calculated using the 17 daily 8-hr averages (EPA, 2015c) noted in the revised O₃ NAAQS compliance determination, which avoid double counting nighttime values across adjacent days. The results indicate that the conventional 6.2 m monitor’s fourth highest daily maximum 8-hr value was 84 ppb, with 12 values ≥70 ppb; the upgraded 6.2 m monitor’s fourth highest was 82 ppb, with 10 values ≥70 ppb; and the upgraded 2 m monitor’s fourth highest was 80 ppb, with 10 values ≥70 ppb.

Hartford NO₂ data

Table 1 lists the three highest daily hourly maximum NO₂ values during the 111-day monitoring period for the FRM, whereas Table 2 lists similar values for the 2B instrument. The NO₂ hourly NAAQS, a 3-yr average of the annual 98th percentiles of daily maximum hourly values, would approximate a second highest daily maximum hourly value within our 111-day period. The estimated differences between the FRM and 2B analyzer values using a second highest metric are about 2 ppb over the monitoring period.

Table 1. Highest three daily maximum FRM NO₂ concentrations recorded at Hartford, CT.

Date, time	FRM NO2	2B NO2	Difference (FRM − 2B)
March 10, 2016, 7:00 a.m.	59 ppb	50 ppb	9 ppb
January 7, 2016, 7:00 a.m.	46 ppb	44 ppb	2 ppb
February 2, 2016, 8:00 a.m.	46 ppb	41 ppb	5 ppb

Table 2. Highest three daily maximum 2B NO₂ concentrations recorded at Hartford, CT.

Date, time	2B NO2	FRM NO2	Difference (2B − FRM)
March 10, 2016, 7:00 a.m.	50 ppb	59 ppb	−9 ppb
January 7, 2016, 7:00 a.m.	44 ppb	46 ppb	−2 ppb
January 26, 2016, 7:00 a.m.	44 ppb	44 ppb	0 ppb

Figure 4 plots both the superimposed hourly conventional TAPI T200U FRM (NO₂) and the 2B 405 nm photometer (2BNO₂) data and their differences from November 24, 2015, through March 13, 2016, at the Hartford, CT, near-road site. The black tips of the daily hourly NO₂ peaks are a measure of the degree of conventional FRM NO₂ monitor interference bias compared with the 2B 405 nm. The narrow black line plot of their observed monitor differences shows variation over time. About 60% of the reported hourly daily maximum FRM NO₂ values exceed the corresponding 2BNO₂ values by an average of 3 ppb, ranging from 1 to 9 ppb; periods of negative differences average about −2 ppb and range from −1 to −4 ppb.

Figure 4. Hourly NO₂ (ppb) time series and NO₂-2BNO₂ (ppb) differences at the near-road Huntley, CT, site.

As with the Westport automated span check data, the percent difference between target (150 ppb for both NO and NO_x channels of the two instruments at Huntley) and response was calculated and showed that the TAPI FRM NO span ranged from −6.7% to +3.8% with an average of −4.4%, whereas the FRM NO_x ranged from −6.1% to +3.9% with an average of −3.5%. The 2B NO span results ranged from a minimum of −4.2% to a maximum of +2.5% with an average of −1.5%, whereas the 2B NO_x span results ranged from −3.9% to +3.1%, averaging −0.5%.

The target values for three channels during automated precision checks at the Huntley site were NO_x = 150 ppb, NO₂ = 35 ppb, and NO = 115 ppb. However, the evaluation of precision point data was hampered by sporadic technical problems with the on-site calibration system for generating those points. On multiple occasions, the NO₂ precision point results exceeded the EPA suggested limit of ±7%, but when a follow-up on-site calibration was manually performed, the TAPI virtually always responded well within the 7% limit, indicating that the on-site Level 3 transfer standard had malfunctioned. It should be noted that due to limited personnel resources, the DEEP could not check the 2B instrument when its precision checks appeared to exceed 7%. In those instances, the zero/span data for the days preceding and following the suspect precision checks were examined and if acceptable (zero within ±3 ppb, span within ±2%), the precision check was deemed to be invalid. Using only precision point check data thought to be valid, the TAPI FRM instrument’s precision expressed as percent difference ranged from −5.6% to +7.9% averaging +3.4%, whereas the 2B instrument’s precision ranged from −8.7% to +6.1% averaging −0.8%.

A least squares linear regression (LSLR) of the 2411 hourly NO₂ pairs from the two monitors is y = 0.90x + 0.80, with an r² value of 0.96, indicating that on average, the FRM monitor reported hourly NO₂ concentrations about 10% higher than those from the direct-measure 2BNO₂ monitor. A similar analysis of the hourly NO data pairs (not shown) was y = 1.06x + 1.10, with an r² value of 0.99, indicating that the FRM monitor reported hourly NO concentrations about 6% lower than the 2BNO₂ monitor. A LSLR analysis of the NO_x study data (not shown) found y = 1.01x − 0.89, with an r² of 0.99, indicating a slight (~1%) underreporting of hourly NO_x values with respect to the 2BNO₂ monitor.

Conclusion

Over the observational period at the Westport, CT, site the scrubber-upgraded 6.2 m O₃ monitor’s fourth highest value was 2 ppb lower than the conventional monitor and recorded two fewer days above 70 ppb; the fourth highest differences were greater between the 6.2 m and 2 m inlets, with the fourth highest value found to be 4 ppb lower at 2 m than at the 6.2 m height with two fewer exceedances of 70 ppb.

Between November 24, 2015, and March 13, 2016, the observed Hartford, CT, NO₂ daily hourly peak levels averaged about 30% of the 100 ppb daily maximum hourly NO₂ NAAQS compliance level and the 2B 405 nm reported lower values than the FRM daily maximum value about 3 of every 5 days. Whether these differences increase at NO₂ levels nearer the standard level is uncertain. Over the 111-day sampling period the 98th percentile hourly NAAQS daily maximum NO₂ value approximates the second highest daily maximum hourly value of 46 ppb; this conventional FRM value is 2 ppb larger than the 2B 405 nm value.

Upgrading of O₃ compliance monitors is not required by EPA, although up to a 20% decrease in O₃ values from lowered inlet heights is reported over the 4 m to 0.5 m range during stagnant O₃-conducive days (Horvath et al., 1998). However, absent adequate data for adjustment of current measurements to 2 m levels, more representative of outdoor nose heights, the agency does encourage network operators to temporarily collocate a suspect analyzer with an FEM NO-CL ozone analyzer to determine if interferences exist. Should interferences be an issue with an UV ozone analyzer, network operators can replace that analyzer with a NO-CL analyzer, or another FEM photometer with a scrubber less susceptible to the interferences present (EPA, 2015a, 2006b).

Additional inlet height studies of the NGOG are important, and we hope such studies in the near future will result in sufficient data accumulation to support adjustments of O₃ data from disparate AGL heights. However, during the interim, network operators should consider upgrading their design value site O₃ instruments and also installing 2 m inlets as a pragmatic remedy of currently underestimated NAAQS compliance and overestimated exposure risk (Johnson et al., 2014). We support the EPA’s mandate to seek NO₂ monitors with upgraded performance at some near-road NO₂ monitoring locations and encourage a similar technology transfer and implementation approach for existing O₃ compliance networks as upgraded O₃ monitors and network practices become available.

Acknowledgment

The authors thank the many staff members of the CT DEEP’s Bureau of Air Management who assisted in these projects, with special thanks to Mark Demko and Dean Tully of the Air Monitoring Division.

Funding

Funding for these studies was provided by the American Petroleum Institute.

Additional information

Funding

Funding for these studies was provided by the American Petroleum Institute.

Notes on contributors

Alan R. Leston

Alan R. Leston is the senior scientist and proprietor of AirQuality Research & Logistics, LLC, in Lebanon, Connecticut.

Will M. Ollison

Will M. Ollison is a senior science advisor at the American Petroleum Institute in Washington, DC.