O₃, CH₄, CO₂, CO, NO₂ and NMHC aircraft measurements in the Uinta Basin oil and gas region under low and high ozone conditions in winter 2012 and 2013

Vigorous ozone formation in the summer under conditions with high levels of solar insolation and abundant O₃ precursor chemicals has long been recognized (Chameides et al., 1992). High levels of O₃ production during winter has only recently been documented in locations such as the Upper Green River Basin, Wyoming and Uinta Basin, Utah, in association with effluents from oil and gas production operations (Schnell et al., 2009; Oltmans et al., 2014; Rappenglück et al., 2014). In these basins, the strong role of meteorology, topography, and high levels of O₃ precursors from oil and gas extraction activities have been identified as key drivers of the wintertime high O₃ events (ibid.). The presence of snow-cover acts as both an enhancer of available ultraviolet (UV) radiation by greatly increasing the surface albedo and as a promoter of strong temperature inversions that trap O₃ precursor emissions in a shallow cold pool surface-based layer (Bader and McKee, 1985; Yu and Pielke, 1986; Schnell et al., 2009; Lareau et al., 2012; Oltmans et al., 2014).

In 2012 and 2013, a multi-agency Uinta Basin Winter Ozone Study (UBWOS) took place in the Uinta Basin in NE Utah (Figure 1) (Lyman and Shorthill, 2013; Stoeckenius and McNally, 2014). Measurements included hundreds of ozone profiles through the boundary layer from tethered ozonesondes (Schnell et al., 2016), non-methane hydrocarbon (NMHC) and nitrogen oxide mole fraction vertical profiles (Helmig et al., 2014), surface chemical constituent observations, and meteorological measurements (Lyman and Shorthill, 2013; Edwards et al., 2013; Stoeckenius and McNally, 2014; Edwards et al., 2014). In the Uinta Basin, intense natural gas production is located mainly on the east side of the Basin and oil production primarily on the west side. Natural gas production levels were similar in 2012 and 2013 with ∼18% greater oil production in 2013 (Utah Oil and Gas Program). Emissions from oil and gas production and processing activities as well as combustion emissions associated with vehicle traffic, drilling rigs, compressor stations and gas processing may be significant sources of both volatile organic compounds (VOCs) (Gilman et al., 2013; Warneke et al., 2014) and nitrogen oxides (NO_x = NO + NO₂), primary O₃ precursors.

This paper presents unique sets of aircraft collected data in the winters of 2012 and 2013 in the Uinta Basin under contrasting meteorological conditions. In the winter of 2012 there were no periods of persistent snow-covered ground in the Uinta Basin and no large enhancements in O₃ while in 2013 the ground was snow-covered during January and February (Oltmans et al., 2014).

In 2012 aircraft measurements were conducted with a single-engine Mooney TLS aircraft (operated by Scientific Aviation; http://www.scientificaviation.com/) and in 2013 a single-engine Cessna 210 aircraft (operated by Kalscott Engineering; http://www.kalscott.com). Instruments drew air through dedicated inlets installed under the aircrafts’ starboard wings (Figure S1). On-board instrumentation included high-frequency analyzers for carbon monoxide (CO) (2013 only), carbon dioxide (CO₂), methane (CH₄), and water vapor (H₂O) (Picarro G2401m), nitrogen dioxide (NO₂) (Los Gatos Research Model 911-0009), and ozone (O₃) (2B Technologies Model 211). NOAA GMD glass flask packages were used to collect discrete air samples that were analyzed for a number of trace gases including CO, CO₂, CH₄, C₃H₈ (propane), C₄H₁₀ (butane), C₅H₁₂ (pentane), C₂H₂ (acetylene), C₆H₆ (benzene) (2012 only) and C₂H₆ (ethane), hexane isomers, and toluene (Helmig et al., 2009, 2014). GPS location, temperature, relative humidity, and ambient pressure collected onboard were synchronized with the gas measurements.

Measurements of trace gases both from the in-situ continuous analyzers and the flask packages are reported on the NOAA/WMO scales as dry air mole fractions (moles per mole of dry air), using methods outlined in Karion et al. (2013a) and online (NOAA GMD Aircraft). O₃ and NO₂ mixing ratios (measured with respect to ambient air) are equivalent to mole fractions when referred to dry air and are denoted herein as mole fractions. In 2012, ozone measurements were made with a 2B Technologies UV photometric analyzer with the solid phase scrubber. In 2013, measurements of O₃ were made with a 2B Technology analyzer that used nitric oxide (NO) in place of a solid phase scrubber to avoid potential inference from aromatics and mercury (Ollison et al., 2014). The O₃ instruments were calibrated before and after each campaign against a NOAA GMD standard that is regularly compared with a U.S. NIST standard. The Los Gatos Research NO₂ instrument was factory calibrated prior to the field deployment.

In 2012 ten research flights were conducted over the period 3-18 February with daytime surface temperatures at or above freezing, snow free ground, and a well-mixed boundary layer (Lyman and Shorthill, 2013). In 2013 seven research flights were conducted over an 8-day period from 31 January to 7 February. In 2013 snow-covered ground, sub-freezing surface temperatures (dipping to below -10° C) low wind speeds, and limited cloud cover supported stable shallow cold pools (∼200 m at their deepest) capped by highly stratified temperature inversions.

Since there was no appreciable photochemical O₃ production in winter 2012, flights focused on determining emissions from oil and gas activities, primarily over the main gas field (Karion et al., 2013b). In 2013, flights were conducted during an episode of intense O₃ production in which the spatial and temporal features of O₃, CH₄, and O₃ precursor NMHCs mole fractions were characterized. During this 8-day period in 2013, surface ozone hourly mole fractions built up from approximately background levels of 45 ppb to over 140 ppb (Figure 2). The first flight of the study (conducted on 31 January) occurred three days after O₃ rich air from a prior event had been flushed out of the Basin by a frontal system. The final flight on 7 February took place during the early stages of an incursion of a Pacific air mass that again flushed O₃ and oil and natural gas field emissions out of the Basin in the subsequent 2 days.

The contrasting vertical temperature structure within the Basin in 2012 and 2013 played a significant role in the way gaseous constituents were distributed both vertically and across the Basin. Typical profiles obtained at a number of locations on 9 February 2012 (Figure 3a) show consistent temperature structure across the Uinta Basin (Figure 3b) with temperature decreasing with altitude. Profiles are plotted with respect to altitude above sea level (asl) to provide consistent altitude registration for the different profiles as terrain varies across the Basin. Also, in 2012 (Figure 3c) O₃ mole fractions fell within a relatively narrow range (∼30–40 ppb) in the sampled altitude range. The highest CH₄ and CO₂ mole fractions (Figure 3d and e) were constrained within the layer that was capped at ∼2000 m asl.

In contrast on 6 February 2013 (Figure 4), the flight day with the highest O₃ mole fractions observed by the aircraft, O₃ in all four profiles was generally >100 ppb below 1650 m asl. Above ∼1800 m asl there was a gradual decrease in all of the constituents with the mole fractions approaching the lower free tropospheric values at ∼2200 m asl where the temperature lapse rate became positive (decreasing temperature with altitude) (Figure 4b). This suggests there was a transition zone resulting from the slow mixing of the higher mole fractions of CH₄ and O₃ out of the top of the boundary layer into the free troposphere. A cold pool layer with a top at 1650–1700 m asl containing the highest constituent mole fractions with a layer of decreasing constituent mole fractions above was characteristic of all of the flights in winter 2013 (Figure S2).

On 6 February the flight track passed through the Bonanza power plant (located in the SE quadrant) plume and encountered enhanced CO₂ as observed in profile 4 in Figure 4e at an altitude of 1775–1850 m asl. CH₄ was not enhanced in the plume (Figure 4d) while O₃ was somewhat depleted (profile 4 in Figure 4c). The plume was located above the cold pool layer that topped out at ∼170 m agl. Similar profile structure was seen on other flight days when the aircraft encountered the plume (Figure S2). Notably, the depression of O₃ mole fractions below values of O₃ measured outside the plume was most likely an indication of O₃ titration by NO emitted from the power plant as shown to be the case in other power plant plumes (Luria et al., 1999, 2003). The buoyant plume of emissions from the power plant, whose stack height is at 192 m agl above ground level or 1815 m asl, is lofted above the stack height (shown in a photograph on February 2 - Figure S3) but then descends somewhat as the plume cools and stabilizes above the top of the temperature inversion capped boundary layer. The combustion product CO₂ observed in the plume (and CO and NO₂ not shown) on each of these encounters declined abruptly immediately below the plume at the top of the cold pool layer, evidence that the plume was not adding substantially to O₃ precursor mole fractions in the cold pool layer and not likely to O₃ production below the inversion.

In 2012 O₃ mole fractions across the Basin varied over a relatively narrow range (∼30–45 ppb) for the 13 flight days. CH₄, on the other hand showed a broader variation across the Basin (Figure S4). The location of the highest mole fractions varied from day to day (Figure S4), consistent with the variable wind conditions during the aircraft flights (Karion et al., 2013b). Peak CH₄ fell in the range 2500–3500 ppb.

In 2013 the buildup of O₃ mole fractions within the cold pool layer ≤1650 m asl across the entire expanse of the Basin during the episode of 31January – 6 February is presented in Figure 5 and for CH₄ in Figure 6. Measurements above the top of the boundary layer inversion are not included in these plots. On 1 & 2 February (Figure 5b and c) the highest O₃ amounts were not coincident with the location of the main gas field (located in the south-east quadrant) while on 4, 5 and 6 February (Figure 5d, e and f) high O₃ (>120 ppb on 5 and 6 February) measured over the Basin included the gas field. A day to day increase across the entire Basin was observed. The spatial distribution of CH₄ was notably different from O₃ (Figure 6). On each day the highest mole fractions of CH₄ were found in the vicinity of the gas field spreading out over the rest of the Basin, indicative of a primary source location for the CH₄.

The mole fractions of O₃ precursor NMHCs in the Basin (measured from flasks collected on the aircraft) plotted against corresponding CH₄ mole fractions are presented in Figure 7. The data are plotted in colors by quadrant for 2013 and indicated by light grey circles for 2012. Note the lower concentrations of NMHCs in 2012 compared to 2013. Details on the correlations for 2013 are presented in Table 1. The regressions are two-sided accounting for errors in both variables.

NMHCs and CH₄ (Figure 7 and Table 1) are tightly correlated, yielding linear regression R² results of 0.69 -.0.91. Although the NMHCs and CH₄ are similarly related in the two years, mole fractions are ∼5 times greater in 2013 compared to 2012. Included in Table 1 are the mean NMHC/CH4 ratios that were observed at the Horsepool ground site in 2012 and 2013 (Helmig et al., 2014). There is a remarkably good agreement between these two data sets, with slope values generally agreeing to within ± 20%. The relatively close similarity in these results indicates that the surface observations from the Horsepool fixed ground site can be deemed relatively representative of the Basin. Another inference is that the NMHCs were distributed across the Basin in a similar manner to CH₄, and that they are co-emitted with CH₄ (Pétron et al., 2012; Peischl et al., 2013) when natural gas is vented or leaks into the atmosphere. The source of these emissions in the SE quadrant is consistent with the presence of a dense array of gas wells, gathering pipelines, compressor stations, and two large processing plants (Chipeta Plant Complex and Stagecoach/QEP) located in this quadrant. It is notable that for most of NMHC species, in particular the lighter NMHC, i.e. propane – pentane, show ∼10–20% higher NMHC/CH₄ slope values for air originating in the SW quadrant compared to the SE. The in general lower NMHC/CH₄ values seen in the SE quadrant point towards a higher CH₄ fraction (dryer gas) in the fugitive emissions from that sector.

The average concentrations of O₃ and CH₄ within the cold pool beneath the temperature inversion for each day of aircraft measurements for the 2013 data presented in Figures 5 and 6 are plotted in Figure 8. On 31 January at the start of the event and just two days after the Basin had been swept out by the passage of a cold front, CH₄ mole fractions over the Basin were close to 3500 ppb This represents a significant enhancement above the mole fractions of ∼1900 ppb measured above the boundary layer (Figure 3), which is similar to the regional background. By 2 February mean CH₄ mole fractions of 6800 ppb were spread throughout the Basin (Figure 6). Between 2 February and 5 February, average CH₄ mole fractions leveled off peaking at just over 7000 ppb, then decreasing to ∼6000 ppb on 7 February. The leveling off and the subsequent decrease of CH₄ with the beginning of the Basin being flushed out is consistent with the time-behavior observed for CH₄ measurements at the surface made at Horse Pool (Edwards et al., 2014-extended data; Helmig et al., 2014).

Over the same time period, mean O₃ mole fractions increased (Figure 8) across the Basin. This was also seen in surface measurements (Helmig et al., 2014) and tether ozonesonde profiles (Schnell et al., 2016). On 31 January average O₃ mole fractions at 1650 m and below were ∼60 ppb increasing steadily to ∼110 ppb by 6 February before decreasing on 7 February with the approaching clean out event.

The multi-species aircraft profile measurements presented in this paper develop a picture of the relationship between various oil and gas field emissions within the Basin and their relationship to wintertime photochemical O₃ production. CH₄ and CO₂ were well correlated in the flight data both in 2012 (Figure S5) and 2013 (Figure 9) with the highest levels of both constituents measured in the vicinity of the gas field in the SE quadrant of the Basin. These correlations reflect a collocated source, with CH₄ directly emitted from extraction and gas processing while CO₂ is a product of combustion related activities also taking place in the vicinity of the gas field. High CO₂ values not corresponding to high CH₄ represent CO₂ sources without accompanying CH₄ such as power plant emissions or mobile sources (cars and trucks). This was seen most prominently in the NE quadrant in 2013 (Figure 9) where Vernal is located (the largest population center in the Basin with extensive vehicle traffic) and also in 2012 (Figure S5). Other constituents associated with combustion such as NO₂ and CO were not consistently related to CH₄ or each other in 2013 (Figure S6), likely due to additional chemical processing (NO₂) or other sources not related to CH₄ emissions (CO). In 2013 there is a strong tendency for higher O₃ mole fractions with higher CH₄ mole fractions throughout the Basin (Figure 10) reflecting the fact that O₃ precursor NMHCs are co-emitted with CH₄. However, there is a group of observations where O₃ was relatively constant with increasing CH₄. These observations are strongly concentrated in the SE quadrant. The highest mole fractions of NO₂ are also found in the SE quadrant (Figure S6). When the observations in Figure 10 are screened for NO₂ mole fractions greater than 10 ppb almost all of the observations where O₃ is relatively constant with increasing CH₄ drop out. This indicates the reduced efficiency of O₃ production in the presence of high NO_x mole fractions (Kleinman, 2005; Edwards et al., 2014; Ahmadov et al., 2015) and is consistent with the highest mole fractions of O₃ often being observed not in the immediate vicinity of the main gas field. O₃ mole fractions are moderately consistent with the variability in CO₂ (Figure 9) but do not show a consistent relationship with NO₂ or CO (Figure S7).

The contrasting meteorological conditions during the winters of 2012 and 2013 highlight the way these conditions led to only modest O₃ formation in winter 2012 and to the extreme high O₃ episodes in winter 2013. The contrasting temperature and constituent profiles between 2012 (Figure 3) and 2013 (Figure 4) show that the air confining geographic characteristics of the Basin also require conditions that prevent efficient vertical mixing and ventilation of air out of the Basin as occurred in 2012. It is worth noting that in 2012 when there was little snow on the ground almost all the incoming radiant heating was transferred to an outgoing heat flux with very little loss to latent heating (R. Zamora, private communication; Stoeckenius and McNally, 2014). Although the ground was snow-covered in 2013 (Oltmans et al., 2014) and daytime temperatures were always relatively cold, a cold pool up to ∼200 m thick with nearly constant temperature and constituent mole fractions indicated a mixed layer resulting from daytime heating of the surface and convective mixing. It was within this layer that the highest mole fractions of constituents emitted from the surface as well as elevated O₃ were measured.

In air above this mixed layer, mole fractions of all constituents declined to values representative of background levels. Assuming that CH₄ emissions remain relatively constant in the Basin and due to the long chemical lifetime of CH₄, the stabilization of CH₄ mole fractions after the first few days of the episode (Figure 8) is consistent with some advection of air out of the Basin. However, O₃ mole fractions continued to increase (Figure 8) across the Basin through the flight period (also seen in other surface and profile measurements; Helmig et al., 2014; Schnell et al., 2016). During the course of the O₃ event, nighttime O₃ minimum values continued to rise (Figure 2 and Schnell et al., 2016), indicating that nighttime chemical loss, surface deposition, and transport out of the boundary layer did not fully remove O₃ from the previous day’s production. Although CH₄ (and likely also accompanying NMHC) levels stabilized after a few days, they remained sufficiently high to fuel O₃ production and increasing O₃ mole fractions throughout the event captured by the aircraft flights.

The spatial pattern of emitted constituents in 2013, such as CH₄ (Figure 6) and CO₂ (Figure S8), is indicative of strong emissions in the region of the natural gas field in the SE quadrant of the Basin that spread throughout the Basin. Wind measurements throughout the Basin are supportive of this (Schnell et al., 2016) where a diurnal wind pattern in which daytime heating at the edge of the Basin moves air from the central portion of the Basin toward the periphery. Based on the CH₄ and aircraft flask-sampled NMHCs, this transport pattern dispersed precursors throughout the Basin. The availability of O₃ precursors throughout the Basin was reflected in the high O₃ mole fractions that encompass the entire Basin and were not concentrated around the primary source of the precursors (Figure 5) where very high NO_x levels may influence the O₃ production efficiency. On several days the highest O₃ mole fractions were encountered away from the gas field, consistent with the time scales for O₃ production that are not instantaneous. This O₃ spatial pattern in the Basin was a result of strong photochemical production over the entire Basin, not primarily a result of transport from a more localized source near where the precursors were emitted.

The tight relationship between CH₄ and NMHCs in both ground and aircraft data (Table 1) and the magnitude of the reported methane (Karion et al., 2013b) and NMHC (Helmig et al., 2014) emissions suggests that the eventual dispersal of the basin wide natural gas related emissions contribute to the atmospheric methane and NMHC increases that have recently been reported on a broader geographical scale (Helmig et al., 2016; Turner et al., 2016).

Extensive aircraft sampling of O₃, CH₄, CO, and NO₂, along with numerous gas species from flask measurements, during the winters of 2012 and 2013, demonstrated the strongly contrasting conditions in both the spatial and vertical dimensions of wintertime O₃ production in these two years. In 2012 O₃ mole fractions were confined to a narrow range of 30–45 ppb throughout the Basin with only small variations with altitude to the top of the measured profiles that extended to above the rim of the Basin. In 2012 CH₄ across the Basin varied from flight-to-flight, although the highest levels were found near the gas field and in a pattern where CH₄ was higher in the direction of the prevailing winds (Karion et al., 2013b). O₃, on the other hand, was much more uniform over the Basin reflecting the lack of connection with precursor emissions under conditions with minimal O₃ production.

The strongest O₃ production in 2013 occurred in a shallow surface layer with a strong temperature inversion topped near 1650±50 m asl. Above the cold pool was a gradient layer extending to ∼2200 m asl with elevated constituent mole fractions that gradually declined to near lower free tropospheric values.

The structure of constituent profiles measured through the plume of a power plant located in the eastern portion of the Basin did not identify a pathway for mixing chemical precursors found in the power plant plume, which was emitted far above the O₃ maximum, into the boundary layer. Also, since the enhancements in O₃ and emitted constituents were always seen in the layer below the top of the temperature inversion, there is strong evidence that the source of these enhancements was from within the Basin itself.

The contrast seen in the aircraft measurements between 2012 and 2013 demonstrates the dominant role snow cover plays in promoting winter O₃ production in the Uinta Basin. Modeling of the conditions in the 2011–2012 and 2012–2013 winters (Edwards et al., 2013; Neemann et al., 2015; Edwards et al., 2014; Ahmadov et al., 2015) supports the vital role snow cover has on wintertime ozone production as this promotes and sustains strong temperature inversions that leads to highly elevated mole fractions of ozone precursors (Schnell et al., 2009). The snow also enhances UV flux because of its high surface albedo and snow reduces O₃ deposition to the surface. Based on precursor emission levels and meteorological conditions in 2013 and similar conditions observed in 2010 and 2011 in the Uinta Basin (Oltmans et al., 2014), it is likely that high O₃ episodes can be expected in the future under snow-covered conditions if emissions from oil and gas operations remain at 2013 levels in the Uinta Basin in future winters.

Data sources are cited in the text of the manuscript with the URLs listed in the references.

© 2016 Oltmans et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

The NO₂ analyzer used on the aircraft was generously loaned to NOAA/GMD by Russ Dickerson of the University of Maryland. The skilled piloting of the aircraft by Chuck Midyett and Steve Conley was a key element in obtaining the extensive data set used in this study.